Modification Of LiMnPO₄as Cathode Material For Lithium-ion Battery

Olivine framed LiMnPO4is one of the most promising cathode material forlithium ion batteries duo to its high voltage, high energy density, low cost, highthermal stability and environmental friendliness. However, the intrinsically lowelectronic and ionic conductivity limite its application. For the purpose of improvingthe electrochemical performances of LiMnPO4, a solid state reaction and ahydrothermal method are used to synthesize LiMnPO4. Besides, substitution andcarbon coating are used to further improve the electrochemical performance ofLiMnPO4composite materials. The detailed works are as follows:(1) LiMn0.5(V2/3)0.5PO4/C cathode material is synthesized by solid state reaction.The influence of calcination temperature, different carbon content and differentcarbon sources on the structure, morphology and electrochemical properties ofLiMn0.5(V2/3)0.5PO4/C cathode material is investigated by XRD, SEM, TEM, EDX,EIS and Charge-discharge tests. The results show that600°C is the optimumcalcination temperature and sample calcined at600°C shows regular particlemorphology, uniform particle size distribution and small degree of aggregates.Adding appropriate amounts of carbon can not only prevent particle agglomerationor growth, but also improve the conductivity of the material. However, for the addedcarbon is electrochemically inert substance, the electrochemical performance will beaffected when the carbon content is too high. The best carbon content is10%.Carbon sources have little effect on the crystal structure and morphology of thesamples, but it has a great influence on the electrochemical properties. SP isconsidered to be the best carbon source. The electrochemical performance testresults show the initial discharge capacity of sample synthesized at optimalconditions is113.1mAh·g-1at0.1C rate and the capacity is still as high as90.6mAh·g-1at25thcycle.(2) A nutty-cake structural C-LiMn1-xFexPO4-LiFePO4cathode material issynthesized by hydrothermal method and further calcined at different temperatures.The influence of calcination temperature on the electrochemical behavior isinvestigated by XRD, SEM, TEM, EDX, EIS and Charge-discharge tests. And theperformance of C-LiMn1-xFexPO4-LiFePO4materials has a relationship with itscrystal structure. The well-crystallized Sample-600calcined at600℃shows the smallest charge transfer resistance, the largest lithium ion diffusion coefficient (DLi)and the best cycling stability. The discharge capacity of Sample-600holds around112mAh·g-1after the3thcycle at0.1C rate. The performances improvement ofC-LiMn1-xFexPO4-LiFePO4material can be mainly attributed to the iron diffusionfrom the LiFePO4core to the outer LiMnPO4layer under appropriate calcinationtemperature. The impact of different carbon content on C-LiMnxFe1-xPO4-LiFePO4cathode material performance is also studied. The results show that the added carbondoes not affect the crystalline structure of the material. The carbon can not onlyprevent particle agglomeration and particle growth but also improve the electricalconductivity of the grains. However, as the carbon is electrochemically inertmaterial and the content should not be too high. The optimum carbon content shouldbe4%.