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Design Rationale and Device Configuration of Lithium‐Ion Capacitors

Design Rationale and Device Configuration of Lithium‐Ion Capacitors IntroductionNonrenewable fossil fuels are experiencing critical challenges in environmental sustainability and global warming. To deal with these problems, several types of renewable and sustainable energy resources, in particular the wind and solar energy are being explored as replacement power technologies. However, the intermittency of wind and solar power requires electricity storage solutions that can respond on a range of different timescales. Among many electrochemical energy systems, lithium‐ion batteries (LIBs) and electrochemical double‐layer capacitors (EDLCs) are the mostly highlighted for their reliable performance, dynamic power responses, and high energy efficiency (>95%).[1]Since the first report of LiCoO2 in 1980s by Goodenough, LIBs with high energy density (≈200 Wh kg−1) have played a significant role in fundamental research and commercialization for more than 30 years.[2] However, its low power density (≤1 kW kg−1) limits its further application in electric vehicles (EVs) and electric cargo ships etc.[3] Though nowadays there are numerous EVs on the market, complex battery management systems are installed to connect LIBs units both in series and in parallel to balance the power performance and traveling distance.[4] In contrast, EDLCs can provide high power density (≥10 kW kg−1), and is capable for high power system like light rail etc.[5] However, its low energy density (≤10 Wh kg−1) blocks its path to long period power supply.[3] Lithium‐ion capacitors (LICs), as a hybrid of EDLCs and LIBs, are a promising energy storage solution capable with high power (≈10 kW kg−1, which is comparable to EDLCs and over 10 times higher than LIBs) and high energy density (≈50 Wh kg−1, which is at least five times higher than SCs and 25% of the state‐of‐art LIBs).[6] The comparison of device configurations, their charge/discharge profiles as well as performance characteristics of LIBs, EDLCs and LICs are shown in Figure 1. LIBs contain two insertion‐type electrodes as positive and negative electrodes respectively, which store/output energy via Li+ insertion/extraction. The plateaus are observed on charge/discharge curve while peaks can be pointed out in cyclic voltammetry curve, reflecting the insertion/extraction of cation and the redox reactions in the bulk material. With the massive cation storage in the bulk materials, LIBs demonstrate high energy density. However, the power performance of LIBs is limited by the slow ion diffusion in the bulk material, though the self‐discharge rate (<5% of the stored capacity over 1 month) is suppressed.[7] With the high energy density, flammable electrolyte, and chemical reaction during charge/discharge, safety issue is critical for LIBs application.[8] Differently, EDLCs contain two adsorption‐type electrodes which adsorb/desorb ions during charge/discharge. The easily accessible surface ion storage site permits the rapid charge/discharge capability of EDLCs. The physical change during charge/discharge and low‐energy density enable the high safety of EDLCs. But the self‐discharge (≈50–80% loss in energy per day) is serious due to the poor interaction between the ion and the active material surface.[9] Taking the much lower energy density into consideration, the energy‐based cost ($ kW h−1) of EDLCs (≈$10000 kW h−1) is much higher than LIBs ($100–200 kW h−1).[10]1FigureComparison of LIB, EDLC, and LIC: a) device configuration; b) charge/discharge curve (up) and cyclic voltammetry curve (down) profile and c) performance evaluation considering energy/power density, lifespan, safety, cost (energy‐based, $ kW h−1), and self‐discharge rate (low value to high value from inner to outer. The value is adapted from refs. [4–7, 9, 16c, 18].To combine the advantages of both LIBs and EDLCs, the first type of LICs was introduced by Amatucci et al. in 2001, which used an activated carbon cathode capturing PF6− via adsorption/desorption and a nanostructured Li4Ti5O12 anode storing Li+ through insertion/extraction.[11] The typical hybrid configuration of LICs, as shown in Figure 1a, contains a LIBs electrode and an EDLCs electrode with an organic lithium‐ion containing electrolyte, e.g., activated carbon (AC)//graphite and AC//Li4Ti5O12 (LTO).[12] Later, in order to improve the power density, capacitor//capacitor asymmetric LICs, like AC//AC, and AC//MXene were investigated.[13] The introduction of pseudocapacitive (PC) materials enables LICs to minimize the gap between bulky diffusion‐controlled ion storage of LIBs and surface adsorption ion storage of EDLCs to build up an asymmetric device demonstrating both high power and high energy performances. LICs not only possess higher power density than LIBs, but also show wider potential range and superior energy density than EDLCs with reduced self‐discharge rate.[14] Besides, LICs also deliver a comparable life span (5000 to 10 000 cycles) as EDLCs (≥10 000 cycles), which is much longer than LIBs (≤2000 cycles) and more suitable for long‐term use. Further, the safety of LICs sits between LIBs and EDLCs. Though with higher energy density than EDLCs, due to the unmatured production and market, the cost of LICs remains the highest among LIBs and EDLCs.[15]Although there have been significant reviews detailing various aspects of LICs, they primarily focus on the material perspective but seldom on the device configuration.[16] We contribute a dedicated review on LICs with focus on the device configurations, and the R&D gaps for LICs from lab bench to market. The vital importance of active materials in the device performance is with no doubt. The correct selection of material and the efficient configuration of device together can optimize the potential of the materials for more practical uses. For instance, the imbalanced kinetics between the battery and EDLC electrodes could result in low power density or low energy density. It is still challenging to obtain a LIC with three to five times higher energy density but comparable power density (≥10 kW kg−1, material‐based) and lifespan (≥10 000 cycles) to EDLCs without optimizing the configuration of LICs.[17] This review will try to provide some insights for the connection between basic and applied studies.LICs Design RationalesThe key challenges of LIBs and EDLCs are to deliver on rapidly evolving energy storage demands. To deal with the problems, lithium‐ion capacitors are introduced.[19] A new design of LIC with PC electrode replacing the battery/EDLC electrode is also put forward to improve the power performance.[20]It is noteworthy that these three types of LICs active materials demonstrate different power/energy performance with unique galvanic charge/discharge (GCD) and cyclic voltammetry (CV) curves as illustrated in Figure 2.[21] EDLC materials demonstrate a linear GCD curve and a rectangle shape cyclic voltammetry (CV) curve, in ideal scenarios, which is due to the pure physical change during charge/discharge with surface ion adsorption/desorption.[22] On the other hand, battery materials show a GCD curve with redox plateau(s) and CV curve redox peak(s). Nevertheless, another kind of faradic material, PC material presents a nonlinear GCD curve with/without obvious redox voltage plateau(s) and nonideal rectangular CV curve.[23] Normally, the capacity of materials has an order of battery material > PC material > EDLC material, while the fast charge/discharge capability of materials shows a reverse order.[24] Notably, battery material would also demonstrate pseudocapacitive behavior with nanosized structure as more active sites are exposed to electrolyte, e.g., 6 nm LiCoO2.[25] With different energy storage kinetics and performance, it is important for us to figure out how we can configure a LIC with these materials. In this section, we will discuss the design principles and strategies of LIC first, then the configurations of battery//capacitor LICs (battery//EDLC and battery//PC) and capacitor//capacitor LICs (EDLC//PC and PC//PC) as well as the design of Li‐rich LICs.2FigureIllustration of GCD and CV curve of EDLC material (top), pseudocapacitive (PC) material (middle), and battery material (bottom).LICs Design and Configuration PrincipleChoose and Balance Active MaterialThe choice and combination of active materials determine the performance of the LICs considering rapid charge/discharge capability, energy density, lifespan, etc. For those devices require high energy density and low self‐discharge rate, the EDLC//battery combination or EDLC with battery hybrid material would be the ideal option, although the power performance of the device would be limited. However, the improvement in energy density is limited by the narrow electrochemical window (EW) due to the faradic reaction voltage plateau of battery material as shown in Figure 3a,b. For battery//EDLC configuration, balancing the cathode/anode mass ratio, prelithiating the anode active materials, and screening battery active materials with fast kinetics along with long lifespan that pairs with EDLC active materials, to obtain equal capacity of two electrodes especially under high current densities, could maximize the performance of device.[26] Oppositely, the combination of capacitor//capacitor LICs, which is lithium‐ion electrolyte‐based supercapacitors with EDLC or PC active material, could meet the demand for fast charge/discharge capability. With this capacitor//capacitor design, the EW of as‐assembled LICs could be expanded to the oxidation/reduction decomposition or lithium plating voltage of the system as displayed in Figure 3c, compensating the energy density of LICs in regard of the low capacity.[27]3FigureIllustration of LICs designs: a) LICs with an EDLC cathode and a battery/pseudocapacitive anode which has a high faradic reaction voltage plateau, resulting in a limited electrochemical window (EW); b) LICs with a battery/pseudocapacitive cathode with has a low faradic reaction voltage plateau and an EDLC anode resulting to a limited EW; c) LICs with an EDLC cathode and a pseudocapacitive anode with low or no faradic reaction voltage plateau, resulting in a maximized EW; d) comparison between LICs with an EDLC cathode and a pseudocapacitive anode with/without pre‐lithiation. The LICs with a prelithiated anode demonstrate higher capacity within the same EW.Design Li‐Rich System by PrelithiationPrelithiation is one of the key chemical steps to increase the energy density and cycling stability of LICs by designing a Li‐rich system. From the electrode perspective, prelithiation could introduce extra lithium source into the electrodes to permit high energy density and good cycling stability: 1) to provide sufficient Li+ shuttles between cathode and anode during cycling, 2) to compensate for the Li+ loss on anode due to the irreversible Li+ insertion/extraction and continuous SEI decomposition/formation, and 3) to optimize the EW of anode.[28] While from the device design as illustrated in Figure 3d, higher capacity is achievable with the lithiated anode within the same EW as the LICs without a prelithiated anode, which could promote the energy density. However, the cost of LICs production goes up with the extra lithium source introduced, e.g., lithium metal or lithium metal oxide, and the complexity in device fabrication increased, e.g., precycling electrode or mixing lithium compound with active materials.[28a] It is worth pointing out that the development of the one‐step prelithiation method without significant volume/mass change, e.g., the use of scarification lithium compound, would be one of the major research directions and shows potential industrial application.[29]Optimize the Electrode FabricationDespite selecting active materials and balancing the mass ratio of cathode/anode to achieve maximum energy density as per mentioned, the electrode fabrication should be optimized. For a typical hybrid LIC, which contains a EDLC cathode and a battery anode, higher mass loading on cathode side is expected due to the lower specific capacity of EDLC material. However, the low density of porous EDLC materials results in a much thicker cathode in the LICs, limiting the improvement in volumetric performances.[30] The thickness difference between the thick electrode and thin electrode should be minimized to achieve high volumetric performance with the application of high gravimetric capacity, high density active materials.[31]Apart from the electrode thickness balance, it is of significance to balance the gravimetric/areal/volumetric performance and power/energy density by optimizing the thickness, mass loading, and density of the electrode.[32] Although a thinner/lower mass loading electrode demonstrates a better power performance, the energy density of the device would be dragged back considering the lower portion of active materials.[33] Oppositely, the electrode should not be too thick, which might limit the power performance of the device with the increased internal resistance. It is necessary to find a suitable range of the mass loading, thickness, and density of the electrode when designing the LICs.In summary, the selection and mass ratio of cathode/anode active materials, the prelithiation strategies, the thickness, mass loading, and density of electrodes, are key factors in the design of high‐performance LICs. After going through these four aspects in the design and configuration of LICs, design of LICs by maximizing the energy density will be discussed in the next section.Configuration of LICsIn the field of electrochemical energy storage, there is a variety of active material selections with the structure from 0D nanoparticle and quantum dot, 1D nanowire, nanotube and nanobelt, 2D nanoflake, and nanosheet to 3D nanoporous skeleton, or with the mechanism from surface ion‐adsorption, fast redox reaction/ion intercalation and ion insertion.[34] From the perspective of device configuration, LICs can be classified into four sorts based on the combination of electrode mechanisms: battery//EDLC, EDLC//PC, PC//PC, and battery//PC device, as illustrated in Figure 4. It is postulated that EDLC//PC device could be very competitive in power density while battery//PC device should be more advanced for energy density. The rest configurations would perform moderately. Despite many reports on EDLC//battery and EDLC//PC types, the study on PC//PC and battery//PC is less reported. The real performance of LICs with different combinations is discussed below.4FigureIllustration of LICs combining an EDLC electrode and a battery electrode (EDLC//battery); an EDLC electrode and a pseudocapacitive (PC) electrode (EDLC//PC); two PC electrodes (PC//PC) and a PC electrode with a battery electrode (PC//Battery) (left) and their estimated performance including power/energy density, cycle life and self‐discharge rate (SDC) (right).Battery//EDLCAt the very first beginning, LICs are combined with a battery electrode and a capacitor electrode as a hybrid device, e.g., AC//Li4Ti5O12 (LTO),[11] AC//Li2TiSiO5,[35] AC//TiO2,[36] AC//SnO2/C,[37] single‐walled carbon nanotubes (SWCNT)//V2O5,[38] porous carbon//3D TiC,[39] AC//Si,[40] AC//lithium–aluminum alloy,[41] etc. In such a design, however, the unbalanced cathode/anode kinetics limits the improvement in power/energy density and lifespan. One of the typical battery//EDLC combinations is AC//hard carbon (HC) cell.[42] Sun et al. reported a LICs design with activated carbon as cathode and prelithiated hard carbon as anode, the specific energy and specific power of which reach 73.6 Wh kg−1 and 11.9 kW kg−1 based on the mass of the active materials in both cathode and anode, with a maximum specific energy of 18.1 Wh kg−1 and a maximum specific power of 3.7 kW kg−1 based on the mass of whole pouch cell.[43]Besides, various organic matter‐derived porous carbons were introduced.[44] Li et al. synthesized a corncob‐derived mesoporous/macroporous activated carbon, which showed superior performance in the LICs with Li4Ti5O12 (LTO) as anode as shown in Figure 5a.[45] The as‐assembled device demonstrated a maximum energy density of 79.6 Wh kg−1 with a power density of 200 W kg−1, which is much higher comparing with those commercial activated carbon‐based devices and similar constructions employing carbonaceous materials as the cathode and LTO as the anode.[46] Apart from this, Chaturvedi et al. assembled an asymmetric LIC with AC cathode and TiSe0.6S1.4 anode.[47] Although the cell delivered high energy density (≈50 Wh kg−1) at small current rate, its rate performance was limited by the ion insertion into the battery electrode.