Two-dimensional hetero-spectral Raman/XAS correlation analysis of Li[Ni _0.45Co _0.18Mn _0.25Al _0.12]O ₂ cathode in the overcharged state

We investigated an electrochemical reaction on the cathode surface of an overcharged Li[Ni _0.45Co _0.18Mn _0.25Al _0.12]O ₂/Li system at the molecular level by using Raman, X-ray absorption spectroscopy (XAS), and 2D hetero-spectral correlation analyses. Changes in the Raman spectra during the overcharging process showed that variations in the local atomic environment occurred with increasing extraction of Li ⁺ ions. XAS spectra obtained during the overcharging process showed that oxidized transition metals coexisted with inoxidizable transition metals. Additionally, synchronous 2D hetero-spectral Raman/XAS correlation spectra clearly elucidated the correlation between the Raman and XAS bands, confirming their band assignments.