While lithium metal possesses one of the highest specific energy storage densities for secondary batteries at 3.86 A-hr g^-1, its use is problematic. Li metal anodes are known to form dendrites during charging cycles and cause device failure. If the dendrite extends to the cathode it shorts and heats up the battery to the flashpoint of the solvent and creates a pressure build up in the casing. The device becomes ripened for fire and explosion. Here we focus on the direct imaging of first the formation of the solid-electrolyte interphase (SEI) on a glassy carbon electrode using in situ electrochemical scanning transmission electron microscopy (ec-STEM) [1]. After SEI formation we follow the early stages of Li dendrite nucleation and growth and speculate on the role that the SEI has upon Li electrodeposition and dendrite growth. We expand the used of Z-contrast scanning TEM imaging to estimate the density of the SEI and correlate the amount of lithium deposited onto the electrode in view with the current measured during the cyclic voltammogram.   

We are able to record high angle annular dark field (HAADF) STEM images of SEI formation and Li deposition by utilizing a microfluidic electrochemical cell that is placed within a tip of a TEM holder. We will briefly discuss the small-scale STEM microfluidic electrochemical liquid cell platform which is formed by compressing two silicon microchip devices together. Each microchip supports a 50 nm thick electron-transparent silicon nitride membrane with a rectangular viewing area of 50 x 200 μm. Three platinum electrodes is patterned one microchip's surface with the central electrode having glassy carbon deposited on it to be used as a working electrode. The lower microchip has a layer of SU-8 that acts as a spacer and flow channel (400 μm wide, 800 μm in the vacuum TEM environment) for transport of electrolyte between the two microchips when inserted into the TEM holder. Battery-grade 1.2 M LiPF₆EC:DMC electrolyte was delivered to the microfluidic electrochemical cell by a syringe pump.

In this work, HAADF STEM imaging was used to facilitate Z-contrast conditions, which allows for quantitative image analysis, to estimate thickness and density of the SEI layer and Li deposits. It also allows for providing definitive proof of Li electrodeposition though contrast analysis. The electrochemical window of the electrolyte is generally taken as 1.2 to 4 V vs Li/Li⁺.  We show that during a thin film developed on the glassy carbon's edge during potential cycling more negative than 1.5 V due to the reduction of the ethylene carbonate. EC reduces to carbonate, ethylene gas, and intermediate radical species that can polymerize. The dynamics of the SEI in response to the change in electric field is directly imaged; the SEI swells and becomes denser with each potential cycle. We also show how Li dendrites seem to grow from defects in the SEI that form about the deposits which suggests that there is a competition between EC reduction to Li₂CO₃(or some other passivating species) and Li electrodeposition.

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.

[1] Sacci, R. L., Black, J. M., Balke, N., Dudney, N. J., More, K. L., & Unocic, R. R. (2015). Nanoscale imaging of fundamental li battery chemistry: solid-electrolyte interphase formation and preferential growth of lithium metal nanoclusters. Nano Letters, 15(3), 2011–2018.