Potassium\nAlloy Reference Electrodes for Potassium-Ion\nBatteries: The K‑In and K‑Bi Systems

Potassium-ion batteries\n(KIBs) are a promising alternative\nto conventional\nlithium-ion batteries with reduced critical mineral dependency but\naccurate three-electrode characterization is hindered by the lack\nof a suitable reference electrode. Potassium metal is frequently used\nas a reference electrode out of necessity, but its high reactivity\nand unstable potential limit its reliability. Here we investigate\nthe K-In and K-Bi alloy systems, synthesize two-phase In-In<sub>4</sub>K and Bi-Bi<sub>2</sub>K alloys, and identify Bi-Bi<sub>2</sub>K\nas a promising material owing to its stable potential of 1.07 V vs\nK<sup>+</sup>/K. We prove the use of Bi-Bi<sub>2</sub>K as a reference\nelectrode by cycling graphite in three-electrode cells and demonstrate\nthat it results in significantly less electrolyte reduction than potassium\nmetal, facilitating the accurate electrochemical characterization\nnecessary to accelerate KIB development.