[48] Sun et al. designed a LIC with the alloying‐type Sn–C anode, which, in a cell with mesoporous carbon cathode, exhibited high energy densities of 196 to 85 Wh kg−1 at power density from 731 to 24 375 W kg−1, as well as good cycling stability with 70% retention after 5000 cycles.[49] Similarly, Kim et al. managed to design LICs with fully etched crumpled graphene (CG) cathode, and partially etched CG wrapped spiky iron oxide particles as anode. The as‐assembled device shows a wide working potential range of 4 V, with a specific capacitance of 238 F g−1 at a current density of 0.2 A g−1.[50] Besides, another battery anode material, BiVO4, was applied in LICs by Dubal et al.[51] The LIC with partially reduced graphene oxide cathode and BiVO4 nanorod anode demonstrated an energy density of 152 Wh kg−1 at 384 W kg−1 and 42 Wh kg−1 at 3861 W kg−1 with a 81% capacity retention rate after 6000 cycles at 0.9 A g−1.5Figurea) Illustration of the LIC assembled with activated carbon (AC) cathode and a free‐standing Li4Ti5O12 (LTO) anode. Reproduced with permission.[45] Copyright 2017, Elsevier. b) Schematic for a LIC with a cathode mixing battery type material and capacitive type material. c) Ragone plots of LIBs and LICs based on different cathodes on the gravimetric bases. Reproduced with permission.[53] Copyright 2017, Springer Nature.(Battery+EDLC)//BatteryOn the other hand, unlike those battery//EDLC hybrid device, mixing battery active material and EDLC active material together, e.g., mixing LiFePO4 (LFP) with AC, seeding LTO onto graphene or coating graphene onto LiMn2O4 (LMO), is one of the main streams.[52] In this case, the EDLC material is correlated with the power performance while the battery material contributes more to the energy performance. Nonetheless, the lifespan is closely related to that of the battery material. Zheng and co‐workers reported combining the advantages of both LIB and LIC via a synergistic combination of LiCoO2 (LCO) and AC in one electrode as shown in Figure 5b.[53] The hybrid capacitor performed two‐fold energy density comparing with the AC//HC LIC at low power density, and five times energy density than LCO//AC device at high power operation, as reflected by the Ragone plot in Figure 5c. For this type of device, the mixing electrode demonstrates both high energy and power densities. However, due to the faradic noncapacitive behavior of battery materials, the power density of the full device is falling behind when comparing to capacitor//capacitor LICs.EDLC//PCWith the aim to obtain a LIC with high power and energy output, 1D/2D/3D PC materials, including N‐doped carbon, polypyrrole nanopipes, graphene, graphdiyne, fluorinated contorted‐hexabenzocoronene, MXene, MoS2, Nb2O5, and MoO3, were used as the active material of LICs.[54] With the similar kinetics of EDLC and PC materials, high power performance and long lifespan are achievable with such a configuration. However, the energy density is limited by the low capacity of these capacitor materials. Carbon‐based material is widely applied in the field of supercapacitors and batteries. Cai et al. reported a full carbon‐based LIC as shown in Figure 6a, in which the carbon material stores energy via PF6− adsorption as cathode and stores energy via Li+ intercalation as anode.[55] The device with defect‐rich O‐doped hierarchical porous carbon (OHPC) demonstrated an energy density of 144 Wh kg−1 at 200 W kg−1 and a long cycling life with a capacity retention rate of 92% at 2 A g−1 and 70.8% at 10 A g−1 after 10 000 cycles. Xia et al. designed a symmetric lithium‐ion capacitor with B/N co‐doped 3D carbon nanofibers, which can achieve 220 Wh kg−1 at 225 W kg−1 and 104 Wh kg−1 at 22.5 kW kg−1, which is superb.[56] Wang et al. reported a N‐doped graphene based LIC with working voltage window of 4.5 V and 187.9 Wh kg−1 at 22.5 kW kg−1, as well as a capacity retention of 93.5% after 3000 cycles.[57] The application of graphdiyne in LICs system was studied by Shen et al. by using activated carbon cathode and F‐doped graphdiyne anode, which delivered 200.2 Wh kg−1 at 131.2 W kg−1 and 1 122.4 Wh kg−1at 13.1 kW kg−1.[54k]6FigureIllustration of capacitor//capacitor LICs. a) Schematic of the charge‐storage mechanisms for the defect‐rich O‐doped hierarchical porous carbon (OHPC)//OHPC LIC. Reproduced with permission.[55] Copyright 2020, The Royal Society of Chemistry. b) Charging process of CTAB‐Sn(IV)@Ti3C2//AC LIC. Reproduced with permission.[61] Copyright 2016, American Chemical Society.Transition metal compounds were also studied with potential to increase the energy density. Wang et al. introduced graphene layer into 2D MoS2 to tune the layer distance, which showed superior energy and power density of 188 Wh kg−1 at 200 W kg−1 and 45.3 Wh kg−1 at 40 kW kg−1.[58] MXene is recently used as a new 2D layered ion‐intercalation host.[59] Comparing to bulk 2D layered materials (e.g., graphite and LiMnO2) and exfoliated 2D materials (e.g., graphene sheet), 2D MXene shows the potential to be the most competitive LICs active material to perform both high energy and power densities.[60] Luo et al. reported MXene‐based LICs as shown in Figure 6b, by using pillared Ti3C2 MXene (CTAB−Sn(IV)@Ti3C2) with adjusted interlayer space by changing the size of intercalation agents.[61] With the help of the pillared layer channel and the existence of Sn4+, the ion‐diffusion rate in the layer is increased, leading to a capacitance of 25 F g−1 even under a high current density of 5 A g−1. Besides, MXene can be combined with conversion‐type material Fe3O4 as LICs anode (Fe3O4/C/MXene), which delivered an energy density of 130 Wh kg−1 at 250 W kg−1 and 31 Wh kg−1 at 25 kW kg−1, with a capacity retention rate of 86.5% after 5000 cycles under a current density of 1 A g−1.[62]PC//PCConducting polymers, e.g., polyaniline (PANi) and polypyrrole (Ppy), are attracting more focus recently as a PC cathode materials storing energy by anion doping.[63] With the capacitive behavior and fast kinetics of PC materials, the as‐fabricated LICs could possibly demonstrate moderate energy/power performance and lifespan. Han et al. reported a PC//PC LIC with PANi@carbon nanofiber (CNF) as cathode and CNF as anode. In this design, cathode stores energy via PF6− doping on PANi while anode stores Li+ by ion intercalation into CNF as shown in Figure 7a.[64] The as‐fabricated device demonstrated a maximum energy density of 106.5 Wh kg−1 at 769.0 W kg−1 and a maximum power density of 15.1 kW kg−1 at 64.5 Wh kg−1, with a capacity retention rate of 70.3% at 10 A g−1 after 7000 cycles.7Figurea) Working principle of polyaniline (PANi)//carbon nanofiber (CNF) LICs. Reproduce with permission.[64] Copyright 2020, American Chemical Society. b) Charge‐storage mechanism of polypyrrole (Ppy)@carbon nanotube (CNT)//Fe3O4@C‐based LICs. Reproduced with permission.[66] Copyright 2019, American Chemical Society.In addition, Byeon et al. attempted to apply Nb2CTx MXene as both cathode and anode for PC//PC LICs.[65] Nb2CTx‐CNT was precycled in a half‐cell with Li metal as counter and reference electrode to ≈0.1 V versus Li/Li+ to form lithiated‐Nb2CTx‐CNT prior to the assembly of Nb2CTx‐CNT//lithiated‐Nb2CTx‐CNT LIC. With Li+ intercalation/de‐intercalation into/from both positive and negative electrodes, the device demonstrates a maximum energy density of 38 Wh kg−1 at ≈25 W kg−1 within an operational voltage window of 0.0–3.0 V. Though the performance of such device is not comparable to the other state‐of‐art LICs yet, it is a proof of concept that MXene materials can also work as LICs cathode, unlocking the potential design of symmetric LICs with 2D layered materials.PC//BatteryTo further enhance the energy density of LICs with PC electrode, PC//battery design was introduced. Though the energy density could be enhanced with battery material, the lifespan and power density may be limited. Han et al. designed a LIC combining a PC cathode of Ppy/CNT as presented in Figure 7b.[66] In such a design, Ppy stores/outputs energy via pseudocapacitive PF6− doping/de‐doping. With Fe3O4‐based battery anode, the as‐obtained device demonstrated a maximum energy density of 101 Wh kg−1 at a high power density of 2.7 kW kg−1 and a maximum power density of 17.2 kW kg−1 at 70 Wh kg−1, with a capacity retention rate of 79.5% after 2000 cycles. In this design, battery electrode contributes to high energy density while the capacitor electrode delivers high power performance. On the other hand, Nb2CTx MXene is also introduced as PC anode by Byeon et al.[65] Coupling with a LFP cathode, the device demonstrates a maximum energy density of 43 Wh kg−1 at ≈10 W kg−1. The low energy density of this design is mainly due to the low working potential range from 0.0 to 3.0 V. Lately, Wu et al. reported a battery//PC‐based LICs configuration with LiNi0.815Co0.15Al0.035O2/carbon as cathode and T‐Nb2O5 as anode.[67] The as‐assembled LICs demonstrated a performance of 165 Wh kg−1 at 105 W kg−1 and 9.1 kW kg−1 at 83 Wh kg−1 based on a Li+ rocking‐chair mechanism. Such a LICs configuration will not consume ions from electrolyte, which could potentially enable the LICs design with lean electrolyte. Notably, although the PC//battery configuration has the potential of demonstrating high energy density, it is necessary to combine a high working voltage cathode with low working voltage anode to maximize the voltage of a fully charged device over 4.0 V for further energy density improvement.Prelithiation for Li‐Rich LICsAlthough the LIC is well designed, large amount of Li+ would be consumed during cycles because of the irreversibility of redox reactivity and the electrolyte decomposition/SEI layer formation on the anode surface.[68] Moreover, due to the difference of the reaction rate between cathode and anode, performance fading caused by lithium loss during cycling is unavoidable, especially at high power output.[28c,69] Therefore, prelithiation becomes more popular to improve the performance of LICs, which could be classified into three sections: pretreated anode, direct addition of Li metal, and sacrificial lithium compound.Pretreated AnodeOne of the most common prelithiation methods for anode pretreatment is based on a two‐step procedure: 1) charge the electrode in a half‐cell (with lithium metal as counter electrode) or in a full cell (with LIB cathode as positive electrode) to a suitable state of charge (SOC), 2) disassemble the cell and collect the prelithiated electrode, following by assembling the LICs with the treated electrode and positive electrode.[70] Li et al. prelithiated the mesocarbon microbead (MCMB) by charging MCMB in a half‐cell with Li‐metal as counter electrode before making the AC//MCMB LICs.[71] Similarly, Aluminum anode, a promising candidate for high capacity lithium storage, was applied in a AC//Al LIC designed by Ou et al., where the Al anode was prelithiated via electrochemical alloying in a Li half‐cell along with the formation of LiF‐rich artificial solid‐electrolyte interphase (SEI).[41] The irreversible consumption of Li+ is reduced and the cycling stability of anode is enhanced. Apart from the GCD method, Cai et al. introduced Li+ into multiwalled carbon nanotube (MWCNTs)/graphite composite anode by the internal short circuit method, in which the anode is in direct contact with Li metal with electrolyte in between as shown in Figure 8a.[72] The Li metal would be oxidized into Li+ and then travels into graphite electrode, with the help of the potential difference of graphite and lithium metal. However, these two‐step anode pretreatment methods are complicated.8FigureIllustration of some mostly used prelithiation strategies. a) Structure diagram of internal short circuit method. Reproduced with permission.[72] Copyright 2018, Springer Nature. b) A schematic diagram of an activated carbon/stable lithium metal powder (SLMP) surface applied hard carbon LICs configuration. Reproduced with permission.[42] Copyright 2012, Elsevier. c) Schematic representation of the cascade‐type pre‐lithiation strategy using a source of electrons from pyrene electro‐polymerization and, a source of lithium ions from a chemical reaction involving an insoluble inorganic base. (Csp: Carbon SuperP and PVDF, YP‐80F: a state‐of‐art supercapacitor carbon, YP‐10‐Li: YP‐80F, pyrene, Li3PO4, PVDF) Reproduced with permission.[28d] Copyright 2019, Wiley.Direct Addition of Li MetalPrelithiation could be obtained with one‐step process by introducing Li metal into the device as extra Li source. Cao and Zheng applied stable lithium metal powder (SLMP) onto the electrode surface to play the role of Li+ source as shown in Figure 8b.[42] However, this configuration is not economic‐friendly as the SLMP is expensive. Also, it may cause volume change of the whole device as the surface Li metal translating into Li+, which may lead to the poor contact between cell components. Furthermore, it is unsure whether the SLMP was separated homogeneously. To mix SLMP with active material uniformly, Lei et al. introduced SLMP into the active material slurry directly. First, graphite and polyvinylidene difluoride (PVDF) was mixed into a slurry with N‐methyl‐2‐pyrrolidone (NMP) as solvent. Then a thick layer mixture was cast onto Cu foil and followed by drying to form the PVDF‐coated graphite powder. The prepared powder as mixed with SLMP as well as styrene‐butadiene rubber (SBR) with toluene as solvent to form the slurry that is coated onto Cu foil to prepare the electrode.[73] Nonetheless, this method is not applicable in industry as it complicated the slurry preparation procedure. Tsuda and co‐workers designed a porous electrode that can allow Li+ transmit from Li metal counter electrode to the working electrode in the full cell.[74] However, extra separators and Li metal electrodes are added into the cell, resulting in lower volumetric energy/power density of LICs and higher safety risk during manufacture.Sacrificial Lithium CompoundTo avoid the potential volume change of the cell and simplify the process, sacrificial Li+ source, including lithium metal oxide that can be added into the electrode directly, are attracting increasing interest.[75]Zhang et al. introduced highly irreversible Li‐rich compound, Li2CuO2, into AC positive electrode in the AC//graphite LICs, whereas in‐situ prelithiation takes place during charging through Reactions (1) and (2), accordingly.[76]1Li2CuO2→LiCuO2+Li++e−\[\begin{array}{*{20}{c}}{{\rm{L}}{{\rm{i}}_2}{\rm{Cu}}{{\rm{O}}_2} \to {\rm{LiCu}}{{\rm{O}}_2} + {\rm{L}}{{\rm{i}}^ + } + {{\rm{e}}^ - }}\end{array}\]2LiCuO2→CuO+Li++1/2O2+e−\[\begin{array}{*{20}{c}}{{\rm{LiCu}}{{\rm{O}}_2} \to {\rm{CuO}} + {\rm{L}}{{\rm{i}}^ + } + 1{\rm{/}}2{{\rm{O}}_2} + {{\rm{e}}^ - }}\end{array}\]Despite the role of primary Li+ source, Li2CuO2 also contributes extra energy to the LICs. While operating in wider potential range (2.0–4.0V), Li2CuO2 stores/outputs energy via the reversible Li+ insertion/extraction to help improve the energy density. Apart from studying the effect of Li2CuO2 additive, there are three tips in choosing suitable lithium source material: a) high lithium content; b) delithiation takes place at lower potentials than the upper operating limit potential of the positive electrode, and c) relithiation either occurs at potentials below the lower operating limit potential of the positive electrode or is substantially irreversible. Another kind of sacrificial pre‐lithiation agent, Li3N, is introduced by Sun et al. and Liu et al.[28e,77] Extra Li+ is generated via electrochemical decomposition of Li3N during the initial charge, in which the decomposed Li+ is inserted into anode while N2 is released into electrolyte. However, Li3N is not stable under ambient atmosphere or in aprotic polar solvents, e.g., NMP. Hence, Sun et al. prepared the electrode with dry ball milling and cold pressing under protective atmosphere while Liu et al. introduced N,N‐dimethylformamide (DMF) as solvent for electrode slurry preparation, trying to extend the application of Li3N in LICs system from research to commercialization. To push prelithiation forward on the industry‐relevant path, a cascade‐type method is reported by Anothumakkool et al. as shown in Figure 8c.[28d] Pyrene monomers and an insoluble lithium compound, Li3PO4, are mixed in cathode, which enable two consecutive reactions during initial charge. Electrons and protons will be released from pyrene moieties during oxidative electrochemical polymerization when charging the electrode. Then the released protons are captured by Li3PO4, which would exchange Li+ into the electrolyte by the formation of H3PO4. Also, the polymerization reaction of pyrene is enhanced due to the consumption of protons. The authors suggested that such a method could be adapted to industry with high flexibility that both the pyrene monomer and Li3PO4 are low cost and their reactivity towards electrode slurry solvent, air, and electrolyte is tunable. Also, the redox potential of monomer is adjustable based on the nature of the pyrene substituent.