Chemical vs. electrochemical solid electrolyte interphase formation in potassium-ion batteries

direct reaction with the electrolyte or electrochemically during potassium plating. In contrast, during charging of a graphite electrode, SEI formation occurs exclusively through electrochemical reduction. It therefore remains unclear to what extent the SEI formed on potassium metal is representative of that formed on graphite. Here, we employ X-ray photoelectron spectroscopy (XPS) to probe differences between chemical and electrochemical SEI formation on potassium metal, graphite, and inert electrodes in two contrasting potassium bis(fluorosulfonyl)imide (KFSI)-based electrolytes: 1 m KFSI in tetraethylene glycol dimethyl ether (G4) and 1 m KFSI in 1,3-dioxane (13-DX). Four-dimensional scanning transmission electron microscopy (4D-STEM) phase mapping provides complementary structural insight. We establish relationships between SEI chemistry, electrolyte composition, electrode material, formation pathway, and applied potential, offering guidance for rational electrolyte design.