In situ formation of porous LiCuVO4/LiVO3/C nanotubes as a high-capacity anode material for lithium ion batteries

Rong Cui a, Jiande Lin a, Xinxin Cao a, Pengfei Hao a, Xuefang Xie a, Shuang Zhou a, Yaping Wang a, Shuquan Liang ab and Anqiang Pan *ab
aSchool of Material Science and Engineering, Central South University, Changsha 410083, Hunan, China. E-mail: pananqiang@csu.edu.cn
bKey Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Central South University, Changsha 410083, Hunan, China

Received 18th August 2019 , Accepted 2nd October 2019

First published on 2nd October 2019


LiCuVO4 has attracted increasing attention as an anode material for lithium ion batteries due to its good structural stability. However, the rate capability is limited by the inferior electronic conductivity and low lithium ion diffusion efficiency. Herein, we report the synthesis of LiCuVO4/LiVO3/C porous nanotubes by an electrospinning method with subsequent calcination in air. The unique morphology, carbonaceous material in the composite and the existence of LiVO3 enable the as-prepared composite to have high capacity, superior cycling stability and rate capability. The LiCuVO4/LiVO3/C electrode possesses a capacity of 636 mA h g−1 which is higher than the theoretical capacity of LiCuVO4 (576 mA h g−1). Moreover, it can retain 77% of the capacity at 0.5 A g−1 after 300 cycles. These results demonstrate that LiCuVO4/LiVO3/C nanotubes are promising anode materials for lithium ion batteries.


1. Introduction

The fast depletion of fossil fuel resources and the resulting environmental issues have stimulated the exploration of alternative green and sustainable energy.1–3 Li ion batteries have attracted great attention in energy storage applications.4 Moreover, the rapid development of hybrid electric vehicles (HEVs) and portable electronic devices also inspire the demand for Li ion batteries with high specific capacity, excellent rate capability as well as long reversible cycle life.5–8 Nevertheless, the state-of-the-art commercial graphite anode is unable to meet the increasing requirements of higher capacity, better rate capability and improved safety because of its relatively low theoretical capacity and the formation and growth of lithium dendrite.9–11 Therefore, it would be of great interest to develop advanced anode materials to fulfil the requirements. Vanadate materials as attention-attracting anode materials for Li ion batteries have multiple valence, structural superiority, safe working potential and the ability to deliver a much higher capacity.12–14

Recently, LiMVO4 (M = Co, Ni, Cu) has attracted increasing attention as anode materials for lithium ion batteries because of their high capacity, good structural superiority and favourable safety.15,16 The spinel type structure of LiMVO4 has vacant lattice sites for the insertion of Li ions.17,18 LiCuVO4 has a distorted inverse spinel structure because of the Jahn–Teller effect of Cu ions.19 The CuO6 octahedra chains are coupled along a margin in the cardinal plane and extend along the b axis, and form regular triangles in the ac plane. The LiO6 octahedra chains extend along the a axis and the VO4 tetrahedra chains are disconnected with each other. Hence, many lattice vacancies are created to facilitate Li+ intercalation and de-intercalation.19 LiCuVO4 was first explored in lithium ion batteries with high capacity in 1991 by R. Kanno et al.20 Li et al. studied the Li+ storage mechanism of LiCuVO4 electrodes and found that LiCuVO4 decomposed into Li3VO4 and Cu nanoparticles, in which Cu is not involved in the electrochemical redox reaction but improved the electronic conductivity.17 Recently, Mishra et al. synthesized LiCuVO4 by a solid state method with ball-milling, which had a relatively low discharge voltage platform and good capacitive charge storage.21

