Study On Preparation Andmodification Of Lithium Manganese Phosphate

In view of the low electronic conductivity and Li-ion diffusion rate of lithium manganese phosphate cathode material for Li-ion battery?LiMnPO₄?,modification methods such as adding mineralizers or surfactants and building three-dimensional conductive network among particles were adopted to improve the electrochemical properties of materials.The modification mechanism was also explored.The main contents are as follows:LiMn_0.8Fe_0.2PO₄?LMFP?was prepared by hydrothermal method,and the optimized conditions of material and positive plate preparation were studied.The results showed that the optimum conditions were:the molar ratio of Li?Mn_0.8Fe_0.2?P in the raw material was3?1?1,the LiOH was added dropwise to H₃PO₄ to obtain a white precipitate,and then MnSO₄ and FeSO₄?Li-P-M?were slowly added,the volume ratio of water and ethylene glycol was 3?5,the hydrothermal temperature was 240?,the slurry mixing speed was 3000 rpm,and the coating thickness was 40?m.On this basis,doping of 10% graphene or 20% carbon nanotubes in the Super P conductive agent improved the conductivity of the material,and the initial specific discharge capacities were 154.6 mAh/g and 154.8 mAh/g.After 50 cycles of 0.2 C rate,the capacity retention rates increased by 2.60% and 4.99% respectively compared with 89.96% without addition.The modified LMFP was prepared by adding the mineralizer potassium hydroxide?KOH?,cationic dodecyl dimethyl ammonium bromide?DDAB?,anionic sodium dodecyl sulfate?SDS?and amphoteric glycine?Gly?in the hydrothermal reaction.The results showed that the appropriate amount of KOH effectively promoted the formation of crystal nucleus and made the particle size smaller.When the concentration of KOH was 0.025 mol/L,the material showed the discharge capacity of 154.8 mAh/g at 0.2 C rate.In comparison with SDS and Gly,DDAB reduced particle size and increased particle size distribution uniformity.The LMFP/C-DDAB showed the initial discharge capacity of 153.1 mAh/g at 0.2 C and capacity retention rate of 91.84%after 50 cycles.Both KOH and DDAB reduced the particle size of the material and increased the effective carbon coating rate of the material.However,KOH and DDAB functioned differently in the formation of materials and the effect of the mineralizer KOH was better.The LMFP/C-KOH had a discharge capacities of 157.4,151.9,141.9,130.2,121.2 and 99.2 mAh/g at 0.1,0.2,0.5,1.0,2.0 and 5.0 C rates,respectively.The capacity retention rate was 94.3% after 100 cycles.The high specific area of graphene oxide?GO?and the high length-diameter ratio of carbon nanotubes?CNTs?improved the electron transport among particles.The results showed the discharge specific capacities of LMFP/GO-CNTs/C were 159.7,155.1,144.9,131.8 and 108.1 mAh/g at 0.2,0.5,1.0,2.0 and 5.0 C,which were increased by 5.0%,9.5%,16.9%,19.7% and 24.8%respectively,compared with LMFP/C.Conductive three-dimensional carbon frames prepared by pore-forming agents NaCl and SiO₂ were compounded with the material separately to fill the positive electrode material in the carbon network structure.The results showed,with NaCl as a pore former,the carbon framework prepared by ball milling with NaCl 0h+NaCl/Fe sugar 6h had the best structure and the pore size was large enough for LMFP particles to fill in the holes.The discharge capacity and cycle performance of material were improved.When the concentration of glucose was 8%,with SiO₂ as the pore forming agent,the carbon framework had complete structure and small wall thickness.When 2% carbon frame of 600 nm diameter was added in the hydrothermal process,the LMFP particles entered into the carbon framework,and the three-dimensional conductive network was formed among the particles.The initial discharge capacity reached 161.6 mAh/g at 0.2 C rate,and the capacity retention rate became 95.6% after 30 cycles.