In Situ TEM Studies of Sodium Polysulfides Electrochemistry in High Temperature Na-S Nanobatteries

Understanding polysulfide electrochemistry in high temperature sodium-sulfur (HT-Na-S) batteries is crucial for their practical applications. Currently the discharge capacity of commercial HT-Na-S battery achieves only one third of its theoretical capacity due to polysulfides formation, understanding of which is limited due to technical difficulty in direct imaging polysulfides. Herein, in situ transmission electron microscopy implemented with a microelectromechanical systems (MEMS) heating device is used to investigate the electrochemical reactions of HT-Na-S batteries. The formation and evolution of transient polysulfides during cycling are revealed in real-time. Upon discharge, sulfur transforms to long-chain polysulfides, short-chain polysulfides, and finally Na₂ S or its mixture with polysulfides, and the process is reversible during charge at high temperatures. Surprisingly, by introducing nanovoids into the sulfur cathode to buffer the large volume change thus preserving the integrity of the electronic/ionic pathways and reducing the diffusion distance of Na⁺ ions, the sulfur cathode is fully discharged to Na₂ S rather than the conventionally observed Na₂ S₂ at 300 °C. Moreover, the electrochemical reaction is swift and highly reversible. The in situ studies provide not only new understanding to the polysulfide electrochemistry, but also critical strategies to boost the capacity and cyclability of HT-Na-S batteries for large-scale energy storage applications.