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

TL;DR

This study probes field-driven ionic transport in intrinsically ion-intercalated van der Waals MnO$_2$ by performing all-atom nonequilibrium MD with explicit water and ions. The results reveal that an in-plane electric field drives water segregation into coexisting hydration domains, which modulates interlayer spacing and morphology, producing nonlinear, non-Ohmic conduction and a breakdown of simple diffusion relations. The maximal ionic conductivity arises at intermediate hydration due to competing screening and hopping accessibility; boundary regions between water-rich and water-poor domains dominate transport. The findings supply a molecular-level mechanism linking confinement, electrostatic correlations, and morphology, and offer design rules for water-assisted, robust ionic conductors in vdW layered materials.

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.

Figures (4)

• Figure 1: Modeling of single-crystalline MnO$_2$ layered structures (a) Representative MD snapshot and experimental image of a single-crystalline MnO$_2$ layer from the lateral ($\hat{y}$) direction. The experimental system is composed of 4 to 5 atomic layers serkan_hoca2021. The simulated model is constructed by stacking 4 MnO$_2$ sheets in the $\hat{z}$ direction. b) The atoms constituting the layers, including intercalated water and ions, are given in their vdW description. The unit lattice and Octahedral site of the MnO$_2$ surfaces are given with the same color codes. c) Lateral view of the layers for various water levels and inter-layer thickness averaged over equilibrium (field-free) simulation trajectories. d) Sodium-layer oxygen (Na-O) radial distribution functions of electric-field-free cases for various water levels.
• Figure 2: Effect of electric field and water segregation and layer morphology: a) Representative simulation snapshots for various water levels at the beginning and after 50 ns under an applied electric field. b) The fractions of ions with various coordination number (CN), which defines the number of water molecules in the first hydration shell as a function of time at $E_x = 0.02$ V/Å for one water per ion case (i.e., 1H$_2$O$\cdot$Na water level). c) Average interlayer distance vs CN for 1H$_2$O$\cdot$Na. d) A snapshot demonstrating the undulations in the interlayer height and corresponding layer thickness after 50 ns at $E_x = 0.02$ V/Å.
• Figure 3: Water segregation versus ionic conduction: a) The displacement trajectories averaged over all ions for the 1 1H$_2$O$\cdot$Na system for various electric field strengths. b) The ionic current as a function of water levels at $E_x = 0.02\ \mathrm{V/\AA}$. c) Electric field versus current for one and two water per ions cases, which correspond to the highest- and lowest-conducting systems in (b). d) Visualization of ionic trajectories for MnO$_2$ layers intercalated by various levels of water calculated at the steady state regime between 50 ns--100 ns interval of the simulation; blue lines denote the path of a single ion and blue dots refer to no motion. e) Coordination number vs. expected value of square of ionic displacement calculated over 1-ns-time windows for various water levels at $E_x = 0.02$ V/Å.
• Figure 4: Ionic transport in rigid, fluctuation-free vdW layers. a) Applied electric field versus current for various water levels. b) Ionic current for (left) various interlayer distances at a fixed water level and (right) for various water levels at a fixed interlayer distance. c) Representative simulation snapshots demonstrating the ion and water distribution in a single layer ($d_z = 7 \ \mathrm{\AA}$) of MnO$_2$ at equilibrium and under an applied field of $E_x = 0.01$ V/Å. d) The distributions of the angle between the $x$ axis and the water dipole on the $xy$ plane. e) Displacement histograms of ions at $d_z = 7 \ \mathrm{\AA}$ interlayer spacing for 10 ns-long simulations at various field strengths for 1H$_2$O$\cdot$Na case.