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Improving the cycling stability of Na3V2(PO4)3 nanoparticle in aqueous sodium ion batteries by introducing carbon support

Improving the cycling stability of Na3V2(PO4)3 nanoparticle in aqueous sodium ion batteries by... Mater Renew Sustain Energy (2016) 5:3 DOI 10.1007/s40243-016-0067-z ORIGINAL PAPER Improving the cycling stability of Na V (PO ) nanoparticle 3 2 4 3 in aqueous sodium ion batteries by introducing carbon support 1 1,2 1,3 • • Huajun Zhou Z. Ryan Tian Simon S. Ang Received: 29 July 2015 / Accepted: 21 January 2016 / Published online: 8 February 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Here we report the electrochemical perfor- safety and cost and find limits in large-scale applications. To mances of a Na V (PO ) /C nanocomposite as a cathode address these issues and meet the vast need for energy storage, 3 2 4 3 material for aqueous sodium-ion batteries (SIBs). Com- recently aqueous sodium-ion batteries (SIBs) with their pared to a previously reported Na V (PO ) microparticle, capacities comparable to those of LIBs have received con- 3 2 4 3 this nanocomposite demonstrated much improved cycling siderable interests [1–5]. On one hand, aqueous systems are stability. While the improvement mainly attributed to the safer as they could be neither flammable nor toxic; on the right pH and the carbon matrixmediated protection against other hand, sodium is much less expensive than lithium due to the electrolyte, the capacity fade was mainly due to the its much more abundant natural reserve. Furthermore, the deterioration of crystallinity and structure of the high ionic conductivity [2] of aqueous electrolytes can sup- nanocomposite caused by various interactions between the port fast charge–discharge processes. nanocomposite and electrolyte. This work not only help to Among electrode materials candidates for aqueous SIBs, understand the degradation of Na V (PO ) in aqueous sodium vanadium phosphates with sodium (Na) super ion 3 2 4 3 SIBs, but also shed light on the design and fabrication of conductor (NASICON) structures such as NaVPO F[6], electrode materials with high cycling stability for aqueous Na V O(PO ) F[7], and Na VTi(PO ) [8] have been 3 2 4 2 3 4 3 SIBs. widely investigated as cathodes for the following reasons. First, this structure features a highly covalent three-di- Keywords Sodium ion batteries  Na V (PO )  Cathode  mensional (3D) framework in which Na-ions can facilely 3 2 4 3 Aqueous batteries  Carbon coating diffuse in and out [9]; second, the strong covalent bonds can provide structural stability and safety. Though exten- sive studies have been investigated on the use of Na V (- 3 2 Introduction PO ) as electrode materials in non-aqueous SIBs [10–15], 4 3 the use of Na V (PO ) as electrode materials in aqueous 3 2 4 3 Currently widely used lithium-ion batteries (LIBs) employ SIBs was first studied by Ji and Banks only very recently flammable and toxic organic solvents and expensive lithium- [16]. In that work, Na V (PO ) microparticle was tested as 3 2 4 3 containing compounds, and thus present two major issues of the cathode material, and its capacities were found to decay in a rather significant fashion (ca. 31 % capacity retention for the 30th cycle). The mechanism responsible for the & Huajun Zhou capacity fade was not investigated either. hxz001@uark.edu Here we report the use of Na V (PO ) /C nanocom- 3 2 4 3 posite composed of Na V (PO ) nanoparticles and a car- 3 2 4 3 High Density Electronics Center, University of Arkansas, Fayetteville, AR 72701, USA bon matrix as the cathode material for aqueous SIBs. The Na V (PO ) nanoparticle provides shorter Na-ion diffu- 3 2 4 3 Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA sion lengths than its micro-sized counterpart does while the carbon support could enhance the electrical conductivity of Department of Electrical Engineering, University of the electrode [10] and slow down its dissolution [1]. Arkansas, Fayetteville, AR 72701, USA 123 3 Page 2 of 9 Mater Renew Sustain Energy (2016) 5:3 Compared to the previously reported Na V (PO ) nanocomposite in an aqueous system. A large piece of pure 3 2 4 3 microparticle, this nanocomposite indeed demonstrates platinum, a Ag/AgCl/1 M KCl electrode (0.235 V vs. much improved cycling stability. The mechanism respon- NHE), and 1 M Na SO aqueous solution served as the 2 4 sible for the capacity fade was also investigated here. counter electrode, reference electrode, and electrolyte, respectively. The working electrode was prepared by mixing the nanocomposite, super P carbon black, and Experimental methods polytetrafluoroethylene (PTFE) in a weight ratio of 75:20:5 onto a stainless steel foil of *0.3 cm area. The electrodes Syntheses of Na V (PO ) /C nanocomposite were dried in a vacuum oven at 80 C overnight, and then 3 2 4 3 pressed at a pressure of 20 MPa using a PHI manual The nanocomposite was prepared by modifying a published compression presses. The loading of each electrode was -2 procedure [10]. To a round-bottom flask containing 30 mL controlled to be 1–2 mg cm , and the thickness was tetraethylene glycol (TEG) was added 0.246 g/3 mmol *50 lm. Before the cell assembly, the electrolyte was sodium acetate (NaCH COO), 0.697 g/2 mmol vanadium bubbled with Ar for 60 min while the platinum foil was (III) acetylacetonate (VO(C H O ) ), and 0.345 g/3 mmol first rinsed in acetone and then repeatedly ultrasonicated in 5 7 2 2 ammonium dihydrogen phosphate (NH H PO ). The mix- deionized water. Cells were assembled in an Ar atmo- 4 2 4 ture was then stirred overnight at room temperature to sphere. Cyclic voltammetry (CV) was performed on a afford a homogenous green solution, and the resultant Solartron SI 1287 Electrochemical Interface at a scan rate -1 solution was then heated at 320 C for 72 h. The precipi- of 0.5 mV s from 0 to 0.8 V (vs. the reference electrode) tate was collected by centrifugation, washed by ethanol and while galvanostatic tests were performed on a BT 2000 acetone each for 3 times respectively, and dried in a vac- battery tester. The electrochemical performances of this uum oven at 80 C for 2 h. Then the resultant light-brown nanocomposite in a non-aqueous system were also tested as powders were annealed at 650 C under Ar/H (40/2 sccm) a reference. The electrode fabrication was the same as for 6 h and then at 800 C under Ar flow (40 sccm) for 6 h above except that here Al foil was used and electrodes were to afford black powders. *8 mm in diameter. The loading and thickness of each -2 electrode was 1–2 mg cm and *50 lm, respectively. Materials characterizations 2032-type coin cells were assembled in an MBraun glove box (O \ 0.1 ppm, H O \ 2 ppm) while a Celgard 3501 2 2 The phase purity was characterized by a Rigaku MiniFlex II microporous membrane was used as the separator. CV was Desktop X-ray diffractometer using monochromatized Cu- performed on a Solartron SI 1287 electrochemical interface Ka radiation (k = 1.5418 A) at 30 kV and 15 mA. A at a scan rate of 0.05 mV/s while galvanostatic tests were continuous scan mode was used to collect the diffraction performed on a BT 2000 battery tester. 