Structural and Electrochemical Studies on NaFe1-XCoxO2 for Na-Ion Batteries

Introduction
  Layered Na_xMeO₂ (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 NaFe_0.5Co_0.5O₂ exhibits large reversible capacity and excellent rate-capability in Na cells.⁴ The electrode performance of NaFe_0.5Co_0.5O₂ is better than those of end-members, NaFeO₂ and NaCoO₂. In this study, we synthesize solid solution samples between NaFeO₂ and NaCoO₂, NaFe_xCo_1-xO₂ (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 NaFe_0.5Co_0.5O₂ is compared by using first-principles DFT calculations.

Results and Discussion
  Figure 1 shows the XRD patterns of NaFe_xCo_1-xO₂ (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_xCo_1-xO₂, 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 Na_1-xFe_yCo_1-yO₂ (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 NaFe_0.5Co_0.5O₂ 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 NaFe_0.5Co_0.5O₂ 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.