Abstract: A novel solid block copolymer electrolyte material is described. The material has first structural polymer blocks that make up a structural domain that has a modulus greater than 1×107 Pa at 25° C. There are also second ionically-conductive polymer blocks and a salt that make up an ionically-conductive domain adjacent to the structural domain. Along with the second ionically-conductive polymers, there is also a crosslinked network of third polymers, which interpenetrates the ionically-conductive domain. The third polymers are miscible with the second polymers. It has been shown that the addition of such an interpenetrating, crosslinked polymer network to the ionically-conductive domain improves the mechanical properties of the block copolymer electrolyte with no sacrifice in ionic conductivity.