Mechanistic insights into the pseudocapacitive performance of bronze-type vanadium dioxide with mono/multi-valent cations intercalation

Abstract

Bronze-type vanadium dioxide (VO₂(B)) is a promising intercalation pseudocapacitive material due to its special corner and edge-sharing structure. Meanwhile, the utilization of multivalent cations as charge carriers has been considered an effective strategy to further improve its charge storage capability. However, the mechanistic understanding of multivalent cations' intercalation into VO₂(B) is still vague, which greatly limits its practical application. Via a combination of structure characterization, theoretical calculations and electrochemical analysis, we have shown that only ion (de-)intercalation into VO₂(B) occurs in Na₂SO₄ and MgSO₄ electrolytes upon cycling, and their distinct charge storage performance is considered due to the synergistic effects between the ionic radius of electrolyte cations and their polarizing power. In contrast, part of VO₂(B) is reversibly converted to Zn₃(OH)₂V₂O₇·2H₂O in ZnSO₄ electrolyte, followed by Zn²⁺ (de-)intercalation into both phases upon cycling, thus enabling full utilization of the bulk electrode and realizing maximization of the specific capacitance (460 F g⁻¹ at 1 A g⁻¹ current density). When cycled in Al₂(SO₄)₃ electrolyte, the large VO₂(B) nanobelts collapse into small pellets due to the strong electrostatic force between the Al³⁺ ions and host structure, thereby resulting in serious structural instability and inferior pseudocapacitive properties. In general, this work provides valuable insights in understanding the behaviors of mono/multi-valent cations upon intercalation into VO₂(B), which will enable rational design of more layered oxides with excellent charge storage properties or will be extended to other applications.