(226c) Polymerized Ionic Liquid Block Copolymers As Solid-State Polymer Electrolytes for Lithium-Ion Batteries

Replacing liquid-based electrolytes with solid-state polymer electrolytes (SPEs) can alleviate safety and stability concerns, while offering other attractive properties, such as thin-film forming ability, flexibility, and transparency. Ideal SPEs expertly engineering within a solid-state battery will require high lithium ion conductivities and high mechanical strength to result in high overall storage capacity/energy density and high stability and cyclability in a lithium-ion battery. Block copolymers are a well-explored material platform for this application because it combines the properties of high ionic conductivity and high strength in a nanostructured solid-state film due to covalent connection of two dissimilar polymer chains in sequential chain architecture. Herein, we present new block copolymer chemistry, where the conductive block is a polymerized ion liquid (PIL), i.e., PIL block copolymer. PILs possess unique physiochemical properties, such as high solid-state ionic conductivity, high chemical, thermal, and electrochemical stability, and widely tunable physical properties. PIL block copolymers are a new type of SPE that combine the advantages of block copolymers (nanostructured morphology) and PILs (unique physiochemical properties). In this work, the PIL diblock copolymer poly(MMA-b-MUBIm-TFSI) with an ionic PIL component (1-[(2-methacryloyloxy)undecyl]-3-butylimidazolium bis(trifluoromethane)sulfonimide) (MUBIm-TFSI) and a non-ionic component (MMA) was synthesized via reverse addition fragmentation chain transfer (RAFT) polymerization technique followed by post-functionalization and ion exchange. SPEs consisting of this PIL block copolymer and lithium salt/IL were fabricated via solution casting at various lithium salt/IL compositions ranging from 0 to 0.5 mol ratio of (salt/IL)/polymer. Interestingly, the PIL diblock copolymer exhibited no microphase separation over the entire salt/IL composition range explored evidenced by no scattering peak in small-angle X-ray scattering (SAXS) and only one broad glass transition temperature (T_g) in differential scanning calorimetry (DSC). The T_g decreased from 39 to 10 °C with increasing salt/IL composition. The temperature-dependent ionic conductivity increased by three orders of magnitude with increasing salt/IL composition, with a half an order of magnitude increase from 0.3 to 0.4 mol ratio and over one and a half orders of magnitude increase from 0.4 to 0.5 mol ratio, which was surprising for this small T_g depression between each ratio. These results suggest that ionic conductivity can be significantly improved without microphase separation and that salt-polymer compatibility may play a significant role on conductivity.