Operando Time-of-Flight Secondary Ion Mass Spectrometry Reveals Iodine-Mediated Redox Chemistry in Solid-State Li–S Batteries

Understanding dynamic reaction pathways in all-solid-state batteries remains challenging because electrochemical processes occur at buried solid–solid interfaces and transient intermediates are difficult to detect with conventional analytical techniques. While operando techniques typically probe structural or macroscopic changes, direct detection of reactive species during operation remains experimentally demanding. Here, we introduce a dynamic operando time-of-flight secondary ion mass spectrometry platform that enables continuous chemical analysis of processes in a solid-state battery under electrochemical bias. Using this approach, we directly observe a pronounced increase of the I2– signal during charging of a Li–S composite cathode containing LiI, indicating the formation of elemental iodine. Concurrent changes in the sulfur-containing ion signals suggest the involvement of iodine species in the ongoing redox processes. These results reveal transient, low-concentration species at buried interfaces undetectable by other operando techniques, offering a pathway for identifying reactive intermediates in conversion-type solid-state batteries and related electrochemical systems.