Structural and Electrochemical Studies on NaFe1-X
Co
x
O2 for Na-Ion Batteries
Authors: Kubota Kei , Yoshida Hiroaki , Yabuuchi Naoaki , Shiiba Hiromasa , Nakayama Masanobu , Komaba Shinichi 
Published: February 2020

Introduction

Layered Na
x
MeO2 (Me = transition metals) compounds have been intensively studied as electrode materials for Na-ion batteries. Among the Na insertion materials, O3-type (alpha-type) NaFeO2 is a promising candidate as a positive electrode material for rechargeable Na batteries due to the electrode performance and material abundance.1, 2 The electrochemical activity of O3-NaFeO2 as a Na intercalation host was reported by Okada and coworkers.3 A Na//NaFeO2 cell shows a flat voltage profile at 3.3 V vs. Na metal associated with Fe3+/Fe4+ redox. Electrode reversibility is, however, significantly deteriorated by extending higher voltage above 3.5 V and the reversible capacity range is limited to be narrow. Our group recently reported that NaFe0.5Co0.5O2 exhibits large reversible capacity and excellent rate-capability in Na cells.4 The electrode performance of NaFe0.5Co0.5O2 is better than those of end-members, NaFeO2 and NaCoO2. In this study, we synthesize solid solution samples between NaFeO2 and NaCoO2, NaFe
x
Co1-x
O2 (0 < x < 1.0). Its electrode performance in Na cells and reaction mechanism are examined.

Experimental

Solid solution samples between NaFeO2 and NaCoO2 were synthesized by solid-state reaction. Na2O2 or NaCO3, Fe3O4, and Co3O4 were mixed by mechanical milling, pelletized and then heated at 650 – 900 °C for 12 h in air. Crystal structures and morphology of the samples were examined by X-ray diffraction (XRD) measurement and scanning electron microscopy (SEM). Galvanostatic charge/discharge tests were conducted using coin-type cells. Positive electrodes consisted of the active material, acetylene black and polyvinylidenefluoride (PVdF) with a gravimetric ratio of 80:10:10. Metallic sodium was used as a counter electrode. The electrolyte used was 1.0 mol dm-3 NaClO4 / PC : FEC (98 : 2 by volume). Phase stability between O3-type and P3-type structures in layered NaFe0.5Co0.5O2 is compared by using first-principles DFT calculations.

Results and Discussion

Figure 1 shows the XRD patterns of NaFe
x
Co1-x
O2 (x = 0.0, 0.4, 0.5, 0.6, and 1.0). All of Bragg diffraction lines of the samples are indexed based on the O3-type layered oxide with a space group R-3m without any diffraction lines from impurity phases. As the value of x decreases in NaFe
x
Co1-x
O2, the position of the diffraction lines systematically shifts to higher diffraction angles, indicating the formation of the solid solution between NaFeO2 and NaCoO2. Initial charge/discharge curves for the solid solution samples of Na1-x
Fe
y
Co1-y
O2 (y = 0.4, 0.5, and 0.6) are shown in Fig. 2. All the sample electrodes exhibit almost the same discharge capacity of ca. 170 mAh g-1. However, potential hysteresis increased with increase in the amount of Fe. Analysis on XAS spectra for NaFe0.5Co0.5O2 revealed oxidation of Co and Fe ions during charge at low and high voltage region, respectively. Redox reaction of Fe ions is expected to be associated with the hysteresis. Furthermore, ex-situ XRD results for NaFe0.5Co0.5O2 indicated phase transitions from O3-type to P3-type and then to P’3-type phase, which is in good agreement with the phase stability expected from the first-principles calculation. From these results, we will further discuss charge/discharge mechanisms of solid solution samples as electrode materials for Na-ion batteries.

References

[1] J. Zhao, L. W. Zhao, N. Dimov, S. Okada and T. Nishida, J. Electrochem. Soc., 160 (2013) A3077.
[2] N. Yabuuchi, H. Yoshida and S. Komaba, Electrochemistry, 80 (2012) 716.
[3] S. Okada, Y. Takahashi, T. Kiyabu, T. Doi, J.-I. Yamaki and T. Nishida, Layered Transition Metal Oxides as Cathodes for Sodium Secondary Battery, in 210th ECS Meeting, p. 201, The Electrochemical Society, Cancun, Mexico (2006).
[4] H. Yoshida, N. Yabuuchi and S. Komaba, Electrochem. Commun., 34 (2013) 60.