Copolymers of methacrylonitrile are reported. Surprisingly, their curing through urethane cross-links acts as an internal plasticization. This cross-linking prevents any dissolution or leakage, up to 90degreesC, when the networks are swollen by liquid organic electrolytes. These provide high conductivities, and the increase in resistivity with respect to pure liquid electrolytes remains less than or equal to2. The electrochemical investigation shows a clear improvement of stability in reduction as compared to polyacrylonitrile. The first evaluations of lithium insertion in graphite are encouraging.