| Summary: | With a higher theoretical specific capacity (1675 mAh g<sup>−1</sup>) and energy density (2600 Wh kg<sup>−1</sup>), the lithium-sulfur (Li-S) battery is considered as a promising candidate for a next-generation energy storage device. However, the shuttle effect of polysulfides as well as the large interfacial impedance between brittle solid electrolyte and electrodes lead to the capacity of the Li-S battery decaying rapidly, which limits the practical commercial applications of the Li-S battery. Herein, we reported a facile in situ ultraviolet (UV) curing method to prepare a flexible quasi-solid-state composite electrolyte (QSSCE) of poly(propylene glycol)-co-pentaerythritol triacrylate/Li<sub>1.5</sub>Al<sub>0.5</sub>Ge<sub>1.5</sub>(PO<sub>4</sub>)<sub>3</sub> (PPG-co-PETA/LAGP). By combining the high Li-ion conductivity and mechanical strength of inorganic NASICON-structure LAGP and good flexibility of the crosslinked PPG-co-PETA with nanopore structure, the flexible QSSCE with 66.85 wt% LAGP exhibited high Li-ion conductivity of 5.95 × 10<sup>−3</sup> S cm<sup>−1</sup> at 25 °C, Li-ion transference number of 0.83 and wide electrochemical window of ~5.0 V (vs. Li/Li<sup>+</sup>). In addition, the application of QSSCE in the Li-S battery could suppress the shuttle effect of polysulfides effectively, thus the Li-S battery possessed the excellent electrochemical cyclic performance, showing the first-cycle discharge-specific capacity of 1508.1 mAh g<sup>−1</sup>, the capacity retention of 73.6% after 200 cycles with 0.25 C at 25 °C and good rate performance.
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