Study On The Synthesis And Electrochemical Performance Of LiMn_1-xFe_xPO₄/C Cathode Material For Lithium-ion Battery

As a new type of environment-friendly chemical power source,lithium-ion battery has been paid more and more attention in electric vehicles and solar energy storage with its continuous development.Therefore,it has higher requirements to the performance of lithium ion battery,especially the energy density.As the core part of the battery,cathode materials are the key to improve the battery performance.Phosphate cathode materials have good thermal stability to improve the safety of the lithium-ion battery.The reason is that in the phosphate system with olivine structure(LiMPO4,M=Fe,Mn),all the oxygen ions form a stable(PO4)3-polyanion group with P5+ through strong covalent bonds,the oxygen in the lattice is not easily lost.The above reasons make the material more safety.The electrode potential of Fe3+/Fe2+ relative to Li+/Li is only 3.4V,while the electrode potential of Mn3+/Mn2+relative to Li+/Li is 4.1V,which is exactly in the stable electrochemical window of the existing electrolyte liquid system.The high potential of 4.1V gives this material the high specific energy.At the same time,the theoretical specific capacity of LiMnPO4 and LiFePO4 is 170 mAh/g.When both of the actual capacity is equal,the energy density of LiMnPO4 will be higher than LiFePO4.However,the Jahn-Teller effect of Mn3+in the two-phase transition of LiMnPO4→MnPO4 makes its volume change about 9.5%,which limits the application of LiMnPO4.Due to the similar atomic radius of Mn and Fe,and the formation of phosphate system is olivine structure,partial substitution of Fe for Mn in LiMnPO4 can effectively improve the electrochemical performance of LiMnPO4.Based on the above discussion,LiMn1-xFexPO4/C materials were prepared by high temperature solid-state method.The effects of Mn/Fe ratio,carbon source type and calcination temperature on the structure,morphology and electrochemical properties of the materials were studied.(1)The ratio of Mn/Fe affects the electrochemical performance of the material.When the content Fe=0.3,the initial charge-discharge capacity of LiMn0.7Fe0.3PO4/C is 130.5 and 125.64 mAh·g-1,respectively.The material has a complete crystalline structure and no obvious impurities,showing a uniform spherical morphology.It exhibites good electrochemical properties.(2)Compared with the other two carbon sources(glucose and melamine),sucrose could more effectively hinder the growth and agglomeration of LiMn0.7Fe0.3PO4/C particles during carbon coating.Reducing the particle size is conducive to shorten the particle diffusion path Li+.(3)The crystallinity of LiMn0.7Fe0.3PO4/C prepared at 650℃ is good,and the specific discharge capacities are 127.26,126 and 113 mAh·g-1 at 0.1,0.2 and 0.5C(1C=170 mAh·g-1),respectively.The charge transfer resistance(Rct)is only 32Ω.