[78] On the other hand, lithium salt, e.g., high concentration lithium bis(trifluoromethane) sulfonimide (LiTFSI), can also act as the lithium source in the LICs.[79] Jezowski and fellow researchers introduced a sacrificial organic lithium salt, 3,4‐dihydroxybenzonitrile di‐lithium salt (Li2DHBN), which is insoluble before delithiation and turns soluble after de‐lithiation, into the positive electrode as the lithium‐ion source for the AC//graphite LICs.[54] During first charge/discharge cycle, 1.9 Li+ per formula unit extracts from Li2DHBN irreversibly. Meanwhile, the product of this reaction, 3,4‐dioxobenzonitrile (DOBN), shows good solubility in carbonate‐based electrolyte and shows no influence on the ion conductivity.[29] However, Li2DHBN is not stable under air, which would increase the cost during industrial processing. Another di‐lithium compound, dilithium squarate (Li2C2O4), which is air‐stable, safe, highly irreversible and cost‐effective with a decomposition voltage from 3.5 to 4.5 V versus Li/Li+, was studied as a sacrificial lithium source mixing with cathode active material in a AC//hard carbon (HC)‐based LIC.[80] The pre‐lithiated LIC demonstrated a long lifespan with a capacity retention rate of 84% after 48 000 cycles at 1 A g−1. Note that this lithium salt can be transformed into sodium/potassium salt by solvent exchange for the pre‐metalation of sodium/potassium‐ion capacitors.The characteristics of LICs with various configurations are summarized in Table 1. Here, both EDLC//battery and PC//battery configurations are classified as capacitor//battery while the EDLC//PC and PC//PC configurations are cataloged as capacitor//capacitor.1TableSummary of typical state‐of‐the‐art lithium‐ion capacitor configurationsConfigurationPrelithiationVoltage [V]EMaxPMaxCyclabilityYear of publicationRefs.Capacitor//battery(+) Activated carbon (AC)//graphite (−)Sacrificial lithium compound4.2196.0 Wh kg−1 at 456.0 W kg−1_96.5% at 0.5 mA cm−2 after 100 cycles2017[76](+) AC//Li4Ti5O12/graphene (−)_4.046.0 Wh kg−1 at 625.0 W kg−12.5 kW kg−1 at 26.0 Wh kg−183.0% at 1 A g−1 after 4000 cycles2017[45](+) AC//Li3VO4 (−)_4.0136.4 Wh kg−1 at 532.0 W kg−111.0 kW kg−1 at 24.4 Wh kg−187.0% at 2 A g−1 after 1500 cycles2017[81](+) AC//Si/C (−)Precycled anode4.0227.0 Wh kg−1 at 1146.0 W kg−132.6 kW kg−1 at 181.0 Wh kg−1>90.0% at 6.4 A g−1 after 16 000 cycles2017[82](+) AC//Sn/C (−)Precycled anode4.5195.7 Wh kg−1 at 731.0 W kg−124.4 kW kg−1 at 84.6 Wh kg−170.0% at 2 A g−1 after 5000 cycles2017[49](+) Reduced graphene oxide (rGO)//BiVO4 nanorod (−)Precycled anode4.0152.0 Wh kg−1 at 384.0 W kg−13.9 kW kg−1 at 42.0 Wh kg−181.0% at 0.9 A g−1 after 6000 cycles2018[51](+) Graphene//graphene/Fe2O3 (−)_4.0121.0 Wh kg−1 at 200.0 W kg−118.0 kW kg−1 at 60.1 Wh kg−187.0% at 1 A g−1 after 2000 cycles2018[50](+) AC//Si/Cu nanowire (−)Precycled anode4.2210.0 Wh kg−1 at 193.0 W kg−199.0 kW kg−1 at 43.0 Wh kg−190.0% at 10 A g−1 after 30 000 cycles2018[83](+) AC//F‐enriched graphdiyne (−)Precycled anode4.0200.2 Wh kg−1 at 131.2 W kg−113.1 kW kg−1 at 122.4 Wh kg−180% at 2 A g−1 after 5000 cycles and 80% at 5 A g−1 after 6000 cycles2019[54k](+) Polypyrrole/CNT//Fe3O4/C (−)Precycled anode4.0101.0 Wh kg−1 at 2709 W kg−117.2 kW kg−1 at 70.0 Wh kg−179.5% at 10 A g−1 after 2000 cycles2019[66](+) B/N co‐doped carbon//SnS2/rGO (−)Precycled anode4.5149.5 Wh kg−1 at ≈92.0 W kg−135 kW kg−1 at ≈65 Wh kg−190.5% at 5.0 A g−1 after 10 000 cycles2020[84](+) AC//soft carbon (−)Sacrificial lithium compound4.074.7 Wh kg−1 at 75.1 W kg−112.9 kW kg−1 at 21.5 Wh kg−191.0% at 0.5 A g−1 after 10 000 cycles2020[28e](+) AC//hard carbon (HC) (−)Sacrificial lithium compound4.2120.0 Wh kg−1 at ≈110.0 W kg−1≈3.0 kW kg−1 at ≈90 Wh kg−190.0% at ≈5.0 A g−1 after 10 000 cycles2020[77](+) AC//Fe3O4/C/Ti3C2Tx (−)Precycled anode4.5130.0 Wh kg−1 at 250.0 W kg−125.0 kW kg−1 at 31.0 Wh kg−186.5% at 1 A g−1 after 5000 cycles2020[62](+) AC//HC (−)Sacrificial lithium compound4.073.0 Wh kg−1 at 299.0 W kg−15.8 kW kg−1 at 45.0 Wh kg−184% at 1 A g−1 after 48 000 cycles2020[80](+) AC//Li‐Al alloy (−)Pre‐cycled anode4.5≈489.3 Wh kg−1 at 450.0 W kg−17.2 kW kg−1 at ≈283.8 Wh kg−185.6% at 0.8 A g−1 after 2000 cycles2020[41](+) LiNi0.815Co0.15Al0.035O2/carbon//T‐Nb2O5 (−)−3.0165.0 Wh kg−1 at 105.0 W kg−19.1 kW kg−1 at 83.0 Wh kg−191.0% at 1 A g−1 after 400 cycles2021[67](+) AC//KNb3O8/carbon cloth (−)−3.569.0 Wh kg−1 at 346.0 W kg−13.8 kW kg−1 at 36.0 Wh kg−188.0% at 2.0 A g−1 after 1000 cycles2021[85](+) Boron carbonitride nanotubes//Si/graphene aerogel (−)−4.5197.3 Wh kg−1 at 225.0 W kg−14.5 kW kg−1 at 127.1 Wh kg−182.4% at 10.0 A g−1 after 10 000 cycles2021[86]Capacitor//capacitor(+) AC//vanadium nitride/rGO (−)Precycled anode4.0162.0 Wh kg−1 at 20.0 W kg−110.0 kW kg−1 at 64.0 Wh kg−183.0% at 2 A g−1 after 1000 cycles2015[87](+) Carbon fibres//TiNb2O7/C (−)_4.2110.4 Wh kg−1 at 99.6 W kg−15.5 kW kg−1 at 20.0 Wh kg−177.0% at 0.2 A g−1 after 1500 cycles2015[88](+) N‐doped carbon//MnO/graphene (−)Internal short‐circuit4.0127.0 Wh kg−1 at 125.0 W kg−125.0 kW kg−1 at 83.3 Wh kg−181.0% at 5 A g−1 after 2000 cycles2015[89](+) Nb2CTx/carbon nanotube (CNT)//lithiated‐Nb2CTx/CNT (−)Precycled anode3.038.0 Wh kg−1 at ≈25.0 W kg−10.35 kW kg−1 at ≈8.0 Wh kg−1>70.0% at 0.25 A g−1 after 900 cycles2016[65](+) rGO//SnO2/rGO (−)Pre‐cycled anode4.1186.0 Wh kg−1 at 142.0 W kg−110.0 kW kg−1 at 12.0 Wh kg−170.0% at 3 A g−1 after 5000 cycles2017[90](+) N‐doped carbon nanosheet//ZnMn2O4/graphene (−)Internal short‐circuit4.0202.8 Wh kg−1 at 180.0 W kg−121.0 kW kg−1 at 98.0 Wh kg−177.8% at 5 A g−1 after 3000 cycles2017[91](+) B, N‐doped porous carbon nanofibers//B, N‐doped porous carbon nanofibers (−)Internal short‐circuit4.5220.0 Wh kg−1 at 225 W kg−122.5 kW kg−1 at 104.0 Wh kg−181.0% at 2 A g−1 after 5000 cycles2017[56](+) AC//MoS2/rGO (−)Precycled anode4.0188.0 Wh kg−1 at 200.0 W kg−140.0 kW kg−1 at 45.3 Wh kg−180.0% at 2 A g−1 after 10 000 cycles2017[58](+) AC//pillared‐Ti3C2 MXene (−)Precycled anode3.0105.6 Wh kg−1 at 495.0 W kg−110.8 kW kg−1 at 45.3 Wh kg−171.1% at 2 A g−1 after 4000 cycles2017[61](+) rGO//N‐doped carbon nanopipes (−)Precycled anode4.0262.0 Wh kg−1 at 450.0 W kg−19.0 kW kg−1 at 78.0 Wh kg−191.0% at 0.9 A g−1 after 4000 cycles2018[92](+) Polypyrrole nanopipes//N‐doped carbon nanopipes (−)Precycled anode4.5107.0 Wh kg−1 at ≈25.0 W kg−110.0 kW kg−1 at ≈30.0 Wh kg−193.0% at 1 A g−1 after 2000 cycles2019[54j](+) AC//carbon black (−)Sacrificial lithium compound4.463.0 Wh kg−1 at ≈440.0 W kg−1_>95.0% at 0.3 A g−1 after 2000 cycles2019[28d](+) AC//Nb2O5/C/rGO (−)Precycled anode3.271.5 Wh kg−1 at 247.0 W kg−13.9 kW kg−1 at 18.3 Wh kg−194.0% at 0.2 A g−1 after 2500 cycles2019[93](+) Activated N‐doped graphene//N‐doped graphene (−)_4.5187.9 Wh kg−1 at 2250.0 W kg−111.3 kW kg−1 at 111.4 Wh kg−193.5% at 2 A g−1 after 3000 cycles2019[57](+) Sponge‐like carbon//Sponge‐like carbon (−)Pre‐cycled anode3.591.0 Wh kg−1 at 145.0 W kg−118.5 kW kg−1 at 22.0 Wh kg−199.0% at 50 A g−1 after 100 000 cycles2019[94](+) PANi/CNF//CNF(−)Precycled anode4.061.5 Wh kg−1 at 444.1 W kg−18.7 kW kg−1 at 37.3 Wh kg−170.3% at 10 A g−1 after 7000 cycles2020[64]Design of Multifunctional LICsWith the advantage in energy storage, LICs could also work as power supply for wearable electronics and micro devices, which requires multifunctional device design, including flexibility, quasi solid‐state and miniaturized structure. However, the study on these is lacking. Sharing similar device configurations with regular LICs in previous sessions, the major challenges in the development of multifunctional LICs are the solid‐state electrolyte with high ionic conductivity, wide electrochemical window, low electrolyte leakage risk, and high flexibility. Some design strategies in the multifunctional batteries and supercapacitors can be applied onto the design of multifunctional LICs. Notably, the current design of multifunctional LICs mainly focuses on minimizing the device footprint and designing flexible devices. In comparison with other multifunctional batteries and supercapacitors, which are moving much further onto the perspectives of rollable, stretchable, transparent, self‐healing, self‐chargeable, or degradable devices,[95] the development of multifunctional LICs is still in its early stage.Design Strategies of Multifunctional LICsDevelopment of Solid‐State Electrolyte with High SafetyConsidering the application environment of multifunctional LICs, e.g., in wearable devices and microgrids, electrolyte leakage would increase the risk of system failure. Herein, quasi‐solid‐state electrolyte is the favorite option to solve this issue. With conventional Li‐ion containing organic electrolyte absorbed in poly(vinylidene fluoride)‐co‐hexafluoropropylene (PVDF‐HFP) or polyacrylonitrile (PAN)‐based polymer framework, gel‐type electrolytes are widely studied and used in multifunctional LICs and LIBs for their comparable ionic conductivity to liquid electrolyte at room temperature, potentially with wider electrochemical window and excellent flexibility. However, the assessment onto the mechanical property and safety of these electrolytes, including flammability and toxicity, are not comprehensive. Especially for those multifunctional LICs used in wearable electronics, the development of non‐flammable and nontoxic electrolyte is desired.[96]Development of Flexible Electrode and Current CollectorIn the conventional device electrode design, active material in micrometer size is preferred for its high packing density. However, the contact between particles would be weakened during banding, resulting in the reduced electrochemical performance of multifunctional LICs. Replacing the micrometer‐sized active material particle with nano/porous/twisted structured particles is regarded as a solution to enhance the connectivity between active materials and maintain electronic conductivity.[97] Alternatively, the deformation of metallic foil during flexing limits the application of Al and Cu foils as current collectors in multifunctional LICs directly. The anchoring of active materials on metallic foils can be strengthened by structure engineering, e.g., honeycomb‐patterning.[98] Also, advanced substrates with high mechanical strength and electronic conductivity, e.g., CNT film, graphene film, and carbon cloth, are suggested to be used in flexible LICs for good stability and high active material mass loading.[99]Development of Advanced Package Material and Manufacture TechniqueUnlike conventional LICs that are packed with cylinder or Al‐plastic laminated foil, thin, soft, lightweight but robust package materials with low water vapor/air permeability (comparable to Al‐plastic laminated foil) are preferred in multifunctional LICs encapsulation.[99] However, the research from this aspect is in lack. Some researchers are still sealing their flexible LICs with Al‐plastic laminated foil, which shows poor flexibility and large thickness. On the other hand, polydimethylsiloxane (PDMS) may be another choice for encapsulation for its superior mechanical property, though the full assessment including air permeability is necessary. Apart from the package materials, fast, scalable and cost‐effective manufacture techniques, e.g., 3D printing, are the key for the large‐scale production of multifunctional LICs.[100]Flexible and Quasi‐Solid‐State LICsHere, some examples showing the development of flexible and quasi‐solid‐state LICs are discussed. Deng et al. introduced an in‐plane assembled orthorhombic Nb2O5 nanorod films on carbon cloth with high Li+ intercalation rate.[101] The LICs assembled with activated carbon cathode (AC) and Nb2O5 nanorod film anode performed outstanding high gravimetric and volumetric energy/power densities of 95.55 Wh kg−1 at 5350.9 W kg−1 and 6.7 mWh cm−3 at 374.63 mW cm−3, which is due to the fast electronic transportation through the single crystalline Nb2O5 and the rapid Li+ migration in the interconnected pores of the electrode film as well as the short solid‐state Li+ diffusion path, respectively. Wang et al. reported a quasi‐solid‐state LIC with graphene nanosheet as cathode, graphene nanosheet‐wrapped TiO2 as anode.[102] The PVDF‐HFP gel works as electrolyte host by absorbing conventional organic electrolyte. The as‐fabricated device demonstrated a maximum energy density of 72 Wh kg−1 at 303 W kg−1, a maximum power density of 2 kW kg−1 at 10 Wh kg−1. With similar electrolyte system, a flexible LIC was designed by Que et al. with activated carbon (AC)/carbon nanotube (CNT) as cathode and TiO2‐x/CNT as anode as shown in Figure 9a.[103] The device demonstrated a maximum energy density of 104 Wh kg−1 at 250 W kg−1 and a maximum power density of 5 kW kg−1 at 32 Wh kg−1.9Figurea) Schematic illustration of the TiO2‐x/CNT//AC/CNT flexible hybrid device. Reproduced with permission.[103] Copyright 2018, Wiley. b) Schematic of the on‐chip prototype with AC/graphite configuration and interdigital 3D electrode structure supported by the separator. Reproduced with permission.[104] Copyright 2015, Elsevier.Micro LICsApart from the flexible device design, micro LICs also show the potential to power the wearable electronics. Li et al. fabricate a micro LIC with AC as cathode and pre‐lithiated graphite as anode with micro electromechanical systems as demonstrated in Figure 9b.[104] The device with conventional 1 m LiPF6 in organic solvents electrolyte is encapsulated with PDMS. It outputs an energy density ≈1750 mJ cm−2 at 0.5 mA cm−2, which is higher than the AC‐based micro supercapacitor with similar design. Another flexible micro LIC with planar design is reported by Zheng et al. by using layer‐by‐layer mask assisted deposition to assemble activated graphene (AG) and Li4Ti5O12 (LTO) on nylon membrane substrate as cathode and anode.[164] With LiTFSI‐P14TFSI‐PVDF‐HFP ionogel as electrolyte, the quasi‐solid‐state micro LIC demonstrates a maximum energy density of 53.5 mWh cm−3 at ≈0.1 W cm−3 and a maximum power density of 4.6 W cm−3 at ≈10 mWh cm−3 with a capacity retention rate of 98.9% after 6000 cycles at 0.4 mA cm−2.[105]The device design of multifunctional LICs is listed in Table 2. Comparing the reported devices with multifunctional batteries and supercapacitors, it could be pointed out the development of multifunctional LICs is still at a very early stage and the study from electrode/electrolyte to encapsulation and manufacture is in need.2TableSummary of state‐of‐the‐art multifunctional LICsConfigurationElectrolyteFlexibility? (Y/N)Maximum energy densityMaximum power densityLifespanYear of publicationRefs.(+) AC//Prelithiated graphite (−)Liquid 1 m LiPF6 in organic solvent (Encapsulated by PDMS)N81.0 at 220.0 W kg−1_71% after 1000 cycles at 1 mA cm−22015[104](+) Nanographene sheet//nano graphene sheet/TiO2 (−)PVDF‐HFP containing 1 m LiPF6 in organic solventN72.0 at 303 W kg−12000 at 10 Wh kg−168% after 1000 cycles at 1.5 A g−12016[102](+) Graphene sheet//Li3VO4/CNF (−)PVDF‐HFP containing 1 m LiClO4 in organic solventN110.0 at 173.0 W kg−13870 at 28 Wh kg−186% after 2400 cycles at 0.4 A g−12017[106](+) AC/CNT//TiO2‐x/CNT (−)PVDF‐HFP containing 1 m LiPF6 in organic solventY104.0 at 250.0 W kg−15000 at 32.0 Wh kg−171% after 2000 cycles at 1 A g−12018[103](+) AC/carbon fiber//Nb2O5/carbon fiber (−)Liquid 1 m LiPF6 in organic solvent (encapsulated by a thin plastic film)Y95.6 at 191.0 W kg−15350.9 at 65.4 Wh kg−173% after 1000 cycles at 0.5 A g−12018[101](+) AC//maleic acid@PVDF(−)Liquid 1 m LiPF6 in organic solvent (encapsulated by Al‐plastic foil)Y153.1 at ≈215.0 W kg−1≈4300 at 22.1 Wh kg−1_2018[107](+) Activated graphene//LTO (−)a)LiTFSI‐P14TFSI‐PVDF‐HFP ionogelY53.5 mWh cm−3 at ≈0.1 W cm−34.6 W cm−3 at ≈10 mWh cm−398.9% after 6000 cycles at 0.4 mA cm−22018[105](+) PANi@CNF//CNF (−)b)PAN containing 1 m LiPF6 in organic solventY≈172.9 mWh cm−2 at ≈0.8 mW cm−2≈16 mW cm−2 at ≈74.7 mWh cm−2_2020[64](+) AC//Li3VO4/graphene (−)a)LiTFSI‐P14TFSI in PVDF‐HFPN51.4 mWh cm−3 at 53.2 mW cm−3511 mW cm−3 at 34.1 mWh cm−390.0% after 1000 cycles at 0.02 mA cm−22021[108]a)Only areal and volumetric performance reportedb)The data is converted from the information provided in the literature.Possible Roadmap for LICs from Academy to IndustryStatus of Commercial LICsEDLC//battery is the only commercialized LICs configuration by Musashi Energy Solutions Co., Ltd.(ULTIMO),[109] TAIYO YUDEN,[110] NEC Tokin,[111] and General Capacitor.