A Mechanistic Study on the Improvement of the Thermal Stability of Delithiated Li_1-xNiO₂by Co Substitution for Ni

The mechanism of thermal stabilization of delithiated Li_1-xNiO₂by cobalt substitution for nickel was closely studied from the structural point of view by thermogravimetry, X-ray diffraction, and X-ray absorption analysis. Delithiated Li_1-xNiO₂with hexagonal (R3̄m) or monoclinic (C2/m) structure was decomposed to a spinel phase (cubic, Fd3m) at temperatures around 220°C and then converted to a rock-salt phase (cubic, Fm3m) at higher temperatures. Cobalt substitution of nickel in Li_1-xNiO₂stabilized the spinel phase, formed from the thermal decomposition of Li_1-xNiO₂, and suppressed the decomposition of this spinel phase to a rock-salt phase. While the highly delithiated Li_1-xNiO₂was eventually converted to a rock salt phase with NiO structure during heating, Co₃O₄spinel structure was locally formed around the cobalt ions in Li_1-xNi_0.85Co_0.15O₂. The improvement of the thermal stability of highly delithiated Li_1-xNiO₂by cobalt addition could be explained by local formation of Co₃O₄spinel structure around the cobalt ions in Li_1-xNi_0.85Co_0.15O₂. This spinel structure around the cobalt ions was relatively stable at high temperature and therefore, depressed the decomposition of Li_1-xNiO₂to a rock-salt phase.