Advanced Block Copolymer Design for Polymer Electrolytes: Prospects of Microphase Separation

Herein, we report on an advanced design for polymer electrolytes (PEs) based on our previously reported microphase-separated poly(vinyl benzyl methoxy poly(ethylene oxide) ether)-block-polystyrene block copolymers (PVBmPEO-b-PS). Usually, such block copolymers are characterized by a high mechanical stability provided by the PS domain, while the PEO-based domain features decent ionic conductivities, however, mostly only at higher temperatures. To enable suitable ionic conductivities at lower temperatures, we selectively implemented two ionic liquids (ILs) as a model plasticizer for the PEO domain. Since those ILs are nonmiscible with PS, the latter domain is unaffected, thus still providing a great mechanical stability. To maintain the necessary self-standing film forming ability, we adjusted the size of the PS domain to match with the conducting PEO-based domain. For this, a series of four block copolymers with different PS:PVBmPEO block ratios were synthesized, thus enabling the study of the influence of different amounts of IL. Further, all derived polymer electrolytes were thoroughly characterized by thermal, rheological, morphological, and electrochemical analyses. We could prove the microphase-separated morphology with long-range order and a good thermal and mechanical stability as well as the selective mixing of the ILs within the conducting domain. Consequently, electrochemical impedance spectroscopy revealed a significant increase in ionic conductivity up to 2 orders of magnitude and a reduced interfacial resistance in comparison to a nonplasticized reference sample. Moreover, exhaustive studies of the lithium-ion transference number showed not only the importance of such detailed analysis for IL-containing PEs but also the true increase of the effective lithium-ion conductivity. Finally, we conducted a full cycling in Li||LiFePO4 (LFP) cells to clearly demonstrate the applicability of our approach.