Solid Electrolytes for Li-S Batteries: Solid Solutions of Poly(ethylene oxide) with LiPON- and LiSiPON-Based Polymers

We report here efforts to synthesize free-standing, dry polymer electrolytes that exhibit superior ionic conductivities at ambient for Li-S batteries. Co-dissolution of poly(ethylene oxide) (PEO) (M_n 900k) with LiPON and LiSiPON polymer systems at a ratio of approximately 3:2 followed by casting provides transparent, solid-solution films 25-50 μm thick, lowering PEO crystallinity, and providing measured impedance values of 0.1-2.8 × 10^-3 S/cm at ambient. These values are much higher than simple PEO/Li⁺ salt systems. These solid-solution polymer electrolytes (PEs) are (1) thermally stable to 100 °C; (2) offer activation energies of 0.2-0.5 eV; (3) suppress dendrite formation; and (4) enable the use of lithium anodes at current densities as high as 3.5 mAh/cm². Galvanostatic cycling of SPAN/PEs/Li cell (SPAN = sulfurized, carbonized polyacrylonitrile) shows discharge capacities of 1000 mAh/g_sulfur at 0.25C and 800 mAh/g_sulfur at 1C with high coulumbic efficiency over 100 cycles.