Diurea cross-linked poly(oxyethylene)/siloxane ormolytes for lithium batteries
Journal article, 2005
Poly(oxyethylene) (POE)/siloxane hybrids (di-ureasils) doped with a wide concentration range of lithium triflate (LiCF3SO3) were investigated. The host matrix of these materials [d-U(2000)] is a sol-gel-derived siliceous framework to which POE chains with about 40.3 repeat units are bonded through urea linkages. Xerogels with ∞ ≥ n ≥ 5 (n is the molar ratio OCH2CH2/Li+) were obtained as amorphous monoliths thermally stable up to at least 340°C. A crystalline POE/LiCF3SO3 complex was detected spectroscopically in samples with n ≤ 10. Below 90°C the ormolyte with n = 20 exhibits the highest conductivity (5.8 × 10-6 n-l cm-1 at 26°C). The redox stability domain of this material spans 4.1 V. Although Fourier transform infrared spectroscopy data suggest that the Li+ ions are complexed by the POE ether oxygen atoms at n ≤10, this threshold composition is probably located at slightly lower salt content "Free" triflate ions and weakly coordinated anions, present in all the samples examined, must be the main charge carriers of the d-U(2000) 20LiCF3SO3 xerogel. Ion pairs (Li +CF3SO3-) or negatively charged triplets ([Li(CF3SO3)2]-) are formed at n ≤ 40. At n ≤ 5 positively charged triplets [Li2(CF 3SO3)]+ also appear. Divalent positively charged multiplets [Li3(CF3SO3)]2+ occur at n = 1.