NMO is a contractual structure of Na₄Mn₉O₁₈ and has an orthorhombic lattice structure. MnO₅ quadrangular pyramid and MnO₆ octahedron constitute the entire unit cell structure. At the same time, Mn ions are distributed in different regions. All Mn⁴⁺ ions and half of the Mn³⁺ ions are located at the octahedral site of MnO₆, and Mn³⁺ are located at the position of the MnO₅ square pyramid within the lattice [25]. The tetrahedron connects two octahedral chains shared by two sides and one three sides, forming a large S-shaped tunnel stabilized by four rows of Na⁺ and a smaller tunnel stabilized by one sodium ion [26–28]. A tunnel channel can maintain a smooth position in the channel through a discontinuous polygon tunnel, and the channel does not meet the requirements of Na(1) and Na(2) in Fig. 1d [29], the position in the S-shape. The Mn particle within the structure has Mn³⁺ and Mn⁴⁺ cornice domain that area unit charge-neutral, given as Na₄¹⁺Mn₄³⁺Mn₅⁴⁺O₁₈²⁻. NMO monocrystals were synthesized by the flow method in 1173 K for the first time. The characteristic Mn³⁺-O coordination for the Mn1 and Mn2 sites in the present NMO structure are shown in Fig. 1e [25]. Na⁺ tend to move toward the center in the S-type tunnel. The Jahn-Teller distortion can be determined by the key distance variance. The Jahn-Teller effect describes the instability of a nonlinear molecular system in a degenerate electron state, which eliminates degeneracy and forms a new system with low symmetry and low energy. In the nonlinear MnO₆ octahedral field, Mn³⁺ generally exists in a high spin state and has a very large magnetic moment, which causes the linear MnO₂ arrangement to elongate along the axial direction, resulting in Jahn-Teller distortion. The high spin Mn³⁺ ion has only one electron in the doubly degenerate e orbital, which leads to the asymmetry of its electron distribution, and the electrons in the end orbital show different shielding effects on Mn nucleus in different directions. To stabilize the Mn³⁺ ion, the two longitudinal Mn-O bonds will be elongated and the four horizontal Mn-O bonds will be shortened. Therefore, the dramatic structural changes caused by the Jahn-Teller effect caused by high spin Mn³⁺ ions seriously affect the electrochemical cycling performance for NMO materials. The transformation of high spin Mn³⁺ ions into low spin states will also expecting suppress the Jahn-Teller effect. The content percentage of Mn⁴⁺ and Mn³⁺ can also be determined by the diversity of Mn sites. It provides a way to understand the relative charge of the Mn site. Kruk et al. [30] reported the relationship between the share of manganese sites associated with sodium sites. The discrimination spectrum of Na_0.4(2)-MnO₂ was obtained by Rietveld analysis of the optical phenomenon information of X-ray synchrotron. From the analysis of soft X-ray absorption spectroscopy (sXAS), the process of absorbing incident photons may lead to the release of electrons from the atomic nucleus of the vacancy state in the left panel. In principle, after the central electron is excited by collision, the excited state shows various characteristics. Otherwise, under the influence of intermediate holes, the shell electrons are excited, vibrated and rotated, and the charge is excited. After the last shell electron is excited and attenuated, as shown in Fig. S1 (Supporting information).

High structural stability is one of the most prominent advantages of sodium manganese oxide tunnel-type structure. Baudrin et al. first proposed a mechanism for medium storage of sodium in 2007 [27]. The combination of potentiostat intermittent titration technique and in situ X-ray diffraction measurement can prove that there were six two-phase transitions in the set voltage range (relative to Na⁺/Na). The insertion process was completely reversible in the composition range 0.25 < x < 0.65 and provides a certain degree of irreversibility when x was less than 0.25. More detailed mechanisms were studied by combining electrochemical characterization, such as cyclic voltammetry, structural determination, infrared spectroscopy and thermal barrier calculation. Chae et al. [29] showed that the diffusion barrier of Na(1) site was significantly higher than that of Na(2) and Na(3) sites. The reason for the faster diffusion of sodium ions in NMO structure is that the activation barrier of three sodium sites is weaker than that of other sites. The redox state of manganese atom can be determined by the distance between particles. Due to the relatively small attraction of manganese bonds, the length of Mn-O increases slightly from 1.906 Ω to 1.980 Ω with slight Jahn-Teller distortion.

