| As one of cathode materials for lithium-ion batteries,the olivine-type LiMnPO4 has received considerable attention for the advantages of high voltage,high energy density,good safety and high compatibility with mainstream electrolyte electrochemical window.Unfortunately,LiMnPO4 suffers from intrinsic drawbacks of the sluggish lithium-ion diffusion and low electronic conductivity,which influence the electrochemical properties and become the main obstacles to its commercialization.In this paper,based on Fe doping on Mn-sites,The modified LiMnPO4 was prepared by solvothermal and solid-state methods through polyanion-site doping,carbon coating,constructing three-dimensional conductive network between particles and compounding with Li3V2?PO4?3.X-ray diffraction,partical size analysis,raman spectroscopy,scanning electron microscopy,charge/discharge test,cyclic voltammetry and impedance spectroscopy were used to study the relationship among structure,morphology and electrochemical properties.The contents are as follows:LiMn0.8Fe0.2PO4 was synthesized via substitution with Fe on Mn-site by solvothermal method and the process conditions were optimized.The results indicated that Fe element entered the crystal structure and formed solid solution with LiMnPO4.The LiMn0.8Fe0.2PO4/C materials showed the best electrochemical performance and its discharge capacity was 159.8 mAh/g at 0.1C when adopting hydrothermal time 4 h,wet milling and tablet pressure of 4 MPa.P position of polyanionic sites occupied by B in pure phase LiMnPO4 was investigated.The specific capacity of LiMnP0.98B0.02O4-???represents the amount of oxygen vacancy?was 135.8 mAh/g at 0.1 C when the B doping amount was0.02.The Si and B doped on the polyanion P-site was investigated on the basis of LiMn0.8Fe0.2PO4.Li1-xNa2xMn0.8Fe0.2P1-xSixO4,LiMn0.8Fe0.2P1-xBxO4-?,LiMn0.8Fe0.2MgxP1-x-BxO4 and Li1+2xMn0.8Fe0.2P1-xBxO4 were prepared by solvothermal method,separately.The results indicated that the electrochemical properties of Li-rich Li1+2xMn0.8Fe0.2P1-xBxO4materials with B occupies P position were higher than those of the other three kinds of doping methods and the Li1.04Mn0.8Fe0.2P0.98B0.02O4 material showed excellent electrochemical performance among all the materials.It had a discharge capacity of 157.9,153.2,144.0,133.0,127.2 and 102.7 mAh/g at 0.1,0.5,1.0,2.0 and 5.0 C,respectively.It shows that B doping can improve the ionic and electrical conductivity of the material compared with pure LiMn0.8Fe0.2PO4.The ethyl cellulose ethoce?EC?was selected as a carbon source for surface coating owning to its good film-forming ability and low carbon residual rate.The results indicated that the material with 2%carbon content with EC as carbon source could get better electrochemical performance.What's more,the electronic transmission ability on the surface of the material was improved due to the high degree of graphitization of the carbon layer formed by EC.The three-dimensional carbon frame?3DC?with suitable pore size was prepared by using NaCl as the pore-forming agent and glucose as carbon source.The LiMn0.8Fe0.2PO4prepared by adding 3DC in solvothermal process showed the best electrochemical performance.The initial discharge specific capacity of the material with 3%three-dimensional carbon frame was 157.0 mAh/g and the capacity retention was 94.5%after 30 cycles at 0.1C.The electrochemical performance tests showed that the Li-ion diffusion was faster and the polarization was smaller during charge and discharge.SEM results showed that the pore structure in 3DC was filled with LiMn0.8Fe0.2PO4 particles in the process of solvothermal.Three kinds of composite methods of LiMn0.7Fe0.3PO4 and Li3V2?PO4?3 were studied by solid-state method.The results showed that LiMn0.7Fe0.3PO4·Li3V2?PO4?3 composite material which was prepared from the pre-sintering products of LiMn0.7Fe0.3PO4 and Li3V2?PO4?3 showed the best electrochemical performance.It had a discharge capacity of167.0,162.3,158.2,138.5,128.0 and 110.9 mAh/g at 0.1,0.5,1.0,2.0 and 5.0 C,respectively.The discharge capacity was 161.1 mAh/g and the rate retention was 96.5%when it went back to 0.1 C after 30 cycles under different current density. |
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