1 M NaClO in EC/ data from 10 to 60 at a speed of 0.2/min. The mor- DMC (w/w = 1:2) and Na foil were used as the electrolyte phologies were investigated by an FEI Nova NanoSEM and counter electrode. scanning electron microscope (SEM) equipped with a field emission gun operated at 10 kV and by an FEI Titan 80–300 transmission electron microscope (TEM). To minimize Results and discussion charging problems, samples for SEM were coated with thin Au layers. Raman spectra were recorded on a homemade In the crystal structure of Na V (PO ) , each VO octahe- 3 2 4 3 6 lRaman spectroscope that was composed of a 632.8 nm dron is connected to three PO4 tetrahedrons via corners to 3- He–Ne excitation laser, an iHR 550 HORIBA spectrometer, afford [V (PO ) ] ribbons along the c axis, and these 2 4 3 and a Si CCD detector. Prior to Raman the measurement, ribbons are connected to each other through PO tetrahe- the system was calibrated using a bulk Si. Elemental anal- drons along the a axis to afford an open 3D framework ysis was done in Atlantic Microlab Inc. to determine the where two kinds of Na ions reside (Fig. 1a). Na1 atoms 3- weight percentage (*20wt %) of carbon species in the reside between two adjacent [V (PO ) ] units within the 2 4 3 Na V (PO ) /C nanocomposite. ICP-MS analysis revealed same ribbons while Na2 atoms reside between adjacent 3 2 4 3 the mole ratio of Na:V:P to be 3.00:2.00:3.00, further sug- ribbons. The channels along the a axis (Fig. 1a) and the gesting the pure phase of Na V (PO ) /C. b axis are the same in dimension, and are favored routes for 3 2 4 3 Na-ion diffusion while the channel along the a axis is more Electrochemical measurements favored [17]. The PLATON program [18] suggests that neither the pristine Na V (PO ) nor the anionic framework 3 2 4 3 3- A three-electrode beaker-type cell was used to test the [V (PO ) ] can support physical adsorption of water. As 2 4 3 electrochemical performances of Na V (PO ) /C confirmed by the space-filling model of the anionic 3 2 4 3 123 Mater Renew Sustain Energy (2016) 5:3 Page 3 of 9 3 Fig. 1 a Ball stick model of a unit cell structure of Na V (PO ) viewed along the 3 2 4 3 a axis; orange lines represent cell edges; b space-filling model of the anionic framework 3- [V (PO ) ] in which Na-ions 2 4 3 reside, indicating the channels are too small for physical adsorption of water molecules; the scale bar represents the diameter of a water molecule framework (Fig. 1b, the radii of all atoms were assigned electronic conductivity. The lower intensity of G-band according to Shannon’s work [19]), the cross section of the could be due to the interaction between the Na V (PO ) 3 2 4 3 channel along the a axis in the anionic framework is nanoparticles and carbon [1]. During the synthesis, tetra- *2.60 9 0.12 A in dimension, making physical adsorption ethylene glycol (TEG) molecules played a dual role: First, water (*2.76 A in diameter) through the channels impos- they provided a reducing environment [21] to stabilize sible. During the repeated charge–discharge cycles, Na V(III) in the presence of oxygen; Second, they functioned 3- V (PO ) is the Na-inserted phase while NaV (PO ) is the as a capping agent [10] on the as-synthesized Na V (PO ) 2 4 3 2 4 3 3 2 4 3 Na-extracted phase [12]. Compared to Na V (PO ) , nanoparticles and were then be converted to carbon species 3 2 4 3 NaV (PO ) maintains the same structure but decreased upon annealing. 2 4 3 lattice sizes (*8.3 % volume decrease) [12]. Therefore The electrochemical performances of this nanocom- water adsorption cannot interfere with the Na-ion extrac- posite were first investigated in non-aqueous electrolyte tion-insertion during cycling. Na V (PO ) is different from (Fig. 3) as a reference. Figure 3a shows the cyclic 3 2 4 3 some frameworks such as spinel-type lithium manganese voltammograms (CVs) in the voltage range of 2.3–3.9 V oxide that can support simultaneous insertion-extraction of vs. Na /Na at the scan rate of 0.05 mV/s. During the first water and metal ions during cycling [20]. scan, a pair of sharp peaks is observed: the oxidation (Na- The powder X-ray diffraction (PXRD) pattern of the ion extraction) and reduction peak (Na-ion insertion) are at Na V (PO ) /C nanocomposite obtained after annealing at 3.48 and 3.28 V. The average voltage (3.38 V) is close to 3 2 4 3 4? 3? 800 C confirmed the NASICON structure with the R-3c the equilibrium voltage of V /V redox couple of this space group (Fig. 2a) [9]. The SEM image in Fig. 2b compound [11]. In subsequent up to ten scans, the peak indicates that Na V (PO ) nanoparticles were aggregated positions also remain unchanged while the currents just 3 2 4 3 with their grain sizes being in the range of 60–120 nm. decreased slightly, indicating the good reversibility for Na- TEM analysis confirmed the presence of the carbon support ion extraction and insertion and the stability of the struc- (Fig. 2c). Raman spectrum in Fig. 2d clearly shows two ture. Figure 3b shows the charge–discharge profiles at a distinct peaks, the D-band associated with sp3-hybridized current of 1 C in the range of 2.3–3.9 V (note that 1 C -1 carbon at *1338 cm and the G-band associated with means the full capacity can be charged or discharged in 1 h -1 sp2-hybridized carbon at *1589 cm . The intensity ratio and here 1 C & 110 mA/g since the theoretical discharge of the D- and G-bands (ID/IG) is 1.10, indicating there are capacity of Na V (PO ) is c.a. 110 mAh/g). An evident 3 2 4 3 large amount of sp -carbon capable of providing high voltage plateau around 3.4 V was observed for both charge 123 3 Page 4 of 9 Mater Renew Sustain Energy (2016) 5:3 Fig. 2 Structural characterizations of the Na V (PO ) /C nanocomposite. 3 2 4 3 a Powder XRD pattern confirming the composition of Na V (PO ) ; b SEM image 3 2 4 3 indicating the sizes; c TEM image indicating Na V (PO ) 3 2 4 3 nanoparticles are embedded into a carbon matrix; d Raman spectrum indicating the characteristic of carbon species Fig. 3 Electrochemical performances of this Na V (PO ) /C nanocomposite 3 2 4 3 using 1 M NaClO in EC/DMC (w/w = 1:2) as the non-aqueous electrolyte. a Cyclic voltammograms at the scan rate of 0.05 mV/s up to ten cycles. b Galvanostatic charge– discharge cycling profiles at 1 C. c Initial charge–discharge cycling profiles at 5 C, 10 C, and 20 C. d Discharge capacity evolution for 40 cycles at 1 C currents and discharge, and the average polarization (i.e. the voltage the initial charge capacity of 108.0 mAh/g and discharge difference between charge and discharge plateaus) is about capacity of 91.3 mAh/g (Note that the mass was based on 0.25 V. The Na V (PO ) in this nanocomposite delivers that of Na V (PO ) while the carbon species constituted 3 2 4 3 3 2 4 3 123 Mater Renew Sustain Energy (2016) 5:3 Page 5 of 9 3 *wt 20 % of the nanocomposite). In comparison, different carbon species have been proven to significantly affect the initial discharge capacities of Na V (PO ) /C composites 3 2 4 3 which range from *30 mAh/g [11], to *70 mAh/g [22], and to 104 mAh/g [10]. The discharge capacity here is 91.5 mAh/g after 10 cycles (Fig. 3b), further suggesting the good and reversible cycling performances in non-aqueous systems. As the currents increase, the polarization increa- ses and the capacities decrease (Fig. 3c). While the initial discharge capacity of this nanocomposite at 5 C is 57.4 mAh/g, the discharge capacities at 10 C and 20 C are 25.0 and 9.9 mAh/g and much smaller than that at 1 C. At 1C, the discharge capacities remain quite stable after forty cycles (Fig. 3d). pH has been proved to significantly affect the cycling performances of many electrode materials in aqueous LIBs and SIBs [23, 24]. Figure 4 shows CV curves of the nanocomposite in 1.0 M aqueous Na SO solution at dif- 2 4 ferent pH in an oxygen-free environment in which the electrode hydrolysis was inhibited [23]. For the first scan at the initial pH = 4.0 (Fig. 4a), there is a pair of well-defined redox peaks: *0.465 V for the oxidation peak and *0.373 V for the reduction peak. From the second scan to the fifth scan, while both the oxidation currents at *0.465 V and reduction currents at *0.373 V decreased, the positions and shapes of these redox peaks remained the same. Meanwhile, shoulder peaks close to *0.373 V appeared in the reduction processes and the currents of those shoulder peaks only decreased very slightly possibly due to very slight dissolution of Na V (PO ) nanoparticles. 