Advanced nanoengineering strategies endow high‐performance layered transition‐metal oxide cathodes for sodium‐ion batteries

Considering the abundance and low price of sodium, sodium‐ion batteries (SIBs) have shown great potential as an alternative to existing lithium‐based batteries in large‐scale energy storage systems, including electric automobiles and smart grids. Cathode materials, which largely decide the cost and the electrochemical performance of the full SIBs, have been extensively studied. Among the reported cathodes, layered transition‐metal oxides (LTMOs) are regarded as the most extremely promising candidates for the commercial application of the SIBs owing to their high specific capacity, superior redox potential, and suitable scalable preparation. Nevertheless, irreversible structural evolution, sluggish kinetics, and water sensitivity are still the critical bottlenecks for their practical utilization. Nanoengineering may offer an opportunity to address the above issues by increasing reactivity, shortening diffusion pathways, and strengthening structural stability. Herein, a comprehensive summary of the modification strategies for LTMOs is presented, emphasizing optimizing the structure, restraining detrimental phase transition, and promoting diffusion kinetics. This review intends to facilitate an in‐depth understanding of structure–composition–property correlation and offer guidance to the further development of the LTMO cathodes for next‐generation energy storage systems.


| INTRODUCTION
The ever-increasing demand for energy has greatly accelerated the consumption of traditional fossil fuels (coal, petroleum, and natural gas), which, in recent years, has resulted in energy crises and significant environmental pollution.][3][4] However, these green renewable resources are commonly intermittent and geographically distributed, which are hard to provide stable energy output for our daily life and production.Therefore, compact and fastresponding energy storage systems are necessarily explored for the storage and release of the energy produced by green resources. 5,6Among diverse reported technologies, stationary secondary batteries have been considered one of the most promising solutions for this issue due to their high safety, low cost, environmental friendliness, and so on. 7,80][11] Unfortunately, low reserves and the uneven distribution of lithium in nature raise concerns about the price and availability of LIBs for grid-scale energy storage systems.Compared with lithium, sodium, which belongs to the same alkali metal series and shows analogous chemical properties, is earthabundant and extensively distributed, subsequently giving rise to the low price of sodium-containing raw materials. 2Moreover, many preceding studies have suggested that sodium-ion batteries (SIBs) behave with a similar working principle to LIBs. 3,6][14][15][16][17] Early research on SIBs can be dated back to almost the same period as LIBs, but the pursuit of LIBs with high energy density brings about the negligence for SIBs and impedes their further development.][20] Meanwhile, some techniques and knowledge obtained from the successful commercial application experience of LIBs can be directly adapted to SIBs on account of their similar properties, which tremendously expedites the rapid development of SIBs.2][23][24] Up to now, various kinds of materials such as layered transition-metal oxides (LTMOs), [25][26][27][28] Prussian blue analogs, 29,30 polyanionictype frameworks, 31,32 and organic substances, 33 have been extensively explored as positive electrodes for SIBs.5][36] To be more specific, P or O represents the prismatic or octahedral coordination environment of Na, while the number reveals the oxygen packing sequence.P-type cathodes can provide direct and wide diffusion pathways for Na + , realizing better rate performance and cycling life.O-type structures possessing high Na contents behave at high capacity and undergo complex phase revolutions during the sodiation/desodiation process. 15,19,282][43][44] (iii) This kind of material is sensitive to air and water, which will produce an insulated inorganic layer composed of Na 2 CO 3 and NaOH.In addition, the H 2 O molecule can insert into the Na layer or interchange the Na + with H + , forming an adverse hydration phase. 45,46All these parasite reactions bring about an inhibited ability for Na + insertion/ extraction and unfavorable electrochemical performance.Nanoengineering, which endows electrode materials with numerous advantages, including short diffusion paths, improved structural stability, enhanced reactivity, small polarization, and so on, is beneficial to solve these above-mentioned problems and realize better cycle life and rate performance. 18,21These phenomena suggest that it is significant to optimize the structure and composition of the LTMO electrode materials to achieve positive progress of the SIBs.
In this review, we comprehensively summarize the existing modification strategies to ameliorate the electrochemical performance of LTMO cathodes for SIBs, focusing primarily on ion substitution, coating technique, structure design, and mixed phases.The tactics described above are beneficial for enhanced structural stability, promoted fast kinetics, and restrained irreversible phase transition.In the meantime, the relationship between the inner Na + storage mechanism and structural transformation behind the sodiation/ desodiation process is discussed in detail to further clarify the strengths of various methods.At last, some perspectives for the future development of LTMO electrode materials for SIBs are also introduced.We expect that the insights in this review can expedite the progress toward the triumphant commercial application of SIBs.

