As early as 1981, Delmas et al. (1981) pioneered Na_xCoO₂ system by solid state reaction between 500°C and 800°C under oxygen atmosphere, four different phases were obtained, including P′3 phase (0.55 ≤ x ≤ 0.6), P2 phase (0.64 ≤ x ≤ 0.74), O′3 phase (x = 0.77) and O3 phase (x = 1) and they were all indexed into layered structure. Unlike P3 phase and O3 phase, P2 phase not only has lamellar displacement, but also involves rotation of (CoO₆) octahedron and Co-O bond rupture when the phase transfers to P3 or O3. Thus, the P2 structure remains unchanged throughout the electrochemical process. However, for O3 type NaCoO₂, a reversible phase transition O3−P3−P′3 in the electrochemical process can be observed in Figure 2B. Jean et al. (1971) respectively synthesized Na_0.7CoO₂ intercalation compound by using different preparation techniques such as solid-stated reaction, sol-gel and hydrothermal method. The results showed that the cathode material with uniform ball-shaped morphology synthesized by sol-gel method demonstrated the best performance. The contribution of Na⁺ ions to the conductivity was also confirmed, but the corresponding electrochemical performance was not mentioned. The electrochemical properties of Na_0.71CoO₂ were systematically investigated. In the relatively narrow voltage range of 2–3.5 V, it delivered 70.4 mAh g⁻¹ reversible capacity at 0.08 C in the first week with an initial coulomb efficiency of 90% (Figure 2C). At the magnification of 0.08 C, the cyclic life was long and stable, almost reaching 100% (Figure 2D). In-situ XRD (Figure 2E) results showed that the diffraction peaks of (002), (004) and (100) shifted with the insertion/extraction of Na⁺, indicating the composition range of sodium has changed slightly. Carlier et al. (2011) successfully prepared P2-type Na_2/3Co_2/3Mn_1/3O₂, the discharge profile shown in Figure 2F presents nine different platforms in the x range 0.5–0.9. When x = 1/2 and 2/3, a large voltage step could be observed, indicating that Na⁺/vacancy was arranged in the structure. When Mn⁴⁺ partly replaced Co⁴⁺, Na_1/2Co_2/3Mn_1/3O₂ with well-ordered structure was formed. Moreover, P2-phase Na_0.74CoO₂ prepared by Ding J.-J. et al. (2013) exhibited 107 mAh g⁻¹ initial discharge capacity at 0.1 C with 94% reversible capacity remained after 40 weeks under 2–3.8 V, showing good cycling performance.

In recent years, Na_0.7CoO₂ microspheres using CoO₃ microspheres as precursor self-template has been investigated (Zhu et al., 2005). Due to its unique structure, the material exhibited high reversible specific capacity (125 mAh g⁻¹), excellent rate property (64 mAh g⁻¹ reversible specific capacity at 16 C rate). Sabi et al. (2017) prepared P2-Na2/3Co1/2Ti1/2O2 in air, and compared the electrochemical behavior of the obtained material with NaxCoO2. The constant current charge-discharge cycle showed that the nine potential steps of NaxCoO2 were effectively reduced by the substitution of Co by Ti (Figure 2E). In addition, the substitution of Co by Ti has promoted the improvement of electrochemical performance. The P2-Na2/3Co1/2Ti1/2O2 delivered 100 mAh g−1 initial specific discharge capacity in Na half cell between 2.0 and 4.2 V with 98% capacity retention after 50 rounds. Moreover, Huang et al. (2021) proposed a Mg/Zn co-doping strategy to increase the interfacial spacing thus providing a broad ion diffusion channel for rapid Na + insertion/extraction. The modified material possessed a 67.2 mAh g−1 at 10 C. Zhu et al. (2016) synthesized P2-Na0.67Co0.5Mn0.5O2 cathode, which was easy to quickly diffuse Na+ and establish stable structure.

Recently, in order to further optimize the Na + storage in P2 type Na-Mn-Co-O cathode, cationic doping/substitution has been studied deeply to regulate the layer spacing, suppressing active Mn3+ ions and stabilizing the lattice structure. Wang X. et al. (2021) synthesized Na0.67Co0.20Mn0.79Ce0.01O2 using solid-state reaction, where Ce occupied the Mn-site. The capacity retention was 92.3% after 100 cycles at 0.1 C and 91.7% after 400 rounds at 1 C. The cathode demonstrated excellent cyclic performances. The work proved that small amount of Ce could effectively enhance the stability, which alleviated the Jahn-teller effect and guarantee the phase transition to be highly reversible. Chen Y. et al. (2020) synthesized micron sized Na0.65[Ni0.17Co0.11Mn0.72]O2 by coprecipitation, benefitting from the spherical morphology, the material provided sufficient interface between electrode and electrolyte solution, so the transmission path of sodium ions and electrons was shortened, the as-prepared cathode delivered 187 mAh g^-1 initial capacity at 12 mA g^-1. After 500 cycles, its abnormal capacity retention rate reached 74.7% at a current density as high as 600 mA g−¹.

