HollowâCuboid Li3VO4

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Hollow-cuboid Li3VO4/C as HighPerformance Anodes for Lithium-Ion Batteries Changkun Zhang, Chaofeng Liu, Xihui Nan, Huanqiao Song, Yaguang Liu, Cuiping Zhang, and Guozhong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09810 • Publication Date (Web): 10 Dec 2015 Downloaded from http://pubs.acs.org on December 16, 2015

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ACS Applied Materials & Interfaces

Hollow-cuboid Li3VO4/C as High-Performance Anodes for Lithium-Ion Batteries Changkun Zhanga, Chaofeng Liua, Xihui Nana , Huanqiao Songa, Yaguang Liua, Cuiping Zhanga, and Guozhong Cao*ab

a

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences,

Beijing, China b

Department of Materials Science and Engineering, University of Washington,

Seattle, Washington, United States. E-mail: [email protected]

Keywords: Li3VO4, hollow-cuboid, F127, anode, lithium ion batteries Abstract

Li3VO4 has been demonstrated as a promising anode material for lithium-ion batteries with a low, safe voltage and large capacity. However, its poor electronic conductivity hinders its practical application particularly at a high rate. This work reports Li3VO4 coated with carbon was synthesized by a one-pot, two-step method with F127 ((PEO)100–(PPO)65–(PEO)100) as both template and carbon source yielding a micro-cuboid structure. The resulting Li3VO4/C cuboid shows a stable capacity of 415 mAh g-1 at 0.5 C and excellent capacity stability at high rates (e.g., 92% capacity retention after 1,000 cycles at 10 C = 4 A g-1). The lithiation/delithiation process of Li3VO4/C was studied by ex-situ X-ray diffraction and Raman spectra, which confirmed that Li3VO4/C underwent a reversible intercalation reaction during discharge/charge 1

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processes. The excellent electrochemical performance is attributed largely to the unique micro-hollow structure. Not only can the voids inside hollow structure provide more space to accommodate volume change during discharge/charge processes, but also allow the lithium ions insertion and extraction from both outside and inside the hollow structure with much large surface area or more reaction sites and shorten the lithium ions diffusion distance, which leads to smaller overpotential and faster reaction kinetics. Carbon derived from F127 through pyrolysis coats Li3VO4 conformably and thus, offers good electrical conduction. The results in this work provide convincing evidence that the significant potential of hollow-cuboid Li3VO4/C for high power batteries.

1. Introduction The development of high energy and high density rechargeable batteries with good safety and long cycle life is imperative for the next generation of portable electronics and hybrid and all electric cars1-4. In spite of the great success in commercialization and the wide spread applications, the current lithium ion batteries (LIBs) with graphite as the anode generally suffer from poor performance in fast charge/discharge processes with potential safety concerns1, 5. Alternative anode materials with high storage capacity and high rate capacity are needed, in addition, the alternate anode must also meet safety requirements. Recently, Li3VO4 (LVO) has been studied as one of the alternate anode material for the next generation LIBs6-7. LVO consists of corner-sharing VO4 and LiO4 tetrahedrons7. LVO has small volume changes during lithiation/delithiation processes and fast diffusion of lithium ions8-9. 2


