| Three types of composite solid polymer electrolytes based on polyethylene oxide (PEO) have been developed to enhance lithium ion conductivities at ambient temperatures: (i) composites of PEO(600K) and a ceramic filler, silanated silica; and (ii) blends of either PEO(600K) or methylcellulose, and POSS-PEO(n=4) 8.; Plain fumed silica was found to be an inert filler in PEO matrix, and did not improve the conductivity at any temperature. Silanated silica, surface modified fumed silica with an oligomeric PEO-silane, was found to be more compatible with the PEO matrix, decreased crystallinity of the matrix, and therefore moderately enhanced the conductivity at ambient temperature.; The temperature dependent conductivities of POSS-PEO(n)8, as a function of salt concentration for LiClO4 and LiN(CF 3CF2SO2)2, and the calorimetric properties of the electrolytes have been investigated. Glass transition temperatures, Tg, increased with increasing salt concentration and PEO chain length (n). The effect of anion on conductivity was also evaluated. At high temperatures, the less associative salts had greater conductivities. At low temperatures, the salts with lower Tg had higher conductivities.; Solid polymer electrolytes prepared with LiClO4 and blends of POSS-PEO(n=4) 8 and PEO(600K) microphase separated into two amorphous phases (O/Li = 8/1 and 12/1), or a crystalline phase and two amorphous phases (O/Li = 16/1), and showed improved low temperature conductivities compared with PEO(600K). For O/Li = 12/1 and 16/1, improvements in conductivity were due to suppression of crystallization. However, the tendency of PEO(600K) to crystallize limited the amount of POSS-PEO(n=4)8 that could be incorporated into the blends. For O/Li = 8/1, only if the POSS-rich phase was the continuous phase, was there appreciable conductivity improvement. The highest RT conductivity, 8 x 10-6 S/cm, was obtained for a 60% POSS-PEO(n=4)8/40%PEO(600K)/LiClO4 (O/Li = 12/1) blend.; POSS-PEO(n=4)8 and methyl cellulose (MC) formed microphase separated blends, and LiClO4 partitioned in the POSS-PEO(n=4) 8 domain. Good mechanical properties resulted from the crystalline MC matrix, and the significantly enhanced low temperature conductivities were due to the continuous POSS-PEO(n=4)8-rich phase. A typical composition was the blend of 80% POSS-PEO(n=4)8, 20% MC, and LiClO4 with O/Li = 16/1, which provided attractive conductivity, ∼ 10 -5 S/cm, at RT. |
