Effect of Aqueous Electrolytes on LiCoO2 Surfaces: Role of Proton Adsorption on Oxygen Vacancy Formation

Aqueous electrolytes are a safer, greener, and cheaper solution for energy storage applications. However, aqueous Li-ion batteries (ALIBs) suffer from faster degradation and poorer cyclability. The presence of H + and O loss have often been claimed to deteriorate electrode materials in aqueous electrolytes. Understanding the surface reactivity of the commercial LiCoO 2 cathode with respect to aqueous electrolytes and O loss is essential for designing cathode materials in such aqueous electrochemical cells. In this work, we use density functional theory calculations to investigate the stability and structure of several low-index surfaces of layered Li 1– x CoO 2 (0 ≤ x ≤ 0.5) before and after H + adsorption. We compute the binding energies of H + from low to full coverage regimes. By employing ab initio atomistic thermodynamics, we determine the stability of O vacancies on protonated and nonprotonated layered LiCoO 2 surfaces. Our computations demonstrate that O loss is energetically favorable on the lowest energy surfaces, i.e., on the most exposed surface terminations. We suggest that the O vacancy formation is directly related to the transition metal (Co) coordination. Finally, the role of H + on O loss is investigated, showing that H + can facilitate the generation of O vacancies in some surface terminations.