3 2 4 3 Currently, the mechanisms of the Na-ion extraction from Na V (PO ) during charge processes in non-aqueous 3 2 4 3 systems are still being debated. Gu, Xi, and Hu suggested Na1 ions at the M1 (6b) sites tend to remain immobilized, only Na2 ions at the M2 (18e) sites could be extracted, and the Na-ion extraction process involve a two-phase trans- formation from Na V (PO ) to NaV (PO ) [12]. In con- 3 2 4 3 2 4 3 trast, Ji and Banks suggested that Na-ions at both sites could be extracted and the transformation from Na V (- 3 2 PO ) to NaV (PO ) involve Na V (PO ) as an inter- 4 3 2 4 3 2 2 4 3 mediate [25]. From the second scan to the fifth scan in repeated CV scanning (Fig. 4a), should peaks near *0.373 V showed up and the oxidation peaks at *0.465 V are apparently broader than the reduction peaks at *0.373 V (Fig. 4a). All these observations suggest Na V (PO ) could serve as an intermediate in the two-step 2 2 4 3 Fig. 4 Cyclic voltammograms (CVs) for Na V (PO ) /C in 1 M 3 2 4 3 aqueous Na2SO4 solution up to ten cycles at the scan rate of 0.5 mV/s Na-ion insertion process but not the one-step Na-ion at the initial pH = 4.0 (a), 7.0 (b), and 9.0 (c), respectively extraction process. Such difference indicates mechanisms or kinetics of Na-ion insertion and extractions differ, and similar phenomena in the metal-ion insertion and extrac- tion in aqueous systems were observed before [26]. From reduction peaks at *0.373 V shifted positively slightly the fifth scan to the tenth scan, the oxidation peaks shifted (*0.390 V for the tenth scan), suggesting possible struc- negatively slightly (*0.457 V for the tenth scan) while the tural reorganization. 123 3 Page 6 of 9 Mater Renew Sustain Energy (2016) 5:3 After the fifth scan of CVs at pH = 4.0, an additional evolution. Figure 5a shows the charge–discharge profiles at pair of weaker redox peaks (*0.270 and *0.130 V) a current of 1 C in the range of 0–0.8 V vs. Ag/AgCl. Upon started to intensify (top left insets in Fig. 4a). These redox assembly, the cell was charged to 0.8 V first and interest- peaks are more prominent than those at pH = 7.0 and 9.0 ingly two close plateaus (*0.390 and *0.430 V with the (top left inset in Fig. 4b, c), suggesting such redox reac- average being *0.410 V) were observed for the first time ? ? tions could be related to the H intercalation. Such H possibly due to Na V (PO ) being as the intermediate 2 2 4 3 intercalation in aqueous LIBs was observed to be irre- during the charge (sodium extraction) process [25] or some versible [27]. Since Na V (PO ) has large interstitial kind(s) of structural reorganization. During the first dis- 3 2 4 3 spaces, Na V (PO ) may support reversible H insertion- charge process, a major voltage plateau at *0.405 V 3 2 4 3 extraction. Such redox peaks were not reported before in showed up. The low polarization (*0.005 V) is much both aqueous [10] and non-aqueous SIBs [7] using Na smaller than 0.25 V observed in the non-aqueous elec- 3- V (PO ) as cathode materials. The reason could be due to trolyte and indicates excellent ionic and electronic con- 2 4 3 the later-described deterioration of crystallinity and struc- ductivities during this cycle. As the cycling continued, the ture that could favor the subsequent H insertion-extrac- charge–discharge profiles from the second to the tenth tion. The corresponding redox currents being more than cycle demonstrated the following trends: (1) the polariza- one order of magnitude lower than the major redox currents tion for the redox peaks at *0.410 V kept increasing suggest low extent of H insertion-extraction, which was (0.029, 0.034, and 0.054 V for the second, fifth, and tenth confirmed by ICP analysis (the mole ratios of Na: P for cycles), suggesting continuously decreased ionic and electrodes before cycling and after five scans were found to electronic conductivities which could come from the be 1.00 and 0.98, respectively). deterioration of the Na V (PO ) nanoparticle’s crys- 3 2 4 3 From the fifth scan to the tenth scan at pH = 4.0, the tallinity and structure and/or the breakage of the carbon redox currents decreased in a much slower fashion than matrix; (2) an unobvious plateau showed up at *0.200 V those at pH = 7.0 and 9.0 (Fig. 4a–c). The reason could during discharge processes, and could attribute to the be due to higher solubility of this nanocomposite’s Na H -insertion processes; (3) the charge–discharge plateaus 3- V (PO ) at higher pH, which was previously observed in at *0.410 V became narrower while the discharge pla- 2 4 3 the case of vanadium oxide [24]. teaus at *0.200 V became wider. Accordingly, the It is worthy to note that during cycling the pH of elec- capacity fade around the voltage plateaus of *0.410 V trolyte changed only slightly, especially for the cases of was somehow compensated by the capacity increase pH = 4.0 and 9.0. Equations (1) and (2) show the elec- around those of *0.200 V. In the tenth cycle, the voltage trochemical processes during charge in both the working plateau around 0.410 V during the charge process was not electrode and counter electrode: strictly flat. All these trends match those observed during repeated CV scanning. As the currents increase, both the Na VðÞ PO ! NaVðÞ PO + 2Na +2e ð1Þ polarization increases and the capacity decreases (Fig. 5b) 3 2 4 2 4 3 3 were less significant than those in the non-aqueous system 2H O + 2Na +2e ! 2NaOH + H ð2Þ 2 2 due to faster kinetics in the aqueous system. The initial Since the total mass of each electrode is only about discharge capacities of this nanocomposite at 5 C, 10 C, 0.3 mg while the electrolyte is about 10 mL, the and 20 C are 33.7, 18.3, and 11.6 mAh/g respectively. It is concentration of generated OH during charge is about worthy to note the discharge capacity at 20 C in the -5 7.9 9 10 M. During the subsequent discharge, these aqueous system is even slightly higher than that in the non- OH are consumed. Therefore, the pH of the electrolyte aqueous system (9.9 mAh/g). with the initial pH being 4.0 is expected to range from 4.0 The charge and discharge capacities decrease relatively to 4.7 during cycling. The electrolytes with the initial pH fast from the first cycle to the fifth cycle and then decrease being 7.0 and 9.0 are expected to be in the range of 7.0–9.9 much slower from the fifth cycle to the tenth cycle. Such and 9.0–9.95 respectively during cycling. Our experimental trends match those of the redox currents at *0.465 and pH values matched these expected values. More in-depth *0.373 V during repeated CV scanning. To understand investigations of the Na V (PO ) -water interactions, its such trends, ex situ PXRD analysis was used to analyze the 3 2 4 3 pH-dependent solubility, and optimization of electrolytes pristine electrode and the electrode after five charge–dis- are beyond the scope of the paper and will be published charge cycles at pH = 4.0 (Fig. 5c). All the major peaks separately. remained, indicating the main