Chen T. et al. (2020) designed and synthesized multilayer directional lamination Na0.67Mn0.6Ni0.2Co0.1Cu0.1O2 with P2 structure by a simple sol-gel method, a large three-dimensional framework was gained, which enabled sodium ions to diffuse. A reversible capacity of 131.3 mAh g-1 was acquired at 0.1 C, the outstanding retention rate of 80.0% at 1 A g−1 after 500 cycles was obtained. Such remarkable electrochemical performance was attributed to high Na + mobility and low Na + diffusion resistance. Pang et al. (2019) prepared P2 type layered oxide Na2/3Mn1/2Co1/3Cu1/6O2. Due to the collaborative improvement of polymetal ions, the electrode showed good cyclic stability and excellent sodium ion transport performance. And the capacity retention was still achieved at 83.5% after 100 cycles at 100 mAh g−1.

Besides the above efforts, some other transition metal elements have been employed to modify the layered structures. Wang et al. (2019) prepared Na0.67Co0.25Mn0.75-xNbxO2 with different x value ranging from 0 to 0.045 through simple solid-stated method. Because of Nb5+, the structural stability was greatly enhanced and the charge transfer resistance was reduced. The corresponding Nb-doped cathode exhibited 126.7 mAh g−1 initial discharge capacity and the cycling stability was obviously developed than that of Na0.67Co0.25Mn0.75O2. Furthermore, Wang Y. et al. (2018) prepared P2-type Na0.70Mn0.80Co0.15Zr0.05O2 and suggested that Zr could significantly increase the rate and cycling properties of P2 type material, after 50 cycles, the capacity remained 88% of the initial cycle.

The layered Na_xMnO₂ cathodes were firstly explored by Parant et al. (1971) and Mendiboure et al. (1985) reproduced the crystal chemistry of the Na_xMnO₂ system in 1985. In the x range of 0–0.44, the structure was found to be three-dimensional, however, when x > 0.5 the structure turned to be two-dimensional. Due to different synthesis conditions and stoichiometric ratios, the layered oxides of Mn group are mainly divided into P2-Na_0.7MnO_2+y, monocline O′3 type α-NaMnO₂ and orthogonal P2 type β-NaMnO₂ (Figure 3A). The sodium storage properties of these three compounds were systematically studied in this paper. Shibata et al. (2014) studied the diffusion of lamellar Na_xMnO₂ at (0.49 < x < 0.75) and compared it with Na_xCoO₂. In 1994, Doeff et al. (2002) prepared Na_xMnO₂. When x is less than 0.45, the cathode material had a tunnel structure, such as Na_0.44MnO₂. When x was greater than 0.45, the crystal structure of the cathode material was generally layered. β-NaMnO₂ has a layered structure which was different from the traditional NaMO₂ structure. The crystal shape of the traditional layered structure (O3, P2, P3, etc.) depends on the stacking order of the planar layers of the MnO₆ octahedron. β-NaMnO₂ was consist of a serrated layered edge shared MnO₆ octahedron, in which sodium ions was located at the octahedral position as shown in Figure 3B. Figure 3C shows the symbiotic illustration between α and β-NaMnO₂ (Billaud et al., 2015). In Figure 3D, when the 2θ value between 12^o and 15^o, the intensity of (001) peak diminished, while the new peak at lower angles increased. However, from the in-situ XRD data, it could be concluded that the long-range structure collapsed significantly and many peaks disappeared at low sodium levels. In most cases, the remaining peaks showed significant broadening as many peaks disappeared. β-NaMnO₂ showed high capacity (about 190 mAh g⁻¹ at 0.05 C), good rate capacity (142 mA h g^-1 at 2 C) and excellent cycling stability (100 mA h g^-1 at 2 C rate after 100 cycles) in Figures 3E,F (Huang et al., 2021). Jo et al. (2014) synthesized pure α-NaMnO₂ using solid state reaction of Mn₂O₃ and Na₂CO₃. The material displayed 146 mAh g⁻¹ initial discharge capacity and remained 136 mAh g⁻¹ after 20 cycles.

