In Situ Converting Conformal Sacrificial Layer Into Robust Interphase Stabilizes Fluorinated Polyanionic Cathodes for Aqueous Sodium‐Ion Storage

Sodium vanadium oxy-fluorophosphates (NVOPF), as typical fluorinated polyanionic compounds, are considered high-voltage and high-capacity cathode materials for aqueous sodium-ion storage. However, the poor cycle life caused by interfacial degradation (especially the attack of specific HF by-products) greatly hampers their application in aqueous electrolytes. Here, it is shown that in situ converting harmful HF derivate to F-containing cathode electrolyte interphase (CEI) can overcome the above challenge. As a proof-of-concept, a conformal Al₂O₃ sacrificial layer is precoated on NVOPF for on-site generating robust AlF₃-rich CEI while eliminating continuous HF release. The evolved CEI chemistry mitigates interfacial side reactions, inhibits vanadium dissolution, and promotes Na⁺ transport kinetics, thus significantly boosting cycling stability (capacity retention rate increased to 3.15 times), rate capability, and even low-temperature performance (≈1.5 times capacity improvement at -20 °C). When integrated with pseudocapacitive zeolite-templated carbon anode and adhesive hydrogel electrolyte, a unique 2.3 V quasi-solid-state sodium-ion hybrid capacitor is developed, exhibiting remarkable cycle life (77.0% after 1000 cycles), high energy and power densities, and exceptional safety against extreme conditions. Furthermore, a photovoltaic energy storage module is demonstrated, highlighting the potential use in future smart/microgrids. The work paves new avenues for enabling the use of unstable electrode materials via interfacial engineering.