Nanocomposite gel electrolytes based on silica particles dispersed in lithium bis (perfluoroethylsulfonyl)imide (LiBETI) salt + mixed-carbonate solvent are examined as an electrolyte system for a lithium-ion battery. Gel behavior is observed with both hydrophilic (A200 ) and hydrophobic (R805) fumed silicas. The silica nanoparticles affect a small decrease in conductivity but increase mechanical strength significantly (elastic modulus similar to 10(5) Pa). Chronoamperometry and linear sweep voltammetry results show that an Al current collector is stable in 1 M LiBETI carbonates up to similar to 5 V. Cycling Li (Ni)/electrolyte/Li cells shows that silica nanoparticles improve the coulombic efficiency and interfacial stability in the order: 10% R805 LiBETI gel > LiBETI liquid > LiPF6 liquid. Cycling Li/ LiFePO4 cells shows that both liquid and 10% R805 gel electrolytes provide good capacity and cycle performance, but the average charge/ discharge voltages for the latter are more stable. Both Li/ LiMn2O4 and Li/graphite cells have less capacity fade using LiBETI than LiPF6 electrolyte. The gel electrolyte provides better cycle performance than its liquid counterpart because of its increased interfacial stability due to improved rheology and ability to scavenge residual moisture. Silica- based LiBETI carbonate nanocomposite gel electrolytes appear to be a promising candidate for lithium- ion batteries. (c) 2007 The Electrochemical Society.