Tailoring Breathing Behavior of Solid Electrolyte Interphases Unraveled by Cryogenic Transmission Electron Microscopy

Abstract The cycling stability of batteries is closely related to the dynamic evolution of solid electrolyte interphases (SEIs) in response to the discharging/charging processes. Here, the state‐of‐the‐art cryogenic transmission electron microscopy (cryo‐TEM) and spectroscopy are utilized to probe the SEI breathing behavior induced by discharging/charging on the conversion‐type anode made of Fe 2 O 3 quasi‐cubes. The incorporation of the identical‐location strategy allows the tracking of the evolution of the same SEIs at different charge states. SEI breathing is shown to involve swelling (contracting) upon lithiation (de‐lithiation) driven by the reversible compositional change. Bare Fe 2 O 3 anodes develop an unstable SEI layer due to the intermixing with the lithiation product Li 2 O, which exhibits a large thickness variation upon breathing as well as excessive growth. A transition from organic to inorganic‐type SEI is also identified upon cycling, which gives rise to significantly increased SEI resistance. To tailor the SEI behavior, N‐doped carbon coating is applied on Fe 2 O 3 (Fe 2 O 3 @CN), which can effectively separate the lithiation product from SEI. A thinner and chemically more stable SEI layer develops on Fe 2 O 3 @CN, resulting in remarkably enhanced cycling stability compared to bare Fe 2 O 3 . This work demonstrates the importance of understanding and optimizing the dynamic behavior of SEIs to achieve better battery performance.