Preparation And Modification Of LiNi_0.5Mn_1.5O₄ Cathode Material For 5V Lithium-Ion Battery

The energy and power density of lithium- ion batteries have been paid more and more attention, with the aggravation of environmental pollution and the development of electric vehicles. LiNi_0.5Mn_1.5O₄ has a 4.7 V discharge platform and relatively high discharge capacity as 5 V cathode material. Nowadays it has become one of the key researches of cathode materials.The main content of the study includes preparation of LiNi_0.5Mn_1.5O₄ and doping modification of Cr³⁺, Al³⁺ and F^- by solid-state method and sol- gel method. The research focuses on improving rate performance of LiNi_0.5Mn_1.5O₄ by optimizing the synthesis process conditions and ion doping to meet the requirement of new power battery.LiNi_0.5Mn_1.5O₄ material prepared by solid state method has good spinel structure and high crystallinity, its particles have good dispersion after grinding. The initial discharge capacity is 119.4 mAh/g at 0.5 C while it is only about 60 mAh/g at 5 C. Capacity fading is serious at high rate, thus the rate performance needs to be improved. After Cr³⁺ doping, the structural stability of material is obviously improved, but there are still some Lix N i1-xO impurity phases. The LiNi_0.45Cr_0.1Mn_1.45O₄ material has the highest crystallization and excellent electrochemical performance, the initial discharge capacity is 126.6 mAh/g at 0.5 C, and it delivers 99.2 mAh/g at 5 C. The high rate discharge capacity is enhanced. The cell parameter of Al³⁺ doping material further decreases, particle size is mostly 3 μm, but the aggregation still exists. LiNi_0.45Al_0.1Mn_1.45O₄ has the ideal discharge capacity of 141.9 mAh/g at 0.5 C, and it reaches 99.1 mAh/g at 5 C. Compared to the 1 C before, the discharge capacity is back to 133.8 mAh/g when returning to 1 C, showing capacity retention of 99.4% significantly. The reason may be that Al³⁺ doping reduces the Warburg resistance, increasing the rate and proportion of Li+ insertion. C haracterization shows the particle size becomes smaller after F^- doping, particle size distribution of LiNi_0.5Mn_1.5O_3.9F_0.1 is the most uniform which is mostly between 1 and 2 μm. Compared to cations doping, the discharge capacity is lower at low rate, while the capacity retention is relatively higher at high rate, discharge capacity of LiNi_0.5Mn_1.5O_3.9F_0.1 is 80.1 mAh/g at 5 C. After 50 cycles, the capacity retention is 85.6% at 1 C. F^- doping improves the conductivity of material, thus the rate performance is improved.Because the impurity phase is produced easily and discharge capacity is not high enough by solid-state method, so the preparation of LiNi_0.5Mn_1.5O₄ cathode material by sol- gel method is adopted. The sintering temperature is determined by TG-DTA curve. Its structure has a space group of Fd3 m and no impurity phase exists, the crystallinity and structural stability are high and no agglomeration exists. Particle size is about 2 μm. Discharge capacity is 141.6 mAh/g at 0.5 C, closely to the theoretical capacity, and it remains 111.8 mAh/g at 5 C, which has excellent rate performance. Characterization shows that no impurity phase exists after F^- doping, LiNi_0.5Mn_1.5O_3.9F_0.1 has the most uniform distribution, which is about 2³ μm. It has the best electrochemical performance, coulombic efficiency of first charge-discharge cycle is 94.5%, initial discharge capacities at 0.5 C and 5 C are 139.8 and 108.9 mAh/g respectively. When the rate returns to 1 C, it can reaches 129.9 mAh/g, the capacity retention is as high as 97.0% compared to the first 1 C. CV curve shows that it has smaller irreversible potential after F^- doping, which improves structural stability and conductivity of the material. After Cr³⁺ doping, characterization shows the agglomeration of particles is slightly more than the F^-doped material. LiNi_0.45Al_0.1Mn_1.45O₄ has the highest discharge capacity of 127.1 mAh/g at 0.5 C, the coulomb efficiency of first cycle is 90.4%, while discharge capacity is only 72.8 mAh/g at 5 C. The impedance increases and the polarization is more serious than LiNi_0.5Mn_1.5O₄ cathode material, which deteriorates the electrochemical performance.