Supporting Data for "The Influence of Sodium Impurities in Metallic Potassium on Photoemission Spectra and Electrochemical Data"

The supporting data for the article entitled "The Influence of Sodium Impurities in Metallic Potassium on Photoemission Spectra and Electrochemical Data" includes XPS data (raw data and corresponding curve fits), peak data tables, as well as cycling data of the electrochemical results and the in-house analytical scripts to process the data. RSC Appl. Interfaces (2026) https://doi.org/10.1039/D6LF00207B Abstract to manuscript: Potassium-ion batteries (KIBs) have emerged as promising candidates for low-cost, high-energy storage systems, driven by their fast ionic conductivity and high operating voltage. In the pursuit of high energy and high power KIBs, material evaluation is often first performed in half-cell tests using highly reactive potassium metal as anode. Herein, we demonstrate that potassium from different suppliers and purity grade can lead to ambiguous results, related to parasitic reactions between metal anode and electrolyte and associated cross-talk processes. In our hands, potassium labelled as lower purity chemical lead paradoxically to higher capacities and lower voltage hysteresis in half cell tests against hard carbon electrodes. Complementary, x-ray photoelectron spectroscopy (XPS) as highly surface-sensitive technique and inductively coupled plasma optical emission spectrometry (ICP-OES) for bulk elemental analysis suggest substantial amounts of sodium impurities in the potassium metal. Potassium comprising higher sodium contents appear to affect the electrochemistry of hard carbon electrodes adversely. Our results are yet another example of how even subtle differences (here in Na impurities) can lead to major changes in material characterization in potassium-ion systems. As the origin of potassium, as well as its quality likely differs across different research facilities, it is critical to become aware of potential performance differences when comparing results reported by different laboratories.