The chemical diffusion coefficient of Na⁺ was improved by the Intermittent Galvanostatic Titration Technique test and by the Electrochemical Impedance Spectroscopy test. The yield of the electrodes depends on the density of the active Na⁺ transport planes (d), the solid phase diffusion in the electrode (D) and on the diffusion length (L). That is Eq. 3 [31]:

The geometric diffusion parameters of the materials (L) and the density of the active transmission planes (d) can be vital for the velocity characteristics of the materials. As the depth of sodium removal increases, the charge transfer resistance first decreases and then increases, and the diffusion coefficient of Na⁺ in solid materials first increases and then decreases. As the number of cycles increases, the charge transfer resistance of the material increases, the diffusion coefficient of Na⁺ in the material decreases, and the electrochemical performance of the material deteriorates. From a crystallographic point of view, the diffusion path of Na⁺ through the Na(2) and Na(3) positions is more accessible to identify than the Na(1) position. Therefore, at the Na(1) position, Na⁺ operate by a hopping mechanism at this position, and at the Na(2) and Na(3) positions, Na⁺ operate through diffusion this position. The Na⁺ at the Na(2) and Na(3) positions have triangular prism coordination with the Na-O bond at a distance of 2.3–2.4 Å During the intercalation/deintercalation, Na⁺ are transmitted along the c-axis in the specified direction.

The solid line part of the sXAS spectrum at the edge of Mn-L shows that Mn²⁺ collected on the electrode, Mn³⁺ and Mn⁴⁺ are in the initial charge (Na_0.22MnO₂, 1Ch) and initial discharge (Na_0.66MnO₂, 1D). The dotted line is the calculated spectrum of the electrode sample. The spectrum was a linear combination of three reference spectra, which can directly measure Mn²⁺, Mn³⁺ and Mn⁴⁺ [32]. Also, the sXAS of the NMO electrode were studied comprehensively. A comparison diagram of the collected experimental data and calculated experimental data with solid lines and dashed lines under different electrochemical potentials. A graph showing the relationship between manganese ion concentration and sodium ion concentration of different valence states in the electrode during the reflection process [33], as shown in Fig. S2 (Supporting information).

The special quasi-random structure (SQS) methodology is applied to mimic the disordered NMO, and supported this, the simulation of molecular dynamics and the calculations of the primary principle are used to study the Na⁺ diffusion mechanism and the transport pathways after reasonable approximations [34]. The best adjustable equivalent circuits were shown within the part of Figs. 2a and b. To look at the diffusion mechanism obtained by molecular dynamics (MD), the bond valence-energy state-landscape mapping (BVEL) was shown in Fig. 2c. In the part of Figs. 2d–h [34], all possible trajectories of the minimum energy paths (MEPs) between two ions along the axial direction are shown. The graph shows the energy distribution of the two ions in the migration process. Fig. 3 clearly shows the structural changes of Na⁺ during intercalation and deintercalation. The 2D in-situ XRD patterns and charge-discharge curves of the first three cycles and the XRD patterns of Ti (004) peak after the first three cycles are shown in Fig. 3a. Fig. 3b shows the XRD patterns of peak T2 (102) (200), the lattice parameters of a-axis and c-axis T1 structure, the lattice parameters of P2 structures A and C, and the lattice parameters of a and c in T2 system. Fig. 3c shows the volume changes of T1 structure, P2 structure and T2 structure in three periods [35].

In recent years, the research of tunnel manganese oxides has made extensive progress. The specific capacity synthesis methods, electrochemical properties and voltage range of tunnel-type manganese oxides are summarized in this paper of Table 1.

The above equation shows the relation between the cationic potential and the weighted average Na ionic potential . Ion potential is an index that characterizes the surface charge density of ions and reflects the polarization power of cations. The ion potential increases with the oxidation state and atomic mass. Larger cation potential means stronger transition metal (TM) electron cloud extension and interlayer electrostatic repulsion, resulting in P2-type structure, with more covalent TM-O bonds and increased d(O-Na-O) distance. On the contrary, the larger average Na⁺ potential obtained by increasing the Na content increases the shielding of the electrostatic repulsion between the TMO₂ sheets, which is beneficial to the O3 type structure. Considering the different Na content, oxidation states of TMs, the composition of TMs, the cationic potential of Na, TMs and oxygen and the ratio of total TMs are summarized in Table 2.

