A new series of hyperbranched polymer electrolytes for use as membranes for lithium batteries are described. Electrolytes were made by polymerization between cyanuric chloride and diamino-terminated poly(ethylene oxide)s, followed by cross-linking via a sol-gel process. The cross-linking improves the mechanical integrity of the polymer films over other PEO-based electrolytes previously described without deleteriously affecting the ionic conductivity. Thermal analysis and lithium conductivity of free-standing polymer films were studied. The effects of several variables on conductivity were investigated, such as length of backbone PEO chain, length of branching PEO chain, extent of branching, extent of cross-linking, salt content, and salt counterion. Polymer films with the highest percentage of PEO were found to be the most conductive, with a maximum lithium conductivity of 3.9 x 10(-5) S/cm at 25 degrees C, but more mechanically robust films with as much as 50% cross-linker were only slightly lower in conductivity (3.7 x 10(-5) S/cm) at 25 degrees C.