[112] With porous carbon‐based EDLC cathode and carbon‐based battery anode with/without pre‐lithiation, these commercialized LICs demonstrate device‐based maximum energy density from 8 to 45 Wh kg−1 (9.1 to 90 Wh L−1) and maximum power density from 4000 to 14000 W kg−1 (3400 to 15 000 W L−1) with voltage up to 4 V. Also, lifespan from 50 000 to 1 000 000 cycles is achievable.[28c] These high‐performance products showcase the promising future of LICs in electrochemical energy storage. Notably, other LICs configurations, e.g., EDLC//PC, PC//PC, and PC//battery are absent from the market.With large specific surface area, EDLC material suffers from significant side reactions when operating at low potential, e.g., electrolyte decomposition and continuous SEI formation, which blocks its application as efficient anode in LICs.[113] Meanwhile, PC material demonstrates both fast kinetics and large capacity, which is an emerging active material in LICs. However, the mechanism understanding of PC material is shallow, comparing with conventional EDLC and battery materials.[114] Further, there is in lack of PC material with wide EW (>3.5 V or <1.0 V vs Li/Li+) that could substitute EDLC cathode or battery anode in LICs.[115] Apart from active material selection and device configuration, simplified and low cost prelithiation method is in great demand for safer and more eco/environmental‐friendly LICs production.[69b]Path for Advanced LICs from Academy to IndustryIn this session, we would try to explain the possible roadmap from academy to industry for advanced LICs with various configurations and simplified prelithiation process. The summary of LICs reported in literatures in Table 1 shows that, the active material‐based maximum energy density of state‐of‐the‐art LICs ranges from 38.0 to 489.3 Wh kg−1, while the maximum power ranges from 2.5 to 350.0 kW kg−1, depending on the configuration of device and the selection of active materials. However, the performance reported based on the mass of active materials only fails to reflect the real performance of industrial product with the same design. In the conventional SCs, taking all the other components into consideration, including current collectors, electrolytes and etc., a factor of 3 to 4 will be used to convert the material‐based performance into the device‐based one.[116] Here, a factor of 3 is applied to convert the laboratory LICs performance into the industrial one in comparison with conventional LIBs and SCs, the value of which is adapted from refs. [18] and [33]. The converted device‐based LICs performance ranges from 12.3 to 163.1 Wh kg−1 in maximum energy density and 0.8 to 116.7 kW kg−1 in maximum power density, which shows a significant advantage comparing to the commercial LICs. As the inserted Ragone plot shown in Figure 10, with the recent development, LICs are catching up with EDLCs in fast energy storage/output capability with superior energy density and demonstrating better power performance than LIBs with comparable energy density. Besides, battery//capacitor devices demonstrate higher energy density while the capacitor//capacitor device is equipped with superior power performance.10FigureThe inserted Ragone plot compares the device‐based performance of SCs, LICs, and LIBs. The value of battery//capacitor and capacitor//capacitor LICs performance is converted from the material‐based energy/power density listed in Table 1 (with extreme value excluded) and a factor of 3 is applied during calculation.[116] The performance of LIBs and SCs is adapted from refs. [1b, 16f].With the rapid development of LICs, it is foreseeable that the scale‐up LICs could achieve 50% or even 80% in maximum energy density of the LIBs (≈200 Wh kg−1 for state‐of‐the‐art) with a maximum power density over 10 kW kg−1 (which is comparable to the commercial EDLCs) as illustrated in Figure 11.[117] Also, excellent stability with cycle life over 10 000 cycles and high safety without electrolyte leakage and flammable issues are preferred. Furthermore, sharing the similar set up of LIBs but a lower (≈50%) energy density, the energy cost ($ kW h−1) of LICs is expected to be nearly double of LIBs, which is less than $500 kW h−1. With such advantages, LICs are suggested to replace LIBs in high‐power applications and EDLCs in the situations that both high‐power, high‐energy are required, e.g., hybrid vehicles, crane plants, industrial machines, and electronic processors etc. The possible roadmap for LICs from laboratory to commercialization is discussed below.11FigureIllustration of a possible roadmap for LICs from lab product to industrial device. (The cost projection of LICs in future development is based on the factors that, LICs are sharing the similar set up, but demonstrating ≈50% energy density of current LIBs).High‐Performance Active MaterialWhile there are numerous potential LICs anode materials (e.g., carbon‐based materials, transition metal oxides and MXenes) with their associated energy storage mechanisms, the choice for cathode materials appears to be restricted due to the low operational potential Li/Li+. Especially for those capacitive materials, carbon‐based materials with different atom doping and morphology are still the most popular candidate to stores energy via surface ion adsorption, which provides high power performance with no doubt. However, their energy density is poor when compared to battery cathodes due to the low capacity. The lack of PC materials with higher energy density and good power performance for LICs cathode seems to be the bottleneck that limits the further development of device at this stage.Despite porous carbons, there is in lack of anion host for reasons: the anion size is large and ion diffusion channel in host materials is narrow. Despite this, anion intercalation host with larger ion diffusion channel could be the focus point. Based on this, the anion/cation‐intercalation host‐type LICs, which will show much higher capacitance theoretically, and may be the next generation high‐performance LICs.Nevertheless, self‐discharge is another key factor in the performance judgment of LICs. LICs with EDLC cathode demonstrate comparable self‐discharge behavior to EDLCs due to the physical ion adsorption in EDLC active material.[118] Formation of passivation layer on the surface of EDLC materials could minimize the self‐discharge effectively.[119] Meanwhile, great effort in detailed mechanism understanding and investigation into effective and feasible self‐discharge suppression is necessary to further improve the performance of LICs.Active Material Selection and BalancingLICs in both academy and industry share the similar design principles, e.g., voltage window expansion, mass ratio balance and cathode/anode kinetic balance etc., to achieve high energy/power performance with ultralong lifespan. From the recent development of LICs reported, it is foreseeable that LICs could achieve a high energy density over 100 Wh kg−1 with comparable power density and cycling life to those supercapacitors. Though gravimetric performance of LICs is widely reported, the areal/volumetric performance is of equal importance in industry for the design of compact device. Active materials with high density (low porosity) and excellent conductivity are desired, which enable the design of electrodes with high mass loading, high density but low electrolyte uptake, leading to the design of LICs with high areal/volumetric performance. Also, cathode/anode thickness balance shows significant influence onto the design of compact LICs.[120]Minimizing the Offsetting Effect of Nonactive ComponentsSignificant differences in the amount of nonactive components, e.g., electrolyte, conductive agent, and binder, are observed between laboratory and commercial devices. Porous or nanosized active materials may require flooding electrolyte uptake in electrode for surface‐like ion diffusion or anion source. However, high portion of electrolyte limits the performance on an electrode/device‐based discussion. Especially in commercial devices, lean electrolyte system is preferable. By minimizing the volume of electrolyte per device, it enables the design of lightweight and compact LICs with low cost. However, the insufficient electrolyte may induce the polarization, reducing the energy/power performance.[121] On the other hand, the development of high ion conductive electrolyte with wide operation voltage range is important for both academy and industry, which could increase the columbic efficiency during cycling by minimizing the side reaction between electrode/electrolyte and reduce the ionic polarization under high current density.Apart from electrolyte, the high portion of active material in electrode, including the low content of conductive agent and binder, and high active material mass loading, in those conventional products are required to achieve higher energy density, which would result to the high electronic resistance and limit the power performance of LICs.[122] Notably, active materials tend to be exfoliated from the substrate under high mass loading or in flexible LICs during flexing, resulting to a reduced lifespan. Herein, the development of advanced current collectors and electrode fabrication methods that anchors the material onto substrate strongly are desired.Simplifying of Skipping PrelithiationPrelithiation technique is essential for LICs activation and achieving high power/energy performance with excellent stability due to the lack of sufficient lithium source in the device. However, the methodologies involved, e.g., pretreating anode and introducing extra lithium source in electrode etc., and the materials used, e.g., Li metal and lithium salt etc., complicate the device fabrication process with cost increased. Though researchers in the field are putting great efforts to develop techniques and materials that are suitable for industrial application, e.g., integrating sacrificial Li‐compounds into electrode, the necessity of pre‐lithiation may still block the commercialization of LICs. As summarized from Table 1 and to our best knowledge, there is in lack of Li‐rich active material with capacitive behavior for LICs. The investigation into highly reversible Li‐rich capacitive electrode can improve the performance of LICs greatly. Meanwhile, the pre‐lithiation step could be skipped, simplifying the LICs fabrication process and lowering the cost.Safety ImprovementLike LIBs, it should be noticed that the issues of electrolyte leakage and flammability remain critical in device commercialization with liquid organic electrolytes, which limit the application of LICs, especially in flexible devices. To get rid of this drawback, it is necessary to develop solid‐state electrolyte with both wide operational potential range as well as high ionic conductivity to maintain the high power/energy density advantages of LICs without the risk of leakage.[123] Notably, the use of solid‐state electrolyte could also enable the possibility of flexible LICs and reduce the self‐discharge behavior.[124] Nevertheless, the development of nonflammable electrolyte, is also urgent for those organic Li‐ion electrolyte systems.[125] A conventional LiPF6‐based electrolyte bending with fluorinated cyclic phosphate solvent that reported recently demonstrates excellent thermal stability and high safety for LIBs and shows the potential to be extended to the system of LICs.[126]Reducing the Production CostThe LICs are more expensive than LIBs in the value of $ kWh−1, with the much lower energy density, unmatured production and market. With the similar components to LIBs, e.g., electrolyte, separator, production line, etc., and higher energy density than EDLCs, the cost of LICs should sit between EDLCs and LIBs. With the future development on performance and fabrication process simplification, it is possible to reduce the cost of LICs to the value close to LIBs. On the other hand, considering the power‐based price, $ kW−1, with the comparable power performance to EDLCs, LICs are targeted with a lower power cost than LIBs.Setting Up Standardized Evaluation ProtocolSetting standardized protocols in performance testing and reporting as well as taking the industrial requirements into consideration in research could further boost the development of next generation LICs. The tested LICs performance may vary from each other with different electrode mass loading, thickness, mass ratio between active material and non‐active materials, volume of electrolyte used, testing temperature etc. Herein, it is recommended to report the testing condition together with the electrode/device performance. When reporting the performance of device, the calculation should base on the total mass/area/volume of two electrodes. Further, LICs assembly with high electrode mass loading and minimized volume of electrolyte infilled could better reflect the real performance of the material/device design in commercialization. Despite those widely studied aspects reported in literatures, the industry‐related evaluations, including LICs performance within wide operational temperature range, thermal characterization (reflecting the irreversible lifespan reduction and potential safety issues due to the heat generated by the electronic resistance of electrode and ionic resistance of electrolyte during charge/discharge), lifespan prediction and self‐discharge rate etc., could give more insight onto the commercialization possibility of the lab products.[127]From LICs to Post LICsFinally, we would like to extend the knowledge gained from LICs research to other electrochemical energy systems. Post LICs, e.g., sodium‐ion capacitors (NICs) and potassium‐ion capacitors (KICs), are attracting numerous interests for their high performance and potentially low cost.[128] Due to the larger size of sodium ion (1.02 Å) and potassium ion (1.38 Å) to lithium ion (0.76 Å),[129] the current cation host in LICs may not be applicable to NICs and KICs. More efforts in searching for suitable active materials in post LICs are necessary. Apart from that, sharing the similar working mechanisms, the general LICs design rational and device configurations are adaptable to post LICs, including: a) optimizing the pre‐metalation process, e.g., replacing the direct use of highly active alkaline metal (lithium, sodium, and potassium) with other sacrificial alkaline metal‐rich components to improve the safety and sustainability; b) developing high‐performance active materials with the consideration of high power/energy, long cycle life and low self‐discharge rate; and c) balancing cathode/anode capacity and kinetics to prolong the lifespan and maximize the energy‐power performance of devices etc. The experiences learnt from LICs may provide a shortcut to the development of post LICs.OutlookIn this review, we distinguish LICs from their electrochemical energy storage counterparts EDLCs and LIBs. The comparative advantages and disadvantages of the different LICs device designs (i.e., battery//capacitor and capacitor//capacitor) were reviewed with state‐of‐the‐art example devices being used to exemplify different aspects of device design and functionality. Finally, key differences between laboratory and industrial metrics of LICs were identified, with possible development roadmap pointed out. The experiences learnt from LICs research could be further extended to post LICs. We conclude that, although LICs can provide higher energy density than EDLCs and faster responses than LIBs, they also demonstrate some deficiencies. With the further improvements of performance and safety, and reduction in cost, LICs would show unparalleled competence comparing with LIBs and EDLCs to meet the high power and high energy demand market.AcknowledgementsThis work was financially supported by the Australian Research Council Discovery Project (DP190101008), Future Fellowship (FT190100058), and the UNSW Scientia PhD Scholarship.Open access publishing facilitated by University of New South Wales, as part of the Wiley ‐ University of New South Wales agreement via the Council of Australian University Librarians.Conflict of InterestThe authors declare no conflict of interest.a) N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung, J. Thomas, Adv. Mater. 2017, 29, 1605336;b) Poonam, K. Sharma, A. Arora, S. K. Tripathi, J. Energy Storage 2019, 21, 801;c) P. Poizot, J. Gaubicher, S. Renault, L. Dubois, Y. Liang, Y. Yao, Chem. Rev. 2020, 120, 6490.K. Mizushima, P. C. Jones, P. J. Wiseman, J. B. Goodenough, Mater. Res. 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Design Rationale and Device Configuration of Lithium‐Ion Capacitors

Liang, Jiaxing; Wang, Da‐Wei
Advanced Energy Materials , Volume 12 (25) – Jul 1, 2022
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Wiley
