Ionic transport and interfacial stability of sulfonate-modified fumed silicas as nanocomposite electrolytes

Degussa A200 and R711 fumed silica surfaces were modified by attaching lithium sulfonate groups through alkyl or oligomer chains, respectively, in an attempt to form single-ion conducting fumed silicas: A200-lithium propanesulfonate (A200-LiPS), R711-poly(lithium vinylsulfonate) (R711-pLiVS), and R711-poly(lithium 2-acrylamido-2-methyl-1-propanesulfonate) (R711-pLiAMPS). Conductivity, lithium transference number, and Li/electrolyte interfacial stability measurements were conducted on nanocomposite electrolytes prepared by dispersing the conducting fumed silicas into solvents consisting of oligomeric polyethylene glycol dimethyl ether (PEGdm), polyethylene oxide (PEO), or PEGdm/PEO blends. Among the three sulfonate-modified fumed silicas, the highest conductivity was always obtained using R711-pLiAMPS. A maximum room-temperature conductivity of 4.5x10(-6) S cm(-1) was obtained at a surface Li+ concentration of 4.2 nm(-2) and a Li:O mole ratio of 1:100 (15.8 wt % filler). The maximum lithium transference number achieved for the same R711-pLiAMPS-based system is 0.78 at a surface Li+ concentration of 4.2 nm(-2) and a Li:O mole ration of 1:20 (48.5 wt % filler). Adding lithium salts to the solvent, such as lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(perfluoroethylenesulfonyl)imide (LiBETI), lithium bis(oxalato)borate (LiBOB), and lithium phosphate (Li3PO4), increases room-temperature conductivity and interfacial stability while maintaining relatively high lithium transference numbers. (c) 2005 The Electrochemical Society.