framework remained after On the basis of the above observations, pH = 4.0 was five cycles. However, the PXRD pattern of the electrode found to be an optimal condition for evaluating the cycling after cycling is indeed different from that of the pristine performances. pH lower than 4.0 was not employed since it electrode in two aspects: First, the two pairs of split peaks could induce more H intercalation and/or even hydrogen in the PXRD pattern of the pristine electrode, namely 123 Mater Renew Sustain Energy (2016) 5:3 Page 7 of 9 3 Fig. 5 a Galvanostatic charge– discharge cycling profiles of Na V (PO ) /C nanocomposite 3 2 4 3 in a 1 M Na SO electrolyte at 2 4 pH = 4.0 at 1 C. b Initial charge–discharge cycling profiles at pH = 4.0 at 5 C, 10 C, and 20 C. c PXRD patterns of electrodes before and after five cycles (unfilled square and filled square represent peaks from the stainless steel foil and super P carbon black, respectively); d Discharge capacity evolution for 40 cycles at 1 C currents (104)/(110) and (211)/(116), were found to merge into two previously reported Na V (PO ) microparticle discharged 3 2 4 3 broad peaks, indicating the crystallinity of Na V (PO ) at 8.5 C in the first cycle (209 F/g, i.e. *46.4 mAh/g), but 3 2 4 3 nanoparticles deteriorated during the cycling. Second, the the capacity retention rate of this nanocomposite (Fig. 5d) intensity of the starting third-strong (300) peak relative to is much higher than that of the microparticle (50 vs. 31 % the strongest (116) peak was found to significantly decrease after thirty cycles) [16] though less than that observed in after the cycling. In contrast, such Na V (PO ) nanopar- the non-aqueous system (Fig. 3c). It is worthy to note that 3 2 4 3 ticles could maintain good crystallinity even after being the capacity fade is faster at current rates lower than 1C, cycled at 100 C for 1000 cycles in non-aqueous electrolytes possibly due to the slow dissolution of the electrode [10]. Therefore, repeated Na-ion diffusion along the a axis materials and/or the interactions between electrode mate- should not be responsible for the deterioration of both rials and electrolytes during cycling [31]. crystallinity and structure of Na V (PO ) nanoparticles. 3 2 4 3 Possible reasons could be the interactions between water molecules and Na V (PO ) nanoparticle including its Conclusions 3 2 4 3 crystallographic planes since both phosphates [28] and vanadium oxides [29] turned out to have interesting inter- In summary, compared to Na V (PO ) microparticle [16], 3 2 4 3 facial chemistry in aqueous systems. this Na V (PO ) /C nanocomposite demonstrated compa- 3 2 4 3 Based on all these observations, we propose that the rable discharge capacity during the first cycle in aqueous capacity fade during cycling should be mainly due to the SIBs but much higher cycling stability in subsequent deterioration of crystallinity and structure of the cycling. The carbon support capable of helping protect the nanocomposite during the initial several charge–discharge electrode materials from aqueous electrolyte along with the cycles, some H insertion-extraction, and slow dissolution optimal pH should be responsible for the significant of Na V (PO ) nanoparticles. improvement. The capacity fade should be mainly due to 3 2 4 3 The discharge capacity of our Na V (PO ) /C (1) the deterioration of both crystallinity and structure of 3 2 4 3 nanocomposite at 1 C rate in the first cycle in aqueous SIBs the Na V (PO ) nanoparticle and (2) the possible break- 3 2 4 3 is 44.7 mAh/g. Considering the discharge capacities of age of the carbon matrix. The former should be due to the both aqueous LIBs and SIBs are generally lower than those interactions between Na V (PO ) nanoparticle and elec- 3 2 4 3 of their non-aqueous counterparts [23, 30], the capacity of trolytes and could lead to the decreased ionic and elec- this nanocomposite is quite decent. It is also close to that of tronic conductivities while the latter could lead to 123 3 Page 8 of 9 Mater Renew Sustain Energy (2016) 5:3 based on Na CuFe(CN) -NaTi (PO ) intercalation chemistry. decreased electronic conductivities. H intercalation and 2 6 2 4 3 ChemSusChem. 7(2), 407–411 (2014) slow dissolution of electrode materials are also partially 6. Qin, H., Song, Z.P., Zhan, H., Zhou, Y.H.: Aqueous rechargeable responsible for the capacity fade. alkali-ion batteries with polyimide anode. J. Power Sources 249, Therefore, to further optimize the cycling stability of 367–372 (2014) 7. Kumar, P.R., Jung, Y.H., Lima, C.H., Kim, D.K.: Na V O (- Na V (PO ) micro/nanoparticles in aqueous SIBs, the 3 2 2x 3 2 4 3 PO ) F : a stable and high-voltage cathode material for 4 2 3-2x interactions between Na V (PO ) particles and water need 3 2 4 3 aqueous sodium-ion batteries with high energy density. J. Mater. to be minimized via either effective coating or wrapping of Chem. 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Jian, Z.L., Zhao, L., Pan, H.L., Hu, Y.S., Li, H., Chen, W., Chen, amount [1] of carbon species were proved to significantly L.Q.: Carbon coated Na V (PO ) as novel electrode material for 3 2 4 3 sodium ion batteries. Electrochem. Commun. 14, 86–89 (2012) affect the cycling performances of NASCICON nanopar- 12. Jian, Z.L., Yuan, C.C., Han, W.Z., Lu, X., Gu, L., Xi, X.K., Hu, ticles in both non-aqueous and aqueous SIBs. Optimization Y.S., Li, H., Chen, W., Chen, D.F., Ikuhara, Y., Chen, L.Q.: of such carbon coating/wrapping on Na V (PO ) 3 2 4 3 Atomic structure and kinetics of NASICON Na V (PO ) cath- x 2 4 3 nanoparticles for aqueous SIBs is ongoing. ode for sodium-ion batteries. Adv. Funct. Mater. 24, 4265–4272 (2014) 13. Plashnitsa, L.S., Kobayashi, E., Noguchi, Y., Okada, S., Yama- Acknowledgments We appreciate Mr. Tyler Chism, Dr. Mourad kia, J.I.: Performance of NASICON symmetric cell with ionic Benamara, and Erik D. Pollock at the University of Arkansas for their liquid electrolyte. J. Electrochem. Soc. 157(4), A536–A543 help in materials synthesis, TEM and ICP-MS analysis. We also (2010) appreciate Mr. Seyed A. Ghetmiri and Dr. Shui-Qing Yu at the 14. Aragon, M.J., Lavela, P., Ortiz, G.F., Tirado, J.L.: Effect of iron University of Arkansas for their assistance in Raman measurement. substitution in electrochem. Performance of Na3V2(PO4)3 as Dr. Jian-jun Zhang at Dalian University of Technology is also cathode for Na-ion batteries. J. Electrochem. Soc. 162(2), appreciated for his help in using PLATON to calculate the porosity of A3077–A3083 (2015) crystal structures. 15. Jian, Z.L., Han, W.Z., Lu, X., Yang, H.X., Hu, Y.S., Zhou, J.: Superior electrochemical performance and storage mechanism of Open Access This article is distributed under the terms of the Na V (PO ) cathode for room-temperature sodium-ion batteries. Creative Commons Attribution 4.0 International License (http:// 3 2 4 3 Adv. Energy Mater. 3, 156–160 (2013) creativecommons.org/licenses/by/4.0/), which permits unrestricted 16. Song, W.X., Ji, X.B., Zhu, Y.R., Zhu, H.J., Li, F.Q., Chen, J., Lu, use, distribution, and reproduction in any medium, provided you give F., Yao, Y.P., Banks, C.E.: Aqueous sodium-ion battery using a appropriate credit to the original author(s) and the source, provide a Na V (PO ) electrode. ChemElectrochem 1, 871–876 (2014) link to the Creative Commons license, and indicate if changes were 3 2 4 3 17. Song, W.X., Ji, X.B., Wu, Z.P., Zhu, Y.R., Yang, Y.C., Chen, J., made. Jing, M.J., Li, F.Q., Banks, C.E.: First exploration of Na-ion migration pathways in the NASICON structure Na V (PO ) . 