Stacking-Dependent Interfacial Water Dynamics Govern the Aqueous Degradation of Layered LiCoO₂ Cathodes

Despite their high energy density, layered cathode materials suffer from instability in aqueous lithium-ion batteries. LiCoO₂, as a prototypical layered oxide cathode, exhibits a pronounced dependence of aqueous stability on oxygen stacking, with markedly different stabilities for its O3- and O2-type structures, yet the underlying atomistic mechanism remains elusive. Here, we investigate the aqueous degradation of O3- and O2-LiCoO₂ using large-scale molecular simulations enabled by a high-fidelity machine learning potential developed in this work, focusing on representative hydrogenated (001) surfaces. Our simulations reveal that oxygen stacking reconstructs the interfacial water structure, leading to distinct proton transfer kinetics. O3-LiCoO₂ stabilizes a disordered and dynamically labile hydration layer that enhances proton accessibility and transfer, whereas the more ordered water on O2-LiCoO₂ kinetically suppresses proton transfer, despite similar solvent exchange rates. This stacking-dependent proton accessibility stabilizes transition states for lattice oxygen dissolution, resulting in a lower dissolution barrier on O3-LiCoO₂. Lattice oxygen loss subsequently triggers cobalt dissolution via vacancy-mediated coordination rearrangements, yet O2-LiCoO₂ consistently exhibits higher dissolution barriers. Our results identify interfacial water as an active kinetic gate that links stacking-dependent interfacial water dynamics to the aqueous durability of layered oxide cathodes.