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In comparison with other anodic materials under intensive study, lithium ions can be intercalated to LVO between 0.5 and 1.0 V vs. Li/Li+, for example, appreciably lower than that of Li4Ti5O12 (~1.56 V vs. Li/Li+), and has a high theoretical capacity of 394 mAh g-1, corresponding to x = 2 in Li3+xVO4 with the end discharging voltage of 0.2 V9. However, the pristine LVO exhibited poor rate capacity due to its low electrical conductivity (~5.8× 10-6 S cm-1 for LVO10). Hybridization with carbon has proved as an effective way to enhance electrical conductivity, leading to high performance9, 11-15. For example, carbon encapsulated LVO synthesized presented exceedingly good rate capability (a reversible capability of 450, 340, and 106 mAh g-1 at 0.1 C, 10 C, and 80 C, respectively) and long cycling performance (80% capacity retention after 2000 cycles at 10 C)11. Other carbon materials like graphite13, carbon nanotube15, and graphene9, 14, 16 have also been reported to coat on LVO. Shi et al.9 synthesized a hollow LVO/graphene microbox composites by sol-gel method And the composites showed a reversible capacity of 378 mAh g-1 at 0.1 C rate. Hollow LVO/carbon nanotube has also been reported15, though the high rate performance was not attained. This paper reports a one-pot, two-step method for the synthesis of hollow LVO/C micro-cuboid composites with excellent lithium ion storage properties. The Pluronic®F127 was not only selected as template to form hollow structure, but also served as carbon source in this method. The synthesis was accomplished via a sol-gel method and can potentially be scalable to large quantities for industrial production. The resulting unique hollow LVO/C micro-cuboid composite exhibits high capability of 3



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481 mA g-1 at 0.1 C. When at 5 C, and 20 C, it can yield a reversible capability of 329 and 230 mA h g-1 respectively. After 1000 cycles at 10 C, the hollow LVO/C material can deliver 92% capacity retention. The synthesis mechanism and the relationship between the nano-/micro-structure and much enhanced electrochemical properties of the resulting LVO/C cuboid composite anode have been discussed.

2. Experimental section Material Synthesis and Characterization. The LVO/C samples were synthesized by a sol-gel method. The Pluronic®F127 block copolymer was first dissolved in water and ethanol solution. And then 0.41 g V2O5 (0.225 mol L-1, TianJin·FUChemical, ＞ 99.0%) and 0.568 g LiOH·H2O (1.35 mol L-1, Sinopharm Chemical Reagent, ＞98.0%) was added, the mixture was then sonicated for 10 min and stirred for 5 h at room temperature. The solution pH is 12.3, we did not control the pH during the process. The precursor (LVO-F127) was dried and collected at 55oC. The precursors were subsequently treated at 450, 600, and 750oC respectively for 2 h in 1 atm Ar (＞ 99.999%) to form LVO/C-450, LVO/C-600, and LVO/C-750 composites. The F127 concentration in the solution was set as 0.5, 1.0, and 5.0 % (w/v) respectively. The LVO without F127 template was also synthesized, and the annealed process was set at 600 oC for 2 h in Ar. The crystalline structure of the samples was detected by X-ray diffraction (XRD, MXP21 VAHF) using the Cu Kα radiation (λ=1.5418Å). The morphologies were characterized by the field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM, JEOL JEM-2010). The surface area was test 4


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by

N2

adsorption-desorption

analyses

(ASAP

2020

HD88).

Thermogravimetry/differential scanning calorimetry (TG/DSC) was conducted on a Simultaneous Thermal Analyzer (STA 449F3, NETZSCH) from 25 to 600oC at 10 oC min-1. X-ray photoelectron spectra (XPS) were measured on an X-ray photoelectron spectrometer (ESCALAB 250Xi) with referencing the C 1s peak to 284.6 eV. Raman spectra were collected with a Horiba JOBIN YVON Raman system (LabRAM HR Evolution) using an argon ion laser (532 nm) as the excitation source. Electrochemical Characterization. Electrochemical performances were evaluated with standard CR2032 coin cells. The electrode consisted of 75 wt % of active material, 20 wt % of acetylene black and 5 wt % of sodium carboxymethyl cellulose (CMC) as binder on Cu foil with a loading mass of 1 mg cm-2. Then the electrode was dried at 120 oC for 12h. The separator was NKK TF4840, and the electrolyte was 1 M LiPF6 solution in ethylene carbonate (EC)/dimethyl carbonate (DMC) (EC/DMC=1:1 in volume). All cells were assembled in a glove box filled with water and oxygen contents ＜0.1 ppm using Li metal as the counter and reference electrode. The galvanostatic charging/discharging measurements were conducted on a LAND (Wuhan, China) automatic battery tester. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) was measured on a Solartron Instrument. The frequency of EIS measurement ranged from 100 kHz to 0.1 Hz.