Most of the research on sodium ion battery materials is carried the basis of the research on lithium-ion battery materials. Therefore, its synthesis method also draws on the synthesis method of lithium ions battery materials. The synthesis method of Na_xMnO₂ is similar with Li_xMnO₂. Various artificial techniques for preparation of NMO were according as well as solid-state [34,37–45], molten salt [46–49], hydrothermal [41,50–54], microwave [55], sol-gel [56–58], spray pyrolysis [59], polymer-pyrolysis [60], electrospinning [61,62], combustion [63], reverse microemulsion [64–66], coprecipitation [67–69], glycine nitrate method [70], sonochemical methods [71] and other methods [72,73] are summarized in Fig. 4.

The high-temperature solid phase method is one of the commonly used methods in the synthesis of inorganic materials. This method has the advantages of simple operation process, easy control of reaction conditions, and low production cost. Therefore, it is widely used in the synthesis of various inorganic materials in industrial production. However, the disadvantages of this method are uneven composition, easy agglomeration of particles and wide distribution of particle diameter. The synthesis of Na_xMnO₂ by the high-temperature solid phase method is mainly to mix the sodium source and the manganese source uniformly according to a certain stoichiometric ratio, and the mixture undergoes a chemical reaction under high temperature conditions. Sodium-to-manganese ratio, roasting temperature and roasting time have a certain influence on the deintercalation process of Na⁺ in the material. With the increase of sodium-to-manganese ratio, roasting temperature and roasting time, the solid electrolyte interphase (SEI) film diffusion activation energy W and interface of the synthesized material both the electrochemical reaction activation energy ΔG and the solid phase diffusion activation energy Ea show a trend of first decreasing and then increasing. Among them, the ratio of sodium to manganese has a small effect on the activation energy F of the diffusion of the synthetic material SEI membrane, and has a greater effect on the activation energy E_a of sodium ion diffusion in the solid phase, while the baking temperature and baking time have a greater effect on the activation energy E_a of sodium ion diffusion in the solid phase. The sol-gel method and the molten salt method can overcome this shortcoming, because the advantages of this method include the small particle size of the synthesized cathode material, the size can be from nanometer to micron, and the shape can be silk, thin and spherical, thus improving the performance. There are also some problems with sol-gel method, the raw materials used are expensive, some are organic, harmful to health, the reaction takes a long time, and many gasses and organic compounds will overflow during the drying process. Microwave is the electromagnetic wave between 300 MHz and 300 GHz. Its advantages are non-direct heating in order to avoid overheating on the surface of the material. It can be applied to various heating equipment such as domestic microwave ovens. But the amounts of materials synthesized by microwave method is small, and the instrument is more expensive than other methods. The advantages of electrospinning are simple and easy to operate. However, it is difficult to separate nanofibers from each other by electrospinning, and the current production of electrospinning machines is very low. When burning organic matter, it releases a lot of heat to reduce the final burning temperature. At the same time, when burning organic matter, it produces a lot of gas to reduce the agglomeration of products and the smaller particles of products. The products synthesized by this method have uniform particle size and composition, and the low sample synthesis temperature reduces the energy consumption. However, this method has small treatment amount and increases the cost after adding organic substances. For example, nanometer particles with higher crystallinity have better performance, and cathode materials that are intercalation/deintercalation along a given direction have relatively better performance [74]. The prepared electrode has good electrochemical performance. After 50 mA/g cycles in the range of 3.5–4.25 V, the initial discharge capacity is 145.5 mAh/g, and the capacity retention rate is 86.1%. After 50 cycles of 3.5–4.5 V, the discharge capacity reached 177.9 mAh/g, and the capacity retention rate was 85.6%. Zhou et al. had synthesized the Na_xMnO₂ (x = 0.66) three-dimensional tunnel-type cathode materials by chemical pretreatment with sodium biphenyl reagent. The initial charge capacity of cathode material obtained by this method is excellent [75]. In addition, increasing the surface area and shortening the diffusion length make the application of nanoparticles in rechargeable batteries very attractive. One of the foremost effective ways to boost power density is to reduce the cathode particles as much as possible. The nanoscale powder is calcined at 600 ℃ for six hours and then washed with a large quantity of water to get rid of impurities. Under the same cycle conditions, the capacity of the cathode material obtained is increased to 100 mAh/g. When cathode material is the nanoscale influence of anisotropic characteristics of the electronic structure, this can be achieved by electron energy loss spectroscopy (EELS) to prove that the spectra can be revealing the reduction of element valence state near electrode and metal oxide layer stack direction, which can reveal the cathode material anisotropy and explain the advantage of nanometer size to improve the electrochemical performance [76–78]. When the particle morphology is needle-like or rod-like, the cycling performance of the cathode material is better. This special morphology is characterized by a large surface area and volume, and it can still show good cycling performance when larger discharge current is generated. Excess sodium content plays a vital role in the formation of nanorods, which grow perpendicular to the grain direction. Due to the release of energy by Na⁺ of deintercalation, the current and voltage characteristics of the battery formed by nanorods show six voltage platforms. Many mathematical models can be used to simulate and fit experimental data in NMO, which is thermodynamically consistent with the characteristics of a simple system [52,79–81].

