LifePO₄Cathode Material For Lithium Ion Batteries Prepared By Supercritical/Subcritical Hydrothemal Synthesis

Lithium iron phosphate is considered as a promising cathode material for the power lithium ion batteries in terms of its low cost, cycling stability, safety and free from contamination. Hydrothermal method is an effective way to synthesize ultrafine LiFePO4particles with small size, narrow size distribution and good uniformity. But it has the shortcoming of too long reaction cycle. Based on the preparation of ultrafine LiFePO4powder via traditional hydrothermal method, fast synthesis process can be achieved by super/suberitical hydrothermal synthesis. The synthetic cycle is greatly shortened resulting in saving time and energy consumption. So a new way can be provided for the industrialization of the LiFePO4material by super/subcritical hydrothermal synthesis.In this paper, LiFePO4with high capacity was prepared by super/subcritical hydrothermal synthesis and solid phase methods. The effects of hydrothermal reaction parameters (reaction pressure, temperature, time and the adding amount of sucrose) and calcining process parameters (calcining time and calcining temperature) on the the structure, morphology and electrochemical performance of the material were well investigated. In order to improve the electrochemical properties of LiFePO4, different templates and different solvents were adopted to control the particle morphology and particle size in the hydrothermal synthesis process. The influence of different carbon-coating ways on the morphology and electrochemical performance of the material was studied in the end. The main results of this study were obtained as following:1. LiFePO4were synthesized under the super/subcritical hydrothermal conditions with sucrose as the template by controlling the pressure and temperature of the hydrothermal reaction respectively. The synthesized samples were sub-micron and micron massive particles, the increasing pressure led to an increase in the particle size, and the increasing temperature resulted in the smaller particle. The sample prepared at the optimized synthetic temperature of400℃and pressure of25MPa demonstrated the specific discharge capacity of114.5mAh/g at0.1C.2. LiFePO4were synthesized with sucrose as the template by controlling the reaction time. The particle size of the synthesized samples decreased with the extend reaction time. The samples prepared within1min and10min demonstrated very similar morphology, particle size and specific discharge capacity, also the charge and discharge efficiency and capacity retention of the two samples were exactly the same. The synthesis process of LiFePO4was considered to be almost completed within1min.3. Taking PVP as template, the optimized calcining temperature was700℃, calcining time was1h, and the specific discharge capacity of this sample was141.2mAh/g at0.1C.4. The growth of LiFePO4particles in the reaction process was effectively controlled with the template of PVP. The sample prepared with PVP as template showed the highest specific discharge capacities of113.6mAh/g,108.6mAh/g,96.1mAh/g and86.7mAh/g respectively at1C,2C,5C and IOC. The cycling stability, rate capability and high-current discharge capacity of this sample were excellent.5. The adding of ethanol as reaction solvent could accelerate the nucleation of LiFePO4and reduce the grain size, resulting in the improvement of the discharge capacity. The discharge capacities of this sample were151.4mAh/g and128.0mAh/g respectively at0.1C and1C.6. The discharge capacity and cycle stability of LiFePO4were successfully improved by adding sucrose as carbon source and calcination after the material was synthesized. The sample demonstrated the discharge capacities of153.0mAh/g and144.0mAh/g at0.1C and1C. The capacity retention rate was up to96.2%after100cycles at1C.