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Most O3-type materials generally transform in the O3 → P3 → O3 (→ O1) sequence by gliding of the MO₂ slab during Na extraction during the charging process (Kaufman and Van der Ven 2019) as shown in Figures 1.5 and 1.6(a). Note that O1-type CoO₂ was reported to be obtained via electrochemical Li extraction from LiCoO₂ in a Li cell (Amatucci et al. 1996). However, defect-free O1-type MO₂ including O1-CoO₂ has not been reported so far, although some literature reports defective O1-type MO₂ having migrated transition metal ions into the interslab spacing after Na extraction from NaMO₂ (Mariyappan et al. 2018b; Wang et al. 2019b). Non-hexagonal structures of O’3- and P’3-type phases are also formed in the compositional regions surrounding those of non-distorted O3- and P3-type phases. Furthermore, Na⁺/vacancy orderings in the interslab spacing are often observed for x = 2/3, 1/2 and 1/3 in Na_xMO₂ (Figure 1.6(b)), leading to voltage jumps in the charge-discharge profiles (Zandbergen et al. 2004; Toumar et al. 2015; Kaufman and Van der Ven 2019).

Figure 1.5. Schematic illustrations of crystal structures reported for partly desodiated O3- and O’3-type Na_xMO₂. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip

Figure 1.6. (a) Calculated formation energies versus composition for Na_xCoO₂ configurations on the local convex hull of each host structure (top). Calculated zerotemperature equilibrium voltage curve (black) compared to experiment from Kubota et al. (2016) (gray) (bottom). (b) P3 ground-state orderings ζ and ∆, with Na shown in blue (top). O3 orderings with Na in yellow on the local convex hull of O3 for x = 1/3 and 1/2 in Na_xCoO₂ (bottom). Asterisks indicate that the ordering is above the global hull. Reprinted with permission from Kaufman and Van der Ven (2019). Copyright 2019, American Physical Society. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip