Effects of pore size and surface charge on Na ion storage in carbon nanopores

Na ion batteries (NIBs) are considered as a promising low cost and sustainable energy storage technology. To better design nanoporous carbons as anode materials for NIBs, molecular dynamics simulations have been employed to study the behavior of Na⁺ ions (as well as PF₆^- ions) confined within carbon nanopores, in the presence of non-aqueous (organic) solvent. The effects of pore size and surface charge density were quantified by calculating ionic density profiles and concentration within the pores. Carbon slit pores of widths 0.72-10 nm were considered. The carbon surfaces were charged with densities of 0 (neutral pores), -0.8e nm^-2, -1.2e nm^-2, and -2e nm^-2. Organic solutions of Na⁺ and PF₆^- at 1 M concentrations were considered under operating conditions of sodium ion batteries. As the surface charge density increases, more Na⁺ ions enter the pores. In all pores, when the surface is highly charged the Na⁺ ions move toward the negatively charged graphene surfaces because of counterion condensation effects. In some instances, our results reveal the formation of multiple layers of adsorbed Na⁺ inside the pores. Both the nanopore width and surface charge alter the density profiles of ions and solvent inside the pores.