Unveil the Failure of Alkali Ion‐Sulfur Aqueous Batteries: Resolving Water Migration by Coordination Regulation

Abstract Sulfur aqueous battery (SAB) is promising owing to its high theoretical capacity and cost competitiveness. Although decoupled electrolyte design has successfully endowed transition metal ion‐SABs with customizability to achieve high energy density, its effectiveness in alkali ion‐SABs remains problematic. Here, we identify for the first time an intractable phenomenon of alkali‐ion‐driven water migration between decoupled electrolytes through ex situ NMR, which is recognized as the origin of the irreversible sulfur redox reactions. To address the challenge, we propose an alkali‐ion‐H 2 O‐poor coordination strategy to effectively regulate water migration by incorporating low molecular polarity index (MPI) anions. In situ Raman, synchrotron spectroscopy, and molecule dynamic simulations reveal that the repulsion of low MPI anions to water effectively disrupts the hydration patterns around the alkali cations, and thereby minimizes the concomitant water migration. The elaborated Na + ‐SAB achieved an ultrahigh capacity of 1634 mAh g −1 (97.7% sulfur utilization) and prolonged stability over 500 cycles. Furthermore, the versatility of the alkali‐ion‐H 2 O‐poor coordination strategy is further substantiated in Li + ‐SAB and K + ‐SAB batteries, boosting the scope of the following SAB systems.