We designed, synthesized and characterized several novel hybrid inorganic/organic nanocomposite electrolytes that consist of poly(ethylene oxide) (PEO) based polymer grafted from silica nanoparticles. Poly(ethylene glycol)methyl ether methacrylate (PEGMA) was tailored on the silica surface through atom transfer radical polymerization (ATRP). A series of silica–polymers were synthesized with different lengths of PEO side chains. Electrolytes were prepared from the functionalized particles and low-molecular weight polyethylene glycol dimethyl ether (PEGDME) with the addition of LiI. Upon the introduction of particles, electrolytes became viscous and gel-like. With the increase of PEO side chains, the viscosity of the electrolytes increased dramatically, among which, silica–poly(PEGMA-1100) became solid-state. The room temperature conductivities of the hybrid silica–polymer electrolytes are in the range of 6 × 10⁻⁵ to 1.2 × 10⁻⁴ S cm⁻¹. Silica–poly(PEGMA-475) and silica–poly(PEGMA-1100), with higher viscosity, exhibited better ionic conductivity. Surface-initiated copolymerization was also conducted to optimize the electrochemical performance of polymer coated silica nanoparticles.