Intercalation of Ag ions into ammonium vanadate toward high-performance and long-lifespan aqueous zinc-ion battery cathodes

Technical Summary: Intercalation of Ag ions into ammonium vanadate toward high-performance and long-lifespan aqueous zinc-ion battery cathodes

Problem Statement
While aqueous zinc-ion batteries (AZIBs) offer significant potential for large-scale energy storage due to their low cost, safety, and eco-friendly nature, their commercialization is hindered by the limitations of cathode materials. Specifically, layered ammonium vanadate (NH₄V₄O₁₀, NVO) suffers from sluggish Zn²⁺ diffusion kinetics, low specific capacity, and poor long-cycle stability. The inherent issues include the blocking of Zn²⁺ migration by excessive interlayer NH₄⁺ ions and the irreversible structural collapse (pulverization) of the layered framework during repeated cycling. Existing modification strategies often yield single optimization effects or involve complex synthesis procedures, failing to simultaneously address electronic conductivity, structural stability, and active site availability.

Methodology
To address these challenges, the authors developed a facile one-pot liquid-phase (hydrothermal) strategy to synthesize Ag-doped ammonium vanadate (Ag-NVO or ANVO).

• Synthesis: The process involved dissolving ammonium metavanadate (NH₄VO₃) in deionized water, adjusting the pH to ~2 with dilute hydrochloric acid, and introducing silver nitrate (AgNO₃) as the dopant source. The mixture was hydrothermally treated at 180 °C for 24 hours. A pristine NVO sample was prepared identically without silver nitrate for comparison.
• Characterization: Comprehensive structural and chemical analyses were conducted using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron microscopy (SEM, TEM, HRTEM, HAADF-STEM with EDS mapping).
• Electrochemical Testing: Coin cells (CR2016) were assembled using zinc foil anodes, 3 M Zn(CF₃SO₃)₂ electrolyte, and glass fiber separators. Performance was evaluated via galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and long-term cycling tests. Kinetic analyses, including b-value calculations and capacitive/diffusive current separation, were performed to understand the storage mechanism.

Key Contributions and Findings
The study demonstrates that Ag ion intercalation serves a dual function: acting as structural pillars and inducing defect engineering.

1. Structural Stabilization and Defect Engineering: XRD and FTIR analyses confirmed that the monoclinic layered framework of NVO was preserved after Ag doping, with Ag species acting as interlayer pillars to stabilize the structure. Crucially, Ag incorporation triggered the formation of abundant oxygen vacancies and a high concentration of low-valence V⁴⁺ active sites, verified by XPS and Raman spectroscopy.
2. Enhanced Kinetics and Conductivity: The introduction of Ag and the resulting defects optimized the electronic configuration, significantly lowering the charge-transfer resistance (Rct decreased from 13.56 Ω for NVO to 6.45 Ω for ANVO). This facilitated faster Zn²⁺ diffusion and charge transfer.
3. Electrochemical Performance:
   • Capacity: The ANVO cathode delivered a high reversible capacity of 509.66 mAh g⁻¹ at 0.5 A g⁻¹, significantly outperforming pristine NVO (359.88 mAh g⁻¹).
   • Rate Capability: ANVO retained a capacity of 246.89 mAh g⁻¹ even at an ultrahigh current density of 20 A g⁻¹, whereas NVO performance degraded sharply.
   • Cycling Stability: The material exhibited exceptional durability, maintaining 92.17% of its initial capacity after 2000 consecutive cycles at 5 A g⁻¹. In contrast, pristine NVO retained only 63.48% after 100 cycles at 0.5 A g⁻¹.
4. Storage Mechanism: Kinetic analysis revealed that Zn-ion storage in ANVO is dominated by highly reversible capacitive contributions (increasing from 68% to 84% as scan rates increased), which accelerates migration kinetics and accounts for the superior rate performance.

Significance
The authors posit that this work establishes a facile and effective methodology combining metal doping with defect engineering. By simultaneously tailoring the electronic configuration, introducing lattice defects (oxygen vacancies and V⁴⁺), and stabilizing the intrinsic layered framework via Ag intercalation, the study offers an instructive route for developing high-performance, long-lifespan vanadium-based cathodes. The results suggest that Ag doping coupled with defect engineering effectively mitigates structural deterioration and capacity fading, providing a valuable reference for advancing aqueous zinc-ion battery technology.