(NH4)2V7O16 as a Cathode Material for Rechargeable Calcium-Ion Batteries: Structural Transformation and Co-Intercalation of Ammonium and Calcium Ions

Calcium-ion batteries (CIBs) are viable alternatives to lithium-ion batteries. However, few cathode materials can reversibly intercalate Ca ions in anhydrous electrolytes. Most high-capacity materials contain crystal water, causing unwanted reactions on the anode. Herein, we report a crystal-water-free ammonium vanadate, (NH4)2V7O16, as a CIB host material. Synthesized via a microwave-assisted hydrothermal method, (NH4)2V7O16 exhibits a layered structure with stacked V7O16 layers and interlayer ammonium ions hydrogen-bonded to adjacent oxygen atoms. We demonstrate the reversible electrochemical intercalation of Ca2+ ions into (NH4)2V7O16, achieving a reversible capacity of 89 mA h g–1 and an average discharge voltage of ∼3.21 V vs Ca/Ca2+. Although (NH4)2V7O16 displays poor rate capability and cycling performance, we reveal a unique reaction mechanism. During the initial charge, an irreversible structural change occurs, removing all ammonium ions and inserting a small amount of Ca ions, forming Ca0.37V7O16. This suggests an ion-exchange reaction between calcium and ammonium ions. Subsequent cycles exhibit the reversible coinsertion and coextraction of calcium and ammonium ions. We observe that V7O16 lacks structural stability without interlayer cations. Our findings offer insight into electrochemical reaction processes in crystal-water-free layered materials containing interlayer ammonium ions, highlighting the importance of cointercalation between ammonium and carrier ions for reversible cycling.