| ELEMENT SUBSTITUTION
Element substitution, also known as element doping, is one of the most common ways to regulate the crystal structure of LTMO cathode materials and is feasible for mass production.This strategy means to supersede some of the composition elements in the original sample with a few other ions, which can be added during the preparation of the undoping electrode, contributing to enhanced rate performance, stable structure, high energy density, and so forth. 47,48Element substitution is mainly subdivided into two categories: cationic and anionic substitution, according to the charge of the doping elements.Moreover, modulation of the microstructure with both kinds of ions to accommodate the volume change and optimize performance has also received much attention.

| Cation substitution
Cation substitution manifests doping some ions with a positive charge (Li + , Mg 2+ , Zn 2+ , Al 3+ , etc.) into the transition-metal (TM) site or Na + site. 1,35,42The properties of ultimate samples adjusted at the nanoscale may vary from the amount and types of the elements, which suggests the adjustability of this approach.And different elements may provoke synergistic effects when doped into the host structure at the same time.4][55] Then, LTMO cathodes with optimized composition may protect the material from erosion caused by water or air, greatly reducing the cost of storage and transportation. 56Finally, unusual site-selective substitution in the Na site serves as a pillar to stabilize the structure by increasing electrostatic cohesion between neighboring TMO 2 layers, inducing a pinning effect to achieve a zero-strain layered cathode, reducing the activation energy to improve diffusion kinetics, thus realizing enhanced rate performance. 57,58a + /vacancy ordering induced by ionic radii and Fermi level of TM occurred when Na + was extracted from the host structure leads to fast capacity decay and slow Na + mobility.Moreover, irreversible phase transitions caused by layer gliding after Na + extraction during the charge process give rise to large voltage hysteresis and large volume changes.59 Thus, the asprepared positive material demonstrates a solid-solution reaction mechanism when deeply charged, confirmed by the in-situ X-ray diffraction (XRD) measurement (Figure 1A).And it achieves excellent rate performance and outstanding cycle stability simultaneously.Xu and coworkers reported an interesting Ti-substituted electrode material Na 0.67 [Li 0.21 Mn 0.59 Ti 0.2 ]O 2 , which can show a high reversible capacity up to 231 mA h/g caused by the synergistic redox reaction of manganese and oxygen when cycled at 0.2 C. 60 Moreover, benefiting from the reduced repulsion between adjacent TM layers and suppressed irreversible manganese migration caused by additional oxygen redox, this material maintains its P2type structure during the whole process with an extremely small volume strain (0.7%) with almost no change in parameter c during the charging process, which is much smaller than the variation of c-axis in NaMnO 2 when changed into Na 0.70 MnO 2 (5.807-16.737Å) (Figure 1B).27,60 Zhou's group introduced three cationic ions (e.g., Ni 2+ , Cu 2+ , and Mg 2+ ) into the Na x MnO 2 host structure to replace part Mn and the prepared Na 0.8 Mn 0.6 Ni 0.2 Cu 0.1 Mg 0.1 O 2 (NaMNCuMg) electrode material.61 The obtained cathode can maintain 82.9% of its initial specific capacity after 500 cycles at a high rate of 500 mA/g because doping postpones the adverse P2-O2 phase to a high voltage range.In addition, divalent cation doping increases the average oxidation state of Mn, thereby eliminating the Jahn-Teller effect of Mn 3+ .Meanwhile, Cu doping endows this material with excellent air/water stability, confirmed by the aging test (Figure 1C).
As illustrated by the calculated ion-removing energies, Shen et al. found that Ca 2+ is inclined to dope into the Na + site, which can act as a "pillar" to stabilize the crystal structure (Figure 1D). 62More specifically, this pillar effect directly refrains from the detrimental Na + / vacancy ordering to ensure enhanced Na + diffusion kinetics and inhibits the gliding of TM layers at a deep desodiation state to avoid P2-O2 phase transition.Compared with Na-O bonds, the bonding energy of the Ca-O bond becomes higher on account of the highly valent and small radius of Ca 2+.As a result, the stable configuration enables reversible anionic redox activities.Peng and co-workers prepared a co-substituted [Na 0.67 Zn 0.05 ]Ni 0.18 Cu 0.1 Mn 0.67 O 2 cathode in which Zn and Cu occupy Na and TM sites of the original material, respectively. 63The Cu 2+ doping can stabilize the metal layers, while the O 2− -Zn 2+ -O 2− "pillar" with strong electrostatic cohesion mitigates crack formation and facilitates interfacial diffusion kinetics.Therefore, this electrode achieves pre-eminent long-cycle stability at high current density (Figure 1E).Chen et al. modified the representative P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 cathode with Li, Mg, and Ti, which can achieve high theoretical capacity, strengthen structural stability, and promote high redox potential, respectively. 64Moreover, Li substitution increases the Na + content and changes local structure, contributing to increased TM layers gliding energy barriers.Therefore, the as-prepared material shows excellent cycling life. 64More interestingly, introducing more cation ions even contributes to generating high entropy materials, which represent multicomponent elements co-existing in a single phase without the formation of other impurities, as compared to ordinary metal oxide systems with limited elements.This kind of high entropy material shows eminent structural robustness, greatly ameliorating the cycling performance of LTMO cathode. 65,66

