Li-salt dissociation and polymer microstructural changes in composite polymer electrolytes

Li-S batteries (LSBs) are considered as one of the most attractive alternatives in the future of aviation industry, particularly for developing electric vertical takeoff and landing aircrafts. They possess high specific energy and capacity, high thermal stability, mechanical strength, and non-flammability. LSBs however are also prone to formation of Li-dendrites and long chain polysulfides (LiPS), resulting in poor cyclability and reduced long-term electrochemical performance. To solve these problems, we employ composite polymer electrolytes (CPEs), which contain ceramic fillers that not only provide mechanical robustness but can also enhance the poor ionic conductivity in polymers. These properties can be further improved with the right selection of Li-salts. LiFSI for example, can form a more robust SEI on the Li anode compared to LiTFSI, prolonging the cyclability of the battery. In contrast, LiTFSI outperforms LiFSI in terms of salt dissociation because of its higher anion basicity. This work seeks to understand the effect of mixing LiTFSI and LiFSI in PEO and PEO – ENR (epoxidized natural rubber) blends in the presence of varying amounts of Bi-doped LLZO (LLZBO) nanoparticles, as well as different EO:Li ratios. Adding LLZBO to PEO -LiFSI, decreases ionic conductivity of the CPE, whereas for the case of PEO - LiTFSI an increase in ionic conductivity is observed when adding LLZBO. Raman spectroscopy was utilized to derive the degree of dissociation of the salts and EIS was employed to measure ionic conductivity in CPEs as a function of temperature. We employ DSC, XRD and polarized light microscopy to investigate changes in the polymer morphology and microstructure brought about by the incorporation of filler nanoparticles and salts.