Article pubs.acs.org/JPCC
In situ Formed Li2NaV2(PO4)3: Potential Cathode for Lithium Ion Batteries Saravanan Karuppiah and Kalaiselvi Nallathamby* CSIR-Central Electrochemical Research Institute, Karaikudi-630 003, Tamilnadu, India S Supporting Information *
ABSTRACT: The characteristic three plateau electrochemical behavior of Li3V2(PO4)3, criticized as a drawback of the electrode against its practical applications, has been surpassed through the in situ formed rhombohedral Li2NaV2(PO4)3 analogue obtained upon aging, especially when Na3V2(PO4)3 is explored as a cathode in the rechargeable lithium cell assembly. By incorporating an eco-benign and waste driven biocarbon, viz., human hair derived carbon in the cathode formulation, one can demonstrate the potential cathode behavior of the resultant Li2NaV2(PO4)3/HHC composite that exhibits superior electrochemical performance and a negligible polarization of 0.40 V upon extended cycles. Interestingly, Li2NaV2(PO4)3/HHC cathode of the present study exhibits a capacity of 109 mAh g−1 at 0.1 C for 200 cycles and tolerates as high as 20 C rate for about 5000 cycles with excellent capacity retention behavior, thus qualifying itself as a first ever strain free cathode for lithium-ion batteries.
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INTRODUCTION With a view to qualify lithium-ion technology with ensured safety in device applications, phosphate family electrodes replace the oxide cathodes for obvious reasons.1 In particular, the covalency driven conductivity and thermal safety advantages of eco-benign polyanionic phosphate matrix plays a vital role in validating lithiated polyanionic phosphates such as Li3V2(PO4)3, otherwise known as LVP, as a probable cathode for lithium ion batteries (LIBs).2,3 It is well-known that LVP could be prepared in two forms, viz., rhombohedral and monoclinic LVP, corresponding to the extraction of two and three lithium per formula unit, permissible as per structural advantages. Herein, the stringent synthesis conditions involved in preparing rhombohedral LVP leaves ample scope to investigate upon monoclinic LVP for its exploitable applications.4,5 However, monoclinic LVP poses practical difficulties by virtue of its characteristic three plateau cycling behavior, thus making it not suitable for device applications.6 As a result, one needs to address two critical issues, viz., single-step two-electron transfer possibilities in the electrode matrix and the requirement of simple and an easy-to adopt synthesis protocol to prepare such an electronic composition. The said combination could be realized by choosing an alternative rhombohedral Na3V2(PO4)3, viz., NVP matrix, commonly known as a practically viable sodium battery electrode, involving simple facile and scalable synthesis protocols for its application in LIBs. It is important to note that NVP has been chosen instead of LVP, as the later requires fuzzy synthesis conditions to obtain rhombohedral LVP. Further, the chosen rhombohedral NVP is bestowed with the © 2017 American Chemical Society
currently desired one-step two-electron transfer kinetics, due to which the preference for NVP instead of LVP gets justified.6,7 Hence, the study aims at the site selective ion exchange mechanism driven in situ formation of rhombohedral Li2NaV2(PO4)3, represented as LNVP from NVP as the ultimate electroactive material for LIBs to combat the twin intricacies associated with the synthesis method and electrochemical behavior of simple and rhombohedral LVP. Despite the efforts made to address the synthesis and multiple plateau related issues, yet another inherent drawback related to the poor electronic conductivity of phosphate family electrodes needs to be addressed, prior to qualify the chosen electrode matrix for its global acceptance.1,8 Toward this direction, dual heteroatom containing biocarbon obtained from a universal solid waste, viz., human hair, bestowed with advantages such as graphene sheet-like structure, heteroatom(s) triggered enhanced electronic conductivity, porosity, and surface area driven faster transport kinetics of shuttling ion has been chosen to form LNVP/HHC composite and evaluated further as a cathode in LIB assembly.9 In short, the study deals with an ever first simple approach to demonstrate rhombohedral NVP driven LNVP/HHC composite (despite the presence of sodium counterpart) for its suitability as a potential cathode for LIBs bestowed with insignificant strain, especially upon extended cycles, by synergistically reaping the advantages of LNVP and HHC Received: May 2, 2017 Revised: June 5, 2017 Published: June 8, 2017 13101
