A soft matter solid electrolyte was prepared by polymerizing a monomer trimethylolpropane trimethylacrylate (TMPTMA) using in situ thermal polymerization into a non-ionic plastic crystal electrolyte, which is consisted of 5mol% lithium bis-trifluoromethanesulfonimide dissolved in succinonitrile (SN). X-ray diffraction, differential scanning calorimetry and conductivity measurements are used to investigate the structural and electrochemical performance of the polymer electrolyte films. It exhibits high ionic conductivities, wide electrochemical window and excellent mechanical strength. The solid electrolyte with 7.5wt% TMPTMA displays a high initial discharge capacity of 160.3mAhg−1 at 0.1C when combined with a LiFePO4 cathode and excellent capacity retention. With their beneficial properties, the polymer electrolytes are considered to have significant potential applications for lithium ion batteries.