3D graphene-encapsulated Li3V2(PO4)3 microspheres as a high-performance cathode material for energy storage

Journal of Alloys and Compounds 723 (2017) 873e879

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3D graphene-encapsulated Li3V2(PO4)3 microspheres as a high-performance cathode material for energy storage Yisheng Hu a, c, *, Xin Ma b, Ping Guo a, Frederike Jaeger c, Zhouhua Wang a a

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, PR China School of Science, Southwest University of Science and Technology, Mianyang 621000, PR China c Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 February 2017 Received in revised form 27 June 2017 Accepted 29 June 2017 Available online 29 June 2017

In this study, we report a promising structural design of the 3D graphene-encapsulated Li3V2(PO4)3 microspheres (3D-Li3V2(PO4)3/G) by using a facile spray-drying method with one-step calcination. XRD results indicate that the as-prepared composite shows a single monoclinic Li3V2(PO4)3 without any impurity phases. SEM and TEM images reveal that all the particles of 3D-Li3V2(PO4)3/G are spherical with diameters of about 5 mm and the surface of Li3V2(PO4)3 particles are tightly covered by soft graphene sheets, forming a conductive network. This unique structure of the composite offers a synergistic effect to facilitate the transport of electrons and Liþ ions. As the advanced cathode for lithium-ion batteries, the obtained 3D-Li3V2(PO4)3/G displays good high-rate capability and long cycling performance between 3.0 and 4.8 V (vs. Li/Liþ). It delivers an initial specific capacity of 187 mAh g1 at 0.1 C, which is close to the theoretical maximum value (197 mAh g1). More remarkably, it presents a superior discharge capacity of 146 mAh g1 at 20 C with capacity retention of about 95.7% over 100 cycles. Combined with the advantages of high voltage and high theoretical capacity, the 3D-Li3V2(PO4)3/G cathode material would be a potential cathode material for next-generation lithium-ion batteries. © 2017 Elsevier B.V. All rights reserved.

Keywords: Lithium-ion batteries Li3V2(PO4)3 Graphene sheets 3D microspheres Cathode

1. Introduction Nowadays, with the rapid development of portable electronics and electrical vehicles, it is urgently needed to explore rechargeable batteries with high energy density for next-generation energy-storage devices and systems [1,2]. As the potential candidate, the NASICON-type monoclinic Li3V2(PO4)3 cathode in lithium-ion batteries has been widely investigated due to its outstanding properties. The Li3V2(PO4)3 material has three-dimensional pathways for Liþ ions insertion/extraction, which facilitate fast migration of the Liþ ions inside the bulk material [3e5]. Li3V2(PO4)3 cathode can completely extract all three Liþ ions when it is charged to a higher voltage of 4.8 V, which results in a theoretical capacity of 197 mAh g1 [6e8]. Moreover, the Li3V2(PO4)3 also shows a good safety, low cost, excellent thermal stability, high cell-voltage and high-power density [5,9]. Nevertheless, the poor intrinsic electronic conductivity (ca. 2.4  108 S cm1) [10] of the pure Li3V2(PO4)3 material lead to a low

* Corresponding author. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, PR China. E-mail address: [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.jallcom.2017.06.315 0925-8388/© 2017 Elsevier B.V. All rights reserved.

coulombic efficiency, poor rate capability and bad cycling performance. Actually, this factor limits its practical application for energy storage, which can be attributed to the two separated [VO6] octahedral arrangement [3,11,12]. In order to improve the lithium storage performance of the pure Li3V2(PO4)3 material, many effective approaches have been explored: (I) Reducing the particle size to nanoscale to increase the surface area and shorten the diffusion pathway for electron and Liþ ion [13,14]. For instance, Kim's group reported that the Li3V2(PO4)3/C with nanostructured morphology was fabricated via a one-step pyrosynthesis and it exhibited a high reversible capacity of 190.3 mAh g1 within the potential window of 3e4.8 V [14]. (II) Combining with conductive carbon layer [7,15e18], carbon nanotube [19] or graphene sheet [20e26] to increase the apparent electronic conductivity of the Li3V2(PO4)3-based composite. This strategy is an economic and feasible technique which can supply a highly conductive network for both electrons and Liþ ions, promoting the fast charge/discharge process [27]. Among these carbon materials, graphene sheet is the most promising candidate to improve the properties of material [28]. (III) Doping with an appropriate amount of other metal ions (e.g. Kþ [29], Ge4þ [30], Ti4þ [31]), Fe3þ [32]), which can enhance the intrinsic