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© 2022 Wiley‐VCH GmbH
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1614-6832
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DOI
10.1002/aenm.202200920
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Abstract

IntroductionNonrenewable fossil fuels are experiencing critical challenges in environmental sustainability and global warming. To deal with these problems, several types of renewable and sustainable energy resources, in particular the wind and solar energy are being explored as replacement power technologies. However, the intermittency of wind and solar power requires electricity storage solutions that can respond on a range of different timescales. Among many electrochemical energy systems, lithium‐ion batteries (LIBs) and electrochemical double‐layer capacitors (EDLCs) are the mostly highlighted for their reliable performance, dynamic power responses, and high energy efficiency (>95%).[1]Since the first report of LiCoO2 in 1980s by Goodenough, LIBs with high energy density (≈200 Wh kg−1) have played a significant role in fundamental research and commercialization for more than 30 years.[2] However, its low power density (≤1 kW kg−1) limits its further application in electric vehicles (EVs) and electric cargo ships etc.[3] Though nowadays there are numerous EVs on the market, complex battery management systems are installed to connect LIBs units both in series and in parallel to balance the power performance and traveling distance.[4] In contrast, EDLCs can provide high power density (≥10 kW kg−1), and is capable for high power system like light rail etc.[5] However, its low energy density (≤10 Wh kg−1) blocks its path to long period power supply.[3] Lithium‐ion capacitors (LICs), as a hybrid of EDLCs and LIBs, are a promising energy storage solution capable with high power (≈10 kW kg−1, which is comparable to EDLCs and over 10 times higher than LIBs) and high energy density (≈50 Wh kg−1, which is at least five times higher than SCs and 25% of the state‐of‐art LIBs).[6] The comparison of device configurations, their charge/discharge profiles as well as performance characteristics of LIBs, EDLCs and LICs are shown in Figure 1. LIBs contain two insertion‐type electrodes as positive and negative electrodes respectively, which store/output energy via Li+ insertion/extraction. The plateaus are observed on charge/discharge curve while peaks can be pointed out in cyclic voltammetry curve, reflecting the insertion/extraction of cation and the redox reactions in the bulk material. With the massive cation storage in the bulk materials, LIBs demonstrate high energy density. However, the power performance of LIBs is limited by the slow ion diffusion in the bulk material, though the self‐discharge rate (<5% of the stored capacity over 1 month) is suppressed.[7] With the high energy density, flammable electrolyte, and chemical reaction during charge/discharge, safety issue is critical for LIBs application.[8] Differently, EDLCs contain two adsorption‐type electrodes which adsorb/desorb ions during charge/discharge. The easily accessible surface ion storage site permits the rapid charge/discharge capability of EDLCs. The physical change during charge/discharge and low‐energy density enable the high safety of EDLCs. But the self‐discharge (≈50–80% loss in energy per day) is serious due to the poor interaction between the ion and the active material surface.[9] Taking the much lower energy density into consideration, the energy‐based cost ($ kW h−1) of EDLCs (≈$10000 kW h−1) is much higher than LIBs ($100–200 kW h−1).[10]1FigureComparison of LIB, EDLC, and LIC: a) device configuration; b) charge/discharge curve (up) and cyclic voltammetry curve (down) profile and c) performance evaluation considering energy/power density, lifespan, safety, cost (energy‐based, $ kW h−1), and self‐discharge rate (low value to high value from inner to outer. The value is adapted from refs. [4–7, 9, 16c, 18].To combine the advantages of both LIBs and EDLCs, the first type of LICs was introduced by Amatucci et al. in 2001, which used an activated carbon cathode capturing PF6− via adsorption/desorption and a nanostructured Li4Ti5O12 anode storing Li+ through insertion/extraction.[11] The typical hybrid configuration of LICs, as shown in Figure 1a, contains a LIBs electrode and an EDLCs electrode with an organic lithium‐ion containing electrolyte, e.g., activated carbon (AC)//graphite and AC//Li4Ti5O12 (LTO).[12] Later, in order to improve the power density, capacitor//capacitor asymmetric LICs, like AC//AC, and AC//MXene were investigated.[13] The introduction of pseudocapacitive (PC) materials enables LICs to minimize the gap between bulky diffusion‐controlled ion storage of LIBs and surface adsorption ion storage of EDLCs to build up an asymmetric device demonstrating both high power and high energy performances. LICs not only possess higher power density than LIBs, but also show wider potential range and superior energy density than EDLCs with reduced self‐discharge rate.[14] Besides, LICs also deliver a comparable life span (5000 to 10 000 cycles) as EDLCs (≥10 000 cycles), which is much longer than LIBs (≤2000 cycles) and more suitable for long‐term use. Further, the safety of LICs sits between LIBs and EDLCs. Though with higher energy density than EDLCs, due to the unmatured production and market, the cost of LICs remains the highest among LIBs and EDLCs.[15]Although there have been significant reviews detailing various aspects of LICs, they primarily focus on the material perspective but seldom on the device configuration.[16] We contribute a dedicated review on LICs with focus on the device configurations, and the R&D gaps for LICs from lab bench to market. The vital importance of active materials in the device performance is with no doubt. The correct selection of material and the efficient configuration of device together can optimize the potential of the materials for more practical uses. For instance, the imbalanced kinetics between the battery and EDLC electrodes could result in low power density or low energy density. It is still challenging to obtain a LIC with three to five times higher energy density but comparable power density (≥10 kW kg−1, material‐based) and lifespan (≥10 000 cycles) to EDLCs without optimizing the configuration of LICs.[17] This review will try to provide some insights for the connection between basic and applied studies.LICs Design RationalesThe key challenges of LIBs and EDLCs are to deliver on rapidly evolving energy storage demands. To deal with the problems, lithium‐ion capacitors are introduced.[19] A new design of LIC with PC electrode replacing the battery/EDLC electrode is also put forward to improve the power performance.[20]It is noteworthy that these three types of LICs active materials demonstrate different power/energy performance with unique galvanic charge/discharge (GCD) and cyclic voltammetry (CV) curves as illustrated in Figure 2.[21] EDLC materials demonstrate a linear GCD curve and a rectangle shape cyclic voltammetry (CV) curve, in ideal scenarios, which is due to the pure physical change during charge/discharge with surface ion adsorption/desorption.[22] On the other hand, battery materials show a GCD curve with redox plateau(s) and CV curve redox peak(s). Nevertheless, another kind of faradic material, PC material presents a nonlinear GCD curve with/without obvious redox voltage plateau(s) and nonideal rectangular CV curve.[23] Normally, the capacity of materials has an order of battery material > PC material > EDLC material, while the fast charge/discharge capability of materials shows a reverse order.[24] Notably, battery material would also demonstrate pseudocapacitive behavior with nanosized structure as more active sites are exposed to electrolyte, e.g., 6 nm LiCoO2.[25] With different energy storage kinetics and performance, it is important for us to figure out how we can configure a LIC with these materials. In this section, we will discuss the design principles and strategies of LIC first, then the configurations of battery//capacitor LICs (battery//EDLC and battery//PC) and capacitor//capacitor LICs (EDLC//PC and PC//PC) as well as the design of Li‐rich LICs.2FigureIllustration of GCD and CV curve of EDLC material (top), pseudocapacitive (PC) material (middle), and battery material (bottom).LICs Design and Configuration PrincipleChoose and Balance Active MaterialThe choice and combination of active materials determine the performance of the LICs considering rapid charge/discharge capability, energy density, lifespan, etc. For those devices require high energy density and low self‐discharge rate, the EDLC//battery combination or EDLC with battery hybrid material would be the ideal option, although the power performance of the device would be limited. However, the improvement in energy density is limited by the narrow electrochemical window (EW) due to the faradic reaction voltage plateau of battery material as shown in Figure 3a,b. For battery//EDLC configuration, balancing the cathode/anode mass ratio, prelithiating the anode active materials, and screening battery active materials with fast kinetics along with long lifespan that pairs with EDLC active materials, to obtain equal capacity of two electrodes especially under high current densities, could maximize the performance of device.[26] Oppositely, the combination of capacitor//capacitor LICs, which is lithium‐ion electrolyte‐based supercapacitors with EDLC or PC active material, could meet the demand for fast charge/discharge capability. With this capacitor//capacitor design, the EW of as‐assembled LICs could be expanded to the oxidation/reduction decomposition or lithium plating voltage of the system as displayed in Figure 3c, compensating the energy density of LICs in regard of the low capacity.[27]3FigureIllustration of LICs designs: a) LICs with an EDLC cathode and a battery/pseudocapacitive anode which has a high faradic reaction voltage plateau, resulting in a limited electrochemical window (EW); b) LICs with a battery/pseudocapacitive cathode with has a low faradic reaction voltage plateau and an EDLC anode resulting to a limited EW; c) LICs with an EDLC cathode and a pseudocapacitive anode with low or no faradic reaction voltage plateau, resulting in a maximized EW; d) comparison between LICs with an EDLC cathode and a pseudocapacitive anode with/without pre‐lithiation. The LICs with a prelithiated anode demonstrate higher capacity within the same EW.Design Li‐Rich System by PrelithiationPrelithiation is one of the key chemical steps to increase the energy density and cycling stability of LICs by designing a Li‐rich system. From the electrode perspective, prelithiation could introduce extra lithium source into the electrodes to permit high energy density and good cycling stability: 1) to provide sufficient Li+ shuttles between cathode and anode during cycling, 2) to compensate for the Li+ loss on anode due to the irreversible Li+ insertion/extraction and continuous SEI decomposition/formation, and 3) to optimize the EW of anode.[28] While from the device design as illustrated in Figure 3d, higher capacity is achievable with the lithiated anode within the same EW as the LICs without a prelithiated anode, which could promote the energy density. However, the cost of LICs production goes up with the extra lithium source introduced, e.g., lithium metal or lithium metal oxide, and the complexity in device fabrication increased, e.g., precycling electrode or mixing lithium compound with active materials.[28a] It is worth pointing out that the development of the one‐step prelithiation method without significant volume/mass change, e.g., the use of scarification lithium compound, would be one of the major research directions and shows potential industrial application.[29]Optimize the Electrode FabricationDespite selecting active materials and balancing the mass ratio of cathode/anode to achieve maximum energy density as per mentioned, the electrode fabrication should be optimized. For a typical hybrid LIC, which contains a EDLC cathode and a battery anode, higher mass loading on cathode side is expected due to the lower specific capacity of EDLC material. However, the low density of porous EDLC materials results in a much thicker cathode in the LICs, limiting the improvement in volumetric performances.[30] The thickness difference between the thick electrode and thin electrode should be minimized to achieve high volumetric performance with the application of high gravimetric capacity, high density active materials.[31]Apart from the electrode thickness balance, it is of significance to balance the gravimetric/areal/volumetric performance and power/energy density by optimizing the thickness, mass loading, and density of the electrode.[32] Although a thinner/lower mass loading electrode demonstrates a better power performance, the energy density of the device would be dragged back considering the lower portion of active materials.[33] Oppositely, the electrode should not be too thick, which might limit the power performance of the device with the increased internal resistance. It is necessary to find a suitable range of the mass loading, thickness, and density of the electrode when designing the LICs.In summary, the selection and mass ratio of cathode/anode active materials, the prelithiation strategies, the thickness, mass loading, and density of electrodes, are key factors in the design of high‐performance LICs. After going through these four aspects in the design and configuration of LICs, design of LICs by maximizing the energy density will be discussed in the next section.Configuration of LICsIn the field of electrochemical energy storage, there is a variety of active material selections with the structure from 0D nanoparticle and quantum dot, 1D nanowire, nanotube and nanobelt, 2D nanoflake, and nanosheet to 3D nanoporous skeleton, or with the mechanism from surface ion‐adsorption, fast redox reaction/ion intercalation and ion insertion.[34] From the perspective of device configuration, LICs can be classified into four sorts based on the combination of electrode mechanisms: battery//EDLC, EDLC//PC, PC//PC, and battery//PC device, as illustrated in Figure 4. It is postulated that EDLC//PC device could be very competitive in power density while battery//PC device should be more advanced for energy density. The rest configurations would perform moderately. Despite many reports on EDLC//battery and EDLC//PC types, the study on PC//PC and battery//PC is less reported. The real performance of LICs with different combinations is discussed below.4FigureIllustration of LICs combining an EDLC electrode and a battery electrode (EDLC//battery); an EDLC electrode and a pseudocapacitive (PC) electrode (EDLC//PC); two PC electrodes (PC//PC) and a PC electrode with a battery electrode (PC//Battery) (left) and their estimated performance including power/energy density, cycle life and self‐discharge rate (SDC) (right).Battery//EDLCAt the very first beginning, LICs are combined with a battery electrode and a capacitor electrode as a hybrid device, e.g., AC//Li4Ti5O12 (LTO),[11] AC//Li2TiSiO5,[35] AC//TiO2,[36] AC//SnO2/C,[37] single‐walled carbon nanotubes (SWCNT)//V2O5,[38] porous carbon//3D TiC,[39] AC//Si,[40] AC//lithium–aluminum alloy,[41] etc. In such a design, however, the unbalanced cathode/anode kinetics limits the improvement in power/energy density and lifespan. One of the typical battery//EDLC combinations is AC//hard carbon (HC) cell.[42] Sun et al. reported a LICs design with activated carbon as cathode and prelithiated hard carbon as anode, the specific energy and specific power of which reach 73.6 Wh kg−1 and 11.9 kW kg−1 based on the mass of the active materials in both cathode and anode, with a maximum specific energy of 18.1 Wh kg−1 and a maximum specific power of 3.7 kW kg−1 based on the mass of whole pouch cell.[43]Besides, various organic matter‐derived porous carbons were introduced.[44] Li et al. synthesized a corncob‐derived mesoporous/macroporous activated carbon, which showed superior performance in the LICs with Li4Ti5O12 (LTO) as anode as shown in Figure 5a.[45] The as‐assembled device demonstrated a maximum energy density of 79.6 Wh kg−1 with a power density of 200 W kg−1, which is much higher comparing with those commercial activated carbon‐based devices and similar constructions employing carbonaceous materials as the cathode and LTO as the anode.[46] Apart from this, Chaturvedi et al. assembled an asymmetric LIC with AC cathode and TiSe0.6S1.4 anode.[47] Although the cell delivered high energy density (≈50 Wh kg−1) at small current rate, its rate performance was limited by the ion insertion into the battery electrode.