Study On The Preparation And Modification Of LiMnPO₄ As Cathode Material For Lithium Ion Batteries

Olivine-structured LiMnPO₄ is considered as the most promising cathode material due to its high theoretical energy density,low cost,long cycle life and structure stability.However,the intrinsic drawbacks of high intrinsic electronic resistance and low ionic conductivity originated from the special structure of LiMnPO₄ unit cell limited the development of lithium manganese phosphate materials.The low electronic conductivity and Li⁺diffusivity of LiMnPO₄ severely limited its electrochemical activity especially under high current density,which hindered the remakable electrochemical characteristics.To overcome these drawbacks,many investigations have been attempted to improve the electrochemical properties of LiMnPO₄/C.Here we mainly study the synthesis and modification condition,probing into the influence of synthesis conditions on the structure and properties of the LiMnPO₄/C materials.Feasible and controllable synthesis method was utilized to prepare LiMnPO₄/C electrode material which possessed excellent structural stability and electrochemical performance.1.Carbon coating can effectively increase the electrode conductivity,improve the surface chemistry of the active material,and protect the electrode from direct contact with electrolyte,leading to enhanced electrochemical properties of the batteries.A solvothermal route for synthesizing LiMnPO₄/C nanoplates with a carbon layer from different carbon sources,such as citric acid,glucose,and sucrose was applied to improve their electrochemical performance.Effect of different carbon sources on the electrochemical properties of plate-like LiMnPO₄/C nanocomposites was dicussed in detail.The prepared LiMnPO₄/C nano-sheet composite obtained from glucose as the carbon source possessed large specific surface area and single crystal structure,showing a reversible capacity of 153.3 and 104.4 mAh/g at 0.1and 5 C.The capacity retention rate was 96.6%after 100 cycles,showing excellent electrochemical performance,which is better than that obtained from citric acid and sucrose.2.Novel LiMnPO₄/C nancomposites have been successfully synthesized via a facile solvothermal approach in a mixed solvent of organic solvents and water.Organic solvents such as ethylene glycol,glycerol,PEG400,and PEG600 were used as precursor solvents for the reaction.Detailed investigations indicate that organic solvent plays an important role in the formation of the LiMnPO₄/C material by effectively regulating the morphology,size,and crystal orientation.The phase and crystal morphology of LiMnPO₄ have changed significantly by adjusting the ratio of the mixed solvent and the feeding sequences of the precursor materials.Novel LiMnPO₄ nanocrystals have been successfully synthesized via a facile solvothermal approach in a mixed solvent of ethylene glycol?EG?and water.Benefiting from the special plate-like morphology and uniform thin carbon coating,LiMnPO₄/C composite exhibit a high reversible capacity of 158.5mAh/g at 0.1 C.It is extraordinary and remarkable for the preparation of high-performance LiMnPO4 cathode material,providing a new method for the preparation of electrode materials for Li-ion batteries.3.The plate-like LiMn_1-xMg_xPO₄/C possessed large specific surface area and well-defined mesoporous structure,which were synthesized from the microwave-assisted solvothermal method.In particular,Mg²⁺doping exerts a significant effect on synthesizing flake-like nanocrystal which favors lithium-ion extraction/insertion reactions,improving the electrochemical activity and electrochemical performance of LiMnPO₄/C material.In particular,Mg²⁺doping exerts a significant effect on synthesizing flake-like nanocrystal which favors lithium-ion extraction/insertion reactions,improving the electrochemical activity and electrochemical performance of LiMnPO₄/C material.As a result,the LiMn_0.95Mg_0.05PO₄/C nanoparticle exhibited a high reversible capacity of 141.2 and 95.3 mAh/g at 0.1 C and 5 C,respectively,exhibited outstanding charge/discharge performance and rate capability.Additional,LiMn_1-x-x Fe_x PO₄/C composite have been successfully prepared via a simplified sol-gel method.Fe has been successfully embedded into the LiMnPO₄ lattice,which can strengthen the structural stability of LiMnPO₄ by enhancing the ionic transfer efficiency.The proper substitution amount is beneficial to refine crystal size,increase the electrical conductivity,and improve the mobility of Li⁺.LiMn_0.8Fe_0.2PO₄/C possessed good long-term cycling performance and rate capability with the discharge capacity of 152.2,148.9,142.7,130.5,115.2 and 95.7 mAh/g at 0.1,0.2,0.5,1C,2C and 5C,exhibiting better electrochemical performance.Ion doping is an effective method to improve the cycle stability and rate performance of LiMnPO₄/C,which is suitable for large-scale applications.