A class of electrolytes based on polyethylene oxide (PEO)-functionalized polyhedral oligosilsesquioxanes [Q(8)M(8)(PEO(n)) or PEO(n)POSS] and lithium salts was developed, where n = 2, 4, (8), [12.5] ethylene oxide units. Conductivities (sigma) of these highly branched, nonvolatile (375 degreesC), viscous polymers (M-w - 3000-5000 g/mol) were >10(-3) S/cm at 50 degreesC and >10(-4) S/cm (LiClO4) at room temperature (RT), comparable to conductivities of oligomeric PEOs (M-w = 300-500). The T-g values of Q(8)M(8)(PEO(n=2,4,(8)))/LiClO4 were between -60 and -70degreesC, so that sigma at -20 degreesC approached 10(-6) S/cm. Strong self-supporting films (tensile strength 3.7 MPa) from 10% methyl cellulose and 90% Q(8)M(8)(PEO(n=(8)))/ LiClO4 had RT conductivities >10(-5) S/cm. (C) 2004 The Electrochemical Society.