Study On Electrochemical Performance Of Lithium Manganese Iron Phosphate Cathode Materials

Lithium-ion batteries have become the first choice of power sources for electric vehicles and hybrid electric vehicles,due to the advantages of small self-discharge,large energy density,light weight and low pollution.The cathode material is the core of the lithium-ion battery,and lithium manganese iron phosphate has become a promising cathode material because of its advantages of integrated lithium iron phosphate and lithium manganese phosphate.In this paper,ferrous sulfate and manganese sulfate are used as iron source and manganese source,ascorbic acid is used as antioxidant,polyacrylamide(PAM)is used as surfactant,and carbon source is used as glucose.LiFe1-xMnxPO4/C with different manganese content and non-stoichiometric LiFe0.2Mn0.8PO4/C lithium battery cathode materials were prepared by hydrothermal method,and the effects of synthesis conditions,manganese content,phosphorus and lithium element ratio on the crystal structure,morphology and electrochemical performance of the samples were also studied.LiFe0.2Mn0.8PO4/C cathode material was successfully synthesized by hydrothermal method under the conditions of different reaction temperature and reaction time.XRD results show that the diffraction peak gradually shifts to the right as the reaction temperature increases,and the reaction time is too short and too long to cause the diffraction peak to shift to the left.Scanning results showed that when the reaction temperature was 200 ℃ and the reaction time was 12 h,the powder morphology was regular,uniformly distributed,and the degree of agglomeration was small.This sample has the highest initial discharge specific capacity of 137.5 mAh/g at 0.1 C rate,and the best cycle stability at high rate.A series of LiFe1-xMnxPO4/C(x=0.70,0.75,0.80,0.85,0.90)cathode materials with different manganese contents were successfully synthesized by hydrothermal method at reaction temperature and time of 200℃ and 12 h.XRD results showed that the five groups of samples were all pure phases.XPS test results prove the existence of Mn2+,and EDS test shows that Fe and Mn elements are evenly distributed,which further proves the successful incorporation of manganese.The sample of LiFe0.30Mn0.70PO4/C has the highest first discharge specific capacity of 153.1 mAh/g at 0.1 C rate and the best rate performance due to the smallest particle diameter,which shortens the transmission path of Li+ and weakens the polarization phenomenon.The charge and discharge results show that as the manganese content increases,the discharge specific capacity decreases.The main reason is that the electrochemical activity of LiMnPO4 is worse than that of LiFePO4.The discharge specific capacity of the Fe2+/Fe3+redox pair is basically unchanged,while the linear decrease in the discharge specific capacity of Mn2+/Mn3+redox reaction also confirms this.The two discharge voltages increased first and then decreased with the increase of manganese content,and reached the maximum when the manganese content was 0.80.The reaction conditions were set at 200℃ and 12 h,and a series of LiFe0.2Mn0.8PxO4/C(x=1.00,1.01,1.02,1.04)cathode materials with different phosphorus contents were synthesized by hydrothermal method.X-ray diffraction patterns showed that samples with different phosphorus contents were all pure phases.With the increase of phosphorus content,the morphology of the sample gradually decreased from micron to nanometer,and the regularity of the shape became better.Electrochemical tests show that the first discharge specific capacity of LiFe0.2Mn0.8P1.04O4/C sample at 0.1 C rate is 141.9 mAh/g,which is the highest specific discharge capacity,lower polarization and longer discharge platform.This is related to its good particle morphology,but also due to its small charge transfer resistance and maximum lithium ion diffusion coefficient.With a reaction temperature and time of 200℃ for 12 h and a phosphorus content of 4%,a series of LixFe0.2Mn0.8PO4/C(x=1.00,1.02,1.03,1.04)positive material with different lithium contents were successfully synthesized by hydrothermal method.XRD results showed that all samples were pure phase,and no impurity phase appeared.SEM results showed that when x=1.02,the particle morphology was regular and uniformly distributed;as the lithium content increased further,the morphology became worse and obvious agglomeration appeared.The maximum discharge specific capacity of Li1.02Fe0.2Mn0.8PO4/C sample at 0.1 C rate is 153.4 mAh/g.Compared to samples with a standard molar amount of lithium,the rate performance and cycle performance of the samples containing excess lithium were significantly improved.The Li1.02Fe0.2Mn0.8PO4/C sample still has a high capacity retention rate of 80.84%at a rate of 5 C,and a capacity retention rate of 85.76%after 100 cycles at a rate of 2 C,showing excellent rate performance and relatively Good cycling performance.