Studies On Synthesis And Performance Of Li₃V₂（PO₄）₃ As Cathode Materials Of Lithium Ion Battery With Aqueous Electrolyte

Traditional lithium-ion battery has many advantages, but it still has security issues which need overcome.The security issues limit its wide application in electric vehicles. Aqueous lithium-ion batteries(ALIBs) not only overcome the problem, but also have many advantages such as environmentally friendly, simple preparation process, low cost and low equipment requirements. It is believed to be one of electrochemical power source with development potential in the future. Due to the theoretical water decomposition is low(1.229 V), it need to cautiously select the cathode materials for ALIB and the cathode materials in battery has an important position.In this paper, the mechanism of lithium ion battery was introduced firstly, then the cathode and anode materials of ALIB was elaborated. A series of new cathode materials without being used in ALIBs were researched in this paper. Some physical and electrochemical characterization methods were used to illustrate the excellent electrochemical performances of the newly cathode materials in ALIB. The scanning electron microscopy(SEM) was used to observe morphology, and x-ray diffraction(XRD) to observe the crystal structure.Firstly, we reported a facile sol-gel method for the synthesis of Li3V2(PO4)3 composite with citric acid as chelating agent. The electrochemical properties of Li3V2(PO4)3 calcined at 700℃was studied in saturated lithium nitrate(LiNO3) and saturated lithium sulfate electrolyte. The results showed that the mechanism of lithium intercalation-deintercalation in aqueous electrolyte was same to that in organic electrolyte. The electrochemical performance of Li3V2(PO4)3 was better in saturated lithium nitrate electrolyte than that in saturated lithium sulfate electrolyte. The initial charge capacity of Li3V2(PO4)3 at 1C, 2C, and 5C rate was 103 mAh.g-1, 100 mAh.g-1 and 93 mAh.g-1, respectively. After 30 cycles, the corresponding capacity retention was 80.58%, 81% and 82.26%, respectively.Secondly, Li3V2(PO4)3 was coated with TiO2 to improve its electrochemical properties. Li3V2(PO4)3 was coated with different amounts of TiO2(2%, 3% and 4%), then the electrochemical performance of Li3V2(PO4)3/TiO2 was studied in saturated lithium nitrate electrolyte. The results showed the cycle performance and rate performance of Li3V2(PO4)3/TiO2 had greatly improved, and the coating amounts of 3% with TiO2 exhibited the best electrochemical performance. The initial charge capacity of Li3V2(PO4)3/3%TiO2 at 1C, 5C, 10 C and 20 C rate was 105.5 mAh·g-1, 99.8 mAh·g-1, 93.4 and 81.3 mAh·g-1, respectively. After 50 cycles, the corresponding capacity retention was 77.9%, 79.6%, 78.1 and 82.3%, respectively.Thirdly, Li3V2(PO4)3-Li FePO4 was synthesized by two steps. FeVO4·xH2O was synthesized by wet chemical process, then Li3V2(PO4)3-LiFePO4 was synthesized by solid-phase method. The electrochemical performance of Li3V2(PO4)3-LiFePO4 calcined at 700℃ was studied in 5 M, 7 M and saturated lithium nitrate electrolyte. The results showed that the electrochemical performance of Li3V2(PO4)3-LiFePO4 in saturated lithium nitrate electrolyte was best. The initial charge capacity of Li3V2(PO4)3-Li FePO4 at 1C, 2C, 5C and 10 C rate was 118.0 mAh·g-1、112.3 mAh·g-1、 109.0 mAh·g-1and 90.0 mAh·g-1, respectively. After 50 cycles, the corresponding capacity retention was 67.9%,68.6%,67.2% and 80.4%, respectively.