Superior pseudo-capacitive performance of Co-free P2-Na_2/3Mn_2/3Ni_1/3O₂ for aqueous dual-ion hybrid supercapacitors

Recently, manganese oxides with pre-inserted cations are emerging charge storage materials for electrochemical supercapacitors, due to their higher gravimetric capacitance as well as better cycling stability. The pseudo-capacitance of manganese-based oxides are usually studied in aqueous electrolytes containing either monovalent (Na⁺) or divalent (Mg²⁺) cations. In this study, a mixed electrolyte of 0.5 M NaNO₃ + 0.25 M Mg(NO₃)₂ (SMME) is found to deliver a higher rate performance for hydrothermally synthesized Na_2/3Mn_2/3Ni_1/3O₂ compared to aqueous solutions of 0.5 M NaNO₃ (SNE) or 0.5 M Mg(NO₃)₂ (MNE), although MNE solution provides a higher gravimetric capacitance of 397.0 F g⁻¹ at 0.8 A g⁻¹ compared to 240.0 F g⁻¹ in SNE electrolyte. Interestingly, the specific capacitances of Na_2/3Mn_2/3Ni_1/3O₂ in SMME are found to be 341.0 and 160.0 F g⁻¹ at 0.8 and 10.0 A g⁻¹, respectively, thus providing a superior rate compared to that of 70.9 F g⁻¹ in SNE or 69.0 F g⁻¹ in MNE electrolyte when cycled at 10.0 A g⁻¹. An aqueous hybrid AC||Na_2/3Mn_2/3Ni_1/3O₂ supercapacitor is assembled, which exhibits a specific capacitance of 69.0 Fg_cell⁻¹ (based on both electrode masses), thus providing a specific energy of 39.0 Wh kg⁻¹ with excellent cycle-life (82.3 % retention of capacitance) for 8000 cycles.