First-Ever Use of LiMn₂O₄ Cathode in State-of-the-Art Ammonium-Ion Batteries: Unlocking a New Ametal Charge Carrier

Growing demand for sustainable, economical, and high-efficiency energy storage has intensified research into aqueous ammonium-ion battery systems. These electrochemical storage devices offer inherent safety benefits, environmental compatibility, and cost advantages stemming from ammonium ions' favorable characteristics - their lightweight nature (18 gmol-1) and minimal solvated dimensions (~1.25 Å). A critical research gap exists in developing cathode materials that can stably cycle NH4+ ions in water-based electrolytes. Our investigation focuses on spinel-phase lithium manganese oxide (LiMn2O4) as a potential NH4+ host material, leveraging its distinctive structural advantages. The material's face-centered cubic framework with three-dimensional lithium diffusion pathways enables rapid ionic conduction and stable electrochemical behavior. Beyond its performance merits, LiMn2O4 stands out for its economical precursor materials and reduced environmental impact compared to standard cathode compounds. The material's theoretical energy storage capability reaches approximately 148 mAh g−1, indicating significant potential for energy-dense battery designs. We employed solid-state synthesis at elevated temperatures to produce the material, with XRD analysis verifying the development of an orthorhombic crystalline phase. Electrochemical analysis using cyclic voltammetry indicated a two-step lithium extraction process in ammonium-ion electrolytes. As cycling progressed, redox peaks associated with ammonium-ion insertion and extraction became more defined, highlighting the material's capability for efficient and reversible charge storage. Galvanostatic charge-discharge tests revealed that the LiMnO2-based electrode delivered a stable specific capacity of approximately 47 mAh g−1 during NH4+ intercalation/de-intercalation. The study demonstrates that LiMn2O4 effectively supports ammonium-ion storage, offering a sustainable and high-performance cathode option for next-generation aqueous batteries. These findings provide crucial insights into the material’s electrochemical behavior and potential for advancing ammonium-ion battery technology.