Potassium Alloy Reference Electrodes for Potassium-Ion Batteries
Authors: Jagger Ben , Dhir Shobhan , Aspinall Jack , Pasta Mauro 
Published: December 2024

Potassium-ion batteries (KIBs) represent a promising complementary technology to lithium-ion batteries due to the availability and low cost of potassium. KIBs could also be produced with graphite anodes and Prussian blue analogue (PBA) cathodes, reducing the demand for rare, costly elements necessary in LIBs.1 However, accurate characterization of KIB electrode materials is currently impeded by a lack of suitable reference electrodes. A reference electrode should have a stable rest potential, should be chemically inert, and should operate within the electrochemical stability window of KIB electrolytes.
Many studies currently use potassium metal as a reference electrode out of necessity, which does not satisfy any of the above criteria. The potential of potassium metal has been shown to be irreproducible and it can drift over time,2 which we demonstrated may be caused by the high concentrations of sodium and other impurities present in commercially available potassium metal.3 Potassium metal pre-treatment has therefore been necessary to achieve satisfactory reliability of rest potential.2,3 Potassium metal is also highly reactive, decomposing electrolyte on contact and forming resistive solid electrolyte interphase (SEI) layers. Electrolyte decomposition products formed at potassium metal electrodes have further been observed to travel across cells and deposit on the other electrode as a result of crosstalk.4 The development of a stable reference electrode for KIBs is therefore of critical importance.
Here, we explore the K-In and K-Bi binary alloy systems as potential reference electrode materials and synthesize promising two-phase alloys. These are metallic, and hence electronically conductive, and the two-phase nature should provide a stable potential, independent of random composition fluctuations. The In-KIn4 system displays a rest potential of 0.64 V vs. K+/K but takes tens of hours to stabilize, indicating a kinetic limitation. On the other hand, the Bi-KBi2 system displays a rest potential of 1.07 V vs. K+/K, greatly reducing the driving force for electrolyte reduction, and stabilizes within a few hours.5

We therefore demonstrate the merits and use of Bi-KBi2 as a stable K-ion reference electrode.5 Its implementation in three-electrode cells would enable the accurate assignment of electrochemical behaviour to the correct electrode, facilitating the development of KIB electrode materials.
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Dhir, S., Jagger, B., Maguire, A. & Pasta, M. Fundamental Investigations on the Ionic Transport and Thermodynamic Properties of Non-aqueous Potassium-Ion Electrolytes. Nat.
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Jagger, B., Aspinall, J., Dhir, S., Kotakadi, S., Cattermull, J. & Pasta, M. Potassium Alloy Reference Electrodes for Potassium-Ion Batteries. In Preparation

Figure 1