Carbothermal Reduction Synthesis Of Lithium Iron Phosphate Electrode Process Research

In this paper, LiFePO₄/C composite cathode materials synthesized bycarbothermal reduction method using as-prepared FePO₄,LiOH-H₂O andglucose as raw materials were reported. The reaction mechanism andconditions were also investigated. Besides, the electrode process kineticsof LiFePO₄/C was studied. The main points can be summarized asfollows.The mechanism of carbothermal reduction reaction was investigatedby Thermogravimetry-Differental Thermal Analysis (TG-DTA), ElementalAnalysis (EA),Fourier Transfer Infrared Spectroscopy (FTIR) and X-rayDiffraction (XRD), and the effect of calcination temperature on thecompostion of the products was also analysized. The results show theneeded reduction carbon generates at 250℃, and LiFePO4 is obtained at300℃without any intermediate phase. At 300,400,500℃, Li₃PO₄ andFe³⁺ as impurity phases exist in the samples and the higher temperatureleads to less impurity phases. The sample calcined at 600℃has noimpurity.The effects of calcination temperature,calcination time,carbonsource and carbon content on structure,morphology,conductivity,tapdensity and electrochemical performance were systematically investigated.The optimal conditions of preparing LiFePO₄ are as follows: the rawmaterials containing as-prepared FePO₄, glucose, LiOH-H₂O are firstlyball-milled 2h at a rate of 250 r·min^-1, and then calcined 24h at 600℃, andthe carbon content is 8.31%. The first charge and discharge specialcapacities are 152.5,151.8 mAh·g^-1 at 0.1C, whereas the discharge specialcapacities at 0.5C,1C are 135.5,125.7 mAh·g^-1, respectively. The capacityfade per cycle is 0.17‰(at 0.1C and in 30 cycles),0.32‰(at 0.5C and in40 cycles),0.38‰(at 1C and in 50 cycles).The electrode process kinetics of LiFePO₄/C was studied by CyclicVoltammetry (CV),Potential Step Chronoamperometry (PSCA) andElectrochemical Impedance Spectroscopy (EIS). CV analysis indicatesthat LiFePO₄/C is quasi reversible and Li-ion diffusion coefficients in oxidation,reduction process are 4.753×10^-12,5.889×10^-12cm²·s^-1,respectively. The Li-ion diffusion coefficients calculated by PSCA rangefrom 2.65×10^-13 to 9.84×10^-11cm²·s^-1, which has a minimum(2.65×10^-13cm²·s^-1) next to the potential plateau (3.45V). The EIS revealsthat the film resistance of the LiFePO₄/C electrode (R_f) increases withcharge, and reaches a maximum at 3.4V, and then drops. The filmcapacitance (C_f) of LiFePO₄/C electrode decreases firstly and thenincreases. R_Li,C_Li have similar changes with potentials. The chargetransfer resistance (R_ct) has a minimum at 3.4V.