A High‐Energy and Safe Lithium Battery Enabled by Solid‐State Redox Chemistry in a Fireproof Gel Electrolyte

Abstract Recent years have witnessed thriving efforts in pursuing high‐energy batteries at an unaffordable cost of safety. Herein, a high‐energy and safe quasi‐solid‐state lithium battery is proposed by solid‐state redox chemistry of polymer‐based molecular Li₂S cathode in a fireproof gel electrolyte. This chemistry fully eliminates not only the negative effect of extremely reactive Li metal and oxygen species on cell safety but also the damage of electrode reversibility by soluble redox intermediates. The molecular Li₂S cathode exhibits an exceptional lifetime of 2000 cycles, 100% Coulombic efficiency, high capacity of 830 mA h g⁻¹ with ultralow capacity loss of 0.005–0.01% per cycle and superior rate capability up to 10 C. Meanwhile, it shows high stability in the carbonate‐involving electrolyte for maximizing the compatibility with carbonate‐efficient Si anode. The optimized cell chemistry exerts high energy over 750 W h kg⁻¹ for 500 cycles with fast rate response, high‐temperature adaptability, and no self‐discharge. A fire‐retardant composite gel electrolyte is developed to further strengthen the intrinsic safe redox between the Li₂S cathode and the Si anode, which secures remarkable safety against extreme abuse of overheating, short circuits, and mechanical damage in air/water or even when on fire.