Na_0.7MnO₂ with the P2 structure displayed a high capacity of 200 mAh g⁻¹. However, due to structural and chemical problems, Na_0.7MnO₂ suffers from severe capacity decay during repeated charging and discharging. Phase transitions (P2 to O2 or OP4) in high voltage range (low Na⁺ content) and the transitions between different Na⁺/vacancy ordered modes at specific Na⁺ stoichiometry were the main causes of capacity loss (Baster et al., 2016).

Le et al. (2021) evaluated the effect of different 3D metals (Fe, Co and Ni) replacing Mn on the electrochemical performance of P2-Na_xMe_1/3Mn_2/3O₂ cathodes. The results showed that both Na_xCo_1/3Mn_2/3O₂ and Na_xFe_1/3Mn_2/3O₂ had high specific capacities of larger than 140 mAh g⁻¹, while Na_xCo_1/3Mn_2/3O₂ showed better cyclic performance and had a capacity retention rate of 83% after 50 rounds. In contrast, Na_xNi_1/3Mn_2/3O₂ exhibited an excellent cyclic stability, but the specific capacity was relatively low, which was only 110 mAh g⁻¹ in the initial cycle. A kind of cathode material Ni-Mn-based P2-type Na_0.58Ni_1/3Mn_2/3O_1.95 was synthesized by a co-precipitation reaction, which displayed a high capacity of 153 mAh g^-1 in the initial cycle. After optimizing the voltage window to 2.6–3.8 V, the capacity retention increased from 14% to 94.5% after 500 cycles. According to in-situ XRD, the existence of oxygen anion group [O₂]^x− (0 < x < 4) under high pressure was the main cause of battery capacity attenuation in Figure 3G (Cai et al., 2020). Arjunan et al. (2020) successfully prepared Na_0.67Ni_0.23Zn_0.1Mn_0.67O₂ electrode material by a conventional solid-state method. When the cut-off voltage was exceeded (≥4.2 V), the capacity and cycle performance were attenuated due to stacking fault, and the substitution of Zn²⁺ ions acted as a stabilizer in the P2-layer structure of Na-Ni-Mn-O system.

In addition, Zhang et al. (2020) prepared Na_0.91MnO₂ as the precursor material, and prepared Na_0.67Mn_0.67Ni_0.33O₂ nanoparticles by Ni doping under the same conditions, which effectively improved the specific capacity. Furthermore, Na_0.67Mn_0.67Ni_0.33-xCo_xO₂ was prepared by Co doping. It was found that the electrode material had more stable cyclic performance. The specific discharge capacity in the initial week delivered above 160 mAh g⁻¹ at 0.1 C, and the specific discharge capacity remained 120 mAh g⁻¹ after cycling for 200 rounds. As the current density increased to 1 C, the reversible capacity reached 90 mAh g⁻¹. The results showed that Ni and Co co-doping could effectively improve the specific capacity, rate and cycling stability of electrode materials. Wang H. et al. (2020) systematically prepared and studied the structure and electrochemical performance of P2-type Na_0.67Mn_0.6Ni_0.2Co_0.2O₂. The addition of La reduced Mn³⁺ the amount of manganese trivalent ions in the lattice, alleviated Jahn-Teller distortion, and enhanced Na⁺ diffusion kinetics. Even at 8 C, it could deliver 86 mAh g^-1 reversible discharge capacity. Xiao et al. (2019) proposed Na_2/3Ni_1/6Mn_2/3Cu_1/9Mg_1/18O₂ layered oxide after the morphology and structure optimization, it showed good electrochemical performance under the cut-off voltage of 4.2 V. Kouthaman et al. (2020) prepared Na_0.9Mn_0.55Ni_0.35Cu_0.1O₂ oxide with O3 structure, benefiting from the small amount of copper which effectively improve the O3 structure, the electrochemical performance was greatly enhanced.

Jiang et al. (2022) prepared P2-Na_0.6Ni_0.3–xMn_0.7Cu_xO₂ with different copper content of 0–0.2 and the mechanism of improving electrochemical properties was studied comprehensively. It has been discovered that a small amount of copper can inhibit the P2-O2 phase transition, consequently stabilizing the crystal structure and enhancing the cycling stability. The augmentation of the layer spacing, resulting from copper doping, contributes to the dynamics of charge transfer, thereby ensuring excellent rate performance. Furthermore, copper substitution enhances the moisture stability of the electrode due to the increased initial potential in the charging process from the Cu²⁺/Cu³⁺ redox couple.

