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

Abstract 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‐MnO 2 electrode and reveals the side electrochemical reactions occurring in the (NH 4 ) 2 SO 4 electrolyte. Besides the widely recognized dissolution of MnO 2 , the re‐deposition of MnO 2 and irreversible insertion of NH 4 + exist simultaneously during cycling, resulting in irreversible structural changes of MnO 2 . A portion of β‐/γ‐MnO 2 converts to δ‐MnO 2 , and a layer of 7Mn(OH) 2 ·2MnSO 4 ·H 2 O 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 Mn 2+ is added to the electrolyte to facilitate this mechanism, thereby improving performance. Finally, the AIHSC, featuring the MnO 2 cathode and activated carbon anode in the Mn 2+ ‐added (NH 4 ) 2 SO 4 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.