Preparation And Modification Of Lini_0.5Mn_1.5O₄ As High Voltage Cathode Material For Lithium Ion Batteries

With the development of electric vehicles, hybrid electric vehicles and large capacity storage battery, the requirements for lithium ion batteries’ energy density and power density are put forward. The cathode material is one of the key materials for lithium ion batteries. It is the main way to improve the lithium ion batteries’energy density by research and develope the high potential cathode material. Spinel LiNi0.5Mn1.5O4 cathode material has become the first choice of the EV and HEV power battery for its discharge voltage platform at 4.7V, high energy density, structure stability, excellent cycle performance and low production cost. At present, LiNi0.5Mn1.5O4 is one of the highlights in scientific research of the lithium ion battery cathode material.In the work, oxalate precursors was prepared though co-precipitation reaction using Ni(OOCCH3)2·4H2O, Mn(OOCCH3)2·4H2O as raw materials and H2C2O4·2H2O as precipitant. LiNi0.5Mn1.5O4 material was prepared using oxalate precursors and Li2CO3. The microstructure and morphology of the samples were characterized by XRD and SEM. The electrochemical performance was evaluated by galvanostatic charge/discharge cycling, CV and EIS.At first, effect of pH value, stirring speed, sintering temperature and sintering time on the electrochemical performance of the pure phase LiNi0.5Mn1.5O4 were studied by orthogonal experiments. The experimental results show that nickel and manganese oxalate precursor was prepared under the condition of the solution pH value is 7.0 and the speed of stirring is 700rpm, the pure spinel LiNi0.5Mn1.5O4 preparation from precursor preparation of nickel and manganese compound oxide 550℃ for 5h, and then with the Li2CO3 sintering at 850℃ for 16h, last at 600℃ which had the best performance. The initial discharge capacity of the sample reached 142.09mAh/g at the rate of 0.1C, the charge transfer resistance of the sample Rct was 109.20Ω.In order to improve the electrochemical properties, F- and Fe3+ doping alone in LiNi0.5Mn1.5O4 were prepared. The effect of doping with different content on the micro structure and electrochemical properties of LiNi0.5Mn1.5O4 were investigated. The experimental results show that when F- doped x=0.05 LiNi0.5Mn1.5O3.95F0.05 had the best electrochemical performance. The initial discharge capacity of the sample was 129.07mAh/g, at charge/discharge rate of 1C and 5C at room temperature, its initial discharge capacities were 118.49mAh/g and 92.57mAh/g After 50 cycles, its discharge capacities were 113.51mAh/g and 80.35mAh/g, the retention rate of capacity were 95.8% and 86.8%. The samples were analyzed by CV and EIS, the results show that its redox peak potential difference △E=0.273V, the charge transfer resistance Rct was 48.26Ω, the lithium ion diffusion coefficient DLi+ was 4.49×10’10 cm2·s-1. The right amount of F" doped decreased charge transfer resistance Rct and the polarization, improve the lithium ion diffusion coefficient. When Fe3+ doped y=0.1, LiFe0.1Ni0.45Mn1.45O4 had the best electrochemical performance. The initial discharge capacity of the sample reached 131.54,126.8,121.28,116.49, and 96.82mAh/g at the rate of 0.1C,0.5C,1C,2C and 5C, respectively. At charge/discharge rate of 1C and 5C at room temperature, its initial discharge capacities were 117.03mAh/g and 84.81mAh/g. After 50 cycles, the retention rate of capacity were 96.5% and 87.6%. LiFe0.1Ni0.45Mn0.45O4 s redox peak potential difference AE= 0.291V, it farer less than un-doped LiNio.5Mn1.5O4’s redox peak potential difference AE=0.408. The results show that Fe3+doped ion amounts improve the structure stability and cycling stability of material, decreased the polarization. LiFe0.1Ni0.45Mn0.45O4 had the smallest charge transfer resistance Rct and the largest the lithium ion diffusion coefficient DLi+, its Rct was 42.13Ω and DLi+ was 5.16×10-10 cm2·s-1.The method of F- and Fe3+ ions co-doping was used to modify the LiNi0.5Mn1.5O4 cathode material. The studies showed that F- and Fe3+ ions co-doping did not change the structure of material, the all samples prepared showed spinel structure. Let F-doped x=0.05, when Fe3+ doped y=0.1, the sample showed uniform particle size, better crystalline and single spinel structure. LiFe0.1Ni0.45Mn1.45O3.95F0.05 had the best electrochemical performance. The initial discharge capacity of the sample reached 134.28,128.76,123.61,119.62, and 102.68mAh/g at the rate of 0.1C,0.5C,1C,2C and 5C, respectively. At charge/discharge rate of 1C and 5C at room temperature, its initial discharge capacities were 130.92mAh/g and 92.58mAh/g. After 50 cycles, the retention rate of capacity were 97.5% and 90.16%. LiFe0.1Ni0.45Mn0.45O3.95F0.05’s redox peak potential difference AE=0.269V, it farer less than un-doped LiNi0.5Mn1.5O4 s redox peak potential difference AE=0.408. The results show that F-and Fe3+ ions co-doping can decreased the polarization and improve cycling stability. LiFe0.1Ni0.45Mn0.45O3.95F0.05’s the charge transfer resistance Rct was 24.96Ω and the lithium ion diffusion coefficient DLi+ was 6.39×10-10 cm2·s-1. Compared with un-doped and alone doped, it showed a higher lithium diffusion rate and material conductivity, a larger lithium ion diffusion coefficient. The studies showed that F- and Fe+ co-doping significantly improve the rate capability and cycling stability.