Toward a Safer and Greener Future: Reliable Aqueous Ammonium-Ion Batteries with LiMnO₂ Cathodes

Abstract

Ammonium-ion-based energy storage systems have gained attention as a sustainable alternative for efficient charge storage. In this study, LiMnO₂ is explored for the first time as a cathode material in an aqueous ammonium-ion battery using a 2M (NH₄)₂SO₄ electrolyte. XRD analysis confirms the formation of phase-pure, highly crystalline orthorhombic LiMnO₂, while SEM imaging reveals a nanorod morphology that enhances ion transport. Cyclic voltammetry identifies two distinct charge storage mechanisms: NH₄⁺ insertion/extraction and surface-controlled redox reactions, with oxidation peaks at 0.89 V and 0.72 V vs. Ag/AgCl and reduction peaks at 0.53 V and 0.28 V vs. Ag/AgCl. Galvanostatic charge-discharge testing demonstrates an initial discharge capacity of ~60 mAh/g at 1C, stabilizing at ~50 mAh/g after the second cycle and maintaining excellent capacity retention over 130 cycles. The stable electrochemical performance suggests that LiMnO₂ undergoes minimal structural degradation, while the mildly acidic (NH₄)₂SO₄ electrolyte effectively mitigates Mn dissolution. Electrochemical impedance spectroscopy reveals a moderate increase in charge transfer resistance (Rct) from 135 Ω to ~200 Ω after cycling, indicating stable interfacial kinetics. The successful demonstration of LiMnO₂ as a hosting material in an aqueous ammonium-ion battery highlights its potential for next-generation energy storage applications.