Preparation And Doping Modification Of LiMnPO₄

As one of the most potential cathode materials for lithium ion batteries,olivine structured lithium manganese phosphate（LiMnPO₄）has many merits,such as high specific capacity（170.0mAh/g）,structural stability,higher voltage plateau than the commercial lithium iron phosphate（LiFePO₄）and compatible with the electrochemical window of commercial electrolyte.However,the poor intrinsic electrical conductivity and low ionic diffusion rate of LiMnPO₄ largely restrict its wide application.Based on hydrothermal method,cation doping and co-doping are tried to improve the electrochemical properties of LiMnPO₄.X-ray diffraction,scanning electron microscopy,partical size analysis,charge/discharge test,cyclic voltammetry and impedance spectroscopy were used to study the relationship among structure,morphology and electrochemical properties.The effect of cation doping and co-doping on the improvement of electrochemical properties were explored and summarized.Firstly,composite materials were prepared by hydrothermal method,the effect of carbon content,solution concentration,the speed of ball-milling and the rolling thickness of laminate on the electrochemical performance of materials were systematically discussed.The optimized conditions were as follows:the carbon content was 5wt%,the concentration of solution was 0.15mol/L,the speed of ball-milling was 300r/min,the rolling thickness of laminate was 0.16mm.A series of LiMn_1-xFe_xPO₄/C（x=0,0.1,0.2,0.3,0.4）materials with different amount of Fe²⁺doping were prepared based on the former studies,the electrochemical performance of the material was the best when x=0.3,it delivered a capacity of 132.9mAh/g at 1C and remained 95.1%of the initial capacity after 30 cycles.Secondly,a series of LiMn_0.7Fe_0.3-xMg_xPO₄/C（x=0.02,0.05,0.07）materials were synthesized by co-doping of Fe²⁺and Mg²⁺.The results showed that the interplanar distance of（101）plane of LiMnPO₄ can be enlarged by Mg²⁺doping,thus increasing the diffusion channels of Li⁺,which led to the reversed rate performance at 1C.In addition,the rate capability and cycling performance were improved to some extent due to the effect of Mg²⁺,which could stabilize the lattice,the electrochemical properties of LiMn_0.7Fe_0.28Mg_0.02PO₄/C were the best,it had the smallest particle size of 242.3nm and the discharge capacities was137.9 mAh/g at 1C.Thirdly,LiMn_0.7Fe_0.3-x.3-x Cu_xPO₄/C（x=0.02,0.05,0.07）materials were synthesized by co-doping of Fe²⁺and Cu²⁺.As the valence electron configuration of Cu²⁺was 3d⁹,which was similar to Mn²⁺（3d⁵）and Fe²⁺（3d⁶）,the doping mechanism was different from Mg²⁺,Cu²⁺could not only stabilize the lattice structure,but also improved the electronic conductivity of the materials effectively.The results showed that when x=0.02,it had the smallest particle size of 261.9nm and the highest discharge capacity,which was 159.7,143.8,138.8,129.6 and 114.7mAh/g at 0.1,0.5,1,2 and 5C respectively.At last,considering the three-dimensional channel of Li₃V₂（PO₄）₃,which were benefit for the diffusion of Li⁺and were expected to improve the rate performance of the materials,a series of xLiMn_0.7Fe_0.3PO₄·yLi₃V₂（PO₄）₃/C（x:y=1:0,1:1,3:1,5:1,7:1）composites were synthesized.The results of X-ray diffraction and cyclic voltammetry showed that the composites were a mixture of LiMn_0.7Fe_0.3PO₄ and Li₃V₂（PO₄）₃,the electrochemical activity of LiMn_0.7Fe_0.3PO₄ was significantly improved.The electrochemical properties of LiMn_0.7Fe_0.3PO₄·Li₃V₂（PO₄）₃/C were the best,it had a capacity of 170.6,149.0,139.1,123.6 and 93.5mAh/g at 0.1,0.5,1,2 and 5C respectively.