Understanding the Impact of Li⁺ vs Na⁺ Salt on the Mechanical and Dynamic Properties of Polyzwitterion-supported Ionogel Electrolytes

Polyzwitterion (polyZI) supported ionogel electrolytes have emerged as alternatives for designing mechanically robust electrolytes with high room temperature conductivities. This is due to the PolyZI side chains playing a dual role as noncovalent crosslinking sites and mobile ion dissociation promoters. In this study, we used atomistic molecular dynamic simulations to probe the impact of cation identity (Li⁺vs Na⁺ ) on the elastic moduli and conductivity of single-salt (Li⁺ or Na⁺) and mixed-salt (Li⁺ and Na⁺) ionogels as a function of ZI weight percentage. Our model system is comprised of ZI poly (2-methacryloyloxyethyl phosphorylcholine) (pMPC) loaded with 0.5M X + TFSI/N-butyl-N-methylpyrrolidinium (BMP) TFSI ionic liquid electrolyte (where X ⁺ = Li⁺, Na⁺, or Li ⁺ and Na⁺ ). For both single- and mixed-salt doped ionogel electrolytes, with increasing ZI weight percentage, we observe an increasing association between Li⁺ and Na⁺ ions with the ZI polymer. This observation is accompanied by a decrease in the interaction between Li⁺ and Na⁺ ions with the ionic liquid and salt common anion. Our simulations also reveal a preferential association of Li + - ZI over Na + - ZI in both cases. We also present the probability of ion-pair associations to identify the mechanistic origins of structural and dynamical property differences in both single- and mixed-salt ionogels. Lastly, we show cluster analysis to rationalize the experimentally observed elastic moduli and conductivity results.