3 2 4 3 J. Mater. Chem. A 2, 5358–5362 (2014) 18. Spek, A.L.: PLATON, Version 1.62, University of Utrecht, 1999 References 19. Shannon, R.D.: Revised effective ionic radii and systematic studies of interatomic distances in halides and chaleogenides. 1. Li, X.N., Zhu, X.B., Liang, J.W., Hou, Z.G., Wang, Y., Lin, N., Acta Crystallogr. A 32, 751–767 (1974) Zhu, Y.C., Qian, Y.T.: Graphene-supported NaTi (PO ) as a 20. Feng, Q., Miyai, Y., Kanoh, H., Ooi, L.: Hydrothermal synthesis 2 4 3 high rate anode material for aqueous sodium ion batteries. of lithium and sodium manganese oxides and their metal ion J. Electrochem. Soc. 161, A1181–A1187 (2014) extraction-insertion reactions. Chem. Mater. 7, 1226–1232 (1995) 2. Li, Z., Young, D., Xiang, K., Carter, W.C., Chiang, Y.M.: 21. Kim, D.H., Kim, J.: Synthesis of LiFePO4 nanoparticles in polyol Towards high power high energy aqueous sodium-ion batteries: medium and their electrochemical properties. Electrochem. The NaTi (PO ) /Na MnO system. Adv. Energy. Mater. 3, Solid-State Lett. 9(9), A439–A442 (2006) 2 4 3 0.44 2 290–294 (2013) 22. Li, S., Dong, Y.F., Xu, L., Xu, X., He, L., Mai, L.Q.: Effect of 3. 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Wu, X.Y., Sun, M.Y., Shen, Y.F., Qian, J.F., Cao, Y.L., Ai, X.P., 2730–2735 (1996) Yang, H.X.: Energetic aqueous rechargeable sodium-ion battery 123 Mater Renew Sustain Energy (2016) 5:3 Page 9 of 9 3 25. Song, W.X., Cao, X.Y., Wu, Z.P., Chen, J., Huangfu, K.L., 30. Wang, G.J., Fu, L.J., Zhao, N.H., Yang, L.C., Wu, Y.P., Wu, Wang, X.W., Huang, Y.L., Ji, X.B.: A study into the extracted ion H.Q.: An aqueous rechargeable lithium battery with good cycling number for NASICON structured Na V (PO ) in sodium-ion performance. Angew. Chem. Int. Ed. 46, 295–297 (2007) 3 2 4 3 batteries. Phys. Chem. Chem. Phys. 16, 17681–17687 (2014) 31. Luo, J.Y., Cui, W.J., He, P., Xia, Y.Y.: Raising the cycling sta- 26. Kanoh, H., Tang, W.P., Makita, Y., Ooi, K.: Electrochemical bility of aqueous lithium-ion batteries by eliminating oxygen in intercalation of alkali-metal ions into birnessite-type manganese the electrolyte. Nat. Chem. 2, 760–765 (2010) oxide in aqueous solution. 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Parks, G.A.: The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems. Chem. Rev. 65(2), 177–198 (1965) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals

Improving the cycling stability of Na3V2(PO4)3 nanoparticle in aqueous sodium ion batteries by introducing carbon support

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Material Science; Materials Science, general; Renewable and Green Energy; Renewable and Green Energy
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Mater Renew Sustain Energy (2016) 5:3 DOI 10.1007/s40243-016-0067-z ORIGINAL PAPER Improving the cycling stability of Na V (PO ) nanoparticle 3 2 4 3 in aqueous sodium ion batteries by introducing carbon support 1 1,2 1,3 • • Huajun Zhou Z. Ryan Tian Simon S. Ang Received: 29 July 2015 / Accepted: 21 January 2016 / Published online: 8 February 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Here we report the electrochemical perfor- safety and cost and find limits in large-scale applications. To mances of a Na V (PO ) /C nanocomposite as a cathode address these issues and meet the vast need for energy storage, 3 2 4 3 material for aqueous sodium-ion batteries (SIBs). Com- recently aqueous sodium-ion batteries (SIBs) with their pared to a previously reported Na V (PO ) microparticle, capacities comparable to those of LIBs have received con- 3 2 4 3 this nanocomposite demonstrated much improved cycling siderable interests [1–5]. On one hand, aqueous systems are stability. While the improvement mainly attributed to the safer as they could be neither flammable nor toxic; on the right pH and the carbon matrixmediated protection against other hand, sodium is much less expensive than lithium due to the electrolyte, the capacity fade was mainly due to the its much more abundant natural reserve. Furthermore, the deterioration of crystallinity and structure of the high ionic conductivity [2] of aqueous electrolytes can sup- nanocomposite caused by various interactions between the port fast charge–discharge processes. nanocomposite and electrolyte. This work not only help to Among electrode materials candidates for aqueous SIBs, understand the degradation of Na V (PO ) in aqueous sodium vanadium phosphates with sodium (Na) super ion 3 2 4 3 SIBs, but also shed light on the design and fabrication of conductor (NASICON) structures such as NaVPO F[6], electrode materials with high cycling stability for aqueous Na V O(PO ) F[7], and Na VTi(PO ) [8] have been 3 2 4 2 3 4 3 SIBs. widely investigated as cathodes for the following reasons. First, this structure features a highly covalent three-di- Keywords Sodium ion batteries  Na V (PO )  Cathode  mensional (3D) framework in which Na-ions can facilely 3 2 4 3 Aqueous batteries  Carbon coating diffuse in and out [9]; second, the strong covalent bonds can provide structural stability and safety. Though exten- sive studies have been investigated on the use of Na V (- 3 2 Introduction PO ) as electrode materials in non-aqueous SIBs [10–15], 4 3 the use of Na V (PO ) as electrode materials in aqueous 3 2 4 3 Currently widely used lithium-ion batteries (LIBs) employ SIBs was first studied by Ji and Banks only very recently flammable and toxic organic solvents and expensive lithium- [16]. In that work, Na V (PO ) microparticle was tested as 3 2 4 3 containing compounds, and thus present two major issues of the cathode material, and its capacities were found to decay in a rather significant fashion (ca. 31 % capacity retention for the 30th cycle). The mechanism responsible for the & Huajun Zhou capacity fade was not investigated either. hxz001@uark.edu Here we report the use of Na V (PO ) /C nanocom- 3 2 4 3 posite composed of Na V (PO ) nanoparticles and a car- 3 2 4 3 High Density Electronics Center, University of Arkansas, Fayetteville, AR 72701, USA bon matrix as the cathode material for aqueous SIBs. The Na V (PO ) nanoparticle provides shorter Na-ion diffu- 3 2 4 3 Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA sion lengths than its micro-sized counterpart does while the carbon support could enhance the electrical conductivity of Department of Electrical Engineering, University of the electrode [10] and slow down its dissolution [1]. Arkansas, Fayetteville, AR 72701, USA 123 3 Page 2 of 9 Mater Renew Sustain Energy (2016) 5:3 Compared to the previously reported Na V (PO ) nanocomposite in an aqueous system. A large piece of pure 3 2 4 3 microparticle, this nanocomposite indeed demonstrates platinum, a Ag/AgCl/1 M KCl electrode (0.235 V vs. much improved cycling stability. The mechanism respon- NHE), and 1 M Na SO aqueous solution served as the 2 4 sible for the capacity fade was also investigated here. counter electrode, reference electrode, and electrolyte, respectively. The working electrode was prepared by mixing the nanocomposite, super P carbon black, and Experimental methods polytetrafluoroethylene (PTFE) in a weight ratio of 75:20:5 onto a stainless steel foil of *0.3 cm area. The electrodes Syntheses of Na V (PO ) /C nanocomposite were dried in a vacuum oven at 80 C overnight, and then 3 2 4 3 pressed at a pressure of 20 MPa using a PHI manual The nanocomposite was prepared by modifying a published compression presses. The loading of each electrode was -2 procedure [10]. To a round-bottom flask containing 