Principles of interlayer-spacing regulation of layered vanadium phosphates for superior zinc-ion batteries

Abstract

Layered vanadium phosphate (VOPO₄·2H₂O) is reported as a promising cathode material for rechargeable aqueous Zn²⁺ batteries (ZIBs) owing to its unique layered framework and high discharge plateau. However, its sluggish Zn²⁺ diffusion kinetics, low specific capacity and poor electrochemical stability remain major issues in battery application. In this work, a group of phenylamine (PA)–intercalated VOPO₄·2H₂O materials with varied interlayer spacing (14.8, 15.6 and 16.5 Å) is synthesized respectively via a solvothermal method for the cathode of aqueous ZIBs. The specific capacity is quite dependent on the d-spacing in the PA–VOPO₄·2H₂O system following an approximate linear tendency, and the maximum interlayer spacing (16.5 Å phase) results in a discharge capacity of 268.2 mA h g⁻¹ at 0.1 A g⁻¹ with a high discharge plateau of ∼1.3 V and an energy density of 328.5 W h kg⁻¹. Both of the experimental data and DFT calculation identify that the optimal 16.5 Å spacing can boost fast zinc-ion diffusion with an ultrahigh diffusion coefficient of ∼5.7 × 10⁻⁸ cm⁻² s⁻¹. The intercalation of PA molecules also significantly increases the hydrophobility in the aqueous electrolyte, resulting in the inhibition of the decomposition/dissolution of VOPO₄·2H₂O and remarkably improved cycling stability over 2000 cycles at 5.0 A g⁻¹ with a capacity retention of ∼200 mA h g⁻¹. Our study provides a feasible solution for the sluggish Zn²⁺ diffusion kinetics and poor cyclic stability, and also shows a clear understanding of the interlayer chemistry principle of layered phosphates toward high-performance zinc-ion batteries.