| Anion substitution
Partial cationic substitution has been proven to be a useful way to enhance the overall electrochemical performance of LTMO cathodes.However, most of these selected doping elements are electrochemically inactive, so that the improved cycling life is brought at the expense of some specific capacity owing to reduced redox center. 26Reported results show that anionic substitution (F − ) in the oxygen site is also a valid measure to ameliorate the structural stability and increase the specific capacity because the strong electronegativity of F − strengthens the chemical bonds and less negative charge decreases the TM valence.8][69] Kang et al. prepared a series of Fdoping samples Na 0.6 Mn 0.7 Ni 0.3 O 2−x F x (x = 0, 0.03, 0.05, and 0.07) and found that the modified electrodes retain the original P2 structure but with changed crystal parameter (Figure 2A). 70Apparently, the left shift of (002) peaks indicates the enlarged adjacent layer distance along c-axis, which is in favor of accelerated reaction kinetics.Moreover, the blue shift of E g and A 1g bands in Raman spectra proves that F − ions replace partial oxygen, leading to shorter TM-O bond lengths.Through evaluating the specific capacities, cycling performance, and rate capability, the optimal value is F-0.05.Liu and coworkers proposed that F − substitution alters local interatomic distance and is prone to reduce the valence of Mn ions from +4 to +3 rather than having an effect on the oxidation state of Ni ions in the Na 2/3 Ni 1/3 Mn 2/3 O 1.95 F 0.05 cathode, confirmed by the X-ray absorption near edge structure (XANES) (Figure 2B). 71Redistribution of Mn/Ni and ordering disruption of Na + induced by F doping stabilizes the phase structure so that this cathode delivers distinguished capacity retention over 2000 cycles at 55°C (75.6% at 10 C).This means this battery system can behave with improved output power because the increased temperature contributes to accelerated reaction kinetics.

