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Cycling properties of Na3V2(PO4)2F3 as positive material for sodium-ion batteries

Cycling properties of Na3V2(PO4)2F3 as positive material for sodium-ion batteries The research into sodium-ion battery requires the development of high voltage cathodic materials to compensate for the potential of the negative electrode materials which is usually higher than the lithium counterparts. In this framework, the polyanionic compound Na V (PO ) F was prepared by an easy-to-scale-up carbothermal method and characterized to evaluate its electrochemical per- 3 2 4 2 3 −1 formances in half cell vs. metallic sodium. The material shows a specific capacity (115 mAh g ) close to the theoretical limit, good coulombic efficiency (>99%) and an excellent stability over several hundred cycles at high rate. High-loading free-standing electrodes were also tested, which showed interesting performances in terms of areal capacity and cyclability. . . . . Keywords Fluorophosphates NASICON Sodium-ion battery Positive electrodes Self-standing electrodes Introduction and lithium, respectively) and the higher standard reduc- tion potential (−2.71 and −3.04 V vs. SHE) which leads to Nowadays, rechargeable batteries are fundamental for both lower energy densities [3]. While it is true that the weight + + static and mobile applications. With their long-term history of cyclable Na and Li are just a small fraction of the mass and deep investigations, lithium-ion batteries (LIBs) rule the of the whole electrodes (no practical penalty in terms of landscape of electrochemical energy storage, but this may energy density loss) [4], it is clear that the electrode poten- change in the future due to the low abundance of lithium in tial gap (about 300 mV) needs to be considered to make the earth’s crust and its enormous request in order to satisfy NIBs competitive to LIBs. Moreover, sodium’sbiggerra- the growing battery market [1, 2]. Sodium-ion batteries dius becomes an issue for the stability of several intercala- (NIBs) have been proposed as potential candidates in the re- tion cathodic materials, such as layered oxides. For exam- placement of LIBs for several applications thanks to the ple, while the intercalation process takes place in Li 1- physico-chemical similarities between the two elements and CoO with a narrow potential range (from 4.3 to 3.5 vs. x 2 the abundance and ubiquity of sodium on earth. Li /Li for x=0.5), Na CoO is characterized by several x 2 There are, however, some differences between the two drops of the insertion potential of Na during reduction ions, reflected in their electrochemical properties such as [5]. This is to attribute to the several changes in the oxide’s −1 the higher molar mass (23.2 and 6.9 g mol for sodium structure to better accommodate sodium ions, resulting in an overall cathodic potential that is generally about 1V less with respect of lithium’s counterpart, thus affecting the energy density of the full battery. In order to overcome This paper is dedicated to the late Professor Claudio Maria Mari, who inspired the research here reported these issues, it is necessary to focus on different solutions, such as high voltage cathodic materials which can be * Riccardo Ruffo achieved, for example, with polyanionic compounds. riccardo.ruffo@unimib.it Since the use of olivine (LiFePO ) for LIBs, the phos- phate anion has been widely studied for both lithium and Dipartimento di Scienza dei Materiali, Università di Milano Bicocca, sodium-ion batteries due to its intrinsic thermal stability. 20125 Milano, Italy Furthermore, when partially substituted with fluoride, the National Reference Center for Electrochemical Energy Storage resulting network allows for a higher insertion electrical (GISEL) - Consorzio Interuniversitario Nazionale per la Scienza e potential of alkali ions, due to the stabilization of the Tecnologia dei Materiali (INSTM), 50121 Firenze, Italy 1854 Ionics (2021) 27:1853–1860 antibonding d-orbitals of the metal thanks to the inductive From the literature analysis, it is clear that the carbothermal effect in the M-F bond [6]. For those reasons, nowadays synthesis seems to be the more suitable and scalable method fluorophosphate materials are considered valid alternatives for preparing materials and simultaneously disperse them in- to transition metal oxides. side a carbonaceous (conductive) matrix. The aim of this work Several phosphates and fluorophosphates have been was to reproduce and optimize the synthetic procedure to pro- proposed during the past years, such as Na V (PO ) (op- duce NVPF active materials, which are then formulated in 3 2 4 3 erating voltage of ~3.4V Na /Na) [7], Na FePO F(operat- electrodes with different mass loads and tested to report the 2 4 ing voltage ~3.5V vs. Li /Li) [8], NaVPO F (operating corresponding rate capabilities. A long cycling test is also voltage of ~3.7V vs. hard carbon in a SIB) [9]and reported. Moreover, as attempt to produce high load elec- −2 Na M (PO ) F (with M = Ti, Fe, V) [10, 11]. Among trodes (> 20 mg cm ), we report the result about a NVPF 3 2 4 2 3 all these materials, Na V (PO ) F shows the highest po- free-standing membrane which can be processed from aque- 3 2 4 2 3 tential against metallic sodium (~3.8V) and a relatively ous solution. −1 high theoretical capacity of 128.3 mAh g . With its tetrag- onal crystal lattice (P4 /mnm), it has channels in which Na ions can move fast, and it is to be considered a Materials and methods NASICON-like structure. However, the separation of va- nadium ions by the phosphates combined with the strong Na V (PO ) F was prepared with a slightly modified 3 2 4 2 3 ionicity of the V-F bond leave this material with poor elec- procedure as what suggested by Weixin et al. [16]. tronic conductivity. Firstly, stochiometric amounts of NH H PO ,V O and 4 2 4 2 5 The Na V (PO ) F phase (NVPF) was firstly prepared NaF were dissolved/dispersed in distilled water, stirred 3 2 4 2 3 by J. Barker in 2006 [11] using the carbothermal reduc- and dried under air flux at 50°C. Then, 5 wt% of Super tion. In further works of the same group [12, 13], the P as a carbon source was grounded with the other reagents material was tested in half lithium cells, i.e. after the