Determination of mechanical properties of the SEI in sodium ion batteries via colloidal probe microscopy

• First investigation of mechanical properties of the SEI in sodium ion batteries.
• Young's Modulus varies over 3 orders of magnitude in a 625 µm2 area.
• Evidence that a softer layer forms on top of a more rigid layer.
• Colloidal probe technique can be used for electrolyte performance comparisons.

The solid electrolyte interphase (SEI) is a layer that forms at the anode surface for all alkali metal ion batteries which utilize liquid electrolytes. It acts as an electronic insulator and ion conductor, but the formation of the SEI results in an irreversible capacity loss. For high capacity anodes in sodium ion batteries (NIBs), continued cycling of the battery ruptures the SEI and exposes new areas of the anode surface where the electrolyte will reduce. In this work, we utilize colloidal probe microscopy to investigate the mechanical properties of the SEI layer in NIBs for use in future anode designs. Our results indicate a lateral inhomogeneity in the Young's Modulus on the micron scale spanning 3 orders of magnitude. This technique can be used to investigate the differences in the SEI formed by different electrolytes and electrolyte additives and ultimately to determine the best liquid electrolyte for NIBs.

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