| Anion/cation dual-substitution
In addition to single cationic or anionic ion doping, dualsubstitution with both of them has been considered a useful method to ameliorate the performance of LTMO cathodes, which can combine the positive effect of cations and anions as mentioned above. 74,75For instance, Chae et al. doped Al and F into tunnel-type Na 0.44 MnO 2 (NMO), successfully preparing a new layered structure with 2D diffusion pathways. 72The homogeneous distribution of Al/Mn and F/O without ordering is confirmed by Fourier synthesis maps, which also clearly point out the two types of Na + (Figure 2C).Compared to the original sample, the as-synthesized Na 0.46 Mn 0.93 Al 0.07 O 1.79 F 0.21 (AFNMO) electrode shows improved rate performance (125.5 mA h/g at 5 C) and capacity retention (89.1% over 500 cycles).From the bond valence energy landscape (BVEL) calculations and ex situ XRD measurements, this codoping material shows lower diffusion energy barriers, restrained volume change, and mitigated Jahn-Teller effect of Mn 3+ , thereby realizing high electrochemical performance.Cui and coworkers proposed a significant ex situ F and in situ Mg dualsubstitution tactic to prepare the Na 0.524 Mg 0.146 Ni 0.15 -Fe 0.20 Mn 0.65 F 0.05 O 1.95 (NM-NFMF005) cathode in which Mg 2+ ions were introduced into the Na sites by a simple electrochemical method. 73Moreover, F doping further shortens the interatomic bond length and enlarges the dspacing for Na + diffusion.Therefore, this dual-site doped electrode shows a reinforced structure, enhanced electrochemical performance, and a solid-solution reaction mechanism without the emergence of a new phase (Figure 2D).

| COATING TECHNIQUE
Some reports have found that LTMO cathode materials are prone to react with electrolytes during the sodiation/de-sodiation process. 6,14,16,22,39These parasite reactions would exfoliate the active materials from the current collector, produce thick and uneven insulating cathode-electrolyte interface layers (Na 2 CO 3 ), and cause harmful phase transition. 46,54In addition, the generated HF product from the degradation of the component (NaPF 6 ) in the electrolyte further attacks the inner active material and consumes more Na + , bringing about fast capacity attenuation.To resolve these issues, researchers explored coating strategy as another valid way to modify LTMO cathode materials.These protective nanolayers on the cathode surface can prevent active materials from direct contact with electrolytes, thus inhibiting harmful side reactions. 76,77On the other hand, they can also minimize large volume change, mitigate irrevocable phase transition, and restrain metal dissolution.In the meantime, these coating layers are prioritized for the materials with good conductivity so that they decrease the interface resistance of LTMO cathodes  73 and enhance the rate performance and cycling stability, and some can even increase the specific capacity. 78,79

