Unlocking Polyanion-Type Materials through High-Entropy Effect for Aqueous Potassium-Ion Batteries.

Exploration of electrode materials for aqueous batteries has predominantly followed the established principles and design strategies derived from organic electrolyte-based systems. However, this conventional approach faces inherent limitations. Although V-based polyanion materials (e.g., Na3V2(PO4)3) are compelling for organic electrolyte-based alkali metal ion batteries, their applications in aqueous K-ion batteries remain untapped, probably due to the limited cation electroactivity and uncontrollable dissolution. Herein, we unlock reversible and stable aqueous K+ storage in V-based polyanion materials via a high-entropy strategy. Unlike the phase-transition mechanism in traditional polyanionic electrodes, in situ spectroscopic characterizations reveal a solid-solution process in the entropy-tuned polyanionic electrode, facilitated by reduced steric hindrance during K+ uptake. Time-of-flight secondary ion mass spectrometry and density functional theory simulations further confirm the suppressed Na+/K+-migration barrier and solubility of the entropy-tuned polyanionic cathodes. As a result, the high-entropy V-based polyanion cathodes are demonstrated promising for aqueous K-ion batteries, even in dilute aqueous electrolytes, achieving an ultrahigh initial Coulombic efficiency of 98.7%, rate capability at 36C, and impressive cycling durability up to 3500 cycles. This work uncovers the charge storage gap between organic and aqueous electrolyte-based systems and provides insights into activating the electroactivity of other materials in an aqueous environment.