Study On The Performance Improvment Of LiNi_0.5Mn_1.5O₄ Coated By LiAlTiO₄

In comparison with other cathode materials for lithium ion batteries,high voltage spinel LiNi_0.5Mn_1.5O₄（LNMO）has the prominent features of high operating voltage and high energy density,and is a promising cathode material for lithium ion batteries.However,its cycle performance needs to be further improved due to its poorer performance at elevated temperature.One of the main reasons is that the chemical action of the electrolyte on the electrode material dissolves the transitional metal.At present,one of the most effective methods is to coat a surface on LNMO in order to improve the cycling performance,especially the cycling performance at elevated temperature.The effect of LiAlTiO₄（LATO）coating on the morphology,structure and electrochemical performance of LNMO was studied.First,the effects of different heat treatment mechanisms on the structure of LATO and Li_0.7Al_1.9Ti_0.4O₄ materials in solid state synthesis process were investigated.XRD and SEM results showed that the obtained Li_0.7Al_1.9Ti_0.4O₄ morphology was irregular and contained impurities.However,the LATO synthesized at 900℃ for 12h and then annealed at 670℃for 15h crystallized well,which established the optimal conditions for the following experiments.Secondly,LNMO was synthesized by a sol-gel method.Through the study on its structure,morphology and charge-discharge curve,it is found that the material had good crystal structure,regular morphology and obvious Mn^3+/4+ redox plateau（～4.0 V）.The specific charge and discharge capacities were 137 and 121.8 mAh/g at 25℃,respectively,and 153.9 and 119.7 mAh/g at 55℃,respectively.The corresponding coulombic efficiencies were 88.84 and 77.78%respectively.The specific discharge capacity was 113.3 mAh/g after 150 cycles at 25℃,compared to 77.2 mAh/g at 55℃.Finally,the LNMO synthesized by the sol-gel method was surface-coated by using the optimal synthesis conditions of LATO,and the coating amount was 3,5 and 7%（weight ratio of LATO to LNMO）.The obtained materials were characterized by XRD,SEM,EDS,TEM and particle size distribution.At the same time,the dQ/dV,EIS and the charge-discharge curves at room temperature and high temperature were performed.The effects of coating amount on the electrochemical properties of LNMO were compared.The study showed that after coating,there was a layer of LATO product with a loose structure.The results of charge and discharge curves exhibited that the first charge-discharge capacity of the coated LNMO decreased at both room temperature and elevated temperature.The material with 3%coating amount,denoted as LNMO@3LATO,delivered charge-discharge capacities of 117.8 and 112.8 mAh/g in the first cycle,respectively.The material with the coating amount of 5%,marked as LNMO@5LATO,delivered charge-discharge capacities of 118.8 and 117 mAh/g,respectively.The material with the coating amount of 7%was marked as LNMO@7LATO,and its specific charge and discharge capacities were 122 and 115.4 mAh/g,respectively.The cycling performance of the coated material had been improved,especialy at elevated temperatures.The discharge capacity decay rates of LNMO,LNMO@3LATO,LNMO@5LATO and LNMO@7LATO at room temperature were 0.0787,0.066,0.0393 and 0.0608 mAh/g each time.Their capacity decay rates at high temperature were 0.2833,0.126,0.1113 and 0.1307 mAh/g each time.When the coating amount is 5%,LNMO has the best comprehensive electrochemical performance.