Polymer electrolytes based on biogenic and commercially available substrates have been crosslinked thermally to improve their mechanical properties with only negligible effects on their conductivity. The new materials consist of oligo(ethylene oxide) side chains grafted via ether linkages to a cellulose backbone [poly(ethylene oxide)-2-hydroxypropylcellulose (PEO-HPC)]. PEO-HPC with a degree of side-chain substitution up to 3.0, as determined by NMR, has been blended with lithium bis(trifluoromethylsulfone)imide in molar ratios of 0.02 <= [Li]/[0] <= 0.2. The effect of the salt concentration on the thermal behavior and ionic conductivity has been investigated by impedance spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The uncrosslinked materials are viscous liquids. The PEO-HPC/lithium salt blends have been thermally crosslinked with the application of urethane chemistry. The crosslinking reactions have been monitored by dynamic mechanical analysis. Crosslinking renders the liquid materials into mechanically stable films with a storage modulus of 105 Pa at 100 degrees C. Surprisingly, their conductivities remain rather unaffected and reach at best 2.5 x 10(-4) S/cm. The applicability of the PEO-HPC/ lithium salt blends as separators in lithium-ion batteries has been demonstrated by the construction of a storage cell that is rechargeable. A freshly constructed cell reveals a voltage of about 3.2 V and can be recharged with a constant direct-current voltage up to the open circuit voltage of 3.65. (C) 2007 Wiley Periodicals, Inc.