(356k) Ionic Solvation in Dual-Zwitterion Electrolytes: A Molecular Simulation Study

ABSTRACT: Conducting polymer electrolytes have applications in batteries for flexible electronic devices and medical appliances due to their flexibility, and better safety over liquid electrolytes. However, realizing high mechanical toughness often results in low ionic conductivity of polymer electrolytes. Zwitterionic polymers have emerged as candidates to achieve high mechanical strength without sacrificing ionic conductivity due to their unique chemical structures: zwitterionic molecules (ZW) possess both cationic and anionic groups. The ZW molecules can not only associate with themselves but also dissociate ionic associations between cations and anions. The self-associations among zwitterionic monomers give zwitterionic polymers the high mechanical strength. Experiments have shown that different zwitterionic molecular structures play different roles on regulating elastic modulus of lithium and sodium electrolytes. Particularly, the sulfobetaine vinylimidazole ratio in polymer composition has deterministic effect on elastic modulus in lithium electrolytes while to a smaller degree in sodium electrolytes. To reveal the governing mechanism behind this phenomenon, we investigate the zwitterionic electrolytes containing sulfobetaine vinylimidazole (SBVI), 2-methacryloyloxyethyl phosphorylcholine (MPC), and 0.34M lithium/sodium bis(trifluoromethylsulfonyl)imide (LiTFSI/NaTFSI) in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMPTFSI) at 340K, 500K and 600K using molecular dynamics simulations. The mole percentage of SBVI molecules are varied as 0, 25, 50, 75, and 100% in ZW mixtures while the total mole percentage of all ZW molecules remain 15%. We analyzed association numbers and lifetimes of associations among ZW molecules, Li⁺/Na⁺, BMP⁺ and counterions. The simulation results show that while increasing SBVI percentage affect self-associations among SBVI and MPC molecules in a similar way between Li⁺ and Na⁺ systems, it enhances the association lifetime of SBVI-MPC for Li⁺ while not for Na⁺ systems. The longer lifetime helps zwitterionic electrolytes with higher content of SBVI to have better mechanical strength in case of Li⁺ systems.