The development of an energy storage system with abundant elements is a key challenge for a sustainable society, and the interest of Na intercalation chemistry is extending throughout the research community. Herein, the impact of Ti integration into NaMnO2 in a binary system of x NaMnO2–(1–x) TiO2 (0.5≤x≤1) is systematically examined for rechargeable Na battery applications. Stoichiometric NaMnO2, which is classified as an in-plane distorted O′3-type layered structure, delivers a large initial discharge capacity of approximately 200 mAh g-1, but insufficient capacity retention is observed, most probably associated with dissolution of Mn ions on electrochemical cycles. Ti-substituted samples show highly improved electrode performance as electrode materials. However, the appearance of a sodium-deficient phase, Na4Mn4Ti5O18 with a tunnel-type structure, is observed for Ti-rich phases. Among the samples in this binary system, Na0.8Mn0.8Ti0.2O2 (x=0.8), which is a mixture of a partially Ti-substituted O′3-type layered oxide (Na0.88Mn0.88Ti0.12O2) and tunnel-type Na4Mn4Ti5O18 as a minor phase elucidated by Rietveld analysis on both neutron and X-ray diffraction patterns, shows good electrode performance on the basis of energy density and cyclability. Both phases are electrochemically active as evidenced by in situ X-ray diffraction study, and the improvement of reversibility originates from the suppression of Mn dissolution on electrochemical cycles. From these results, the feasibility of Mn-based electrode materials for high-energy rechargeable Na batteries made from only abundant elements is discussed in detail.

To realize the development of rechargeable sodium batteries, new positive electrode materials without less abundant elements are explored. Enrichment of sodium contents in host structures is required to increase the theoretical capacity as electrode materials, and therefore Na‐excess compounds are systematically examined in a binary system of Na2TiO3–NaMnO2. After several trials, synthesis of Na‐excess compounds with a cation disordered rocksalt structure is successful by adapting a mechanical milling method. Among the tested electrode materials, Na1.14Mn0.57Ti0.29O2 in this binary system delivers a large reversible capacity of ≈200 mA h g−1, originating from reversible redox reactions of cationic Mn3+/Mn4+ and anionic O2−/On− redox confirmed by X‐ray absorption spectroscopy. Holes in oxygen 2p orbitals, which are formed by electrochemical oxidation, are energetically stabilized by electron donation from Mn ions. Moreover, reversibility of anionic redox is significantly improved compared with a former study on a binary system of Na3NbO3–NaMnO2 tested as model electrode materials.

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LiCoO2 is generally used as a positive electrode material for lithium ion batteries. However, the abundance of cobalt in the Earth’s crust is limited to be less than 100 ppm, and therefore cobalt is expensive element. Iron-based electrode materials would be attractive as positive electrode materials for battery applications because iron is the most abundant transition metal ion. However, as a counterpart of LiCoO2, layered LiFeO2 is electrochemically inactive.[1] In contrast, layered NaFeO2 is electrochemically active, but cyclability is not enough associated with the irreversible structural change upon charge.[2, 3] In this study, we examine different factors affecting the electrode properties of NaFeO2 and discuss possibility as the positive electrode material for sodium ion batteries. O3-type NaFeO2 was prepared by solid-state reaction from stoichiometric amount of Na2O2 and Fe2O3 as described by Takeda et al.[4] These starting materials were mixed using a mortar and pestle in an argon-filled globe box, and then pressed into a pellet. The pellet was heated at 650 oC for 12 h in O2. After the sample was prepared, the sample was mixed with acetylene black (NaFeO2: acetylene black = 90: 10 wt%) by using the planetary ball mill. Figure 1 shows the XRD patterns and SEM images of O3-type NaFeO2 before and after mixing with acetylene black (denoted as as-prepared and carbon-composite NaFeO2, respectively). The as-prepared sample crystallizes into an O3-type layered structure as previously reported, and, from the XRD patterns, no change in the structure is found after milling with carbon. However, the particle size of the sample is clearly reduced by milling with carbon. Charge/discharge curves of O3-type NaFeO2 before and after milling with carbon are compared in Fig 2. A voltage profile of the as-prepared sample with a well-defined voltage plateau at 3.3 V is similar to that of the previous reports, and capacity degradation on electrochemical cycles is clearly observed. In contrast, carbon-composited NaFeO2 delivers a reversible capacity of over 100 mA h g-1 with much improved capacity retention. From these results, we will discuss factors affecting electrode reversibility of O3-type NaFeO2 and possibility of rechargeable batteries made from only abundant elements. References [1] K. Ado et al., J. Electrochem. Soc., 144, L177 (1997). [2] S. Okada et al., ESC Meeting Abstract, 602, 201 (2006). [3] N. Yabuuchi et al., Electrochemistry, 80, 716 (2012). [4] Y. Takeda et al., Mater. Res. Bull., 15, 1167 (1980). Figure 1