Suppressing Vanadium Dissolution in "Water-in-Salt" Electrolytes for 3.2 V Aqueous Sodium-Ion Pseudocapacitors

Low-cost sodium-ion-based electrochemical energy storage devices, especially vanadium-based sodium-ion pseudocapacitors, are receiving increasing attention. However, the inevitable dissolution of vanadium in aqueous electrolytes usually leads to poor cycling stability and a narrow electrochemical stability window (ESW). In this study, we prepared layered (NH₄)₂V₁₀O₂₅·8H₂O with a hierarchical flower-like structure and an ultralarge layer spacing and evaluated its potential as a sodium-ion pseudocapacitive material. Ex situ X-ray diffraction (XRD) measurement and kinetic analysis demonstrate the reversible intercalation and deintercalation of Na⁺ in (NH₄)₂V₁₀O₂₅·8H₂O in NaClO₄ electrolytes. Significantly improved durability and a large voltage window of 3.2 V are achieved in the high-concentration NaClO₄ electrolyte. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis and molecular dynamics (MD) simulations reveal that the dissolution of vanadium in the high-concentration NaClO₄ electrolyte can be effectively suppressed. An asymmetric sodium-ion capacitor with a wide voltage window of 3.2 V was successfully assembled, and it delivered a high energy density of 53.1 Wh kg^-1 at a power density of 3.2 kW kg^-1.