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electronic conductivity of Li3V2(PO4)3 material. Although many efforts have been devoted, the high-rate capability and long cycling performance are still worth exploring deeply. As the effective strategy to improve the lithium storage performance of Li3V2(PO4)3 material, various synthetic approaches such as solid-state method [8,33], microwave heating route [34], sol-gel method [17,26], hydrothermal route [14,35], carbothermal reduction method [36,37], spray-drying method [38] and electrospinning route [39e41], have been proposed. Among these mentioned approaches, the spray-drying method is a simple and effective way to synthesize the spherical powders [42]. Besides, the particle size distribution of the product is controllable from the micrometer to the nanometer range, and the composition of the powder is easy to control. For instance, Cao's group [42] reported a facile spraydrying method to prepare the graphene-decorated Na3V2(PO4)3 microspheres in which Na3V2(PO4)3 nanoparticles are embedded in graphene sheets to form porous microspheres. In this regard, this method has also been employed to synthesize spherical Fe- and Aldoped Li3V2(PO4)3 cathode material for application in lithium-ion batteries [38]. The obtained Li3V2(PO4)3/C microspheres present excellent electrochemical performance with high power density. Therefore, the spray-drying approach is inferential to be suitable for preparing 3D graphene-scaffolded electrodes for electrochemical energy storage [43]. To the best of our knowledge, there is no research focusing on the fabrication of 3D grapheneencapsulated Li3V2(PO4)3 microspheres with high tap-density using this simple and effective spray-drying method. In this work, we first report the graphene-decorated Li3V2(PO4)3 microspheres with a 3D structure as cathode material for lithiumion batteries using the spray-drying and subsequent calcination method. In the composite, the crumpled 3D graphene not only increases the electronic conductivity and reactive sites for the electrochemical reaction but also exhibits excellent stability during the charge/discharge process. As a comparison, the 3D-Li3V2(PO4)3 was also prepared by the same method. The structures, morphologies and electrochemical properties of the products have been schematically investigated in this study. 2. Experimental 2.1. Preparation and characterization The 3D-Li3V2(PO4)3/G sample was prepared by a facile spraydrying route followed by a one-step annealing process. Note that the added graphene was fabricated through a modified Hummers method, which can be applied to the treatment of petroleum production wastewater. The precursor solution for atomization was an aqueous mixing solution of Li2CO3, V2O5, NH4H2PO4, citric acid and graphene oxide. Here, the citric acid was only employed as the reduction agent. In a typical synthesis, the citric acid was firstly dissolved in the deionized water and then stoichiometric amounts of Li2CO3, V2O5, and NH4H2PO4 were dispersed into the solution with continuous stirring for 40 min. Thirdly, the graphene oxide solution (3 mg mL1) was added under stirring for 2 h. After ultrasonically exposed for 15 min, the solution was dried in a spray-dryer at 170  C to form a solid precursor. Finally, the obtained precursor was calcined at 750  C for 12 h under flowing Ar/H2 (95:5, v/v) atmosphere in a tube furnace to yield the 3D-Li3V2(PO4)3/G composite. For comparison, the pristine 3D-Li3V2(PO4)3 sample was also prepared using the same process without adding graphene oxide to the precursor solution. The crystal structures of the as-prepared powders were analyzed with an X-ray diffraction measurement (XRD, Rigaku DMAX-33) using Cu Ka radiation in a 2q range from 15 to 60 . The morphologies and microstructures were investigated by scanning electron

