ABSTRACT High‐voltage sodium‐ion batteries employing layered oxide cathodes (e.g., P2‐type Na 0.66 Li 0.22 Mn 0.78 O 2 ) 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 PF 6 − /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 O 2 formation and irreversible O 2 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 0.66 Li 0.22 Mn 0.78 O 2 cathode. Under harsh conditions—including high voltage (4.5 V), elevated temperature (55°C), and moisture exposure (200 ppm H 2 O)—the Na 0.66 Li 0.22 Mn 0.78 O 2 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.