DOI: 10.1021/acs.jpcc.7b04160 J. Phys. Chem. C 2017, 121, 13101−13105
Article
The Journal of Physical Chemistry C
sites, 6b is occupied by one sodium atom (M1) and the remaining two sodium atoms occupy the 18e site of M2. Further, the structure is bestowed with the presence of VO6 octahedra and PO4 tetrahedra to form lanterns and the presence of channels formed out of vacancies facilitates the facile intercalation/deintercalation of host ions.19 With the said structural arrangement, the intentionally triggered ion exchange mechanism of the present study (by treating the NVP/HHC powder externally with the electrolyte) promotes the facile exchange of M2 site occupied sodium by lithium atoms to form LNVP.19 Such a reaction mechanism that replaces M2 site occupied sodium by two lithium atoms is further substantiated by the currently recorded 7Li and 23Na NMR. To illustrate the existence of one sodium in M1 and two sodium atoms in the M2 site of NVP, we have recorded the 23 Na NMR spectrum of pristine NVP powder. Simultaneously, the currently formed LNVP, believed to be obtained through the site selective ion-exchange mechanism has also been subjected to 23Na NMR analysis with a view to confirm the mechanism of formation of LNVP from NVP (Figure 2a). It is evident from 23Na NMR that the peak at 91 ppm corresponds to the presence of two sodium atoms in M2 site, and the less pronounced peak at −54 ppm is due to the presence of one sodium atom in the M1 site of pristine NVP (Figure 2a). Interestingly, the product of the mimic experiment to evidence the in situ formation of LNVP from NVP exhibits only one peak at −13 ppm, corresponding to the presence of unexchanged sodium atom present in the M1 site (Figure 2b). Notably, peak corresponding to the presence of two sodium atoms in M2 site is missing, which is in support of the formation of LNVP from NVP through site selective ion exchange mechanism.19 In parallel, in situ formed LNVP for its lithium content has been investigated using 7Li NMR. As a customary approach, 7Li NMR spectrum of LVP has been recorded and reproduced in Figure 2c. As expected, the 7Li NMR spectrum of the mimic experiment exhibits peaks in support of the presence of lithium, thus substantiating the in situ formation of LNVP from NVP (Figure 2d). Having confirmed the in situ formation of LNVP from rhombohedral NVP, the target electroactive material, viz., LNVP/HHC has been evaluated for its electrochemical performance in LIBs using cyclic voltammetry, charge/ discharge, and rate capability studies. LNVP/HHC, formed through the in situ ion exchange reaction was examined as cathode in the Li half cell assembly, restricted to the potential region of 3.0−4.3 V. Cyclic voltammetry result for LNVP/ HHC cathode recorded at a scan rate of 0.05 mV s−1 is shown in Figure 3a. A sharp redox pair, seen around 3.80/3.70 V (Figure 3a), could be assigned to the formation of reversible V4+/V3+ redox pair, which is accompanied by the insertion/ extraction of two lithium ions.1,12−18 After the first cycle, CV curves of subsequent cycles almost overlap with each other, indicating excellent reversibility upon progressive cycles. Figure 3b shows the charge−discharge profile of LNVP/ HHC cathode. Upon charging, the currently observed flat charge plateau located around 3.80 V could be correlated to the extraction of two lithium ions and the duly associated corresponding phase transition from Li2NaV2(PO4)3 to NaV2(PO4)3. Subsequently, the discharge plateau located around 3.70 V could be attributed to the insertion of already extracted lithium ions, which is associated with the phase transition from NaV2(PO4)3 to Li2NaV2(PO4)3.1,12−18 Hence, the cyclic reversibility of LNVP/HHC cathode could be
matrixes. It is noteworthy that literature is replete with reports on zero strain anodes, leaving behind no citation on zero strain or strain free cathodes, to the best of our knowledge.10,11 Hence, the study is bestowed with novelty and fascinating importance, supported further by the currently chosen LNVP/ HHC cathode in terms of realization of theoretical capacity with negligible polarization up to 200 cycles and an outstanding tolerance to rated current as high as 20 C for 5000 cycles.