microscopy (SEM, JEOL JSM-5600LV) and transmission electron microscopy (TEM, JEOL JEM-2100F). Raman spectra of 3D-Li3V2(PO4)3/G sample was carried out on a laser Raman spectrometer (Bruker, SENTERRA) with the 514 nm line of an Ar ion laser as the excitation source. The chemical composition of 3D-Li3V2(PO4)3/G was analyzed using the energy dispersive X-ray spectroscopy (EDX). 2.2. Electrochemical measurements The electrochemical performances of the obtained 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G as cathode materials for lithium-ion batteries were tested using CR2032 coin-type cells. The working electrodes were fabricated by mixing 85 wt% active materials, 10 wt% Super p and 5 wt% polyvinylidene fluorides (PVDF) dissolved in the Nmethylpyrrolidinone (NMP) solvent. Then, the mixed slurry was stirred and spread uniformly on an aluminum foil. The coated foil was dried in the vacuum oven at 100  C for 15 h. The electrodes were punched in the form of 14 mm diameter disks and the typical working electrode loading was about 2.9 mg cm2. The theoretical capacity of Li3V2(PO4)3 coin cell was about 0.747 mAh. Finally, the cells were assembled and sealed in a high-purity argon-filled glove box using metal lithium (theoretical capacity, 177 mAh) as the anode, Cellgard 2400 film as the separator, and 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, v/v) as the electrolyte. The galvanostatic charge/discharge tests were performed at various current rates between 3.0 and 4.8 V (Li/Liþ) using a LAND CT2001 tester (Wuhan, China). The actual capacity of cathode material was calculated based on the weight of Li3V2(PO4)3 material without the carbon (Super p, graphene) and PVDF. The electrochemical impedance spectroscopy (EIS) was conducted on an electrochemical workstation (CHI660A) with an ac amplitude of 5 mV in the frequency range from 100 kHz to 0.01 Hz. The cyclic voltammetric (CV) test was performed with the CHI660A electrochemical analyzer at a scan rate of 0.1 mV s1 in the voltage range of 3.0 and 4.8 V (Li/Liþ). The electronic conductivities of the as-prepared 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G cathode materials were tested using the fourelectrode method. In the experimental section, the galvanostatic charge/discharge measurements were tested by using the twoelectrode coin cells. However, the potential polarizability of lithium anode which may be caused by the passive surface films is existed in this two-electrode system, and this phenomenon has been evidenced in the previously reported literature [44e47]. Although the anode polarizability has a negative effect on the lithium storage performance of Li3V2(PO4)3 cathode, it is equal to all the coin cells in this research. 3. Results and discussion XRD patterns of the as-prepared 3D-Li3V2(PO4)3 and 3DLi3V2(PO4)3/G powders are shown in Fig. 1a. All the diffraction peaks for both samples can be assigned to the single phase of monoclinic Li3V2(PO4)3 (JCPDS: 080-1515) with P21/n space group [48], confirming the successful conversion of the precursor prepared by spraydrying method into a pure phase after one-step calcination. The results are consistent well with that of carbon-decorated Li3V2(PO4)3 spheres fabricated using the spray-drying route [37,38]. Furthermore, the characteristic graphene diffraction peak of (002) crystal face at around 26 can be also vividly observed for the 3D-Li3V2(PO4)3/G composite. There is no peaks related to the graphene oxide at 11, which demonstrates that the graphene oxide has been reduced to graphene during the annealing process. The 3D-Li3V2(PO4)3/G composite was further investigated by Raman spectra analysis as illustrated in Fig. 1b. Obviously, there are two peaks located at around 1345 and 1572 cm1 in the profile, which are assigned to the D-band and G-band, respectively. This

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Fig. 1. (a) XRD patterns of the obtained 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G samples; (b) Raman spectra of 3D-Li3V2(PO4)3/G composite.

