Unveiling the Superior Rate Performance of Sodiated Manganese Oxides for Ca-Ion Hybrid Supercapacitors

Sodiated manganese oxides are promising electrode materials for hybrid supercapacitors, which possess higher gravimetric capacitance (Cg) in aqueous electrolytes containing divalent cations compared to that of monovalent cations. This study explores the electrochemical capacitive performances of hydrothermally synthesized sodiated manganese oxide (NaMnO2) in the aqueous electrolytes of 0.5 M Mg(NO3)2 (MNE) and 0.5 M Ca(NO3)2 (CNE) containing divalent cations of Mg2+ and Ca2+ ions. The Cg values of NaMnO2 (NMO) are found to be 432 F g–1 (583 mF cm–2) and 308 F g–1 (462 mF cm–2), respectively, in MNE and CNE electrolytes when cycled at 0.6 A g–1. Very impressively, the prepared NMO exhibits Cg values of 166 F g–1 (249 mF cm–2) and 115 F g–1 (155 mF cm–2) in CNE and MNE electrolytes, respectively, at a very high specific current (Is) of 10 A g–1, indicating its high-rate performance in the CNE electrolyte. The variation in the electrochemical performance of the NMO between Mg2+ and Ca2+ is correlated to the charge density, size of the hydrated ions, and desolvation energy. Furthermore, AC||NMO hybrid capacitors are fabricated, which deliver a higher Cg of 63.3 F g–1 in MNE compared to that of 54.0 F g–1 in the CNE electrolyte at 0.2 A g–1. The hybrid devices exhibit energy densities (Ed) of 35.0 Wh kg–1 and 30.6 Wh kg–1 at 0.2 kW kg–1, whereas they are found to be 4.2 and 12.4 Wh kg–1 in Mg2+ and Ca2+-based electrolytes, respectively, at a power density (Pd) of 7 kW kg–1, thus demonstrating its high energy and high-power characteristics in an aqueous Ca(NO3)2 electrolyte.