Cathode Properties of Perovskite-Type Strontium Ferrite for Magnesium Rechargeable Batteries
Authors: Kubota Yuki , Hibino Mitsuhiro , Ogasawara Yoshiyuki , Yamaguchi Kazuya , Mizuno Noritaka 
Published: February 2020

Introduction
Development of high performance electrical energy storage devices has been in great demand for solving environmental and energy issues. Metallic magnesium can potentially deliver high volumetric capacity and operational safety when it is used as an anode material. At the same time, magnesium is abundant in natural resources and inexpensive. Therefore, it is a good candidate for an anode material of rechargeable batteries. However, divalent Mg2+ ions diffuse in an intercalation cathode much more slowly than Li+ ions because of their stronger electrostatic interaction. It has been reported that some perovskite-type oxides allowing electrochemical extraction and reinsertion of oxide ions can be used as cathode materials of lithium ion batteries and sodium ion ones[1, 2]. By combining such kind of oxide and Mg as a cathode and an anode, respectively, Mg battery not restricted by the low diffusivity of Mg2+ ions in a solid is expected to be attained. In this study, charge-discharge properties of a perovskite-type SrFeO3cathode in a Mg anode battery were investigated.
Experimental
SrFeO3 was obtained by oxidizing SrFeO2.5 with NaClO4. The SrFeO2.5 was synthesized by a solid state method. A SrFeO3 powder, which was wet-pulverized beforehand, acetylene black as a conducting additive, and polytetrafluoroethylene as a binder were mixed at a weight ratio of 100:40:5, and pressed on a Pt mesh to serve as a cathode. Well-polished Mg ribbons and a 1.5 M triglyme solution of Mg(ClO4)2were used as an anode and an electrolyte, respectively. Discharge and charge tests were conducted in constant current modes. Various chemical and spectroscopic analyses were carried out ex-situ for cathodes taken from cells discharged or charged to various depths
Results and Discussion
The voltage curves of SrFeO3 during discharge and charge at a current density of 1.4 mA/g at 80oC are shown in Fig. 1. The voltage gradually decreases from 2.1 V in the earlier period of discharge, reaches a plateau of 1.9 V and then drops. The specific capacity is 59 mAh/g, corresponding to a charge transfer of 0.42 electrons per SrFeO3. In the following charge, a considerable polarization is observed. No plateau is observed and the voltage changing rate during charge is higher than that during discharge. Figure 2 shows the XRD profiles of the cathodes before and after discharge, and after the following charge. The peaks of SrFeO3 disappear and those attributed to a brownmillerite-type SrFeO2.5 appear in the profile after discharge. The conversion from SrFeO3 to SrFeO2.5with 0.42 electrons implies that the following reaction proceeded during discharge:
SrFeO3 + 1/4 Mg2+ + 1/2 e- → SrFeO2.5 + 1/4 MgO2. (1)
MgO2, however, was not detected by XRD. This might be due to its presence in amorphous, nano-sized, and/or quite poorly crystallinity state. It is possible that the formed MgO2 reacted with an electrolyte solution. On the other hand, the peaks of SrFeO2.5 disappear and the peaks similar to those of SrFeO3 but with shifted positions appear in the XRD profile after charge. While SrFeO
x
crystallizes in cubic perovskite-type structure at x = 3, it does in tetragonal and orthorhombic ones at x = 2.875 and 2.75, respectively [3]. Since the lattice distortion of tetragonal and orthorhombic SrFeO
x
from cubic symmetry is considerably small, a SrFeO
x
sample with low crystallinity shows a XRD pattern exactly similar to that with cubic symmetry. Since the cell parameter calculated from XRD peaks indexed with a pseudo-cubic cell has a linear relation with x averaged in a sample, the charged sample was analyzed on the basis of a pseudo cubic structure to estimate x. As a result, the calculated cell parameter was 0.3865(6) nm and x was determined to be 2.77(5). This indicates that during charge SrFeO2.5 underwent reinsertion of oxide ions to be SrFeO2.77, though it could not completely return to SrFeO3.
From the above, although SrFeO3 might be insufficient, it was revealed that an oxide allowing electrochemical extraction and reinsertion of oxide ions has a potential for a cathode material of Mg anode batteries.
Acknowledgment
This work was conducted with the support of JSPS Grants-in-Aid for Scientific Researches (B) Grant Number 26289371.
References
[1] Y. Ogasawara et al., J. Electrochem. Soc.
2014, 161, A792-A797.
[2] M. Hibino et al., J. Am. Chem. Soc.
2014, 136, 488-494.
[3] J. P. Hodges et al., J. Solid State Chem.
2000, 151, 190-209.

Figure 1