phenomenon indicates that the Li3V2(PO4)3 nanoparticles are embedded in the conductive graphene network. Here, the D-band is attributed to the defects and disordered portions of carbon (sp3coordinated), whereas the G-band corresponds to the ordered graphitic crystallites of carbon (sp2-coordinated) [49,50]. The peak intensity ratio of D-band and G-band (ID/IG) can offer an important index for studying the degree of graphitization [51]. According to Fig. 1b, the ID/IG of 3D-Li3V2(PO4)3/G is calculated to be 1.09, indicating a relatively high degree of graphitization. To investigate the morphologies of the obtained 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G samples prepared by spray-drying method, SEM was carried out shown in Fig. 2. It can be seen from Fig. 2a that the 3D-Li3V2(PO4)3 product consists of secondary quasi-spherical particles with diameters of about 4 mm. These spherical particles can significantly increase the tap-density of electrode for lithiumion batteries with high-power density [42]. The SEM also indicate that each microsphere is actually a random aggregate of primary Li3V2(PO4)3 with particle size of about 100e300 nm. For the 3DLi3V2(PO4)3/G sample (Fig. 2b), it exhibits the spherical particles with diameters largely distributed around 5 mm. Besides, there is a typical wrapped and crinkled morphology of graphene sheet on the surface of the composite. This formed 3D conductive network between the Li3V2(PO4)3 and graphene is benefit to facilitate the transport of electrons and Liþ ions. The nanostructures of 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G were further analyzed using TEM, and the typical TEM images are presented in Fig. 3. According to Fig. 3a, the image indicates that the 3D-Li3V2(PO4)3 particles have spherical shapes. The TEM observation on the edge of an individual microsphere also reveals that its texture is smooth with no carbon layer visible on the surface. As shown in Fig. 3b and c, the spherical particles of 3D-Li3V2(PO4)3/G are composed of a large number of graphene sheets and nanosized Li3V2(PO4)3 grains with the size of about 100 nm. These graphene sheets are uniformly distributed on/in the micrometer-sized 3DLi3V2(PO4)3/G spheres, which could further establish a conductive network through the whole microsphere. In Fig. 3d, the graphene sheets are clearly visible and the Li3V2(PO4)3 particles are anchored onto the surface of graphene sheets. The inset in Fig. 3d shows the TEM image of 3D-Li3V2(PO4)3/G after 100 cycles at 20 C. It is found that the graphene sheets and Li3V2(PO4)3 nanoparticles can still be clearly identified and the overall electrode morphology is retained even after 100 cycles. This designed structure would benefit to the electrochemical performance of the 3D-Li3V2(PO4)3/G composite. The typical TEM image (Fig. 4a) and the corresponding elemental mapping (Fig. 4bee) for 3D-Li3V2(PO4)3/G composite confirm the presence of V, P, O and C elements in the individual particles. Determined from the element analysis, the total carbon contents in

Fig. 2. SEM images of (a) 3D-Li3V2(PO4)3 and (b) 3D-Li3V2(PO4)3/G products prepared by the spray-drying method.

the 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G samples are estimated to be about 0.5 wt% and 6.4 wt%, respectively. The Li storage performances of the as-prepared 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G electrodes were tested in coin-type cells. Fig. 5 shows the 1st and 30th charge/discharge curves for the materials at a low rate of 0.1 C in the potential range of 3.0 and 4.8 V. When the cells were charged to 4.8 V, all three Liþ ions could be

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Fig. 3. Typical TEM images recorded for the as-prepared (a) 3D-Li3V2(PO4)3 and (bed) 3D-Li3V2(PO4)3/G powders.

Fig. 4. TEM image (a) of 3D-Li3V2(PO4)3/G and the corresponding elemental mapping of (b) vanadium, (c) phosphorous, (d) oxygen and (e) carbon.

