Design of Stretchable and Self-Healing Gel Electrolytes via Fully Zwitterionic Polymer Networks in Solvate Ionic Liquids for Li-Based Batteries

An emerging class of ion-dense electrolytes consisting of complexed lithium cations and weakly basic anions, known as solvate ionic liquids, possess many desirable attributes for lithium-based electrochemical energy storage. In this study, a series of fully zwitterionic (f-ZI) polymer scaffold-supported solvate ionogels are synthesized via UV-initiated free-radical (co)polymerization of two zwitterionic monomers, 2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine vinylimidazole (SBVI), in situ within the solvate ionic liquid [Li(G4)][TFSI], which is prepared from an equimolar mixture of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and tetraglyme (G4). Systematically varying the MPC:SBVI molar ratio within the f-ZI polymer network enables one to widely tune the mechanical properties of the poly(MPC-co-SBVI)-supported solvate ionogel composites. For a fixed polymer content of 20 mol %, gel compressive elastic modulus values are observed to span 2 orders of magnitude, from 23 kPa to 7.3 MPa, while the room temperature ionic conductivity values remain fairly unchanged (between 0.48 and 0.70 mS cm–1). MPC-rich copolymer formulations lead to solvate ionogels that exhibit substantial plastic deformation, exceeding 200% tensile strain prior to failure, and Li-ion transference numbers as high as 0.60, which represents a 5-fold increase compared to the neat solvate ionic liquid electrolyte. A 20 mol % poly(MPC-co-SBVI)-supported solvate ionogel having a 3:1 MPC:SBVI molar ratio successfully enables the galvanostatic cycling of a lithium-ion battery prototype for 100 cycles at a rate of C/2, demonstrating the viability of these safer gel electrolytes for lithium-based energy storage devices.