This work presents a detailed study of the electrochemical performance of polyphosphazene based electrolyte membranes consisting of a linear polymer with –(NPR2)– units, grafted with ethylene oxide side chains of the type R = –(OCH2CH2)3OCH3 and containing LiTFSI and LiBOB as dissolved lithium salts. The average molecular weight was 105 g mol−1. Mechanical stability was achieved by UV induced in-situ cross-linking of the thin polymer electrolyte films. Favorable properties of this type of polymer electrolytes are the good thermal and electrochemical stability of the electrolyte membranes, the broad electrochemical stability window ranging between 0 V and 4.7 V versus the Li/Li+ reference and a very good interface stability at lithium metal electrodes where a stable SEI was formed during initial contact. Total ionic conductivities up to 10−4 S cm−1 were measured at 30 °C. The transference numbers of lithium ions at 50 °C ranged between 0.06 and 0.07 and hence are lower by a factor of about three as compared to other typical polymer electrolytes. Nevertheless, the partial lithium ion conductivity estimated from the product of total conductivity and lithium ion transference number is as high or slightly higher compared to PEO based polymer electrolytes.