Aqueous potassium-ion batteries are a promising alternative to lithium-ion batteries owing to their safety and low cost. However, not much is known about the properties of the solid-electrolyte interphases (SEI) that form between the electrode and the aqueous electrolyte. To address this knowledge gap, researchers from Japan have now conducted a study using advanced scanning electrochemical microscopy and operando electrochemical mass spectrometry. Their findings provide a deeper understanding of SEI in next-generation batteries.