Preparation And Modification Of The Carbon Coated LiMPO₄（ M = Fe,Mn）

Olivine-type LiFePO4 and LiMnPO4 have received much attention as the new generation cathode materials for lithium ion batteries because of their low cost, high safty, large theoretical capacity and stable cycle performance. However, low Li+ diffusion rate and poor conductive performance seriously limit LiFePO4 and LiMnPO4 commercialization process and their applications. In order to improve their electrochemical properties, LiFePO4 and LiMnPO4 were synthesised by hyperthermal solid-state reduction and were doped by carbon coating and ion doping. The materials were characterized by means of scanning(SEM),electrochemical impedance spectroscopy(EDS), X-ray diffraction(XRD), energy-dispersive spectroscopy(EDS) and charge/discharge test, etc.LiMPO4/C(M=Fe,Mn) were synthesised by hyperthermal solid-state reduction in this paper. First, the synthetic conditions were optimized. Then different carbon sources such as glucose, citric acid, Poly propulene(PP) were applied to synthesis LiMPO4/C(M=Fe, Mn)composite materials under the optimized conditions. It was found that Poly propulene(PP)showed better cycle performance and rate performance. The effect of carbon-coated content of LiMPO4 was studied and found that the optimum carbon content was 15% for LiFePO4 and10% for LiMnPO4, respectively.The modification of Na+ doping were also studied. Results suggested that Sodium polyacrylate(ASAP) as a carbon source can fulfill carbon-coating and Na+ doping at the meantime. Nax Li1-x MPO4 /C prepared with ASAP displayed a better olivine structure and a more uniform particle size distribution than that with PP. NaxLi1-xFePO4 exhibited better electrochemical performance with an initial discharge capacity of 167 mAh/g at 0.1 C. The sample also showed a favorable cycling stability that the discharge retention specific capacity was 154 mAh/g ater 50 cycles or equivalent to 92.2% of the initial discharge capacity.NaxLi1-xMnPO4 showed a discharge capacity of 157 mAh/g at 0.1C and an excellent cycle performance with a discharge retention specific capacity of 152mAh/g after 50 cycles or equivalent to 96.8% of the initial discharge capacity. These results indicate that the proposed method is an efficient and practical technology to prepare LiMPO4/C(M=Fe,Mn).