Polyethylene glycol functionalized polyoctahedral silsesquioxanes as electrolytes for lithium batteries

The conductivity of polyoctahedral silsesquioxanes [POSS-PEO (n = 2, 3, 4, 6, 8 , 12.5 )(8)]-based electrolytes has been investigated as a function of O/Li ratio for LiClO4, LiN(CF3CF2SO2)(2), LiN(CF3SO2)(2), LiCF3SO3, LiPF6, LiAsF6, and LiBF4. T-g increased with increasing salt concentration and poly(ethylene oxide) chain length (n). At high temperatures, T > T-g, corresponding to low-viscosity conditions, conductivity increased with increasing salt concentration and the conductivities of the less-associative salts [LiClO4, LiN(CF3CF2SO2)(2), LiN(CF3SO2)(2), LiPF6, and LiAsF6] were greater than those of the more associative salts [LiBF4 and LiCF3SO3] due to increased numbers of charge carriers. At low temperatures, T -> T-g, corresponding to high-viscosity conditions, the salts [LiBF4, LiCF3SO3, LiN(CF3SO2)(2), and LiN(SO2CF2CF3)(2)] with lower T-g (204 - 206K) had higher conductivities than the higher T-g (211 - 215K) salts [LiClO4, LiAsF6, and LiPF6] due to increased mobility of the ethylene oxide units. Decreased Tg resulted from: (i) plasticization effects of the large, flexible (CF2CF3SO2)(2)N- and (CF3SO2)(2)N- anions and (ii) decreased Li+/PEO complexation for the associative LiBF4 and LiCF3SO3 salts. The flexible, bulky anion (CF3CF2SO2)(2)N- decreased the crystallinity of semicrystalline POSS PEO (n = 8 )(8) more than the smaller ClO4-. SiO1.5 acted like an inert filler. The highest conductivity, 1.1 x 10(-4) S/cm, was obtained for POSS-PEO (n = 4)(8)/LiN(CF3CF2SO2)(2) (O/Li = 16:1). (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2139869] All rights reserved.