All-solid-state Li-S batteries with fast solid-solid sulfur reaction

With promises for high specific energy, high safety and low cost, the all-solid-state lithium-sulfur battery (ASSLSB) is ideal for next-generation energy storage^1-5. However, the poor rate performance and short cycle life caused by the sluggish solid-solid sulfur redox reaction (SSSRR) at the three-phase boundaries remain to be solved. Here we demonstrate a fast SSSRR enabled by lithium thioborophosphate iodide (LBPSI) glass-phase solid electrolytes (GSEs). On the basis of the reversible redox between I^- and I₂/I₃^-, the solid electrolyte (SE)-as well as serving as a superionic conductor-functions as a surficial redox mediator that facilitates the sluggish reactions at the solid-solid two-phase boundaries, thereby substantially increasing the density of active sites. Through this mechanism, the ASSLSB exhibits ultrafast charging capability, showing a high specific capacity of 1,497 mAh g^-1_sulfur on charging at 2C (30 °C), while still maintaining 784 mAh g^-1_sulfur at 20C. Notably, a specific capacity of 432 mAh g^-1_sulfur is achieved on charging at an extreme rate of 150C at 60 °C. Furthermore, the cell demonstrates superior cycling stability over 25,000 cycles with 80.2% capacity retention at 5C (25 °C). We expect that our work on redox-mediated SSSRR will pave the way for developing advanced ASSLSBs that are high energy and safe.