Great efforts have been devoted to improve the electrochemical performances of LiCuVO4 electrodes. However, the rate performance is still not satisfactory because of the large and heterogeneous grain size, and inferior electronic conductivity. One possible strategy to solve this problem is by making composites with conductive materials, and has been proven to be an effective way to improve the rate performances of other electrode materials.22–24 For example, Zhang et al. reported the fabrication of V2O5/carbon dodecahedra by a template-based approach, which exhibit enhanced rate capability and cycling stability.25 Wang et al. reported the fabrication of V2O5/graphene composites by a solvothermal method with subsequent annealing in air, which exhibit superior rate capability.26 However, the synthesis of LiCuVO4/carbon composites is rarely reported. The difficulty is mainly attributed to the required high temperature and oxidation atmosphere for the synthesis of LiCuVO4, while carbonaceous materials normally cannot tolerate high temperature. Another strategy to optimize the electrochemical performance is by designing the structures of the electrode materials rationally, which can enlarge the contact area between the electrode materials and the electrolyte, reduce the lithium ion diffusion distances and improve the structural stability upon cycling.27–29 Nanostructured LiCuVO4 with a high specific surface area has been reported as a kind of high performance electrode material.30 Although LiCuVO4 has relatively high theoretical capacity, it would be great if the specific capacity can be further improved when it is combined with other materials with a higher capacity. To date, reports on making LiCuVO4 based composites with other active materials with high capacity are limited. LiVO3 is another material with a higher theoretical capacity, but with inferior capacity retention.31 It would be interesting to mimic the advantages of both electrode materials by making a LiCuVO4/LiVO3 composite, which would possibly result in higher capacity and improved cycling stability.

Herein, we report the in situ formation of LiCuVO4/LiVO3/C nanotubes by an efficient electrospinning method with subsequent calcination in air. As an anode material for lithium ion batteries, the prepared composite electrode exhibits a higher capacity than the theoretical one for LiCuVO4. Moreover, it shows superior rate capability. This excellent electrochemical performance is attributed to the presence of carbonaceous materials in the composite, porous tube-like structure and the higher capacity contribution from LiVO3.

2. Experimental section

Synthesis of materials

Analytical grade copper acetate monohydrate (Cu(CH3COO)2·H2O), lithium acetate dihydrate (CH3COOLi·2H2O), vanadium acetone oxide (C10H14O5V), polyacrylonitrile (PAN) and N,N-dimethylformamide (DMF) were used as received without further treatment. 2 mmol Cu (CH3COO)2·H2O, 2 mmol CH3COOLi·2H2O, 2 mmol C10H14O5V, and 0.5 g PAN were dispersed in 5 ml of DMF under vigorous stirring for 12 h at room temperature. Then the obtained dark blue solution was transferred to an injection syringe for electrospinning. The solution was pumped continuously at a rate of 0.06 mm min−1, and the high voltage on the syringe needle was 20 kV. The collector, which is placed 15 cm away from the needles, was a cylindrical stainless roller covered by aluminium foil. Then the as-spun composite fibers were first annealed at 250 °C for 40 min to stabilize the polymer using a muffle furnace and then calcined in air at 450 and 500 °C for 0.5 h to get LiCuVO4/LiVO3/C. The obtained material was designated as LCVO-450 and LCVO-500, respectively.

Materials characterization

The morphology and structure of the samples were examined using a scanning electron microscope (SEM, FEI Nova Nano SEM230) and a transmission electron microscope (TEM, JEOL JEM-2100F). X-ray diffraction (XRD, Rigaku D/Max 2500) with non-monochromated Cu Kα radiation (λ = 1.54178 Å) was used to investigate the crystallographic structures of the resulting products. Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449C analyzer in air from room temperature to 650 °C with a heating rate of 10 °C min−1. Raman spectroscopy was performed by using a Raman spectrometer (Lab RAM HR800). The chemical and electronic states of each elements in the sample were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher-VG Scientific, UK).

Electrode fabrication and electrochemical measurement

The electrochemical properties of the as-prepared LiCuVO4 were tested by assembling coin-type half cells with lithium foil as the counter electrode. The working electrodes were prepared by coating slurry, in which LiCuVO4/LiVO3/C composites as active materials, Super P as the conductive additive, and carboxymethyl cellulose (CMC) as the binder dissolved in distilled water were mixed in a weight ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1 on a copper foil current conductor. The electrode films were dried in a vacuum oven at 100 °C for 10 hours in succession. The mass loading of the LiCuVO4/LiVO3/C anode material for coin cell testing was about 0.8–1.5 mg cm−2. All coin cells were assembled in a glovebox filled with ultra-high pure argon gas. The separators of the cells were Celgard 2320 membranes. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) with a volume ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]8. Galvanostatic charge–discharge cycling was then evaluated at room temperature on a Land-2001A (Wuhan, China) automatic battery tester in a voltage range of 0.01–3.0 V vs. Li+/Li. The cyclic voltammetry (CV) curves were detected by using an electrochemical workstation (CHI660E, China) in the range from 0.01–3.0 V vs. Li+/Li.