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RESULTS AND DISCUSSION In order to demonstrate the in situ formation of LNVP through site selective ion exchange mechanism from NVP, we have disassembled the cell (fabricated with NVP/HHC as cathode against lithium and 1 M LiPF6 dissolved in 1:1 v/v ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolyte in 2032 coin cell assembly) after aging and subjected the electrode to characterization studies such as XRD and NMR. With a view to offer a mimic experiment in support of the in situ formed LNVP, we have added the as-prepared NVP powder in the LIB electrolyte consisting of 1 M LiPF6 dissolved in 1:1 v/v ethylene carbonate (EC) and dimethyl carbonate (DMC) and kept undisturbed for about 24 h (to match with the aging time of fabricated cells, prior to cycling). To our expectation, LNVP was obtained through site selective ionexchange mechanism, according to eq 1, which is quite interesting. lithiation
Na3V2(PO4 )3 ⎯⎯⎯⎯⎯⎯⎯⎯→ Li 2NaV2(PO4 )3 (or)aging
(1)
Figure 1 depicts the XRD recorded for Li2NaV2(PO4)3 formed through two different approaches viz., NVP/HHC
Figure 1. XRD pattern of LNVP/HHC composite cathode disassembled from the fabricated cell after aging time and the intentionally formed LNVP when NVP powder is soaked externally in the LIB electrolyte consisting of 1 M LiPF6 dissolved in 1:1 v/v ethylene carbonate (EC) and dimethyl carbonate (DMC).
electrode in lithium-ion cell assembly after aging and the deliberate soaking of NVP powder, treated externally with the lithium-ion battery electrolyte. Interestingly, the XRD pattern of both the cases matches with the standard JCPDS pattern (54−0736) of rhombohedral LNVP (R) with R3c space group.1 All the peaks are indexable with the standard pattern, and the calculated lattice parameter values are a = 8.325 and c = 22.49, which are in good agreement with the reported results.12−18 Hence, the formation of LNVP through site selective ion exchange mechanism could be understood, which is as follows: NVP, bestowed with the 3D open framework structure, is known to contain the presence of sodium ions in two different sites, viz., 6b site of M1 and 18e site of M2. Among the two 13102
DOI: 10.1021/acs.jpcc.7b04160 J. Phys. Chem. C 2017, 121, 13101−13105
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The Journal of Physical Chemistry C
Figure 2. 23Na MAS spectra (acquired at 298 K and at the magic angle spinning speed of 5, 9, and 11 kHz to understand the ion-exchange mechanism) of (a) pristine NVP powder, (b) pristine NVP powder deliberately soaked in lithium-ion battery electrolyte for 12 h, and 7Li MAS spectra of (c) pristine LVP and (d) NVP powder soaked in lithium-ion battery electrolyte for 12 h.
Figure 3. (a) CV behavior, (b) charge−discharge profile, (c) cycling performance, and (d) rate capability behavior of LNVP/HHC cathode at 3.0− 4.3 V potential region.
delivers a capacity of 109 mAh g−1 (84 and 92% of capacity retention with respect to the currently observed initial capacity and the theoretical capacity of LNVP, respectively) with an admissible capacity fade behavior. In particular, extraction and maintenance of closer to theoretical capacity value for about 200 cycles with a more or less zero strain behavior has never
understood. Figure 3c displays the cycling performance of LNVP/HHC cathode conducted at the rate of 0.1 C. LNVP/ HHC cathode delivers an initial capacity of 130 mAh g−1, which is higher than the theoretical capacity (i.e., 118 mAh g−1) and approaches its theoretical capacity after few cycles. Interestingly, even after completing 200 cycles, LNVP/HHC cathode 13103
DOI: 10.1021/acs.jpcc.7b04160 J. Phys. Chem. C 2017, 121, 13101−13105
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The Journal of Physical Chemistry C
Figure 4. Ex situ XRD patterns of the LNVP cathode at various states of charge and discharge indicated in the voltage profile.
