In Situ TEM Observations of Discharging/Charging of Solid-State Lithium-Sulfur Batteries at High Temperatures

Understanding the structural evolution of Li₂ S upon operation of lithium-sulfur (Li-S) batteries is inadequate and a complete decomposition of Li₂ S during charge is difficult. Whether it is the low electronic conductivity or the low ionic conductivity of Li₂ S that inhibits its decomposition is under debate. Furthermore, the decomposition pathway of Li₂ S is also unclear. Herein, an in situ transmission electron microscopy (TEM) technique implemented with a microelectromechanical systems (MEMS) heating device is used to study the precipitation and decomposition of Li₂ S at high temperatures. It is revealed that Li₂ S transformed from an amorphous/nanocrystalline to polycrystalline state with proceeding of the electrochemical lithiation at room temperature (RT), and the precipitation of Li₂ S is more complete at elevated temperatures than at RT. Moreover, the decomposition of Li₂ S that is difficult to achieve at RT becomes facile with increased Li⁺ ion conduction at high temperatures. These results indicate that Li⁺ ion diffusion in Li₂ S dominates its reversibility in the solid-state Li-S batteries. This work not only demonstrates the powerful capabilities of combining in situ TEM with a MEMS heating device to explore the basic science in energy storage materials at high temperatures but also introduces the factor of temperature to boost battery performance.