Effects of a Solid Solution Outer Layer of TiO₂ on the Surface and Electrochemical Properties of LiNi_0.6Co_0.2Mn_0.2O₂ Cathodes for Lithium-Ion Batteries through the Use of Thin-Film Electrodes

Thin-film electrodes are considered to be desirable for understanding the detailed surface characteristics of active materials for rechargeable batteries. This study attempts to elucidate the effects of a solid solution outer layer (SSOL) of TiO₂ on the surface and electrochemical properties of LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) cathodes by using thin-film electrodes synthesized by a spin-coating technique. The SSOL phase is induced on the NCM622 thin-film surface by a post-annealing process after the TiO₂ coating using atomic layer deposition. Structural and morphological analyses revealed that the bare NCM622 thin-film electrode without a thin SSOL has a spinel-like derivative phase induced by an oxygen vacancy at the surface, which is considered to be the crucial factor for the poor electrochemical properties of Ni-rich NCM. In particular, additional measurements including in situ Raman spectroscopy revealed that the spinel-like derivative phase rapidly makes the surface structure become corrupt and change to the amorphous state during electrochemical reactions. In contrast, the oxygen vacancy can be eliminated by forming a SSOL phase at the surface of the NCM622 thin-film through the rapid migration of Ni, Ti, and O atoms during the post-annealing process, significantly enhancing the structural stability, which ultimately improves the electrochemical performance, including cyclability and Coulombic efficiency.