Research On Surface Coating And Cell Performance Of LiMPO₄ Cathode Materials For Lithium-ion Batteries

Rechargeable lithium-ion batteries (LIBs) which have attracted extensive attention due to superior energy density, long lifetime and environmental benignity, have been regarded as promising energy storage and conversion devices for applications in electric vehicles (EVs) and smart grids. Olivine LiFePO4 cathode material due to its good structure stability, high safety and excellent cycling performance has already been widely used in commercial lithium ion batteries. Li3V2(PO4)3 material which belong to the phosphate system has also attracted extensive attention with high voltage platform (more than 4V) and high theoretical capacity. But the poor electronic conductivity and low lithium ion diffusion coefficient of phosphate materials restricted their application and development. In this article, we devoted to improve the electrochemical performances of the LiFePO4 and Li3V2(PO4)3 materials through coating. The main aspects are as follows:Firstly, a series of LiFePO4/C materials were prepared by two step ball milling, and the PVA was used as the carbon source with different amounts (Owt%,5wt%,10wt% and 20wt%) in this paper. The effect of different amounts of PVA on the phase structures and electrochemical performances was studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and constant current charge-discharge tests. The results showed that better electrochemical performance appeared when the amount of PVA was 10wt%. When the electrode was charged and discharged at 0.1C, its discharged specific capacity of LiFePO4/C was 155mAh/g. And capacity retention ratio of this material got up to 98% at 0.5C for 100 cycles. With the discharge capacities of about 155,148,146,142,124 and 103 mAh g-1 at 0.1,0.5,1,2,5 and 10 C, respectively. It was also found that all prepared materials had phosphor-olivine type structure without impurity appeared and the different content of carbon had different effect on its particles size by means of SEM and XRD.Secondly, the V2O3-C dual-layer coated LiFePO4 cathode materials with excellent rate capability and cycling stability were prepared by carbothermic reduction of V2O5. X-ray powder diffraction, elemental analyzer, high resolution transmission electron microscopy and Raman spectra revealed that the V2O3 phase co-existed with carbon in the coating layer of LiFePO4 particles and the carbon content reduced without graphitization degree changing after the carbothermic reduction of V2O5. The electrochemical measurement results indicated that small amounts of V2O3 improved rate capability and cycling stability at elevated temperature of LiFePO4/C cathode materials. The V2O3-C dual-layer coated LiFePO4 composite with 1 wt.% vanadium oxide delivered an initial specific capacity of 167 mAh g-1 at 0.2 C and 129 mAh g-1 at 5 C as well as excellent cycling stability. Even at elevated temperature of 55℃, the specific capacity of 151 mAh g-1 was achieved at 1 C without capacity fading after 100 cycles.Finally, Li3V2(PO4)3/C composites were prepared by carbothermal reduction method using the PVA as the carbon source, and the effect of different amounts of PVA on the phase structures and electrochemical performances were studied. The results showed that the prepared samples have the highest voltage platform and bestest rate performance when the amount of PVA was 10wt%.