Layered Na x Cr x Ti1-X O2 As Bi-Functional Electrode Materials for Rechargeable Sodium Batteries

Abstract

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 Na0.6Cr0.6Ti0.4O2 has been reported as a bi-functional electrode material, which is used for both positive and negative electrodes. Moreover, P2-type Na0.6Cr0.6Ti0.4O2 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 x Cr x Ti1-x O2) are systematically examined. Na x Cr x Ti1-x O2 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 x Cr x Ti1-x O2 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 Na2/3Cr2/3Ti1/3O2 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 Na0.58Cr0.58Ti0.52O2 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 x Cr x Ti1-x O2 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.