关键词: 锂层状氧化物, 首圈库仑效率, 表面残碱, 锂镍阳离子混排

Abstract:

Lithium layered oxide LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) is one of the most promising cathode materials in high-energy lithium-ion batteries for electric vehicles. However, one drawback for NCM622 is that its initial coulombic efficiency (ICE) is only about 87%, which is at least 6% lower than that of LiCoO₂ or LiFePO₄. In this work, we investigated the effects of surface chemical residues (e.g., LiOH and Li₂CO₃) and Li/Ni cation disorder resulted during the sintering on the ICE. We found that the ICE of the as-prepared samples could be boosted from 80.80% to 86.68% as the sintering temperatures were increased from 825 to 900 ^oC. The corresponding Li/Ni cation disorder and surface chemical residues were also reduced with the increasing sintering temperatures. Furthermore, the ICE of the sample sintered at 825 ^oC could be enhanced by 3.57% after washing with HNO₃ solution to remove the surface residues despite the Li/Ni cation disorder being increased. These results demonstrate that minimizing the amount of surface residuals and the degree of Li/Ni cation disorder through an appropriate sintering process and post-treatment technology is critical to achieve a high ICE and improve the electrochemical performances of NCM622.

Key words: lithium layered oxide cathode, initial coulombic efficiency, surface chemical residues, Li/Ni cation disorder