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in Na-ion chemistry.

      1.4.2. P2-Na_2/3[Mn,Co,M]O₂

      As mentioned above, P2-type single 3d transition metal systems are found in only P2-Na_2/3VO₂, P2-Na_0.6Mn_0.9O₂, P’2-Na_2/3MnO₂ and P2-Na_2/3CoO₂. Since Na_xMnO₂ delivers the largest reversible capacities of >200 mAh g⁻¹ among single 3d transition metal P2-type systems (Kumakura et al. 2016), various solid solutions are synthesized based on P’2-Na_2/3MnO₂ and P2-Na_2/3CoO₂ as parent oxides and their Na (de)intercalation properties have been extensively investigated in Na cells.

      A solid solution between P’2-Na_2/3MnO₂ and P2-Na_2/3CoO₂ is crystallized in any ratio of Mn/Co (Paulsen and Dahn 1999; Wang et al. 2013). From the late 1990s to the early 2000s, the solid solution was studied as positive electrode materials for Li-ion batteries after Li⁺/Na⁺ exchange (Paulsen and Dahn 1999; Lu and Dahn 2001b; Eriksson et al. 2003). Na intercalation properties of the sodium solid solution were first reported in P2-Na_2/3Mn_1/3Co_2/3O₂ by Carlier et al. (2011). P2-Na_2/3Mn_1/3Co_2/3O₂ delivers a reversible capacity of 112 mAh g⁻¹ in the range of 1.5–4.0 V, as shown in Figure 1.14. The voltage profiles of P2-Na_2/3Mn_1/3Co_2/3O₂ are relatively smooth except for the voltage jump at x = 2/3 and 1/2 in P2-Na_xMn_1/3Co_2/3O₂ compared to the end-members of P’2-Na_2/3MnO₂ and P2-Na_2/3CoO₂, suggesting that the phase transitions and Na⁺/vacancy ordering in the Mn/Co-disordered phase are of less significance (Beck et al. 2014). According to XAS data of Na_2/3Mn_1/3Co_2/3O₂ during charge/discharge in Na cells (Cheng et al. 2014), LS Co³⁺ is oxidized into LS Co⁴⁺ by Na extraction and HS Mn⁴⁺ is maintained during charging to 4.0 V. On discharge to 2.5 V, LS Co⁴⁺ is reversibly reduced into LS Co³⁺. P2-Na_2/3Mn_1/3Co_2/3O₂ can accommodate more Na in the structure and both HS Mn⁴⁺ and LS Co⁴⁺ were reported to reduce into HS Mn³⁺ and HS Co²⁺, respectively, with major modification of hybridization between M 3d and O 2p orbitals by Na insertion on discharge below 2.5 V. The electronic structures significantly change with the formation of Jahn–Teller active Mn³⁺ and the reduction from LS-Co³⁺ to HS-Co²⁺ on discharge, resulting in much lower voltage than P2-Na_2/3CoO₂ below 2.5 V in the Na cells (Cheng et al. 2014; Watanabe et al. 2019). The extra 0.16 Na insertion into Na_2/3Mn_1/3Co_2/3O₂ and the large reversible capacity on discharging from 2.5 to 1.25 V is attractive, but Na compensation is necessary for the practical use. Even for P2-Na_2/3[Mn,Co,Ni]O₂, the low working voltage and indispensableness of external Na compensation are drawbacks.

