Synthesis And Electrochemical Performance Study Of Submicron/micron LiNi_0.5Co_0.22Mn_0.3O₂

With the development of electric cars and hybrid vehicles, the requirement of high performance lithium ion battery becomes increasingly urgent. LiNi_0.5Co_0.22Mn_0.3O₂, which has high specific capacity and low cost, becomes one of potential lithium ion battery cathode materials. However, to be applied on electric vehicles, it needs a further improve on rate capability and better understanding of the fading mechanism to increase stability.Based on the electrospinning method and sol-gel method, two kind of materials with particle size 300 nm and 1μm were prepared respectively, the influence of preparation conditions on material phase were discussed in detail. The results show the as-electrospun fibers could inhibit the elements migration between fibers, and the larger specific surface area was advantageous to sintering, so electrospinning is a good way to control material particle size and the sintering time was reduced to 5h. However, the larger specific surface area will exacerbate lithium evaporation, so a 10% lithium overdose was needed in the preparation process. The capacity of the two materials which were prepared by two methods at their best possible conditions at 0.1C were 166 m Ah/g and 167 m Ah/g, in the potential range of 2.8⁴.3V, but when the discharge current were set as large as 5C, the capacity were 130 m Ah/g and 103 m Ah/g for the submicron material and micron material, respectively, this phenomenon can be explained by different polarization which were caused by different lithium ion and electron diffusion distance. Under 55℃, both materials showed good cyclic stability, and the rate capability is similar, the capacity at 5C increased to 141 m Ah/g and 135 m Ah/g as high temperature increased the lithium ion migration rate. But in the potential range of 2.5⁴.6V, the submicron material had a significant performance attenuation because of large specific surface area and surface activity, the 100 th cycle capacity at 0.5C was only 55 m Ah/g.For further understanding of the fading mechanism and improve the high potential range stability, carbon, copper oxide and zinc oxide were coated on both materials by thermal decomposition. Results show that in the coating process of pyrolysis, a reducing atmosphere were formed, and the reducing atmosphere can lead to lattice damage, which resulted in loss of activity. For micron material coated by 1mass% copper oxide, in the potential range of 2.8⁴.3V and 2.5⁴.6V, the capacity were 98 m Ah/g and 100 m Ah/g respectively at 5C, while the 3mass% material decreased to 65 m Ah/g and 46 m Ah/g, because of copper oxide have poor electronic and lithium ion conductivities and the coating layer might reduce the number of transmission channels of lithium ions and electrons, the polarization of materials were increased, resulting in a decline in material performance and high voltage properties. However, for the corresponding micron material coated by 1mass% zinc oxide, the capacity were 109 m Ah/g and 131 m Ah/g at 5C in the potential range of 2.8⁴.3V and 2.5⁴.6V, although the 3mass% material decreased to 102 m Ah/g and 98 m Ah/g, the high potential range stability was improved. The reason can be explained by the good electronic conductivity of zinc oxide that the electronic conduction resistance was reduced under the appropriate amount of cladding, and the electronic and lithium ion transport channels were also been separated, which protected the material from high potential fading. The capacity of submicron material coated by 1mass% copper oxide at 5C decreased to 101 m Ah/g, but the high potential range stability was improved significantly, the 100 th cycle capacity at 0.5C was upto 126 m Ah/g, the reason can be explained as follow: after reducing of the material particle size, the polarization of material itself has reached a very small value, surface modification of material increased the polarization, leaded to a decline in rate capability, but at the same time, the coating layer can weaken the surface activity of material, which may inhibit the destruction effect of high potential on the material.Study on the responses of AC impedance between different materials, within the scope of the existing research, the response frequence of SEI layer on submicron and micron material was about 10000 Hz and the bulk electronic transmission process was about 1000 Hz, a process of electronic through SEI layer was also observed in submicron material. By the relationships between the AC impedance at various potentials and the electrochemical performance of different particl size materials or different coating stations, the dramatic increase of the mid-high frequence arc at high potential corresponds to the material damage effect while the increase of mid-low frequence arc corresponds to poor rate capability.