A lithium-ion battery contains a negative anode and a positive cathode. Graphite or silicon are commonly used as anode materials due to their ability of intercalating Li-ions in their host structures during battery charging. At the boundary of the solid anode and the liquid battery electrolyte an interphase, the so called SEI (solid electrolyte interphase), is forming. This ultra-thin (ca. 30-50 nm) layer is the product of degradation reactions between the electrolyte and the anode during the first charging- and discharging cycles and protects the battery components from further decomposition. The SEI is conductive for Li-cations, the energy carriers in the electrolyte, and has low electron transfer kinetics.
The scanning electrochemical microscope (SECM) is capable of monitoring the local electron transfer kinetic with high resolution and translate it into a 2D image. Like in a flicker book, the changes of the SEI layer on an identical image section can be tracked by a series of images.
With a newly designed measurement setup the nascent SEI formation on the highly relevant anode material lithium metal was tracked under realistic current densities.1 In addition a self-discharging phenomenon on silicon electrodes could be correlated to the SEI formation process with this technique.2 In the past, different battery electrode materials were already investigated by SECM in the Wittstock group.3–5
(1) Krueger, B.; Balboa, L.; Dohmann, J. F.; Winter, M.; Bieker, P.; Wittstock, G. Solid Electrolyte Interphase Evolution on Lithium Metal Electrodes Followed by Scanning Electrochemical Microscopy Under Realistic Battery Cycling Current Densities. ChemElectroChem 2020, 7, 1–8. DOI: 10.1002/celc.202000441.
(2) Bärmann, P.; Krueger, B.; Casino, S.; Winter, M.; Placke, T.; Wittstock, G. Impact of the Crystalline Li15Si4 Phase on the Self-Discharge Mechanism of Silicon Negative Electrodes in Organic Electrolytes. ACS Appl. Mater. Interfaces 2020, 12 (50), 55903–55912. DOI: 10.1021/acsami.0c16742.
(3) dos Santos Sardinha, E.; Sternad, M.; R. Wilkening, H. M.; Wittstock, G. Nascent SEI-Surface Films on Single Crystalline Silicon Investigated by Scanning Electrochemical Microscopy. ACS Applied Energy Materials 2019, 2 (2), 1388–1392. DOI: 10.1021/acsaem.8b01967.
(4) Bülter, H.; Peters, F.; Schwenzel, J.; Wittstock, G. Comparison of Electron Transfer Properties of the SEI on Graphite Composite and Metallic Lithium Electrodes by SECM at OCP. J. Electrochem. Soc. 2015, 162 (13), A7024-A7036. DOI: 10.1149/2.0031513jes.
(5) Bülter, H.; Peters, F.; Schwenzel, J.; Wittstock, G. Detektion lokaler und zeitlicher Veränderungen der Elektrodengrenzschicht in Lithium-Ionen-Batterien mit dem elektrochemischen Rastermikroskop. Angew. Chem. 2014, 126 (39), 10699–10704. DOI: 10.1002/ange.201403935.