Synthesis Of Polyanion-type Cathode Materials And Study Their Electrochemical Properties

In this paper, we main focused on the research of polyanion-type cathode materials(LiFePO4and Li2MnSiO4). the controlling synthesis of LiFePO4and Li2MnSiO4, and their electrochemical properties in lithium-ion batteries were studied: nanosize/microsize LiFePO4were synthesized using hydrothermal and solvothermal methods, and their electrochemical properties were studied. We also synthesized nanospherical Li2MnSiO4through solid-state reaction using spherical SiO2as precursor, then their electrochemical properties were also tested. The details list as follows:1. LiFePO4rhombus-plates with a thickness of200nm, length of2μm and width of1μm were synthesized by a simple hydrothermal method under160℃; The XRD figure showed that LiFePO4rhombus-plates has a strongest peaks at30degree, which was corresponding to (211/020) face. The further study revealed that the LiFePO4rhombus-plates expose large scale of (010) face, and the thinnest part of the rhombus-plates just corresponding to the [010] direction. Which means that we successful control the synthesis of LiFePO42. Spherical LiFePO4materials were synthesized by a solvothermal method using PEG400as solvent at200℃. Moreover, the size of spherical LiFePO4particles were about1μm. The spherical LiFePO4materials were come from the aggregation of nanoparticles. After heat treatment at high temperature, electrochemical measurements showed that the spherical LiFePO4materials have a discharge capacity of104mAh g-3. LiFePO4nanopolyhedrons with the size of～200nm have been solvothermally synthesized in the binary solvent of benzyl alcohol and ethylene glycol. As citric acid added, microellipsoids with an average size of1μm were formed. LiFePO4nanopolyhedrons and microellipsoids were coated by carbon layers using cellulose acetate as carbon source. Electrochemical measurements showed that the LiFePO4/C nanopolyhedrons could deliver a discharge capacity of145mAh g-1under1C and had a capacity decay rate of <11%after200cycles. while the LiFePO4/C microellipsoids had a discharge capacity of123mAh g-1under I C and the capacity decay rate was less than8%after200cycles.4. Uniform Li2MnSiO4/C nanospheres have been obtained by PEG-600assisted solid-state reaction using spherical SiO2as precursor. research showed that Li2MnSiO4nanospheres with size of50nm are embedded in the three-dimensional (3D) nest-like carbon network. In order to further improve the electrochemical performance of Li2MnSiO4, we used mixed carbon sources (glucose, cellulose acetate and GO) to get Li2MnSiO4/C/grapheme composites. Electrochemical measurements reveal that the composites exhibit first discharge capacity of215.3mAh g-1under1/20C, together with a stable discharge capacity of175mAh g-1after40cycles. The3D carbon network and the coating of carbon and graphene are favorable for improving the cycling stability..