A mixed dual-ion electrolyte for high-rate performance and remarkable cycling stability of NaMnO2 in hybrid supercapacitors

Abstract

Recently, pre-sodiated manganese oxides are gaining more attention due to their higher specific capacitance and better cycling stability compared to pristine manganese oxides. However, previous reports on their electrochemical performances are limited to the aqueous electrolytes of Na+ and K+ ions. In this study, the electrochemical capacitance properties of hydrothermally synthesized NaMnO2 are investigated in aqueous electrolytes of 0.5 M NaNO3, 0.5 M Mg(NO3)2, and the mixed dual-ion electrolyte of 0.5 M NaNO3 and 0.25 M Mg(NO3)2. Although 0.5 M Mg(NO3)2 enables a higher specific capacitance of 427 F g−1 compared to that of 267 F g−1 in NaNO3 electrolyte, both electrolytes provide limited rate capability. On the other hand, the mixed dual-ion electrolyte of 0.5 M NaNO3+0.25 M Mg(NO3)2 enables highly improved rate capability along with a high specific capacitance of 325 F g−1, signifying the importance of the mixed dual-ion electrolyte for optimizing the specific capacitance and the high-rate capability of NaMnO2. An aqueous hybrid supercapacitor of activated carbon||NaMnO2 demonstrates excellent cycling stability of 98.9 % capacitance retention in the mixed electrolyte as compared to 96.9 % in NaNO3 electrolyte after 6000 cycles.