Counter‐ion insertion of chloride in Mn₃O₄ as cathode for dual‐ion batteries: A new mechanism of electrosynthesis for reversible anion storage

Irreversible reductive insertion of Zn 2+ transforms Mn 3 O 4 such that the resulting Zn 0.2 Mn 3 O 4 exhibits highly reversible storage properties of chloride ions, thus rendering Zn 0.2 Mn 3 O 4 an excellent cathode of aqueous dual‐ion batteries. With Zn 2+ trapped, Zn 0.2 Mn 3 O 4 delivers the chloride‐storage capacity over 200 mAh/g at an average potential of 1.6 V vs Zn 2+ /Zn by reversibly forming a new ionic compound equivalent to Zn 0.2 Mn 3 O 4 Cl 1.7 . Electrochemical quartz crystal microbalance results suggest chloride as the primary charge carrier in the reversible oxidative anion insertion. The Mn 3 O 4 anion‐hosting cathode couples with Zn metal anode in a full‐cell dual‐ion battery, demonstrating stable cycling in practical pouch cells with an energy density of 150 Wh/kg based on the mass of both electrodes.