Influence of Stacking on H+ Intercalation in Layered ACoO2 (A = Li, Na) Cathode Materials and Implications for Aqueous Li-Ion Batteries: A First-Principles Investigation

Li- and Na-ion batteries are effective energy storage technologies. Nonetheless, currently used organic-electrolyte batteries present well-known safety problems. Therefore, the research community is intensively looking for potential alternatives. Aqueous batteries based on low-cost salts in water could be an interesting choice since they are safe and environmentally benign. However, working with aqueous electrolytes brings new detrimental mechanisms such as proton intercalation. Understanding the (de)intercalation chemistry of protons and alkali is one of the keys for designing cathode materials in such aqueous electrochemical cells. In this work, we carry out density functional theory calculations to investigate the H+/alkali exchange in layered LiCoO2 and NaCoO2 materials. By computing the grand potential phase diagram and voltage–composition plots, we determine the relative stability of several orderings of protons, alkali, and vacancies. The fully protonated CoO2 lattice (CoO(OH)) is revealed to be the most stable insertion product due to the formation of interlayer hydrogen bonds. Our computations demonstrate the key role of layer stacking: H+ insertion is favored in prismatic (P) stacking, while Li favors octahedral (O) stacking. While the fully protonated layered cobalt oxide is the thermodynamically favored product when protons and alkali compete, we show that mixing protons and lithium is energetically disfavored because of the different stacking preferences. We suggest that the kinetic difficulty in nucleating fully protonated phases in the layered oxide prevents proton insertion when cycling LiCoO2 in an aqueous electrolyte. The good cyclability and lack of proton insertion in LiCoO2 are, therefore, a result of the slow kinetics of protonation in partially lithiated cobalt oxide. On the other hand, we demonstrate that NaCoO2 is prone to proton and alkali mixing due to the different stacking preferences for sodium. We hypothesize that this could lead to proton intercalation and poor performances in aqueous batteries for NaCoO2 cathodes.