30 mL controlled to be 1–2 mg cm , and the thickness was tetraethylene glycol (TEG) was added 0.246 g/3 mmol *50 lm. Before the cell assembly, the electrolyte was sodium acetate (NaCH COO), 0.697 g/2 mmol vanadium bubbled with Ar for 60 min while the platinum foil was (III) acetylacetonate (VO(C H O ) ), and 0.345 g/3 mmol first rinsed in acetone and then repeatedly ultrasonicated in 5 7 2 2 ammonium dihydrogen phosphate (NH H PO ). The mix- deionized water. Cells were assembled in an Ar atmo- 4 2 4 ture was then stirred overnight at room temperature to sphere. Cyclic voltammetry (CV) was performed on a afford a homogenous green solution, and the resultant Solartron SI 1287 Electrochemical Interface at a scan rate -1 solution was then heated at 320 C for 72 h. The precipi- of 0.5 mV s from 0 to 0.8 V (vs. the reference electrode) tate was collected by centrifugation, washed by ethanol and while galvanostatic tests were performed on a BT 2000 acetone each for 3 times respectively, and dried in a vac- battery tester. The electrochemical performances of this uum oven at 80 C for 2 h. Then the resultant light-brown nanocomposite in a non-aqueous system were also tested as powders were annealed at 650 C under Ar/H (40/2 sccm) a reference. The electrode fabrication was the same as for 6 h and then at 800 C under Ar flow (40 sccm) for 6 h above except that here Al foil was used and electrodes were to afford black powders. *8 mm in diameter. The loading and thickness of each -2 electrode was 1–2 mg cm and *50 lm, respectively. Materials characterizations 2032-type coin cells were assembled in an MBraun glove box (O \ 0.1 ppm, H O \ 2 ppm) while a Celgard 3501 2 2 The phase purity was characterized by a Rigaku MiniFlex II microporous membrane was used as the separator. CV was Desktop X-ray diffractometer using monochromatized Cu- performed on a Solartron SI 1287 electrochemical interface Ka radiation (k = 1.5418 A) at 30 kV and 15 mA. A at a scan rate of 0.05 mV/s while galvanostatic tests were continuous scan mode was used to collect the diffraction performed on a BT 2000 battery tester. 1 M NaClO in EC/ data from 10 to 60 at a speed of 0.2/min. The mor- DMC (w/w = 1:2) and Na foil were used as the electrolyte phologies were investigated by an FEI Nova NanoSEM and counter electrode. scanning electron microscope (SEM) equipped with a field emission gun operated at 10 kV and by an FEI Titan 80–300 transmission electron microscope (TEM). To minimize Results and discussion charging problems, samples for SEM were coated with thin Au layers. Raman spectra were recorded on a homemade In the crystal structure of Na V (PO ) , each VO octahe- 3 2 4 3 6 lRaman spectroscope that was composed of a 632.8 nm dron is connected to three PO4 tetrahedrons via corners to 3- He–Ne excitation laser, an iHR 550 HORIBA spectrometer, afford [V (PO ) ] ribbons along the c axis, and these 2 4 3 and a Si CCD detector. Prior to Raman the measurement, ribbons are connected to each other through PO tetrahe- the system was calibrated using a bulk Si. Elemental anal- drons along the a axis to afford an open 3D framework ysis was done in Atlantic Microlab Inc. to determine the where two kinds of Na ions reside (Fig. 1a). Na1 atoms 3- weight percentage (*20wt %) of carbon species in the reside between two adjacent [V (PO ) ] units within the 2 4 3 Na V (PO ) /C nanocomposite. ICP-MS analysis revealed same ribbons while Na2 atoms reside between adjacent 3 2 4 3 the mole ratio of Na:V:P to be 3.00:2.00:3.00, further sug- ribbons. The channels along the a axis (Fig. 1a) and the gesting the pure phase of Na V (PO ) /C. b axis are the same in dimension, and are favored routes for 3 2 4 3 Na-ion diffusion while the channel along the a axis is more Electrochemical measurements favored [17]. The PLATON program [18] suggests that neither the pristine Na V (PO ) nor the anionic framework 3 2 4 3 3- A three-electrode beaker-type cell was used to test the [V (PO ) ] can support physical adsorption of water. As 2 4 3 electrochemical performances of Na V (PO ) /C confirmed by the space-filling model of the anionic 3 2 4 3 123 Mater Renew Sustain Energy (2016) 5:3 Page 3 of 9 3 Fig. 1 a Ball stick model of a unit cell structure of Na V (PO ) viewed along the 3 2 4 3 a axis; orange lines represent cell edges; b space-filling model of the anionic framework 3- [V (PO ) ] in which Na-ions 2 4 3 reside, indicating the channels are too small for physical adsorption of water molecules; the scale bar represents the diameter of a water molecule framework (Fig. 1b, the radii of all atoms were assigned electronic conductivity. The lower intensity of G-band according to Shannon’s work [19]), the cross section of the could be due to the interaction between the Na V (PO ) 3 2 4 3 channel along the a axis in the anionic framework is nanoparticles and carbon [1]. During the synthesis, tetra- *2.60 9 0.12 A in dimension, making physical adsorption ethylene glycol (TEG) molecules played a dual role: First, water (*2.76 A in diameter) through the channels impos- they provided a reducing environment [21] to stabilize sible. During the repeated charge–discharge cycles, Na V(III) in the presence of oxygen; Second, they functioned 3- V (PO ) is the Na-inserted phase while NaV (PO ) is the as a capping agent [10] on the as-synthesized Na V (PO ) 2 4 3 2 4 3 3 2 4 3 Na-extracted phase [12]. Compared to Na V (PO ) , nanoparticles and were then be converted to carbon species 3 2 4 3 NaV (PO ) maintains the same structure but decreased upon annealing. 2 4 3 lattice sizes (*8.3 % volume decrease) [12]. Therefore The electrochemical performances of this nanocom- water adsorption cannot interfere with the Na-ion extrac- posite were first investigated in non-aqueous electrolyte tion-insertion during cycling. Na V (PO ) is different from (Fig. 3) as a reference. Figure 3a shows the cyclic 3 2 4 3 some frameworks such as spinel-type lithium manganese voltammograms (CVs) in the voltage range of 2.3–3.9 V oxide that can support simultaneous insertion-extraction of vs. Na /Na at the scan rate of 0.05 mV/s. During the first water and metal ions during cycling [20]. scan, a pair of sharp peaks is observed: the oxidation (Na- The powder X-ray diffraction (PXRD) pattern of the ion extraction) and reduction peak (Na-ion insertion) are at Na V (PO ) /C nanocomposite obtained after annealing at 3.48 and 3.28 V. The average voltage (3.38 V) is close to 3 2 4 3 4? 3? 800 C confirmed the NASICON structure with the R-3c the equilibrium voltage of V /V redox couple of this space group (Fig. 2a) [9]. The SEM image in Fig. 2b compound [11]. In subsequent up to ten scans, the peak indicates that Na V (PO ) nanoparticles were aggregated positions also remain unchanged while the currents just 3 2 4 3 with their grain sizes being in the range of 60–120 nm. decreased slightly, indicating the good reversibility for Na- TEM analysis confirmed the presence of the carbon support ion extraction and insertion and the stability of the struc- (Fig. 2c). Raman spectrum in Fig. 2d clearly shows two ture. Figure 3b shows the charge–discharge profiles at a distinct peaks, the D-band associated with sp3-hybridized current of 1 C in the range of 2.3–3.9 V (note that 1 C -1 carbon at *1338 cm and the G-band associated with means the full capacity can be charged or discharged in 1 h -1 sp2-hybridized carbon at *1589 cm . The intensity ratio and here 1 C & 110 mA/g since the theoretical discharge of the D- and G-bands (ID/IG) is 1.10, indicating there are capacity of Na V (PO ) is c.a. 110 mAh/g). An evident 3 2 4 3 large amount of sp -carbon capable of providing high voltage plateau around 3.4 V was observed for both charge 123 3 Page 4 of 9 Mater Renew Sustain Energy (2016) 5:3 Fig. 2 Structural characterizations of the Na V (PO ) /C nanocomposite. 