Nanoscale Imaging of Lithium Ion Distribution During In Situ Operation of a Battery Electrode and Electrolyte

Abstract

The integration of renewable, and often intermittent, energy sources such as solar and wind into the energy landscape, as well as the electrification of transportation, requires dramatic advances in electrical energy conversion and storage technologies including fuel cells, batteries and supercapacitors. TEM detection of lithium through a liquid is difficult, because lithium is a weak elastic scatterer and multiple scattering from the liquid overwhelms the inelastic core-loss signal in electron energy-loss spectroscopy (EELS). In this work, we successfully observed the lithiation state by valence energy-filtered TEM (EFTEM), which probes the low-energy regime (~1-10 eV), and allows us to work in thicker liquid layers than core-level A Baseline for Electrochemistry in the TEMWe use a liquid cell holder developed by Protochips using chips we designed to mimic a typical electrochemical cell (Figure 1a-b). The tip of the holder is a microfluidic flow cell with silicon nitride viewing membranes that confine a liquid, shown in cross section in Figure 1a. Figure 1b illustrates the top chip, with three patterned electrodes optimized for electrochemical cycling and imaging. Traditional silicon-processing methods use a chromium adhesion layer and gold electrodes. However, chromium diffuses rapidly 6 through gold (especially at grain boundaries) and can affect and even dominate the electrochemical signal. In addition, high-atomic-number electrodes such as Pt and Au obscure imaging. Instead, we used a carbon working electrode which only weakly scatters electrons and is commonly used in bulk electrochemistry, and titanium adhesion layers under platinum reference and counter electrodes. This allows us to image through the electrode with little loss in spatial resolution and contrast, which is dominated by scattering in the liquid instead. As a practical matter, and discussed below, spatial resolution is often limited by the low doses needed to control radiation damage than by beam spreading in the cell.To demonstrate that in situ electrochemistry reproduces well-established criteria, we performed cyclic voltammetry of a film of platinum, shown in Figure 1c-d, in the TEM.This control experiment represents a test case for quantitative electrochemistry, since the features are surface effects -including hydrogen adsorption and desorption and oxide formation and reduction -which are sensitive to contaminants at the sub-monolayer level.The in situ electrochemistry reproduced the characteristic voltametric profile of a polycrystalline platinum electrode at an appropriate current scale, regardless of the electron beam. In thin liquid layers, the ohmic drop in the solution becomes significant, as evidenced by the slanted curve in Figure 1d. This implies an inherent compromise between the highest spatial resolution imaging and quantitative electrochemistry.Accounting for ohmic drops in solution, this setup replicates results of a conventional electrochemical cell while obtaining nanometer resolution. 7Having established the electroch...

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