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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 cells with the molten salt of sodium bis(fluorosulfonyl)amide (NaFSA) and potassium bis(fluorosulfonyl)amide (KFSA) as electrolyte, and battery and safety performances were reported (Nitta et al. 2013). Despite the toxicity issue of chromium, O3-NaCrO₂ has been used as a standard positive electrode material to evaluate performance of Na-ion batteries. Furthermore, Na_0.5CrO₂ shows high thermal stability in NaPF₆-based electrolyte, higher than those of Li_0.5CoO₂ and Li_xNi_1/3Mn_1/3Co_1/3O₂ in a LiPF6-based electrolyte, which is evidenced by accelerating rate calorimetry (Xia and Dahn 2012) and explained with phase transitions without oxygen generation below 477°C (Yabuuchi et al. 2016).