(192c) Effect of Zwitterionic Molecules on Solvation and Transport of Sodium and Potassium Cations in Ethylene Oxide-Based Electrolytes

The transport of ions in electrolytes is governed mainly by their ionic interactions. Zwitterionic (ZW) molecules can affect electrolytes' lithium-ion (Li⁺) conductivity by adjusting Li⁺ solvation. Many newly developed batteries are using cations like sodium (Na⁺) and potassium (K⁺). It remains unclear if the ZW molecule could regulate the transport of Na⁺ and K⁺ in electrolytes and which mechanisms govern this regulation. To address this issue, we investigate the dissociation effect of three ZW molecules (MPC, CB, and SB) on Na⁺ and K⁺ in ethylene oxide (EO)-based electrolytes using molecular dynamics simulations (MPC: 2-methacryloyloxyethyl phosphorylcholine, SB: sulfobetaine ethylimidazole, CB: carboxybetaine ethylimidazole). We simulate model systems of NaTFSI/EO₁₀ electrolytes and KTFSI/EO₁₀ electrolytes at a cation:O(EO₁₀) molar ratio of 1:6 and with addition of ZW at a ZW:cation molar ratio of 1:5 (NaTFSI: sodium bis(trifluoromethylsulfonyl)-imide, and KTFSI: potassium bis(trifluoromethylsulfonyl)-imide). The simulation results unravel the difference in the ionic association and transport of Na⁺ and K⁺ and how the effect of ZW structures depends on cationic size using radial distribution functions, association number, association lifetime, dissociation percentage, and diffusion coefficients. K⁺ with a larger cationic size has weaker associations with [TFSI]^-, EO₁₀ and ZW molecules, thus presenting higher diffusion than Na⁺. Adding ZW molecules dissociates the cation-EO10 and cation-[TFSI]^- associations, which could help transport cations. MPC and SB present the highest and lowest dissociation ability among the three ZW structures. The dissociation ability of ZW molecules correlates with the strength of their associations with cations. MPC and SB also have the strongest and weakest associations with cations. However, these strong cation-ZW associations can slow down cationic transport. The competition of these two effects results in enhancing or decreasing cationic diffusion. MPC reduces the K⁺ diffusion by 40% while CB and SB induce a similar decrease of 25%. Diffusion of Na⁺ remains unvaried with the presence of MPC and enhanced by 20% with CB addition and decreased by 20% with adding SB. Our study suggests the important role of ZW structures and cationic size on the ionic dissociation effect of ZW.