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 (NH4 Ac) electrolyte. The NH4 Ac 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 NH4 Ac, 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.
Keywords: batteries; energy storage; hydrogen bonding; manganese; phase transformations.
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