Syndiotactic polypropylene-b-poly(ethylene oxide)-b-syndiotactic polypropylene (PEOP) triblock copolymers were synthesized and solid polymer electrolytes were prepared by mixing with lithium bis(trifluoromethane) sulfonimide (LiTFSI) salt. PEOP formed strongly-segregated morphologies in the absence and presence of LiTFSI. LiTFSI inhibited poly(ethylene oxide) crystallization without affecting polypropylene crystallinity. The conductivity exhibited a non-monotonic dependence on molecular weight (M-n) with a maximum near 20 kg/mol. In contrast, polystyrene-b-poly(ethylene oxide) electrolytes exhibit conductivity increasing monotonically with M. up to a plateau in the high-M-n limit. This suggests that non-conducting semi-crystalline microphases interfere with conducting pathways, while non-conducting amorphous microphases formed well-connected conducting pathways. Published by Elsevier B.V.