3. Results and discussion

The formation process of the porous LiCuVO4/LiVO3/C nanofiber network and its advantages as an anode material for lithium ion batteries are illustrated in Fig. 1. Cu (CH3COO)2·H2O, CH3COOLi·2H2O, C10H14O5V, and PAN were dissolved in DMF under vigorous stirring to form a dark blue solution. Then the homogeneous solution was electrospun into a wathet blue nanofiber membrane. After stabilization in air at 250 °C and subsequent treatment at 450 °C, the nanofibers were turned into porous LiCuVO4/LiVO3/C nanotubes. The carbonaceous materials can improve the conductivity of the electrode materials and the porous tube-like structure can increase the penetration of the electrolyte and reactive site for redox reactions. Moreover, the existence of LiVO3 in the composite can pull the capacity limit of the LiCuVO4/LiVO3/C composite electrodes.
image file: c9qi01040h-f1.tif
Fig. 1 Schematic illustration of the fabrication process and the proposed structural advantages of LiCuVO4/LiVO3/C nanotubes.

Fig. 2a shows the XRD pattern of the LCVO-450 composite. The Rietveld refinement method (using GSAS-EXPGUI software package) was used to refine the X-ray diffraction pattern to analyze the crystal structures and phases of the material. The lattice parameters and phase contents of LiCuVO4 and LiVO3 in the LCVO-450 composite are listed in Table S1. The sample was scanned in the range between 10° and 80° (2θ) with a scanning rate of 2° min−1. The composite is composed of orthorhombic LiCuVO4 (space group: Imma, 15836-ICSD) and monoclinic LiVO3 (space group: C2/c (15), 2899-ICSD), with no evidence of undesired parasitic phases. The observed and calculated patterns match well, and the reasonably small R factor (<10.0%) indicates that the refinement results are convincing. The mass contents of LiCuVO4 and LiVO3 are 24.6% and 75.4% based on the multi-phase refinement. Fig. S1 displays the XRD pattern of the LCVO-500 composite, which is also indexed to the LiCuVO4 and LiVO3 phases. More concretely, the characteristic peaks located at 18.4°, 20.4°, 34.6° and 36.3° are attributed to (001), (002), (103) and (121) planes of the LiCuVO4 phase. Moreover, the characteristic peaks located at 14.0°, 26.6°, 29.1°, and 32.4° can be indexed to (110), (021), (311) and (002) planes of the LiVO3 phase. LiVO3 is normally formed in air between 350 °C to 550 °C.32 As LiVO3 has a higher theoretical capacity for lithium ion storage, the existence of LiVO3 in the composite may further improve the capacity of the prepared composite materials.31,33–35


image file: c9qi01040h-f2.tif
Fig. 2 (a) XRD pattern of LCVO-450, (b) Raman scattering spectra of LCVO-450 and LCVO-500 samples, (c) TG/DSC curves of the porous LCVO-450 nanotubes annealed from room temperature to 650 °C at a temperature ramping rate of 10 °C min−1 in air.

Fig. 2b displays the Raman scattering spectra of the as-prepared samples. The Raman spectrum of the LCVO-450 exhibits two characteristic peaks at 1333 and 1612 cm−1, which correspond to the D and G bands of carbonaceous materials.36 The G band derives from sp2 hybridized carbon and the D band results from disordered carbon.37 The peak intensity ratio of ID/IG is about 2.16, suggesting the existence of many disorder sites and defects in LCVO-450. However, no characteristic peaks of the D and G bands are detected for LCVO-500. The Raman spectrum confirms the existence of carbon in the LCVO-450. To figure out the formation of carbon in the as-prepared LCVO-450, the Raman spectroscopy of the precursor (LCVO-Precursor) and precursor after stabilization (LCVO-250) was performed and the results are shown in Fig. S2. No characteristic peaks corresponding to the D or G band of carbon were detected for LCVO-Precursor. However, LCVO-250 shows two weaker peaks at 1349 and 1550 cm−1 related to the D and G band of carbon, which meant polyacrylonitrile starts to decompose during the preliminary stabilization. The LCVO-250 peak intensity ratio of ID/IG is about 1.2. When the calcination temperature reaches 450 °C, the peak intensity of the target samples became much stronger. Moreover, the ratio of ID to IG for LCVO-250 was larger than that of LCVO-250, indicating the pyrolytic carbon at 450 °C has more defective sites.

Thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses were carried out to quantify the content of carbon in the composites. The two samples were heated from room temperature to 650 °C with a heating rate of 10 °C min−1 in air. The detection of mass loss from the TG curve and the exothermic peak from the DSC curve of LCVO-450 between 400 and 550 °C (Fig. 2c) indicate the oxidation of carbon.38 The endothermic peak at 598.1 °C is related to the melting point of the LCVO-450 composite. The weight loss above 600 °C is on account of lithium volatizing at a high temperature. According to the TG analysis result, the weight percentage of carbon in the LCVO-450 is about 4.07%. It is well known that residual carbon is helpful to improve the conductivity of the electrode, which is important for redox based electrochemical reactions.22,39 However, neither the obvious weight loss in the TG curve and nor the exothermic peak in the DSC curve are detected for the LCVO-500 (Fig. S3), which demonstrates no carbon in LCVO-500.

The morphology and microstructure of the samples were characterized by SEM and TEM techniques. Fig. 3a and b show the SEM images of LCVO-450, which are composed of uniform porous nanotubes connecting with each other to form a network morphology. The nanotubes are about 300 nm in diameter. As shown in Fig. 3b, clear porous-like morphologies are observed on the surface of the nanotubes. The elemental mapping results (Fig. 3c and d) demonstrate the homogenous distribution of Cu, V, O, and C in LCVO-450 nanotubes. Furthermore, the TEM image (Fig. 3e) reveals the hollow interior of the nanotubes and the tube-like morphology. The formation of the hollow structure can be attributed to the oxidation of the carbonaceous materials in the nanofibers. The high-resolution TEM (HRTEM) image (Fig. 3f) shows the lattice fringes with the interlayer distance of 0.435 nm and 0.211 nm, which are in good accordance with the (002) crystal plane of LiCuVO4 and the (040) plane of LiVO3, respectively. For comparison, the LCVO-500 sample loses the tube morphology (Fig. S4) and exhibits a short rod-like structure. The average diameter of the short rods is about 1 μm. Besides, the rods intersect with each other and stack up to form a coral-like morphology.


image file: c9qi01040h-f3.tif
Fig. 3 (a and b) SEM images, (c and d) elemental mapping images, (e) TEM image, and (f) HRTEM image of the porous LCVO-450 nanotubes.

XPS was employed to further investigate the chemical components and element valences of the LCVO-450. As is shown in Fig. 4a, the survey scan spectrum indicates that the LCVO-450 is composed of lithium, copper, vanadium, oxygen and carbon. In the high resolution Cu 2p spectrum (Fig. 4b), the peaks at the binding energies of 934.2 and 954.0 eV are indexed to Cu2+ 2p3/2 and Cu2+ 2p1/2, respectively. Besides, a strong satellite peak at 942.4 eV can correspond to a typical character of Cu2+ 2p3/2.30 In the high resolution V 2p spectrum (Fig. 4c), the peak at V 2p3/2 (517.3 eV) and V 2p1/2 (524.7 eV) can be assigned to V5+. In conclusion, the valence of Cu and V in the prepared LCVO-450 is 2+ and 5+, respectively. In the high resolution C1s spectrum (Fig. 4d), there are three peaks located at 284.8 eV, 285.9 eV, and 289.2 eV corresponding to sp2 carbon atoms, sp3 carbon atoms and O–C[double bond, length as m-dash]O, respectively.40,41


image file: c9qi01040h-f4.tif
Fig. 4 XPS result of the LCVO-450 sample. (a) XPS survey, and high-resolution XPS spectra of (b) Cu 2p, (c) V 2p, (d) C 1s of the LCVO-450.

