A clear understanding of the electrochemical stability of metal components is needed to ensure the sustainability of potassium-ion battery (KIB) materials. Herein, the effect of highly concentrated 7 M potassium-bis (fluorosulfonyl)imide (KFSI) in dimethoxyethane (DME) electrolyte is investigated to determine the electrochemical stability of the Cu, Al, and 316L stainless steel components. Dynamic- and transient-mode polarization reveal that the Cu is passivated with a Cu–O (CuO or Cu2O) layer below 3.55 V vs. K+/K. Above this threshold potential, dissolution of Cu2+ ions is inevitable, causing general corrosion of the Cu metal. Strikingly, Al and 316L stainless steel are passivated even at 5 V vs. K+/K in the highly concentrated electrolyte, with two double layers; namely, an outermost M