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 ( C g ) 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 (NaMnO 2 ) in the aqueous electrolytes of 0.5 M Mg(NO 3 ) 2 (MNE) and 0.5 M Ca(NO 3 ) 2 (CNE) containing divalent cations of Mg 2+ and Ca 2+ ions. The C g values of NaMnO 2 (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 C g 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 ( I s ) of 10 A g –1, indicating its high-rate performance in the CNE electrolyte. The variation in the electrochemical performance of the NMO between Mg 2+ and Ca 2+ 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 C g 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 ( E d ) 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 Mg 2+ and Ca 2+ -based electrolytes, respectively, at a power density ( P d ) of 7 kW kg –1, thus demonstrating its high energy and high-power characteristics in an aqueous Ca(NO 3 ) 2 electrolyte.