Modulating Anion-Reinforced Solvation Chemistry for Stable Anionic Redox Reaction in High-Voltage Sodium-Ion Batteries

High-voltage sodium-ion batteries employing layered oxide cathodes (e.g., P2-type Na_0.66Li_0.22Mn_0.78O₂) face challenges of interfacial degradation and irreversible anionic redox reaction (ARR), which limit their cycling stability and practical application. This study integrates ARR chemistry with interface engineering through a “pre-anchoring and post-decomposition” protocol: DFOB⁻ first adsorbs on the cathode surface to form a B-F/NaF-rich film while priming itself to scavenge the incoming reactive oxygen species, after which the PF₆⁻/DFOB⁻-enriched solvation sheath facilitates the formation of a dense, NaF-dominated and B-containing cathode/electrolyte interphase (CEI) during cycling. Such a functional interface inhibits trapped O₂ formation and irreversible O₂ release in the initial cycle, thereby enabling reversible ARR for long-term cycling. Besides, the robust CEI layer effectively suppresses dissolution of Li/Mn ions, thus refraining irreversible phase transformation and preserving the structural integrity of Na_0.66Li_0.22Mn_0.78O₂ cathode. Under harsh conditions—including high voltage (4.5 V), elevated temperature (55°C), and moisture exposure (200 ppm H₂O)—the Na_0.66Li_0.22Mn_0.78O₂ cathode exhibits exceptional cycling stability. This study presents a universal high-voltage electrolyte design strategy to achieve long-term cyclability in cathodes with ARR activity.