LNVP owing to the addition of extracted lithium ions, which clearly demonstrates the maintenance of structural stability and reversibility due to the occurrence of reversible intercalation/ deintercalation mechanism upon electrochemical cycling.1,12−18 In short, ex situ XRD results (at different state of charge and discharge conditions) conclude that the mechanism of lithium storage in LNVP cathode is a characteristic two phase reaction, which is similar to that of its sodium counterpart.6 23 Na and 7Li NMR spectrum of the LNVP cathode after completing one charge and one cycle is shown in Figure S2. As expected, no peak corresponding to the presence of sodium in M2 site is seen with LNVP cathode after cycling (Figure S2a,b). Such an observation clearly indicates that the M2 site is occupied by two lithium atoms and are duly replaced by the corresponding number of sodium atoms through intercalation/ deintercalation of lithium ions.19 In addition, appearance of peak at 0 ppm, corresponding to the presence of lithium atom (after completing one cycle) once again proves the presence of lithium atom in LNVP/HHC matrix (Figure S2d).4 With a view to prove the role of HHC in imparting structural stability and to demonstrate the more or less strain free behavior of LNVP/HHC cathode upon cycling, we have carried out and compared the results of ex situ XRD analysis of LNVP/ HHC cathode after completing one and 200 cycles (Figure S3). Surprisingly, even after the completion of 200 cycles, the XRD pattern of LNVP/HHC cathode is exactly matching with that of pristine LNVP after the first cycle, which in turn is strongly supporting the strain free behavior and the structural stability of LNVP/HHC cathode, wherein the buffering matrix effect of HHC that plays a vital role in maintaining the structure of LNVP is better understood.1 In addition, we have taken the TEM images of LNVP/HHC cathode after the completion of 200 cycles (Figure S4). The complete wrapping of LNVP particles by graphene sheet-like carbon is found to be intact even after the completion of 200 cycles, which once again confirms the role of HHC in improving the electrochemical behavior and structural stability of LNVP cathode. In short, the excellent electrochemical behavior of strain-free LNVP/HHC cathode in LIBs is attributed to the multiple advantages of the composite architecture, wherein HHC that has been added as a composite additive ensures the structural stability upon prolonged cycling. In addition, HHC provides sufficient electronic conductivity for active material. Moreover,
been demonstrated as far as lithium-ion battery cathodes are concerned, and hence, the current study falls under first of its kind demonstration of LNVP/HHC cathode possessing insignificant capacity loss behavior in LIBs. Figure 3d shows the rate performance of LNVP/HHC cathode from 0.1 to 20 C. It is obvious that LNVP/HHC cathode exhibits reasonably good cycling performance at various current rates. At 10 C rate, about 50% of the theoretical capacity is retained. It is noteworthy that at 20 C rate, LNVP/ HHC cathode of the current study still delivers a capacity of 32 mAh g−1 for 5000 cycles (Figure S1a), corresponding to a capacity retention of 99%, which indicates that the material displays an excellent extended cycle life performance at high rate condition also. When the current rate is brought back to 0.1 C, LNVP/HHC cathode is capable of retaining 99% of its theoretical capacity (118 mAh g−1) (Figure 3d), which is noteworthy. Further, it is important to mention here that the rate performance of LNVP/HHC cathode is superior than the literature reported values of LNVP/C cathode at various high rate conditions (Table S1) and hence deserves to recommend as a potential high rate cathode for LIBs.1,12−18 In order to demonstrate the involvement of the intercalation/deintercalation