3. Results and Discussion

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Fig. 1 Schematic diagram of the synthesis procedure of the hollow LVO/C micro-cuboid composite.

Fig. 1 shows the schematic diagram of the synthesis procedure. The Pluronic®F127 copolymer has the (PEO)100–(PPO)65–(PEO)100 structure with a central block of poly(propylene oxide), PPO, and end blocks of poly(ethylene oxide), PEO. In the sol-gel method, the F127 was first dissolved in water and ethanol solution. And then V2O5 and LiOH was added, the yellow mixture turned to a white dispersion after sonication for 10 min and stirred further for 5 h at room temperature. The precursor LVO-F127 was dried at 55oC. XRD pattern shown in Fig. 2a reveals that the precursor LVO-F127 has already transformed into LVO completely (JCPDS No. 38-1247). The total reaction equation was as follows16:

V2O5 +2OH- →2VO3- +H2O

(1)

LiVO3 +2LiOH→Li3VO4 +H2O

(2)

The TGA result at O2 atmosphere (Fig. S1) reveals that the template F127 would decomposed above 350 oC. The precursor was subsequently treated at high temperatures for 2 h to form LVO/C composites. The XRD analysis in Fig. 2a exhibits that all the samples could be the LVO orthorhombic phase, without any preferential crystal orientation and thus suggesting a polycrystalline material. In addition, there are no detectable shift of XRD peaks, indicating the different annealing temperature did introduce neither impurities nor ionic defects. In Fig. 2b, the slowly mass 6


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decreasing from the TG at O2 can be attributed to the carbon oxidation along with the increasing of temperature. The carbon content in the LVO/C-600 composite is about 7.1 wt.%, which corresponding to the endothermic DSC peak at about 350 oC.

Fig. 2 (a) XRD patterns of the different composites. (b) The TG/DSC curves of the LVO/C-600. (c) V 2p XPS spectra of the LVO/C-600. (d) C 1s XPS of the LVO/C-600.

Fig. 2c-d shows the V 2p and C 1s XPS spectra of LVO/C-600 . The peak at 525.5 and 517.6 eV can be ascribed to V2p1/2 and V2p3/2 electrons for V in the pentavalent state which matches well with previous report for pure LVO13. The peak at 523.9 eV corresponds to an X-ray satellite of O 1s17. Combined with the XRD and XPS results, It can be concluded that the residual carbon in Ar gas at 1 atm would not reduce the vanadium ions from pentavalent to tetravalent state. The results are different from the findings reported in our previous paper11, which can be attributed to the different synthetic method. In the present study, the LVO phase has already formed before heat treatment, while in our earlier work the in-stiu carbon coating through 7



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reduction of organic molecules was introduced during the synthesis and formation of LVO with vanadium(IV) acetylacetonate and lithium hydroxide as precursors11. A small amount of tetravalent vanadium ions could be found in other pentavalent vanadium oxides from metal doping such as Mo-LiV3O818 and Ni19, Sn20 doping V2O5 and insert atmosphere annealing like N2 annealed V2O5 xerogel21. There have three peaks at 284.6, 286.2, and 289.5 eV for C 1s. The first peak arises from C=C on the surface of LVO, and the other two suggest the existence of C-O and C=O, respectively indicating the formation of carbonated species22-23. The morphologies of the composites were characterized using the SEM and TEM systems. In Fig. S-2a, without surfactant F127 as template, the pristine LVO particles have an irregular shape with a particle size of 0.8-2.0 μm. With a low concentration of F127 (0.5 w/v %), the synthesized LVO/C particles are of approximately 350-500 nm, which is much smaller than that of the pristine LVO, and some particles aggregated to form sheets (Fig. S-3a-b). Small particle size in LVO/C with large surface area would facilitate the lithium ions transport. Fig S-4 shows the results of nitrogen sorption analyses; the specific surface area was found to be 11.9 and 2.7 m2 g-1 for LVO/C-600 and LVO samples. With an increased concentration of F127 (1.0 w/v %, Fig. S3c), more LVO/C sheets were formed. Some sheets turned to form porous micro-hollow structure. When the concentration of F127 increased further to 5.0 w/v %, only a hollow micro-cuboid structured LVO with size of ~13 × 5 μm formed with the wall thickness of 450 nm (Fig. 3a-c). The above results clearly indicate the presence of surfactant F127 plays an important role in the formation of cuboid 8