The hydrothermal method is one of the commonly used methods for synthesize materials in the liquid phase method. The principle is that the reactants are placed in a high temperature and high-pressure kettle and the raw materials are synthesized by using water or other solvents as the medium. The material synthesized by hydrothermal method has high purity, uniform particle size and excellent crystallinity. The morphology of the product can be controlled by controling the reaction conditions. Hydrothermal method requires high equipment, technical and safety performance. The hydrothermal synthesis of Na_xMnO₂ is mainly based on the soluble sodium salt and manganese metal oxides as raw materials, with water or other solvents as the medium, in the reaction kettle for chemical reaction.

Su et al. synthesized Na_0.7MnO₂ nanosheets by hydrothermal method with a current density of 40 mA/g and a specific capacitance of 163 mAh/g, showing good electrochemical performance [82]. Liu et al. have synthesized Na-Mn-O composite materials by hydrothermal method. Using manganese sulfate and manganese nitrate as raw materials and H₂O₂ as oxidant, the metal oxide of manganese was prepared by electrodeposition method as a precursor, and then the precursor hydrothermal reaction took place with sodium hydroxide. After heat treatment, the reaction product was prepared with sodium manganese oxide composite material. At the current test density of 0.1 C, the first specific discharge capacity of the synthesized material is 185 mAh/g, which shows good electrochemical performance [83]. Tan et al. used the hydrothermal method to take Mn₂O₃ and NaOH as raw materials. The raw materials were placed in an autoclave and reacted for 46 to 96 h at 205 ℃. After the reaction products were filtered, washed and dried, they were calcined at 600 ℃ for two hours, finally obtaining Na_xMnO₂ cathode materials [84]. Hosono et al. synthesized NMO cathode material by hydrothermal method using manganese tetroxide and sodium hydroxide as raw materials, first, 0.1 g of manganese tetroxide powder was weighed and dispersed in 50 mL of sodium hydroxide solution, then the solution was transferred to an autoclave for 96 h reaction at 205 ℃, and the final product was filtered and washed and dried in vacuum at room temperature. The specific discharge capacity of the composite material was 115 mAh/g at the current test density of 0.42 C, and the capacity retention rate was 88% after 20 cycles at the current test density of 8.3 C [53]. Liu et al. synthesized NMO nanorods using KMnO₄, MnSO₄ and NaOH as raw materials by hydrothermal method, and investigated the effect of adding carbon nanotubes on the electrochemical properties of the materials. With the addition of carbon nanotubes, the material was tested at a test current density of 0.1 A/g. After 50 cycles, the capacity retention rate of the material increased from 70 ℃ to 82%, and the specific discharge capacity of the material increased from 45 to 99 mAh/g at a test current density of 2 A/g. The results show that the addition of carbon nanotubes is beneficial to improve the cyclic stability and rate performance of the materials [50]. Li et al. synthesized nanowire NMO by hydrothermal method. The Mn₂O₃ powder was dispersed in NaOH aqueous solution and reacted in a 200 ℃ reactor for two days. After centrifugation and washing, the reaction product was reacted with the same NaOH solution in a 200 ℃ reactor for two days. The final product was obtained by calcination at 600 ℃ for two hours. SEM results showed that the product was nanowire structure less than 100 nm [85].

