Voltage regulation toward stable cycling of sodium vanadium oxy-fluorophosphates for high-performing, mechanically robust aqueous sodium-ion hybrid capacitors

Sodium vanadium oxy-fluorophosphates (Na3V2O2x(PO4)2F3−2x, NVOPF, 0 ≤ x ≤ 1) are promising cathodes for aqueous sodium-ion hybrid capacitors (ASIHCs) due to their high theoretical capacity and operation potential. However, the extremely poor cycle life and unclear failure mechanism greatly hinder their application in ASIHCs. Here, the intrinsic capacity degradation mechanism of NVOPF is elucidated and a unique voltage regulation strategy is proposed. By slightly compressing the charge cut-off voltage to 1.0 V (vs. SCE, saturated calomel electrode), the vanadium dissolution of NVOPF nanocomposite is significantly suppressed and superior cycling stability is achieved (79.67 % after 800 cycles), among the best reported for NVOPF operating in aqueous electrolytes. Microporous zeolite-templated carbon (ZTC) with ultra-large surface area is selected as a unique capacitive anode for pairing and intentionally activated via an electrochemical oxidation, providing huge pseudocapacitance that is ∼23 times higher than pristine ZTC. Furthermore, a high-performing 2.1 V quasi-solid-state NVOPF-based ASIHC is developed by designing an electrode-compatible polyacrylamide (PAM)-17 m (mol kg−1) NaClO4 hydrogel electrolyte with high ionic conductivity and adhesiveness, which not only provides recordable cycle stability, superior rate performance and high energy/power density, but also demonstrates exceptional safety, flexibility, and robust durability against extreme conditions, including short circuit, nail penetration and mechanical damage.