Advanced ammonium salt materials for electrochemical energy storage: Recent progress and future perspectives

• A comprehensive and thorough review about emerging ammonium salt materials for energy storage applications was given. • Diverse modification strategies for ammonium salt materials as ZIB electrode were outlined. • Advanced characterization techniques and theoretical calculation methods towards structural evolution were presented. • The future challenges and directions of emerging ammonium salt materials were summarized. The development of new high-performance materials is essential for robust electrochemical energy storage (EES). In recent years, ammonium salt materials, as an emerging class of layered materials, have attracted considerable attention as electrode materials for EES because of their abundance resource, simple synthesis, low cost, and high specific capacity. This review aims to comprehensively summarize the recent progress of ammonium salt materials in EES field. Firstly, the crystal structures and preparation methods of typical ammonium salts including ammonium vanadate, ammonium metal phosphate, and ammonium metal molybdate are discussed. Subsequently, their application progress in supercapacitors (SCs) and various metal-ion batteries including monovalent alkali ion (Li + , Na + , and K + ) and ammonium ion (NH 4 + ) batteries, zinc ion (Zn 2+ ) batteries, and multivalent alkali-earth ion (Mg 2+ and Ca 2+ ) batteries is thoroughly introduced. Likewise, the structure-activity relationships between the layered structure of ammonium salts and electrochemical performance are clarified. In particular, diverse modification strategies including composites, defect and doping engineering etc. for ammonium salt materials as Zn-ion battery electrode were presented. Thereafter, advanced characterization techniques together with theoretical calculations are expounded to further explain the internal structure evolution and reaction mechanism of ammonium salt materials. Finally, a short conclusion and outlook, along with the current challenges and future opportunities of ammonium salts for EES, are proposed.