Fluorine-Doping Strategy To Improve the Surface and Electrochemical Properties of LiNi0.6Co0.2Mn0.2O2 Cathodes for Use in Lithium-Ion Batteries

This study sought to achieve fluorine doping into the surface structure of LiNi0.6Co0.2Mn0.2O2 (NCM622) cathodes for use in lithium-ion batteries by the atomic layer deposition method to improve structural stability and electrochemical performance. Thin-film electrodes were used to clearly understand the fundamental effects of surface properties. Based on our observations, the oxygen vacancy that basically exists in the surface of the NCM622 thin-film electrode triggers the additional formation of oxygen derivatives with a spinel-like phase, leading to poor cycling stability and severe deterioration of the surface structure. Contrary, the fluorinated NCM622 thin-film electrode exhibits improved electrochemical properties including satisfactory cycling stability, which benefits from the higher binding energy of fluorine that positively influences the stability of the surface oxygen layer by being doped into the unstable oxygen site, and thus the formation of oxygen derivative phases is successfully suppressed during electrochemical reactions.