High Temperature Synthesis Of LiFePO₄ By Solid-state Method And The Modification Research

Along with the increasing popularization of the portable electronic products, battery as a portable power source receives high attention in the world. LiFePO₄ is emerging as a promising cathode material for lithium-ion batteries because of its security, low cost and environmental compatibility. The conductivity and the ion diffusibility of LiFePO₄ are poor due to the restriction of its crystal structure, so the practical specific capacity is low and the discharge performance at high rates is poor. The modification research is focused on the enhancement of the conductivity and the ion diffusibility.In this paper, LiFePO₄ cathode material was prepared by solid-state method and modified by carbon coating and Nb⁵⁺, SO^2- doping. The effects of thermal processing conditions, carbon coating, ion-doping and particle size on the phase-purity, particle size, morphology and electrochemical performance of the materials are investigated by XRD, SEM, CV, EIS.The results indicated that the optimized method of synthesizing LiFePO₄/C material with Li₂CO₃, FeC2O₄·2H2O, NH4H2PO₄ and glucose is presintering the precursor at 300℃for 6h in N₂ atmosphere, and then roasting at 650℃for 16h. The most optimized carbon-coating content should be controlled in the 6wt.%.In order to improve the performance of LiFePO₄ cathode materials, the LiFePO₄/C doping with Ti⁴⁺, V⁵⁺, Nb⁵⁺ was synthesized. The results indicated that metal ion doping could improve the electrochemical performance of LiFePO₄/C, especially Nb5+ doping. The discharge capacity of LiFePO₄/C doped with Nb⁵⁺ reached 154mAh·g^-1 at the C/5 rate, reaching 91% of theoretical capacity, and 148mAh·g^-1 at 0.1C rate. Even at the 1C rate, a discharge capacity of 117mAh·g^-1 was obtained.The LiFeP1-xSxO₄/C (x=0.03, 0.05, 0.08) material was synthesized through the medthod of doping defined amount of SO₄2- into the precursors. The results showed that the sample of 5% S-doped obtained a capacity of 140.3mAh·g^-1 at 0.05C rate, and 132mAh·g^-1 at 0.1C rate after 20 cycles. Even at 1C rate it could stay at the level of 90mAh·g^-1 after 60 cycles.The amount of carbon-coating and Nb, S ion doping was optimized through an orthogonal experiment. The results indicated that the best preparation method is 6wt.% carbon coating and the optimized Nb, S doping amount is 0.05 and 0.08. In this condition, the LiFePO₄/C material synthesized with optimized method was pure well-ordered olivine phase with no impurities, and obtained a discharge capacity of 141.1mAh·g^-1 at 0.05C, and 114mAh·g^-1 even at 1C.