Preparation And Electrochemical Properties Of Rare Earth Ions Doped Lithium Iron Phosphate Cathode Materials

Li Fe PO₄ cathode material is widely used in energy storage equipment due to its safety,low cost,environmental friendliness and excellent electrochemical performance.However,the application in high-rate Li-ion batteries（LIBs）is limited by its poor electronic conductivity and slow ion diffusion.In order to break these limitations,in this study,many efforts such as rare earth ion doping,carbon coating,controlling the particle size and morphologies have been taken to increase electronic conductivity and ion diffusion,and thus to speed up the process of its application in high-rate LIBs.Firstly,in this paper,the synthesis process was optimized.The influence of different sintering temperatures（500,600,700,800℃）on the material was researched.It was found that sintering at600℃is more conducive to the crystallization of the material.The effects of different carbon sources（ascorbic acid and glucose）on materials were studied.It was found that the materials coated with ascorbic acid had better graphitization degree,which made the materials have better electrical conductivity.Compared with the solid phase method,the liquid phase method can better mix the raw material evenly,and the hydrothermal reaction crystallized into uniform Li Fe PO4crystal firstly,so the material achieved better electrochemical performance.Li Fe PO₄/C was prepared under the optimal synthesis conditions,and the electrochemical performance of the material was tested by electrochemical workstation and battery tester.The specific capacity Li Fe PO4/C is 150.35 m Ah g^-1 at 0.2C.After 50 cycles of 1C,the specific capacity is 141.80 m Ah g^-1,and the material has good specific capacity and cycle performance.According to the optimized technology,Li Fe PO₄ doped with lanthanum and yttrium was synthesized,and the different doping amounts of Li Fe PO₄/C were obtained.The influence of doping amount on structure and morphology was analyzed by XRD,SEM and XPS.XRD results show that doping of La³⁺or/and Y³⁺can enlarge the lattice of Li Fe PO4 and widen the migration path of Li⁺.The XPS test results show that doping can reduce the electrostatic binding of surrounding atoms to Li⁺.These test results indicate that Li⁺is more easily embedded/removed,which will enhance the electrochemical performance of the material.The electrochemical performance of all materials was tested.The results show that the specific discharge capacity of La1-LFP/C was 157.17 m Ah g^-1 at 0.2C,and the specific discharge capacity of Y1-LFP/C was158.93 m Ah g^-1.The specific discharge capacity of co-doped La·Y2-LFP/C reached 160.56 m Ah g^-1.The rate performance and cycle performance of the materials with the best doping amount were tested,in which the capacity retention rate of La·Y2-LFP/C was 99%after 100 times charging and discharging at 5C.Co-doping combined with the advantage of La³⁺/Y³⁺single doping,it has the better Li⁺diffusion rate and electron conductivity,resulting in excellent electrochemical performance of the material.The mechanism of doping La³⁺and Y³⁺to improve the electrochemical properties of the materials was further analyzed by the theoretical studies.The structural optimization results show that the expansion of lattice parameters improves the ion diffusion channel,which is conducive to improving the Li⁺diffusivity of Li Fe PO₄.The calculation results of band gap indicate that the electrical conductivity of the doped material is enhanced,and the DOS explains the electronic change of the system,which confirms that the doped material has good electronic properties and can effectively improve the electronic conductivity of Li Fe PO₄.