Ionic transport in spontaneously ion-intercalated van der Waals layered structures

Abstract: Understanding ionic transport under strong confinement is crucial for the design of next-generation energy, catalytic, and information-processing materials; however, repeated field-driven ion motion often degrades conventional solid electrolytes. Van der Waals layered materials offer an alternative by providing structurally resilient ion-transport channels, yet the microscopic origins of their nonequilibrium transport behavior remain poorly understood. Here, we investigate field-driven ionic conduction in sodium-intercalated layered MnO$_2$ as a model self-intercalated van der Waals solid, using all-atom nonequilibrium molecular dynamics simulations that explicitly capture ion-water correlations and layer morphology. We demonstrate that ionic conductivity depends nonlinearly on the applied electric field, interlayer spacing, water content, and lattice flexibility. The applied electric field induces spatial segregation of water coupled to distortions of the MnO$_2$ sheets, producing coexisting regions populated by highly hydrated and weakly hydrated ions with suppressed conductivity. Concurrently, ionic transport exhibits a nonmonotonic dependence on the total amount of intercalated water, with boundary domains of weakly hydrated ions displaying relatively higher mobility. In fluctuation-free layers, ion transport transitions from single-particle motion to a collective conduction regime characterized by elongated, same-charge ionic clusters that violate Nernst-Einstein behavior. Together, these findings provide a molecular-level mechanism linking confinement-induced electrostatic correlations and structural response to the emergent nonlinear transport observed experimentally in ion-intercalated MnO$_2$, and suggest general design principles for robust, water-assisted ionic conductors.