Carbon Layer Modification On Li-ion Battery Cathode Material LiFePO₄/C

Accompany with the serious environmental problem caused by the decreasing of the fossil energy and the using of the traditional energy sources,our human are ready to explore the new energy to remit the energy crisis and the contradiction between the environment and the human race.Except for the nuclear energy,new energy sources such as the wind-energy,hydro energy and solar energy need effective energy storage devices because of the intermittent.In many kinds of electronic storage devices in social use,lithium ion battery has been the most competitive materials for its high energy density,suitable electrochemical window,proper heat stability and low self-discharge property.Olivine-structured Li FePO₄ is a promising cathode material because of the long cycle life,low-cost price and environmental friendliness.However,there are two weakness in the Li FePO₄ material: small electronic conductivity and weak Li + diffusion coefficient,which limit its use in the power battery and long-life storage devices.Aimed to improve the above-mentioned weakness,plenty of trails have been applied to promote the electrochemical property of LiFePO₄-based cathode material including narrowing particle size or changing its morphology,alien ion doping or substitution and adding conductive agents onto the surface.The original carbon coating can improve the electronic conductivity at an appropriate level,but it cannot meet the requirement in high-rate performance and the long cycle life.In this work,we use the Li FePO₄ prepared via hydrothermal method and sovolthermal method as the material and incorporated nitrogen and boron into its carbon layer according to the optimized ratio and the adding sequence.We have successfully got the improved electrochemical performance and subsequently applied in Li FePO₄ for commercial use.N+B co-doped carbon coating tremendously enhances the electrochemical property: at rate of 20 C,the co-doped sample LiFePO₄/C-N+B?prepared in lab?can elevate the discharge capacity of LFP/C from 101.1 mAh g-1 to 121.6 mAh g-1,and the co-doped product based on commercial c-LiFePO₄/C-N+B?prepared using commercial product?shows the discharge capacity of 78.4 mAh g-1 rather than 48.1 mAh g-1.The positive doping strategy leads to synergistic electrochemical activity distinctly compared with single N?or B?doped materials?even much better than the sum of them in capacity at 20C?.We also carried out the pore-forming process in the carbon layer to elevate the lithium ion diffusion rate using naphthalene as the pore former,which successfully improve the discharging capacity and the long cycle life of LiFePO₄ at high rate.According to the BET characterization,the porous carbon material LiFePO₄/C-n10 reveals the expansion of the surface area and enlarges the pore volume from 0.015 cc g-1 to 0.098 cc g-1,therefore this method has elevate the lithium ion diffusion rate efficiently and is responsible for the higher electrochemical performance.Through the characterization XRD,Raman,XPS,SEM,HRTEM,TG,BET analysis?N2 and CO2 adsorption?,four-probe electronic method,LAND electrochemical performance test and EIS characterization,it is performed that the surface modification in carbon has no influence on the crystal structure and the particle size of Li FePO₄,meanwhile,the improvement of the electronic conductivity and the lithium diffusion coefficient can elevate the electrochemical performance of LiFePO₄ effectively.