1.4.3. P2-Na_2/3[Mn,Fe,M]O₂

      A solid solution of P2-Na_2/3[Mn,Fe]O₂ was also synthesized and the battery performances were investigated in Li cells in the early 2000s (Eriksson et al. 2003; Matsumura et al. 2003). Our group evaluated Na (de)intercalation properties of P2-Na_2/3Fe_1/2Mn_1/2O₂ and P2-Na_2/3Fe_1/2Mn_1/2O₂ deliver a reversible capacity of ca. 190 mAh g⁻¹ and exhibit good rate performance in Na cells in the range of 1.5–4.3 V (Yabuuchi et al. 2012a), as shown in Figure 1.14. P2-Na_2/3Fe_1/2Mn_1/2O₂ consists only of the abundant elements in the Earth’s crust without cobalt and shows acceptable capacity retention different from O3-NaFeO₂, which is probably due to less or reversible Fe migration into interslab space. The P2-Na_2/3Fe_1/2Mn_1/2O₂ phase transforms into an OP4-like phase by Na extraction on charge to 4.3 V and tetrahedral sites exist in the O-type interslab space. P2-Na_2/3Fe_1/2Mn_1/2O₂ shows capacity decay during cycles, which is thought to be caused by irreversible migration of transition metals into the interslab space. Talaie et al. successfully detected migrated transition metals at tetrahedral sites in the interslab space by pair distribution function (PDF) analysis on the XRD patterns of chemically oxidized Na_0.1Fe_1/2Mn_1/2O_2, Na_0.15Fe_1/2Mn_1/2O₂, Na_0.15Fe_0.3Mn_0.6Ni_0.1O₂, Na_0.15Fe_0.2Mn_0.65Ni_0.15O₂ and Na_0.1Fe_0.2Mn_0.66Cu_0.14O₂ (Talaie et al. 2015, 2017). The amount of Fe migration is higher in Na_xFe_1/2Mn_1/2O₂ than in Cu and Ni doped materials. Indeed, cycling stability of the doped samples is better than that of P2-Na_2/3Fe_1/2Mn_1/2O₂. The P2-Na_2/3Fe_1/2Mn_1/2O₂ sample, however, generally contains crystal domains of O3-NaFe_1/2Mn_1/2O₂, leading to broad 10l Bragg diffraction peaks in the XRD (Yabuuchi et al. 2012a). The difficulty of synthesizing pure P2-Na_2/3Fe_1/2Mn_1/2O₂ complicates our understanding of the reaction mechanism. As Talaie et al. also mentioned in the literature, reversible charge–discharge behaviors are observed for P2-type materials in Na cells even if transition metals are migrating to interslab space, which is quite different from O3-NaFeO₂ and O3-NaCrO₂ (Figure 1.8). Mortemard de Boisse et al. (2014) confirmed well-crystallized phases and no significant structural evolution before and after sodium extraction/insertion for P2-Na_xFe_1/2Mn_1/2O₂. Except for the capacity decay, P2-Na_2/3Fe_1/2Mn_1/2O₂ has some drawbacks: (1) voltage hysteresis in between charge and discharge as seen above 3 V in Figure 1.14, (2) Na deficiency in the as-prepared state and (3) hygroscopic property like O3-type materials. First, the significant voltage hysteresis appears after charging beyond 4.1 V (Talaie et al.

2015). Despite extensive research, the mechanism of the voltage hysteresis has not been elucidated yet. Redox reaction of oxide ion (Talaie et al. 2015) and P2/OP4-like phase transition accompanied by formation of stacking faults (Mortemard de Boisse et al. 2014) and by migration of transition metals (Talaie et al. 2015) are estimated to cause the voltage hysteresis. Second, Na deficiency is a considerable issue in all P2-type materials showing capacity below 2.5 V and the large reversible capacity is achieved by Na compensation from external Na sources such as Na metal counter electrode in a half-cell and pretreatment with Na metal by ball milling (Zhang et al. 2016), but Na metal should be avoided in the Na-ion battery full cells and synthesis process due to safety issues. Recently, the addition of sacrificial Na sources such as NaN₃ and Na₂CO₃ is proposed to compensate for Na in the full cells without Na metal (Singh et al. 2013; Martinez De Ilarduya et al. 2017; Sathiya et al. 2017). Third, as discussed in the hygroscopic property of O3-type materials above, P2-type materials are also generally hygroscopic except for P2-Na_2/3Ni_1/3-xCu_xMn_2/3O₂ (0 ≤ x ≤ 1) (Lu and Dahn 2001b; Zheng et al. 2017). Air stable P2-Na_2/3[Mn,Fe]O₂ is achieved by Cu doping (Li et al. 2015; Talaie et al. 2017). Doping with Cu²⁺ and alkaline earth metals would be effective to enhance the waterand moisture-resistant properties of P2-type materials. Ternary and quaternary Fe–Mn-based P2-type materials such as P2-Na_x[Fe,Mn,Ti]O₂ (Han et al. 2016), P2-Na_x[Fe,Mn,Co]O₂ (Thorne et al. 2014b), P2-Na_x[Fe,Mn,Ni]O₂ (Talaie et al. 2015; Bai et al. 2016), P2-Na_x[Fe,Mn,Co,Ni]O₂ (Kaithwas and Kundu 2015; Li et al. 2017) and P2-Na_x[Fe,Mn,Cu]O₂ (Li et al. 2015; Talaie et al. 2017) have also been extensively studied and Fe in P2-type materials would also assist Na diffusion at the end of charge due to the flexibility of Jahn–Teller active Fe⁴⁺ (Liu et al. 2015) as observed in O3-type materials (Li et al. 2016). The metal

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