Exceptionally Highly Stable Cycling Performance and Facile Oxygen-Redox of Manganese-Based Cathode Materials for Rechargeable Sodium Batteries
Authors: Konarov Aishuak , Myung Seung-Taek 
Published: February 2020

Recent finding on oxygen redox is considered as a brand-new chemistry in rechargeable battery systems using monovalent charge carries such as lithium and sodium. This kind of redox species can generate extra electron in addition to transition-metal redox. This combination enables achievement of high capacity, so that this chemistry becomes more important to provide sustainable electrode materials for lithium- and sodium-ion batteries. These interests have been applied towards in sodium-ion batteries; however, most of the compounds are employing expensive transition metals such as Ru, Ir, etc. In this study we introduce a new system, P2-Na2/3[Mn0.7Zn0.3]O2 and its derivatives, of which a hybridized chemistry stemming from oxygen-redox and transition-metal redox that are highly reversible over cycles.
The composites were synthesized by combustion method. The aqueous solution of stoichiometric values of sodium nitrate (98%), manganese (II) nitrate hexahydrate (97%), and zinc (II) nitrate hexahydrate (98%) were added to aqueous citric acid solution (nitrates : citric acid, 1 : 0.5 in weight). The solution was heated on a hot plate at 100 °C for overnight under constant stirring to evaporate the solvent. Then dried powder was further heated to 200 °C for auto combustion of citric acid. Burnt powder further heated at 500 °C for 3 h to decompose the nitrates and yield a homogeneously mixed amorphous powder containing carbon residues. The obtained decomposition product was pelletized and heated in a tube furnace at 700 °C (heating rate – 5 °C / min) for 10 h in air atmosphere and then slowly cooled to room temperature. The obtained powder was transferred to Ar-filled glove box to avoid the contact with moisture in the air.
X-ray diffraction (XRD, Xpert, PANalytical) using Cu-Kα radiation was employed to characterize the crystal structure of the synthesized powders. XRD measurement was carried out in the 2θ range of 10−80° with a step size of 0.03°. The FULLPROF Rietveld program was used to analyze the observed powder diffraction patterns. Structural studies during cycle were examined by means of operando X-ray diffraction (XRD) and ex situ X-ray Absorption Spectroscopy (XAS). The ex situ XAS measurements were performed at the 7D beamline and 4D beamline of the Pohang Accelerator Laboratory (PAL), Pohang, South Korea, respectively.
Electrochemical properties were studied in an half-cell configuration assembling a R2032 coin-type cell using sodium metal as the negative electrode in an Ar-filled glove box. The electrolyte solution comprised 0.5 M NaPF6 in propylene carbonate and fluorinated ethylene carbonate (PC:FEC, 98:2 in volume). The cells were charged and discharged between 1.5 V and 4.6 V at a rate of 0.1C at 25 °C.
The calculated XRD pattern of the sample was obtained by Rietveld refinement using the FullProf software, as shown in Figure 1a. The result was well matched with the P2-type structure (Figure 1a) with P63/mmc space group, with an acceptable reliability factor (Rwp) of approximately 10%. The initial charge-discharge profiles of P2-Na2/3[Mn0.7Zn0.3]O2 electrode are presented in Figure 1b.
During initial charge, the electrode exhibited a long plateau above 4 V delivering a capacity of 120 mAh g−1. The average oxidation state of Mn for the electrode was 3.9+, such that the deliverable capacity is theoretically expected to be 26 mAh g−1 in the voltage range. However, the delivered capacity is over than theoretical value. The explanation for the unexpected capacity is the oxygen redox. So, at higher voltage the delivered capacity is driven by O2-/O1- redox and lower by Mn3+/Mn4+ redox. More detailed experimental and theoretical studies will be presented at the meeting.

Figure 1. (a) Rietveld refinement and crystal structure of P2-Na2/3[Mn0.7Zn0.3]O2; (b) Initial charge-discharge profiles of P2-Na2/3[Mn0.7Zn0.3]O2 electrode.

Figure 1