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structure. Annealing at 450 and 600 oC in Ar has little effect on the morphologies of the LVO/C composites (Fig. S-5a); however annealing at 750 oC resulted in a little broke of some hollow LVO cuboids (Fig. S-5b). From SEM images of sample LVO/C-600 in Fig. 3a-c, one can see that the hollow LVO/C micro-cuboid is assembled by small primary particles of 400 nm in diameter. The HRTEM image of LVO/C-600 composite (Fig. 3d) shows the lattice space of 0.39 nm which can be identified to the (011) plane.

Fig. 3 (a-c) SEM images of the LVO/C-600 sample; (d) HRTEM image of the LVO/C-600 sample.

The introduction of F127 has resulted in the formation of much smaller LVO particles, suggesting the presence of F127 has promoted the nucleation of LVO. F127 may serve as initial nucleation sites and may also retard the diffusion of growth species. The concentration of surfactant F127 used in the present study for the synthesis of LVO is much lower than the critical micelle-formation concentration24-25. Although, the 9



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subsequent solvent evaporation concentrates the solution in water and F127 and LVO species, the LVO/C with low F127 concentration (0.5 and 1.0 w/v %) did not receive the hollow structure (Fig. S-4a-c), which confirmed the LVO phase has already formed during the stirring process. At low F127 concentration, the F127 in the solution favors the LVO nucleation resulting in small particle. With the increasing of F127 concentration, more PPO group of F127 interacted with hydrophobic LVO induces the particles to assembly sheet and cuboid structure, and the subsequent solvent evaporation strengthens this interaction. The hollow-cuboid structure has collapsed when annealing the LVO-F127 at Air atmosphere (Fig. S-4d) where F127 has been total removed which exhibits the F127 has important effects to form the hollow-cuboid structure. The phenomenon is different from other metal oxides because of the easy synthesis of LVO at room temperature26-27. The energy-dispersive X-ray (EDX) mapping images of C, O, and V elements shown in Fig. 4 were used to analyse the elementary distribution in LVO/C-600 sample. The elemental mapping images in Fig. 4 display the existence and uniform distribution of C, O, and V elements, which indicates that C distributes uniformly the LVO/C. It is demonstrated that F127 in this work not only can be taken as template to form hollow/sheet structure LVO, but also as carbon source coated onto the LVO uniformly.

10


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Fig. 4 SEM image, EDX, and mapping of the C, O and V elements for the LVO/C-600 sample.

The coin cells were overcharged between 0.2 and 3.0 V vs. Li/Li+ at 40 mA g-1 (0.1 C). Galvanostatic discharge and charge curves shown in Fig. 5a-b demonstrated the lithium ions intercalation occurs mainly at a voltage range of 1.0 V and 0.5 V vs. Li/Li+. Form the charge/discharging flat difference of the electrodes, the LVO/C-600 has lower polarization than the pristine LVO especially at second cycle. The lower polarization can be derived from the carbon coated onto the LVO uniformly in the LVO/C-600 sample. The first discharge capacities of the cells with LVO and LVO/C-600 displayed 469 and 604 mAh g-1. The LVO/C-600 electrode delivered a higher discharge capacity than the bare LVO. At second cycle, the discharge capacities of LVO and LVO/C-600 electrodes are 326 and 481 mAh g-1 which are lower than the initial discharge, resulting from the formation of the solid electrolyte interface (SEI) film. The phenomenon can also be seen in other anode materials such as SnO228, CoO29, and CoSx30. The discharge capacity of LVO/C-600 is higher than the theoretical value (394 mAh g-1). Although, LVO/C composites in other works also have high 11



