Homogeneous Surface Modification Of Layered Cathode Material And Research On Its Electrochemical Performance

The layered cathode material has better cycle performance.However,its surface is easy to react with the electrolyte during cycling.Therefore,to suppress this phenomenon,in this paper,different coating methods were used to modify the surface of LiCoO₂（LCO）and LiNi_0.6Co_0.2Mn_0.2O₂（NCM622）,respectively,and the effects of different coating technologies on the cycle performance were explored and analyzed.The main contents are as follows: 1.The comparative study of Al₂O₃-coated LCO through the aluminum isopropoxide（AIP）and aluminum nitrate（ANO）hydrolysis technology.Through the comparative study of two different coating methods,it was found that compared with ANO method,the Al₂O₃ coating layer obtained by AIP method had more excellent uniformity and better cycle performance.After 80 cycles,the initial discharge capacity and retention rate was 137.5 mAh g^-1,98.1% between 3.0 and 4.2 V,166.7 mAh g^-1,94.1% between 3.0 and 4.4 V,186.2 mAh g^-1,95.0% between 3.0 and 4.5 V and 219.2 mAh g^-1,63.2% between 3.0 and 4.6 V,respectively.The ICP and AC impedance analysis suggested that the Al₂O₃ coating layer from AIP method could more effectively inhibit the cobalt dissolution and the increase of Rp during cycling.2.The comparative study of Al₂O₃-coated LCO through the improved AIP method and sodium aluminate（SMA）hydrolysis technology.Through both the improved AIP and SMA coating methods,it was found that the Al₂O₃-I coating layer derived from improved AIP method was amorphous and flocculent,and had a thickness of 20-50 nm,which showed better coating uniformity.Besides,charge and discharge test suggested that for AIP method,the discharge capacity retention at 4.4,4.5 V was 91.8% and 91.2%,respectively.For SMA method,the corresponding retention was 83.7% and 91.2%,respectively.Therefore,compared with the SMA coating method,the improved AIP coating method had better cycle stability.3.The comparative study of coated NCM622 through three different oxide coating technologies.Through the comparative study of three oxides coating methods,it is found that the SiO₂ coating layer were more uniform,its thickness was 10 nm.However,the uniformity of both Al₂O₃ and TiO₂ coating layer were poor,the thickness of each coating layer was was about 5 nm and 30 nm.Besides,charge and discharge cycle test indicated that the cycle stability of three coated samples was greatly enhanced.Particularly,after 50 cycles at 50℃,the initial discharge capacity and retention rate of Al₂O₃,SiO₂ and TiO₂-coated NCM622 were 197 mAh g^-1,93%,190 mAh g^-1,89% and 185 mAh g^-1,83%,respectively.As seen from this,the best coating effect was from Al₂O₃ and the worst was from TiO₂.In addition,the AC impedance suggested that solution impedance Rs,diffusion impedance R1 and charge transfer resistance Rct of each oxide coating were different,thereinto,Al₂O₃ coating could better effectively prevent the increase of various impedance and reduce the interface impedance.