Electrochemical Properties of Composites of Sodium Peroxide(Na2O2) and Transition Metal Oxide (MnOx) As a Positive Electrode Material for the Sodium Ion Battery
Authors: Kataoka Riki , Kiyobayashi Tetsu , Takeichi Nobuhiko 
Published: February 2020

Introduction

Sodium ion batteries have attracted reserchers’ attention due to the abundance of Na resources.
Sodium containing metal oxides, phosphates, etc., which are used as the positive electrode material, are usually prepared by the heat treatment. Recently, FeOF has been successfully synthesized without any heat treatment, but by an electrochemical conversion reaction of the NaF/FeO composite, and proved to work as a host structure of Na ion(1).
Sodium oxide would also react with transition metal oxides in the same mechanism to form a composite which would work as the positive electrode material of the sodium ion battery.
In this study, we prepared Na2O2 / MOx (M : transition metal, x= 1~1.5) composites by the mechanical milling and investigated their electrochemical performance. In addition, the structural change during the charge and discharge process were also investigated. The performance of a full cell, in which the composite was combined with a hard carbon electrode, was also investigated.

Experimental

The composites of sodium peroxide and various manganese oxides were prepared by mechanically milling in a planetary mixer at 500 rpm for 2 ~ 20 hours in an Ar atmosphere. The electrochemical properties of the samples were evaluated by assembling the two-electrode setup in an IEC R2032 coin-type cell. The working electrode was prepared by mixing the composite, Ketjen black as the conductive additive and polytetrafluoroethylene (PTFE) as the binder in the weight ratio of 84 : 8 : 8. A metallic sodium foil was used as the counter electrode and 1 M NaPF6 / EC-DEC was used as the electrolyte. The constituent phases in the samples were identified by X-ray diffraction.

Results and discussion

Figure 1 shows the XRD patterns of the Na2O2/Mn3O4 composite with molar ratio of (a) 1 : 1 (Sample A), (b) 1.5 : 1 (SampleB) and Mn3O4 precursor. The Mn3O4 phase was detected in the both samples, but Na2O2 phase was not detected. The peak width of Mn3O4 phase in the both samples broadens compared with the Mn3O4 precursor and no peak shift was observed. We consider that Na2O2 and Mn3O4 did not react with each other during the milling process.
Figure 2 shows the charge and discharge profiles of the composite (a) Sample A and (b) Sample B. The initial charge capacity of Sample B is 210 mAhg-1, which is higher than that of SampleA; i.e., 174 mAhg-1, probably due to the initial excess Na content. On the other hand, the subsequent discharge capacity of Sample A is higher value of 194 mAhg-1 than that of Sample B, 210 mAhg-1, since Sample A contains more redox active manganese ions. The discharge capacities after 20 cycles remain 88% (Sample A) and 85% (Sample B) of their initial discharge capacities.
The electrochemical performance of the full cell performance using the hard carbon as the negative electrode and the structural change of the composite during the charge and discharge process will be discussed in the presentation.

Reference

1. A. Kitajo et al., 54th Battery Symp. Japan, 2013 3D20.

Figure 1