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 AlO sacrificial layer is precoated on NVOPF for on-site generating robust AlF-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.