Nanostructural and Electrochemical Evolution of the Solid-Electrolyte Interphase on CuO Nanowires Revealed by Cryogenic-Electron Microscopy and Impedance Spectroscopy

Battery performance is critically dependent on the nanostructure and electrochemical properties of the solid-electrolyte interphase (SEI)-a passivation film that exists on most lithium-battery anodes. However, knowledge of how the SEI nanostructure forms and its impact on ionic transport remains limited due to its sensitivity to transmission electron microscopy and difficulty in accurately probing the SEI impedance. Here, we track the voltage-dependent, stepwise evolution of the nanostructure and impedance of the SEI on CuO nanowires using cryogenic-electron microscopy (cryo-EM) and electrochemical impedance spectroscopy (EIS). In carbonate electrolyte, the SEI forms at 1.0 V vs Li/Li+ as a 3 nm thick amorphous SEI and grows to 4 nm at 0.5 V; as the potential approaches 0.0 V vs Li/Li+, the SEI on the CuO nanowires forms an 8 nm thick inverted multilayered nanostructure in ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte with 10 vol % fluoroethylene carbonate (FEC) and a mosaic nanostructure in EC/DEC electrolyte. Upon Li deposition, the total SEI thickness grows to 16 nm, and significant growth of the inner amorphous layer takes place in the inverted multilayered nanostructure, indicating that electrolyte permeates the SEI. Using a refined EIS methodology, we isolate the SEI impedance on Cu and find that the SEI nanostructure directly correlates to macroscopic Li-ion transport through the SEI. The inverted layered nanostructure decreases the interfacial impedance upon formation, whereas the mosaic nanostructure continually increases the interfacial impedance during growth. These structural and electrochemical findings illustrate a more complete portrait of SEI formation and guide further improvements in engineered SEI.