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Sun et al. (2014) also synthesized O3-NaNi_0.4Fe_0.2Mn_0.4-x Ti_xO₂ and O3-NaNi_0.4Fe_0.2Mn_0.2Ti_0.2O₂ exhibits superior cycling stability to that of O3-NaNi_0.4Fe_0.2Mn_0.4O₂. In addition to the Fe-substituted O3-Na[Mn,Ni]O₂, Qi et al. (2016), Zheng and Obrovac (2017), and Wang et al. (2017b) reported Ti-substituted one and the electrochemical performance of the Na cells. O3-Na_x[Mn,Ni,Ti]O₂ delivers almost the same discharge capacity of 120–135 mAh.g-¹: in general we are using a as O3-NaMn_1/2Ni_1/2O₂ (Wang et al. 2017b). The Ti-substitution changes the voltage profiles from stepwise into smooth ones and enhances capacity retention. Qi et al. (2016) and Yao et al. (2017) anticipated that Ti-substitution disturbs both the Na/vacancy ordering in the interslab space and the Ni²⁺/Mn⁴⁺ ordering in the slab due to the substantial difference in Fermi level between Ti⁴⁺ and Ni²⁺/Mn⁴⁺ and decrease in the electronic localization as Wang et al. (2015) proposed in P2-Na_0.6Cr_0.6Ti_0.4O₂. Mariyappan et al. (2018a) proposed that substitution of HS Mn⁴⁺ (3d³; t_2g³e_g⁰) in O3-NaMn_1/2Ni_1/2O₂ by Ti⁴⁺ (3d⁰; t_2g⁰e_g⁰) increases the electronic density on oxygen and enlarges the energy difference between Ni 3d and O 2p orbitals, leading to an increase in the M-O bond ionicity and redox potential. Note that higher Ti content leads to the smoother voltage curves and the higher average charge and discharge voltage, however voltage hysteresis increases (Zheng and Obrovac 2017). Furthermore, metal substitution was conducted to O3-Na_x[Mn_, Ni, Ti]O₂ by Guo’s (Yao et al. 2017), Deng’s (Wang et al. 2019a) and Tarascon’s (Wang et al. 2019b; Mariyappan et al. 2020) groups. Substitution by Ti⁴⁺ (3d⁰; t_2g⁰e_g⁰) for HS Mn⁴⁺ (3d³; t_2g³e_g⁰) and divalent metals such as Cu²⁺ (3d⁹; t_2g⁶e_g³) or Zn²⁺ (3d¹⁰; t_2g⁶e_g⁴) for LS Ni²⁺ (3d⁸; t_2g⁶e_g²) in O3-NaMn_1/2Ni_1/2O₂ achieve excellent capacity retention and rate capability in the range of 2.0–4.0 V (Yao et al. 2017) and even in the range of 2.0–4.5 V (Wang et al. 2019b). O3-NaNi_1/2−yCu_yMn_1/2−zTi_zO₂ (y = 0, 0.05, 0.1; z = 0.1, 0.2) solid solution phases deliver reversible capacities of ca. 125 and 200 mAh g⁻¹ in 2.0–4.0 V and 2.0–4.5 V, respectively, with smooth voltage profiles. Structures of O3-NaNi_0.4Cu_0.1Mn_0.4Ti_0.1O₂ are evolving in the O3 → O′3 → P3 → P’3 sequence by Na extraction during charging to 4.0 V (Yao et al. 2017; Wang et al. 2019b) and then from P’3 → P3-O3-O1 intergrowth by charging from 4.0 to 4.5 V, which is confirmed by operando XRD patterns and ED patterns as well as high-angle annular dark field (HAADF) STEM images (Wang et al. 2019b; Mariyappan et al. 2020). Wang et al. revealed with the HAADF-STEM images that the O1-type phase (Figure 1.5), having migrated transition metal cations in the interlayer octahedral interstices of ~5.1 Å in distance, randomly co-exist with P3 domains having a wider interlayer space with ~7.1 Å. The nucleation of the O1 stacking domains in the P3 ones and the significantly different interlayer distances reduce the lattice contraction and overall lattice changes, thus improving the cycle life (Wang et al. 2019b). Furthermore, DFT calculations estimate that the Cu and Zn substitution in O3-Na_x[Mn,Ni,Ti]O₂ decreases in the energy difference between P and O stackings and leads to a continuous transition between Pand O-stacking phases, explaining the sloping solid solution–like charging profile above 4.0 V (Wang et al. 2019b) unlike the flat charging voltage profiles corresponding to the biphasic P3 → O3 transition in O3-NaMn_1/2Ni_1/2O₂ (Figure 1.12). Tarascon’s group fabricated the prototype 18 650 cells of the O3-phase positive electrode with a hard carbon negative electrode and demonstrated a reversible specific capacity of ca. 154 mAh g⁻¹ with an operating voltage of ca. 3.1 V with gravimetric and volumetric energy densities of ca. 115 Wh kg⁻¹ and ca. 250 Wh L⁻¹ for the total cell weight and volume, respectively, comparable or slightly superior to the polyanionic Na₃V₂(PO₄)₂F₃||hard carbon cells (100 Wh kg⁻¹, 175 Wh L⁻¹) because of the higher tapped density of the layered oxide material (1.9 g cm⁻³) than 1.1 g cm⁻³ of Na₃V₂(PO₄)₂F₃ (Mariyappan et al. 2020).

