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 (MnOx). MnOx experiences morphology and phase transformations during charge/discharge in dilute ammonium acetate (NH4Ac) electrolyte. The NH4Ac concentration plays an important role in NH4+ storage for MnOx. The transformed MnOx 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 NH4Ac, outperforming the state-of-the-art NH4+ hosting materials. Experimental results suggest a solid-solution behavior associated with NH4+ migration in layered MnOx. Spectroscopy studies and theoretical calculations show that the reversible NH4+ insertion/deinsertion is accompanied by hydrogen-bond formation/breaking between NH4+ and the MnOx layers. These findings provide a new prototype (i.e., layered MnOx) for NH4+-based energy storage and contributes to the fundamental understanding of the NH4+-storage mechanism for metal oxides.