To mitigate the Jahn-Teller effect, interfacial instability, and phase transition in Mn-Ni based materials with a P2 structure under high pressure, research has been conducted on cationic doping/substitution as a means to stabilize the lattice structure and enhance the electrochemical performance. For example, Cu-Mg co-doping (Chen T. et al., 2020), Li-doping (Chen et al., 2021) and Mg-doping (Yoda et al., 2020) were proved to be effective to promote the cycling stability of P2-type cathode material. In a recent study, the substitution of copper and cobalt formed stable copper oxide and cobalt oxide octahedrons, and the lattice structure was adjusted to enhance the diffusion kinetics of Na⁺ in layered manganese-based oxide Na_0.67Mn_0.6Ni_0.1Cu_0.20Co_0.1O₂ prepared by Zhao Q. Q. et al. (2021).

NaFeO₂ is a traditional layered oxide which consists of a closed oxygen array as a possible candidate material for rechargeable SIBs, α-NaFeO₂ was also regarded as “O3″ structure by Delmas. Qin et al. (2021) synthesized Na_2/3Fe_1/2Mn_1/2O₂ through sol-gel method. The optimum calcination temperature was 900°C. The material displayed hexagonal plate shape with an average diameter of 2–4 μm, and showed 243 mAh g⁻¹ initial discharge capacity at 26 mA g^-1 with excellent cycling performance.

Sui et al. (2019) prepared Na_2/3Fe_1/2Mn_1/2O₂ by adopting simple spray drying method at low temperature. The as-obtained Na_2/3Fe_1/2Mn_1/2O₂ displayed good rate and cycling performances. The initial discharge capacities was 217.9, 171.3 and 117.4 mAh g⁻¹ at 0.1 C, 0.5 C and 2 C, respectively. This oxide was expected to be used as the suitable cathode for SIBs. Ding et al. (2017) studied the electrochemical properties of Na_2/3Fe_1/3Mn_2/3O₂ oxide in ionic liquid electrolyte which was heated to 90°C for the first time. At 20 mA g^-1, the electrode demonstrated a 227 mAh g⁻¹ initial discharge capacity. Furthermore, Kumar et al. (2021) found that the electrochemical performance of P2-Na_0.67Mn_0.5Fe_0.5O₂ decreased slowly due to surface reconstruction, ingrain cracks, reduction and dissolution of transition metal and decomposition of electrolyte, and suggested that element doping is an effective way to solve the above defects.

Recently, Darbar et al. (2021) reported a titanium doping strategy based on a wet chemical method to improve electrochemical properties of Na_0.67Mn_0.5Fe_0.5O₂. Ti⁴⁺ replaces Mn and Fe atoms in the material and effectively reduced the Jahn-teller distortion. It was found that Ti⁴⁺ could increase the thickness of sodium layer and minimize the volume strain in the layered structure, showing enhanced structural stability. Yoda et al. (2020) synthesized O3-Na_5/6Fe_1/2Mn_1/2O₂ and Na_5/6Fe_1/3Mn_1/2Me_1/6O₂ (Me = Mg or Cu). Although the undoped sample provided more than 150 mAh g⁻¹ discharge capacity, its exhibited poor wet-air stability and cycling stability in aprotic sodium batteries, but the addition of copper and magnesium can improve the wet air stability. The following effort results will provide ideas to improve the electrochemical performance of SIBs.

Wang S. et al. (2021)) designed a submicron O3-NaCrO₂ microsphere (s-NaCrO₂), which had a higher apparent Na⁺ diffusion coefficient. The as-prepared s-NaCrO₂ showed impressive electrochemical performances, which had a very high reversible capacity of 90 mAh g⁻¹ at 50 C. Moreover, when cycled at a quite high rate of 20 C, it remained 87% of its initial discharge capacity after 1,500 cycles. In addition, s-NaCrO₂ had advantages at high temperature (50°C) and low temperature (−10°C). Hamani et al. (2011) observed that P2-Na_0.7VO₂ generated a very reversible charge-discharge behavior of ∼105 mAh g⁻¹ within the voltage range of 1.2–2.6 V Na₂RuO₃ reached 140 mAh g⁻¹ with average potential of 2.8 V/Na⁺. In the 20 cycles, the capacitance retention and coulomb efficiency were close to 100%, indicating that the electrochemical reaction has high reversibility. Siriwardena et al. (2019) synthesized iron substituted Na_1.33Ru_0.67O₂ (NRFO_x) phase by conventional solid-state method, NRFO₅ showed the highest specific discharge capacity of 110 mAh g⁻¹ and 61 mAh g⁻¹ at 0.2 C and 2 C rates in the voltage range of 2.0–3.7 V. The results showed that the level of iron substitution significantly affected the structural stability and discharge capacity.

The exploration of layered Na₂MO₃ is of great necessity to prepare new TMOs, although the structural evolution and electrochemical performance optimization during Na⁺ insertion and extraction still need to be further studied.