Effect of Zwitterionic Additives on Solvation and Transport of Sodium and Potassium Cations in (Ethylene Oxide)₁₀: A...

Sodium- (Na⁺) and potassium- (K⁺) ion batteries are cost-effective alternatives to lithium-ion (Li⁺) batteries due to the abundant sodium and potassium resources. Solid polymer electrolytes (SPEs) are essential for safer and more efficient Na⁺ and K⁺ batteries because they often exhibit low ionic conductivity at room temperature. While zwitterionic (ZW) materials enhance Li⁺ battery conductivity, their potential for Na⁺ and K⁺ transport in batteries remains unexplored. In this study, we investigated the effect of three ZW molecules (ChoPO4, i.e., 2-methacryloyloxyethyl phosphorylcholine, ImSO3, i.e., sulfobetaine ethylimidazole, and ImCO2, i.e., carboxybetaine ethylimidazole) on the dissociation of Na⁺ and K⁺ coordination with ethylene oxide (EO) chains in EO-based electrolytes through molecular dynamics simulations. Our results showed that ChoPO4 possessed the highest cation-EO₁₀ dissociation ability, while ImSO3 exhibited the lowest. Such dissociation ability correlated with the cation-ZW molecule coordination strength: ChoPO4 and ImSO3 showed the strongest and the weakest coordination with cations. However, the cation-ZW molecule coordination could slow the cationic diffusion. The competition of these effects resulted in accelerating or decelerating cationic diffusion. Our simulated results showed that ImCO2 enhanced Na⁺ diffusion by 20%, while ChoPO4 and ImSO3 led to a 10% reduction. For K⁺, ChoPO4 reduced its diffusion by 40%, while ImCO2 and ImSO3 caused a similar decrease of 15%. These findings suggest that the ZW structure and the cationic size play an important role in the ionic dissociation effect of ZW materials.