3 2 4 3 a Powder XRD pattern confirming the composition of Na V (PO ) ; b SEM image 3 2 4 3 indicating the sizes; c TEM image indicating Na V (PO ) 3 2 4 3 nanoparticles are embedded into a carbon matrix; d Raman spectrum indicating the characteristic of carbon species Fig. 3 Electrochemical performances of this Na V (PO ) /C nanocomposite 3 2 4 3 using 1 M NaClO in EC/DMC (w/w = 1:2) as the non-aqueous electrolyte. a Cyclic voltammograms at the scan rate of 0.05 mV/s up to ten cycles. b Galvanostatic charge– discharge cycling profiles at 1 C. c Initial charge–discharge cycling profiles at 5 C, 10 C, and 20 C. d Discharge capacity evolution for 40 cycles at 1 C currents and discharge, and the average polarization (i.e. the voltage the initial charge capacity of 108.0 mAh/g and discharge difference between charge and discharge plateaus) is about capacity of 91.3 mAh/g (Note that the mass was based on 0.25 V. The Na V (PO ) in this nanocomposite delivers that of Na V (PO ) while the carbon species constituted 3 2 4 3 3 2 4 3 123 Mater Renew Sustain Energy (2016) 5:3 Page 5 of 9 3 *wt 20 % of the nanocomposite). In comparison, different carbon species have been proven to significantly affect the initial discharge capacities of Na V (PO ) /C composites 3 2 4 3 which range from *30 mAh/g [11], to *70 mAh/g [22], and to 104 mAh/g [10]. The discharge capacity here is 91.5 mAh/g after 10 cycles (Fig. 3b), further suggesting the good and reversible cycling performances in non-aqueous systems. As the currents increase, the polarization increa- ses and the capacities decrease (Fig. 3c). While the initial discharge capacity of this nanocomposite at 5 C is 57.4 mAh/g, the discharge capacities at 10 C and 20 C are 25.0 and 9.9 mAh/g and much smaller than that at 1 C. At 1C, the discharge capacities remain quite stable after forty cycles (Fig. 3d). pH has been proved to significantly affect the cycling performances of many electrode materials in aqueous LIBs and SIBs [23, 24]. Figure 4 shows CV curves of the nanocomposite in 1.0 M aqueous Na SO solution at dif- 2 4 ferent pH in an oxygen-free environment in which the electrode hydrolysis was inhibited [23]. For the first scan at the initial pH = 4.0 (Fig. 4a), there is a pair of well-defined redox peaks: *0.465 V for the oxidation peak and *0.373 V for the reduction peak. From the second scan to the fifth scan, while both the oxidation currents at *0.465 V and reduction currents at *0.373 V decreased, the positions and shapes of these redox peaks remained the same. Meanwhile, shoulder peaks close to *0.373 V appeared in the reduction processes and the currents of those shoulder peaks only decreased very slightly possibly due to very slight dissolution of Na V (PO ) nanoparticles. 3 2 4 3 Currently, the mechanisms of the Na-ion extraction from Na V (PO ) during charge processes in non-aqueous 3 2 4 3 systems are still being debated. Gu, Xi, and Hu suggested Na1 ions at the M1 (6b) sites tend to remain immobilized, only Na2 ions at the M2 (18e) sites could be extracted, and the Na-ion extraction process involve a two-phase trans- formation from Na V (PO ) to NaV (PO ) [12]. In con- 3 2 4 3 2 4 3 trast, Ji and Banks suggested that Na-ions at both sites could be extracted and the transformation from Na V (- 3 2 PO ) to NaV (PO ) involve Na V (PO ) as an inter- 4 3 2 4 3 2 2 4 3 mediate [25]. From the second scan to the fifth scan in repeated CV scanning (Fig. 4a), should peaks near *0.373 V showed up and the oxidation peaks at *0.465 V are apparently broader than the reduction peaks at *0.373 V (Fig. 4a). All these observations suggest Na V (PO ) could serve as an intermediate in the two-step 2 2 4 3 Fig. 4 Cyclic voltammograms (CVs) for Na V (PO ) /C in 1 M 3 2 4 3 aqueous Na2SO4 solution up to ten cycles at the scan rate of 0.5 mV/s Na-ion insertion process but not the one-step Na-ion at the initial pH = 4.0 (a), 7.0 (b), and 9.0 (c), respectively extraction process. Such difference indicates mechanisms or kinetics of Na-ion insertion and extractions differ, and similar phenomena in the metal-ion insertion and extrac- tion in aqueous systems were observed before [26]. From reduction peaks at *0.373 V shifted positively slightly the fifth scan to the tenth scan, the oxidation peaks shifted (*0.390 V for the tenth scan), suggesting possible struc- negatively slightly (*0.457 V for the tenth scan) while the tural reorganization. 123 3 Page 6 of 9 Mater Renew Sustain Energy (2016) 5:3 After the fifth scan of CVs at pH = 4.0, an additional evolution. Figure 5a shows the charge–discharge profiles at pair of weaker redox peaks (*0.270 and *0.130 V) a current of 1 C in the range of 0–0.8 V vs. Ag/AgCl. Upon started to intensify (top left insets in Fig. 4a). These redox assembly, the cell was charged to 0.8 V first and interest- peaks are more prominent than those at pH = 7.0 and 9.0 ingly two close plateaus (*0.390 and *0.430 V with the (top left inset in Fig. 4b, c), suggesting such redox reac- average being *0.410 V) were observed for the first time ? ? tions could be related to the H intercalation. Such H possibly due to Na V (PO ) being as the intermediate 2 2 4 3 intercalation in aqueous LIBs was observed to be irre- during the charge (sodium extraction) process [25] or some versible [27]. Since Na V (PO ) has large interstitial kind(s) of structural reorganization. During the first dis- 3 2 4 3 spaces, Na V (PO ) may support reversible H insertion- charge process, a major voltage plateau at *0.405 V 3 2 4 3 extraction. Such redox peaks were not reported before in showed up. The low polarization (*0.005 V) is much both aqueous [10] and non-aqueous SIBs [7] using Na smaller than 0.25 V observed in the non-aqueous elec- 3- V (PO ) as cathode materials. The reason could be due to trolyte and indicates excellent ionic and electronic con- 2 4 3 the later-described deterioration of crystallinity and struc- ductivities during this cycle. As the cycling continued, the ture that could favor the subsequent H insertion-extrac- charge–discharge profiles from the second to the tenth tion. The corresponding redox currents being more than cycle demonstrated the following trends: (1) the polariza- one order of magnitude lower than the major redox currents tion for the redox peaks at *0.410 V kept increasing suggest low extent of H insertion-extraction, which was (0.029, 0.034, and 0.054 V for the second, fifth, and tenth confirmed by ICP analysis (the mole ratios of Na: P for cycles), suggesting continuously decreased ionic and electrodes before cycling and after five scans were found to electronic conductivities which could come from the be 1.00 and 0.98, respectively). deterioration of the Na V (PO ) nanoparticle’s crys- 3 2 4 3 From the fifth scan to the tenth scan at pH = 4.0, the tallinity and structure and/or the breakage of the carbon redox currents decreased in a much slower fashion than matrix; (2) an unobvious plateau showed up at *0.200 V those at pH = 7.0 and 9.0 (Fig. 4a–c). The reason could during discharge processes, and could attribute to the be due to higher solubility of this nanocomposite’s Na H -insertion processes; (3) the charge–discharge plateaus 3- V (PO ) at higher pH, which was previously observed in at *0.410 V became narrower while the discharge pla- 2 4 3 the case of vanadium oxide [24]. teaus at *0.200 V became wider. Accordingly, the It is worthy to note that during cycling the pH of elec- capacity fade around the voltage plateaus of *0.410 V trolyte changed only slightly, especially for the cases of was somehow compensated by the capacity increase pH = 4.0 and 9.0. Equations (1) and (2) show the elec- around those of *0.200 V. In the tenth cycle, the voltage trochemical processes during charge in both the working plateau around 0.410 V during the charge process was not electrode and counter electrode: strictly flat. All these trends match those observed during repeated CV scanning. As the currents increase, both the Na VðÞ PO ! NaVðÞ PO + 2Na +2e ð1Þ polarization increases and the capacity decreases (Fig. 5b) 3 2 4 2 4 3 3 were less significant than those in the non-aqueous system 2H O + 2Na +2e ! 