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 (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 C_g 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 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 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 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 Mg²⁺ and Ca²⁺-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(NO₃)₂ electrolyte.