Efficient modulation of vanadium spin state via poly(3,4-ethylenedioxythiophene) functionalization for enhanced zinc ion storage

Abstract

Layered VOPO₄·2H₂O (VOP) cathodes exhibit promising advantages for aqueous zinc-ion capacitors (ZICs) but hindered by inferior structural stability and poor bulk conductivity. Herein, a PEDOT-VOPO₄·2H₂O (PEDOT-VOP) cathode has been developed, achieving an enlarged interlayer spacing of 9.7 Å (compared to 7.4 Å for single VOP) through effective polymer intercalation. The π-conjugated chain of PEDOT forms a delocalized hybrid state with the orbitals of vanadium, introducing asymmetrical spin polarization and thereby enhancing the intrinsic electronic conductivity of VOP. Furthermore, the PEDOT insertion modifies the local crystal field symmetry of VOP, resulting in the reorganization of orbital splitting energy levels. This significantly modifies the bond strength between V and O by altering the original [VO₆] octahedral structure into a distorted VO₅ pyramidal structure, which in turn enhances structural stability. Consequently, the PEDOT-VOP cathode demonstrates a specific capacity of 512.7 mAh g⁻¹ at a wide potential window of 1.5 V in aqueous electrolyte under a current density of 1 A g⁻¹. In-situ Raman and XRD analysis confirm the excellent reversibility and long-term stability of the cathode, with 88 % of its initial capacity retained over 10,000 cycles. This study provides profound insights into the development of high-voltage, stable, and high-capacity cathode materials for ZICs by modulating the electronic structure through enhanced spin polarization of d-orbitals.