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

Abstract

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 LiNi0.6Co0.2Mn0.2O2 (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.

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

“…The XRD characteristics of NCM622‐40F and NCM622‐60F thin‐film electrodes are shown in Figure S1 (Supporting Information). The NCM622‐bare thin‐film without any impurity phases of transition metal oxides showed patterns similar to those in the literature for NCM622 powder (ICSD #291469), suggesting that the prepared NCM622 thin‐film electrodes have a similar hexagonal layer structure that can be assigned to the space group, [28,29] as shown in our previous work [12] . As for the NCM622‐F thin‐films, no peak shift or impurity phase formation was observed in the XRD pattern.…”

“…The NCM622-bare thin-film without any impurity phases of transition metal oxides showed patterns similar to those in the literature for NCM622 powder (ICSD #291469), suggesting that the prepared NCM622 thin-film electrodes have a similar hexagonal layer structure that can be assigned to the space group, [28,29] as shown in our previous work. [12] As for the NCM622-F thin-films, no peak shift or impurity phase formation was observed in the XRD pattern. All of the samples displayed an almost identical peak position, indicating that the structure of thin-film electrodes after fluorination was well maintained in the layered structure without an obvious change in structure at a bulk scale.…”

“…The quantified results are shown in Table S1. Based on previous works, a peak at 529.7 eV (red line) can be assigned to the lattice O 2− in the layered structure of the NCM622‐bare thin‐film, while a peak with a high binding energy at 531.2 eV (green line) is attributed to the formation of oxygen derivatives with spinel‐like phase [12,32,33] . These derivative phases can be induced by the unstable nickel‐oxygen bond on the surface, which tends to trigger phase transformation and form an additional oxygen vacancy, which becomes especially strong in thin‐films comprised of a nanocrystal with a flatter and larger surface.…”

“…Characteristics of NCM622-bare thin-film electrodes, which were synthesized by the sol-gel method, including morphological images such as transmission electron microscopy were described in our previous report. [12] To compare the structural characteristics between NCM622-bare and NCM622-F thin-films after fluorination, first, the XRD profile was compared with that of NCM622-bare thin-film electrodes. The NCM622 thin-film fluorinated for 10 cycles is denoted NCM622-F, as shown in Figure 1a.…”

“…Based on previous works, a peak at 529.7 eV (red line) can be assigned to the lattice O 2À in the layered structure of the NCM622-bare thin-film, while a peak with a high binding energy at 531.2 eV (green line) is attributed to the formation of oxygen derivatives with spinel-like phase. [12,32,33] These derivative phases can be induced by the unstable nickel-oxygen bond on the surface, which tends to trigger phase transformation and form an additional oxygen vacancy, which becomes especially strong in thin-films comprised of a nanocrystal with a flatter and larger surface. As determined by these XPS spectra, it is convincible that NCM622-bare thin-films possess high ratios of spinel-like derivative phases on the surface rather than usually adsorbed contaminants (Li 2 CO 3 , LiOH, etc.).…”