Study On Preparation And Low Temperature Performance Of LFP/C-P Composite Cathode Material For Lithium-ion Battery

Lithium iron phosphate?LiFePO₄?has lots of advantages such as stable working voltage,good cycle performance,low cost and environmental protection etc.It is considered to be a potential cathode material in lithium ion batteries and has been widely used in the electric vehicle industry,fixed energy storage and other fields.However,LiFePO₄?LFP?has some disadvantages,such as low conductivity and slow diffusion rate of Li⁺,which will become more prominent at low temperature and high rate.In order to solve these problems,people try to modify it by reducing particle size,coating carbon,doping metal or non-metallic elements and introducing conductive polymers.In this thesis,graphite and triphenylphosphine are used as carbon source and phosphorus source,respectively,carbon-coated and phosphorus-doped LFP/C-P composite cathode material is prepared by sol-gel method.The powder of LFP/C-P composite cathode material is spherical,the particles belong to nanometer size and the particle size distribution is consistent,and it disperses evenly into the network structure formed by carbon and phosphorus.The results of electrochemical measurements at room temperature show that the performance of charge-discharge capacity and cycle rate of LFP/C-P composite cathode are improved significantly comparing with LFP,LFP/C?carbon-coated lithium iron phosphate?and LFP-P?phosphorus-doped lithium iron phosphate?cathode materials.The specific discharge capacity of LFP/C-P composite cathode can reach 168.8 mAh g^-1 at 0.1 C,and the capacity retention rate can still reach 98.8%after 100 cycles.The low temperature properties of LFP/C-P composite cathode materials are further studied in this paper.The study shows that the modification of carbon coating can form a uniform carbon layer,and the conductivity of the LFP/C-P cathode material is improved;the modification of phosphorus doping is beneficial to the formation of a network structure of carbon and phosphorus,and the generation of the Li⁺diffusion channel is promoted.Therefore,the synergetic effect of carbon coating and phosphorus doping can reduce the resistance of Li⁺release and intercalation during charge and discharge,and effectively increase the migration rate of Li⁺.In addition,coating carbon protects the surface of the LFP/C-P cathode material and inhibits the corrosion and damage of the electrolyte on the cathode material,and reduces the diffusion barrier of Li⁺in the charge-discharge reaction,thus increasing the specific capacity,rate performance and structural stability of the cathode material at low temperature.So that the overall electrochemical properties of LFP/C-P composite cathode materials can be greatly improved at low temperature.In the low temperature environment at-40?,the first discharge specific capacity of the LFP/C-P composite cathode material at 0.1 C can reach 85.8 mAh g^-1,and the capacity retention rate can still reach more than 90%after 50 cycles.The comprehensive performance of the LFP/C-P battery has been improved obviously,thus extending the practical application range of LFP material.Therefore,the technology of the combination of the coating and the doping provides a new idea for the modification and industrial application of LFP material,and it has very important practical significance.