Poly[oxymethylene-oligo(oxyethylene)] (PEM) cross-linked by UV radiation will solvate LiClO4 and demonstrates ionic conductivities similar to those with linear PEM, as well as improved dimensional stability. The PEM(24)LiClO(4)/Li interface has an impedance of 1000 Omega cm(2); and demonstrates behavior consistent with the formation of a solid electrolyte interface (SEI). Grass-linked films of PEM do not form a stable interface with lithium; however, upon treatment with methyl iodide R(i) stabilizes at 2000 Omega cm(2).