first with a mortar. The resulting powders were pelletized and in situ desodiation, it was evaluated as positive material heated in argon atmosphere in a single synthesis com- for LIBs. The preliminary results in Na ion electrolyte posed by two steps: the first at 350°C for 4h and the were not so promising [13]. The first example of success- second at 650°C for 8h. The two-step heating procedure ful use in Na half cell was reported in 2012 using 1 M was introduced to form oxides and phosphate (first step) NaClO as electrolyte [14]. Despite the ball milling pro- and then promote the reaction between them (second cess with 20% of carbon and the low electrode loading step). Between the two treatments, we ground and pellet- −2 (1.85mgcm ), the material was able to provide 110 ize the precursors in order to make the contact between −1 mAh g at low current. A rate test was performed; how- them more homogenous and intimate. ever, no data were reported about cyclability. As-synthesized powders were studied with X-ray pow- Subsequently, several electrolytes were tested, both in der diffraction (XRD), performed with a Rigaku Miniflex half cell vs. Na and full cells with hard carbon [15]. 600. The analysis was made from 10 to 80 degrees (2θ) However, the electrode was fabricated with a large with a step size of 0.02 degrees, an angular velocity of 0.1 amount of carbon (27%) and low mass loading (2 mg degrees per minute and using a copper source. Scanning −2 cm ). An improved preparation route, also based on electron microscopy was performed with a Zeiss® Gemini carbothermal reduction, was presented in 2014 [16]. The SEM 450. Even though all samples (powder and elec- electrodes were fabricated with 80% of active material trodes) had some content of carbon inside, each of them −2 with higher mass loading(4.5mgcm ), and the diffusion was covered with graphite in order to reduce the charging coefficient was investigated. Cyclability tests were per- effects. Optical images were collected with a Leica® opti- formed with materials obtained by sol gel [17]and wet cal microscope equipped with a Leica DFC280 digital col- chemistry [18]. Electrodes were produced with 70% of our camera. −2 active material and a mass loading of 1.5/2.0 mg cm , Electrodes’ slurries were prepared by mixing together and they showed remarkable cyclability and rate capabil- the active material (80 wt%) with Super P 10 wt%) and ities (more than 1000 cycles at C/rate of 10C or higher) PVdF (10 wt%) as binder, using NMP as solvent. The obtained thanks to the use of carbon shells [17]ornano- resulting slurries were deposited on aluminium foils composites [18]. The only example of high load electrode (30 μm of thickness) with different thicknesses (5, 10 −2 (about 10 mg cm ) was reported in 2017, showing prom- and 30 mils), then dried under vacuum at 80°C overnight ising results but without presenting cyclability data [19]. and pressed with a calender. Self-standing high-load elec- More recent works focused on the composite preparation trodes were prepared following a previously reported (e.g. with graphene oxide [20]) without further investigat- route [21] by grounding NVPF together with Super P ing the pristine materials. conductive carbon in a mortar and mixing the obtained Ionics (2021) 27:1853–1860 1855 powder with a suspension of PTFE in water (Sigma- reduction reaction was evaluated with CHNS measurement, Aldrich, 60 wt%) to obtain a homogenous dough. The corresponding to 5.4% of the whole mass. amount of suspension was calculated in order to have The morphological features of supported and free-standing the same ratio of active material/binder as for the support- electrodes were characterized by optical and electronic mi- ed electrode, i.e. 8:1. The dough was then calendered croscopy (Fig. 3). The Al-supported electrode (Fig. 3a)shows several times reducing the thickness up to obtain a flexi- a compact surface with an overall good homogeneity. The ble film with final thickness of 180 μm. R2032 coin cells electrode’s thickness was evaluated via optical microscopy were assembled in an argon-filled glove box, using me- (insertion in Fig. 3a), showing a total average thickness of tallic sodium as counter electrode and a 1M solution of about 49±4 μm(34±4 μm excluding the aluminium foil). At NaClO in PC + FEC (2 wt%) as electrolyte, and were higher magnification (Fig. 3b), it is possible to appreciate the tested using a Bio-Logic VMP3 battery tester. binder filaments, while it is harder to distinguish between active materials and Super P particles. The self-standing elec- trode (Fig. 4a) seems porous and homogenous in composition; also in this case (Fig. 4b), the binder filaments are clearly Results and discussion observed. The electrode thickness is constant and it has been measured as 237±7 μm. X-ray diffraction technique has been used both for evaluating The quasi-thermodynamic electrochemical behaviour of the preparation products and the presence of impurities. As Na V (PO ) F was explored with PCGA (potentiodynam- 3 2 4 2 3 shown in Fig. 1, the Rietveld refinement shows that the main ic cycling with galvanostatic acceleration) technique. The phase is NVPF (97%), with a small amount of Na V (PO ) measurement was indeed performed by applying 3 2 4 3 (NVP) as impurity. The average particle dimension of crystal- potentiostatic steps of 4 mV until the current dropped be- −1 lites was also evaluated by means of the Scherrer equation, low a limit value (in our case 0.02C, 2.56 mA g ). This obtaining a value of about 45 nm, considering the two most procedure allows to extract/insert sodium inside the struc- intense (002) and (222) peaks. ture with very little overpotentials, thus operating near the SEM images of the as-synthesized NVPF powders are equilibrium, and collecting all the available charge thanks showninFig. 