A modified polyphosphazene was synthesized using a mixed substitution at phosphorus consisting of 2-(2-methoxyethoxy)ethoxy side groups and anionic trifluoroborate groups. The primary goal was to increase the low lithium ion conductivities of the conventional lithium salt containing poly[2-(2-methoxyethoxy)ethoxy-phosphazene] (MEEP) by the immobilized anionic groups. As in previous studies, the mechanical stability was stabilized by UV induced radiation cross linking. By variation of the molar ratio between different side groups, mechanical and electrochemical properties are controllable. The polymer demonstrated large electrochemical stability windows ranging between 0 and 4.5 V versus the Li/Li+ reference. Total and lithium conductivities of 3.6 × 10−4 S·cm−1 and 1.8 × 10−5 S·cm−1 at 60 °C were revealed for the modified MEEP. When observed in special visualization cells, dendrite formation onset time and short-circuit time were determined as 21 h and 90 h, respectively, under constant current polarization (16 h and 65 h for MEEP, both with 15 wt % LiBOB), which hints to a more stable Li/polymer interface compared to normal MEEP. The enhanced dendrite suppression ability can be explained by the formation of a more conductive solid electrolyte interphase (SEI) and the existence of F-contained SEI components (such as LiF). With the addition of ethylene carbonate–dimethyl carbonate (EC/DMC) to form MEE-co-OBF3P gel polymer, both total and lithium conductivity were enhanced remarkably, and the lithium transference numbers reached reasonable values (σtotal = 1.05 mS·cm−1, σLi+ = 0.22 mS·cm−1, tLi+ = 0.18 at 60 °C).