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

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2024-09, Vol.495, p.153445, Article 153445
• Main Authors: Gong, Peng, Xia, Jiale, Chen, Chenyang, Zhao, Zelin, Liu, Dan, Li, Yuanyuan, Liu, Jinping
• Format: Article
• Language: English
• Subjects: Aqueous sodium-ion hybrid capacitors; Hydrogel electrolyte; Sodium vanadium oxy-fluorophosphates; Vanadium dissolution; Zeolite-templated carbon
• ISSN: 1385-8947
• DOI: 10.1016/j.cej.2024.153445

A simple voltage regulation strategy is proposed to stabilize the sodium vanadium oxy-fluorophosphates cathode in aqueous electrolyte with comprehensively understanding the capacity fade mechanism, leading to the best reported cycling stability. By pairing with intentionally activated zeolite-templated carbon anode with huge pseudocapacitance and highly conductive hydrogel electrolyte with excellent adhesion ability, a 2.1 V high-performing, mechanically robust sodium-ion hybrid supercapacitor is developed. [Display omitted] •The degradation mechanism of sodium vanadium oxy-fluorophosphates in aqueous electrolytes is comprehensively elucidated.•The voltage regulation strategy improves stability of sodium vanadium oxy-fluorophosphates cathode in aqueous electrolytes.•The pseudocapacitance of activated zeolite-templated carbon is 147.0 mA h g−1, ∼1.5 times larger than activated carbon.•Constructed 2.1 V flexible quasi-solid-state aqueous sodium-ion hybrid capacitor shows superb safety and robust durability. 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 condit