Design of Pyrrolidinium-PEG Ionic Copolyesters for Li-Ion Transport Channels in Polymer Network Gel-Polymer Electrolytes

Since electric vehicle (EV) accidents from flammable lithium-ion batteries (LIBs) continue to increase, solving the safety issue of LIBs has emerged as an important task. Highly volatile liquid solvents that help solvate and transport Li⁺ are the major cause of the flammability of LIBs. Therefore, replacing liquid electrolytes with solid electrolytes may overcome this safety issue. Even though solid-state electrolytes have advantages over liquid electrolytes, restricted ion transportation in solid-state electrolytes causes poor room temperature ionic conductivity (≈10⁻⁵ to 10⁻² mS cm⁻¹), which makes solid-state electrolytes difficult to apply to commercial LIBs.<br/><br/>In this work, ionic liquid-based linear polyesters were adapted on solid-state electrolytes. Several ionic polyesters prepared from imidazolium ionic liquid monomers have been previously reported. Imidazolium-polyethylene glycol (Imidazolium-PEG) copolyesters have also been researched, and their electrical and thermal properties are dependent on the length of the PEG segment. However, because pyrrolidinium ionic liquids are more stable and show wider electrochemical windows than imidazolium ionic liquids, pyrrolidinium ionic liquid based polyesters have been prepared and used for solid-state electrolytes. To fabricate a solid-state semi-IPN polymer electrolyte system, a UV-crosslinkable polymer matrix of ethoxylated trimethylolpropane triacrylate (ETPTA) and tailored linear pyrrolidinium-PEG copolyester with carbonate liquid electrolyte were mixed. Well-dispersed pyrrolidinium-PEG copolyester (P_NPEG, N: average molecular weight of PEG) could accelerate Li⁺ transportation between the cathode and anode in the polymer matrix. To optimize battery performance, P_NPEG was synthesized from PEGs with different average molecular weights and pyrrolidinium Bis(trifluoromethanesulfonyl)imide (pyrrolidinium-Tf₂N) ionic monomers. The PEG backbone in P_NPEG promotes Li⁺ transportation through the interactions between ether groups and Li⁺ ions. In addition, ionic liquid monomers in P_NPEG contribute to ion solvation and selective cation transport.<br/>In summary, we reported an in situ UV-cured gel polymer electrolyte (GPE) incorporating novel pyrrolidinium-PEG copolyester materials to promote lithium ion transport properties. Well-dispersed P₁₀₀₀PEG in the GPE formed ion channels, which were confirmed by SEM images and electrochemical tests. The amorphous polymer structure and low <i>T</i>g of P₁₀₀₀PEG-GPE supported the higher ion conductivity results obtained by XRD and DSC analysis, respectively. In addition, MD simulations showed a lower coordination number between P_NPEG and Li⁺ than between pure PEG1000 and Li⁺. As the optimized proportion of P₁₀₀₀PEG improved the ion conductivity and stability of the solid-state electrolytes, this study is expected to support advances in solid-state batteries applying P₁₀₀₀PEG-based ion channels beyond liquid electrolytes.