A 3-D Phase Evolution Panorama Uncovered Using a Grid-in-a-Coin Cell Method for Conversion Reaction Electrodes in Lithium-ion Batteries

In recent years, in situ transmission electron microscopy (TEM) studies have been performed to investigate the electrochemically driven phase conversions in lithium-ion batteries. However, most of these studies primarily focused on non-volatile environments and lacked information on interfaces between commercially used organic liquid electrolytes and battery electrodes. Furthermore, due to limitations of TEM sample geometry in these studies, electrode-electrolyte interfaces were simplified as a point/line contact and thus failed to represent the electrode materials’ three-dimensional (3-D) immersion in electrolyte that occurs in practical battery cells. Other groups have attempted quasi in situ observation of energy materials supported on TEM grids in liquid electrochemical cells. However, due to the limited loading allowed on a TEM grid, an accurate and reproducible charging/discharging rate (C-rate) control is difficult to achieve but is essential for lithium-ion battery systems. Here, we report a direct visualization of phase conversion in a high performance anode material in a coin cell configuration [1]. In the coin cell, we pressed a holey carbon supported TEM grid, which was loaded with a small amount of the anode material, against the bulk anode to ensure the materials on the grid are submerged in the same electrolyte environment as the bulk anode (Fig. 1). This configuration allows us to accurately control the C-rate for cycling; additionally the TEM grid can be dissembled from the cell to facilitate TEM imaging and spectroscopy. This grid-in-a-coin cell methodology has enabled direct probing of the 3-D morphology and charge state distribution of the anode material and evolution of AEI at different stages of electrochemical cycling with atomic-scale TEM imaging, electron tomography, electron energy loss spectroscopic mapping as well as ensemble-averaged synchrotron X-ray spectroscopy.