In situ and operando infrared spectroscopy of battery systems: Progress and opportunities

In situ and operando infrared spectroscopies are powerful techniques to support the design of novel materials for batteries and the development of new battery systems. These techniques can support the study of batteries by identifying the formation of new species and monitoring electrochemical energy stability. However, few works have employed these techniques, which can be used to investigate various materials, including systems beyond lithium-ion technology, in the research of batteries. Therefore, this review presents a comprehensive overview focusing on the main contributions of in situ and operando infrared spectroscopy for lithium-ion batteries (LIBs) and other battery systems. These techniques can successfully identify the formation of species during the electrolyte reduction, electrode degradation, and the formation of the solid-electrolyte interphase (SEI) layer. From these outcomes, it is possible to conclude that this characterization approach should be employed as a protocol to overcome remaining issues in batteries, consequently supporting battery research. This review aims to be a guide on how infrared spectroscopy can contribute to monitoring battery systems and to lead researchers interested in applying this technique.