Synthesis And Performance Of LiMnPO₄ By Solid State Method

Olivine structured lithium manganese phosphate（LiMnPO₄） is considered to be one of the most potential cathode materials for lithium ion batteries due to its low cost, structural stability, environmental friendliness, high theoretical capacity of 170 m Ah/g and high voltage plateau of 4.1V which is compatible with the electrochemical window of commercial electrolyte. However, the low intrinsic electric conductivity and low ionic diffusion rate largely limit the practical application of LiMnPO₄.At present, traditional modification strategies to improve the electrochemical properties of LiMnPO₄ are particle size reduction, carbon coating and metal cation doping. Based on the solid-state method, cation doping and co-doping on Mn-site are tried to improve the electric conductivity and thus improving the electrochemical properties of LiMnPO₄. X-ray diffraction, elemental analysis, thermogravimetry, partical size analysis, scanning electron microscopy, charge/discharge test, cyclic voltammetry and impedance spectroscopy were used to study the structure, morphology and electrochemical properties. The mechanism of metal ion doping and co-doping was discussed. The main work of this paper is as follows:Firstly, the Mn-site was doped by ferric ion. Li Mn0.7Fe0.3PO4/C materials were prepared and the synthesis conditions were opimized. The opitimized synthesis conditions were as follows: dry ball milling, carbon content was 7wt%, tablet pressure was 10 MPa, sintering time was 8h, sintering temperature was 550℃. Compared with LiMnPO₄/C and Li Mn0.7Fe0.3PO4/C, the result showed both materials were olivine structured, the particle size of Li Mn0.7Fe0.3PO4/C was small and uniform, which had a Z-Average size of 296.6nm and PDI of 0.165, the discharge capacity of Li Mn0.7Fe0.3PO4/C at 0.1C and 1C was 136.4m Ah/g and 104.2m Ah/g and the capacity retention was 96.3% and 96.0% after 50 cycles, respectively. CV and EIS tests showed that Fe2+ doping significantly increased the electric conductivity and the electrochemical properties were greatly improved.Secondly, the Mn-site was doped by magnesian ion, Li Mn1-x Mgx PO4/C（x=0.02, 0.04, 0.06, 0.08） materials were synthesized and the results showed that all samples were olivine structured. When x=0.04, the particle size had a well distribution, which had the smallest Z-Average size of 147.5nm and PDI of 0.154. The discharge capacity showed a trend of increase at first and then decrease with the doping amount of magnesian ion. When x=0.04, the electrochemical properties of Li Mn0.96Mg0.04PO4/C were best. The discharge capacity of Li Mn0.96Mg0.04PO4/C at 0.05 C, 0.1C and 1C was 107.5m Ah/g, 105.4m Ah/g and 77.1m Ah/g respectively. Mg2+ doping improved the electrochemical properties of LiMnPO₄/C, but the effect was not obvious compared with Fe2+ doping.Based on the former studies, Li Mn0.7Fe0.3-x Mgx PO4/C（x=0.02, 0.04, 0.06, 0.08） composites were synthesized by co-substitution of Fe2+ and Mg2+ on Mn-site. The result showed that all samples were homogeneous phase olivine structured. With the increase of doping amount, the specific capacity and energy density of Li Mn0.7Fe0.3-x Mgx PO4/C had a trend of increase first and then decrease. When x=0.04, the ratio of Fe and Mg is 0.26∶0.04, the particle size of Li Mn0.7Fe0.26Mg0.04PO4/C had a well distribution, which had the smallest Z-Average size of 272.8nm and PDI of 0.109. It showed the best electrochemical performance, and the discharge capacities was 159.7m Ah/g, 154.3m Ah/g, 148.2m Ah/g, 143.9m Ah/g, 134.7m Ah/g and 110.7m Ah/g at 0.1C, 0.2C, 0.5C, 1C, 2C and 5C respectively. The capacity retention rate was 94.5% after 50 cycles at 1C and it showed an excellent rate capability and cycling performance. It was the synergistic effect of co-substitution of Fe2+ and Mg2+ that increase the electric conductivity and lattice stability. Non-stoichiometric ratio Li Mn0.7Fe0.23 Mgx PO4/C（x=0.035, 0.105） materials were synthesized. The result showed that Li Mn0.7Fe0.23Mg0.035PO4/C with inadequate Mg2+ had the best electrochemical properties, and highest discharge capacity was 158.6m Ah /g at 0.1 C sintered at 550℃.