Study On Preparation And Modification Of Nanosheet LiMnPO₄/C Composite

As a cathode material for lithium ion batteries,LiMnPO₄ is considered as an alternative material for commercial LiFePO₄ due to its high voltage platform,abundant raw material source and good safety.However,its electronic conductivity is due to its own crystal structure.The shortcomings such as low and low ion mobility rate restrict the performance of its electrochemical performance.In order to solve problems mentioned above,the electrochemical properties of the materials were improved by synthesizing nanosheets,three-dimensional porous nitrogen-doped graphene modified LiMnPO₄ and expanded graphite induced synthesis?010?maximum exposed surface LiMnPO₄.LiMnPO₄ with different morphologies were synthesized by controlling the molar ratio of LiOH to H₃PO₄ in the solvothermal process.When the molar ratio of LiOH to H₃PO₄ was 2.5:1,the LiMnPO₄ particles were nanosheet-like morphology.On the one hand,primary particle nanocrystallization can shorten the Li⁺transport path in the crystal and increase the migration rate of Li⁺.On the other hand,the coated carbon layer can significantly improve the electronic conductivity of the material.The material has a specific discharge capacity of 130 mAh g^-1 at 0.2 C and a reversible capacity of 50 mAh g^-1 at a large current density of 10 C.In order to further improve the conductivity of the material and inhibit the agglomeration of the primary particles of the material,a solvothermal method is used to grow LiMnPO₄ in situ on the surface of the N-doped graphene aerogel.The three-dimensional conductive network composed of N-doped graphene improves the electron conductivity of the material.Large specific surface area provides more electrochemical active sites and nitrogen doping modification enhances wettability to organic electrolytes.The composite achieve a specific discharge capacity of 143 mAh g^-1 at a current density of 0.2 C.And a reversible capacity of 130 mA hg^-1is obtained at 1 C with a capacity retention rate of 90%for 100 cycles.At a large rate of 10 C,the discharge specific capacity is as high as 92 mAh g^-1.The solvothermal method is used to induce the formation of LiMnPO₄ with?010?dominant crystal plane on the surface of expanded graphite by utilizing the electronegativity of the surface of the expanded graphite and the strong interaction of the lithium manganese phosphate crystal with the hydrogen bond on the?010?plane.?010?crystal plane orientation can shorten the lithium ion migration path inside the crystal,and the face-to-face bonding mode between LiMnPO₄ and expanded graphite accelerates the lithium ion and electron transport.The material deliver a initial discharge of 130 mAh g^-1with capacity retention rate of 100%after 200 cycles at 1 C.At the large rate of 10 C,the initial discharge specific capacity 102 mAh g^-1 is obtained with the capacity retention rate of 72%after1000 cycles.