Anodic decomposition of surface films on high voltage spinel surfaces - Density function theory and experimental study

Oxidative decomposition of organic-solvent-based liquid electrolytes at cathode material interfaces has been identified as the main reason for rapid capacity fade in high-voltage lithium ion batteries. The evolution of "cathode electrolyte interphase" (CEI) films, partly or completely consisting of electrolyte decomposition products, has also recently been demonstrated to correlate with battery cycling behavior at high potentials. Using density functional theory calculations, the hybrid PBE0 functional, and the (001) surfaces of spinel oxides as models, we examine these two interrelated processes. Consistent with previous calculations, ethylene carbonate (EC) solvent molecules are predicted to be readily oxidized on the Li_xMn₂O₄ (001) surface at modest operational voltages, forming adsorbed organic fragments. Further oxidative decomposition of such CEI fragments to release CO₂ gas is however predicted to require higher voltages consistent with Li_xNi_0.5Mn_1.5O₄ (LNMO) at smaller x values. We argue that multistep reactions, involving first formation of CEI films and then further oxidization of CEI at higher potentials, are most relevant to capacity fade. Mechanisms associated with dissolution or oxidation of native Li₂CO₃ films, which are removed before the electrolyte is in contact with oxide surfaces, are also explored.