Study On Hydrothermal Synthesis And Modification Of Cathode Materials LiFePO₄ For Lithium Ion Battery

Olivine-structured LiFePO₄ is gaining particular interest as a potential candidate cathode material for rechargeable lithium-ion batteries from both economic and environmental points of view. In the industrial applications, it shows more advantages and potential especially as the dynamic battery in "electric power tools and cars". There are two ways including coating and doping to improve the electrochemical performance of LiFePO₄. In this paper, LiFePO₄ doped with rare-earth and transition elements have been synthesized by the solvothermal method, and the effect of different carbon sources and coating temperatures has been studied on electrochemical performance. Other ways of modification including introducing the CNT and Ag have been also researched in the paper.LiFePO₄ and Li(RE, Fe)PO₄ (RE = La, Ce, Nd) have been synthesized by the solvothermal method using FeSO₄, H₃PO₄, LiOH and rare-earth elements as raw materials, So the nano-sized samples free of any electronically conductive materials could be used to research the effect of doping. The XRD results indicate that the as prepared LiFePO₄ is well-crystallized with olivine-structure, and the synthesized powders have a single-size between 50 and 100 nm as observed by FESEM. As a result, the electronic conductivity of LiFePO₄ was enhanced by around 1～3 orders of magnitude by element doping. Accordingly the capacities of the composites were improved greatly by doping, about 2～5 times higher than the pure sample, especially for the Nd doped sample, which had the largest range. As an RE element, Nd has larger possibility to be doped into LiFePO₄ compared with the other RE elements, and much more effect as a donor to increase current carriers and intrinsic conductivity, which can finally improved the poor electrochemical performance of pure LiFePO₄. When LiFePO₄ is doped by 1.5 mol % Nd, the specific capacity of LiNd_0.015Fe_0.985PO₄ is the best.Li(Mn, Fe)PO₄, Li(Co, Fe)PO₄, Li(Ni, Fe)PO₄ and corresponding carbon-coated composites have been synthesized by the solvothermal method. As a result, doping transition metals enhanced the conductivities of LiFePO₄ obviously; and improved the redox kinetics of Fe～(3+)/Fe～(2+) couple to reduce polarization. Accordingly, the capacities of corresponding Li(M, Fe)PO₄/C(M=Mn, Co, Ni) composites are also increased markedly, especially for the Mn and Ni doped samples.Comparing the effects of different carbon sources as glucose and polypropylene on LiFePO₄, polypropylene has a better result at the appropriate range of temperature (600℃～ 700℃), which is related to the degree of graphitization. The best coating temperature for glucose is 650℃; while that for polypropylene is 600℃, but the capacities of the composites declined sharply when coating temperature raised to 750℃. Because PO₄^3- can be reduced to pyrophosophoric acid by hydrogen from polypropylene at the high temperature, which has the impact on the electrochemical performance of pure LiFePO₄.The comparison between LiFePO₄/CNT prepared by the in -situ solvothermal method and the pure LiFePO₄ shows that introducing CNT can reduce the polarization and enhance the electronic conductivity coming from the intimate contact between LiFePO₄ particles. In addition, LiFePO₄/Ag composites were synthesized by silver mirror reaction combined with chemically reductive reaction. The addition of Ag conductor is effective to improve the electrochemical properties of LiFePO₄. However the structure and chemical reaction of Ag in the process of charge and discharge are unpredictable. LiFePO₄/Ag is instability as a cathode material for rechargeable lithium-ion batteries, which needs further research.