[48] Sun et al. designed a LIC with the alloying‐type Sn–C anode, which, in a cell with mesoporous carbon cathode, exhibited high energy densities of 196 to 85 Wh kg−1 at power density from 731 to 24 375 W kg−1, as well as good cycling stability with 70% retention after 5000 cycles.[49] Similarly, Kim et al. managed to design LICs with fully etched crumpled graphene (CG) cathode, and partially etched CG wrapped spiky iron oxide particles as anode. The as‐assembled device shows a wide working potential range of 4 V, with a specific capacitance of 238 F g−1 at a current density of 0.2 A g−1.[50] Besides, another battery anode material, BiVO4, was applied in LICs by Dubal et al.[51] The LIC with partially reduced graphene oxide cathode and BiVO4 nanorod anode demonstrated an energy density of 152 Wh kg−1 at 384 W kg−1 and 42 Wh kg−1 at 3861 W kg−1 with a 81% capacity retention rate after 6000 cycles at 0.9 A g−1.5Figurea) Illustration of the LIC assembled with activated carbon (AC) cathode and a free‐standing Li4Ti5O12 (LTO) anode. Reproduced with permission.[45] Copyright 2017, Elsevier. b) Schematic for a LIC with a cathode mixing battery type material and capacitive type material. c) Ragone plots of LIBs and LICs based on different cathodes on the gravimetric bases. Reproduced with permission.[53] Copyright 2017, Springer Nature.(Battery+EDLC)//BatteryOn the other hand, unlike those battery//EDLC hybrid device, mixing battery active material and EDLC active material together, e.g., mixing LiFePO4 (LFP) with AC, seeding LTO onto graphene or coating graphene onto LiMn2O4 (LMO), is one of the main streams.[52] In this case, the EDLC material is correlated with the power performance while the battery material contributes more to the energy performance. Nonetheless, the lifespan is closely related to that of the battery material. Zheng and co‐workers reported combining the advantages of both LIB and LIC via a synergistic combination of LiCoO2 (LCO) and AC in one electrode as shown in Figure 5b.[53] The hybrid capacitor performed two‐fold energy density comparing with the AC//HC LIC at low power density, and five times energy density than LCO//AC device at high power operation, as reflected by the Ragone plot in Figure 5c. For this type of device, the mixing electrode demonstrates both high energy and power densities. However, due to the faradic noncapacitive behavior of battery materials, the power density of the full device is falling behind when comparing to capacitor//capacitor LICs.EDLC//PCWith the aim to obtain a LIC with high power and energy output, 1D/2D/3D PC materials, including N‐doped carbon, polypyrrole nanopipes, graphene, graphdiyne, fluorinated contorted‐hexabenzocoronene, MXene, MoS2, Nb2O5, and MoO3, were used as the active material of LICs.[54] With the similar kinetics of EDLC and PC materials, high power performance and long lifespan are achievable with such a configuration. However, the energy density is limited by the low capacity of these capacitor materials. Carbon‐based material is widely applied in the field of supercapacitors and batteries. Cai et al. reported a full carbon‐based LIC as shown in Figure 6a, in which the carbon material stores energy via PF6− adsorption as cathode and stores energy via Li+ intercalation as anode.[55] The device with defect‐rich O‐doped hierarchical porous carbon (OHPC) demonstrated an energy density of 144 Wh kg−1 at 200 W kg−1 and a long cycling life with a capacity retention rate of 92% at 2 A g−1 and 70.8% at 10 A g−1 after 10 000 cycles. Xia et al. designed a symmetric lithium‐ion capacitor with B/N co‐doped 3D carbon nanofibers, which can achieve 220 Wh kg−1 at 225 W kg−1 and 104 Wh kg−1 at 22.5 kW kg−1, which is superb.[56] Wang et al. reported a N‐doped graphene based LIC with working voltage window of 4.5 V and 187.9 Wh kg−1 at 22.5 kW kg−1, as well as a capacity retention of 93.5% after 3000 cycles.[57] The application of graphdiyne in LICs system was studied by Shen et al. by using activated carbon cathode and F‐doped graphdiyne anode, which delivered 200.2 Wh kg−1 at 131.2 W kg−1 and 1 122.4 Wh kg−1at 13.1 kW kg−1.[54k]6FigureIllustration of capacitor//capacitor LICs. a) Schematic of the charge‐storage mechanisms for the defect‐rich O‐doped hierarchical porous carbon (OHPC)//OHPC LIC. Reproduced with permission.[55] Copyright 2020, The Royal Society of Chemistry. b) Charging process of CTAB‐Sn(IV)@Ti3C2//AC LIC. Reproduced with permission.[61] Copyright 2016, American Chemical Society.Transition metal compounds were also studied with potential to increase the energy density. Wang et al. introduced graphene layer into 2D MoS2 to tune the layer distance, which showed superior energy and power density of 188 Wh kg−1 at 200 W kg−1 and 45.3 Wh kg−1 at 40 kW kg−1.[58] MXene is recently used as a new 2D layered ion‐intercalation host.[59] Comparing to bulk 2D layered materials (e.g., graphite and LiMnO2) and exfoliated 2D materials (e.g., graphene sheet), 2D MXene shows the potential to be the most competitive LICs active material to perform both high energy and power densities.[60] Luo et al. reported MXene‐based LICs as shown in Figure 6b, by using pillared Ti3C2 MXene (CTAB−Sn(IV)@Ti3C2) with adjusted interlayer space by changing the size of intercalation agents.[61] With the help of the pillared layer channel and the existence of Sn4+, the ion‐diffusion rate in the layer is increased, leading to a capacitance of 25 F g−1 even under a high current density of 5 A g−1. Besides, MXene can be combined with conversion‐type material Fe3O4 as LICs anode (Fe3O4/C/MXene), which delivered an energy density of 130 Wh kg−1 at 250 W kg−1 and 31 Wh kg−1 at 25 kW kg−1, with a capacity retention rate of 86.5% after 5000 cycles under a current density of 1 A g−1.[62]PC//PCConducting polymers, e.g., polyaniline (PANi) and polypyrrole (Ppy), are attracting more focus recently as a PC cathode materials storing energy by anion doping.[63] With the capacitive behavior and fast kinetics of PC materials, the as‐fabricated LICs could possibly demonstrate moderate energy/power performance and lifespan. Han et al. reported a PC//PC LIC with PANi@carbon nanofiber (CNF) as cathode and CNF as anode. In this design, cathode stores energy via PF6− doping on PANi while anode stores Li+ by ion intercalation into CNF as shown in Figure 7a.[64] The as‐fabricated device demonstrated a maximum energy density of 106.5 Wh kg−1 at 769.0 W kg−1 and a maximum power density of 15.1 kW kg−1 at 64.5 Wh kg−1, with a capacity retention rate of 70.3% at 10 A g−1 after 7000 cycles.7Figurea) Working principle of polyaniline (PANi)//carbon nanofiber (CNF) LICs. Reproduce with permission.[64] Copyright 2020, American Chemical Society. b) Charge‐storage mechanism of polypyrrole (Ppy)@carbon nanotube (CNT)//Fe3O4@C‐based LICs. Reproduced with permission.[66] Copyright 2019, American Chemical Society.In addition, Byeon et al. attempted to apply Nb2CTx MXene as both cathode and anode for PC//PC LICs.[65] Nb2CTx‐CNT was precycled in a half‐cell with Li metal as counter and reference electrode to ≈0.1 V versus Li/Li+ to form lithiated‐Nb2CTx‐CNT prior to the assembly of Nb2CTx‐CNT//lithiated‐Nb2CTx‐CNT LIC. With Li+ intercalation/de‐intercalation into/from both positive and negative electrodes, the device demonstrates a maximum energy density of 38 Wh kg−1 at ≈25 W kg−1 within an operational voltage window of 0.0–3.0 V. Though the performance of such device is not comparable to the other state‐of‐art LICs yet, it is a proof of concept that MXene materials can also work as LICs cathode, unlocking the potential design of symmetric LICs with 2D layered materials.PC//BatteryTo further enhance the energy density of LICs with PC electrode, PC//battery design was introduced. Though the energy density could be enhanced with battery material, the lifespan and power density may be limited. Han et al. designed a LIC combining a PC cathode of Ppy/CNT as presented in Figure 7b.[66] In such a design, Ppy stores/outputs energy via pseudocapacitive PF6− doping/de‐doping. With Fe3O4‐based battery anode, the as‐obtained device demonstrated a maximum energy density of 101 Wh kg−1 at a high power density of 2.7 kW kg−1 and a maximum power density of 17.2 kW kg−1 at 70 Wh kg−1, with a capacity retention rate of 79.5% after 2000 cycles. In this design, battery electrode contributes to high energy density while the capacitor electrode delivers high power performance. On the other hand, Nb2CTx MXene is also introduced as PC anode by Byeon et al.[65] Coupling with a LFP cathode, the device demonstrates a maximum energy density of 43 Wh kg−1 at ≈10 W kg−1. The low energy density of this design is mainly due to the low working potential range from 0.0 to 3.0 V. Lately, Wu et al. reported a battery//PC‐based LICs configuration with LiNi0.815Co0.15Al0.035O2/carbon as cathode and T‐Nb2O5 as anode.[67] The as‐assembled LICs demonstrated a performance of 165 Wh kg−1 at 105 W kg−1 and 9.1 kW kg−1 at 83 Wh kg−1 based on a Li+ rocking‐chair mechanism. Such a LICs configuration will not consume ions from electrolyte, which could potentially enable the LICs design with lean electrolyte. Notably, although the PC//battery configuration has the potential of demonstrating high energy density, it is necessary to combine a high working voltage cathode with low working voltage anode to maximize the voltage of a fully charged device over 4.0 V for further energy density improvement.Prelithiation for Li‐Rich LICsAlthough the LIC is well designed, large amount of Li+ would be consumed during cycles because of the irreversibility of redox reactivity and the electrolyte decomposition/SEI layer formation on the anode surface.[68] Moreover, due to the difference of the reaction rate between cathode and anode, performance fading caused by lithium loss during cycling is unavoidable, especially at high power output.[28c,69] Therefore, prelithiation becomes more popular to improve the performance of LICs, which could be classified into three sections: pretreated anode, direct addition of Li metal, and sacrificial lithium compound.Pretreated AnodeOne of the most common prelithiation methods for anode pretreatment is based on a two‐step procedure: 1) charge the electrode in a half‐cell (with lithium metal as counter electrode) or in a full cell (with LIB cathode as positive electrode) to a suitable state of charge (SOC), 2) disassemble the cell and collect the prelithiated electrode, following by assembling the LICs with the treated electrode and positive electrode.[70] Li et al. prelithiated the mesocarbon microbead (MCMB) by charging MCMB in a half‐cell with Li‐metal as counter electrode before making the AC//MCMB LICs.[71] Similarly, Aluminum anode, a promising candidate for high capacity lithium storage, was applied in a AC//Al LIC designed by Ou et al., where the Al anode was prelithiated via electrochemical alloying in a Li half‐cell along with the formation of LiF‐rich artificial solid‐electrolyte interphase (SEI).[41] The irreversible consumption of Li+ is reduced and the cycling stability of anode is enhanced. Apart from the GCD method, Cai et al. introduced Li+ into multiwalled carbon nanotube (MWCNTs)/graphite composite anode by the internal short circuit method, in which the anode is in direct contact with Li metal with electrolyte in between as shown in Figure 8a.[72] The Li metal would be oxidized into Li+ and then travels into graphite electrode, with the help of the potential difference of graphite and lithium metal. However, these two‐step anode pretreatment methods are complicated.8FigureIllustration of some mostly used prelithiation strategies. a) Structure diagram of internal short circuit method. Reproduced with permission.[72] Copyright 2018, Springer Nature. b) A schematic diagram of an activated carbon/stable lithium metal powder (SLMP) surface applied hard carbon LICs configuration. Reproduced with permission.[42] Copyright 2012, Elsevier. c) Schematic representation of the cascade‐type pre‐lithiation strategy using a source of electrons from pyrene electro‐polymerization and, a source of lithium ions from a chemical reaction involving an insoluble inorganic base. (Csp: Carbon SuperP and PVDF, YP‐80F: a state‐of‐art supercapacitor carbon, YP‐10‐Li: YP‐80F, pyrene, Li3PO4, PVDF) Reproduced with permission.[28d] Copyright 2019, Wiley.Direct Addition of Li MetalPrelithiation could be obtained with one‐step process by introducing Li metal into the device as extra Li source. Cao and Zheng applied stable lithium metal powder (SLMP) onto the electrode surface to play the role of Li+ source as shown in Figure 8b.[42] However, this configuration is not economic‐friendly as the SLMP is expensive. Also, it may cause volume change of the whole device as the surface Li metal translating into Li+, which may lead to the poor contact between cell components. Furthermore, it is unsure whether the SLMP was separated homogeneously. To mix SLMP with active material uniformly, Lei et al. introduced SLMP into the active material slurry directly. First, graphite and polyvinylidene difluoride (PVDF) was mixed into a slurry with N‐methyl‐2‐pyrrolidone (NMP) as solvent. Then a thick layer mixture was cast onto Cu foil and followed by drying to form the PVDF‐coated graphite powder. The prepared powder as mixed with SLMP as well as styrene‐butadiene rubber (SBR) with toluene as solvent to form the slurry that is coated onto Cu foil to prepare the electrode.[73] Nonetheless, this method is not applicable in industry as it complicated the slurry preparation procedure. Tsuda and co‐workers designed a porous electrode that can allow Li+ transmit from Li metal counter electrode to the working electrode in the full cell.[74] However, extra separators and Li metal electrodes are added into the cell, resulting in lower volumetric energy/power density of LICs and higher safety risk during manufacture.Sacrificial Lithium CompoundTo avoid the potential volume change of the cell and simplify the process, sacrificial Li+ source, including lithium metal oxide that can be added into the electrode directly, are attracting increasing interest.[75]Zhang et al. introduced highly irreversible Li‐rich compound, Li2CuO2, into AC positive electrode in the AC//graphite LICs, whereas in‐situ prelithiation takes place during charging through Reactions (1) and (2), accordingly.[76]1Li2CuO2→LiCuO2+Li++e−\[\begin{array}{*{20}{c}}{{\rm{L}}{{\rm{i}}_2}{\rm{Cu}}{{\rm{O}}_2} \to {\rm{LiCu}}{{\rm{O}}_2} + {\rm{L}}{{\rm{i}}^ + } + {{\rm{e}}^ - }}\end{array}\]2LiCuO2→CuO+Li++1/2O2+e−\[\begin{array}{*{20}{c}}{{\rm{LiCu}}{{\rm{O}}_2} \to {\rm{CuO}} + {\rm{L}}{{\rm{i}}^ + } + 1{\rm{/}}2{{\rm{O}}_2} + {{\rm{e}}^ - }}\end{array}\]Despite the role of primary Li+ source, Li2CuO2 also contributes extra energy to the LICs. While operating in wider potential range (2.0–4.0V), Li2CuO2 stores/outputs energy via the reversible Li+ insertion/extraction to help improve the energy density. Apart from studying the effect of Li2CuO2 additive, there are three tips in choosing suitable lithium source material: a) high lithium content; b) delithiation takes place at lower potentials than the upper operating limit potential of the positive electrode, and c) relithiation either occurs at potentials below the lower operating limit potential of the positive electrode or is substantially irreversible. Another kind of sacrificial pre‐lithiation agent, Li3N, is introduced by Sun et al. and Liu et al.[28e,77] Extra Li+ is generated via electrochemical decomposition of Li3N during the initial charge, in which the decomposed Li+ is inserted into anode while N2 is released into electrolyte. However, Li3N is not stable under ambient atmosphere or in aprotic polar solvents, e.g., NMP. Hence, Sun et al. prepared the electrode with dry ball milling and cold pressing under protective atmosphere while Liu et al. introduced N,N‐dimethylformamide (DMF) as solvent for electrode slurry preparation, trying to extend the application of Li3N in LICs system from research to commercialization. To push prelithiation forward on the industry‐relevant path, a cascade‐type method is reported by Anothumakkool et al. as shown in Figure 8c.[28d] Pyrene monomers and an insoluble lithium compound, Li3PO4, are mixed in cathode, which enable two consecutive reactions during initial charge. Electrons and protons will be released from pyrene moieties during oxidative electrochemical polymerization when charging the electrode. Then the released protons are captured by Li3PO4, which would exchange Li+ into the electrolyte by the formation of H3PO4. Also, the polymerization reaction of pyrene is enhanced due to the consumption of protons. The authors suggested that such a method could be adapted to industry with high flexibility that both the pyrene monomer and Li3PO4 are low cost and their reactivity towards electrode slurry solvent, air, and electrolyte is tunable. Also, the redox potential of monomer is adjustable based on the nature of the pyrene substituent.