Preparation Of Modified Lithium Ironphosphate Cathode Material And Study On Low Temperature Electrochemical Properties

In order to improve the electronic conductivity and structural stability of LiFePO4,and then improve the electrochemical properties and low temperature electrochemical properties of LiFePO4,the preparation process was optimized to synthesize LiFePO4 materials with better comprehensive electrochemical properties,and the materials were modified both in room temperature and low temperature.LiFePO4?LFP?with the best electrochemical performance was synthesized by single factor control variable method and modified by carbon coating.Porous-LiMn0.03Fe0.97PO4/C materials with surface microporous modification,Mn body-doping and carbon coating were synthesized by one-step solvothermal method on the basis of LFP.On the basis of LFP,LFP/Ag/RGO composites were synthesized by two-step hydrothermal method with 1.0M LiPF6 as solute and EC:DMC:DEC:EMC=1:1:1:3?wt.?as solvent.The electrolyte had the best low temperature performance.XRD,TEM,SEM,BET,EIS,CV and charge-discharge tests were used to characterize the crystal structure,morphology and electrochemical properties of the synthesized materials.The following conclusions are drawn:?1?The effects of pH,solvothermal reaction temperature,reaction time and calcination temperature on morphology,crystallinity,particle size,particle dispersion and electrochemical properties of LiFePO4 were systematically investigated by single factor controlled variable method,and the preparation process was optimized.The prepared LPF was initially modified by carbon coating under the optimum preparation conditions.The optimum preparation conditions of LiFePO4 with narrow particle size distribution,rod-like morphology were as followed:using H3PO4,FeSO4.7H2O,LiOH.H2O as raw materials,L-ascorbic acid as reducing agent,ethylene glycol as solvothermal solvent,then LFP was obtained by solvothermal method and calcination.The first specific capacity of charge-discharge at 0.2C was about 125 mAh.g^-1,and the capacity retention rate of 50 cycles at 0.2C was 93.3%.The first discharge capacity of LFP/C modified by 5wt%carbon coating was increased from 128 mAh/g to 145 mAh/g at 0.2C,and the specific capacity was increased to 13.4%.The sum of total resistance Rsei and Rct in the high and medium frequency regions decreased by 50?.?2?A series of Porous-LiMnxFe1-xPO4 materials were prepared by CTAB surface microporous modification and Mn doping modification of LFP.Mn:Fe?atomic ratio?=3:97had the best electrochemical performance.Carbon-coated LFP composites with high specific capacity,low internal resistance and excellent cyclic stability were obtained.The first charge-discharge specific capacity of Porous-LMFP material reached 151 mAh/g at 0.2C rate,and the capacity retention rate of 70 cycles at different rates was 93.3%.At 10C high current density,the discharge specific capacity could also reach 74.5mAh/g.Compared with different Mn doping content,the first charge-discharge specific capacity of Porous-LiMn0.03Fe0.97PO4material at 0.2C condition was the best.After carbon coating,the first discharge capacity of0.2C reached 152.4mAh/g,and the capacity retention rate was 90.7%after 100 cycles.?3?GO prepared by improved Hummers two-step oxidation method had thinner lamellar structure,more oxygen groups in the lamella and good quality.Ag nanoparticles with60-70 nm diameter were prepared by Lee-Meisel method,and the nano-Ag had better dispersivity.LFP/Ag/RGO was synthesized by secondary hydrothermal method.The loads of Ag and GO were 3 wt%and 1 wt%respectively.The first charge-discharge specific capacity of modified LFP/Ag/RGO at-15?0.2C ratio was 94 mAh/g,polarization voltage was0.422V,and the sum of solid electrolyte impedance?Rsei?and charge transfer impedance?Rct?of LFP/Ag/RGO was 321.3?.As low temperature electrode material,LFP/Ag/RGO was found that the discharge capacity of C electrolyte EC:DMC:DEC:EMC=1:1:1:3?wt.?was higher than that of C electrolyte EC:DMC:DEC:EMC=1:1:3?wt.?at low temperature of-15??109mAh/g at 0.2C?.The lowest impedance of LFP/Ag/RGO in C electrolyte was247.9?.at-15?LFP/Ag/RGO exhibited good cyclic stability in C electrolyte.The capacity retention rate reached 94.6%after 100 cycles.The synergistic effect of material modification LFP/Ag/RGO and electrolyte improvement greatly improved the low-temperature electrochemical performance of the material.