Hydrothermal Synthesis And Doping Modification Of Cathode Material LiFePO₄for Li-ion Batteries

With the development of human society and economy, the demand for andconsumption of various kinds of new energies have been increasing. However, thestore, transfer and utilization of new energies are based on storage devices of highperformances. Li-ion batteries, with the advantages of high voltage, smallself-discharge, non-memory effect and environmentally friendly, have become thenew mobile energy storage devices. The olive LiFePO4has become one of the mostpromising cathode materials owing to such advantages as low cost, high theoreticalcapacity, long cycle life, good thermostability, no memory effect and so on. However,LiFePO4suffers from low electronic conductivity and lithium ion diffusivity, resultingin low specific capacity and bad high-rate performance, which has hindered itslarge-scale production and commercial application. Methods have been developed toovercome these intrinsic drawbacks thus improve the performance of the material:particle size reduction, doping and surface modification.In this paper, LiFePO4and alien ion doped LiFePO4were synthesized byhydrothermal method. Through Mn-doping and Ni-doping orthogonal experiments,we systematically investigated the effect of dopant concentration, reactingtemperature and reacting time on the electrochemical performance of the material.The results indicated that in the nickel doped experiments, the dopant concentra tionhas a greater influence on the capacity of the material at0.1C. Under the bestexperiment condition, the sample delivered a capacaity of150.9mAh g-1. For the Mndoped samples, the factors mainly influenced the high rate capacity of the material,the best sample delivered127mAh g-1at10C.This paper also investigated the effect of reacting temperature on the Ni dopingprocess and the final performance of the material. Through ICP and FTIRinvestigation, it was indicated that the reacting temperature influenced the dopantconcentration in the final crystals and the anti-site concentration in the crystals. Animproved capacity of the doped sample can be obtained at the reacting temperatureshigher than210oC. The best sample was achieved at the reacting temperature of240oC, which delivered a capacity of98.5mAh g-1at20C with improved cyclingperformance. At last, this paper also investigated the effect of high valence dopants as Al3+andTi4+on the performance of the material. The results indicated that these high valencedopants were prone to hydrolyze in the hydrothermal reaction thus altered the reactingand crystallization environment, leading to the deterioration of the phase purity andparticle morphology and growth of the particle size. And this effect can be improvedwith increasing temperature. However, the electrochemcial performances were notimproved after Al3+or Ti4+doping.