[78] On the other hand, lithium salt, e.g., high concentration lithium bis(trifluoromethane) sulfonimide (LiTFSI), can also act as the lithium source in the LICs.[79] Jezowski and fellow researchers introduced a sacrificial organic lithium salt, 3,4‐dihydroxybenzonitrile di‐lithium salt (Li2DHBN), which is insoluble before delithiation and turns soluble after de‐lithiation, into the positive electrode as the lithium‐ion source for the AC//graphite LICs.[54] During first charge/discharge cycle, 1.9 Li+ per formula unit extracts from Li2DHBN irreversibly. Meanwhile, the product of this reaction, 3,4‐dioxobenzonitrile (DOBN), shows good solubility in carbonate‐based electrolyte and shows no influence on the ion conductivity.[29] However, Li2DHBN is not stable under air, which would increase the cost during industrial processing. Another di‐lithium compound, dilithium squarate (Li2C2O4), which is air‐stable, safe, highly irreversible and cost‐effective with a decomposition voltage from 3.5 to 4.5 V versus Li/Li+, was studied as a sacrificial lithium source mixing with cathode active material in a AC//hard carbon (HC)‐based LIC.[80] The pre‐lithiated LIC demonstrated a long lifespan with a capacity retention rate of 84% after 48 000 cycles at 1 A g−1. Note that this lithium salt can be transformed into sodium/potassium salt by solvent exchange for the pre‐metalation of sodium/potassium‐ion capacitors.The characteristics of LICs with various configurations are summarized in Table 1. Here, both EDLC//battery and PC//battery configurations are classified as capacitor//battery while the EDLC//PC and PC//PC configurations are cataloged as capacitor//capacitor.1TableSummary of typical state‐of‐the‐art lithium‐ion capacitor configurationsConfigurationPrelithiationVoltage [V]EMaxPMaxCyclabilityYear of publicationRefs.Capacitor//battery(+) Activated carbon (AC)//graphite (−)Sacrificial lithium compound4.2196.0 Wh kg−1 at 456.0 W kg−1_96.5% at 0.5 mA cm−2 after 100 cycles2017[76](+) AC//Li4Ti5O12/graphene (−)_4.046.0 Wh kg−1 at 625.0 W kg−12.5 kW kg−1 at 26.0 Wh kg−183.0% at 1 A g−1 after 4000 cycles2017[45](+) AC//Li3VO4 (−)_4.0136.4 Wh kg−1 at 532.0 W kg−111.0 kW kg−1 at 24.4 Wh kg−187.0% at 2 A g−1 after 1500 cycles2017[81](+) AC//Si/C (−)Precycled anode4.0227.0 Wh kg−1 at 1146.0 W kg−132.6 kW kg−1 at 181.0 Wh kg−1>90.0% at 6.4 A g−1 after 16 000 cycles2017[82](+) AC//Sn/C (−)Precycled anode4.5195.7 Wh kg−1 at 731.0 W kg−124.4 kW kg−1 at 84.6 Wh kg−170.0% at 2 A g−1 after 5000 cycles2017[49](+) Reduced graphene oxide (rGO)//BiVO4 nanorod (−)Precycled anode4.0152.0 Wh kg−1 at 384.0 W kg−13.9 kW kg−1 at 42.0 Wh kg−181.0% at 0.9 A g−1 after 6000 cycles2018[51](+) Graphene//graphene/Fe2O3 (−)_4.0121.0 Wh kg−1 at 200.0 W kg−118.0 kW kg−1 at 60.1 Wh kg−187.0% at 1 A g−1 after 2000 cycles2018[50](+) AC//Si/Cu nanowire (−)Precycled anode4.2210.0 Wh kg−1 at 193.0 W kg−199.0 kW kg−1 at 43.0 Wh kg−190.0% at 10 A g−1 after 30 000 cycles2018[83](+) AC//F‐enriched graphdiyne (−)Precycled anode4.0200.2 Wh kg−1 at 131.2 W kg−113.1 kW kg−1 at 122.4 Wh kg−180% at 2 A g−1 after 5000 cycles and 80% at 5 A g−1 after 6000 cycles2019[54k](+) Polypyrrole/CNT//Fe3O4/C (−)Precycled anode4.0101.0 Wh kg−1 at 2709 W kg−117.2 kW kg−1 at 70.0 Wh kg−179.5% at 10 A g−1 after 2000 cycles2019[66](+) B/N co‐doped carbon//SnS2/rGO (−)Precycled anode4.5149.5 Wh kg−1 at ≈92.0 W kg−135 kW kg−1 at ≈65 Wh kg−190.5% at 5.0 A g−1 after 10 000 cycles2020[84](+) AC//soft carbon (−)Sacrificial lithium compound4.074.7 Wh kg−1 at 75.1 W kg−112.9 kW kg−1 at 21.5 Wh kg−191.0% at 0.5 A g−1 after 10 000 cycles2020[28e](+) AC//hard carbon (HC) (−)Sacrificial lithium compound4.2120.0 Wh kg−1 at ≈110.0 W kg−1≈3.0 kW kg−1 at ≈90 Wh kg−190.0% at ≈5.0 A g−1 after 10 000 cycles2020[77](+) AC//Fe3O4/C/Ti3C2Tx (−)Precycled anode4.5130.0 Wh kg−1 at 250.0 W kg−125.0 kW kg−1 at 31.0 Wh kg−186.5% at 1 A g−1 after 5000 cycles2020[62](+) AC//HC (−)Sacrificial lithium compound4.073.0 Wh kg−1 at 299.0 W kg−15.8 kW kg−1 at 45.0 Wh kg−184% at 1 A g−1 after 48 000 cycles2020[80](+) AC//Li‐Al alloy (−)Pre‐cycled anode4.5≈489.3 Wh kg−1 at 450.0 W kg−17.2 kW kg−1 at ≈283.8 Wh kg−185.6% at 0.8 A g−1 after 2000 cycles2020[41](+) LiNi0.815Co0.15Al0.035O2/carbon//T‐Nb2O5 (−)−3.0165.0 Wh kg−1 at 105.0 W kg−19.1 kW kg−1 at 83.0 Wh kg−191.0% at 1 A g−1 after 400 cycles2021[67](+) AC//KNb3O8/carbon cloth (−)−3.569.0 Wh kg−1 at 346.0 W kg−13.8 kW kg−1 at 36.0 Wh kg−188.0% at 2.0 A g−1 after 1000 cycles2021[85](+) Boron carbonitride nanotubes//Si/graphene aerogel (−)−4.5197.3 Wh kg−1 at 225.0 W kg−14.5 kW kg−1 at 127.1 Wh kg−182.4% at 10.0 A g−1 after 10 000 cycles2021[86]Capacitor//capacitor(+) AC//vanadium nitride/rGO (−)Precycled anode4.0162.0 Wh kg−1 at 20.0 W kg−110.0 kW kg−1 at 64.0 Wh kg−183.0% at 2 A g−1 after 1000 cycles2015[87](+) Carbon fibres//TiNb2O7/C (−)_4.2110.4 Wh kg−1 at 99.6 W kg−15.5 kW kg−1 at 20.0 Wh kg−177.0% at 0.2 A g−1 after 1500 cycles2015[88](+) N‐doped carbon//MnO/graphene (−)Internal short‐circuit4.0127.0 Wh kg−1 at 125.0 W kg−125.0 kW kg−1 at 83.3 Wh kg−181.0% at 5 A g−1 after 2000 cycles2015[89](+) Nb2CTx/carbon nanotube (CNT)//lithiated‐Nb2CTx/CNT (−)Precycled anode3.038.0 Wh kg−1 at ≈25.0 W kg−10.35 kW kg−1 at ≈8.0 Wh kg−1>70.0% at 0.25 A g−1 after 900 cycles2016[65](+) rGO//SnO2/rGO (−)Pre‐cycled anode4.1186.0 Wh kg−1 at 142.0 W kg−110.0 kW kg−1 at 12.0 Wh kg−170.0% at 3 A g−1 after 5000 cycles2017[90](+) N‐doped carbon nanosheet//ZnMn2O4/graphene (−)Internal short‐circuit4.0202.8 Wh kg−1 at 180.0 W kg−121.0 kW kg−1 at 98.0 Wh kg−177.8% at 5 A g−1 after 3000 cycles2017[91](+) B, N‐doped porous carbon nanofibers//B, N‐doped porous carbon nanofibers (−)Internal short‐circuit4.5220.0 Wh kg−1 at 225 W kg−122.5 kW kg−1 at 104.0 Wh kg−181.0% at 2 A g−1 after 5000 cycles2017[56](+) AC//MoS2/rGO (−)Precycled anode4.0188.0 Wh kg−1 at 200.0 W kg−140.0 kW kg−1 at 45.3 Wh kg−180.0% at 2 A g−1 after 10 000 cycles2017[58](+) AC//pillared‐Ti3C2 MXene (−)Precycled anode3.0105.6 Wh kg−1 at 495.0 W kg−110.8 kW kg−1 at 45.3 Wh kg−171.1% at 2 A g−1 after 4000 cycles2017[61](+) rGO//N‐doped carbon nanopipes (−)Precycled anode4.0262.0 Wh kg−1 at 450.0 W kg−19.0 kW kg−1 at 78.0 Wh kg−191.0% at 0.9 A g−1 after 4000 cycles2018[92](+) Polypyrrole nanopipes//N‐doped carbon nanopipes (−)Precycled anode4.5107.0 Wh kg−1 at ≈25.0 W kg−110.0 kW kg−1 at ≈30.0 Wh kg−193.0% at 1 A g−1 after 2000 cycles2019[54j](+) AC//carbon black (−)Sacrificial lithium compound4.463.0 Wh kg−1 at ≈440.0 W kg−1_>95.0% at 0.3 A g−1 after 2000 cycles2019[28d](+) AC//Nb2O5/C/rGO (−)Precycled anode3.271.5 Wh kg−1 at 247.0 W kg−13.9 kW kg−1 at 18.3 Wh kg−194.0% at 0.2 A g−1 after 2500 cycles2019[93](+) Activated N‐doped graphene//N‐doped graphene (−)_4.5187.9 Wh kg−1 at 2250.0 W kg−111.3 kW kg−1 at 111.4 Wh kg−193.5% at 2 A g−1 after 3000 cycles2019[57](+) Sponge‐like carbon//Sponge‐like carbon (−)Pre‐cycled anode3.591.0 Wh kg−1 at 145.0 W kg−118.5 kW kg−1 at 22.0 Wh kg−199.0% at 50 A g−1 after 100 000 cycles2019[94](+) PANi/CNF//CNF(−)Precycled anode4.061.5 Wh kg−1 at 444.1 W kg−18.7 kW kg−1 at 37.3 Wh kg−170.3% at 10 A g−1 after 7000 cycles2020[64]Design of Multifunctional LICsWith the advantage in energy storage, LICs could also work as power supply for wearable electronics and micro devices, which requires multifunctional device design, including flexibility, quasi solid‐state and miniaturized structure. However, the study on these is lacking. Sharing similar device configurations with regular LICs in previous sessions, the major challenges in the development of multifunctional LICs are the solid‐state electrolyte with high ionic conductivity, wide electrochemical window, low electrolyte leakage risk, and high flexibility. Some design strategies in the multifunctional batteries and supercapacitors can be applied onto the design of multifunctional LICs. Notably, the current design of multifunctional LICs mainly focuses on minimizing the device footprint and designing flexible devices. In comparison with other multifunctional batteries and supercapacitors, which are moving much further onto the perspectives of rollable, stretchable, transparent, self‐healing, self‐chargeable, or degradable devices,[95] the development of multifunctional LICs is still in its early stage.Design Strategies of Multifunctional LICsDevelopment of Solid‐State Electrolyte with High SafetyConsidering the application environment of multifunctional LICs, e.g., in wearable devices and microgrids, electrolyte leakage would increase the risk of system failure. Herein, quasi‐solid‐state electrolyte is the favorite option to solve this issue. With conventional Li‐ion containing organic electrolyte absorbed in poly(vinylidene fluoride)‐co‐hexafluoropropylene (PVDF‐HFP) or polyacrylonitrile (PAN)‐based polymer framework, gel‐type electrolytes are widely studied and used in multifunctional LICs and LIBs for their comparable ionic conductivity to liquid electrolyte at room temperature, potentially with wider electrochemical window and excellent flexibility. However, the assessment onto the mechanical property and safety of these electrolytes, including flammability and toxicity, are not comprehensive. Especially for those multifunctional LICs used in wearable electronics, the development of non‐flammable and nontoxic electrolyte is desired.[96]Development of Flexible Electrode and Current CollectorIn the conventional device electrode design, active material in micrometer size is preferred for its high packing density. However, the contact between particles would be weakened during banding, resulting in the reduced electrochemical performance of multifunctional LICs. Replacing the micrometer‐sized active material particle with nano/porous/twisted structured particles is regarded as a solution to enhance the connectivity between active materials and maintain electronic conductivity.[97] Alternatively, the deformation of metallic foil during flexing limits the application of Al and Cu foils as current collectors in multifunctional LICs directly. The anchoring of active materials on metallic foils can be strengthened by structure engineering, e.g., honeycomb‐patterning.[98] Also, advanced substrates with high mechanical strength and electronic conductivity, e.g., CNT film, graphene film, and carbon cloth, are suggested to be used in flexible LICs for good stability and high active material mass loading.[99]Development of Advanced Package Material and Manufacture TechniqueUnlike conventional LICs that are packed with cylinder or Al‐plastic laminated foil, thin, soft, lightweight but robust package materials with low water vapor/air permeability (comparable to Al‐plastic laminated foil) are preferred in multifunctional LICs encapsulation.[99] However, the research from this aspect is in lack. Some researchers are still sealing their flexible LICs with Al‐plastic laminated foil, which shows poor flexibility and large thickness. On the other hand, polydimethylsiloxane (PDMS) may be another choice for encapsulation for its superior mechanical property, though the full assessment including air permeability is necessary. Apart from the package materials, fast, scalable and cost‐effective manufacture techniques, e.g., 3D printing, are the key for the large‐scale production of multifunctional LICs.[100]Flexible and Quasi‐Solid‐State LICsHere, some examples showing the development of flexible and quasi‐solid‐state LICs are discussed. Deng et al. introduced an in‐plane assembled orthorhombic Nb2O5 nanorod films on carbon cloth with high Li+ intercalation rate.[101] The LICs assembled with activated carbon cathode (AC) and Nb2O5 nanorod film anode performed outstanding high gravimetric and volumetric energy/power densities of 95.55 Wh kg−1 at 5350.9 W kg−1 and 6.7 mWh cm−3 at 374.63 mW cm−3, which is due to the fast electronic transportation through the single crystalline Nb2O5 and the rapid Li+ migration in the interconnected pores of the electrode film as well as the short solid‐state Li+ diffusion path, respectively. Wang et al. reported a quasi‐solid‐state LIC with graphene nanosheet as cathode, graphene nanosheet‐wrapped TiO2 as anode.[102] The PVDF‐HFP gel works as electrolyte host by absorbing conventional organic electrolyte. The as‐fabricated device demonstrated a maximum energy density of 72 Wh kg−1 at 303 W kg−1, a maximum power density of 2 kW kg−1 at 10 Wh kg−1. With similar electrolyte system, a flexible LIC was designed by Que et al. with activated carbon (AC)/carbon nanotube (CNT) as cathode and TiO2‐x/CNT as anode as shown in Figure 9a.[103] The device demonstrated a maximum energy density of 104 Wh kg−1 at 250 W kg−1 and a maximum power density of 5 kW kg−1 at 32 Wh kg−1.9Figurea) Schematic illustration of the TiO2‐x/CNT//AC/CNT flexible hybrid device. Reproduced with permission.[103] Copyright 2018, Wiley. b) Schematic of the on‐chip prototype with AC/graphite configuration and interdigital 3D electrode structure supported by the separator. Reproduced with permission.[104] Copyright 2015, Elsevier.Micro LICsApart from the flexible device design, micro LICs also show the potential to power the wearable electronics. Li et al. fabricate a micro LIC with AC as cathode and pre‐lithiated graphite as anode with micro electromechanical systems as demonstrated in Figure 9b.[104] The device with conventional 1 m LiPF6 in organic solvents electrolyte is encapsulated with PDMS. It outputs an energy density ≈1750 mJ cm−2 at 0.5 mA cm−2, which is higher than the AC‐based micro supercapacitor with similar design. Another flexible micro LIC with planar design is reported by Zheng et al. by using layer‐by‐layer mask assisted deposition to assemble activated graphene (AG) and Li4Ti5O12 (LTO) on nylon membrane substrate as cathode and anode.[164] With LiTFSI‐P14TFSI‐PVDF‐HFP ionogel as electrolyte, the quasi‐solid‐state micro LIC demonstrates a maximum energy density of 53.5 mWh cm−3 at ≈0.1 W cm−3 and a maximum power density of 4.6 W cm−3 at ≈10 mWh cm−3 with a capacity retention rate of 98.9% after 6000 cycles at 0.4 mA cm−2.[105]The device design of multifunctional LICs is listed in Table 2. Comparing the reported devices with multifunctional batteries and supercapacitors, it could be pointed out the development of multifunctional LICs is still at a very early stage and the study from electrode/electrolyte to encapsulation and manufacture is in need.2TableSummary of state‐of‐the‐art multifunctional LICsConfigurationElectrolyteFlexibility? (Y/N)Maximum energy densityMaximum power densityLifespanYear of publicationRefs.(+) AC//Prelithiated graphite (−)Liquid 1 m LiPF6 in organic solvent (Encapsulated by PDMS)N81.0 at 220.0 W kg−1_71% after 1000 cycles at 1 mA cm−22015[104](+) Nanographene sheet//nano graphene sheet/TiO2 (−)PVDF‐HFP containing 1 m LiPF6 in organic solventN72.0 at 303 W kg−12000 at 10 Wh kg−168% after 1000 cycles at 1.5 A g−12016[102](+) Graphene sheet//Li3VO4/CNF (−)PVDF‐HFP containing 1 m LiClO4 in organic solventN110.0 at 173.0 W kg−13870 at 28 Wh kg−186% after 2400 cycles at 0.4 A g−12017[106](+) AC/CNT//TiO2‐x/CNT (−)PVDF‐HFP containing 1 m LiPF6 in organic solventY104.0 at 250.0 W kg−15000 at 32.0 Wh kg−171% after 2000 cycles at 1 A g−12018[103](+) AC/carbon fiber//Nb2O5/carbon fiber (−)Liquid 1 m LiPF6 in organic solvent (encapsulated by a thin plastic film)Y95.6 at 191.0 W kg−15350.9 at 65.4 Wh kg−173% after 1000 cycles at 0.5 A g−12018[101](+) AC//maleic acid@PVDF(−)Liquid 1 m LiPF6 in organic solvent (encapsulated by Al‐plastic foil)Y153.1 at ≈215.0 W kg−1≈4300 at 22.1 Wh kg−1_2018[107](+) Activated graphene//LTO (−)a)LiTFSI‐P14TFSI‐PVDF‐HFP ionogelY53.5 mWh cm−3 at ≈0.1 W cm−34.6 W cm−3 at ≈10 mWh cm−398.9% after 6000 cycles at 0.4 mA cm−22018[105](+) PANi@CNF//CNF (−)b)PAN containing 1 m LiPF6 in organic solventY≈172.9 mWh cm−2 at ≈0.8 mW cm−2≈16 mW cm−2 at ≈74.7 mWh cm−2_2020[64](+) AC//Li3VO4/graphene (−)a)LiTFSI‐P14TFSI in PVDF‐HFPN51.4 mWh cm−3 at 53.2 mW cm−3511 mW cm−3 at 34.1 mWh cm−390.0% after 1000 cycles at 0.02 mA cm−22021[108]a)Only areal and volumetric performance reportedb)The data is converted from the information provided in the literature.Possible Roadmap for LICs from Academy to IndustryStatus of Commercial LICsEDLC//battery is the only commercialized LICs configuration by Musashi Energy Solutions Co., Ltd.(ULTIMO),[109] TAIYO YUDEN,[110] NEC Tokin,[111] and General Capacitor.