Improvement On Performance Of LiFePO₄/C Cathode Materials For Lithium Ion Batteries

As the applications of electric vehicles and increasing needs of storage of solar-energy and wind energy, cathode materials for lithium ion batteries have attracted wide attention. Among them, LiFePO4 is considered as the first candidate as the potential cathode materials for high power batteries, because of its high structural stability, reliability and abundance resources. However, there are still some problems in the actual application:poor performance at low temperature and low energy density, and so on. Aiming at sovling these problems, the superfine LiFePO4 particles and carbon-shell LiFePO4/C composite were prepared by MA-one-step solid-state method. And secondary microsphere LiFePO4/C was also synthetised by hydro-thermal method.In order to improve the specific capacity and tap density of LiFePO4 materials, superfine LiFePO4 particles were synthesized via a ball-milling route followed by one step solid-state reaction. The effect of sintering temperature on the electrochemical performance was investigated, and the optimal sintering temperature was 550℃. The initial capacity is very high but the capacity fading is very fast. By the analysis of CV and EIS, it is because the fine active materials without carbon coating were directly exposed to electrolyte and the Fe2+was dissolving in the electrolyte.A nanosized LiFePO4/C composite with a complete and thin carbon-shell was synthesized via a ball-milling route followed by solid-state reaction using polv (vinvl alcohol) as carbon source. The LiFePO4/C nanocomposite delivers discharge capacities of 159,141,124 and 112 mAh g-1 at 1 C,5 C,15 C and 20 C, respectively. Even at a charge-discharge rate of 30 C, there is still a high discharge capacity of 107 mAh g-1 and almost no capacity fading after 1000 cycles. Based on the analysis of cyclic voltammograms, the apparent diffusion coefficients of Li ions in the composite are in the region of 2.42×10-11 cm2 s-1 and 2.80×10-11 cm2 s-1. Electrochemical impedance spectroscopy and galvanostatic intermittent titration technique are also used to calculate the diffusion coefficients of Li ions in the LiFePO4/C electrode, they are in the range of 10-11 to 10 cm2 s-1. In addition, at -20℃. it can still deliver a discharge capacity of 122 mAh g-1,90 mAh g-1 and 80 mAh g-1 at the charge-discharge rates of 0.1 C,0.5 C and 1 C. respectively.A secondary mesoporous LiFePO4/C microsphere was synthesized by simple and controllable hydro-thermal method using low-cost Fe3+ salt as a raw material. Benefiting from this unique hierarchical architecture, these microspheres can be densely packed together, giving a relatively high tap density of 1.5 g cm-3. Simultaneously, the mesoporous structure can probide a huge electrochemically available surface for the active materials and electrolyte, and LiFePO4/C microsphere has the improved electrochemical performance. It delivers capacities of 137 mAh g-1, 118 mAh g-1,109 mAh g-1 and 102 mAh g-1 at 1 C.5 C,10 C and 20 C, respectively.