behavior and to substantiate the structural stability of target cathode (LNVP/HHC) upon extended cycles, postcycling XRD and NMR studies have been carried out by carefully collecting the electrode from the disassembled cell after cycling. Figure 4 shows the XRD pattern of LNVP cathode with various state of charge and discharge conditions, viz., (A) after aging, (B) half-charged, (C) fully charged, (D) half discharge, and (E) fully discharged state. When the LNVP cathode is fully charged, new diffraction peaks are observed at 2θ = 21°, 24.7°, 29.9°, 32.6°, and 33.7°, corresponding to the formation of NaV2(PO4)3 due to the deintercalation/extraction of lithium ions from LNVP. In addition, the shift from lower to higher angle side could be correlated to the decrease in the crystalline interplanar spacing due to the extraction of lithium ions. Further, the coexistence of two phases such as Li2NaV2(PO4)3 and NaV2(PO4)3 has been observed, especially when LNVP cathode is half charged or half discharged, which in turn is an evidence for the involvement of deintercalation/ intercalation mechanism. Interestingly, when the cathode is fully discharged (one cycle), the diffraction peaks revert back to their original position, corresponding to the formation of 13104
DOI: 10.1021/acs.jpcc.7b04160 J. Phys. Chem. C 2017, 121, 13101−13105
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The Journal of Physical Chemistry C
Crystalline VO2-Graphene Ribbons as Cathodes for Ultrafast Lithium Storage. Nano Lett. 2013, 13, 1596−1601. (3) Hercule, K. M.; Xu, L.; Minhas-Khan, A.; Zhang, Q. Z. One-Pot Synthesized Bicontinuous Hierarchical Li3V2(PO4)3/C Mesoporous Nanowires for High-Rate and Ultralong-Life Lithium-ion Batteries. Nano Lett. 2014, 14, 1042−1048. (4) Gaubicher, J.; Wurm, C.; Goward, G.; Masquelier, C.; Nazar, L. Rhombohedral Form of Li3V2(PO4)3 as a Cathode in Li-Ion Batteries. Chem. Mater. 2000, 12, 3240−3242. (5) Huang, H.; Yin, S. C.; Kerr, T.; Taylor, N.; Nazar, L. F. Nanostructured Composites: A High Capacity, Fast Rate Li3V2(PO4)3/Carbon Cathode for Rechargeable Lithium Batteries. Adv. Mater. 2002, 14, 1525−1528. (6) Jian, Z.; Han, W.; Liang, Y.; Lan, Y.; Fang, Z.; Hu, Y.-S.; Yao, Y. Carbon-Coated Rhombohedral Li3V2(PO4)3 as both Cathode and Anode Materials for Lithium-ion Batteries: Electrochemical Performance and Lithium Storage Mechanism. J. Mater. Chem. A 2014, 2, 20231−20236. (7) Morcrette, M.; Leriche, J.-B.; Patoux, S.; Wurm, C.; Masquelier, C. In Situ X-Ray Diffraction during Lithium Extraction from Rhombohedral and Monoclinic Li3V2(PO4)3. Electrochem. Solid-State Lett. 2003, 6, A80−A84. (8) Zhu, C. B.; Song, K. P.; van Aken, P. A.; Maier, J.; Yu, Y. CarbonCoated Na3V2(PO4)3 Embedded in Porous Carbon Matrix: An Ultrafast Na-Storage Cathode with the Potential of Outperforming Li Cathodes. Nano Lett. 2014, 14, 2175−2180. (9) Karuppiah, S.; Vellingiri, S.; Nallathamby, K. Newer Polyanionic Bio-composite Anode for Sodium ion Batteries. J. Power Sources 2017, 340, 401−410. (10) Ohzuku, T.; Ueda, A.; Yamamota, N. Zero-Strain Insertion Material of Li [Li1/3Ti5/3]O4 for Rechargeable Lithium Cells. J. Electrochem. Soc. 1995, 142, 1431−1435. (11) Yi, T.-F.; Yang, S.-Y.; Xie, Y. Recent Advances of Li4Ti5O12 as a Promising Next Generation Anode Material for High Power Lithiumion Batteries. J. Mater. Chem. A 2015, 3, 5750−5777. (12) Cushing, B. L.; Goodenough, J. B. Li2NaV2(PO4)3: A 3.7 V Lithium-Insertion Cathode with the Rhombohedral NASICON Structure. J. Solid State Chem. 2001, 162, 176−181. (13) Du, K.; Guo, H.; Hu, G.; Peng, Z.; Cao, Y. Na3V2(PO4)3 as Cathode Material for Hybrid Lithium ion Batteries. J. Power Sources 2013, 223, 284−288. (14) Song, W.; Ji, X.; Pan, C.; Zhu, Y.; Chen, Q.; Banks, C. E. A Na3V2(PO4)3 Cathode Material for use in Hybrid Lithium ion Batteries. Phys. Chem. Chem. Phys. 2013, 15, 14357−14363. (15) Tang, Y.; Wang, C.; Zhou, J.; Bi, Y.; Liu, Y.; Wang, D.; Shi, S.; Li, G. Li2NaV2(PO4)3: A Novel Composite Cathode Material with High Ratio of Rhombohedral Phase. J. Power Sources 2013, 227, 199− 203. (16) Song, W.; Ji, X.; Yao, Y.; Zhu, H.; Chen, Q.; Sun, Q.; Banks, C. E. A Promising Na3V2(PO4)3 Cathode for use in the Construction of High Energy Batteries. Phys. Chem. Chem. Phys. 2014, 16, 3055−3061. (17) Mao, W.