Controlling Interfacial Reactivity of Cyclic Sulfonyl Additives via in‐Ring S–O Bonds for Stable Sodium‐Ion Batteries

ABSTRACT Rational design of electrolyte additives is vital for stabilizing electrode/electrolyte interfaces in battery systems. This study presents a molecular engineering strategy using cyclic sulfonyl compounds, revealing that the number of in‐ring S–O bonds serves as a precise structural parameter to tune bond energy, orbital levels, and interfacial decomposition reactivity. Comparing three representative additives shows that 1,3‐propanesultone (PS), with one in‐ring S–O bond, exhibits optimal reactivity. It facilitates in‐situ formation of a thin, robust, and stable cathode‐electrolyte interphase (CEI) with a balanced organic‐inorganic structure, enhancing interfacial passivation and cycling stability. In contrast, sulfolane (SUL) with no in‐ring S–O bonds shows insufficient reactivity, while ethylene sulfate (DTD) with two leads to continuous decomposition due to excessive reactivity. Benefiting from the tailored interfacial chemistry, the PS‐based electrolyte enables a Na||Na 0.67 MnO 2 half‐cell to retain 75.92% capacity after 800 cycles and demonstrates practical viability in an Ah‐level pouch full cell, achieving 84.48% capacity retention after 2000 cycles. This work establishes the in‐ring S–O bond count as a key descriptor for additive reactivity, offering a rational design paradigm for high‐stability sodium‐ion battery electrolytes.