| Metal phosphate coating
Metal phosphate coating is one of the most common ways to modify LTMO cathodes with protective layers to improve the overall performance, which can be prepared by facial sol-gel, melt-impregnation, and chemical coprecipitation methods.To be more specific, the high electronegativity of the anionic phosphate groups is beneficial for increasing the resistance to prevent active materials from reacting with electrolytes. 76Besides, NaTi 2 (PO 4 ) 3 (NTP) coating material with NASICONtype structure belonging to the phosphate family is composed of corner-shared TiO 6 and PO 4 polyhedrons, and it shows robust structure and provides fast diffusion pathways for Na + during the intercalation/deintercalation process owing to the three-dimensional crystal framework.][82][83][84] Jo et al. adopted a bioinspired β-NaCaPO 4 nanolayer (5-10 nm) on the surface of the popular P2-Na 2/3 [Ni 1/ 3 Mn 2/3 ]O 2 cathode via a high-temperature calcination reaction between Ca-P-O-based compound and residual sodium residues (Na 2 CO 3 and NaOH). 81Due to the strong bonding among Ca 2+ and PO 4 − ions, this ecofriendly and nontoxic protective layer shows excellent chemical and thermal stability.The coating layer can effectively clear away HF and H 2 O, originating from the decomposition of the electrolyte component.Therefore, the coated sample not only suggests prolonged cycling life (Figure 3A), but also displays stable thermal properties in contrast to the bare material verified by thermogravimetry (TG) and differential scanning calorimetry tests (Figure 3B).The same group also reported another NaPO 3 coating material synthesized by the meltimpregnation method at 300°C using the NH 4 H 2 PO 4 and sodium-containing byproducts as reactants to stabilize the surface (Figure 3C). 82It plays a similar role to β-NaCaPO 4 mentioned above.And benefiting from the good effect of the NaPO 3 nanolayer with a thickness of 10 nm on the electrode surface, the obtained composite material can maintain 73% of its initial specific capacity after 300 cycles when assembled in full batteries with hard carbon as the anode.Moreover, this coating material successfully inhibits oxygen release at a deep charge state because it can postpone the formation of manganic-manganous oxide.Another interesting NTP material with a NASICON-type structure has been extensively studied as a coating layer to optimize the performance of LTMO cathodes due to its special open structural framework. 83,84In 2020, Li and coworkers successfully introduced the NTP protective layer on the surface of Na 0.67 Co 0.2 Mn 0.8 O 2 (NCM) cathode by a simple wet chemical method and as confirmed by the transmission electron microscope (TEM) measurement, the layer thickness is about 20 nm (Figure 3D). 85The as-prepared NCM@NTP composite shows better cycling stability (capacity retention of 92  86 Besides that, the Ti 4+ with an ion radius (0.61 Å) smaller than Na + (1.02 Å) doped into the host structure decreases the thickness of TM layers while increasing the d-spacing for Na + diffusion, as verified by the shift of (002) peaks in the XRD patterns.What is more, the ameliorated diffusion kinetics was confirmed by the calculated activation barrier energy, further suggesting the merit of NTP modification (Figure 3E).

| Metal oxide coating
Apart from metal phosphates, metal oxide coating is also a good option to optimize the properties because they isolate the active material and electrolyte, thereby preventing the parasite reaction and dissolution of TM ions.8][89] The atomic layer deposition (ALD) technique has been widely used in the coating field for electrode materials owing to the merits, including controllable thickness and homogeneous coating layers, which is difficult to achieve by traditional methods as well as suitable for lots of different coating materials (TiO 2 , SnO 2, and Al 2 O 3 ). 90or instance, Hwang et al. coated nanosized Al 2 O 3 on the surface of microspherical Na[Ni 0.6 Co 0.2 Mn 0.2 ]O 2 cathode through the facile dry-balling method (Figure 4A). 91The densely covered Al 2 O 3 scavenges the HF generated by the decomposition of NaPF 6 in the electrolyte and forms new stable AlF 3 material on the outmost edge of the Al 2 O 3 protective layer, further protecting the active materials from HF attack.Thus, the modified composite shows excellent structural stability and maintains 75% of its initial specific capacity after 300 cycles in a pouch-type full cell.5 (E) The activation barrier energy for Na + diffusion of pristine NMCNO and NMNCO@NTP materials.Reproduced with permission: Copyright 2020, Elsevier B.V. 86 cathode with chemically stable TiO 2 via a simple solidstate reaction. 93The TiO 2 coating accompanying partial Ti 4+ doping effectively alleviates the Jahn-Teller effect of Mn 3+ by decreasing its ratio and shrinking the TM-O and O-O bonds that are beneficial for the stable structure.Therefore, the as-prepared MFN@TiO 2 composite electrode achieves reversible phase transition (O3-P3) as compared to pristine MFN (Figure 4D).A homogeneous Al 2 O 3 layer was introduced into Na 2/3 Ni 1/ 3 Mn 2/3 O 2 by Alvarado et al. via the ALD technique (Figure 4E).This protective layer decreases inorganic species (carbonate) in the cathode electrolyte interphase (CEI) layer and reduces the resistance, thereby enhancing the Coulombic efficiency and mechanical stability. 94eanwhile, Ji et al. anchored different metal oxide nanolayers by using the ALD method to prevent deformation of the CEI layer on the Na 2/3 Ni 1/3 Mn 2/3 O 2 (NNMO) surface and dissolution of TM ions, which  95 caused serious capacity degradation. 95These protective layers effectively suppress the side reactions and contribute to the formation of a simple and stronger CEI layer, so modified samples show excellent cycling life (Figure 4F).More interestingly, the NNMO-Al 2 O 3 ALD electrode shows better performance than other samples because of its highest formation energy of Mn vacancy.