To improve the performance of the cathode material, later in the research process, the method widely recognized by researchers is to shorten the ion diffusion length and expand the area of the electrode and electrolyte contact surface [41], synthesized nanoscale materials, various shapes such as stick [62,86–88], tablet [89], powder [59,90], super long submicron flat shape [91], nanofibers [65], polygon [61], nanowires [53,79,92], which further increases the interface charge dynamics, improving the electrochemical performance of sodium-ion intercalated. Liu et al. reported nanowires through electrospinning, and the process of synthesizing nanowires. The preparation of composite electrode can improve electronic conductivity. Liu et al. reported a NMO nanorod@CNA, the cyclic properties of the final composite material [53], which was a comparison of the cyclic performance of the composite electrode and the pure electrode at 0.1 A/g after 50 cycles. The discharge capacity of the composite electrode is about 4% higher than that of the pure electrode. Through comparison, the capacity retention rate of the pure electrode was much lower than that of the composite electrode, which was about 82% and 70%, respectively. NMO@rGO nanocomposite was prepared by using highly conductive reduced graphene oxide (rGO) by Fu et al. [93]. Due to the greatly increased ionic and electronic performance, and its electrochemical performance was significantly enhanced to 124 mAh/g at 0.2 C. PVP combustion synthesis technology has the best electrochemical performance among many synthesis methods. Through X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy to analyze its morphology and structure, all samples show good magnification ability, and the performance of the synthesized sample at 900 ℃ was the best discharge capacity 122.9 mAh/g at 5 C [63].

In addition to the above methods, there are also combustion and spray pyrolysis methods for the synthesis of NMO cathode materials. The principle of combustion method is that the polymer is pyrolyzed at high temperature. According to the experimental process, combustion method is also a sol-gel method. The difference is that the polymer releases a large amount of gas at high temperature, thereby inhibiting the growth of particle size of the material. Ferrara et al. [94] used combustion method to synthesize NMO cathode materials. Manganese nitrate tetrahydrate, sodium nitrate and urea were used as raw materials. The raw materials were dissolved in deionized water at a stoichiometry of 80 ℃ under continuous stirring condition. The final product was obtained by heat treatment from 500 ℃ to 700 ℃ for 2 h. At the test current density of 2 C and 5 C, the specific discharge capacity of the synthesized material was 95 mAh/g and 78 mAh/g, respectively. After 200 cycles, the capacity retention rate of the synthesized material was still 75.7%. Bucher et al. [95] synthesized sodium manganese oxide cathode materials by combustion method. With sodium nitrate and manganese acetate as raw materials, they were dissolved in deionized water at a certain stoichiometric ratio to form a solution. Then concentrated nitric acid and a small amount of gelatin were added to the solution, and the precursor powder was obtained by spontaneous combustion of the solution at 205 ℃. The precursor was heat treated at 800 C for 4 h to obtain Na-Mn oxide cathode material. The specific discharge capacity of the synthesized material is 138 mAh/g at the test current density of 0.3 C, and 75 mAh/g after 20 cycles. Dai et al. [63] used NaHCO₃ and Mn(OAc)₂·4H₂O as raw materials and polyethylpyrrolidone as chelating agent to prepare NMO cathode material by gel-combustion method. The electrochemical performance of the synthesized material was tested in the voltage range of 2.4 V. The specific discharge capacity of the material was 122.9 mAh/g at the test current density of 0.2 C. It shows a high discharge specific capacity. Spray pyrolysis method is to dissolve the raw material in the solvent, and quickly evaporate the solvent in the solution with the spray device to get new solid particles. The particle size and morphology of the synthesized material are relatively uniform.

