Ammonium‐Ion Storage Using Electrodeposited Manganese Oxides

NH₄ ⁺ ions as charge carriers show potential for aqueous rechargeable batteries. Studied here for the first time is the NH₄ ⁺ -storage chemistry using electrodeposited manganese oxide (MnO_x ). MnO_x experiences morphology and phase transformations during charge/discharge in dilute ammonium acetate (NH₄ Ac) electrolyte. The NH₄ Ac concentration plays an important role in NH₄ ⁺ storage for MnO_x . The transformed MnO_x with a layered structure delivers a high specific capacity (176 mAh g^-1 ) at a current density of 0.5 A g^-1 , and exhibits good cycling stability over 10 000 cycles in 0.5 M NH₄ Ac, outperforming the state-of-the-art NH₄ ⁺ hosting materials. Experimental results suggest a solid-solution behavior associated with NH₄ ⁺ migration in layered MnO_x . Spectroscopy studies and theoretical calculations show that the reversible NH₄ ⁺ insertion/deinsertion is accompanied by hydrogen-bond formation/breaking between NH₄ ⁺ and the MnO_x layers. These findings provide a new prototype (i.e., layered MnO_x ) for NH₄ ⁺ -based energy storage and contributes to the fundamental understanding of the NH₄ ⁺ -storage mechanism for metal oxides.