Christos E. Athanasiou,1,2,6,7,* Cole D. Fincher,3,6,* Colin Gilgenbach,3 Huajian Gao,1,4,5 W. Craig Carter,3 Yet-Ming Chiang,3,* and Brian W. Sheldon1,*

Fundamental understanding of stress buildup in solid-state batteries is elusive due to the challenges in observing electro-chemomechanical phenomena inside solid electrolytes. In this work, we address this problem by developing a method to directly measure stresses within solid-state electrolytes. As a proof -of -concept, we provide the first direct measurements of the stress fields generated around the lithium metal dendrites in a model garnet electrolyte, Li6.75La3Zr1.75Ta0.25O12, and show that these are consistent with the predictions for an internally loaded crack in an elastic solid. The measurements are based on employing the principle of photoelasticity to probe the stress fields during operando electrochemical cycling in a plan -view cell. This new experimental methodology provides a means to access chemo-mechanical events in solid-state batteries and has the potential to provide insight into a variety of chemo-mechanical failure modes.