Advanced Characterizations of Solid Electrolyte Interphases in Lithium-Ion Batteries

Solid electrolyte interphases (SEIs) in lithium-ion batteries (LIBs) are ionically conducting but electronically insulating layers on electrode/electrolyte interfaces that form through the decomposition of electrolytes. And although SEIs can protect electrodes from the co-intercalation of solvent molecules and prevent the continued decomposition of electrolytes, their formation can consume active lithium and electrolytes and build up impedance for ion conduction. Therefore, the control of SEI structures and properties to allow for stability and ionic conductivity has become a critical but highly challenging task in battery designs. However, several factors contribute to the difficulty in SEI research. First, the chemical and electrochemical reactions leading to SEI formation are immensely complex and heavily influenced by numerous factors including electrolyte solvents, lithium salts, additives, electrode materials and charge/discharge conditions. Second, the chemical nature of film-formation products such as SEI constituents and their distribution and arrangement in the SEI are complex. Finally, SEIs are in situ formed at the electrode/electrolyte interface in assembled batteries, making the direct observation of SEIs difficult. To address these challenges, the development of advanced characterization techniques is key in the fundamental understanding of SEIs in LIBs. Based on this, this review will provide an overview of the progress in SEI characterization, including methods to investigate electrochemical performance, surface morphology, chemical composition, and structure and mechanical properties, with state-of-the-art characterization techniques developed in recent years being emphasized. And overall, the scientific insights obtained by using these advanced methods will help researchers to better understand electrode/electrolyte interfaces toward the development of high-performance secondary batteries.