The Preparation Of LiFePO₄ Cathode Material For Lithium Ion Battery By Hydrothermal Reaction

LiFePO₄ cathode materials were prepared by hydrothermal reaction method with precursors at different conditions. The precursors were synthesized by precipitation named one step and two steps separately. The study on doping modification of LiFePO₄ was carried out. The principle of hydrothermal reaction synthesizing LiFePO₄ was investigated. At the same time, influences of the technology parameters and doping modification on structure, morphology, electrochemical properties and tap density of the cathode materials were discussed.The mechanism of precursors prepared by precipitation of one step and two steps separately was investigated. The results indicate that there were Li and Fe in the precursors. However, the Li and Fe in the solution didn't transfer to precipitation by stoichiometric ratio. X-ray diffraction analysis of the precursors suggests that the precursor is amorphous. Therefore it was a further precipitation of precursors, a crystallization and growth of LiFePO₄ during the hydrothermal reaction.The precursors were crystallized by hydrothermal reaction. Influences of the process of synthesizing the precursor, time and temperature of hydrothermal reaction on tap density, charging and discharging properties of the cathode materials were discussed. The optimized conditions of hydrothermal reaction were obtained. One is the sample with reaction temperature 150°C and reaction time 1h in precipitation of one step, the other is the sample with reaction temperature 190°C and reaction time 1h in precipitation of two steps. The special surface area of powders analysis indicates that the special surface area decrease gradually by the increasing of temperature, and the smaller one exhibit better flat voltage.The tap density of cathode powder was tested. The results suggest that, the tap density is gradually decreased by increasing reaction time and enhancing the reaction temperature. The fluidity of the cathode powder deteriorated because of the powder absorbing dampness. The tap density of the sample obtained with a 300 mesh sieve is very low. However, influence of the powder absorbing dampness on tap density of the sample obtained with a 300 mesh sieve is sensitive. Most of the cathode tap density is between 1 g·cm^-1 and 1.5 g·cm^-1. The results of tap density are close to the tap density of commercial products. The doping modification of LiFePO₄ was studied, the effects of Mg doping on the tap density, charging and discharging properties of the LiFePO₄ cathode material were discussed. The results reveal that, the doping of 5 mol. % Mg is benefit for the growth of the crystal grain by XRD and SEM, the grain size is decreased, there are charge and discharge flats of cathode material obviously, the specific capacity of the doping of 5 mol. % Mg are increased than the no doping one, however the tap density of the doping of 5 mol. % Mg is more decreased than the no doping one.