2 together with the histogram of the particle size to the low current threshold. Figure 4a depicts the voltage- obtained by averaging different images. The microstructure capacity curves of three cycles of sodiation/desodiation of consists nano-sized particles with dimension picked around NVPF. As expected, the reaction with sodium happens at 60–80 nm but partially coalesced in agglomerations of few three different potentials, roughly 3.4V, 3.6V and 4.0V vs. micrometres. The residual carbon left from carbothermal Na /Na. These electrochemical events are better shown in the differential capacity curves derived from PCGA (Fig. 4b). As stated by Weixin et al. [16], the two low-potential peaks are associated to the insertion/extraction of the first equivalent of sodium through a two-phase reaction, while Experimental data the highest peaks involve the second equivalent of Na in a Calculated pattern mono-phase domain. This can easily be appreciated from I -I exp Calc the shape of the peaks which appear sharp-spiked and Bragg peaks broad-bellied for the low and potential processes, respec- tively. From PCGA it was also possible to calculate the mean potential at which NVPF operates with very low overpotentials. This value was calculated in the half cell using the ratio between the energy (the integral of the V/ charge curve) and the capacity, both obtained during sodiation/desodiation, respectively. For the cathodic NVPF NVP sodiation, NVPF operates at a mean operative potential of 3.78 V. The total charge stored in the material is 120 mAh −1 g , a value close to the theoretical one. However, during the first desodiation, an irreversible capacity of about 30 −1 10 20 30 40 50 60 70 80 mAh g is observed due probably to the formation of the cathodic electrolyte interface (CEI) on both the active ma- 2θ (°) terials and the carbon additive [22, 23]. Fig. 1 XRD pattern of the obtained powders NVPF (red dots) and The kinetic behaviour of NVPF was observed by Rietveld results using the Na V (PO ) F and Na V (PO ) reference 3 2 4 2 3 3 2 4 3 materials means of galvanostatic measurements. Figure 4c shows Intensity (a.u.) 1856 Ionics (2021) 27:1853–1860 200 nm 1 µm 0 20 40 60 80 100 120 140 Particle size (nm) Fig. 2 (a) SEM images of as-synthesized NVPF taken at different magnitudes (10k and 50k, respectively) and (b) particle size distribution histogram the achieved capacity obtained during subsequent cycling currents cycling as well as stability. When returning to −1 at different currents (0.1C, 0.2C, 0.5C, 1C, 5C and 10C). 0.1C, indeed, the capacity resets back to 105 mAh g , Achievable capacity showed an inverse proportion with whichis95% of theinitial value, andthendecreased less −1 −1 respect to the C-rate, losing about 20% (88 mAh g with than 3 mAh g during thenext50cyclesatlow currents. −1 respect to the starting value of 110 mAh g )after in- Mean potentials were also evaluated for each cycle (Fig. creasingthe current from0.1Cto1C, butwithanincrease 4d). NVPF showed relatively low polarization at low C- in the coulombic efficiency (> 99%), as an indication of rates, with an increase of the mean potential of about 60– the presence of anodic parasitic reactions. Moreover, 70 mV between scanning at 0.1C and 1C, while higher NVPF showed a very good capacity recovery after high overpotentials (200/400 mV) can be seen for currents of Fig. 3 Images of the two electrodes’ configuration Al-supported and self-standing. (a, b) SEM images of the Al-supported electrode. (c, d) SEM images of the self-standing one. The two inserted images are optical microscope’s images of the cross section of the two electrodes Frequency Ionics (2021) 27:1853–1860 1857 Fig. 4 (a) Voltage-gravimetric 3.61V 4.2 charge curves of PCGA ab measurement. (b)Differential 4.03V st 1 cycle capacities curves derived from nd 2 cycle rd 3.9 3 cycle PCGA. (c) Specific capacity vs. 3.39V st 1 cycle nd cycle number for NVPF at 2 cycle rd 3 cycle various C-rates. (d) Sodiation and 3.36V desodiation mean voltages vs. 3.6 -2 cycle number 4.02V -4 3.3 3.60V -6 3.2 3.4 3.6 3.8 4.0 4.2 0 20 40 60 80 100 120 140 160 -1 Voltage vs Na /Na (V) Gravimetric Charge (mAh g ) 180 4.2 4.1 4.0 3.9 0.2C 0.1C 3.8 0.1C 0.5C 1C 5C 10C 0.1C 0.1C 0.2C 92 3.7 80 0.5C 1C 3.6 5C 3.5 Charge Charge 40 Efficiency cd Discharge Discharge 10C 86 3.4 0 10 2030 405060 7080 90 100 110 0 1020 30405060 7080 Cycle Number Cycle Number 5C and 10C. The stable trend of the mean potential 600 cycles in total and almost zero loss of specific capacity (3.78V) during recovery cycles means that NVPF does at 2C) and high coulombic efficiencies of more than 99% not undergo irreversible changes in structure during mul- at 1C and 2C. tiple sodiations/desodiations. This can be considered an- Electrodes with different mass loading of active mate- other good indication of NVPF’s stability and cyclability. rials were also tested. Figure 6a displays the discharge To better evaluate the stability of NVPF during cycles, a capacity of three electrodes with different mass loadings, −2 long-term measurement was performed: starting from 0.1C 1.5, 3.5 and 6.4 mg cm , respectively, using a common then increasing current to 1C and, in the end, 2C. As cycling procedure (10 cycles at C-rates of 0.1, 0.2, 0.5, 1, displayed in Fig. 5, this material showed outstanding sta- 5, 10 and again 0.1). At low currents, where kinetical bility performances, with very high cyclability (more than effects tend to be negligible and thus materials operate in conditions that are not too far from the thermodynamic limit, the three electrodes showed no significant differ- 180 105 ences. This may suggest that NVPF could be very well dispersed inside the carbonaceous matrix (from electrode formulation but also from carbothermal reduction), in- creasing electronic conductivity and thus reducing 0.1C overpotentials. Still, deviations of capacity retention are present at high currents, where kinetic effects tend to re- duce specific capacity values in electrodes with higher 1C thickness and larger mass loading. Same trends can be observed for both coulombic efficiencies and mean oper- ative voltages. Charge 2C Efficiency 75 Finally, the self-standing, very high mass-loading elec- 40 Discharge −2 trodes (25mgcm ) were also tested and compared to the 20 70 aluminium-supported ones. Despite working at a much 0 100 200 300 400 500 600 700 higher current (almost four times), self-standing electrodes Cycle Number showed identical performances in terms of specific capac- Fig. 5 Specific capacity vs. cycle number for a long-term cycling of ity and cyclability at low C-rates (0.1 and 0.2C, Fig. 7) NVPF -1 Specific Capacity (mAh g ) -1 + Specific Capacity (mAh g ) Voltage