Preparation And Modification Research Of LiMnPO4as Cathode Material For Lithium Ion Battery

Compared with conventional cathode materials for lithium ion batteries, LiMnPO4has many merits, such as high potential plateau, high theoretical capacity of170mAh/g, low cost、 stabilized cycling, good thermostability and environmental friendly, but the lower conductivity and diffusion of Li-ion of LiMnPO4has seriously influenced its electrochemical performance, blocking its application.In this paper, we first synthesized the carbon-coating LiMnP04material by solid-state method to solve the disadvantages of low conductivity and diffusion of Li-ion, we used Mg2+and Fe2+to modify LiMnPO4/C; then, another convenient synthesizing method was presented, we prepared the spherical carbon-coating LiMnPO4by spray drying followed by heat treatment, the influencing factor of the two experiment project was investigated systematicly. X-ray diff usion, scanning electron microscopy(SEM), differential thermoanalysis thermogravimetry (TG/DTA), charge-discharge test, AC impedance test and cyclic voltammetry(CV) were used to research the synthesizing craft, structure characterization, material modified, electrochemical performance and diffusion coefficient of li-ion.The results showed, the metal ion doping could effectively improved its electrochemical performance, appropriate doping content could improve the performance obviously, Mg2+doping could stabilize the material structure, restrain the stress generated by the volume change when charging-discharging, when x=0.04, this value of the amount of Mg doping provides a compromised balance between the positive effect of inactive Mg2+doping and the negative effect of the electrochemically active Mn ion number reduction, exhibited144mAh/g at0.05C; in consideration of lowing the theoretical capacity by Mg doping, we used Fe doping to improve the rate performance of the material, the discharge capacity of the samples increased with Fe content and reached an equilibrium value, the optimized synthesized condition for Fe doping was x=0.5,600℃for10h, the corbon content was10%.The synthesized condition of the samples which was Mg doping of x=0.04and Fe doping of x=0.5was investigated systematicly to explain the performance improvement of LiMnPO4material. The result shows, when x=0.04, it had the smallest impedance; when x=0.5, the discharge capacity at0.1C was147.3mAh/g,115.2mAh/g at2C, the capacity retention rate was97.8%after100cycles, its diffusion coefficient of li-ion was1.35×10-8 cm2/s at3.9V by CV test, both lager than LiMnP04, indicated after carbon coating and Mg or Fe doping, the electrochemical performance was improved.The olivine LiMnPO4/C powder was prepared by spray drying method followed by heat treatment, in which the corresponding acetate salts as the reagents. The product showed good crystallinity, spherical particle and homogenous size distribution. The results showed that high spray drying temperature and smaller nozzle diameter could form bigger precursor particle, the as-prepared precursors were pre-sintered at400℃for2h and re-sintered at600℃for12h, and the resulting LiMnPO4/C powder exhibited good electrochemical performance. The initial discharge capacity of the cathode at0.1C is115.4mAh/g. The particle size of the sample was about3u m, the primary particle was consisted of smaller twice particle, and it was distributed homogeneously. The carbon coating was effective in enhancing the electronic conductivity of LiMnPO4, lowering the polarization of the materials, hindering the particle growth, consequently improving the electrochemical performance.