Deciphering Unresolved Materials in Quantitative Analysis of Lithium Metal Solid Electrolyte Interphases

Integrated quantification of inactive Li0 and Solid Electrolyte Interphase (SEI) materials is a powerful tool to understand Coulombic inefficiency of Li metal anodes, but current approaches cannot distinguish unresolved SEI phases from materials lost during cycling or sample preparation. Here, the significance of these previously unresolvable materials was investigated in four electrolytes, leveraging gas chromatography, inductively coupled plasma atomic emission spectroscopy, and nuclear magnetic resonance. Li0 and SEI losses were tracked across the Cu electrode, separator, and wash solvent, bringing the inventoried Li within error of the electrochemical capacity loss. Ultimately, tracking all irreversible Li in the cell did not impact relative trends in inactive Li0 formation or SEI composition across electrolytes. This work demonstrates the robustness of capacity loss analysis to minor losses of material during sample processing, validating simpler approaches for cross-electrolyte comparison while offering a procedure for cases requiring highly precise material inventory.