The LCVO-450 and LCVO-500 samples were assembled into coin cells to evaluate their electrochemical performances. Fig. 5a shows the cyclic voltammetry (CV) curves of the LCVO-450 electrode at a scan rate of 0.1 mV s−1 in a voltage range from 0.01 to 3.0 V (vs. Li+/Li). In the first cathodic scan, the sharp peak which appears at about 0.2 V can be ascribed to the decomposition of the electrolyte and the formation of solid electrolyte interface (SEI) films.42 Besides, two reduction peaks at 1.68 V and 1.18 V are detected, which are related to the reduction of Cu2+ to Cu metal with the insertion of lithium ions and the insertion of lithium ions to Li3VO4 associated with the reduction of V5+ to V3+.43–45 Moreover, the peak at 1.89 V in the first anodic scan can be attributed to the extraction of Li ion from Li3+yVO4.17,46 Besides, the anodic peak corresponding to the oxidation from Cu0 to Cu2+ at about 2.6 V was not detected, which indicates that the reduced metallic copper is not oxidized in the following cycles and is beneficial for promoting the electronic conductivity of the electrode material.30 The following CV curves are largely overlapped, implying a high reversibility of the LCVO-450 anode upon cycling. Fig. 5b displays the galvanostatic charge/discharge curves of the LiCuVO4 electrode at the 1st, 2nd, 10th and 50th cycles at the current density of 100 mA g−1. An obvious platform at 1.18 V in the initial discharge curve is in good consistence with the CV curves (Fig. 5a). Moreover, the initial discharge/charge capacities of the LCVO-450 electrode were 910 mA h g−1 and 590 mA h g−1, respectively, with a coulombic efficiency of 64.8%. After 10 cycles, the coulombic efficiency increases gradually to 98.7% and it reaches 99.5% at the 50th cycle. The relatively low coulombic efficiency of the first cycle may be attributed to the decomposition of the electrolyte and the formation of a SEI layer on the surface of the LCVO-450 electrode.


image file: c9qi01040h-f5.tif
Fig. 5 The electrochemical performances of the LCVO-450 and LCVO-500 electrodes in the potential range of 0.01–3 V versus Li+/Li. (a) The CV curves of LCVO-450 at a scan rate of 0.1 mV s−1 and (b) galvanostatic discharge/charge profiles of LCVO-450 at 0.1 A g−1, (c) cycling performance of the LCVO-450 and LCVO-500 electrodes at a current density of 0.1 A g−1, (d) rate performance of the LCVO-450 and LCVO-500. (e) Long-term cycling performance of the LCVO-450 and LCVO-500 at 0.5 A g−1.

As shown in Fig. 5c, LCVO-450 possesses a higher capacity and better cycling performance than LCVO-500 at a current density of 0.1 A g−1. An initial capacity of 910 mA h g−1 was obtained and it decreased to 576 mA h g−1 after 50 cycles. The capacity fading rate become slower for the later cycles. The LCVO-450 electrode has higher capacity than the theoretical capacity of LiCuVO4 of 576 mA h g−1,17 which can be attributed to the existence of LiVO3 in the composite and its unique structures. Although LiVO3 is normally reported as a cathode material for lithium ion batteries, it has been rarely reported as an anode material for lithium ion batteries which may be due to its inferior cycling stability. In our work, by making the composites of LiCuVO4 and LiVO3, the capacity of LiCuVO4 can be enlarged and the cycling stability of LiVO3 can be improved. As reported by other researchers,47,48 the one-dimensional tube morphology could offer a higher specific surface area, shorter Li+ diffusion pathways and faster electron transfer along the longitudinal direction that lead to excellent cycling performance. Fig. 5d shows the rate performances of both LCVO-450 and LCVO-500 electrodes between 0.01 and 3 V vs. Li+/Li. The LCVO-450 delivers the specific discharge capacities of 666, 550, 431, 320, and 129 mA h g−1 at the current densities of 0.1, 0.2, 0.5, 1.0 and 2.0 A g−1, respectively. When the current density is reset to 0.1 A g−1, a discharge capacity of 592 mA h g−1 can be recovered, suggesting good rate capability and structural stability of LCVO-450. However, the LCVO-500 electrode delivers a capacity of 469, 400, 323, 238, and 144 mA h g−1 at the same ladder-shaped current densities. Moreover, a capability of 484 mA h g−1 can be obtained when the current capacity changes to 0.1 A g−1. Even at a high active material mass loading of 1.32 mg cm−2, a superior rate performance is still delivered, further indicating the high availability of the active material (Fig. S5). Fig. 5e shows the long-term cycling stability of the two electrodes at the current density of 0.5 A g−1. Likewise, LCVO-450 shows good cyclability and higher capacity at a high density of 0.5 A g−1. An initial discharge capacity of 539 mA h g−1 for LCVO-450 is obtained and it retains a capacity of 408 mA h g−1 (77% of the second discharge capacity) after 300 cycles. According to the cycling performances, both LCVO-450 and LCVO-500 electrodes have good capacity retention. However, the rate capability of LCVO-450 is much better. The more outstanding rate performance of the LCVO-450 is benefited from the tube-like hollow interior and conductive carbonaceous materials in LCVO-450. Interestingly, the LCVO-450 exhibits higher capacity and better rate performance than many previously reported LiCuVO4 electrodes.17,30 The electrochemical performances of the LCVO-450 composite and previously reported relevant materials are compared and summarized in Table S2, which indicates that the LiCuVO4/LiVO3/C composite exhibits enhanced electrochemical performance. The SEM images of the LCVO-450 electrode after rate-capability tests are shown in Fig. S6. It can be observed that a part of the LCVO-450 electrode still maintains a nanofiber morphology, which indicates the good structural stability.

