Electrolyte Engineering Challenges and Opportunities for Next-Generation Aqueous Ammonium-Ion Batteries.

Aqueous ammonium-ion batteries (AAIBs) have emerged as a promising post-lithium energy-storage technology, combining intrinsic safety with sustainable viability. However, unlike other aqueous-ion batteries, the engineering of AAIB electrolytes confronts a fundamental dilemma: their unique hydrogen-bond (HB)-rich environment, which enabling ultrafast NH4 + transport, concurrently exacerbates water decomposition and undermines electrolyte stability. In this review, we present the first comprehensive analysis of the dual role of HB networks in AAIB electrolytes and delineate their consequences for battery performance. We then systematically examine and evaluate the prevailing strategies devised to circumvent this dilemma, specifically anion engineering, salt-concentration tuning, pH regulation, organic co-solvent design, functional-additive introduction, and semi-solid electrolyte construction, while concurrently discussing their respective trade-offs and future outlook. Finally, we outline future directions involving coupled-reaction batteries, interfacial modeling, machine-learning-assisted electrolyte discovery, and sustainable ammonium sourcing. These insights provide a framework for balancing ion-transport kinetics and electrolyte stability in next-generation AAIBs.