Valence Engineering Enhancing NH₄⁺ Storage Capacity of Manganese Oxides

Ammonium ions (NH4+), as non-metallic charge carriers, are attracting attention in aqueous batteries due to its low molar mass, element sufficiency, and non-toxicity. However, the host materials for NH4+ storage are still limited. Herein, an oxygen defects-rich manganese oxide (MnO2-x) for NH4+ storage are reported. The oxygen defects can endow the MnO2-x sample with improved electric conductivity and low interface activation energy. The electrochemical reaction mechanism is also verified by using ex situ X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FT-IR), demonstrating the insertion and extraction of NH4+ in the MnO2-x by formation/breaking of a hydrogen bond. As a result, MnO2-x delivers a high capacity of 109.9 mAh g(-1) at the current density of 0.5 A g(-1) and retention of 24 mAh g(-1) after 1000 cycles at the current density of 4 A g(-1), outperforming the pristine MnO2 sample.