The Synthesis、Modification And Performance Research Of High-voltage LiNi_0.5Mn_1.5O₄ Cathode Material For Lithium-ion Battery

With the popularization of new energy vehicles, lithium-ion battery has become the first choice for motivation equipment of electric vehicles because of its high discharge capacity, high working potential and excellent cycling performance. Lithium-ion battery spinel LiNi_0.5Mn_1.5O₄ cathode material has attracted a lot of attentions due to its advantages of abundant resources, low cost, high safety and high-voltage platform, so it is considered as one of the most promising high-voltage cathode material of lithium-ion battery. However, LiNi_0.5Mn_1.5O₄ has a poor rate performance and the electrolyte will decomposite when sufferring high-voltage, it results in the fast capacity fading and its commercialization is restricted. Therefore, to improve the material’s cycling and rate performance and synthesize LiNi_0.5Mn_1.5O₄ cathode material with excellent performace are the main points of this paper.Spinel LiNi_0.5Mn_1.5O₄ has two different space groups, one is disorded face-centered cubic（Fd3m）, the other is ordered primitive simple cubic（P4332）. The former has the higher electronic conductivity and it results in the the better high-rate capability and cycling performance. It is difficult to synthesize the pure Fd3 m space group LiNi_0.5Mn_1.5O₄ and the Mn3+ in it can result in the disproportionation reaction and Mn₂+ will be produced, it will react with the electrolyte. In this paper, LiNi_0.5Mn_1.5O₄ cathode materials were synthesized via a spray drying method and an electrospinning method. Then the more balanced and outstanding materials were synthesized through the methods of rare-earth elements doping and magnetron sputtering coating. All the researches are as below:（1） LiNi_0.5Mn_1.5O₄ was synthesized via a spray drying method. The effects of different calcinations temperatures and annealing temperatures on the structure, morphology and electrochemical property were investigated. The results indicate that the as-prepared LiNi_0.5Mn_1.5O₄ is well crystallized at calcinations temperature of 900°C for 20 h, and annealing temperature of 600°C for 30 h. The LiNi_0.5Mn_1.5O₄ particles have the typical octahedral shape and the average size is about 2 um. The initial discharge capacity of the sample is 133.7 mAh·g^-1 at 0.1C and the capacity retains 123.1 mAh·g^-1 after 50 cycles.（2） The effects of La and Ho doping on the structure, morphology and electrochemical property were investigated. There is no significant change on the morphology and when the proportion is excessive, the impurity is observed. Both La and Ho doping reduce the initial discharge capacity, while the Ho doping improves the cycling performance and rate capability. LiNi0.5-xHo2xMn1.5-xO₄（x=0, 0.01, 0.02 and 0.03） cathode materials were synthesized, and when the x=0.01, the material exhibited the best electrochemical performance. The initial discharge capacity of the sample is 129.9 mAh·g^-1 at 0.1C and the capacity retains 125.4 mAh·g^-1 after 50 cycles. At the rate of 3C, the initial discharge capacity of the sample is 114.3 mAh·g^-1.（3） Magnetron sputtering was used as a surface modification method, Al and ZnO were directly coated on the electrode. The effects of different sputtering time on the structure, morphology and electrochemical property were investigated. The results indicate that there are two different size particles in the samples. When sputter Al, t=10min, the material exhibits the best electrochemical performance, the initial discharge capacity of the sample is 139 mAh·g^-1 at 0.1C and the capacity retains 132.1 mAh·g^-1 after 50 cycles. At the rate of 3C, the initial discharge capacity of the sample is 123.4 mAh·g^-1. When sputter ZnO, t=5min, the material exhibits the best electrochemical performance. The initial discharge capacity of the sample is 135.3 mAh·g^-1 at 0.1C and the capacity retains 132.3 mAh·g^-1 after 50 cycles. At the rate of 3C, the initial discharge capacity of the sample is 114.3 mAh·g^-1.（4） LiNi_0.5Mn_1.5O₄ was synthesized via an electrospinning method. The effects of different calcinations temperatures on the structure, morphology and electrochemical properties were investigated. The LiNi_0.5Mn_1.5O₄ synthesized at calcinations temperature of 650°C for 3 h has a nanofiber shape with average size of about 300-500 nm. Electrochemical performance tests reveal that the sample shows the initial discharge capacity of 127.8 mAh·g^-1 at 0.1C and the capacity retains 124.1 mAh·g^-1 after 50 cycles. At the rate of 3C, the initial discharge capacity of the sample is 105 mAh·g^-1.