Application Of Lithium-Ion Battery Cathode Material LiFePO₄ In Aqueous Solution

Lithium-ion batteries have become the most important component in the field of batteries for its high specific capacity, stable charge-discharge performance, diverse types of materials and environment-friendly. And have been widely used and developed in cell phone batteries, PC batteries, car power batteries and other fields currently. The lithium-ion battery cathode material LiFePO4 was seen as the most promising commercial vehicle power battery materials currently. However, the using of LiFePO4 was mainly concentrated in the organic electrolyte system which is flammable and environmentally harmful. A new type of lithium ion batteries which applying the electrode materials into aqueous electrolytes is a very effective way to avoid these disadvantages. And the higher ionic conductivity of the battery in aqueous electrolytes will be more conducive to large rate charge and discharge.Firstly, we explored the best aqueous electrolytes for LiFePO4 by electrochemical testing methods like cyclic voltammetry (CV), galvanostatic charge/discharge, electrochemical impedance (EIS) and XRD, ICP and other auxiliary method. The battery using LiFePO4/C as the cathode material and large area of the activated carbon as the anode in pH=75M LiNO3 electrolyte had a good electrochemical performance. The discharging capacity can reach 135mAh/g at the current density of 1C, and the cycle capacity retention rate is 74.1% after 70 charge-discharge cycles. We also propose a material capacity fading mechanism in the charge-discharge process:The Fe, P in LiFePO4 crystal will be dissolved at the same time with the Li+-extraction in charging process. And a complex compound thin film which is constituted by Li, Fe, P will be formed on the surface of LiFePO4 crystal. The film always dissolved to some extent at the discharge process. With the accumulation of film formation, the Li+-extraction and insertion will be affected seriously and resulting in capacity fading of LiFePO4. At the same time, the film dissolves during discharge process will be detected in the electrolyte in the form of white floc.According to the conclusions had been obtained, we improved the electrolyte by using a gelled electrolyte for LiFePO4 and the charge-discharge cycling stability of LiFePO4 was greatly improved. The initial discharge capacity of LiFePO4 in a silica-based gel electrolyte system is 125mAh/g at 0.5C current density. And the cycle capacity retention rate is 76.8% after 308 charge-discharge cycles.Finally, we get the LiFePO4/C composites by hydrothermal method and sol-gel synthesized. These both are very effective methods to get materials with good electrochemical performance by controlling the particle size. The initial discharge capacity of synthesis of LiFePO4/C composite material in silica-based gel electrolyte system is 144mAh/g at 1C current density. The capacity retention after 400 cycles was 75%, and there is not capacity rapid decay phenomenon.Firstly, LiFePO4 capacity fading mechanism in aqueous medium during charging and discharging is studied. According to the conclusions had been obtained, we improved the charge-discharge cycling stability of LiFePO4 in two ways:the electrolyte and the material itself. We have systematically studied the using of LiFePO4 in an aqueous environment.