P2- and P3-Type Na x Cr x Ti1-X O2 Layered Oxides As Bi-Functional Electrode Materials for Rechargeable Sodium Batteries

Abstract

Rechargeable Na batteries are promising to realize sustainable energy development in the future because of the material abundance, and many electrode materials have been actively researched in the world. O3-type NaCrO2 1, 2 is known to show excellent cycle performance and thermal stability3. Additionally, O3-NaCrO2 shows second highest operating voltage among O3-type layered oxides, next to O3-NaFeO2. In this study, to increase of the operating voltage of Cr3+/Cr4+ redox by inductive effect, Ti4+ is substituted for Cr3+, according to the formula of Na x Cr x Ti1-x O2 (0.5 ≤ x ≤ 1) and crystal structures and electrode performance are systematically examined as positive and negative electrode materials for rechargeable Na batteries.Na x Cr x Ti1-x O2 (0.5 ≤ x ≤ 1) layered oxides were synthesized by a solid-state reaction from stoichiometric amounts of 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.). These starting materials were mixed using a mortar and pestle, and then pressed into pellets. The pellets were heated at 800 – 1000 oC in Ar. Samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy, (SEM) and X-ray absorption spectroscopy (XAS). Electrochemical properties were examined by galvanostatic charge/discharge test.After the survey on the Na-Cr-Ti-O ternary oxide system, three different phases were isolated in the range of 0.5 ≤ x < 1. For the sample of x = 0.80 at 950 oC, Na-deficient O3 phase, Na0.8Cr0.8Ti0.2O2, is found. For the sample of x = 0.67 at 1000 oC, P2-type phase, Na2/3Cr2/3Ti1/3O2, is observed. Similar to our work, P2-Na0.6Cr0.6Ti0.4O2 is also found in the literature.[4] Additionally, for the sample of x = 0.58 at 800 oC, Bragg diffraction lines of the sample, Na0.58Cr0.58Ti0.42O2, were assigned into P3-type layered structure. These samples with different layered stacking manners are used as both positive and negative electrodes. Especially, P2-Na2/3Cr2/3Ti1/3O2 as the positive electrode and P3-Na0.58Cr0.58Ti0.42O2 as the negative electrode shows excellent cyclability as shown in Figure 1a, b for continuous 100 cycle tests. Moreover, operating voltage of the P2 phase is much higher than that of O3-NaCrO2. In addition, both samples show excellent rate capability as shown in Figure 1c, d. Large reversible capacities are observed for both samples even at >3,000 mA g-1.From these results, we will further discuss the impact of Ti4+-substitution for Cr3+on electrode performance and reaction mechanisms in Na cells for more details. AcknowledgementsThis 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.