Facilitated Self-Adjusting Mechanism with Mn²⁺ Additive in Electrolyte for Ammonium-Ion Hybrid Supercapacitors

Ammonium-ion hybrid supercapacitors (AIHSCs) have gained extensive attention due to their high safety and environmental friendliness. Manganese oxides are among the most promising cathode materials; however, the side electrochemical reactions occurring in aqueous electrolytes limit their reversible capacities and energy densities. This work prepares the β-/γ-MnO₂ electrode and reveals the side electrochemical reactions occurring in the (NH₄)₂SO₄ electrolyte. Besides the widely recognized dissolution of MnO₂, the re-deposition of MnO₂ and irreversible insertion of NH₄ ⁺ exist simultaneously during cycling, resulting in irreversible structural changes of MnO₂. A portion of β-/γ-MnO₂ converts to δ-MnO₂, and a layer of 7Mn(OH)₂·2MnSO₄·H₂O forms on the electrode surface, modifying the ionic accessibility and structural stability of the electrode. The structural changes, along with the competition among the three types of side reactions, cause capacity decay and uprise during cycling. Accordingly, the self-adjusting mechanism is proposed, and trace Mn²⁺ is added to the electrolyte to facilitate this mechanism, thereby improving performance. Finally, the AIHSC, featuring the MnO₂ cathode and activated carbon anode in the Mn²⁺-added (NH₄)₂SO₄ electrolyte, shows 60.2 mAh g^-1 at 0.5 A g^-1 under 0-2 V. The maximum energy and power densities of 60.2 Wh kg^-1 and 5000 W kg^-1 are achieved.