How reference electrodes improve our understanding of degradation processes in half and full cell potassium-ion battery setups

Abstract

Electrochemical testing of electrodes for K-ion batteries (KIBs) can be strongly affected by interferences from electrolyte degradation reactions and associated crosstalk. This affects half cell measurements in particular. To address this issue, inert and stable reference electrodes are required for reliable 3-electrode measurements that allow to distinguish reversible electrochemical electrode processes more clearly from irreversible parasitic reactions. Therefore, this study evaluated K-metal and a partly charged positive electrode (K₂Fe[Fe(CN)₆], KFF), as two established solutions from the Li-ion battery field. Their electrochemical stability and suitability in various 3-electrode cell setups are evaluated in half and full cell, as well as symmetric cell configurations. Our experiments revealed that the high reactivity of the K-metal as reference or counter electrode interferes considerably with the electrode processes, leading to additional features in the voltage profile of KFF. Furthermore, any amount of K-metal led to a crosstalk-induced self-discharge of KFF. This places considerable limitations on the materials that can be used as a reference electrode. Therefore, we introduced an alternative reference electrode based on Ag/AgCl in a separate cell compartment, with high flexibility in the choice of the electrolyte formulation. To demonstrate the efficacy of this approach, we examine the impact of the electrolyte additive 1,3,2-dioxathiolane 2,2-dioxide (DTD) in 3-electrode setups, with the aim to illustrate the influence of DTD on the electrochemical processes of K-metal, KFF, and graphite electrodes in different cell configurations.