Synthesis And Electrochemical Performance Of Nano-LiFePO₄/C By High Temperature Solid-state Reaction

Lithium ion batteries（LIB） are attractive for use in the field of mobile power source with the favorableproperties of high voltage, long cycling life, high energy density, non-memory effect and pollution-free.LiFePO₄is emerging as a promising cathode material for lithium-ion batteries because of low cost andenvironmental compatibility. In addition, LiFePO₄has a large theoretical capacity of170mAh·g^-1, a flatdischarge potential of3.4V versus Li/Li⁺, the good cycle stability, and the excellent thermal stability.However, the low electronic conductivity and diffusion Li⁺arising from LiFePO₄are the bottleneck of itsdevelopment. Many attempts have been made to overcome these intrinsic limitations by doping with theforeign atoms, decreasing the particle size, coating electronically conductive agents such as carbon andmetals, and doping with the foreign atoms, Obviously, the reduction of the particle size and carbon coatingare still the most effective ways to overcome electronic transport limitations of LiFePO₄. The main aspectsare as follows:（1） Amorphous nano-FePO₄was prepared by spontaneous precipitation from aqueous solutions, usingFe³⁺salt（Fe（NO₃）₃9H₂O） as the iron precursor, with the presence of surfactant. The reasons lead toagglomeration were analyzed，and the mechanism of different dispersion methods for the precursor in themixing process was discussed. Both grinding and ultrasonic can make FePO₄aggregates disperse again andmax with phenolic resin and Li source uniform. The LiFePO₄/C cathode materials prepared with particlesize within200nm, showed excellent electrochemical performances.（2） The water-soluble phenolic resin was used as carbon to synthesize LiFePO₄/C, The effect ofcontent of water-soluble phenolic resin and sintering temperature on the electrochemical performance ofthe LiFePO₄/C cathode materials was discussed. The results show that optimal sample synthesized with 35.7mL/mol（VS LiFePO₄） water-soluble phenolic resin, sintered at700°C for10h and with a carboncontent of8.7%are spherical-like particles, it consisted a carbon shell with a thickness of about5nm andthe LiFePO₄core, and showed excellent high-rate performance and cycling performance as the cathode forlithium ion batteries. The initial discharge capacities at rates of0.2C （1C=170mAh·g-1）,0.5C,1C,2C,5C and10C was151mAh·g-1,150mAh·g-1,146mAh·g-1,142mAh·g-1,132mAh·g-1and119mAh·g-1,respectively. The material still showed initial discharge capacity of105mAh·g-1even at20C and itscapacity doesn’t decay after50cycles.（3） We preliminary studied on B-doped LiFePO₄in P-site in this paper.