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 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 [Ru2/3□1/3]O2 honeycomb structure formed by Na extraction during charge, leading to reduction of stacking faults and progressive ordered stacking of the [Ru2/3□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