Composite polymer electrolytes using surface-modified fumed silicas: conductivity and rheology

We report results from our studies on composite polymer electrolytes based on novel surface-modified fumed silicas. The electrolytes were prepared by dispersing fumed silica in a matrix formed by methyl-capped polyethylene glycol and lithium salt. Silicas with widely different surface chemistries were synthesized in order to study the effects of surface modification, with the attached surface groups ranging from non-polar alkyl moieties (C-1 or C-8) to polar polyethylene oxide (PEO) oligomers (MW similar to 200). We find, rather surprisingly, that the conductivity is independent of the type of surface group present on the silica. Moreover, the conductivity decreases only slightly on addition of fumed silica, even at high weight fraction of solids. In contrast, the rheological properties of the composites are strongly affected by both the silica surface chemistry and weight fraction. Dynamic rheology measurements reveal that fumed silicas with silanol and octyl coverage both flocculate into gels (networks). The resulting materials are mechanically stable, with the elastic modulus of the gel being strongly dependent upon weight fraction of solids. The PEG-modified silica, in contrast, gives rise to a low-viscosity suspension where the silica units exist as distinct, non-interacting species. The findings of this study have significant implications for future work on composite electrolytes, in that we can tailor the mechanical properties of the system without affecting the electrochemical behavior. (C) 1998 Elsevier Science B.V. All rights reserved.