A Molecular Dynamics Study of Na Ion Transport and Storage inside Nanoporous Carbon Electrode for Na-Ion Batteries

Abstract

Hard carbon has emerged as an attractive anode material for sodium ion batteries (NIBs) because of its low cost, high initial coulombic efficiency, high specific capacity, and steady cycling performance. Based on combined molecular dynamics (MD) simulation and density functional theory (DFT) calculations, we present an atomistic description of the electrolyte-electrode interaction of sodium hexafluorophosphate (NaPF6) in mixed ethylene (EC) and dimethyl carbonate (DMC) electrolyte inside nanoporous carbon electrode. We investigate the microstructure of electrolyte in carbon nanopores with an average pore size of 8.9 Å and a specific surface area of 1794 m2g-1 at 0 V and 2 V to gain molecular insight into electrolyte behavior such as solvent packing, preferential adsorption sites, and solvation structure. Our simulation suggests good accessibility of carbon nanopores by electrolyte molecules upon charging. At 2 V, Na+ is desolvated and intercalates into highly confined carbon structures as a result of the strong interaction between Na+ and charged carbon atoms while PF6 - occupies weakly confined carbon nanostructures by forming ion pairs with Na+. Also, we examined the various levels of confinement of electrolyte components using a degree of confinement (DoC) analysis, and present the effects of confinement on electrolyte properties. Our calculations show a 29 % reduction in Na+ adsorption distance for a nanoporous structure compared to a planar structure of carbon, and the preferential adsorption of Na+ at a hydrogen-terminated edge of porous carbon with an adsorption energy of -0.27 eV compared to a basal site with a higher adsorption energy of 0.02 eV. 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.