Carbon-coated Li3V2(PO4)3 derived from metal-organic framework as cathode for lithium-ion batteries with high stability
Introduction
Lithium ion batteries are essential available alternative energy storage devices due to their superior electrochemical potentials and relatively low cost to make commercially viable energy packages. Lithium transition metal phosphates such as LiMPO4 (M = Fe, Co, Ni, Mn) [[1], [2], [3], [4], [5], [6]], Li3M2(PO4)3 (M = Fe, V, Ti) [[7], [8], [9], [10], [11], [12], [13]], and LiVPO4F [14,15] have been regarded as highly promising cathode materials for rechargeable lithium batteries. Among these mentioned phosphates, the monocline Li3V2(PO4)3 cathode has attracted great attention over the years due to its high average potential (4.0 V) and specific capacity (197 mA h g−1) with benign ion mobility and excellent thermal stability. [16] Unfortunately, Li3V2(PO4)3 has an essentially low electronic conductivity (2.4 × 10−7 S cm−1) as LiFePO4 (10−10 to 10−9 S cm−1), [17] which greatly restricts its practical applications.
Hence, various methods were applied to solve this problem, including designing and controlling microstructures [[18], [19], [20]], doping with foreign atoms [[21], [22], [23], [24]], and coating electronically conductive material such as carbon [[25], [26], [27], [28], [29]]. Carbon from different kind of sources has been widely applied to coat the electrode materials in order to improve their electrical conductivities. Besides, the electrochemical performance of Li3V2(PO4)3 was strongly influenced by the nature of carbon sources. For instance, Zhong et al. synthesized the Li3V2(PO4)3 samples by carbon-thermal reduction. It was shown that the leaving residual carbon was beneficial to the stabilization of V3+, resulting in greatly improved electrochemical performance. [30] Rui et al. prepared the carbon coated Li3V2(PO4)3 composites using four different carbon precursors: starch, polyvinylidene difluoride (PVDF), glucose, and citric acid as both carbon source and reducing agent. The best performance was achieved by those obtained by the pyrolysis of PVDF with a carbon content of 12.68%, leading to lower impedance and a discharge specific capacity of 95 mA h g−1 at 5C in 3.0–4.3 V [31].
Recently, the metal-organic frameworks (MOFs) have attracted an increasing attention due to their high specific surface areas and ultrahigh porosities, which are resulted from the enriched organic ligands and metal ions in the MOFs structure [[32], [33], [34], [35]]. MOFs have been also widely applied as precursors for synthesizing metal oxides and the carbon materials used for anodes in batteries [[36], [37], [38]]. However, there are few reports on employing MOFs to produce cathode materials.
In this work, we firstly attempted to employ the vanadium MOF [V3O(BDC)3(H2O)2Cl0.7(HBDC)0.3]·2H2O·0.5EtOH (MIL-101(V) [39]) as both carbon source and vanadium source, instead of V2O5, to synthesis the carbon-coated Li3V2(PO4)3 composite material. MIL-101(V) owns a rigid and mesoporous cages structure, exhibiting the largest specific surface area among vanadium MOFs [[39], [40], [41]]. By using the pyrolysised MIL-101(V) as the reaction processor, the as-synthesized carbon-coated Li3V2(PO4)3, referred to as LVP@M-101 in the following, exhibits laminated structures with some hollow bubbles inside and the residual carbon outside. The electrochemical performances of LVP@M-101 as cathode has also been investigated, especially with long charge-discharge cycles, to compare with the Li3V2(PO4)3 (LVP in the following) prepared using V2O5 as vanadium sources. Furthermore, we have employed the ex-situ XRD and EPR spectroscopy to investigate the mechanism of Li+ intercalation and deintercalation process.
Section snippets
Synthesis of MIL-101(V)
The MIL-101(V) was synthesized following a typically reported method [39]. H2BDC (10 mmol, 1.66 g) and VCl3 (10 mmol, 1.57 mg) were dissolved in 50 mL pure ethanol. After that, the mixture was transferred into 100 mL Teflon-lined autoclave and stirred for 30 min. After sonicating for 15 min, the autoclave was placed in an oven at 120 °C for 48 h. Then the autoclave was cooled down to room temperature in a fume hood. After washing with ethanol, the green powders were collected by centrifugation.
Results and discussion
We firstly show the XRD and TG analysis of the MIL-101(V) MOF prepared in this work in Figs. S1 and S2. It is shown that the diffraction peaks of activated MIL-101(V) match well with those of cubic structured Cr-MIL-101 [43] Therefore, the as-prepared MIL-101(V) compound is isostructural with Cr-MIL-101. TG analysis prove that the thermal decomposition of MIL-101(V) occurred around 200 °C. Further decomposition above 400 °C gave V2O5 as the only final product with a relative mass ratio of
Conclusion
The laminated structure Li3V2(PO4)3 carbon composite material has been synthesized by using the vanadium metal-organic framework (MIL-101(V)) precursor as both vanadium and carbon sources. The as-synthesized LVP@M-101 carbon composite materials exhibits an adequate carbon coating with some hollow bubbles inside, which would enhance specific surface area and decrease volumetric change during charge-discharge process. It is demonstrated that the unique structure in LVP@M-101 carbon composites
Acknowledgement
We are grateful to the financial support from National Natural Science Foundation of China (Grants 21522303, 21373086, and 21703068), National High Technology Research and Development Program of China (Grant No. 2014AA123401), and Large Instruments Open Foundation of East China Normal University. M.S. is grateful for the support from China Postdoctoral Science Foundation (Grant No. 2017M611491). We also would like to thank the support from National Magnetic Field Laboratory of the Chinese
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These authors contributed equally to this work.