Investigations On Hydrothermal Preparation And Electrochemical Performance Of LiMnPO₄/C As Cathode Material

LiMnPO4 has become one of the most promising cathode materials for power batteries due to its high theoretical capacity,high voltage platform,low cost and environmental friendliness.In this paper,LiMnPO4 cathode material was modified by metal ion doping and graphene modification to overcome its own defects of low electronic conductivity and ion diffusion rate.Carbon encapsulation of Li3PO4 with sucrose by ball milling was studied in this paper to obtain Li3PO4/C particles with small particle size and high dispersion degree.Based on that,MnSO4·H2O?PEG400 and sucrose were used as the raw materials to synthesis LiMnPO4 cathode material by hydrothermal method.The effects of sucrose content,calcination temperature,holding time and heating rate on morphology and electrochemical properties of LiMnPO4 were systematically studied.Under the optimum technological parameters(sucrose content of 25wt%,calcination temperature of 600?,holding time of 3 h,heating rate of 5?/min),LiMnPO4/C synthesized by optimization has the primary particle size between 50 and 100 nm,and delivers the specific capacity of 86.3 mAh/g at 0.05 C,which is better than the pure LiMnPO4.In this paper,Li1-nxMxMnPO4/C composites doped by La3+?Zr4+?Nb5+and Na+ were synthesized to improve the electrochemical properties tgrough widening the Li+ diffusion path and generating Li+ vacancies by large radius ion doping.The experimental results show that the particle size and agglomeration degree of LiMnPO4 could be effectively reduced by doping appropriate amount of metal ions.Among all the doped samples,Na-doping sample has the best rate performance and cycle performance,of which the first discharge capacity is 125.4 mAh/g at 0.05 C.After 100 cycles at 0.5 C,the discharge capacity of sample could still reach 102.9 mAh/g with the retention rate of 94.2%.As for Li0.085La0.015MnPO4/C?Li0.09Zr0.01MnPO4/C and Li0.09Nb0.01MnPO4/C samples,the discharge specific capacities are 107.3,101.7 and 115.2 mAh/g respectively,which are significantly better than LiMnPO4/C(86.3 mAh/g).Additionally,Li0.95Na0.05MnPO4/C composite coated with reduced graphene oxide was prepared by freeze-drying method combined with carbothermal reduction for the first time.And the effects of reduced graphene oxide with different content on the morphology and electrochemical properties of Li0.95Na0.05MnPO4/C were studied systematically.The experimental results show that when the content of graphene oxide is 20 mg,the nanoparticles are loaded on the surface of reduced graphene oxide compactly and uniformly with the particle size of?50 nm.At the same time,particles and particles are connected by folded reduced graphene oxide chains and stacked together to form a two-dimensional conductive network,which is propitious to improve the conductivity of material.The results of electrochemical performance testing show that the first discharge capacity of sample is 150.4 mAh/g at 0.05 C,and the discharge capacity can still reach 125.4 mAh/g at 1 C after 100 cycles,with the excellent cycle performance.