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

The pursuit of cost-effective, high-performance, and eco-friendly energy storage solutions has driven increasing interest in aqueous ammonium-ion batteries. These systems provide enhanced safety, sustainability, and affordability, attributed to the low molar mass and small hydrated ionic radius of ammonium ions. However, identifying a cathode material capable of reversible ammonium-ion storage in aqueous electrolytes remains a key challenge. This study explores lithium manganese oxide (LiMn₂O₄) as a promising cathode material for ammonium-ion batteries. The spinel LiMn₂O₄ structure, known for its cubic symmetry and interconnected 3D ion-diffusion channels, ensures efficient charge transport and robust electrochemical performance. Additionally, its low-cost raw materials and environmental advantages make it an attractive alternative to conventional transition metal oxides. With a theoretical capacity of ~148 mAh g⁻¹, LiMn₂O₄ exhibits substantial specific capacity, contributing to improved battery energy density. The material was synthesized via a high-temperature solid-state reaction, and X-ray diffraction (XRD) confirmed the formation of a stable orthorhombic structure. 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 MnO₂-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.