The slow intercalation/deintercalation of sodium ions in tunnel-type SIBs and the slow transmission of sodium ions in the matrix material framework will greatly reduce the specific capacity and rate performance. The volume expansion caused by sodium ion implantation also causes the phase transformation and lattice change of the matrix material, making it difficult to obtain a good electrochemical stability. In addition, due to the low potential of sodium and large atomic mass, the specific energy of sodium ion battery is also lower than that of lithium-ion battery. So, building a cheap sodium-based material with high specific energy remains a challenge, mainly to overcome the low energy density and poor cycle life of SIBs. The theoretical capacity of NMO is about 120 mAh/g, and the capacity retention of NMO varies from hundreds of cycles to dozens of cycles, so there is a great space for improving the performance of tunneling materials. To improve the electrochemical performance of the cathode material, the researchers attempted to replace the manganese site of the NMO cathode with anions [37,38,94,96]. The manganese site can be replaced by titanium ions without affecting the crystal structure, which can prevent the structural changes of Na⁺ during intercalation/deintercalation. The cathode materials Na_0.44Mn_0.78Ti_0.22O₂ and Na_0.44Mn_0.67Ti_0.33O₂ have similar cycle stability and high capacitance retention. Ti⁴⁺ does not have electrochemical activity, and the chemical valence does not change before and after charge and discharge. The discharge capacity of Ti⁴⁺ substituted cathode materials is slightly attenuated compared with that of unsubstituted cathode materials, but the stability is improved. Surprisingly, the materials kept their tunneling structure when the sodium content was raised [96].

Replacing the double ions with the cationic manganese with iron and titanium provided a stable cathode material Na_0.61Mn_0.27Fe_0.34Ti_0.39O₂, which was synthetically prepared in a solid-state using additional dopant precursors of Fe₂O₃ and anatase TiO₂ [43]. Its higher usable capacity is about 90 mAh/g, and its performance was outstanding in all tunneling cathode materials. Fe³⁺ is an electrochemical active ion and participates in redox reaction during charge and discharge, which is beneficial to increase discharge capacity. When hard carbon was used as the anode electrode, the reversible capacity and cycling performance of the rechargeable battery are very excellent, and the discharge capacity can reach 300 mAh/g. This new electrode material has great potential to be a candidate for large-scale application. The synthetic route of the material is simple, and the function is rich. Not only can it be used as cathode material, but also can be extended to the anode material, to guide the development direction of new materials.

The crystal structure of NaAl_0.1Mn_0.9O₂ is changed from pure tunnel-type to hybrid layered structure after the doping of Al ions at the sodium site. The replacement material has high stability of the cathode material due to the formation of Al-O bonds on the surface of the material. The replacement of aluminum has high stability due to forming shape of a very stable Al-O bond on the surface [42]. Although the introduction of aluminum can cause phase transition, it can improve the stability of particles. The surface stability of particles is better because of the formation of Al-O bonds than that of tunnel-type NMO. NaAl_0.1Mn_0.9O₂ showed a high discharge capacity. The initial discharge capacity was 145 mAh/g, and the capacity retention rate was 71.0% after 100 cycles. Compared with NMO, the capacity retention rate was higher. The high initial discharge capacity of the cathode material is the formation of the orthogonal laminated NaMnO₂ phase.

The performance of zirconium doped Na_0.44Mn_1-xZr_xO₂ cathode material is brilliant [97]. In situ X-ray diffraction shows that the volume change of Na_0.44Mn_0.98Zr_0.02O₂ optimized by Zr doping is less than 5% during the initial charge-discharge process. In addition, the diffusion coefficient of Na⁺ is significantly increased, and the multiplier performance is significantly improved. A high reversible capacity of 112 mAh/g can be obtained at 1 C. The 78 mAh/g can be maintained after 1000 cycles and retains 80% capacity at 5 C to support a long cycle life. It shows that Zr substitution provides a new idea for optimizing excellent performance cathode of sodium-ion battery.

NMO nanorods substituted by B can be synthesized by solid-phase method [98]. As the content of B increases, the appearance of the impurity phase can be seen in the XRD of NaMn_2−xB_xO₄. The temperature of α-Mn₂O₃ is higher than that of another manganese alloy [41,99]. It is shown that the contents of α-Mn₂O₃ and Na(BO₂)₂ sections increase with the decomposition of the NMO phase. The content of boron is reciprocally proportional to the number of nanorods. The number of nanorods is inversely proportional to the amount of boron content. When the boron content increases, the number of nanorods decreases in a particular proportion, and the particle size and strain will decrease accordingly. Because the B ion may be immersed in the NMO phase, the C axis will increase monotonically. At the same time, the increase of boron content will produce the boron impurity phase. By affecting the growth of nanotubes, the morphology of nanorods is concerned, and the surface area is increased, and the length is decreased, which is anticipated to be employed in reversible sodium-ion batteries.

