Transition Metal Compounds for Aqueous Ammonium-Ion Batteries: Storage Mechanisms and Electrode Design

Aqueous ammonium-ion batteries (AAIBs) have recently emerged as promising candidates for next-generation energy storage owing to their intrinsic safety, environmental benignity, and cost efficiency. The unique tetrahedral configuration and hydrogen-bonding capability of NH₄ ⁺ enable fast ion transport and dendrite-free operation, distinguishing AAIBs from traditional metal-ion systems. However, sluggish NH₄ ⁺ intercalation kinetics and electrode structure degradation have limited their practical implementation. Transition metal compounds (TMCs), with flexible oxidation states, rich redox activity, and tunable electronic structures, provide a versatile platform to address these issues. This review systematically summarizes recent progress in TMCs-based electrodes for AAIBs, encompassing oxides, sulfides, carbides, nitrides, and other related compounds. We begin by distinguishing the operational principles of AAIBs in conventional "rocking-chair" and dual-ion configurations, emphasizing their distinct charge-storage pathways and associated performance limitations. Subsequently, we elucidate the fundamental mechanisms governing ammonium-ion storage and hydrogen-bonding dynamics in governing ion transport. Finally, we outline a roadmap aimed at guiding future research efforts, offering material design insights into the commercialization of next-generation safe and sustainable aqueous energy storage technologies. Unlike previous reviews that primarily focused on hydrogen bonding, organic electrodes, or safety chemistry, this review offers a material perspective that bridges inorganic redox chemistry with NH₄ ⁺-ion dynamics.