Synthesis And Modification Of LiFePO₄as Cathode Material For Lithium-ion Batteries

The olivine-structured LiFePO₄is one of the most promising cathode materials of lithium ionbatteries due to its excellent thermal stability, great cycle stability and environmental benignity, but itspoor electronic conductivity (10^-9s/cm) and the low Li-ion diffusion (10^-11s/cm) severely influencethe electrochemical performance. To improve the conductivity of materials, the carbon-coated, tracerare earth and transition elements doped LiFePO₄were prepared in this paper, the microstructure, themorphology and the electrochemical performance of which were carefully investigated.The single factor experiment was used to optimize the calcinations temperature and carbon sourceon the synthesis of LiFePO₄. The XRD and SEM patterns of the simples showed that the sampleshave well-crystallized olivine structure. Meanwhile, the particle size is moderate in good dispersionwhen the synthesis temperature is650℃. The structure and micro-morphologies of LiFePO₄/Ccomposites modificated by four different carbon sources （citric acid, glucose, acetylene black andascorbic acid） were investigated by XRD, SEM, TEM and Raman. It was found that there was a5nm-thick uniform carbon layer on the surface of LiFePO₄using organic carbon. The citric acidcoated LiFePO₄exhibited the first discharge capacity of147mAh/g at0.1C, showing excellentelectrochemical performance.A novel two-step milling method for preparing LiFePO₄/C has been developed.The resultsindicated that the product have a better crystallinity with nano-sized particle in good dispersion. Thematerial exhibited a first discharge capacity of153mAh/g at0.1C and142mAh/g at0.2C with nocapacity fading after50cycles. Compared with the materials got from one-step milling, thenano-sized materials obtained by two-step milling method exhibited better cycling stability and ratecapacity, which could be associated with the reducing of the migrate distance of lithium ions andincreasing of the contact area with electrolyte.LiFePO₄was doped by six kinds of trace rare earth ions Re³⁺（Re=La、Nd、Gd、Ho、Er、Lu） onFe site. The microstructures and electrochemical performance of LiFe_0.99Re_0.01PO₄were invesitaged.The results showed that La and Lu-doped caused the lattice distortion and the the capacity of LiFePO₄was reduced. The samples of LiFe_0.99Nd_0.01PO₄showed the best electrochemical performance with anintial discharge capacity of134mAh/g at0.1C and126mAh/g at0.2C without any capacity fadingafter20cycles.The cathode materials of LiFe_0.98M_0.02PO₄（M=Mn, Co, Ni） have been synthesized doping withtransition metals. The electrochemical tests results showed that doping transition metals can greatly improved the capacity of LiFePO₄, especially the sample with Mn-doped delivered first capacity of140mAh/g at0.1C with an initial discharge efficiency of98%. Otherwise, the specific capacity ofLiFe_0.98Mn_0.02PO₄remained90%after50cycles at0.2C. Transition metals doping can enhance theconcuctivity of LiFePO₄and reduce the polarization in the charge-discharge process, thus theelectrochemical performance could be improved.