Synthesizing ideal interfaces between Li metal and electrolytes is one of the keys to high energy batteries. Thermodynamic calculations suggest that virtually all solid-state electrolytes are not stable with Li metal. However, practical experience with LLZO, Lipon, and other solid-state electrolytes have demonstrated extraordinary apparent stability. This discrepancy is likely due to the kinetics of the reaction with Li metal but makes it difficult to accurately predict practical stability of Li metal with solid-state electrolytes. Here we present a simple electroanalytical technique that enables us to precisely measure the maximum amount of Li that reacts with a solid-state electrolyte and the maximum possible thickness of an interfacial layer. Using this technique, we demonstrate that a maximum of 10 nm of Li reacts with 5 nm of Lipon to form an interfacial layer that is less than 10 nm thick and does not propagate. Finally, we demonstrate that this technique is broadly applicable to a wide variety of Li/solid-state electrolyte systems. This method demonstrates a clean and simple path to evaluate the practical stability of solid-state electrolytes with Li metal that can increase understanding and accelerate their development.

Acknowledgements: This work was supported by the ARPA-E IONICS program, U.S. Department of Energy, award DE-AR0000775.