(Invited) In Situ Electrochemical STEM As a Platform for Interpreting Electrochemical Phenomena
Authors: Unocic Raymond R , Sacci Robert L , Sang Xiahan , Unocic Kinga A , Veith Gabriel M , Dudney Nancy J , More Karren L. 
Published: February 2020

In situ electrochemical STEM (ec-STEM) is a multi-modal characterization technique that allows for the direct interpretation of nanoscale mechanisms and kinetics during electrochemical operation. The major advantage of this method is the ability to perform quantitative electrochemical measurements while simultaneously characterizing nanoscale structural and chemical changes that occur at site-specific electrode/electrolyte interfaces using high spatial resolution imaging, spectroscopy, and/or diffraction; all within a scanning transmission electron microscope (STEM). ec-STEM is being used for battery research by studying the formation mechanisms of the solid electrolyte interphase (SEI) and growth mechanisms of lithium metal dendrites within an organic electrolyte (1.2M LiPF6 EC:DMC). The initial stage of lithium metal dendrite nucleation and growth is observed on the surface and edge of the glassy carbon working electrode during cyclic voltammetry experiments below -3.0V vs. Ptpseudo reference electrode. The Li dendrites grow with a globular morphology during a sequence of bursts, whereby newly formed lithium metal deposits are passivated by the SEI followed by additional growth events associated with localized regions of SEI breakdown. Electron energy loss spectroscopy (EELS) confirms that the deposits are lithium metal with surfaces composed of inorganic compounds commonly found in the SEI. The EEL spectra reveal a Li metal plasmon peak at 7.5eV and a characteristic LiF spectral feature on the Li K-edge at 55eV. Furthermore, a framework for obtaining and analyzing quantitative EELS measurements is demonstrated to directly determine the oxidation state of battery electrodes (LiMn2O4 and Li4Ti5O12) using the “white-lines” of the L2,3 core loss ionization edges of Mn and Ti. By simultaneously acquiring low-loss and core-loss EEL spectra, plural scattering effects caused by scattering through the electrolyte can be removed to quantify the oxidation state of Mn and Ti using the white-line intensity ratio method. RF magnetron sputtering was used to deposit a thin layer of LiMn2O4, a commonly used battery cathode material, directly onto the silicon nitride membrane of the ec-STEM cell. Simultaneous low-loss and core-loss EEL spectra are acquired with and without the presence of dimethyl carbonate (DMC) in the liquid cell (Figure 3c). The low-loss portion of the EEL spectra confirms the presence of the DMC within the liquid cell and aides in the quantification of fluid layer thickness. If the fluid layer thickness is small, it is possible to extract and analyze the Mn L2,3-edge for quantification of the Mn oxidation state using the “white-line” intensity ratio method. Mixed Mn3+/4+ oxidation is measured in the as-deposited LiMn2O4, which shifts to a lower oxidation state when DMC is present in the cell, suggesting that LiMn2O4 may be prone to electron beam induced chemical reduction when immersed in a battery electrolyte during in situ ec-STEM experiments. The information obtained from these studies can help provide a deeper understanding of how batteries function at the nanoscale.
Research supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Department of Energy’s Office of Basic Energy Sciences Division and by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.