vs Na /Na (V) Coulombic Efficiency (%) CoulombicEfficiency(%) -1 -1 V vs Na /Na (V) MEAN dQ/dV (Ah g V ) 1858 Ionics (2021) 27:1853–1860 Fig. 6 Comparison among electrodes with different loadings of active mass during cycling. (a) Cathodic specific capacity. (b) Mean discharge voltage. (c) Coulombic efficiency 20 -2 6.4mgcm 4.0 -2 3.5 mg cm 3.8 0.1C 0.1C 0.2C 0.5C 5C 1C 60 3.6 10C 3.4 -2 1.5 mg cm -2 0 10 203040 506070 1.58 mg cm 3.2 -2 3.52 mg cm Cycle Number -2 6.44 mg cm 3.0 0 1020 3040506070 Cycle Number while failing at higher currents. This can be due to ineffi- Conclusions ciencies in the working electrode itself but also to issues in the sodium counters used in the half cells that tend to form Na V (PO ) F was prepared by means of a carbothermal 3 2 4 2 3 thick SEI layers as well as dendrites at high currents during reduction synthesis with high crystallinity. A small to the long operation of the cell. Still, self-standing elec- amount of NVP impurity was detected (3%). The material trodes can be considered good candidates for full cell NIBs showed good performances as a positive electrode for because of the capacity (neither affected by the high mass sodium-ion batteries able to operate at a mean voltage of nor by the absence of the current collector), the coulombic almost 3.8 V under quasi-thermodynamic conditions. Low efficiency (> 98–99% even at low C-rates) and the average load electrodes also showed outstanding cyclability prop- voltage (3.8V vs. Na /Na). The obtained areal capacities of erties with coulombic efficiency always above 98–99%. −2 theself-standing andthe 6.4gcm electrodes are 2.45 and The stable trend of the mean voltage, with relatively low −2 0.65 mAh cm , respectively. overvoltage at current up to 1C, makes this material Fig. 7 Comparison between Al- 4.2 4.2 supported (active mass loading: ab −2 3.9 6.4mgcm ) and self-standing (active mass loading: 25 mg 3.6 4.0 −2 cm ) electrodes. (a) Surface 3.3 charge density of the 5th cycle for Charge 0 20 406080 100 Discharge -1 both systems with the gravimetric 3.8 Gravimetric Charge (mAh g ) Efficiency charge as insert and (b) specific 96 capacity for the self-standing 3.6 electrode at two different C-rates 0.2C (0.1 and 0.2C) 0.1C Al-supported 3.4 Self-standing 3.2 90 0.0 0.5 1.0 1.5 2.0 2.5 05 10 15 20 -2 Surface Charge Density (mAh cm ) Cycle Number + + V vs Na /Na (V) Voltage vs Na /Na (V) MEAN Voltage vs Na /Na (V) -1 Specific Capacity (mAh g ) Coulombic Efficiency (%) Coulombic Efficiency (%) Ionics (2021) 27:1853–1860 1859 7. Jian Z, Zhao L, Pan H, Hu YS, Li H, Chen W, Chen L (2012) suitable for low-to-mid power delivery systems, and it is Carbon coated Na3V2(PO4)3 as novel electrode material for sodi- also a factor that affects the energy efficiency, which um ion batteries. Electrochem Commun 14:86–89. https://doi.org/ maintains higher than 94%. Moreover, NVPF synthesized 10.1016/j.elecom.2011.11.009 by carbothermal reduction appeared to be conductive 8. Ellis BL, Makahnouk WRM, Makimura Y, Toghill K, Nazar LF (2007) A multifunctional 3.5V iron-based phosphate cathode for rechargeable enough in order to allow low-to-no kinetical effects when batteries. Nat Mater 6:749–753. https://doi.org/10.1038/nmat2007 increasing mass loadings on electrodes. This is particular- 9. Navpo F, Barker J, Saidi MY, Swoyer JL (2003) A sodium-ion cell ly true considering that even self-standing electrodes with based on the fluorophosphate compound NaVPO4F. Electrochem high thicknesses (180 μm) andhighmassloadings(25 mg Solid-State Lett 6:11–14. https://doi.org/10.1149/1.1523691 −2 cm ) show very good performances in terms of coulom- 10. Chihara K, Kitajou A, Gocheva ID, Okada S, Yamaki JI (2013) Cathode properties of Na3M2(PO4)2F3[M = Ti, Fe, V] for sodium- bic efficiency and achieved capacity. ion batteries. J Power Sources 227:80–85. https://doi.org/10.1016/j. jpowsour.2012.10.034 11. Barker J, Saidi MY, Swoyer JL, (2002) US Patent 6, 387,568 Funding Open access funding provided by Università degli Studi di 12. Gover RKB, Bryan A, Burns P, Barker J (2006) The electrochem- Milano - Bicocca within the CRUI-CARE Agreement. This work has ical insertion properties of sodium vanadium fluorophosphate, been funded by the Italian Ministry of University and Research (MIUR) Na3V2(PO4)2F3. Solid State Ionics 177:1495–1500. https://doi. through grants “Dipartimenti di Eccellenza 2017 – Materials for energy” org/10.1016/j.ssi.2006.07.028 and PRIN 2017 “Towards sustainable, high-performing, all-solid-state 13. Barker J, Gover RKB, Burns P, Bryan AJ (2007) sodium-ion batteries”. Li4 3Ti5 3O4∥Na3V2(PO4)2F3: an example of a hybrid-ion cell using a non-graphitic anode. J Electrochem Soc 154:A882–A887. Declarations https://doi.org/10.1149/1.2756975 14. Shakoor RA, Seo DH, Kim H, Park YU, Kim J, Kim SW, Gwon H, Lee S, Kang K (2012) A combined first principles and experimental Conflict of interest The authors declare no competing interests. study on Na3V2(PO4)2F3 for rechargeable Na batteries. J Mater Open Access This article is licensed under a Creative Commons Chem 22:20535–20541. https://doi.org/10.1039/c2jm33862a Attribution 4.0 International License, which permits use, sharing, adap- 15. Ponrouch A, Dedryvère R, Monti D, Demet AE, Ateba Mba JM, tation, distribution and reproduction in any medium or format, as long as Croguennec L, Masquelier C, Johansson P, Palacín MR (2013) you give appropriate credit to the original author(s) and the source, pro- Towards high energy density sodium ion batteries through electro- vide a link to the Creative Commons licence, and indicate if changes were lyte optimization. Energy Environ Sci 6:2361–2369. https://doi.org/ made. The images or other third party material in this article are included 10.1039/c3ee41379a in the article's Creative Commons licence, unless indicated otherwise in a 16. SongW, JiX, Wu Z, YangY,ZhouZ,LiF,ChenQ, Banks CE credit line to the material. If material is not included in the article's (2014) Exploration of ion migration mechanism and diffusion ca- Creative Commons licence and your intended use is not permitted by pability for Na3V2(PO4)2F3 cathode utilized in rechargeable statutory regulation or exceeds the permitted use, you will need to obtain sodium-ion batteries. J Power Sources 256:258–263. https://doi. permission directly from the copyright holder. To view a copy of this org/10.1016/j.jpowsour.2014.01.025 licence, visit http://creativecommons.org/licenses/by/4.0/. 17. 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Electrochim Acta 79:95–101. https:// Publisher’snote Springer Nature remains neutral with regard to jurisdic- doi.org/10.1016/j.electacta.2012.06.082 tional claims in published maps and institutional affiliations. 23. Li W, Dolocan A, Oh P, Celio H, Park S, Cho J, Manthiram A (2017) Dynamic behaviour of interphases and its implication on http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Ionics Springer Journals

