An Ambient Temperature Electrolyte with Superior Lithium Ion Conductivity based on a Self-Assembled Block Copolymer

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^-1 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^-1 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 10⁷  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.