O3-type binary, ternary and quaternary transition metal systems exhibit a larger reversible capacity, better rate performance and capacity retention with smooth voltage profiles in Na cells compared to those of the single transition metal system. Most of the O3-type multiple transition metal systems reported so far overcome the severe issues for the practical use faced by the single transition metal systems: (1) an irreversible structural change related to the migration of transition metals into interlayers during the charging process; (2) stepwise voltage profiles originating from Na/vacancy orderings and phase transitions; (3) slow kinetics of Na (de)insertion; and (2) less moist air stability (Sathiya et al. 2012). Lowering the content of migrating cations such as Fe³⁺ reduces probability of the migration and enhances structural stability during charge and discharge (Li et al. 2016). The substitution by both Ti⁴⁺ and divalent cations such as Cu²⁺ and Zn²⁺ leads to sloping voltage profiles and reduces the number of phase transitions (Mariyappan et al. 2020). Furthermore, Ti⁴⁺-substitution seems to stabilize a P3-type phase having a large interslab space (Mariyappan et al. 2018a), in which Na⁺ ions at prismatic sites directly migrate to a neighbor face-shared prismatic site through the wide rectangle bottleneck in P3 and P’3 structures, suggesting high Na⁺ diffusion in the P3-type stacking structure. Thus, the P3-type phase exhibits high Na⁺ diffusion, and a wider compositional range for P3-type Na_xNi_0.4Cu_0.1Mn_0.4Ti_0.1O₂ stabilized by the metalsubstitution leads to excellent rate capability (Yao et al. 2017). Consequently, O3-NaNi_0.4Cu_0.1Mn_0.4Ti_0.1O₂ also has higher moist air stability than that of the non-substituted O3-NaMn_1/2Ni_1/2O₂ (Yao et al. 2017).

      1.3.3. Moist air stability of O3-NaMO₂ and surface coating

Most O3-type NaMO₂ are moisture sensitive and the stability of O3-NaMO₂ against a moist environment and the easiness of handling are important for practical applications (Kubota and Komaba 2015; Zhang et al. 2019). During exposure to moist air, the water molecules react with air-instable Na_xMO₂ by (1) H₂O insertion into the interlayer spacing between MO₂ slab and the Na layer (Park et al. 2007), (2) oxidation of transition metal ions with the simultaneous removal of Na⁺ ions (Huang et al. 2005; Monyoncho and Bissessur 2013) and (3) H⁺/Na⁺ exchange (Blesa et al. 1993). (1) H₂O insertion is often observed for Na-deficient phases such as O3-related Na_0.3NiO₂ and P2-type Na_xMO₂ (Legoff et al. 1993; Paulsen and Dahn 1999) and leads to the expansion of the interlayer spacing and then to exfoliation and crack generation at the particle surface after a drying process for cell assembly. Na removal in the (2) and (3) processes results in the further reaction with CO₂ and H₂O in air to form NaOH, NaHCO₃·nH₂O, Na₂CO₃·nH₂O, etc. (Huang et al. 2005; Monyoncho and Bissessur 2013) and the sodium salts containing water are formed on the particle surface, which was confirmed by You et al. with subnanometer surface-sensitive time-of-flight secondary ion mass spectroscopy for O3-NaNi_0.7Mn_0.15Co_0.15O₂ (You et al. 2019). They further found that nickel ions gradually

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