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capacity beyond the theoretical value14, 31-32, there is no explanation about it. Jian et al. considered that the higher capacity of the LVO/graphene was contributed from graphene which was also taken as conductive agent in the electrode16. It is likely a result of the acetylene black’s contribution33, which has been discussed in our previous work11.

Fig. 5 (a-b) Discharge and charge curves of LVO and LVO/C-600 electrodes for the first and second cycles at 0.1 C. (c-d) CV curves of the LVO and LVO/C-600 electrodes for the first and second cycles with the scan rate of 0.2 mV s-1.

Fig. 5c-d show the CV curves of the electrodes for the first and second cycles at 0.2 mV s-1. In the 1st cycle of the LVO/C-600, two reduction peaks were found at 0.56 and 0.36 V, which are attributed to the insertion of Li+ into LVO and the formation of solid electrolyte interphase (SEI)9, 13, and two oxidation peaks at 1.09 V and 1.35 V corresponds to the oxidation of LVO. At the 2nd cycle, the oxidation peaks shifted from 1.09 V to 1.10 V and from 1.35 V to 1.34 V, and the two reduction peaks shifted 12


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from 0.56 V to 0.77 V and from 0.36 to 0.52 V. These changes are usually ascribed to the occurrence of side reactions on the electrode surfaces and interfaces due to the SEI formation as well as phase transformation16 which correspond to the differences of the charging/discharging curves (Fig. 5a-b). While for the pristine LVO electrode, only one pair of broad redox peaks was found at 1st cycle at ~ 0.45 V for reduction and 1.09 V for oxidation. However, two pairs of redox peaks appeared at the 2nd cycle, very similar to the LVO/C-600, all four peaks appeared at almost identical voltage positions as that of the LVO/C-600. It might be related to the formation of the SEI films and the insertion instability in the initial stage. In other works, CV curves show various amounts of peaks in the first cycle, but all show two peaks in the second cycle which correspond to 2 Li+ insertion in LVO9, 11-12, 16. It should be noted, however, that the LVO/C-600 exhibits higher current density suggesting that the LVO/C-600 electrode has better kinetic properties, suggesting better rate performance. To

investigate

the

structural

reversibility

of

LVO/C

upon

lithium

ions

insertion/extraction, we conducted the ex-situ XRD and Raman measurements on the LVO/C-600 electrode at different discharge/charge states. As shown in Fig. 6a, the main diffraction peaks of LVO can be observed for the electrode at open voltage (2.95 V). When discharged to 1.0 V, XRD pattern remains the same to that obtained at open voltage, showing maintenance of the LVO structure. The original LVO peaks become much weaker with the increase of depth of discharge. At 0.2 V, the LVO structure has changed largely; however, no new phase was found, which is different 13



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from the previous works, probably due to the instability of the new phase at ambient conditions6, 12, 14. In the following charge, all diffraction peaks of LVO were detected except for the peak at about 37.5. Meanwhile, the recovered peaks when charging to 3.0 V become a little broader indicating decreased crystallization of the original structure8, 12.

Fig. 6 (a) Ex-situ XRD patterns and (b) Raman spectra of electrochemically cycled LVO/C-600 electrode during the first discharge/charge process.