| Metal fluoride coating
Except for the above-mentioned materials, metal fluorides, which were widely applied to modify cathode materials for LIBs, have attracted much attention as promising coating materials owing to their inert property and stable structure. 96Sun et al. found that bare Na [Ni 0.65 Co 0.08 Mn 0.27 ]O 2 cathode showed numerous intraparticle cracks after cycling because the electrolyte eroded the material surface and further damaged the bulk structure, leading to the destruction of mechanical integrity and fast capacity loss.After coating AlF 3 via a simple dry ball-milling method, this stable, protective material successfully shields the cathode from electrolyte attack, thus inhibiting the infiltration of electrolyte into the bulk structure and realizing prolonged cycle stability. 96Liu et al. discovered that AlF 3 -coated Na 0.5 Ni 0.25 Mn 0.75 O 2 (NNMO) cathode greatly alleviates the damage or pulverization of the bare NNMO structure ascribing to accumulated inner stress during repeated sodiation/ desodiation process, which brings about the destruction of SEI layer and the exfoliation of active material (Figure 5A). 97

| Carbon and polymer coating
Though these inorganic coating materials enhance cycle stability and inhibit the side reaction, their effects on improving the rate performance are not outstanding due to their relatively low conductivity.In contrast, conductive carbon coating materials with porous structures, which facilitate electron transfer, are conducive to rate performance amelioration other than stabilizing the crystal structure. 100,101Uniform polydopamine-derived carbon coating layer synthesized via a conventional method was reported to improve the electrochemical properties of the P2-type Na 0.80 Ni 0.22 Zn 0.06 Mn 0.66 O 2 cathode (Figure 5B). 98he carbon layer with a thickness of 5 nm restrains the Na + loss at the cathode surface, which is inclined to bring forth detrimental Na 2 CO 3 /NaOH during electrode fabrication.Compared with the original sample, the obtained composite electrode shows tremendously enhanced rate capacity (Figure 5C,D).Polymer coating is also a good choice not only due to its low-temperature synthesis but also for the mechanical flexibility different from other coating materials.Furthermore, it helps encapsulate cathode materials with consecutive ionic conductive coating layers.Some strong polar atoms or groups on the polymer materials even anchor TM ions on the surface of the active material to achieve a more stable interface nanolayer. 102,103Lu et al. coated polypyrrole (PPy) on Na 0.7 MnO 2.05 hollow microspheres (NMOHS) by a facile chemical ice water bath route (Figure 5E). 99The hollow structure mitigates volume change and shortens the diffusion pathway for ion/electron, while the PPy coating layer ameliorates the conductivity, inhibits Mn ion dissolution, and stabilizes the structure.Therefore, the as-synthesized NMOHS@PPy composite cathode has better cycling life and rate performance than the pure electrode.