Lithium exchange has been attempted at sodium sites [90]. Some dopants such as cobalt led to phase transformation from the original tunnel-type structure towards the P2-type layer structure [100]. Na_0.44Mn_0.89Co_0.11O₂ substituted with cobalt offers greater discharge capacity with excellent retention capacity [101].

Fluoride doping at oxygen sites can reduce the bond energy between oxygen atoms and enhance the diffusion of Na⁺. Na_0.44MnO_1.93F_0.07 material can be synthesized by the sol-gel method, and the cathode material obtained has high discharge specific capacity and cycling performance [102]. The results show that fluorine substitution may be an effective method to change NMO tunnel-type structure and enhanced electrochemical activity. It also provides a high-level argument for superior performance composite structural cathodes for SIBs that introduce halogen anions to exchange structural oxygen.

The Mg²⁺ was introduced into the hinge of the cathode material of the Mn-based tunnel structure. The tunnel-type structure NMO promotes the transport of Na⁺ and reduces the released energy of sodium. By adding electrons near the Fermi level, the stability of the front cycle and the performance of the various rate are enhanced to some extent, and highly reversible electrochemical reactions and phase transitions are realized in the cathode. The initial discharge capacity of Na_0.44Mn_0.95Mg_0.05O₂ remains 67% after 800 cycles at 2 C, which is significantly higher than that of without doped NMO [103]. This method can improve the properties of cathode materials for tunnel structures.

Although the doped crystal structure can improve the electrochemical performance to a certain extent, this modification method will change the pure tunnel-type structure into another phase. The electrochemical performance can also be enhanced by co-doping the two elements [104], which reports the substitution of manganese for Fe and Ti cathode materials. In addition, the co-doping elements are chromium and lithium, which can still form a tunnel structure. However, the structure of Al-doped Mn site is a mixed phase of layered and tunnel structure, and the substitution of cobalt and fluoride at Mn site will result in the transformation of crystal structure from tunnel structure to P2-type layered structure. Single cationic and cationic doping or co-doping can further study the doped ions and doping mechanism.

Another way to modify materials is to apply a coating to the surface of the material. The coated cathode material can form a protective layer on the surface of the cathode material, which acts as a buffer and can promote the diffusion of surface charge to avoid the occurrence of side reactions. Zhao et al. [36] coated NMO with a layer of Na₂MoO₄ by molten impregnation, which can promote electron transfer in SIBs. The surface layer of Na₂MoO₄ has a significant contribution to the stability of cathode materials in high-rate sodium storage. The coated material has improved capacity and high efficiency, and even at the rate of 60 C, it also has about 100% excellent performance until 1000 cycles. It was surprising that the NMO electrode has an amazing capacity retention of up to 1000 cycles at 60 ℃. For 5–10 nm coating on the surface can show the excellent cycle performance of the electrode at high speed. The SEM comparison diagram at low magnification was not shown in this paper, and the influence of coating amount on particle agglomeration can be further explored, as shown in Fig. S3 (Supporting information).

Strikingly, 2 wt% NaTi₂ (PO₄)₃ coating NMO cathode materials synthesized by sol-gel method [67] or hydrothermal method [105] inhibits cationic dissolution [106]. Na₂Ti₆O₁₃ has high conductivity, this advantage can suppress the phase change process of the positive electrode material coated with this material and enhance the stability of the crystal structure as presented. As the pictures show, the cyclic voltammetry characteristic curves show six redox peaks. And there are two kinds of phase transitions in the whole process of charge and discharge. In agreement with the CV results, six voltage platforms conjointly appeared on the initial charge and discharge curves of the NMO and NaTi₂(PO₄)₃ (NTP) coated samples. The polarization of the NTP coated sample is slightly less than that of the original sample. For the cycling and rate properties, the coated sample was significantly improved.

The chemical stability will be affected in the high voltage region, and the chemical stability will be improved by Al₂O₃ coating on the cathode material surface [107]. The performance of the cathode material passed through the alumina layer is better than that of the original submicron rod NMO. Using Al₂O₃ as the layer can prevent the direct contact between the submicron rod and the electrolyte, and more importantly, it can prevent the volume expansion in the electrochemical process, which has certain reference significance for the surface coating. The direction of the research has always been to prepare materials with good stability and good diffusion kinetics.