Cycling properties of Na3V2(PO4)2F3 as positive material for sodium-ion batteries

Ionics , Volume 27 (5) – Apr 2, 2021

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Abstract

The research into sodium-ion battery requires the development of high voltage cathodic materials to compensate for the potential of the negative electrode materials which is usually higher than the lithium counterparts. In this framework, the polyanionic compound Na V (PO ) F was prepared by an easy-to-scale-up carbothermal method and characterized to evaluate its electrochemical per- 3 2 4 2 3 −1 formances in half cell vs. metallic sodium. The material shows a specific capacity (115 mAh g ) close to the theoretical limit, good coulombic efficiency (>99%) and an excellent stability over several hundred cycles at high rate. High-loading free-standing electrodes were also tested, which showed interesting performances in terms of areal capacity and cyclability. . . . . Keywords Fluorophosphates NASICON Sodium-ion battery Positive electrodes Self-standing electrodes Introduction and lithium, respectively) and the higher standard reduc- tion potential (−2.71 and −3.04 V vs. SHE) which leads to Nowadays, rechargeable batteries are fundamental for both lower energy densities [3]. While it is true that the weight + + static and mobile applications. With their long-term history of cyclable Na and Li are just a small fraction of the mass and deep investigations, lithium-ion batteries (LIBs) rule the of the whole electrodes (no practical penalty in terms of landscape of electrochemical energy storage, but this may energy density loss) [4], it is clear that the electrode poten- change in the future due to the low abundance of lithium in tial gap (about 300 mV) needs to be considered to make the earth’s crust and its enormous request in order to satisfy NIBs competitive to LIBs. Moreover, sodium’sbiggerra- the growing battery market [1, 2]. Sodium-ion batteries dius becomes an issue for the stability of several intercala- (NIBs) have been proposed as potential candidates in the re- tion cathodic materials, such as layered oxides. For exam- placement of LIBs for several applications thanks to the ple, while the intercalation process takes place in Li 1- physico-chemical similarities between the two elements and CoO with a narrow potential range (from 4.3 to 3.5 vs. x 2 the abundance and ubiquity of sodium on earth. Li /Li for x=0.5), Na CoO is characterized by several x 2 There are, however, some differences between the two drops of the insertion potential of Na during reduction ions, reflected in their electrochemical properties such as [5]. This is to attribute to the several changes in the oxide’s −1 the higher molar mass (23.2 and 6.9 g mol for sodium structure to better accommodate sodium ions, resulting in an overall cathodic potential that is generally about 1V less with respect of lithium’s counterpart, thus affecting the energy density of the full battery. In order to overcome This paper is dedicated to the late Professor Claudio Maria Mari, who inspired the research here reported these issues, it is necessary to focus on different solutions, such as high voltage cathodic materials which can be * Riccardo Ruffo achieved, for example, with polyanionic compounds. riccardo.ruffo@unimib.it Since the use of olivine (LiFePO ) for LIBs, the phos- phate anion has been widely studied for both lithium and Dipartimento di Scienza dei Materiali, Università di Milano Bicocca, sodium-ion batteries due to its intrinsic thermal stability. 20125 Milano, Italy Furthermore, when partially substituted with fluoride, the National Reference Center for Electrochemical Energy Storage resulting network allows for a higher insertion electrical (GISEL) - Consorzio Interuniversitario Nazionale per la Scienza e potential of alkali ions, due to the stabilization of the Tecnologia dei Materiali (INSTM), 50121 Firenze, Italy 1854 Ionics (2021) 27:1853–1860 antibonding d-orbitals of the metal thanks to the inductive From the literature analysis, it is clear that the carbothermal effect in the M-F bond [6]. For those reasons, nowadays synthesis seems to be the more suitable and scalable method fluorophosphate materials are considered valid alternatives for preparing materials and simultaneously disperse them in- to transition metal oxides. side a carbonaceous (conductive) matrix. The aim of this work Several phosphates and fluorophosphates have been was to reproduce and optimize the synthetic procedure to pro- proposed during the past years, such as Na V (PO ) (op- duce NVPF active materials, which are then formulated in 3 2 4 3 erating voltage of ~3.4V Na /Na) [7], Na FePO F(operat- electrodes with different mass loads and tested to report the 2 4 ing voltage ~3.5V vs. Li /Li) [8], NaVPO F (operating corresponding rate capabilities. A long cycling test is also voltage of ~3.7V vs. hard carbon in a SIB) [9]and reported. Moreover, as attempt to produce high load elec- −2 Na M (PO ) F (with M = Ti, Fe, V) [10, 11]. Among trodes (> 20 mg cm ), we report the result about a NVPF 3 2 4 2 3 all these materials, Na V (PO ) F shows the highest po- free-standing membrane which can be processed from aque- 3 2 4 2 3 tential against metallic sodium (~3.8V) and a relatively ous solution. −1 high theoretical capacity of 128.3 mAh g . With its tetrag- onal crystal lattice (P4 /mnm), it has channels in which Na ions can move fast, and it is to be considered a Materials and methods NASICON-like structure. However, the separation of va- nadium ions by the phosphates combined with the strong Na V (PO ) F was prepared