Non-monotonic hydration dependence of ionic transport in atomically-thin MnO2 sheets
The interlamellar space in van der Waals materials has served as a test bed for studies of strongly confined fluid and ion transport. Despite theoretical and experimental progress, it remains unclear how electrostatic correlations arising from interactions between the confined, hydrated ions and the substrate layer can be reconciled with non-equilibrium ionic transport and the substrate's morphological response. Here, we investigate field-driven ionic conduction in sodium- and water-intercalated layered manganese oxides ($\mathrm{MnO_2}$), a self-intercalated metal oxide nanofluidic system, as a model intrinsically-intercalated van der Waals solid, using nonequilibrium all-atom molecular dynamics simulations that explicitly capture ion-water correlations and layer morphology. We demonstrate that electric-field bias induces spontaneous nanoscale segregation of water within the interlayer space, producing coexisting hydrated and weakly hydrated ionic domains coupled to local lattice distortions. This feedback between hydration and morphology gives rise to heterogeneous transport pathways and leads to a non-monotonic dependence of ionic conductivity on water content. Suppressed conduction arises either from layer collapse at low water levels or from ionic hop blockage by excess water at high water levels. Concurrently, ionic transport exhibits a maximum at intermediate levels of intercalated water, where less hydrated ions can move at the boundaries of strongly hydrated ion clusters. While such non-monotonicity could explain the experimentally observed memristive response of $\mathrm{MnO_2}$ single crystals, these findings also provide a molecular-level mechanism linking intercalated water levels to ionic metal oxide nanofluidic systems, suggesting general design principles for robust, water-assisted ionic conductors.