Alloying anodes exhibit very high capacity when used in potassium-ion batteries, but their severe capacity fading hinders their practical applications. The failure mechanism has traditionally been attributed to the large volumetric change and/or their fragile solid electrolyte interphase. Herein, it is reported that an antimony (Sb) alloying anode, even in bulk form, can be stabilized readily by electrolyte engineering. The Sb anode delivers an extremely high capacity of 628 and 305 mAh g–1 at current densities of 100 and 3000 mA g–1, respectively, and remains stable for more than 200 cycles. Interestingly, there is no need to do nanostructural engineering and/or carbon modification to achieve this excellent performance. It is shown that the change in K+ solvation structure, which is tuned by electrolyte composition (i.e., anion, solvent, and concentration), is the main reason for achieving this excellent performance. Moreover, an interfacial model based on the K+-solvent-anion complex behavior is presented. The electronegativity of the K+-solvent-anion complex, which can be tuned by changing the solvent type and anion species, is used to predict and control electrode stability. The results shed new light on the failure mechanism of alloying anodes, and provide a new guideline for electrolyte design that stabilizes metal-ion batteries using alloying anodes.