Cryo-EM Revealing the Origin of Excessive Capacity of the Se Cathode in Sulfide-Based All-Solid-State Li–Se Batteries

Selenium (Se), whose electronic conductivity is nearly 25 orders higher than that of sulfur (S) and whose theoretical volumetric capacity is 3254 mAh cm–3, is considered as a potential alternative to S to overcome the poor electronic conductivity issue of the S cathode in the lithium (Li)-S battery. However, the study of the Li–Se battery, particularly a Li–Se all-solid-state battery (ASSB), is still in its infancy. Herein, we report the performance of Li–Se ASSBs at both room temperature (RT) and high temperature (HT, 50 °C), using a Li10Si0.3PS6.9Cl1.8 (LSPSCl) solid-state electrolyte and Li-In anode. With a Se loading of 7.6 mg cm–2, the Li–Se battery displayed a record high reversible capacity of 6.8 mAh cm–2 after 50 cycles at HT, which exceeds the theoretical areal capacity of 5.2 mAh cm–2 for Se. Moreover, the RT Li–Se ASSB delivered an initial areal capacity of about 2 mAh cm–2 at a current density of 1 A g–1 for 1200 cycles with a capacity retention of 67%. Cryo-electron microscopy revealed that the excessive capacity of Se at HT can be attributed to the formation of a previously unknown S5Se4 phase during charging, which participated reversibly in a subsequent redox reaction. The formation of the S5Se4 phase originated from the reaction of Se with S, which was generated by the decomposition of LSPSCl at HT. These results unlock the electrochemistry of a Li–Se ASSB, suggesting that a Li–Se ASSB is a viable alternative to a Li–S battery for energy storage applications.