-F.; Ma, Y.; Liu, S.-K.; Tang, Z.-Y.; Fu, Y.-B. Facile Synthesis of Hybrid Phase Li2NaV2(PO4)3 and its Application in Lithium ion Full Cell: Li2NaV2(PO4)3∥Li2NaV2(PO4)3. Electrochim. Acta 2014, 147, 498−505. (18) Zhang, Y.; Nie, P.; Shen, L.; Xu, G.; Deng, H.; Luo, H.; Zhang, X. Rhombohedral NASICON-Structured Li2NaV2(PO4)3 with Single Voltage Plateau for Superior Lithium Storage. RSC Adv. 2014, 4, 8627−8631. (19) Jian, Z.; Yuan, C.; Han, W.; Lu, X.; Gu, L.; Xi, X.; Hu, Y.-S.; Li, H.; Chen, W.; Chen, D.; Ikuhara, Y.; Chen, L. Atomic Structure and Kinetics of NASICON NaxV2(PO4)3 Cathode for Sodium-Ion Batteries. Adv. Funct. Mater. 2014, 24, 4265−4272.
mesopores present in HHC may increase the electrode electrolyte contact area, reduce the path length for the facile diffusion of lithium ions, and hence, outstanding rate capability could be realized. In addition, defect triggered faster reaction kinetics of lithium ions is possible due to the presence of dual heteroatoms in HHC. Precisely, the synergistic effect of rhombohedral structure of LNVP along with the presence of dual heteroatom doped HHC as composite additive in LNVP/ HHC composite qualifies the LNVP/HHC as a high rate and strain-free cathode for LIBs.
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CONCLUSIONS By triggering a simple in situ and site selective ion exchange mechanism through aging process in the cell assembly, the three plateau cycling behavior of monoclinic Li3V2(PO4)3 electrode has been reduced to the desired single flat plateau behavior, corresponding to the one-step two-electron transfer kinetics of Li2NaV2(PO4)3, produced from rhombohedral Na3V2(PO4)3 with the preferred characteristics for wider application in LIBs. The in situ formed LNVP, in synergy with the added HHC in its LNVP/HHC composition, exhibits negligible polarization at 0.1 C up to 200 cycles with the outstanding theoretical capacity of 118 mAh g−1, thus deserving itself to be recommended as a more or less zero strain cathode for practical LIB applications, endorsed further with the outperforming rate capability behavior at 20 C rate for 5000 cycles. To our knowledge, the in situ formed LNVP/HHC cathode of the present study is the ever first strain free cathode demonstrated for its performance in lithium ion cell assembly.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04160. Additional data containing detailed experimental procedures, ancillary structural characterization, and electrochemical performances helpful to an in-depth understanding of the investigated subject (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kalaiselvi Nallathamby: 0000-0002-6782-3234 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.S. is grateful to CSIR for the CSIR-SRF grant. Financial support from Council of Scientific and Industrial Research (CSIR) and Department of Science and Technology through MULTIFUN and GAP-14/16 program is gratefully acknowledged.
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REFERENCES
(1) An, Q.; Xiong, F.; Wei, Q.; Sheng, J.; He, L.; Ma, D.; Yao, Y.; Mai, L. Nanoflake-Assembled Hierarchical Na3V2(PO4)3/C Microflowers: Superior Li Storage Performance and Insertion/Extraction Mechanism. Adv. Energy Mater. 2015, 5, 1401963. (2) Yang, S. B.; Gong, Y. J.; Liu, Z.; Zhan, L.; Hashim, D. P.; Ma, L. L.; Vajtai, R.; Ajayan, P. M. Bottom-up Approach toward Single13105
DOI: 10.1021/acs.jpcc.7b04160 J. Phys. Chem. C 2017, 121, 13101−13105