To get a better insight into the difference in charge storage kinetics of LCVO-450 electrodes, sweep voltammetry measurements at different scan rates were carried out as shown in Fig. 6a. Three typical parts contribute to the total storage capacity, which are the faradaic contribution from the Li ion insertion process, the faradaic contribution from the surface-induced charge transfer process and the non-faradaic contribution from the electrical double-layer effect.49,50 The CV curves show consistent fluctuation at various scan rates from 0.1 to 2.0 mV s−1. By analysing these data according to the power-law relationship between the measured current (i) and sweep rates (v): i = avb where both a and b are adjustable parameters, the electrode reaction kinetics can be characterized.51,52 A b value of 0.5 signifies that the current is proportional to the square root of the scan rate which means the current is controlled by semi-infinite diffusion.53 By contrast, a b value of 1 suggests that the current is controlled by the capacitor-like nature of the kinetics. The b value is determined by the slope of the fitted line of log(v)–log(i) plots. The LCVO-450 exhibits a value of 0.66 (Fig. 6b), which means that the capacity contributions include both the surface-controlled pseudocapacitive behaviour and diffusive behavior.41 The ratio of the capacitive and diffusive contribution at different scan rates is further analysed in detail according to the following equation:

 
i (V) = k1v + k2v1/2(1)
where the current i (V) can be separated into the capacitive-controlled process (k1v) and diffusion contribution (k2v1/2) (k1 and k2 both are constants).54,55 From the results shown in Fig. 6c, 55% of the total capacity is derived from the capacitive contribution at a scan rate of 1 mV s−1. The percentage of the capacitive contribution of LCVO-450 at different scan rates is summarized in Fig. 6d. Along with the increase of the scan rate, the capacitive contribution increases. All the results indicate that the special porous nanotube structure with defective sites caused by carbonization and the composite contributes an appropriate amount of pseudocapacitance, which can provide electrochemical performance improvements.56


image file: c9qi01040h-f6.tif
Fig. 6 (a) CV curves obtained at different scan rates, (b) corresponding log (peak current) versus log (sweep rate) plots and the corresponding fitting line, (c) capacitive contribution and diffusion contribution to lithium storage at 1.0 mV s−1, (d) normalized contribution ratio of capacitive and diffusive contribution at different scan rates for LCVO-450 electrodes.

4. Conclusions

In summary, we prepared porous LiCuVO4/LiVO3/C nanotube networks by an electrospinning method and subsequent annealing in air. The formation of the porous tube-like structure is attributed to the decomposition of PAN and its oxidation in air. The annealing temperature is a key factor for obtaining carbonaceous materials in the composite. As anode materials for lithium ion batteries, LiCuVO4/LiVO3/C nanotubes exhibit superior electrochemical performances, including high capacity, good rate capability and cycling stability.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51874362, 51872334), the Natural Science Foundation of Hunan Province, China (Grant No. 2018JJ1036), and the Innovation Project of Central South University (2017CX001). The authors also thank the Advanced Research Center of CSU for performing the HRTEM examination.

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Footnote

Electronic supplementary information (ESI) available: Supplementary figures and tables. See DOI: 10.1039/c9qi01040h

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