Study On Solid State Synthesis And Modification Of LiMPO₄（M=Fe, Mn） Cathode Materials

In order to improve the conductivity and the energy density of olivine type LiFePO4, the synthesis and modification of LiMPO4(M=Fe, Mn) were studied by using various electrochemical methods in combination with modern analytic techniques, such as XRD, SEM, TEM and so on.The material of LiFePO4/C was synthesized by one-step carbothermal reduction method. The reaction mechanism of synthesis was researched by using TG-DTA analytic technique. The effects of synthesis conditions on the performances of LiFePO4/C were studied. The results show that the sample of LiFePO4/C containing3wt.%carbon being prepared at700℃for12hours by mechanical activation method shows the perfect crystallization, small particle size, excellent rate capability and cyclability.At first, the LiFePO4/C was adopted the modification that it was coated with metal Al. The effect of different technical routes and the sintering temperature and the amounts of Al powder on the performances of the composite was investigated. The results show that the interface reaction occurs between metal Al and LiFePO4/C, resulting in the formation of several by-products and the passive film on the surface of LiFePO4The LiFePO4/Al/C sample containing3wt.%Al being prepared at750℃for18hours shows the best electrochemical performances. The discharge specific capacity is117.8mAh/g at25℃and10C rate. The discharge specific capacity is105.6mAh/g at-20℃and0.1C rate. The discharge capacity at-20℃is73.8%of that at25℃.Olivine LiFexMn1-xPO4/C composites were synthesized by one-step solid-state method combined with mechanical activation method. The results indicated that the LiFe0.8Mn0.2PO4/C sample been produced using Mn3(PO4)2·3H2O for manganese source shows the single-phase olivine structure and small particle size. The discharge specific energy of LiFe0.8Mno.2P04/C increases by13.2%compared to that of LiFePO4/C at IOC rate. And it has good cycle performance at1C rate. The kinetics behaviors of olivine LiFe0.8Mn0.2PO4/C were studied by means of electrochemical impedance spectroscopy and cyclic voltammetry measurements. The results show that doping of Mn leads to lower electrode reaction speed and lithium ion diffusion coefficient, but it could effectively improve the material’s discharge plateaus.