Designing sustainable energy storage and optoelectronic devices requires a fundamental understanding of the mechanisms of intercalation, SEI formation, and ion transport. Visualizing these interactions in-situ through optical techniques offers unique insight. In addition, mid-IR measurements reveal structural changes in the host lattice and the effect of charge transfer at low frequencies during intercalation. However, most battery electrolytes absorb strongly in the IR, which limits the range of optical studies that can be performed. To explore dynamic processes in the visible and IR, we have designed an electrochemical cell that enables us to observe intercalation with cycling in electrolyte (Figure 1a) and electrolyte-free (Figure 1b) environments. We focus on the electrochemical intercalation of bisulfate into bulk and ultrathin graphite as a model for aqueous intercalation since it exhibits well-defined staging and cycles reversibly. Qualitatively, we image intercalation under an optical microscope, and visualize intercalation events during cyclic voltammetry and galvanostatic cycling in the electrolyte (Figure 1c-f) and without the electrolyte (Figure 1g-l). To correlate optical images with charge transfer, we perform in-situ reflectance measurements as well as Raman spectroscopy at distinct locations in bulk and ultrathin graphite. The electrolyte-free region of the cell enables us to perform FT-IR measurements in the mid-IR to gain insight into vibrational and electronic changes that occur upon intercalation. By coupling optical images with in-situ spectroscopy, we gain insight into how intercalation and charge transfer occur in a bulk crystal compared to a few-layer electrode as well as in the absence of an electrolyte. Correlative imaging also showcases the different techniques that this design facilitates: optical microscopy, UV-vis, Raman, and FT-IR spectroscopy. In addition, ex-situ XPS and UPS contrast the composition of the SEI between materials intercalated in an electrolyte with those intercalated in an electrolyte-free environment.

Visualizing electrolyte-free electrochemical intercalation

2019 Fall National Meeting

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