Examining the Impact of Polyzwitterion Chemistry on Lithium Ion Transport in Ionogel Electrolytes

A series of polyzwitterion-supported gels featuring two classes of lithium-containing ionic liquid (IL) electrolytes have been created to examine the impact of different zwitterionic (ZI) group chemistries on lithium ion conductivity in these nonvolatile electrolytes. ZI homopolymer-supported gels containing poly(carboxybetaine methacrylate) (pCBMA), poly(2-methacryloyloxyethylphosphorylcholine) (pMPC), poly(sulfobetaine vinylimidazole) (pSBVI), and poly(sulfobetaine 2-vinylpyridine) (pSB2VP) were realized by rapid, in situ UV photopolymerization. Within a 1 M solution of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in a conventional IL, strong Coulombic interactions between ZI moieties and Li + cations promoted higher ion self-diffusivities for all zwitterion types and generated improved Li + conductivities. In particular, the pCBMA and pMPC gels exhibited improved lithium transference numbers of 0.37 and 0.38, respectively, compared to 0.23 for the IL solution. In the solvate ionic liquid (SIL) prepared from an equimolar mixture of LiTFSI and tetraglyme, the pCBMA scaffold resulted in the largest room temperature Li + conductivity achieved, 0.44 mS cm –1 (vs 0.23 mS cm –1 in the neat SIL). The carboxybetaine ZI motif yielded the largest boost in Li + conductivity in both IL electrolyte types, which was found to be correlated to this monomer generating the largest downfield 7 Li NMR chemical shift in solution. This study illustrates the great potential of polyzwitterions for future application in lithium ion batteries and reveals the importance of zwitterion chemistry when selecting materials for nonaqueous ionogel electrolytes.