The demand for large-scale batteries for energy storage is rapidly increasing, which potentially provides sustainable energy development. In the past few years, research interest on rechargeable sodium batteries is completely renewed for this purpose.[1] Chromium is known as a relatively abundant element in the Earth’s crust as transition metals. Additionally, among O3-type layered oxides, NaCrO₂ [2, 3] shows second highest electrochemical potential next to NaFeO₂.[1] NaCrO₂ delivers a reversible capacity of 120 mAh g^-1, and shows flat operating voltage at 3.0 V vs. Na based on a redox couple of Cr³⁺/Cr⁴⁺.[3]  Electrochemical potential of Cr³⁺/Cr⁴⁺ redox would be increased through an inductive effect, and the use of Ti⁴⁺ is targeted in this study. Recently, P2-type Na_0.6Cr_0.6Ti_0.4O₂ has been reported as a bi-functional electrode material, which is used for both positive and negative electrodes. Moreover, P2-type Na_0.6Cr_0.6Ti_0.4O₂ shows high operating voltage than that of O3-type NaCrO₂.[4]

In this study, crystal structures and electrode performance in a binary system between NaCrO₂ and TiO₂ (Na_xCr_xTi_1-xO₂) are systematically examined. Na_xCr_xTi_1-xO₂ samples were prepared from Na₂CO₃ (99.5 %, Wako Pure Chemical Industries, Ltd.), Cr₂O₃ (98 %, Wako Pure Chemical Industries, Ltd.), and anatase-type TiO₂ (98.5 %, Wako Pure Chemical Industries, Ltd.). Composite electrodes consisted of 80 wt% active materials, 10 wt% acetylene black, and 10 wt% poly(vinylidene fluoride), pasted on aluminum foil as a current collector. Metallic sodium was used as a negative electrode. The electrolyte solution used was 1.0 mol·dm⁻³ NaPF₆ dissolved in propylene carbonate (Kishida Chemical). A glass filter (GB-100R, Advantec) was used as a separator. Two-electrode cells (TJ-AC, Tomcell Japan) were assembled in the Ar-filled glove box. The cells were cycled at a rate of 10 mA g^-1 at room temperature.

Figure 1 shows X-ray diffraction patterns of different Na_xCr_xTi_1-xO₂ samples. By adjusting the chemical compositions and calcination temperatures, different phases, O3, Na-deficient O3, P2, P3 phases, are obtained. Electrochemical properties of the samples in Na cells are shown in Fig. 2. The cells were cycled in both a positive electrode side above 2.5 V and a negative electrode side below 2.0 V. O3-type NaCrO₂ is only used as a positive electrode, and no reversible capacity is observed below 2.0 V because of the absence of vacancies in sodium layers and accessible redox of Ti³⁺/Ti⁴⁺. Small reversible capacity (less than 10 mAh g^-1) originates from the sodium storage by acetylene black. As increase in the titanium contents (and vacancies of sodium sites), P2 and P3 phases become energetically stable phase, instead of the O3 phase.  Among four different phases, P2-type Na_2/3Cr_2/3Ti_1/3O₂ delivers a reversible capacity of ca. 80 mAh g^-1 as a positive electrode with relatively high operating voltage. This sample also delivers a reversible capacity of ca. 90 mAh g^-1 as a negative electrode, and used as bi-functional electrodes.  P3-type Na_0.58Cr_0.58Ti_0.52O₂ delivers a reversible capacity of ca. 110 mAh g^-1 as a negative electrode in a Na cell even though reversible capacity as a positive electrode is inevitably reduced.

              Together with these results, we will further discuss crystal structures, electrode performance, and reaction mechanisms of Na_xCr_xTi_1-xO₂ samples as bi-functional electrode materials for rechargeable sodium batteries operable at room temperature.

Acknowledgements

This study was in part granted by MEXT program “Elements Strategy Initiative to Form Core Research Center”, MEXT; Ministry of Education Culture, Sports, Science and Technology, Japan.

References

[1] N Yabuuchi and S Komaba, Science and Technology of Advanced Materials, 15, 043501 (2014).

[2] J. J. Braconnier, C. Delmas, P. Hagenmuller, Materials Research Bulletin, 17, 993 (1982).

[3] S. Komaba et al., Electrochemistry Communications, 12, 355 (2010).

[4] Y. Wang, R. Xiao, Y.-S. Hu, M. Avdeev, L Chen, Nature Communications, 6, 6954 (2015).