with a slightly modified 3 2 4 2 3 ionicity of the V-F bond leave this material with poor elec- procedure as what suggested by Weixin et al. [16]. tronic conductivity. Firstly, stochiometric amounts of NH H PO ,V O and 4 2 4 2 5 The Na V (PO ) F phase (NVPF) was firstly prepared NaF were dissolved/dispersed in distilled water, stirred 3 2 4 2 3 by J. Barker in 2006 [11] using the carbothermal reduc- and dried under air flux at 50°C. Then, 5 wt% of Super tion. In further works of the same group [12, 13], the P as a carbon source was grounded with the other reagents material was tested in half lithium cells, i.e. after the first with a mortar. The resulting powders were pelletized and in situ desodiation, it was evaluated as positive material heated in argon atmosphere in a single synthesis com- for LIBs. The preliminary results in Na ion electrolyte posed by two steps: the first at 350°C for 4h and the were not so promising [13]. The first example of success- second at 650°C for 8h. The two-step heating procedure ful use in Na half cell was reported in 2012 using 1 M was introduced to form oxides and phosphate (first step) NaClO as electrolyte [14]. Despite the ball milling pro- and then promote the reaction between them (second cess with 20% of carbon and the low electrode loading step). Between the two treatments, we ground and pellet- −2 (1.85mgcm ), the material was able to provide 110 ize the precursors in order to make the contact between −1 mAh g at low current. A rate test was performed; how- them more homogenous and intimate. ever, no data were reported about cyclability. As-synthesized powders were studied with X-ray pow- Subsequently, several electrolytes were tested, both in der diffraction (XRD), performed with a Rigaku Miniflex half cell vs. Na and full cells with hard carbon [15]. 600. The analysis was made from 10 to 80 degrees (2θ) However, the electrode was fabricated with a large with a step size of 0.02 degrees, an angular velocity of 0.1 amount of carbon (27%) and low mass loading (2 mg degrees per minute and using a copper source. Scanning −2 cm ). An improved preparation route, also based on electron microscopy was performed with a Zeiss® Gemini carbothermal reduction, was presented in 2014 [16]. The SEM 450. Even though all samples (powder and elec- electrodes were fabricated with 80% of active material trodes) had some content of carbon inside, each of them −2 with higher mass loading(4.5mgcm ), and the diffusion was covered with graphite in order to reduce the charging coefficient was investigated. Cyclability tests were per- effects. Optical images were collected with a Leica® opti- formed with materials obtained by sol gel [17]and wet cal microscope equipped with a Leica DFC280 digital col- chemistry [18]. Electrodes were produced with 70% of our camera. −2 active material and a mass loading of 1.5/2.0 mg cm , Electrodes’ slurries were prepared by mixing together and they showed remarkable cyclability and rate capabil- the active material (80 wt%) with Super P 10 wt%) and ities (more than 1000 cycles at C/rate of 10C or higher) PVdF (10 wt%) as binder, using NMP as solvent. The obtained thanks to the use of carbon shells [17]ornano- resulting slurries were deposited on aluminium foils composites [18]. The only example of high load electrode (30 μm of thickness) with different thicknesses (5, 10 −2 (about 10 mg cm ) was reported in 2017, showing prom- and 30 mils), then dried under vacuum at 80°C overnight ising results but without presenting cyclability data [19]. and pressed with a calender. Self-standing high-load elec- More recent works focused on the composite preparation trodes were prepared following a previously reported (e.g. with graphene oxide [20]) without further investigat- route [21] by grounding NVPF together with Super P ing the pristine materials. conductive carbon in a mortar and mixing the obtained Ionics (2021) 27:1853–1860 1855 powder with a suspension of PTFE in water (Sigma- reduction reaction was evaluated with CHNS measurement, Aldrich, 60 wt%) to obtain a homogenous dough. The corresponding to 5.4% of the whole mass. amount of suspension was calculated in order to have The morphological features of supported and free-standing the same ratio of active material/binder as for the support- electrodes were characterized by optical and electronic mi- ed electrode, i.e. 8:1. The dough was then calendered croscopy (Fig. 3). The Al-supported electrode (Fig. 3a)shows several times reducing the thickness up to obtain a flexi- a compact surface with an overall good homogeneity. The ble film with final thickness of 180 μm. R2032 coin cells electrode’s thickness was evaluated via optical microscopy were assembled in an argon-filled glove box, using me- (insertion in Fig. 3a), showing a total average thickness of tallic sodium as counter electrode and a 1M solution of about 49±4 μm(34±4 μm excluding the aluminium foil). At NaClO in PC + FEC (2 wt%) as electrolyte, and were higher magnification (Fig. 3b), it is possible to appreciate the tested using a Bio-Logic VMP3 battery tester. binder filaments, while it is harder to distinguish between active materials and Super P particles. The self-standing elec- trode (Fig. 4a) seems porous and homogenous in composition; also in this case (Fig. 4b), the binder filaments are clearly Results and discussion observed. The electrode thickness is constant and it has been measured as 237±7 μm. X-ray diffraction technique has been used both for evaluating The quasi-thermodynamic electrochemical behaviour of the preparation products and the presence of impurities. As Na V (PO ) F was explored with PCGA (potentiodynam- 3 2 4 2 3 shown in Fig. 1, the Rietveld refinement shows that the main ic cycling with galvanostatic acceleration) technique. The phase is NVPF (97%), with a small amount of Na V (PO ) measurement was indeed performed by applying 3 2 4 3 (NVP) as impurity. The average particle dimension of crystal- potentiostatic steps of 4 mV until the current dropped be- −1 lites was also evaluated by means of the Scherrer equation, low a limit value (in our case 0.02C, 2.56 mA g ). This obtaining a value of about 45 nm, considering the two most procedure allows to extract/insert sodium inside the struc- intense (002) and (222) peaks. ture with very little overpotentials, thus operating near the SEM images of the as-synthesized NVPF powders are equilibrium, and collecting all the available charge thanks showninFig. 