Supporting Data for "Potassium Alloy Reference Electrodes for Potassium-Ion Batteries: The K-In and K-Bi Systems"

Potassium-ion batteries (KIBs) are a promising alternative to conventional lithium- ion batteries with reduced critical mineral dependency, but accurate three-electrode characterization is hindered by the lack of a suitable reference electrode. Potassium metal is frequently used as a reference electrode out of necessity, but its high reactivity and unstable potential limit its reliability. Here we investigate the K-In and K-Bi alloy systems, synthesize two-phase In-In4K and Bi-Bi2K alloys, and identify Bi-Bi2K as a promising material owing to its stable potential of 1.07 V vs. K+/K. We prove the use of Bi-Bi2K as a reference electrode by cycling graphite in three-electrode cells and demonstrate that it results in significantly less electrolyte reduction than potassium metal, facilitating the accurate electrochemical characterization necessary to accelerate KIB development.