Fluoride-based hydrogen bond chemistry in a layered double hydroxide cathode toward high-performance aqueous NH4+ storage.

Abstract

In aqueous ammonium-ion storage, hydrogen bonds play a pivotal role in the reversible insertion/extraction of NH₄⁺ within transition metal oxides/hydroxides. Although fluorine (F) is known for its strong electronegativity and potential to form robust hydrogen bonds with NH₄⁺, its specific influence on NH₄⁺ storage remains unexplored. Herein, we systematically investigate the effects of F-based hydrogen bond chemistry within a layered double hydroxide matrix, where F species are introduced and subsequently partially removed via an electrochemical method. Our findings demonstrate that while increasing F doping content accelerates NH₄⁺ diffusion due to F's strong electronegativity, it also triggers crystal shrinkage and depresses storage capacity. To this end, controlled partial removal of F, employing a lye-assistant electrochemical strategy, induces expanded interlayer spacing and distinct edge lattice tearing, thereby facilitating improved NH₄⁺ accommodation. The retained F sites couple with emerging exposed O sites maintain a high hydrogen bonding capability, which is further enhanced by the formation of highly active, curved hydroxyl groups centered around F sites. These manipulations significantly boost the NH₄⁺ storage performance of the electrode, providing insights into leveraging the strongest F-based hydrogen bond chemistry in developing high-performance ammonium-ion energy storage devices.