2 together with the histogram of the particle size to the low current threshold. Figure 4a depicts the voltage- obtained by averaging different images. The microstructure capacity curves of three cycles of sodiation/desodiation of consists nano-sized particles with dimension picked around NVPF. As expected, the reaction with sodium happens at 60–80 nm but partially coalesced in agglomerations of few three different potentials, roughly 3.4V, 3.6V and 4.0V vs. micrometres. The residual carbon left from carbothermal Na /Na. These electrochemical events are better shown in the differential capacity curves derived from PCGA (Fig. 4b). As stated by Weixin et al. [16], the two low-potential peaks are associated to the insertion/extraction of the first equivalent of sodium through a two-phase reaction, while Experimental data the highest peaks involve the second equivalent of Na in a Calculated pattern mono-phase domain. This can easily be appreciated from I -I exp Calc the shape of the peaks which appear sharp-spiked and Bragg peaks broad-bellied for the low and potential processes, respec- tively. From PCGA it was also possible to calculate the mean potential at which NVPF operates with very low overpotentials. This value was calculated in the half cell using the ratio between the energy (the integral of the V/ charge curve) and the capacity, both obtained during sodiation/desodiation, respectively. For the cathodic NVPF NVP sodiation, NVPF operates at a mean operative potential of 3.78 V. The total charge stored in the material is 120 mAh −1 g , a value close to the theoretical one. However, during the first desodiation, an irreversible capacity of about 30 −1 10 20 30 40 50 60 70 80 mAh g is observed due probably to the formation of the cathodic electrolyte interface (CEI) on both the active ma- 2θ (°) terials and the carbon additive [22, 23]. Fig. 1 XRD pattern of the obtained powders NVPF (red dots) and The kinetic behaviour of NVPF was observed by Rietveld results using the Na V (PO ) F and Na V (PO ) reference 3 2 4 2 3 3 2 4 3 materials means of galvanostatic measurements. Figure 4c shows Intensity (a.u.) 1856 Ionics (2021) 27:1853–1860 200 nm 1 µm 0 20 40 60 80 100 120 140 Particle size (nm) Fig. 2 (a) SEM images of as-synthesized NVPF taken at different magnitudes (10k and 50k, respectively) and (b) particle size distribution histogram the achieved capacity obtained during subsequent cycling currents cycling as well as stability. When returning to −1 at different currents (0.1C, 0.2C, 0.5C, 1C, 5C and 10C). 0.1C, indeed, the capacity resets back to 105 mAh g , Achievable capacity showed an inverse proportion with whichis95% of theinitial value, andthendecreased less −1 −1 respect to the C-rate, losing about 20% (88 mAh g with than 3 mAh g during thenext50cyclesatlow currents. −1 respect to the starting value of 110 mAh g )after in- Mean potentials were also evaluated for each cycle (Fig. creasingthe current from0.1Cto1C, butwithanincrease 4d). NVPF showed relatively low polarization at low C- in the coulombic efficiency (> 99%), as an indication of rates, with an increase of the mean potential of about 60– the presence of anodic parasitic reactions. Moreover, 70 mV between scanning at 0.1C and 1C, while higher NVPF showed a very good capacity recovery after high overpotentials (200/400 mV) can be seen for currents of Fig. 3 Images of the two electrodes’ configuration Al-supported and self-standing. (a, b) SEM images of the Al-supported electrode. (c, d) SEM images of the self-standing one. The two inserted images are optical microscope’s images of the cross section of the two electrodes Frequency Ionics (2021) 27:1853–1860 1857 Fig. 4 (a) Voltage-gravimetric 3.61V 4.2 charge curves of PCGA ab measurement. (b)Differential 4.03V st 1 cycle capacities curves derived from nd 2 cycle rd 3.9 3 cycle PCGA. (c) Specific capacity vs. 3.39V st 1 cycle nd cycle number for NVPF at 2 cycle rd 3 cycle various C-rates. (d) Sodiation and 3.36V desodiation mean voltages vs. 3.6 -2 cycle number 4.02V -4 3.3 3.60V -6 3.2 3.4 3.6 3.8 4.0 4.2 0 20 40 60 80 100 120 140 160 -1 Voltage vs Na /Na (V) Gravimetric Charge (mAh g ) 180 4.2 4.1 4.0 3.9 0.2C 0.1C 3.8 0.1C 0.5C 1C 5C 10C 0.1C 0.1C 0.2C 92 3.7 80 0.5C 1C 3.6 5C 3.5 Charge Charge 40 Efficiency cd Discharge Discharge 10C 86 3.4 0 10 2030 405060 7080 90 100 110 0 1020 30405060 7080 Cycle Number Cycle Number 5C and 10C. The stable trend of the mean potential 600 cycles in total and almost zero loss of specific capacity (3.78V) during recovery cycles means that NVPF does at 2C) and high coulombic efficiencies of more than 99% not undergo irreversible changes in structure during mul- at 1C and 2C. tiple sodiations/desodiations. This can be considered an- Electrodes with different mass loading of active mate- other good indication of NVPF’s stability and cyclability. rials were also tested. Figure 6a displays the discharge To better evaluate the stability of NVPF during cycles, a capacity of three electrodes with different mass loadings, −2 long-term measurement was performed: starting from 0.1C 1.5, 3.5 and 6.4 mg cm , respectively, using a common then increasing current to 1C and, in the end, 2C. As cycling procedure (10 cycles at C-rates of 0.1, 0.2, 0.5, 1, displayed in Fig. 5, this material showed outstanding sta- 5, 10 and again 0.1). At low currents, where kinetical bility performances, with very high cyclability (more than effects tend to be negligible and thus materials operate in conditions that are not too far from the thermodynamic limit, the three electrodes showed no significant differ- 180 105 ences. This may suggest that NVPF could be very well dispersed inside the carbonaceous matrix (from electrode formulation but also from carbothermal reduction), in- creasing electronic conductivity and thus reducing 0.1C overpotentials. Still, deviations of capacity retention are present at high currents, where kinetic effects tend to re- duce specific capacity values in electrodes with higher 1C thickness and larger mass loading. Same trends can be observed for