NaMnO_2−xthin nanosheets assembled microspheres as electrode for aqueous asymmetric supercapacitor

Abstract

This study proposes MnO 2 thin nanosheets assembled microspheres with oxygen vacancies and preinsertion of Na ions (NaMnO 2−x ) as highly capable supercapacitor electrodes. The NaMnO 2−x electrode can reach a stable potential window of 1.2 V without oxygen evolution reactions in threeelectrode configuration. The voltage window of the assembled aqueous ASC device can be expanded to 2.4 V (1 M Na 2 SO 4 ) by using NaMnO 2−x electrode and activated carbon electrode as positive and negative electrodes, respectively. The NaMnO 2−x electrode delivers a good specific capacitance of 215 F·g −1 at current density of 1 A·g −1 . It displays high-rate capability and an excellent cycling stability, maintaining 95.5% of its initial specific capacitance at 2 A·g −1 after 2500 cycles. The ASC device shows a high energy density and power density of 28.56 W·h·kg −1 and 1246 W·kg −1 , respectively, at a current density of 1 A·g −1 . OPEN ACCESS RECEIVED

Understanding the NaxMnO2 System: A Thermodynamics and XPS Approach

“…It can be noticed that upon incorporation of Na into the MnO 2 lattice, the Mn-2p peaks could be deconvoluted into two peaks with binding energies of 641.4 and 639.4 eV (for the Mn 2p 3/2 ). The latter represents the Mn 3+ state in Na x MnO 2 , as seen in the NaMnO 2 structure by Xu et al and Cox et al , The Mn 3+ peak grew as a function of the Na content in Na x MnO 2 . The qualitative concentrations of Mn 3+ and Mn 4+ in Na x MnO 2 determined from the area ratios of the deconvoluted peaks of Mn 2p 3/2 using CASA-software with GL(30)-fitting is shown in Table .…”

Electrochemical capacitance properties of pre-sodiated manganese oxide for aqueous Na-ion supercapacitors

“…An energy density of 20.9 W h kg −1 is obtained for this asymmetric supercapacitor, which is comparable with the reported energy density of 8-20 W h kg −1 for MnO 2 and NaMnO 2 -based asymmetric supercapacitors. 36,48,63,65 It should be noted that symmetric capacitors based on AC‖AC can provide an energy density of 5-6 W h kg −1 in aqueous electrolytes, because of low voltage. Thus, the asymmetric supercapacitor based on an aqueous electrolyte exhibits a higher energy density than the symmetric AC supercapacitors.…”

“…However, asymmetric supercapacitors employing two different types of electrodes can be operated with a higher voltage, thus increasing their energy density as compared to symmetric supercapacitors. Among various asymmetric supercapacitors, MnO 2 ‖AC, [43][44][45][46][47] NaMnO 2 ‖AC, [34][35][36]48 and NiCoO 2 ‖AC [49][50][51][52] are reported to exhibit energy density of 10-30 W h kg −1 . Asymmetric supercapacitors based on MnO 2 ‖AC use mostly amorphous MnO 2 , which does not exhibit good cycling performance.…”