Structure and Na Ion Transport in NaPF6/Carbonate Electrolyte inside Carbon Nanopores: A Molecular Dynamics Study

Abstract

Sodium-ion batteries (NIBs) are in the spotlight as highly promising energy storage applications due to the low cost and wide availability of Na sources. To date, computational studies of Na storage and transport in carbon electrodes have been carried out without considering the realistic structural and dynamic properties of a carbon anode - electrolyte, which could be critical to battery performance. Using combined molecular dynamics (MD) simulation and density functional theory (DFT) calculations, we describe the behavior of a Na-based electrolyte, consisting of sodium hexafluorophosphate (NaPF6), ethylene carbonate (EC), and dimethyl carbonate (DMC), near the confined structures of nanoporous carbon electrodes. In our simulation, nanoporous carbons with an average pore size of 8.9 Å were first generated using the Mimetic porous carbon model by quench MD simulation. We also adopted the constant potential method (CPM) to model the electrode carbon to account for the fluctuation of the local charges that are characteristic of realistic battery systems. We investigated the distribution of electrolyte molecules and their microstructures at 0 V and 2 V potentials to provide molecular insights on properties such as solvent packing, degree of confinement (DoC), preferential adsorption sites, and solvation sheath structure. Under the influence of applied potential, we report a good wettability of the electrode by electrolyte molecules as a result of the interconnectivity of nanopores, with the accumulation of electrolyte compounds at the electrode/electrolyte interface as they diffuse into the nanopores. Also, Na+ intercalates into highly confined carbon structures while PF6 - occupies weakly confined carbon nanostructures by forming ion pairs with Na+. We show that nanoporous carbon structures reduce the adsorption distance of Na+ by ~29 % compared to planar electrodes and consequently identify a hydrogen-terminated edge of porous carbon as a preferential adsorption site for Na+ of favorable adsorption energy of -0.27 eV compared to the adsorption energy of 0.02 eV for Na+ insertion onto a basal surface. Finally, an investigation of the solvation sheath structure as a function of DoC shows decreased solvation shell, shortened adsorption distances, and increased counter-charge with increasing DoC.