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

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 sandwichstructured ammonium vanadate (NH4V4O10) and MXene (Ti3C2Tx) composite for the first time and employed it as a high-performance electrode for ammonium ion storage. Benefiting from the unique nanoarchitecture, the developed NH4V4O10/MXene electrode delivered an areal capacitance of 229 mF cm2 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 kg1 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, nonmetal- ion-based charge storage mechanisms. [ABSTRACT FROM AUTHOR]