both coulombic efficiencies and mean oper- ative voltages. Charge 2C Efficiency 75 Finally, the self-standing, very high mass-loading elec- 40 Discharge −2 trodes (25mgcm ) were also tested and compared to the 20 70 aluminium-supported ones. Despite working at a much 0 100 200 300 400 500 600 700 higher current (almost four times), self-standing electrodes Cycle Number showed identical performances in terms of specific capac- Fig. 5 Specific capacity vs. cycle number for a long-term cycling of ity and cyclability at low C-rates (0.1 and 0.2C, Fig. 7) NVPF -1 Specific Capacity (mAh g ) -1 + Specific Capacity (mAh g ) Voltage vs Na /Na (V) Coulombic Efficiency (%) CoulombicEfficiency(%) -1 -1 V vs Na /Na (V) MEAN dQ/dV (Ah g V ) 1858 Ionics (2021) 27:1853–1860 Fig. 6 Comparison among electrodes with different loadings of active mass during cycling. (a) Cathodic specific capacity. (b) Mean discharge voltage. (c) Coulombic efficiency 20 -2 6.4mgcm 4.0 -2 3.5 mg cm 3.8 0.1C 0.1C 0.2C 0.5C 5C 1C 60 3.6 10C 3.4 -2 1.5 mg cm -2 0 10 203040 506070 1.58 mg cm 3.2 -2 3.52 mg cm Cycle Number -2 6.44 mg cm 3.0 0 1020 3040506070 Cycle Number while failing at higher currents. This can be due to ineffi- Conclusions ciencies in the working electrode itself but also to issues in the sodium counters used in the half cells that tend to form Na V (PO ) F was prepared by means of a carbothermal 3 2 4 2 3 thick SEI layers as well as dendrites at high currents during reduction synthesis with high crystallinity. A small to the long operation of the cell. Still, self-standing elec- amount of NVP impurity was detected (3%). The material trodes can be considered good candidates for full cell NIBs showed good performances as a positive electrode for because of the capacity (neither affected by the high mass sodium-ion batteries able to operate at a mean voltage of nor by the absence of the current collector), the coulombic almost 3.8 V under quasi-thermodynamic conditions. Low efficiency (> 98–99% even at low C-rates) and the average load electrodes also showed outstanding cyclability prop- voltage (3.8V vs. Na /Na). The obtained areal capacities of erties with coulombic efficiency always above 98–99%. −2 theself-standing andthe 6.4gcm electrodes are 2.45 and The stable trend of the mean voltage, with relatively low −2 0.65 mAh cm , respectively. overvoltage at current up to 1C, makes this material Fig. 7 Comparison between Al- 4.2 4.2 supported (active mass loading: ab −2 3.9 6.4mgcm ) and self-standing (active mass loading: 25 mg 3.6 4.0 −2 cm ) electrodes. (a) Surface 3.3 charge density of the 5th cycle for Charge 0 20 406080 100 Discharge -1 both systems with the gravimetric 3.8 Gravimetric Charge (mAh g ) Efficiency charge as insert and (b) specific 96 capacity for the self-standing 3.6 electrode at two different C-rates 0.2C (0.1 and 0.2C) 0.1C Al-supported 3.4 Self-standing 3.2 90 0.0 0.5 1.0 1.5 2.0 2.5 05 10 15 20 -2 Surface Charge Density (mAh cm ) Cycle Number + + V vs Na /Na (V) Voltage vs Na /Na (V) MEAN Voltage vs Na /Na (V) -1 Specific Capacity (mAh g ) Coulombic Efficiency (%) Coulombic Efficiency (%) Ionics (2021) 27:1853–1860 1859 7. Jian Z, Zhao L, Pan H, Hu YS, Li H, Chen W, Chen L (2012) suitable for low-to-mid power delivery systems, and it is Carbon coated Na3V2(PO4)3 as novel electrode material for sodi- also a factor that affects the energy efficiency, which um ion batteries. Electrochem Commun 14:86–89. https://doi.org/ maintains higher than 94%. Moreover, NVPF synthesized 10.1016/j.elecom.2011.11.009 by carbothermal reduction appeared to be conductive 8. Ellis BL, Makahnouk WRM, Makimura Y, Toghill K, Nazar LF (2007) A multifunctional 3.5V iron-based phosphate cathode for rechargeable enough in order to allow low-to-no kinetical effects when batteries. Nat Mater 6:749–753. https://doi.org/10.1038/nmat2007 increasing mass loadings on electrodes. This is particular- 9. Navpo F, Barker J, Saidi MY, Swoyer JL (2003) A sodium-ion cell ly true considering that even self-standing electrodes with based on the fluorophosphate compound NaVPO4F. Electrochem high thicknesses (180 μm) andhighmassloadings(25 mg Solid-State Lett 6:11–14. https://doi.org/10.1149/1.1523691 −2 cm ) show very good performances in terms of coulom- 10. Chihara K, Kitajou A, Gocheva ID, Okada S, Yamaki JI (2013) Cathode properties of Na3M2(PO4)2F3[M = Ti, Fe, V] for sodium- bic efficiency and achieved capacity. ion batteries. J Power Sources 227:80–85. https://doi.org/10.1016/j. jpowsour.2012.10.034 11. Barker J, Saidi MY, Swoyer JL, (2002) US Patent 6, 387,568 Funding Open access funding provided by Università degli Studi di 12. Gover RKB, Bryan A, Burns P, Barker J (2006) The electrochem- Milano - Bicocca within the CRUI-CARE Agreement. This work has ical insertion properties of sodium vanadium fluorophosphate, been funded by the Italian Ministry of University and Research (MIUR) Na3V2(PO4)2F3. Solid State Ionics 177:1495–1500. https://doi. through grants “Dipartimenti di Eccellenza 2017 – Materials for energy” org/10.1016/j.ssi.2006.07.028 and PRIN 2017 “Towards sustainable, high-performing, all-solid-state 13. Barker J, Gover RKB, Burns P, Bryan AJ (2007) sodium-ion batteries”. Li4 3Ti5 3O4∥Na3V2(PO4)2F3: an example of a hybrid-ion cell using a non-graphitic anode. J Electrochem Soc 154:A882–A887. Declarations https://doi.org/10.1149/1.2756975 14. 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The images or other third party material in this article are included 10.1039/c3ee41379a in the article's Creative Commons licence, unless indicated otherwise in a 16. SongW, JiX, Wu Z, YangY,ZhouZ,LiF,ChenQ, Banks CE credit line to the material. If material is not included in the article's (2014) Exploration of ion migration mechanism and diffusion ca- Creative Commons licence and your intended use is not permitted by pability for Na3V2(PO4)2F3 cathode utilized in rechargeable statutory regulation or exceeds the permitted use, you will need to obtain sodium-ion batteries. J Power Sources 256:258–263. https://doi. permission directly from the copyright holder. To view a copy of this org/10.1016/j.jpowsour.2014.01.025 licence, visit http://creativecommons.org/licenses/by/4.0/. 17. 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