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

Understanding the structural evolution of Li2S upon operation of lithium-sulfur (Li-S) batteries is inadequate and a complete decomposition of Li2S during charge is difficult. Whether it is the low electronic conductivity or the low ionic conductivity of Li2S that inhibits its decomposition is under debate. Furthermore, the decomposition pathway of Li2S 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 Li2S at high temperatures. It is revealed that Li2S transformed from an amorphous/nanocrystalline to polycrystalline state with proceeding of the electrochemical lithiation at room temperature (RT), and the precipitation of Li2S is more complete at elevated temperatures than at RT. Moreover, the decomposition of Li2S 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 Li2S 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.