extracted from the structure of Li3V2(PO4)3 over four two-phase electrochemical plateaus at 3.60, 3.70, 4.10 and 4.55 V, which correspond to different phases of LixV2(PO4)3 at x ¼ 3.0, 2.5, 2.0, 1.0 and 0.5, respectively [52]. Moreover, three plateaus at around 4.1, 3.6 and 3.5 V can be also observed during the discharge process [35]. A high initial discharge capacity of 187 mAh g1 is achieved for the 3D-Li3V2(PO4)3/G composite at 0.1 C, reaching 95% of its theoretical capacity (197 mAh g1). It can still deliver a specific capacity of 183 mAh g1 after 30 cycles with a capacity retention ratio of 97.5%. For the bare 3D-Li3V2(PO4)3, it shows a low capacity of 156 mAh g1 over 30 cycles which is only 91% of its initial capacity (172 mAh g1). The bad cycling performance can be attributed to

the poor electronic conductivity of Li3V2(PO4)3 material [10]. The rate performances for 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G at various current rates between 3.0 and 4.8 V are illustrated in Fig. 6a. The bare 3D-Li3V2(PO4)3 electrode delivers low reversible capacities of 168, 163, 154, 144, 130 and 115 mAh g1 at current rates of 0.2, 0.5, 1, 2, 5 and 10 C, respectively. Besides, the cycling performance is also poor at each current rate. As expected, the 3DLi3V2(PO4)3/G shows excellent rate capacities, delivering discharge capacities of 185, 183, 180, 174 and 169 mAh g1 at 0.2, 0.5, 1, 2 and 5 C, respectively. Even at high rate of 10 C, it can still reach a specific capacity of 157 mAh g1 with capacity retention of 98.9% over 15 cycles. When the current rate was reduced back to 0.2 C, the capacity

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Fig. 5. The 1st and 30th charge/discharge profiles of the as-prepared 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G electrodes at a low rate of 0.1 C in the potential range of 3.0 and 4.8 V.

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frequency region. It should be mentioned that the semicircle is reflective of the charge-transfer impedance (Rct) which relates to the resistance between the electrolyte and the active material, whereas the line is attributed to the Liþ ions diffusion in the solid phase [56,57]. The fitting value of Rct for the 3D-Li3V2(PO4)3/G composite is 82.6 U, superior to 263.5 U of the 3D-Li3V2(PO4)3 electrode. The decreased Rct value is probably expected to overcome the kinetics restrictions during the charge/discharge process and thus enlarge the depth of Liþ ions insertion/extraction [58]. What's more, the electronic conductivities of 3D-Li3V2(PO4)3 and 3DLi3V2(PO4)3/G materials were tested using the four-electrode method. The results show that the value of conductivity for 3DLi3V2(PO4)3/G is as high as 3.9  102 S cm1, which is much better than that of bare 3D-Li3V2(PO4)3 (6.5  107 S cm1). Furthermore, the Li-ion diffusion coefficient of 3D-Li3V2(PO4)3 and 3DLi3V2(PO4)3/G electrodes are calculated to be on the order of ~1012 cm2 s1 and ~109 cm2 s1, respectively. Thus, it can be seen

Fig. 6. (a) Comparison of the rate performances for 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G electrodes at various current rates between 3.0 and 4.8 V; (b) The cycling performance of 3D-Li3V2(PO4)3/G material at 20 C for 100 cycles.

for 3D-Li3V2(PO4)3/G can almost recover its initial capacity value. The rate performance of 3D-Li3V2(PO4)3/G composite is also better than that of other graphene-decorated Li3V2(PO4)3 electrodes [53e55]. The detailed results are summarized in Table 1. Fig. 6b shows the cycling performance of 3D-Li3V2(PO4)3/G at 20 C for 100 cycles. As expected, the 3D-Li3V2(PO4)3/G exhibits stable cycling performance with a capacity decay rate of 4.3% over 100 cycles. The improved performance of 3D-Li3V2(PO4)3/G can be attributed to the introduction of the graphene sheets which can act as a highly conducting and flexible supporter to facilitate the transport of electrons and Liþ ions [43] and accommodate structural variation during the process of Liþ ions deintercalation/intercalation. To understand the improved electrochemical performance of 3D-Li3V2(PO4)3/G, the EIS measurement was performed. As illustrated in Fig. 7a, both the EIS profiles are composed of a depressed semicircle at the high frequency region and a straight line at the low

