Structural Insight into Protective Alumina Coatings for Layered Li-Ion Cathode Materials by Solid-State NMR Spectroscopy

Layered transition metal oxide cathode materials can exhibit high energy densities in Li-ion batteries, in particular, those with high Ni contents such as LiNiO₂. However, the stability of these Ni-rich materials often decreases with increased nickel content, leading to capacity fade and a decrease in the resulting electrochemical performance. Thin alumina coatings have the potential to improve the longevity of LiNiO₂ cathodes by providing a protective interface to stabilize the cathode surface. The structures of alumina coatings and the chemistry of the coating–cathode interface are not fully understood and remain the subject of investigation. Greater structural understanding could help to minimize excess coating, maximize conductive pathways, and maintain high capacity and rate capability while improving capacity retention. Here, solid-state nuclear magnetic resonance (NMR) spectroscopy, paired with powder X-ray diffraction and electron microscopy, is used to provide insight into the structures of the Al₂O₃ coatings on LiNiO₂. To do this, we performed a systematic study as a function of coating thickness and used LiCoO₂, a diamagnetic model, and the material of interest, LiNiO₂. ²⁷Al magic-angle spinning (MAS) NMR spectra acquired for thick 10 wt % coatings on LiCoO₂ and LiNiO₂ suggest that in both cases, the coatings consist of disordered four- and six-coordinate Al–O environments. However, ²⁷Al MAS NMR spectra acquired for thinner 0.2 wt % coatings on LiCoO₂ identify additional phases believed to be LiCo_1–xAl_xO₂ and LiAlO₂ at the coating–cathode interface. ^6,7Li MAS NMR and T₁ measurements suggest that similar mixing takes place near the interface for Al₂O₃ on LiNiO₂. Furthermore, reproducibility studies have been undertaken to investigate the effect of the coating method on the local structure, as well as the role of the substrate.