Ring-Size-Dependent Lactone Electrolytes for 4.5 V Sodium-Ion Batteries

Sodium-ion batteries cycled to 4.5 V offer low-cost, energy-dense storage but are limited by the poor oxidative stability of conventional carbonate electrolytes. Here, we introduce reaction-pathway engineering of electrolyte oxidation, redirecting oxidation from destructive decomposition into constructive interphase formation. We show for the first time that lactone oxidation at the cathode is self-limiting, building an oxidation-resistant cathode interphase whose ring-size dependence are consistent with, and inferred to arise from, a ring-opening polymerization pathway. Combining a structurally optimized lactone with a low-viscosity diluent and a solvation-regulating cosolvent that improves anode compatibility, we formulate a ternary electrolyte that enhances cycling stability: in HC/NaNFM cells operated to 4.5 V, it retains 60% capacity after 431 cycles versus 66 cycles for the carbonate baseline. A ring-size-dependent reactivity window governs the outcome: six- and seven-membered lactones balance reactivity and oxidation stability. This work reframes electrolyte solvents from passive ion-transport media to active reagents for interphase construction.