Revisiting Anode Fast-Charging Capability with Solid Electrolyte Interface Using Cryogenic Transmission Electron Microscopy

Abstract

The Li+ ion diffusion through the solid electrolyte interphase (SEI) has been widely considered as one of the limiting steps for the Li fast (de)intercalation process. However, a comprehensive understanding of the kinetic limitation of SEI on anode fast-charging remains elusive. Using H-phase (monoclinic) Nb2O5 (H-Nb2O5) as the anode material, we comprehensively studied the fast charging behaviors on both “inorganic SEI free” and “inorganic-rich SEI” anodes with cryogenic transmission electron microscopy (cryo-TEM) and X-ray photoelectron spectroscopy (XPS). The results reveal that there is a negligible difference on the electrochemical performance including fast-charging capability and cycling stability between these H-Nb2O5 anodes in spite of significant discrepancy of SEI structure (e.g. thickness and chemical component). Further, it was observed using cryo-TEM that the discrete decoration of the individual inorganic particles (e.g. Li2O) and amorphous LiNxOy species in the direct SEI does not constitute a dense solid layer. From this study, we conclude a pore diffusion mechanism is dominated across the whole direct SEI layer with those porous organic components, which is extremely faster than the lithium ion knock-off diffusion in the inner inorganic components. Thus, the SEI structures have much less influence on the limitation of the lithium ion transport kinetics than those as commonly accepted if an inner dense inorganic layer is not formed. Our results provide a refreshed understanding on the fundamentals of the lithium ion transport in SEI, and will guide future design of battery materials for fast-charging.