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Lithium Plating Detection and Characterization Utilizing Impedance Changes During the Lithium Deposition

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Lithium plating as a more and more important ageing mechanism in modern lithium-ion batteries in the prospect of the growing fast charging demand in electric vehicles is still not fully understood. Especially the impact of the surface electrode interphase (SEI) layer on plating and vice versa is still subject of many questions. Optical measurements have shown that during the plating process a porous layer is growing on the anode surface that is significantly thicker than the SEI under normal operating conditions. The composition of this layer most likely consists of degenerated electrolyte and possibly inactive metallic lithium that lost electrical contact with the anode and can no longer be utilized in the cell reaction. However, since the SEI layer, metallic lithium and the unidentified plating layer all are very reactive in the presence of oxygen, humidity, nitrogen and carbon dioxide, it is very hard to analyze the layer in detail. To obtain a better understanding of the layer and the plating process in general, focused ion beam (FIB) cross sections of opened 18650 round cell anodes have been optically analyzed. Equivalent samples have also been investigated in their chemical composition using XPS measurements. XPS can be used as a mostly nondestructive way to characterize the chemical compounds present on the surface of a sample. To get depth information of the samples, additional XPS measurements have been performed after etching the surface using an argon-ion beam. The results have then been compared with samples of pristine (non cycled) graphite and non-plated anodes. The results of the etching experiment confirmed the thickness of the plated SEI layer and electron induced lithium plating showed that the SEI of plated anodes contains significantly more lithium than the SEI of non-plated anodes.

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