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

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.