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, Na0.5CrO2 shows high thermal stability in NaPF6-based electrolyte, higher than those of Li0.5CoO2 and LixNi1/3Mn1/3Co1/3O2 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).
1.3.1.3. O’3-NaMnO2
O’3-NaMnO2, generally known as α-NaMnO2, has an advantage in the abundant resources of manganese. Its electrochemical performance in a Na cell was first reported in 1971 by Hagenmuller’s group (Parant et al. 1971) and was recently re-investigated in 2011 by Ceder’s group (Ma et al. 2011; Chen et al. 2018). O’3-NaMnO2 delivers a reversible capacity of 185 mAh g−1 corresponding to ca. 0.8 Na extraction per the formula unit at an initial cycle in the voltage range of 2.0–3.8 V in a Na cell as shown in Figure 1.8. Although phase transitions during charge/discharge are still not fully clear, O3- and O’3-type structures seem to be retained without formation of P3 type (Kubota et al. 2015b; Chen et al. 2018). Despite there being no significant structural changes, the capacity inevitably decays during cycles. Even if the upper cut-off voltage is set to <3.5 V, the capacity fade is not completely avoided (Ma et al. 2011). Further detailed studies are necessary to elucidate the deterioration mechanism, which will be reported in the near future by our group. In addition to O’3-NaMnO2, NaMnO2 has another layered oxide polymorph of β-NaMnO2, of which structure is known to be a zig-zag layered (or corrugated) type (Parant et al.
1971; Fouassier et al. 1975). Zig-zag layered NaMnO2 has orthorhombic symmetry with space group of Pnmm (Abakumov et al. 2014). The zig-zag layered structure consists of corrugated MnO2 slabs alternately stacked with Na along the <100> direction in the orthorhombic lattice and the MnO6 octahedra is cooperatively distorted and elongated along <001> as shown in Figure 1.10(a) (Abakumov et al. 2014). A single phase of zig-zag layered NaMnO2 is hardly obtained due to contamination of O’3-like stacking faults, which is caused by a small difference in the formation energies between O’3 and zig-zag layered ones (Mishra and Ceder 1999; Shishkin et al. 2018), resulting in the presence of local intergrowths of O’3-NaMnO2 and stacking faults (twin planes) within the lattice of zig-zag layered NaMnO2 (Abakumov et al. 2014; Billaud et al. 2014; Orlandi et al. 2018) (Figure 1.10(b)) as evidenced with the XRD, neutron diffraction (ND) and ED patterns (Abakumov et al. 2014; Orlandi et al. 2018) and 23Na NMR spectra (Billaud et al. 2014). It should be noted that zig-zag layered like stacking locally exists even in the O’3-type phase and would influence the electrochemical properties of O’3-NaMnO2 (Figure 1.10(c)).
Figure 1.10. (a) Schematic illustrations of crystal structures for O’3 and zig-zag (corrugated) layered NaMnO2. (b) Schematic illustration of the incommensurate compositional modulated structure of the NaMnO2 polymorphs. Modified with permission from Orlandi et al. (2018). Copyright 2018, American Physical Society. Galvanostatic charge/discharge curves of (c) O’3 and (d) zig-zag layered NaMnO2. The first, second, fifth and tenth Na extraction/reinsertion cycles are represented in black, red, blue and green, respectively. Reprinted with permission from Billaud et al. (2014). Copyright 2014, American Chemical Society. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip
Zig-zag layered NaMnO2 with the stacking faults delivers a reversible capacity of 190 mAh g−1 in a voltage range of 2.0–4.2 V at a current density of 10 mA g−1 as shown in Figure 1.10(d) (Billaud et al. 2014; Kubota et al. 2018a). Recently, our group successfully synthesized zig-zag layered NaMnO2 using Cu-substitution (Shishkin et al. 2018). Differences in structures and electrochemical behaviors between O’3 and zig-zag layered NaMnO2 will be reported in detail elsewhere in the near future.
In addition to the NaMnO2 polymorphs, Na2Mn3O7, which can be described as Na4/7[Mn6/7□1/7]O2 (□: Mn vacancy), is also a layered material (Chang and Jansen 1985). The structure is composed of [Mn6/7□1/7]O2 slabs and has a vacancy at the honeycomb center in the slab. Na+ ions are accommodated at distorted-octahedral and prismatic sites in the interslab space and the structure is regarded as intermediate between O’3- and P’3-type ones with space group of P-1. Due to the Na-deficient composition, Na2Mn3O7 is first reduced down to 1.5 V versus Na to accommodate Na+ ions and delivers a reversible capacity of 160 mAh g−1 with a voltage plateau at 2.2 V based on Mn3+/4+ redox in 1.5–3.0 V (Adamczyk and Pralong 2017) as shown in Figure 1.11(a). Recently, the study on oxygen-redox of layered oxide materials in the high-voltage region above 4.0 V is a hot topic in Li-ion and Na-ion battery fields. The transition metal vacancies in the slab were reported to enhance the redox activity of oxide ions for Na0.78[Ni0.23Mn0.69□0.08]O2 (Ma et al. 2017). Mortemard de Boisse et al. (2014) demonstrated highly reversible charge-discharge reactions delivering a capacity of 75 mAh g−1 with a distinct voltage plateau at ~4.1 V in Na2Mn3O7 as shown in Figures 1.11(a) and (b) (Mortemard de Boisse et al. 2018). Surprisingly, voltage hysteresis between charge and discharge is negligible unlike Li2MnO3 and Li-rich materials having the [Mn2/3Li1/3]O2 honeycomb structure in the slab (Singer et al. 2018). Oxygen redox was also reported for a Na-rich layered oxide of Na2RuO3, which can be described as O3-Na[Ru2/3Na1/3]O2 and has a [Ru2/3Na1/3]O2 honeycomb structure in the slab (Rozier et al. 2015; Mortemard de Boisse et al. 2016, 2019). Mortemard
