(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 Ca_0.37V₇O₁₆. 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.