The Raman spectra shown in Fig. 6b were also confirmed that there is little change for the LVO-600 during the first discharge/charge. The band at about 815 cm-1 can be attributed to symmetric stretching of (VO43-), whereas the band at around 780 cm-1 can be attributed to asymmetric stretching of (VO43-)34. As shown in Table S1, the ratio of the peak intensity at around 815 and 780 cm-1 decreased with different depth 14


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of discharge and recovered when charged to 3.0 V. The peak of asymmetric stretching of VO43- also positively shifted from 815 to 822 cm-1 when discharged to 0.2 V, then come back to 819 cm-1 when recharged to 3.0 V, which can be conducted in the changing of the stretching vibration of VO43- due to lithium ions insertion/ extraction35. The electrochemical performance of the electrode was measured at different current densities. In Fig. 7a, the battery cells were cycled under different rates of 0.1-50 C. The LVO/C-600 electrode exhibited the higher capacity than the bare LVO electrode especially at a high rate. It can be seen that discharge capacities of LVO/C-600 electrode were about 420, 402, 371, 329, 280, and 230 mAh g-1 at discharge rates of 0.5 C, 1 C, 2 C, 5 C, 10 C, and 20 C respectively. For the LVO electrode, the capacities were found to be only 260, 213, 176, 123, 87, and 52 mAh g-1 at the corresponding discharging rates. At a current density as high as 50 C (20 A g-1), the capacity of LVO/C-600 electrode remained at 145 mA h g-1, which is three times more than that of LVO (40 mAh g-1). In Fig. S-6, the capacity of LVO/C-750 electrode is similar with LVO/C-600 electrode at low rate, however it has fast decay at high rate. The better performance of LVO/C-600 might be attributed to the homogeneous integration of the LVO with the carbon at 600 oC30. The electrochemical performance of the LVO/C-600 shows distinct improvement compared with those reported in the literature6, 9, 11-12, 14, 16, 31 (Table S-2).

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Fig. 7 (a) Discharge capacities of the electrodes at various rates; (b) Galvanostatic discharge and charge curves of LVO/C-600 at different rates; (c) Capacity retention and coulombic efficiency of LVO/C-600 at 0.5 C; and (d) Capacity retention and coulombic efficiency of the LVO and LVO/C-600 at 10 C (4 A g-1).

Fig. 7c shows the cycling performance of LVO/C-600 electrode at 0.5 C; the LVO/C-600 exhibits a very stable performance after the first cycle. The discharge capacity is 415 mAh g-1 after 50 cycles and the Coulombic efficiency remains constant close to 100%. The discharge capacity is slightly higher than charge capacity, though the difference is significantly smaller than our earlier results and diminishes with the increasing cycle numbers. We don’t have a definitely explanation yet; however, it is likely due to two possible mechanisms and requires more research. The first one is probably attributable to a little more Li+ insertion than extraction during cycling, the LVO/C composite may consist of either lithium ion vacancies in LVO or some lithium ions inserted and retained in carbon coating. The second is due to some irreversible reaction with impurities in electrolyte. Furthermore, as shown in Fig. 7d, both of LVO 16


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and LVO/C-600 electrodes exhibit long stable and reversible cycling property even at a high rate of 10 C (4 A g-1). A good cycling performance of the LVO/C-600 electrode was obtained with 92% capacity retention after 1000 cycles which is superior to the reported vanadium-based anode materials12.

Fig. 8 (a) Nyquist plots for the electrodes after three cycles (the AC amplitude was 5 mV, and at 2.54 V vs. Li/Li+); Inset: the equivalent circuit and the partial enlarged EIS at high frequencies . (b) the linear fitting of the Z’ vs. ω-1/2 relationship.

