Self-Assembled Layer of Organic Phosphonic Acid Enables Highly Stable MnO2 Cathode for Aqueous Znic Batteries

Manganese dioxide (MnO₂) is an attractive cathode material for aqueous zinc batteries (AZBs) owing to its environmental benignity, low cost, high operating voltage, and high theoretical capacity. However, the severe dissolution of Mn²⁺ leads to rapid capacity decay. Herein, a self-assembled layer of amino-propyl phosphonic acid (AEPA) on the MnO₂ surface, which significantly improves its cycle performance is successfully modified. Specifically, AEPA can be firmly attached to MnO₂ through a strong chemical bond, forming a hydrophobic, and uniform organic coating layer with a few nanometers thickness. This coating layer can significantly inhibit the dissolution of Mn²⁺ by avoiding the direct contact between the electrolyte and cathode, thus enhancing the structural integrity and redox reversibility of MnO₂. As a result, the MnO₂@AEPA cathode achieves a high reversible capacity of 223 mAh g^-1 at 0.5 A g^-1 and a high capacity retention of 97% after 1700 cycles at 1 A g^-1. This work provides new insights in developing stable Mn-based cathodes for aqueous batteries.