Synthesis And Surface Modification Of LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Material

Recently much attention has been paid to LiNi_0.5Co_0.2Mn_0.3O₂ cathode material,since it exhibits the merits of the high capacity,low cost and excellent safety performance.However,there are still some problems for the applications of LiNi_0.5Co_0.2Mn_0.3O₂ cathode material,such as lithium deficiency and cation mixing in the preparation process,low discharge capacity,poor cycle stability and poor rate capability at the high current density.In this work,the electrochemical performance is improved by optimizing of Li content,carbon coating by magnetron sputtering,surface modification of lithium ion conductor materials Li2 Si O3 and Li Ti O2.The main research contents and results are as follows:?1?The LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials was synthesized by calcination.The effect of the Li content on the performance of the cathode material is investigated.The initial discharge capacity of the sample with Li/M=1.15 are 183 m Ah/g and 163.2 m Ah/g at 0.2C and 1C rate within the potential range of 2.7-4.5V,respectively.The charge transfer resistance of sample increases from 140.3 ? to 1218 ? after cycling at 1C rate.It showed that the main reason for the capacity fading was due to side reactions between the electrode and electrolyte,which led to the value increase of Rct.?2?The carbon coating on the cathode surface could be achieved by magnetron sputtering.It could enhance the electronic conductivity of materials.The thickness of amorphous carbon layer was about 20 nm on the surface of particle after 90 min sputtering.The initial capacities reached to 170 m Ah/g at 1C rate.Furthurmore,the capacity was 89.3 m Ah/g even at the high rate of 5C,51.8% higher than that of the pristine one?58.8 m Ah/g?.The value of Rct was 680.2 ? and the third redox peak differences decreased 0.044 V compared with the original.It showed that the carbon layer could reduce the transfer resistance,resulting in the improved electrochemical performances.?3?Li₂SiO₃ modified LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials were prepared via a sol-gel route.The 0.3 wt%-Li2 Si O3 coated cathode exhibits the best electrochemical performance: the initial discharge capacity was 181.0 m Ah/g at 1C rate,capacity retention reached to 90% after 50 cycles;capacity maintained 122 m Ah/g at 5C rate which increased 63 m Ah/g compared to pristine powder;the minimum value of charge transfer resistance?Rct?was 185.1 ?.It showed that Li2 Si O3 coating layer could suppress the side reactions,enhance the stability between electrode and electrolyte,reduce the charge transfer resistance?Rct?,accelerate the diffusion of Li+.?4?LiNi_0.5Co_0.2Mn_0.3O₂ cathode powder was modified by surface coating with solid electrolyte Li Ti O2.The 0.3 mol%-Li Ti O2 coated cathode exhibited the initial specific discharge capacity of 178.0 m Ah/g at 1C rate with retention of 97.3% after 50 cycles;capacity of 130.2 m Ah/g at 5C rate and almost no capacity fading after 10 cycles.The improved cycling performance and rate capability was attributed to the solid-state electrolyte Li Ti O2 coating,which protects the cathode from the corrosion of electrolyte and faster the lithium-ion diffusion.