that the graphene sheets play an important role in enhancing the electronic/ionic conductivity of the 3D-Li3V2(PO4)3/G composite. The CV curves of the 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G electrodes at a scan rate of 0.1 mV s1 are illustrated in Fig. 7b. Obviously, both 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G exhibit four peaks in the charge process and three peaks in the discharge process, indicating that phase conversions occurred during the Li-ion extraction/insertion [24]. This phenomenon is consistent well with the charge/discharge results. Compared with the 3DLi3V2(PO4)3 electrode, the oxidation peaks of 3D-Li3V2(PO4)3/G move to the negative direction while the reduction peaks move to the positive direction. That is to say, the potential difference between the oxidation peaks and reduction peaks for 3D-Li3V2(PO4)3/ G electrode is reduced, which reveals the smaller polarization tendency and easier extraction/insertion of Li-ion in the 3DLi3V2(PO4)3/G lattice.

Table 1 Comparison of the rate capability for the 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G electrodes in this work and other previously reported literature. Samples

Preparation method

Electrochemical performance

3D-Li3V2(PO4)3 3D-Li3V2(PO4)3/G Li3V2(PO4)3/G Li3V2(PO4)3/(GþC) Li3V2(PO4)3/rGO

Spray drying Spray drying Sol-gel Spray drying Solid-state route

168, 185, 168, 182, 195,

163, 154, 144, 130 and 115 mAh g1 at 0.2, 0.5, 1, 2, 5 and 10 C 183, 180, 174, 169 and 157 mAh g1 at 0.2, 0.5, 1, 2, 5 and 10 C 153 and 150 mAh g1 at 0.1, 1 and 2 C 174, 167, 158, 137 and 116 mAh g1 at 0.1, 0.2, 0.5, 1, 2 and 5 C 160 and 117 mAh g1 at 0.5, 2 and 5 C

Ref. In this work In this work [50] [49] [51]

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Fig. 7. (a) Nyquist plots of 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G in the frequency range from 100 kHz to 0.01 Hz; (b) CV curves of the 3D-Li3V2(PO4)3 and 3D-Li3V2(PO4)3/G electrodes at a scan rate of 0.1 mV s1.

4. Conclusions In conclusion, the 3D-Li3V2(PO4)3/G composite has been successfully prepared by a facile spray-drying route followed by a onestep calcination. The obtained product shows a well define monoclinic structure and good crystallization. The particle size of 3DLi3V2(PO4)3/G is 2e5 mm, which is suitable for commercial application. When tested as cathode for lithium-ion batteries, the composite exhibits outstanding rate capability and cycling stability. The excellent electrochemical property can be mainly ascribed to the improved electronic/ionic conductivity arising from the designed 3D conductive network. These primary results illustrate that the 3DLi3V2(PO4)3/G could be a promising cathode for lithium-ion batteries with high energy density. Importantly, this novel strategy provides a potential way for fabricating other 3D graphenedecorated electrodes for energy storage. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652e657. [2] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928e935. [3] W.F. Mao, Y.B. Fu, H. Zhao, G. Ai, Y.L. Dai, D.C. Meng, X.L. Zhang, D.Y. Qu, V.S. Battaglia, Z.Y. Tang, Rational design and facile synthesis of Li3V2(PO4)3@C nanocomposites using carbon with different dimensions for ultrahigh-rate lithium-ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 12057e12066. [4] S.C. Yin, H. Grondey, P. Strobel, H. Huang, L.F. Nazar, Charge ordering in lithium vanadium phosphates: electrode materials for lithium-ion batteries, J. Am. Chem. Soc. 125 (2003) 326e327. [5] X. Rui, Q. Yan, M.S. Kazacos, T.M. Lim, Li3V2(PO4)3 cathode material for lithium-ion batteries: a review, J. Power Sources 258 (2014) 19e38. [6] Q.Z. Ou, Y. Tang, Y.J. Zhong, X.D. Guo, B.H. Zhong, H. Liu, M.Z. Chen, Submicrometer porous Li3V2(PO4)3/C composites with high rate electrochemical performance prepared by sol-gel combustion method, Electrochim. Acta 137 (2014) 489e496.

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