Surface charge-driven sodium-ion migration and secondary desolvation in MXene nanoconfinement

Abstract

Improving the energy storage and power delivery of layered materials relies heavily on a better understanding of the ultrafast ion intercalation dynamics within nanoconfinement. Here we report a molecular dynamics (MD) simulation that directly probes the effect of surface charge strengths on Na ion transport properties in slit MXene (Ti₃C₂(OH)₂) anodes. We found that at high surface charge strengths, the migration of intercalated Na⁺ to the MXene surface is the main reason for the significantly increased interlayer Na⁺ conductivity. More importantly, during the “interlayer-to-surface” process of most intercalated Na⁺, the H-bonding network of some surface water is disrupted by the Na-ions’ electrostatic attraction, causing these water molecules to lose surface affinity and flip away from the MXene surface. This allows further desolvated Na ions to be exposed on the electrode surface, thereby improving the charge transfer between ions and the electrode. This work provides an atomic understanding of Na-ion's secondary desolvation mechanism based on surface water molecular dipole flipping, which may provide new insights into the energy storage mechanism of nanoscale layered materials.