Versatile cation transport in imidazolium based polymerized ionic liquids

Polymerized ionic liquids (PIL) with tethered imidazolium groups are able to conduct a diverse array of cations relevant for energy applications. The well-known complexation of imidazolium with transition metals is exploited to bind ions such as H$+$, Li$^{\mathrm{+}}$, Cu$^{\mathrm{2+}}$, and Ni$^{\mathrm{2+}}$ by doping the neutral PIL with the appropriate Cation-TFSI$^{\mathrm{-}}$ salt. Conductivities were first determined via AC impedance indicating that H$^{\mathrm{+}}$ salts lead to the highest conductivity (due to low ion mass and potential Grotthus mechanism) followed by Cu$^{\mathrm{2+}}$, Li$^{\mathrm{+}}$, Ag$^{\mathrm{+}}$, and Ni$^{\mathrm{2+}}$. The equilibrium constant for imidazolium complexation is larger for Cu$^{\mathrm{2+}}$ relative to Li-, Ag-, and Ni-imidazolium complexes leading to greater salt dissociation and higher conductivities. For LiTFSI and CuTFSI$_{\mathrm{2\thinspace }}$salts, metallic lithium or copper electrodes were employed in battery cells to pass a steady DC current and confirm that the cations are in fact carrying current. Interestingly, the divalent Cu$^{\mathrm{2+}}$ also ionically crosslinks the polymer leading to a plateau in the viscosity. Thus, divalent ions provide an unique route to high conductivity, high modulus polymeric electrolytes. Future studies involving ZnTFSI$_{\mathrm{2}}$ and MgTFSI$_{\mathrm{2}}$ for battery applications are proposed to examine how versatile the PIL platform is for cation transport.