Tailoring Breathing Behaviour of Solid Electrolyte Interphases (SEIs) Unraveled by Cryo-TEM

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 we utilize the state-of-the-art cryogenic transmission electron microscopy (cryo-TEM) and spectroscopy to probe the SEI breathing behaviour induced by discharging/charging on the conversion-type anode made of Fe2O3 quasi-cubes. The incorporation of the identical-location strategy allows us to track the evolution of same SEIs at different charge states, which unequivocally unravels SEI breathing featured by swelling (contracting) upon lithiation (de-lithiation), and the associated compositional change. Bare Fe2O3 anode develops an unstable SEI layer due to the intermixing with the lithiation product Li2O, which exhibits a large thickness variation upon breathing as well as excessive growth, causing substantial capacity fading within 100 cycles. 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 behaviour, we apply N-doped carbon coating on Fe2O3 (Fe2O3@CN), which can effectively separate the lithiation product from SEI. A thinner and chemically more stable SEI layer develops on Fe2O3@CN, resulting in remarkably enhanced cycling stability compared to bare Fe2O3. Our work demonstrates the importance of understanding and optimizing the dynamic behaviour of SEIs to achieve better battery performance.