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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 Na_4/7[Mn_6/7□_1/7]O₂ (□: Mn vacancy), is also a layered material (Chang and Jansen 1985). The structure is composed of [Mn_6/7□_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 Mn^3+/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 Na_0.78[Ni_0.23Mn_0.69□_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 [Mn_2/3Li_1/3]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[Ru_2/3Na_1/3]O₂ and has a [Ru_2/3Na_1/3]O₂ honeycomb structure in the slab (Rozier et al. 2015; Mortemard de Boisse et al. 2016, 2019). Mortemard de Boisse et al. (2014) revealed from the ex situ XRD patterns that coulombic attractive interactions between the slabs are enhanced by the existence of ordered alkali vacancies, i.e. the [Ru_2/3□_1/3]O₂ honeycomb structure formed by Na extraction during charge, leading to reduction of stacking faults and progressive ordered stacking of the [Ru_2/3□_1/3]O₂ slabs upon charging. The cooperatively ordered vacancies generate the electro-active nonbonding 2p orbitals of neighboring oxygen anions and stabilize the phase transformation for highly reversible oxygen-redox reactions (Mortemard de Boisse et al. 2019). Although the operation voltage of O3-Na₂RuO₃ is relatively low as a positive electrode material and further studies are required to enhance the oxygen-redox capacity for Na₂Mn₃O₇, the findings provide a compelling future research direction toward reversible oxygen-redox positive electrode materials for high energy density batteries (Mortemard de Boisse et al. 2018).

Figure 1.11. (a) Potential profile (second cycle) of Na₂Mn₃O₇ upon (de)intercalation in 1.5–4.7 V versus Na/Na⁺. The dashed gray box highlights the high voltage region, where O is the active redox process. (b) Galvanostatic charge/discharge curves in 3.0–4.7 V versus Na/Na⁺ at a rate of C/20. The inset shows a cyclic voltammetry curve at the second cycle at a scan rate of 0.1 mV s⁻¹. Reprinted with permission from Mortemard de Boisse et al. (2018). Copyright 2018, Wiley-VCH. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip