Synthesis And Performance Study On Cathode LiFePO₄/C Material For Lithium-ion Batteries

LiFePO₄is a kind of cathode material for Lithium-ion battery. It has a good safetyperformance and high temperature performance, low cost and environmentally friendly, butthe discharge capacity and rate capability should be improved with low Li+diffusion rate andelectronic conductivity. In this paper, LiMxFe1-xPO₄/C （M for metal ions） are prepared bydoped metal ions using high temperature solid state reaction, the crystal structure,electrochemical properties are investigated by using scanning electron microscopy, X-raypowder diffraction, cyclic voltammetry, electrochemical impedance spectroscopy andgalvanostatic charge-discharge curves.LiFePO₄/C cathode material for Lithium-ion battery is prepared for used Li₂CO₃,FeC₂O₄·2H₂O, NH4H₂PO₄and glucose. The effect of the calcination temperature, calcinationtime and the amount of glucose addition for microstructure, crystal structure andelectrochemical properties of LiFePO₄/C are studied. The results show that LiFePO₄/C wasthe best electrochemical performance with glucose at7%（the mass ratio for glucose qualitycompare with Li₂CO₃+FeC₂O₄·2H₂O+NH4H₂PO₄mixture is7:100）, calcination temperatureat700℃and calcination time at12h. Theinitial discharge specific capacity of LiFePO₄/Cwas141.5mAh/g at0.1C discharge rate, and the capacity retained96.5%after30cycles.LiM_0.01Fe_0.99PO₄/C are prepared by doped content1%（the molar ratio for metal ionscompare with Fe is1:99） Mg²⁺, Zr⁴⁺and V⁵⁺in LiFePO₄/C, respectively. The results showthat there were no changed the olivine crystal structure of LiFePO₄/C, and the initialdischarge specific capacity by doped with Mg²⁺, Zr⁴⁺and V⁵⁺in LiFePO₄/C were147.9,150.9and155.4mAh/g at0.1C rate discharge, and increase4.5%,6.6%and9.8%compared withnon-doped LiFePO₄/C by applying0.1C current, respectively. The initial discharge specificcapacity of LiV0.01Fe0.99PO₄/C were131.3,112.2and103.2mAh/g at1,3and5C dischargerate, and increase7.0%,9.9%and21.3%compared with non-doped LiFePO₄/C by applying1,3and5C current, respectively.There are no changed the olivine crystal structure of LiFePO₄/C by doping content1%,2%and3%V⁵⁺. The discharge specific capacity were135.6,117.9and107.8mAh/g at1,3and5C discharge rate for LiV_0.02Fe_0.98PO₄/C, respectively. And the average diffusioncoefficient for1.2×10-13cm2/s of Li+insertion-disinsertion, the electrochemical migrationresistance for47.7and the best reversibility that it exhibits an excellent electrochemicalperformance. The square-type battery of043046is produced by LiV_0.02Fe_0.98PO₄/C as the cathode andgraphite as the anode. It has a good performance of discharge rate that the discharge specificcapacity at10C rate was above81.8%of1C rate. When1C current cycle at room temperatureand60℃, the capacity retention rate were96.4%and96.2%after1250and300cycles,respectively. The discharge specific capacity at0℃,-10℃and-20℃were84.3%,80.2%and63.3%compared with by applying1C current at room temperature, respectively.LiV_0.02Fe_0.98PO₄/C batteries have an excellent safety performance.