Solid–Liquid Interfacial Coordination Chemistry Enables High‐Capacity Ammonium Storage in Amorphous Manganese Phosphate

Abstract

Ammonium (NH 4 + ) ion as charge carrier is attracting attention in aqueous batteries. Yet, most NH 4 + host materials are still limited by the relatively low capacities. Here, we fabricated a manganese phosphate (MP-20) for NH 4 + ion storage. MP-20 displays a high capacity of 299.6 mAh g À 1 at 1 A g À 1 in ammonium acetate (NH 4 Ac) electrolyte, outperforming other reported NH 4 + host materials. Spectroscopy studies suggest a new NH 4+ /H + co-insertion mechanism. We surprisingly discover that the NH 4 Ac electrolyte plays an important role in improving the charge storage capability of the materials. Experimental and computational results indicate acetate ions can form coordination bonds with the Mn atoms, tailoring the electronic structure of the Mn atoms and the surrounding O atoms, and therefore facilitating the NH 4 + storage process. Our findings provide a new NH 4+ host material and propose the important role of the electrolyte-electrode coordination effect in aqueous ammonium batteries.

Valence Engineering Enhancing NH4+ Storage Capacity of Manganese Oxides

“…Reversibly, the valence state of Mn increases to 2.99 upon the subsequent charging process. [33] The fourier transform infrared spectroscopy (FT-IR) results also confirm the existence of hydrogen bonds between NH 4…”

“…Reversibly, the valence state of Mn increases to 2.99 upon the subsequent charging process. [ 33 ] The fourier transform infrared spectroscopy (FT‐IR) results also confirm the existence of hydrogen bonds between NH 4 + and the MnO 2–x host. Specifically, the stretching peak at 3127 cm −1 is assigned to the unperturbed NH groups, while the stretching peak at 3015 cm −1 is assigned to the hydrogen‐bonded NH.…”

“…+ and the MnO 2-x host. [33,34] Furthermore, the MnO 2-x at different charge/discharge states were also characterized by HRTEM (Figure S11, Supporting Information). For the pristine MnO 2-x , a typical interplanar spacing of 0.25 nm corresponds to the (211) plane.…”

“…As expected, the relative peak intensity at 3015 cm −1 increased from A state to B state and then decreased to C state, suggesting the hydrogen‐bond formation/breakage between NH 4 + and the MnO 2–x host. [ 33,34 ] Furthermore, the MnO 2–x at different charge/discharge states were also characterized by HRTEM (Figure S11, Supporting Information). For the pristine MnO 2–x , a typical interplanar spacing of 0.25 nm corresponds to the (211) plane.…”

Flexible Ammonium-Ion Pouch Cells Based on a Tunneled Manganese Dioxide Cathode

“…It can retain a capacity of 83.2 mA h g –1 even at 10 A g –1 . The battery can still reach an energy density of 68.2 W h kg –1 even at the high power density of 8211.6 W kg –1 , which is more outstanding than most of the reported ABs (Figure d), such as MnO 2 //VO 2 (NH 4 + ), V 2 O 5 //V 2 O 5 (NH 4 + ), NVO//h-WO 3 (NH 4 + ), PBA//WO 3 (H + ), KVO//PTCDI (K + ), HPI-NG//HPI-NG (Li + ), MP-20//MoO x (NH 4 + ), and Na 0.44 MnO 2 //NaTi 2 (PO 4 ) 3 (Na + ) . The long cycling performance of the MnO 2 //PTCDA configuration is displayed in Figure e.…”