In searching for polymer-based electrolytes with improved performance for lithium ion and lithium metal batteries, we studied block copolymer electrolytes with high amounts of bis(trifluoromethane)sulfonimide lithium obtained by macromolecular co-assembly of a poly(isoprene)-block-poly(styrene)-block-poly(ethylene oxide) and the salt from tetrahydrofuran. Particularly, an ultra-short poly(ethylene oxide) block of 2100 g mol was applied, giving rise to 2D continuous lamellar microstructures. The macroscopic stability was ensured with major blocks from poly(isoprene) and poly(styrene), which separated the ionic conductive PEO/salt lamellae. Thermal annealing led to high ionic conductivities of 1.4 mS cm at 20 °C with low activation energy and a superior lithium ion transference number of 0.7, accompanied by an improved mechanical stability (storage modulus of up to 10 Pa). With high Li:O ratios >1, we show a viable concept to achieve fast Li transport in block copolymers (BCP), decoupled from slow polymer relaxation.

Block copolymers are promising materials for electrolytes in lithium metal batteries that can be tuned by changing the individual blocks to independently optimize ion transport as well as electrochemical and mechanical stability. We explored the performance of electrolytes based on modified triblock copolymers, poly­(isoprene)-block-poly­(styrene)-block-poly­(ethylene oxide). Large polyethylene oxide (PEO) blocks with a molecular mass of 53 kg mol–1 allowed only for low lithium salt loadings and led to poor ionic conductivity below 60 °C. However, we found that unusually small molecular weight of the ion solvating PEO blocks down to 2 kg mol–1 enabled polymer-in-salt loadings of up to 5:1 Li/EO. A superior total ionic conductivity greater than 1 mS cm–1 was found for optimized compositions above 0 °C with remarkably low temperature dependence in a wide range from −20 to 90 °C. We believe that highly ordered two-dimensional lamellae from the controlled self-assembly established a beneficial environment for ionic transport with ionic mobility decoupled from segmental polymer motion. This also explains lithium ion transference numbers as high as 0.7 that were obtained for the high conductivity samples.