1Tufts University, Medford, Massachusetts, United States
Ionogel electrolytes can offer many of the advantageous properties of ionic liquids for the realization of safer batteries, such as ultralow volatility, good ionic conductivity, and wide electrochemical stability – all in a non-flowing, leakproof form. Many polymer chemistries have been previously explored to serve as solid, three-dimensional networks for supporting the ionic liquid phase within ionogels. A particularly intriguing class of polymers that has been recently introduced for this role is polyzwitterions, those (co)polymers which contain one or more monomers bearing zwitterionic functional groups. Possessing extremely large permanent dipole moments, zwitterionic side groups endow polyzwitterion scaffolds with an ability to exert significant Coulombic interactions upon the various ionic species present within an ionic liquid-based electrolyte, as well as upon one another. Depending on the specific chemical nature of the zwitterionic motif (e.g. sulfobetaine vs. phosphorylcholine vs. carboxybetaine) and the ionic liquid-based environment, these competing electrostatic interactions amongst the zwitterions and ions present determine the details of both selective ion transport and ionogel mechanical behavior.
We have investigated the compressive stress-strain responses and ionic conductivities of several ionogels supported by scaffolds that feature the zwitterionic monomer 2-methacryloyloxyethyl phosphorylcholine (MPC), for solutions of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt dissolved in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP TFSI). While the zwitterionic homopolymer scaffold was found to substantially increase room temperature lithium ion conductivity compared to that of the ionic liquid/salt solution, statistical copolymer scaffolds formed using a combination of MPC and a non-zwitterionic comonomer (2,2,2-trifluoroethyl methacrylate) did not yield large improvements in selective lithium ion mobility. For both scaffold types, however, our experimental findings support the hypothesis that the zwitterionic side groups form highly stable, noncovalent cross-links that are mediated by lithium ions. For ionogels based on a 1M LiTFSI in BMP TFSI solution, gel elastic modulus values can be readily tuned over several orders of magnitude, between approximately 20 kPa and 10 MPa, simply by varying the MPC content within the scaffold. Remarkably, these noncovalent cross-links, which are based on Coulombic interactions within an ion-dense, nonaqueous ionic liquid environment, persist even as ionogels are heated above 200 °C. Moreover, the ion-mediated cross-links can be successfully formed either in situ (prior to scaffold polymerization) as well as “on demand” (post-polymerization). Ionogels featuring such highly durable, noncovalently cross-linked scaffolds may find practical future use in applications such as lithium-based batteries that can be operated under extreme environmental conditions.