Understanding Nanoscale Battery Processes By Operando (Scanning) Transmission Electron Microscopy
Aberration corrected (scanning) transmission electron microscopy ((S)TEM) has the spatial resolution (typically < 0.1 nm) to directly visualize the atomic scale structural and chemical variations taking place in materials. Historically, such high resolution microscopy has been used to analyze materials before and after a process takes place to infer the dynamics of what happened in between. While there are still great advances that can be made with such analyses (at the very least in providing benchmark structures for nanoscale systems), a major breakthrough in recent years has been the design and implementation of in-situ gas and liquid stages that allow (S)TEM images to be obtained while the transient processes are actually taking place. For battery systems in particular, this means that we can now observe structural and chemical variations that occur in and around the interfaces between the electrodes and a liquid electrolyte during the charge/discharge process.
Here we will discuss the implementation of an operando electrochemical stage within the (S)TEM that has been configured to form a “Li battery” (the same general configuration can be used for Li-ion, Li-air and Li-S batteries, as well as for any other novel battery architecture). One of the first experiments that has been performed is a quantification of the electrochemical processes that occur at the anode during charge/discharge cycling. Of particular importance for these observations is the identification of an image contrast reversal that originates from solid Li being less dense than the surrounding liquid electrolyte and electrode surface. This contrast allows Li to be identified from Li containing compounds that make up the solid-electrolyte interphase (SEI) layer. By correlating images showing the sequence of Li electrodeposition and the evolution of the SEI layer with simultaneously acquired cyclic voltammograms (CV), electrodeposition and electrolyte breakdown processes can be quantified directly on the nanoscale. In addition, changes in the morphology of the Li deposits (dendrites) obtained by introducing additives into the electrolyte can also be readily quantified from these (S)TEM observations.
The experimental conditions needed to obtain these results (including identifying and eliminating the electron beam effect) will be discussed in detail. Preliminary results from Li-air, Li-S and Mg battery systems will be presented to highlight the development of stage technologies for future electrochemical experiments. In addition, the potential for microsecond time resolution experiments in the newly developed dynamic transmission electron microscope (DTEM) and the implementation of Compressive Sensing (CS) methods to identify the initial stages of SEI/dendrite formation will be explored.