Synthesis Of Nano-structured LiFePO₄ And Its Doping & Surface Modifications

As a promising cathode material for next-generation lithium ion battery, lithium iron phosphate exhibits many appealing features, such as low cost, environmental benign, suitable potential plateau, high theoretical capacity, long cycle life, ideal thermal stability, etc. However, its intrinsic electrical conductivity and lithium-ion diffusion velocity are rather poor, which seriously undermines the kinetics of LiFePO4 and thus greatly limits the large-scale application in the field of power battery. In this work, the sol-gel synthesis, doping and surface modifications of LiFePO4 were investigated systematically. The main research work and conclusions are given as follows:An innovative inorganic-based sol-gel route to synthesize nanostructured LiFePO₄/C cathode material with excellent electrochemical performance was introduced. The cheap, environmental friendly inorganic compounds (FeCl₂·4H₂O, H₃PO₄ and Li₂CO₃) were used as raw materials. The influences of the sintering temperature, holding time and residual carbon content on the electrochemical performance of LiFePO₄/C were investigated. The optimized sintering temperature and holding time were 650℃and 15 h, respectively. And the sample with 4.5 wt.% residual carbon exhibited excellent electrochemical performance, at 10 C, its discharge specific capacity was about 108 mAh/g.The supervalent Sn⁴⁺ was firstly introduced as a dopant. The effects of doping amount on the physicochemical and electrochemical performances of nanocrystalline LiFePO4/C were systemically investigated. The charge compensation mechanism of Sn⁴⁺ was studied. It was found that the doping of Sn was a mixed-valence doping. On the basis of the nanosized effect and doping concentration, samples showed pseudocapacitive behavior. When the doping amount was about 3 mol.%, the sample showed excellent electrochemical performance. At 10 C, the discharge specific capacity was about 128 mAh/g.The effects of adding amount of vanadium on the physicochemical and electrochemical properties were investigated in details. The concentration-composition phase diagram was constructed. The increasing adding of vanadium induces 1, the V⁴⁺ substituted for Fe (0 < x≤0.07) within the solid solubility; 2, beyond the solid-solution limit, the excess vanadium formed VO₂(B) coated on the surface of the V-doped LiFePO₄; 3, the excess vanadium formed the secondary phase Li₃V₂(PO₄)₃ (x≥0.11) coexisting with the V-doped LiFePO4. The V⁴⁺ doping contributes to induce the lattice distortion, refine the particle size, increase the electrical conductivity and thus greatly improve the electrochemical performance, especially the rate capability. The surface modification of nano-sized VO₂(B) is helpful to increase the electrical conductivity greatly. Due to the nanosized effect, the sample shows a high energy and power density. The secondary phase Li₃V₂(PO₄) is favorable to increase the electrical conductivity. It plays a paramount role in improving the rate capability of the LiFePO₄-based cathode, although it is not good for the lithium ion transport within the main phase.The electrochemical performances of LiFePO₄ cathode material by nanosized Sb-doped SnO₂ and ZnO coatings were preliminary examined. It was found that the coatings were beneficial to increase the electrical conductivity and enhance the electrochemical performance of LiFePO₄ cathode material.