Direct Observation of SEI Formation and Li Electrodeposition on Au By in Situ Ec-TEM
Authors: Sacci Robert L , Dudney Nancy J , More Karren L. , Unocic Raymond R 
Published: February 2020

Formation of Li dendrites is a major safety concern for Li-ion secondary batteries. 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 because Li metal anodes are known to form dendrites during charging cycles, ultamately leading to device failure. If the dendrite extends to the cathode it causes a short and heats the battery to the flashpoint of the solvent, resulting in pressure build-up in the casing. The device becomes ripened for fire and explosion. Herein we follow the early stages of Li dendrite nucleation and growth in a standard battery electrolyte (1.2 M LiPF6EC:DMC) using in situ electrochemical transmission electron microscopy (ec-TEM). We report the formation a stable solid-electrolyte interphase during Li electrodeposition on gold and possible Li particle formation inside the SEI. Speculations regarding the role that the co-formation of a SEI plays on Li electrodeposition and dendrite growth are explored.

We are able to record TEM images of SEI formation and Li deposition by utilizing a microfluidic electrochemical cell that is placed within a tip of a TEM holder. The cell 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 gold electrodes is patterned one microchip's surface, and are used as the working, counter, and reference electrodes The lower microchip has a layer of SU-8 that acts as a spacer and flow channel (400 μm wide) for transport of electrolyte between the two microchips when inserted into the TEM holder.

The electrochemical window of the electrolyte is generally taken as 1.2 to 4 V vs Li/Li+. During a negative linear-potential sweep, a cathodic current is immediately produced due to the reduction of ethylene carbonate. EC reduces to carbonate, ethylene gas, and intermediate radical species that can polymerize. Shortly after the EC cathodic peak, dendrites form on the Au electrode surface. The nature of the dendrites will be discussed as well as their effect upon Li deposition. The formation of 150 nm nanocrystalline Li (possibly Li-metal) particles within the SEI were observed during Li deposition, which not only may provide electrial conduction through the SEI, but may seed Li dendrite nucleation and growth.

Figure 1: Selected individual frames from Movie shot during a cyclic voltammogram of Li electrodeposition on Au. Dark waves corresponding to Li deposition are highlighted in a) and b). c) shows Li deposition/SEI layer and the inset highlights formation of crystalline features. Voltages correspond to the Au/Ti pseudoreference electrode.

Research supported by the Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program, U.S. Department of Energy (DOE) (RRU and KLM), by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Office of Basic Energy Sciences (BES)-DOE (RLS and NJD), and as part of a user proposal by Oak Ridge National Laboratory's Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, BES-DOE.