2NaOH + H ð2Þ 2 2 due to faster kinetics in the aqueous system. The initial Since the total mass of each electrode is only about discharge capacities of this nanocomposite at 5 C, 10 C, 0.3 mg while the electrolyte is about 10 mL, the and 20 C are 33.7, 18.3, and 11.6 mAh/g respectively. It is concentration of generated OH during charge is about worthy to note the discharge capacity at 20 C in the -5 7.9 9 10 M. During the subsequent discharge, these aqueous system is even slightly higher than that in the non- OH are consumed. Therefore, the pH of the electrolyte aqueous system (9.9 mAh/g). with the initial pH being 4.0 is expected to range from 4.0 The charge and discharge capacities decrease relatively to 4.7 during cycling. The electrolytes with the initial pH fast from the first cycle to the fifth cycle and then decrease being 7.0 and 9.0 are expected to be in the range of 7.0–9.9 much slower from the fifth cycle to the tenth cycle. Such and 9.0–9.95 respectively during cycling. Our experimental trends match those of the redox currents at *0.465 and pH values matched these expected values. More in-depth *0.373 V during repeated CV scanning. To understand investigations of the Na V (PO ) -water interactions, its such trends, ex situ PXRD analysis was used to analyze the 3 2 4 3 pH-dependent solubility, and optimization of electrolytes pristine electrode and the electrode after five charge–dis- are beyond the scope of the paper and will be published charge cycles at pH = 4.0 (Fig. 5c). All the major peaks separately. remained, indicating the main framework remained after On the basis of the above observations, pH = 4.0 was five cycles. However, the PXRD pattern of the electrode found to be an optimal condition for evaluating the cycling after cycling is indeed different from that of the pristine performances. pH lower than 4.0 was not employed since it electrode in two aspects: First, the two pairs of split peaks could induce more H intercalation and/or even hydrogen in the PXRD pattern of the pristine electrode, namely 123 Mater Renew Sustain Energy (2016) 5:3 Page 7 of 9 3 Fig. 5 a Galvanostatic charge– discharge cycling profiles of Na V (PO ) /C nanocomposite 3 2 4 3 in a 1 M Na SO electrolyte at 2 4 pH = 4.0 at 1 C. b Initial charge–discharge cycling profiles at pH = 4.0 at 5 C, 10 C, and 20 C. c PXRD patterns of electrodes before and after five cycles (unfilled square and filled square represent peaks from the stainless steel foil and super P carbon black, respectively); d Discharge capacity evolution for 40 cycles at 1 C currents (104)/(110) and (211)/(116), were found to merge into two previously reported Na V (PO ) microparticle discharged 3 2 4 3 broad peaks, indicating the crystallinity of Na V (PO ) at 8.5 C in the first cycle (209 F/g, i.e. *46.4 mAh/g), but 3 2 4 3 nanoparticles deteriorated during the cycling. Second, the the capacity retention rate of this nanocomposite (Fig. 5d) intensity of the starting third-strong (300) peak relative to is much higher than that of the microparticle (50 vs. 31 % the strongest (116) peak was found to significantly decrease after thirty cycles) [16] though less than that observed in after the cycling. In contrast, such Na V (PO ) nanopar- the non-aqueous system (Fig. 3c). It is worthy to note that 3 2 4 3 ticles could maintain good crystallinity even after being the capacity fade is faster at current rates lower than 1C, cycled at 100 C for 1000 cycles in non-aqueous electrolytes possibly due to the slow dissolution of the electrode [10]. Therefore, repeated Na-ion diffusion along the a axis materials and/or the interactions between electrode mate- should not be responsible for the deterioration of both rials and electrolytes during cycling [31]. crystallinity and structure of Na V (PO ) nanoparticles. 3 2 4 3 Possible reasons could be the interactions between water molecules and Na V (PO ) nanoparticle including its Conclusions 3 2 4 3 crystallographic planes since both phosphates [28] and vanadium oxides [29] turned out to have interesting inter- In summary, compared to Na V (PO ) microparticle [16], 3 2 4 3 facial chemistry in aqueous systems. this Na V (PO ) /C nanocomposite demonstrated compa- 3 2 4 3 Based on all these observations, we propose that the rable discharge capacity during the first cycle in aqueous capacity fade during cycling should be mainly due to the SIBs but much higher cycling stability in subsequent deterioration of crystallinity and structure of the cycling. The carbon support capable of helping protect the nanocomposite during the initial several charge–discharge electrode materials from aqueous electrolyte along with the cycles, some H insertion-extraction, and slow dissolution optimal pH should be responsible for the significant of Na V (PO ) nanoparticles. improvement. The capacity fade should be mainly due to 3 2 4 3 The discharge capacity of our Na V (PO ) /C (1) the deterioration of both crystallinity and structure of 3 2 4 3 nanocomposite at 1 C rate in the first cycle in aqueous SIBs the Na V (PO ) nanoparticle and (2) the possible break- 3 2 4 3 is 44.7 mAh/g. Considering the discharge capacities of age of the carbon matrix. The former should be due to the both aqueous LIBs and SIBs are generally lower than those interactions between Na V (PO ) nanoparticle and elec- 3 2 4 3 of their non-aqueous counterparts [23, 30], the capacity of trolytes and could lead to the decreased ionic and elec- this nanocomposite is quite decent. It is also close to that of tronic conductivities while the latter could lead to 123 3 Page 8 of 9 Mater Renew Sustain Energy (2016) 5:3 based on Na CuFe(CN) -NaTi (PO ) intercalation chemistry. decreased electronic conductivities. H intercalation and 2 6 2 4 3 ChemSusChem. 7(2), 407–411 (2014) slow dissolution of electrode materials are also partially 6. Qin, H., Song, Z.P., Zhan, H., Zhou, Y.H.: Aqueous rechargeable responsible for the capacity fade. alkali-ion batteries with polyimide anode. J. Power Sources 249, Therefore, to further optimize the cycling stability of 367–372 (2014) 7. Kumar, P.R., Jung, Y.H., Lima, C.H., Kim, D.K.: Na V O (- Na V (PO ) micro/nanoparticles in aqueous SIBs, the 3 2 2x 3 2 4 3 PO ) F : a stable and high-voltage cathode material for 4 2 3-2x interactions between Na V (PO ) particles and water need 3 2 4 3 aqueous sodium-ion batteries with high energy density. J. Mater. to be minimized via either effective coating or wrapping of Chem. 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Materials for Renewable and Sustainable EnergySpringer Journals

Published: Feb 8, 2016

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