Understanding the Effects of Li ⁺ Salt Concentration and Zwitterion/Li ⁺ Stoichiometry on Ion Coordination in Zwitterionic Polymer-Supported Ionogel Electrolytes

Ionogel electrolytes are nonflammable, nonleaking alternatives to conventional flammable organic solvent-based electrolytes. In this study, zwitterionic poly(2-methacryloxyethyl phosphorylcholine) (pMPC) homopolymer-supported ionogels were synthesized using solutions consisting of three different molarities of a lithium salt dissolved in an ionic liquid: 0.5, 1, and 2 M LiTFSI dissolved in 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide (BMP TFSI). The zwitterionic monomer (MPC) content was varied relative to the lithium ion concentration in order to realize ionogels containing molar ratios of 0.25, 0.5, and 0.75 mol MPC/mol Li⁺ for each solution. All ionogels exhibited room temperature ionic conductivity values higher than those of their respective lithium salt solutions, with the greatest boost being ∼50%. Elastic modulus values for ionogels created using the 0.5 M solution showed the widest range of tunability (0.5 to 1750 kPa), corresponding to an estimated increase in the noncovalent cross-link density from approximately 9 × 10^-6 to 4 × 10^-2 nm^-3. Deconvoluted Raman spectra revealed that the percentage of TFSI^- anions coordinated with Li⁺ increases from 24 to 71% as the concentration of Li⁺ in the solution increases from 0.5 to 2 M. Within the ionogels synthesized using these solutions, the percentage of TFSI^- anions coordinated with Li⁺ was observed to decrease with increasing MPC/Li⁺ molar ratios. Calculated Li⁺ coordination number (by TFSI^-) values, as well as the normalized ionic conductivity data, were both found to exhibit common behaviors across all three LiTFSI molarities when plotted versus the MPC/Li⁺ molar ratio. These findings demonstrate that stoichiometry (i.e., zwitterionic unit/Li⁺ molar ratio), rather than polymer mass fraction, plays a more fundamental role as a key parameter in determining the properties of poly(MPC)-supported ionogel electrolytes, which are a promising safer alternative to traditional battery electrolyte materials.