This study investigated the effects of an ionic liquid, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), as a doping salt, on the properties of resulting solid biopolymer electrolytes (SPEs) based on all-cellulose composites. The conductivity of the resulting SPEs increased with the [Bmim]Cl loading. Both incubation time and chemical loading affected the tensile properties of the SPEs. The SPEs prepared under the optimum processing conditions exhibited a tensile strength and conductivity of 18.2 MPa and 1.28 x 10(-3) S cm(-1), respectively, when tested at room temperature. FTIR and XPS spectra showed that the concentration of [Bmim]Cl in the SPEs was greater at the surface than at the interior. Treatment with [Bmim]Cl resulted in a decrease in the crystallinity and thermal stability of the SPEs as evidenced by XRD and TGA analyses. This study demonstrates that treatment of cellulosic materials with the ionic liquid [Bmim]Cl can create biomass-based SPEs with high conductivity, improved mechanical properties, good thermal stability and easy processability.