Mechanism of the Loss of Capacity of LiNiO₂ Electrodes for Use in Aqueous Li-Ion Batteries: Unveiling a Fundamental Cause of Deterioration in an Aqueous Electrolyte through <i>In Situ</i> Raman Observation

Abstract

This study investigated the fundamental mechanisms of the loss of capacity of LiNiO₂ (LNO) electrodes for Li⁺ insertion/deinsertion with a special focus on the origin of this deterioration in an aqueous system. <i>In situ</i> Raman spectra revealed that the intercalation of H⁺ ions formed a NiOOH_<i>x</i> film at the surface of LNO during the initial electrochemical cycles; this NiOOH_<i>x</i> film was also confirmed by X-ray photoelectron spectroscopy and transmission electron microscopy analysis. The formation of an electrochemically inactive spinel-like phase (Ni₃O₄) at the subsurface was triggered by the absence of Li in the NiOOH_<i>x</i> film at the surface. These structural changes of LNO, accelerated by the intercalation of H⁺ ions, were considered to be the fundamental cause of the greater loss of capacity in the aqueous system.