[112] With porous carbon‐based EDLC cathode and carbon‐based battery anode with/without pre‐lithiation, these commercialized LICs demonstrate device‐based maximum energy density from 8 to 45 Wh kg−1 (9.1 to 90 Wh L−1) and maximum power density from 4000 to 14000 W kg−1 (3400 to 15 000 W L−1) with voltage up to 4 V. Also, lifespan from 50 000 to 1 000 000 cycles is achievable.[28c] These high‐performance products showcase the promising future of LICs in electrochemical energy storage. Notably, other LICs configurations, e.g., EDLC//PC, PC//PC, and PC//battery are absent from the market.With large specific surface area, EDLC material suffers from significant side reactions when operating at low potential, e.g., electrolyte decomposition and continuous SEI formation, which blocks its application as efficient anode in LICs.[113] Meanwhile, PC material demonstrates both fast kinetics and large capacity, which is an emerging active material in LICs. However, the mechanism understanding of PC material is shallow, comparing with conventional EDLC and battery materials.[114] Further, there is in lack of PC material with wide EW (>3.5 V or <1.0 V vs Li/Li+) that could substitute EDLC cathode or battery anode in LICs.[115] Apart from active material selection and device configuration, simplified and low cost prelithiation method is in great demand for safer and more eco/environmental‐friendly LICs production.[69b]Path for Advanced LICs from Academy to IndustryIn this session, we would try to explain the possible roadmap from academy to industry for advanced LICs with various configurations and simplified prelithiation process. The summary of LICs reported in literatures in Table 1 shows that, the active material‐based maximum energy density of state‐of‐the‐art LICs ranges from 38.0 to 489.3 Wh kg−1, while the maximum power ranges from 2.5 to 350.0 kW kg−1, depending on the configuration of device and the selection of active materials. However, the performance reported based on the mass of active materials only fails to reflect the real performance of industrial product with the same design. In the conventional SCs, taking all the other components into consideration, including current collectors, electrolytes and etc., a factor of 3 to 4 will be used to convert the material‐based performance into the device‐based one.[116] Here, a factor of 3 is applied to convert the laboratory LICs performance into the industrial one in comparison with conventional LIBs and SCs, the value of which is adapted from refs. [18] and [33]. The converted device‐based LICs performance ranges from 12.3 to 163.1 Wh kg−1 in maximum energy density and 0.8 to 116.7 kW kg−1 in maximum power density, which shows a significant advantage comparing to the commercial LICs. As the inserted Ragone plot shown in Figure 10, with the recent development, LICs are catching up with EDLCs in fast energy storage/output capability with superior energy density and demonstrating better power performance than LIBs with comparable energy density. Besides, battery//capacitor devices demonstrate higher energy density while the capacitor//capacitor device is equipped with superior power performance.10FigureThe inserted Ragone plot compares the device‐based performance of SCs, LICs, and LIBs. The value of battery//capacitor and capacitor//capacitor LICs performance is converted from the material‐based energy/power density listed in Table 1 (with extreme value excluded) and a factor of 3 is applied during calculation.[116] The performance of LIBs and SCs is adapted from refs. [1b, 16f].With the rapid development of LICs, it is foreseeable that the scale‐up LICs could achieve 50% or even 80% in maximum energy density of the LIBs (≈200 Wh kg−1 for state‐of‐the‐art) with a maximum power density over 10 kW kg−1 (which is comparable to the commercial EDLCs) as illustrated in Figure 11.[117] Also, excellent stability with cycle life over 10 000 cycles and high safety without electrolyte leakage and flammable issues are preferred. Furthermore, sharing the similar set up of LIBs but a lower (≈50%) energy density, the energy cost ($ kW h−1) of LICs is expected to be nearly double of LIBs, which is less than $500 kW h−1. With such advantages, LICs are suggested to replace LIBs in high‐power applications and EDLCs in the situations that both high‐power, high‐energy are required, e.g., hybrid vehicles, crane plants, industrial machines, and electronic processors etc. The possible roadmap for LICs from laboratory to commercialization is discussed below.11FigureIllustration of a possible roadmap for LICs from lab product to industrial device. (The cost projection of LICs in future development is based on the factors that, LICs are sharing the similar set up, but demonstrating ≈50% energy density of current LIBs).High‐Performance Active MaterialWhile there are numerous potential LICs anode materials (e.g., carbon‐based materials, transition metal oxides and MXenes) with their associated energy storage mechanisms, the choice for cathode materials appears to be restricted due to the low operational potential Li/Li+. Especially for those capacitive materials, carbon‐based materials with different atom doping and morphology are still the most popular candidate to stores energy via surface ion adsorption, which provides high power performance with no doubt. However, their energy density is poor when compared to battery cathodes due to the low capacity. The lack of PC materials with higher energy density and good power performance for LICs cathode seems to be the bottleneck that limits the further development of device at this stage.Despite porous carbons, there is in lack of anion host for reasons: the anion size is large and ion diffusion channel in host materials is narrow. Despite this, anion intercalation host with larger ion diffusion channel could be the focus point. Based on this, the anion/cation‐intercalation host‐type LICs, which will show much higher capacitance theoretically, and may be the next generation high‐performance LICs.Nevertheless, self‐discharge is another key factor in the performance judgment of LICs. LICs with EDLC cathode demonstrate comparable self‐discharge behavior to EDLCs due to the physical ion adsorption in EDLC active material.[118] Formation of passivation layer on the surface of EDLC materials could minimize the self‐discharge effectively.[119] Meanwhile, great effort in detailed mechanism understanding and investigation into effective and feasible self‐discharge suppression is necessary to further improve the performance of LICs.Active Material Selection and BalancingLICs in both academy and industry share the similar design principles, e.g., voltage window expansion, mass ratio balance and cathode/anode kinetic balance etc., to achieve high energy/power performance with ultralong lifespan. From the recent development of LICs reported, it is foreseeable that LICs could achieve a high energy density over 100 Wh kg−1 with comparable power density and cycling life to those supercapacitors. Though gravimetric performance of LICs is widely reported, the areal/volumetric performance is of equal importance in industry for the design of compact device. Active materials with high density (low porosity) and excellent conductivity are desired, which enable the design of electrodes with high mass loading, high density but low electrolyte uptake, leading to the design of LICs with high areal/volumetric performance. Also, cathode/anode thickness balance shows significant influence onto the design of compact LICs.[120]Minimizing the Offsetting Effect of Nonactive ComponentsSignificant differences in the amount of nonactive components, e.g., electrolyte, conductive agent, and binder, are observed between laboratory and commercial devices. Porous or nanosized active materials may require flooding electrolyte uptake in electrode for surface‐like ion diffusion or anion source. However, high portion of electrolyte limits the performance on an electrode/device‐based discussion. Especially in commercial devices, lean electrolyte system is preferable. By minimizing the volume of electrolyte per device, it enables the design of lightweight and compact LICs with low cost. However, the insufficient electrolyte may induce the polarization, reducing the energy/power performance.[121] On the other hand, the development of high ion conductive electrolyte with wide operation voltage range is important for both academy and industry, which could increase the columbic efficiency during cycling by minimizing the side reaction between electrode/electrolyte and reduce the ionic polarization under high current density.Apart from electrolyte, the high portion of active material in electrode, including the low content of conductive agent and binder, and high active material mass loading, in those conventional products are required to achieve higher energy density, which would result to the high electronic resistance and limit the power performance of LICs.[122] Notably, active materials tend to be exfoliated from the substrate under high mass loading or in flexible LICs during flexing, resulting to a reduced lifespan. Herein, the development of advanced current collectors and electrode fabrication methods that anchors the material onto substrate strongly are desired.Simplifying of Skipping PrelithiationPrelithiation technique is essential for LICs activation and achieving high power/energy performance with excellent stability due to the lack of sufficient lithium source in the device. However, the methodologies involved, e.g., pretreating anode and introducing extra lithium source in electrode etc., and the materials used, e.g., Li metal and lithium salt etc., complicate the device fabrication process with cost increased. Though researchers in the field are putting great efforts to develop techniques and materials that are suitable for industrial application, e.g., integrating sacrificial Li‐compounds into electrode, the necessity of pre‐lithiation may still block the commercialization of LICs. As summarized from Table 1 and to our best knowledge, there is in lack of Li‐rich active material with capacitive behavior for LICs. The investigation into highly reversible Li‐rich capacitive electrode can improve the performance of LICs greatly. Meanwhile, the pre‐lithiation step could be skipped, simplifying the LICs fabrication process and lowering the cost.Safety ImprovementLike LIBs, it should be noticed that the issues of electrolyte leakage and flammability remain critical in device commercialization with liquid organic electrolytes, which limit the application of LICs, especially in flexible devices. To get rid of this drawback, it is necessary to develop solid‐state electrolyte with both wide operational potential range as well as high ionic conductivity to maintain the high power/energy density advantages of LICs without the risk of leakage.[123] Notably, the use of solid‐state electrolyte could also enable the possibility of flexible LICs and reduce the self‐discharge behavior.[124] Nevertheless, the development of nonflammable electrolyte, is also urgent for those organic Li‐ion electrolyte systems.[125] A conventional LiPF6‐based electrolyte bending with fluorinated cyclic phosphate solvent that reported recently demonstrates excellent thermal stability and high safety for LIBs and shows the potential to be extended to the system of LICs.[126]Reducing the Production CostThe LICs are more expensive than LIBs in the value of $ kWh−1, with the much lower energy density, unmatured production and market. With the similar components to LIBs, e.g., electrolyte, separator, production line, etc., and higher energy density than EDLCs, the cost of LICs should sit between EDLCs and LIBs. With the future development on performance and fabrication process simplification, it is possible to reduce the cost of LICs to the value close to LIBs. On the other hand, considering the power‐based price, $ kW−1, with the comparable power performance to EDLCs, LICs are targeted with a lower power cost than LIBs.Setting Up Standardized Evaluation ProtocolSetting standardized protocols in performance testing and reporting as well as taking the industrial requirements into consideration in research could further boost the development of next generation LICs. The tested LICs performance may vary from each other with different electrode mass loading, thickness, mass ratio between active material and non‐active materials, volume of electrolyte used, testing temperature etc. Herein, it is recommended to report the testing condition together with the electrode/device performance. When reporting the performance of device, the calculation should base on the total mass/area/volume of two electrodes. Further, LICs assembly with high electrode mass loading and minimized volume of electrolyte infilled could better reflect the real performance of the material/device design in commercialization. Despite those widely studied aspects reported in literatures, the industry‐related evaluations, including LICs performance within wide operational temperature range, thermal characterization (reflecting the irreversible lifespan reduction and potential safety issues due to the heat generated by the electronic resistance of electrode and ionic resistance of electrolyte during charge/discharge), lifespan prediction and self‐discharge rate etc., could give more insight onto the commercialization possibility of the lab products.[127]From LICs to Post LICsFinally, we would like to extend the knowledge gained from LICs research to other electrochemical energy systems. Post LICs, e.g., sodium‐ion capacitors (NICs) and potassium‐ion capacitors (KICs), are attracting numerous interests for their high performance and potentially low cost.[128] Due to the larger size of sodium ion (1.02 Å) and potassium ion (1.38 Å) to lithium ion (0.76 Å),[129] the current cation host in LICs may not be applicable to NICs and KICs. More efforts in searching for suitable active materials in post LICs are necessary. Apart from that, sharing the similar working mechanisms, the general LICs design rational and device configurations are adaptable to post LICs, including: a) optimizing the pre‐metalation process, e.g., replacing the direct use of highly active alkaline metal (lithium, sodium, and potassium) with other sacrificial alkaline metal‐rich components to improve the safety and sustainability; b) developing high‐performance active materials with the consideration of high power/energy, long cycle life and low self‐discharge rate; and c) balancing cathode/anode capacity and kinetics to prolong the lifespan and maximize the energy‐power performance of devices etc. The experiences learnt from LICs may provide a shortcut to the development of post LICs.OutlookIn this review, we distinguish LICs from their electrochemical energy storage counterparts EDLCs and LIBs. The comparative advantages and disadvantages of the different LICs device designs (i.e., battery//capacitor and capacitor//capacitor) were reviewed with state‐of‐the‐art example devices being used to exemplify different aspects of device design and functionality. Finally, key differences between laboratory and industrial metrics of LICs were identified, with possible development roadmap pointed out. The experiences learnt from LICs research could be further extended to post LICs. We conclude that, although LICs can provide higher energy density than EDLCs and faster responses than LIBs, they also demonstrate some deficiencies. With the further improvements of performance and safety, and reduction in cost, LICs would show unparalleled competence comparing with LIBs and EDLCs to meet the high power and high energy demand market.AcknowledgementsThis work was financially supported by the Australian Research Council Discovery Project (DP190101008), Future Fellowship (FT190100058), and the UNSW Scientia PhD Scholarship.Open access publishing facilitated by University of New South Wales, as part of the Wiley ‐ University of New South Wales agreement via the Council of Australian University Librarians.Conflict of InterestThe authors declare no conflict of interest.a) N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung, J. Thomas, Adv. Mater. 2017, 29, 1605336;b) Poonam, K. Sharma, A. Arora, S. K. Tripathi, J. Energy Storage 2019, 21, 801;c) P. Poizot, J. Gaubicher, S. Renault, L. Dubois, Y. Liang, Y. Yao, Chem. Rev. 2020, 120, 6490.K. Mizushima, P. C. Jones, P. J. Wiseman, J. B. Goodenough, Mater. Res. 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Journal

Advanced Energy Materials – Wiley

Published: Jul 1, 2022

Keywords: batteries; electrode materials; lithium‐ion capacitors

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