3D Structural Transition of the Electrodeposited and Electrochemically Dissolved Li Metal onto an Ultramicroelectrode

Abstract

The dendrite growth of Li metal is a very severe problem for a Li metal anode used in rechargeable batteries. In this study, an ultramicroelectrode (UME) was adopted to understand the morphological variation of electrodeposited Li metal. Scanning electron microscopy (SEM) coupled with windowless electron dispersive spectroscopy (EDS) and cryo-orthogonal focused ion beam system (FIB) was utilized to observe not only the surface morphology but also the three-dimensional (3D) structure of the electrodeposited and electrochemically dissolved Li metal. The electrodeposited Li metal from the LiPF6-based electrolyte forms a very uniform column structure. The fluoride-containing solid electrolyte interphase (SEI) layer plays an important role in determining the morphological variation of the dendrite. Furthermore, the fluoride-rich SEI layer shell remains on the UME without collapse after the first cycle. The ultramicroelectrode is a very useful research platform, particularly, for studying drastic dendrite growth and electrochemical dissolution on the Li metal anode.

Characterization of Electrodeposited Li Metal by Cryo-Scanning Transmission Electron Microscopy/Electron Energy Loss Spectroscopy

“…The Li metal and LiF mapping results are shown in Figure (h), where it can be seen that the LiF-rich SEI layer (∼20 nm thickness) uniformly covered the electrodeposited Li metal. These characteristics agree with previously reported SEM-EDS and 3D FIB-SEM images . We expect that LiF forms on the electrodeposited Li metal in the LiPF 6 -containing electrolyte owing to the evolution of HF upon hydrolysis of the LiPF 6 salt by trace amounts of H 2 O (as a contaminant).…”

“…The applied current density was 2 mA cm –2 . As shown in the figure, the electrodeposited Li metal on the UME exhibits a uniform columnar shape, as previously observed by SEM and 3D FIB-SEM imaging . The TEM images show that the electrodeposited Li metal tip possesses a semispherical shape, and the SEI layer is formed on the surface.…”

“…More specifically, the spectrum at point A of Figure (d) exhibited a leading edge at ∼55 eV in addition to three clear peaks, which is typical for Li metal, , as shown Figure (e). In our previous report, a windowless EDS detector detected spectra corresponding to Li metal, so a combination of these results confirm the electrodeposition of Li metal onto the UME. At point B of Figure (f), which is close to the surface, the EELS spectrum exhibits a large peak at ∼62 eV, followed by a long shoulder, as shown in Figure (g).…”

“…The 3D structure of electrodeposited Li metal was previously determined using an orthogonal FIB-SEM technique . In contrast, the UME technique employed herein is appropriate for TEM observations since all electrodes are transferred to the TEM without exposure to air (i.e., using the FIB-SEM lift-out technique).…”

Transparent Ultramicroelectrodes for Studying Interfacial Charge-Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

“…It operates in a variety of modes such as feedback, generation-collection, surface interrogation, hopping, redox competition, tunneling, and so forth to obtain mass transfer and surface reaction kinetic information of an electrode surface. The use of an UME tip with a size of 25 μm or less enables the quantitative electroanalysis with improved temporal , and spatial resolutions. − For instance, SECM was used to study the PEC activities of TiO 2 in the presence of redox mediators of Fe­(CN) 6 4– /Fe­(CN) 6 3– . The surface chemistry activities of photogenerated charge carriers at an unbiased TiO 2 substrate were revealed by illustrating the tip-to-substrate distance dependence of the tip current feedback.…”