EIS measurement has been carried out between 100 kHz and 0.1 Hz at 2.54 V vs. Li/Li+. The Nyquist plots (Figure 8a) show that the charge transfer resistance (Rct) of both LVO and LVO/C-600 electrodes, is for LVO/C-600 (65 Ω) is much smaller than that of the carbon-free pristine LVO (150 Ω), suggesting that the LVO/C-600 electrode have faster kinetics for lithium ions insertion/extraction. The lithium diffusion coefficient, D, could be calculated from the Warburg region using the following equation12:

D = R2T 2 /2A2n4F 4C02σ2

(3)

where R is the gas constant, T is the absolute temperature, F is the Faraday constant, n is the number of electrons transferred per molecule, A is the active surface area of the electrode (0.50 cm2), C0 is the concentration of lithium ions in the cathode (9.8 × 17



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10-3 mol cm-3), D is the apparent ion diffusion coefficient, and σ is the Warburg factor, which is relative to Z’. From the slope of the lines in the inset of Fig. 8b can be obtained12:

Z' = RD + RL +σω−1/2

(4)

Using the value of Equation (3) and (4), the lithium diffusion coefficient D of LVO/C are calculated to be 2.17×10-16 and 1.75×10-15 cm2 s-1 (Table 1) for the LVO and LVO/C-600 electrodes respectively. The lithium diffusion coefficients of the LVO/C-600 composite have increased by an order of magnitude compared to the carbon-free LVO11. These results show that the LVO/C-600 composite presents smaller charge transfer resistance and higher lithium diffusion coefficient, which are favorable for improving the electrochemical performance that is in accordance with electrochemical results. Table 1 The discharge capacity, Rct, and D of the LVO and LVO/C-600 samples. Samples

Capacity at 0.1 C / mAh g-1

Rct / Ω

D / cm2 s-1

LVO

469

150

2.17×10-16

LVO/C-600

604

65

1.75×10-15

The superior battery performance of LVO/C-600 composite is attributed to its unique structural characteristics. First, the carbonization of F127 provides a continuous electron pathway in the LVO/C-600. Second, the LVO/C-600 hollow has the open and porous structure favoring the emission the strain/stress and the retaining good structural stability during discharge/charge process. Third, the LVO/C-600 composite 18


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exist as both smaller primary particles that generates larger surface area and surface energy.

4. Conclusions The carbon coated LVO hollow micro-cuboid material was synthesized by one-pot two-step method. F127 was taken as template to form hollow LVO micro-cuboid structure and then pyrolyzed to carbon uniformly coated onto LVO offering excellent electrical conductivity throughout the composite. The resultant LVO/C-600 composite demonstrated excellent lithium ion storage property with capacities of 481 mAh g-1 at 0.1 C, 366 mA h g-1 at 10 C, and 145 mA h g-1 even at 50 C (20 A g-1). After 1000 cycles at 10 C, the sample maintained 92% of the initial reversible capacity. The voids inside hollow structure provided more space to accommodate volume changes and shorten the lithium ions diffusion distance, which led to small overpotential and fast reaction kinetics. This strategy is facile and effective, and readily extended to the preparation of other carbon coated lithium transition metal oxides to enhance their lithium ion storage properties and the battery performances.

Supporting Information TG curve of F127 in O2; SEM images of the pristine LVO, LVO-F127, Li3VO4/C with 0.5 w/v % F127 and 1 w/v % F127 annealed in Ar at 600 oC; SEM images of the Li3VO4/C with 1 w/v % F127 annealed in Ar at 600 oC and 5 w/v % F127 annealed in Air at 600 o

C; N2 adsorption-desorption isotherm of the pristine LVO and LVO/C-600 samples;

SEM images of the Li3VO4 with LVO/C-450 and LVO/C-750 samples; The peak intensity 19



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of RM at around 815 and 780 cm-1; rate performance of LVO, LVO/C-450 and LVO/C-750; The comparison of the electrochemical performances with precious works. The Supporting Information is available free of charge on the ACS Publications website.

Acknowledgements This work was supported by the "thousands talents" program for pioneer researcher China, Postdoctoral Science Foundation of China (2015M570988), the National Science Foundation of China (51374029), Program for New Century Excellent Talents in University (NCET-13-0668), and Fundamental Research Funds for the Central Universities (FRF-TP-14-008C1).

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