Research On Synthesis And Al Doping Of LiNi_0.5Mn_0.5O₂ As Cathode Material For Lithium-Ion Batteries

LiNi_0.5Mn_0.5O₂ is a promising new type of cathode material for lithium-ion battery with the advantage of environment friendly, low-cost, high specific capacity. And it is one of the most likely to become commercialized alternative LiCoO₂ materials. However, this kind of material now still have disadvantage of large polarization, serious capacity fading, poor rate capability, low conventional voltage capacity and so on. So it is a great significance to further improve the electrochemical properties of the material for accelerating its commercialization. In this paper, three important process conditions for synthesis were selected to optimization the synthesis process of this material. The cation mobility model of material was established in the electrochemical reaction and validated in the experiment. The structural factors which affect the electrochemical performance were found, and the inferior aspects of material electrochemical properties were studied by doping Al.The impact of sintering temperature, the amount of complexing agent and the mixing method was researched in the synthetic process of coprecipitation-lid phase method starting from synthesis. The optimal synthesis condition was adding ammonia of 2 times of the metal ions using ball milling mixing method for sintering at 900℃for 24h. The discharge capacity of LiNi_0.5Mn_0.5O₂ under optimum synthetic process can reach 190mAh·g-1 at the current density of 10 mA·g-1. And the capacity of the material had a serious decline as the charge and discharge current density increased. From the research of charge and discharge system, it is found that materials can show better rate capability under CC-CV charge and discharge system, and it is achieved that make this material maintain good electrochemical performance under large charge or discharge condition.The cation mobility model of LiNi_0.5Mn_0.5O₂ was established in the electrochemical reaction according to the basics of crystal field theory through the crystal structure analysis of LiNi_0.5Mn_0.5O₂. The test results of the material structure and electrochemical properties can validate the model is tenable. The mechanism of structural change and mobile law of the cation in the electrochemical reaction were revealed by introducing the model. And it is reflected accurately for the structural factors of the actual capacity of the material is much less than theoretical capacity and the phenomenon of specific capacity continue decaying.The materials were modified by doped Al based on the assumption of cation model. The cross-layer mobility of Ni ions was inhibited effectively by doping Al, which improved the cycle stability of material significantly. Moreover, Ni ions originally occupied the location of Li ions were restricted effectively by introducing Al ion, which makes specific capacity of the material has significantly improved. The increase of specific capacity can be demonstrated clearly only at low current density because of the effects of electrode polarization in charge and discharge process.