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Sweeping Potential Regulated Structural and Chemical Evolution of Solid-Electrolyte Interphase on Cu and Li as Revealed by Cryogenic Transmission Electron Microscopy

Lithium (Li) metal is one of the most attractive anode materials for rechargeable batteries due to its low electrochemical potential (-3.040 V vs standard hydrogen electrode), ultrahigh theoretical specific capacity (3860 mAh g −1 ), and low density (0.534 g cm −3 ) compared with other present anode materials (1). However, before the Li anode can become a viable technology, formidable challenges need to be overcome. One of the main obstacles restraining the improvement of Li based battery performance is the electrode/electrolyte interphase, which is the key to understand battery electrochemistry, as it is where the electron and Li + ion combine and then get stored in the Li metal (2). However, the structural and chemical evolution of this solid-electrolyte interphase (SEI) with respect to electrochemical operating condition remains barely established. Besides, the reactivity and sensitivity of SEI make it difficult to elucidate the nanostructure and chemistry of SEI. To overcome this challenge, the cryogenic transmission electron microscopy (cryo-TEM) technique widely used in structural biology has been used in battery studies recently, which could not only provide resolution to the sub-angstrom level but also give very high spatial resolution in chemical analysis simultaneously (3, 4). Herein, we develop a unique method for imaging the evolution of SEI formed on the copper (Cu) foil under sweeping electrochemical potential as shown in Figure 1. Then we combine cryo-TEM (high spatial resolution) and energy-dispersive X-ray spectroscopy (EDS) (high chemical sensitivity) techniques to characterize the detailed structure and chemical evolutions of electrochemically formed SEI layers in the vinylene carbonate (VC)-free and VC-containing electrolytes,