The Study On Synthesize And Performance Of Phosphate Cathode Materials For Lithium Battery

With the large-scale application of lithium-ion batteries in electric vehicles and energy storage batteries,this put forward higher requirements on the safety performance and the energy density of the battery.The phospho-olivines LiMPO₄（where M=Fe,Mn,Co）become the first choice to the cathode material due to its high safety.However,the low ionic and electronic conductivity restrict the development of LiMPO₄.Otherwise,the monoclinic Li₃V₂（PO₄）₃ exhibits a much higher Li-ion diffusion coefficient and higher inter-calation potentials,but it has the poor cycle performance.The previous work in literatures has shown that the ion doping and composite material with Li₃V₂（PO₄）₃ can improve the electrochemical performance of LiMnPO₄,respectively.Herein,we combine the ion doping with composite material to further improve the electrochemical performances.Based on the above,this paper mainly studied the following contents:Firstly,Li₃V₂（PO₄）₃ cathode material was studied systematically.The effects of different carbon sources and reaction temperatures to the structure and performance of materials were studied.The experiment results showed that the Li₃V₂（PO₄）₃/C coated by pitch as new carbon source and calcined at 750 °C had the better electrochemical performance than others.Then the spherical Li₃V₂（PO₄）₃/C was synthesized by spray drying method.Compared to Li₃V₂（PO₄）₃/C synthesized by carbon thermal reduction method,the spherical Li₃V₂（PO₄）₃/C had the best tap density and electrochemical performance.Secondly,LiMn_0.5Fe_0.5PO₄/C solid-solution composite was studied preliminary.The structure and performance of solid-solution composite was studied.The XRD result showed that Fe-doped material had the same structure with LiMnPO₄,but the lattice parameters became smaller than those of Li MnPO₄,indicating the Fe²⁺had successfully been doped in lattices of LiMnPO₄.The SEM images indicated that the nanorod grains of LiMn_0.5Fe_0.5PO₄/C had the smaller sizes than LiMnPO₄/C.The electrochemical tests exhibited that the initial discharge capacity and cycle stability of LiMn_0.5Fe_0.5PO₄/C were superior to those of LiMnPO₄/C.Finally,based on the above,the LiMn_0.5Fe_0.5PO₄-Li₃V₂（PO₄）₃ composites were synthesized and studied by rheological phase reaction method.The impacts of calcined temperature and components of the composite material had been studied.The XRD results showed the composite materials were the mixed phases of LiMn_0.5Fe_0.5PO₄ and Li₃V₂（PO₄）₃.The electrochemical test showed that the material with 1:1 mole ratio calcined at 750 °C had the highest capacity and best cycle stability.Also,this composite had better electrochemical properties than the single phased LiMn_0.5Fe_0.5PO₄ and Li₃V₂（PO₄）₃,respectively.