Enabling energy storage in aqueous ammonium-ion batteries: a review of advanced structural design strategies

Abstract

Aqueous ammonium-ion batteries (AAIBs) have emerged as promising sustainable energy storage systems, leveraging the unique advantages of NH₄⁺ as a non-metallic charge carrier. These advantages include low molar mass (18 g mol⁻¹), small hydrated radius (3.1 Å), tetrahedral coordination geometry, and rapid diffusion kinetics enabled by hydrogen bonding. These properties facilitate exceptional rate capability while offering inherent safety, environmental compatibility, and cost-effectiveness compared to resource-limited metal ions (Li⁺, Na⁺, Zn²⁺, etc.). This review comprehensively analyzes NH₄⁺ storage mechanisms across diverse electrode materials, with particular emphasis on structure–property relationships governing intercalation, surface coordination, and redox processes. Critical milestones in the field are contextualized, from Toshima's 1982 discovery of NH₄⁺ intercalation in Prussian blue analogues (PBAs) to Ji's 2017 demonstration of the first rocking-chair AAIB. Despite recent advances, key challenges persist, including limited reversible host materials (<60 mA h g⁻¹ in early PBAs), incomplete mechanistic understanding of NH₄⁺–host interactions, and insufficient cycling stability. We propose targeted materials design principles to overcome these limitations and critically assess future development pathways. This work establishes a framework for the rational design of high-capacity, durable NH₄⁺ storage systems to meet growing demands for sustainable electrochemical energy storage.