UC Santa Barbara

Renewable energy sources are generally abundant but intermittent, with peak production and peak demand occurring at different times. Therefore, storage and controlled distribution of energy is an important component of the shift away from on-demand fuels like petroleum and natural gas. Secondary electrochemical batteries are one convenient method for closing the gap between energy supply and demand. Battery performance and cost are largely restricted by the materials, especially those used for the electrodes. Part of the problem is material instability upon cycling. As the battery is charged (and discharged), the composition of the electrode changes, often resulting in reversible and irreversible phase transitions which result in material degradation. Therefore, battery capacity and lifetime can be limited by thermodynamic instabilities. We employ first-principles methods like density functional theory and Monte Carlo to study phase stability in layered electrode materials. We look at stacking-sequence changes, ion ordering, and atom migration to better understand bulk degradation mechanisms in Li- and Na-ion materials. We use NaxTiS2, NaxTiO2, and LixMO2 (M = Co, Ni, Mn) as model systems to explore phenomena present in a variety of layered transition-metal oxides and sulfides. Our work aids in interpreting experimental observations and in generating design rules for more robust electrode materials.