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the characteristics of each transition metal in O3-NaMO₂ and P2-Na_2/3MO₂, single 3d transition metal O3 and P2 systems are reviewed before each of the multiple transition metal systems.

1.3. O3-type layered materials

      1.3.1. NaMO₂ (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni)

O3-type layered sodium 3d transition metal oxides (including O’3-type ones) were discovered in 1930s–1975s (Goldsztaub 1935; Scholder and Kyri 1952; Dyer et al. 1954; Rüdorff and Becker 1954; Andersson and Wadsley 1962; Hoppe et al. 1965; Jansen and Hoppe 1972) and electrode performances of Ti, V, Cr, Mn, Co and Ni systems in Na cells were first reported by Delmas et al. in 1980–1985 (Braconnier et al. 1980; Braconnier et al. 1982; Delmas et al. 1982; Maazaz et al. 1983; Mendiboure et al. 1985). These oxides are electrochemically active and often evaluated with Na-metal half-cells consisting of the oxide as a positive electrode and metallic Na as a negative electrode for the primary tests. Since NaMO₂ contains Na in the structure, Na-free electrode materials such as carbon are applied to the negative electrode for the practical Na-ion full cells. Charge/discharge tests of NaMO₂ electrodes generally start in the Na-extraction way with simultaneous oxidation of transition metals in both the half-cell and full cell. In this section, electrochemical characteristics of the single 3d transition metal O3 and O’3 systems in Na cells are reviewed in order of increasing atomic number from Sc (3d⁰) to Ni (3d⁹).

      1.3.1.1. O3-NaScO₂, O3-NaTiO₂ and O3-NaVO₂

      As expected from the empty 3d orbital of Sc(III), O3-NaScO₂ (Hoppe et al. 1965) is electrochemically inactive and almost no capacity is observed in the voltage range of 1.5–4.5 V (Kubota et al. 2018b). In contrast, O3-NaTiO₂ is electrochemically active and delivers reversible capacity of ca. 150 mAh g⁻¹ based on Ti^3+/4+ redox in the voltage range of 0.6–1.6 V in a Na cell (Maazaz and Delmas 1982; Maazaz et al. 1983; Wu et al. 2015). In the voltage range, the working voltage is low (ca. 1.0 V) and charging (oxidation) to >1.6 V leads to the irreversible change in the voltage profile and the capacity decay during charge-discharge cycles (Maazaz et al. 1983). O3-NaTiO₂ is, thus, possibly available as a negative electrode material for Na-ion batteries. In contrast to O3-NaTiO₂ (Maazaz and Delmas 1982; Maazaz et al. 1983), Na (de)intercalation properties of O3-NaVO₂ were first reported in 2011 by Didier et al. (2011) and Hamani et al. (2011). O3-NaVO₂ delivers a reversible capacity of 126 mAh g⁻¹ corresponding to 0.5 mol Na extraction/insertion from/into NaVO₂ in the voltage range of 1.4–2.5 V in a Na cell. O3-NaVO₂ represents a very flat voltage plateau at 1.8 V and a large voltage jump from 1.8 to 2.1 V on the charging (Na-extraction) process. In general, the very flat voltage plateau originates from a large difference of formation energies between different Na contents of the terminal phases, e.g. O3-NaVO₂ and O’3-Na_0.5VO₂, and the two phases coexist on the voltage plateau region, while a small difference of formation energies between the different Na-content phases leads to monophasic solid solution–like reactions with sloping voltage curves. In addition, the voltage steps correspond to phase transitions and/or Na⁺/vacancy orderings as described above. Similar to O3-NaTiO₂, charging to >2.5 V suffers capacity decay probably due to irreversible migration of vanadium ions into the interslab space (Didier et al. 2011). Thus, the low working voltage of O3-NaVO₂ is also unavailable as a positive electrode material.

      1.3.1.2. O3-NaCrO₂

      In contrast to O3-NaScO₂, O3-NaTiO₂ and O3-NaVO₂, O3-NaMO₂ (M = Cr, Mn, Fe, Co, Ni) exhibits relatively high working voltages suitable for the positive electrodes of Na-ion batteries. O3-NaCrO₂ delivers a reversible capacity of 110 mAh g⁻¹ with an average working voltage of 3.02 V in a voltage range of 2.5–3.6 V (Figure 1.8) (Komaba et al. 2009, 2010).

      Figure 1.8. Comparison of galvanostatic charge/discharge curves of layered O3 and O’3 type single 3d transition metal oxides (left). Morphology of particles for each sample is also compared (right)

It should be noted that O3-LiCrO₂ is electrochemically inactive in a Li cell (Komaba et al. 2010) and electrochemical behaviors of layered CrO₂ are quite different in between Li and Na cells. O3-NaCrO₂ exhibits reversible structural changes in the O3 → O’3 → P’3 sequence by Na extraction to x = 0.5 in Na_xCrO₂ upon charging to 3.6 V versus Na (Komaba et al. 2009; Chen et al. 2013; Zhou et al. 2013; Kubota et al. 2015a). Further, Na extraction to x < 0.5 in Na_xCrO₂ causes an irreversible structural change, resulting in almost no reversible capacity (Figure 1.9(a)) (Kubota et al. 2015a; Bo et al. 2016).

      Figure 1.9. (a) Initial charge and discharge curves of Na//NaCrO₂ cells at a rate of 12.5 mA g⁻¹ in the ranges of 0.0 ≤ x ≤ 0.5 and 0.0 ≤ x ≤ 0.7 in Na_1−xCrO₂ and in a voltage range of 2.5−4.5 V. Reprinted with permission from Kubota et al. (2015a). Copyright 2015, American Chemical Society. (b) Rietveld refinements for the sample that was charged to the end of the 3.8 V plateau. Reprinted with permission from Bo et al. (2016). Copyright 2016, American Chemical Society. (c) Mechanism of transition metal migration on the sodium extraction process. Reprinted with permission from Kubota et al. (2015a). Copyright 2015, American Chemical Society. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip

The migrated chromium ions into the interslab space were detected with XRD (Figure 1.9(b)), electron diffraction (ED), and X-ray absorption spectroscopy (XAS) (Kubota et al. 2015a; Bo et al. 2016). The presumed migration mechanism is shown as schematic illustrations in Figure 1.9(c). Transition metal ions irreversibly migrate from the slab into interslab, resulting in disturbing Na insertion on discharge (Kubota et al. 2015a; Bo et al. 2016). Even in 2.5–3.6 V, capacity decay is observed in Na cells filled with carbonate ester-based electrolyte, which is probably due to electrolyte decomposition on the surface of O3-NaCrO₂. To suppress the side reaction on the surface, Du et al. synthesized large-grained O3-NaCrO₂ from Na₂Cr₂O₇·2H₂O and demonstrated a large reversible capacity of 123 mAh g⁻¹ and a high tap density of 2.55 g cm⁻³ (Wang et al. 2019c). Yu et al. carried out surface coating on the O3-NaCrO₂ particles with carbon. Carbon-coated O3-NaCrO₂ delivers a larger reversible capacity of 120 mAh g⁻¹ and exhibits superior capacity retention and rate performance in the Na cells and a hard carbon//O3-NaCrO₂ full cell (Yu et al. 2015). Among the single 3d transition metal O3 type systems, charge/discharge behaviors of O3-NaCrO₂ such as large reversible capacity, excellent capacity retention and rate performance, non-stepwise and slightly inclined voltage curves are suitable as a positive electrode material for practical use in Na-ion batteries. Actually, Sumitomo Electric Industries, Ltd. made prototype Zn-Na alloy//O3-NaCrO₂

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