LiMnPO _{Cathode Material 4} Based Lithium-ion Batteries

Lithium-ion batteries, due to its light, portable and high-energy, have been widely used in electric vehicles, camera and laptop devices. However, Developing advanced cathode materials with high energy density, high safety and better to meet the requires of the market is the focus of the present study. Olivine-structured LiMnPO4has recently received increasing attention owing to its higher redox potential (4.1V versus Li/Li+), lower cost and good safety. However, the inherently low ionic and electronic conductivities of LiMnPO4seriously limit its electrochemical performance.Based on the composition of Fe&Mg co-substituted LiMnPO4, and for the purpose of improving the electrochemical performance of LiMn0.8Fe0.19Mg0.01PO4/C, the effect of raw materials on the structure, morphology and electrochemical performance of LiMn0.8Fe0.19Mg0.01PO4/C were studied. The study found that the raw materials play a key role on the formation process and final product of the material. And the material used LiH2PO4as lithium source and phosphorus source, manganese acetate as manganese source show, due to small particle size and uniform distribution, the best electrochemical performance.Next, the influence of the carbon content and heating rate on the electrochemical performance of LiMn0.8Fe0.19Mg0.01PO4/C was carefully studied. Since the poor electrical conductivity of LiMn0.8Fe0.19Mg0.01PO4/C, by coating a small amount of corbon (<10.5wt.%) can effectively improve the electrical conductivity. But when the carbon content exceed the10.5wt.%, with the increase of carbon content, the electrochemical properties will be worse and worse. In this experiment, when the residual carbon is reduced to4.1wt%, the material still present excellent rate performance. Meanwhile, the study found that although the heating rate does not affect the morphology and crystal structure of the materialLiMn0.8Fe0.19Mg0.01PO4/C, but increasing the heating rate can reduce the residual carbon content in the material, and the decreases of carbon content will affect the conductivity and grain size of the material.Finally, in order to improve the electrochemical properties of LiMn0.8Fe0.19Mg0.01PO4, an electronically conductive phosphides, synthesized from a non-stoichiometric mixture of starting materials with an iron:phosphorus excess ratio of2:1, were created on the surface of the LiMn0.8Fe0.19Mg0.01PO4. The study found that this method can effectively improve the rate performance of the material. In addition, the influences of the content of phosphides and sintering temperature on the morphology and dispersion of phosphide were primary discussed, and the influence on the electrochemical properties of material LiMn0.8Fe0.19Mg0.01PO4were also investigated. The experiment found that:(1) Introducing a small amount of phosphides, which can effectively improve the electrical conductivity and electrochemical performance of LiMn0.8Fe0.19Mg0.01PO4, but when the phosphides above the critical amount the material undergo a significant degradation.(3) For the sample sintering at700℃, due to the distribution of phosphide is good for the electric conductivity of material, shows the best rate performance. when discharge at1C,2C,3C,5C,10C, the material can deliver a discharge capacity of145mAh g-1,140mAh g-1,136mAh g-1mAh g-1,131mAh g-1,122mAh g-1respectively. Even charge/discharge at high rates, the material still had excellent rate performance.