Solvothermal Synthesis And Electrochemical Performance Improvement Of Olivine Type LiMPO₄（M=Fe,Mn）as Cathode Materials In Li-ion Batteries

Olivine structure LiMPO4 is the most promising cathode materials for Li-ion battery to achieve high power application. In this paper, after reviewing the current research status of LiMP04 cathode materials, LiFePO4 and LiMnP04 cathode materials are synthesized via P123-assisted and graphene oxide-assisted solvothermal route, respectively. Due to the improvement for fully covering carbon coatings, the obtained LiFePO4 and LiMnP04 cathode materials express excellent electrochemical performances. The main researching results and conclusions are listed as follow:(1) The LiFePO4 nanoparticles which is gathered together gaplessly with nanocrystallites have been successfully prepared by solvothermal method assisted with triblocks copolymer P123. Adsorption of P123 makes the surface of the synthesized LiFePO4 nanoparticles and the boundaries of aggregated nanograins are fully covered with carbon film, showing excellent charge-discharge performance and rate performance. The first charge and discharge specific capacity at 0.1 C are 162.95 mAh/g,153.12 mAh/g, respectively. The discharge capacity at high rate of 10C achieves 127.60 mAh/g, which is 78% of the discharge capacity of 0.1 C.(2) Flower-like LiMnPO4/GO composite nanostructures self-assembled by nanosheets have been successfully prepared by solvothermal method assisted with GO. Due to the adsorption of GO, the primary block nanosheets become much bow and twist, making there exist some gap among the nanosheets. After carbonization of the GO adsorpted on the surface of primary block nanosheets, flower-like LiMnPO4/C composite nanostructures, in which the primary block nanosheets are fully covered with carbon coating, are obtained and deliver excellent electrochemical performance. The first discharge capacity at 0.05 C reached 101.8mAh/g, which was much better than the first discharge capacity of the flower-like LiMnPO4 nanostructures, which are solvothermally synthesized without any GO, about 69.16 mAh/g.(3) The introduction of KOH mineralizer into the ethylene glycol solvothermal system can effectively promote the nucleation of LiMnPO4, making the formed LiMnPO4 grains decreased. Therefore, under the effect of the high concentrations of KOH, the formed nanograins self assemble to single-crystal-like LiMnPO4 ellipsoidal nanoparticles, which show better electrochemical performance due to the shortening of Li-ion diffustion path, the first charge and discharge capacities at 0.1C achieve 58.53mAh/g,55.09 mAh/g, respectively.