An ammonium vanadate/MXene nanocomposite for high-performance ammonium ion storage

Abstract

Energy storage systems with non-metallic charge carriers such as ammonium-ion (NH₄⁺) are inherently safe and enable large-scale storage. Unlike metal ions with spherical symmetry, the intercalation of ammonium ions with a tetrahedral structure is symmetry-specific, leading to high power density and long-term cycling stability. However, developing suitable electrode materials that can reversibly host NH₄⁺ ions to improve electrochemical performance is challenging. To address these issues, here we synthesized a sandwich-structured ammonium vanadate (NH₄V₄O₁₀) and MXene (Ti₃C₂T_x) composite for the first time and employed it as a high-performance electrode for ammonium ion storage. Benefiting from the unique nano-architecture, the developed NH₄V₄O₁₀/MXene electrode delivered an areal capacitance of 229 mF cm⁻² at a specific current of 1 mA cm⁻² with ∼98% retention after 5000 charge–discharge cycles. Electrochemical analyses, supplemented with Raman spectroscopy and X-ray diffractometry, reveal the superior charge kinetics and structural stability of the NH₄V₄O₁₀/MXene electrode under harsh operation conditions. The cell-type ammonium ion asymmetric capacitor (AIC) assembled using NH₄V₄O₈/MXene as the positive electrode and MXene as the negative electrode delivered an energy density of 17.3 W h kg⁻¹ with excellent capacitance retention after 10 000 charge–discharge cycles. These results provide new insights for the development of safe and reliable next-generation clean energy technologies based on unconventional, non-metal-ion-based charge storage mechanisms.