Acetate-assisted Na modification of (NH4)2V6O16 cathodes for high-performance aqueous zinc-ion storage.

Abstract

Layered vanadium-based compounds are considered ideal positive electrode materials for aqueous Zn-ion batteries (AZIBs) due to their high theoretical specific capacity. Nevertheless, the high charge density of Zn2+ induces sluggish diffusion kinetics, and the vanadium-oxygen layers are prone to collapse over the course of cycling, thereby severely impeding their practical utilization. In this study, a series of sodium-modified (NH4)2V6O16 cathode materials were synthesized via a sodium acetate-assisted hydrothermal method to modulate the structural and surface chemical environment of the layered vanadate. Structural and compositional characterization results further indicate that the Na-modified samples retain the host crystal phase of ammonium vanadate and comparable V-O framework vibrational characteristics, while the surface oxygen chemical environment and vanadium valence state distribution are modified, with the V4+/V5+ ratio increasing from 0.18 in pristine NVO to 0.34 in NVONa-2. Electrochemical measurements reveal that the charge transfer resistance of NVONa-2 decreases from 43.5 Ω for NVO to 10.3 Ω, and GITT results further confirm improved Zn2+ diffusion kinetics. The optimized NVONa-2 electrode retains a specific discharge capacity of 219.5 mAh g-1 after 3400 cycles at 5 A g-1, corresponding to a capacity retention of 99.9% calculated relative to the initial specific discharge capacity. These results collectively demonstrate that sodium acetate-assisted precursor composition regulation yields Na-modified ammonium vanadate cathode materials with favorable interfacial charge transfer and Zn2+ diffusion kinetics, achieving superior high-rate long-cycle performance, and thus providing an effective strategy for the development of high-performance cathode materials for aqueous zinc-ion batteries.

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