A wider temperature range polymer electrolyte for all-solid-state lithium ion batteries

Abstract

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.