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

## TL;DR

This paper reviews how transition metal compounds can improve aqueous ammonium-ion batteries by enabling safer and more efficient energy storage.

## Contribution

The paper provides a novel material-focused review of transition metal compounds for ammonium-ion batteries, bridging inorganic redox chemistry with ion dynamics.

## Key findings

- Transition metal compounds offer tunable structures and redox activity for improved ammonium-ion storage.
- Hydrogen-bonding dynamics and ion transport mechanisms are critical for battery performance.
- Structural instability and slow kinetics remain key challenges for practical implementation.

## Abstract

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.

Aqueous ammonium‐ion batteries leverage hydrogen‐bond‐mediated NH4
+ storage in tunable transition metal compounds. Despite progress in Mn‐, V‐, Mo‐, and W‐based compounds, 2D LDHs, and MXenes, challenges like structural instability and slow kinetics persist. Future advances require robust host design, mechanistic understanding via operando studies, and innovative full‐cell architectures for practical, high‐performance AAIBs.