Optimizing the hydrothermal synthesis of micro-sized olivine LiFePO₄

The low tap density of LiFePO₄ is hindering the energy and power density of lithium-ion batteries in portable electronics, electric vehicles, and stationary electricity storage applications. As part of our work to investigate the pathological mechanism of performance degradation in large particle LiFePO₄, micro-sized pristine LiFePO₄ without modifications, such as surface coating or bulk doping, was first prepared hydrothermally by optimizing the synthesis parameters in this work. The influences of precursor concentration, solution pH, hydrothermal temperature, and heating time on the phase structure, particle size, and morphology of the products were systematically investigated. It was found that the particle size of LiFePO₄ increases with decreasing pH value, increasing precursor concentration, increasing hydrothermal temperature, and increasing heating time during hydrothermal synthesis. The performance degradation of large particle LiFePO₄ was demonstrated by these intrinsic samples. The specific discharge capacity decreased from 152 to 80 mAh·g^-1 at 0.1C rate when the particle size was increased from 0.7 to 16.5 μm. Moreover, less capacities were retained after 100 cycles at 1C rate for larger particle materials. Finally, the optimized LiFePO₄ with a distorted diamond shape was prepared for later investigation of the plausible mechanism of performance degradation in large particle LiFePO₄. Its electrochemical performance was preliminarily discussed, and will need to be improved in future to obtain practical high energy/power density LiFePO₄ cathodes for lithium-ion batteries.