Whitacre reported that Na₄Mn₉O₁₈ was used as a cathode material for SIBs for aqueous-electrolyte energy storage devices, The specific capacity of Na₄Mn₉O₁₈ in the neutral electrolyte was 45 mAh/g by solid-state reaction [109,110]. When the mass ratio of positive and anode electrodes is adjusted, the battery can be charged to about 1.7 V with no significant hydroelectric solution. Li et al. [111] study demonstrated that the whole battery in NaTi₂(PO₄)₃/NMO aqueous solution exhibited ultra-fast rate capability and excellent high-rate cycling stability, which was the higher specific capacity of sodium electric system reported so far in the sodium ion aqueous solution system [111]. At a current of up to 90 C (144 mA/cm²), the reversible capacity of the entire cell is 70% of that of 3 C, and the capacity retention rate was about 60% after 700 cycles. Wang et al. [37] prepared a new type of sodium rich tunnel-type cathode material by solid phase method, which was Ti-substituted Na_0.66Mn_0.66Ti_0.34O₂. Using NaTi(PO₄)₃/C as the anode material, NaTi(PO₄)₃/C showed a high specific capacity of 76 mAh/g in 1 mol/L NaSO₄ aqueous electrolyte at a current of 2 C.

Although NMO exhibited good cyclic stability in neutral electrolyte, the previously reported discharge capacity only was 40 mAh/g, which was mainly because H⁺ in water limited the intercalation/deintercalation of Na⁺. The advantage of using alkaline electrolyte is that the activity of H⁺ can be reduced, so that the H⁺ intercalation is affected. The electrochemical properties of NMO in alkaline electrolyte were reported for the first time by Yuan et al. [112]. In the voltage range of 1.95–1.1 V, the reversible capacity of 6 mol/L NaOH electrolyte was 80.2 mAh/g, superior to neutral electrolyte. At the current rate of 20 C, the reversible capacity of the NMO electrode is showing excellent rate performance around 42.7 mAh/g. They also found that the NMO electrode was highly resistant to overcharging, which not only has good rate performance and cycling performance in alkaline electrolyte, but NMO-base alkaline battery also has the advantages of metal corrosion resistance. Increasing the NaOH concentration is beneficial to inhibit the occurrence of hydrogen intercalation reaction and improve the cycle performance and rate performance of the electrode, but at the same time it will also cause the early triggering of the oxygen evolution reaction. If the concentration is too high, the rate performance will be reduced. In addition, we can reduce the side reaction and improve the cycle performance by reducing the voltage window. NMO also shows excellent overcharge resistance in concentrated alkaline electrolyte. This not only indicates that the sodium storage performance of NMO can be greatly improved by optimizing the electrolyte system and test conditions, but also indicates that NMO as the cathode material of aqueous SIBs has a good application prospect in the field of large-scale energy storage.

SIBs can also use ionic liquid electrolytes [113]. Butylmethylpyridine-bis (trifluoromethonyl) imide (BP-TFSI) ionic liquid (IL) or glycine [70] with various sodium solute, namely NaBF₄, NaClO₄, NaTFSi and NaPF₆, acts as electrolytes for rechargeable Na/NMO batteries. Because in the ionic electrolyte, the electrolyte has high stability can be used at high temperature. In the electrolyte containing NaClO₄, the electrode material showed good electrochemical performance. When the charge and discharge ratio is increased to 1 C, the capacity retention rate is 85%. With 1 mol/L NaNO₃ and 0.96 mol/L Mn(NO₃)₂ added to solid glycine as the electrolyte, the cyclic voltammetry characteristic curves of NMO cathode material were tested in the scanning frequency range of 20–400 mV/s. The current density of Na_0.7MnO_2.05 synthesized at 850 ℃ is proportional to the scan rate. Reaction situation is the same as that of reaction a, and the current density of NMO synthesized at 900 ℃ is also proportional, the rate of increase is significantly faster. In summary, it is shown that the cyclic reversibility of Na⁺ is relatively stable and does not change with the change of charge and discharge rate. The performance of this electrolyte is better than that of a traditional electrolyte.