Tautomeric pillars in vanadyl phosphate cathode to screen the divalent charge on Zn2+ for aqueous Zn batteries

Abstract

Cathode materials with high redox potential are important for constructing high-energy-density aqueous zinc batteries. The polyanion compound of layered hydrate vanadyl phosphate (H2O-VOP) provides high potential thanks to the inductive effect. However, the sluggish Zn2+ diffusion in the lattice results in large voltage hysteresis, obstructing the capacity release at high voltage. Herein, tautomeric pillars are inserted in the VOPO4 layers (Tau-VOP) by the desulfhydryl reaction of l-cysteine, aiming to suppress voltage hysteresis with their charge screening effect. Specifically, the Lewis alkaline sites on the tautomers capture inserted Zn2+, and the divalent charge is further delocalized over the conjugated structure of pillars. This reduction of effective charge on cations ensures much weakened electrostatic interactions with the lattice and reduced diffusion barrier. As a result, the Tau-VOP cathode achieves 193 mAh g−1 capacity with 1.47 V/1.6 V redox voltage in zinc cells, which are superior to 110 mAh g−1 capacity and 1.1 V average discharge voltage with large voltage hysteresis obtained with H2O-VOP. The rate performance and energy efficiency are also significantly improved with the help of tautomer pillars. This work presents an effective charge screening strategy by suitable pillars to facilitate multivalent cation transport in cathode structures and enhance electrochemical performance.