Characterization of degeneration phenomena in lithium-ion batteries by combined microscopic techniques

Abstract

The application of detector strategies in scanning electron microscopy (SEM) correlated with computer tomography (CT) and light microscopy (LM) delivered unique new insights in degeneration effects of lithium-ion batteries. There we exemplary studied reference, cycled and storage cells. High-resolution SEM permit to visualize a coating on top of the cathode material of the treated cells for the first time, which also connects the conductive additives and battery active material. This confirms the assumption of a solid permeable interface on top of the cathode. The detection of low-loss reflected backscattered electrons for energies beyond 3 keV increases the available spatial resolution for material contrast. This offered the opportunity to address the atomic number of precipitates in the nm range inside the coating to be above carbon and below Li_1-x(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC). Applying voltage contrast enables to show the difference in electronic conductivity of plate-like features on top of the cycled cell anode, most likely lithium plating. Cross sectional images of the anode delivered a significant change of the surficial-area morphology for the treated cells with increasing porosity. Precipitates were detected on top of the separator foil. An increment in thickness of the entire treated cells by computer tomography was found, which can be explained by the alteration of the anode, separator and cathode.

Keywords: Battery degeneration; High resolution SEM; Lithium-ion batteries; Low-loss reflected backscattered electrons; Voltage contrast.