[Zn1K2]Fe3(PO4)2(P2O7) as a Zn2+/K+ dual-ion cathode with enhanced diffusion kinetics and structural stability for aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) have gained attention as safe and cost-effective alternatives to conventional lithium-ion batteries. However, their practical performance is often hindered by sluggish Zn²⁺ diffusion, primarily due to the large hydrated ionic radius and strong electrostatic interactions. To address this, we present a polyanion-based cathode material, [Zn₁K₂]Fe₃(PO₄)₂(P₂O₇), that enables a reversible dual-ion intercalation mechanism involving both Zn²⁺ and K⁺. Compared to Zn²⁺, monovalent K⁺ interacts more weakly with the PO₄/P₂O₇ framework and desolvates more readily at the electrolyte–cathode interface, enabling the Zn²⁺/K⁺ dual-ion system to achieve faster kinetics than the Zn²⁺-only system. This leads to significantly enhanced power capability and structural reversibility under the AZIB system. The discharge capacity of [Zn₁K₂]Fe₃(PO₄)₂(P₂O₇) is 100.4 mAh g^-1 at C/5 (1C = 100 mA g^-1), corresponding to de/intercalation of 2 mol K⁺ and 0.5 mol Zn²⁺. Even at 5C, it maintains a capacity of 70 mAh g^-1, outperforming the Zn²⁺-only analogue Zn₂Fe₃(PO₄)₂(P₂O₇) (55 mAh g^-1). Galvanostatic intermittent titration technique measurements further confirm the enhanced ionic diffusion kinetics enabled by the Zn²⁺/K⁺ dual-ion system. Moreover, the capacity retention of [Zn₁K₂]Fe₃(PO₄)₂(P₂O₇) is 84.6 % after 100 cycles, while Zn₂Fe₃(PO₄)₂(P₂O₇) deliver only 56.1 %. Advanced structural analyses reveal highly reversible lattice evolution during cycling, supporting the proposed dual-ion storage mechanism.