We report on making a wider operating temperature solid polymer electrolyte (SPE) consisting of polysiloxane, poly(vinylidene fluoride) and lithium bis(trifluoro-methanesulfonyl)imide (LiTFSI). When 30 wt% LiTFSI was added to the electrolyte, the ionic conductivities of 7.9 × 10−5 S cm−1 at 25 °C and 8.7 × 10−4 S cm−1 at 80 °C were measured. However, with 20 wt% LiTFSI the electrolyte has better mechanical properties and a better compatibility with lithium electrodes compared to other LiTFSI concentrations. According to TGA-DSC thermal analysis, the SPE shows a relatively high decomposition temperature of 275 °C. The electrochemical windows up to 5.17 V at 25 °C and 5.05 V at 60 °C were measured, which are higher than currently used liquid organic electrolytes. Batteries employing LiFePO4 as the cathode, lithium foil as the anode and the SPE thin film as the electrolyte were assembled. At 25 °C, the initial discharge capacity at 0.2 C is 144 mA h g−1 and 98% of the capacity is retained at the 100th cycle; the discharge capacity is 115 mA h g−1 after 100 cycles at 1 C and there is no significant decay after 200 cycles. Furthermore, the battery reaches the discharge capacity of 144 mA h g−1 at the 100th charge–discharge cycle at 60 °C and 1 C. Such results indicate the commercial prospect of the electrolyte in all-solid-state lithium ion batteries. The use of this electrolyte will lower the safety risk and extend the operation temperature range of electric devices.