| Monoclinic Li3V2(PO4)3 has been identified as a potential cathodic material for lithium ion battery applications due to its remarkably stable structure, high redox potentials and large specific capacity(197mAh·g-1).Li3V2(PO4)3/C and Li3V2(PO4)3/(C+Cu) composite materials were synthesized by a solid-state method or a solution method in this paper. The micro-structures and morphologies of these composites were investigated by XRD and SEM. The electrochemical performances were evaluated by galvanostatic charge-discharge, cyclic voltammetry and electrochemical impedance spectrum. The effects of the synthesis parameters on the physical and electrochemical properties of Li3V2(PO4)3 cathodic materials have been discussed in detail. In addition, the thermodynamics and the kinetics of lithium extraction/insertion from/into Li3V2(PO4)3 have been preliminarily investigated.The synthesis parameters of the solid-state method, in which acetylene black was used as carbon source, have been studied, and the results showed that the optimal sintering temperature and sintering time are 700℃and 12h, respectively. Based on these results, the effects of different carbon sources, such as graphite, acetylene black and sucrose, on the performances of as-synthesized cathode materials were investigated. It was found that carbon source exerted significant influence on the stuctrues and electrochemical properties of the materials. The results showed that the material carbon-doped by sucrose had the best electrochemical performances due to smaller particles and lower charge-transfer resistance.Sucrose-containing precursors were firstly prepared by the solution method, and Li3V2(PO4)3/C composites were synthesized successfully via one-step heat treatment. According to XRD and electrochemical measurements, the optimal synthesis parameters were obtained, namely, sintering temperature is 700°C, sintering time 3h and the amount of the residual carbon 4.6wt%。The electrochemical performances of the sample, which was synthesized in the optimal synthesis conditions, were tested in three various voltage ranges. In the voltage range of 3.0-4.8V, the sample displays the largest specific capacity of 171mAh·g-1 in the second cycle, and exhibits better cycling stability (142mAh·g-1 at 50th cycle under 0.2C rate) and better rate performance (137mAh·g-1 at 30th cycle under 1C rate). Between 3.0-4.3V, the discharge capacity after 50 cycles is up to 128mAh·g-1 at a rate of 0.2C, when the rate increases from 0.5C to 1C, the initial discharge capacity decreases from 126mAh·g-1 to 117mAh·g-1, meaning lower capacity loss. When charge/discharge was conducted between the voltage ranges of 1.5 and 4.3V, seven extraction plateaus and seven insertion plateaus are observed in the initial charge-discharge curves, indicating that a much more complicated series of seven successive two-phase transitions occur during Li extraction and insertion, respectively. Despite a series of phase transitions in the charge/discharge process, Li3V2(PO4)3/C composite demonstrates a high initial discharge capacity of 246.7mAh·g-1 at C/5 rate, and the discharge capacity is held to be 239.9mAh·g-1 after 50 cycles. These data meaned that the sample had better electrochemical stability in three potential windows.Li3V2(PO4)3/(Cu+C) composite was prepared by the solution method. The discharge capacities and the cycle performances of Li3V2(PO4)3/(Cu+C) composite at various rates are better than those of Li3V2(PO4)3/C, showing that the addition of Cu conductor is effective to improve the electrochemical properties of Li3V2(PO4)3.The thermodynamics of lithium ion insertion into Li3V2(PO4)3 has been preliminarily investigated for the first time. The results demonstrated that open circuit potentials of Li/ Li1+xV2(PO4)3 cell, chemical potentials and activities are independent of the insertion amount of lithium x in the two-phase zones. The insertion free energy of lithium ion△Gi increases approximately in a linear way with the increase of the insertion amount of lithium x, indicating that the△Gi value and the discharge capacity become larger and larger.The chemical diffusion coefficients of lithium ion in Li3-yV2(PO4)3, determined by cyclic voltammetry, electrochemical impedance spectrum and potential step method, respectively, are generally consistent within the same order of magnitude (10-9～10-8 cm2·s-1). Based on the chemical diffusion coefficients determined by electrochemical impedance spectrum, the following results were given: the chemical diffusion coefficient of lithium ion gradually increases with temperature; the diffusion activation energy of Li+ in fully discharged Li3V2(PO4)3 is 33.02kJ·mol-1 within the temperature range of 1565℃; themobility of lithium ion and the component diffusion coefficient are 10-7～10-9 cm2·V-1·s-1 and 10-9～10-11 cm2·s-1, respectively.
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