| STRUCTURE DESIGN
LTMO cathodes for SIBs always suffer from slower reaction kinetics and larger volume change than their counterparts used for LIBs due to the large size of Na + .Rational morphology and structure design are conducive to shortening the diffusion pathways for Na + , mitigating the mechanical stress caused by repeated Na + intercalation/deintercalation, and suppressing the irreversible phase transition to realize better rate capacity and cycle stability. 104,105Furthermore, a special tightly stacked structure with high tap density is beneficial to improve the volumetric energy density, which is a key factor that influences the practical application of LTMO cathodes.A well-designed internal structure can even activate reversible anionic redox to get high specific capacity; therefore, the energy density is also enhanced. 106

| Microsphere structure
Constructing microspherical LTMO cathodes with homogeneous size distribution helps build close-packed arrays to improve the tap density, which means that the space for batteries to be packed can be minimized when used in practical applications.In addition, the fluidity and dispersity of this structure enable the preparation of better electrodes. 107Thus, this distinctive structure endowed with high volumetric energy density goes far toward achieving the commercial application of nextgeneration cathodes.Microspherical P2-Na 0.7 CoO 2 (s-NCO) electrode prepared by facile self-templating method inherits the regular structure of the CoCO 3 precursors; compared with the irregular sample (i-NCO), the s-NCO cathode suggests greatly enhanced cycle life (Figure 6A,B). 108This may be ascribed to the regular structure with high crystallinity, which worked as an effective cushion against volume stress originating from repeated sodiation/desodiation process.Meanwhile, its special morphology effectively reduces the contact area between the active material and the electrolyte to mitigate harmful side reactions and dissolution of TM ions, while primary nanoparticles shorten the diffusion pathways to enhance rate performance. 108Urea-assisted hydrothermal reaction is a facile and effective way to prepare a microsphere structure composed of nanoparticles by comparison with the coprecipitation method, which demands concise control of the pH, stir speed, and other reaction parameters.The Na 0.66 (Ni 0.13 Mn 0.54 Co 0.13 ) O 2 (Na-NMC-180) buckyballs cathode synthesized at  98 (E) Schematic illustration for fabrication of Na 0.7 MnO 2.05 hollow microspheres (NMOHS) and NMOHS@PPy.Reproduced with permission: Copyright 2019, American Chemical Society. 9980°C by this method demonstrates high tap density up to 2.34 g/cm 3 and shows eminent cyclic stability (90% after 150 cycles) at a high cut-off voltage (4.7 V) (Figure 6C). 109Moreover, if the spherical structure is composed of a hierarchical columnar structure, it may achieve interesting properties rather than reduced contact area and high tap density.For instance, designing the structures with inner high Ni composition and outer high Mn composition helps to realize higher specific capacity and better thermal and cycle stability. 111  nanorods (SNAs), which reduced porosity and enhanced the mechanical robustness.Compared with the constantconcentration (CC) counterpart, the primary nanorod particles are longer and align in a single direction to get a more compact structure (Figure 6D,E). 110Benefiting from the grade concentration and special structure, the SNA cathode indicates improved cycle stability even at low temperatures (Figure 6F).

| Nanoflake and nanofiber structures
3][114] Among these reported nanostructures, nanoflake or nanosheet structure is helpful to promote the rate performance of cathode materials. 115Xiao et al. reported a multiple-layer oriented stacking nanosheets cathode which exposed {010} active facets, prepared by a thermal polymerization reaction (Figure 7A). 116The exposed active facets and special stacking structure ensure fast Na + transfer and boost rate capacity by virtue of multiple channels and curtate diffusion distances, respectively.Therefore, the assynthesized O3-type Na[Li 0.05 Ni 0.3 Mn 0.5 Cu 0.1 Mg 0.05 ]O 2 (O3-NaLNMCM) suggests pre-eminent rate capacity even at a high rate density up to 50 C (Figure 7B) and excellent capacity retention (91.9%) when cycled at 5 C after 600 cycles.Another Na 2/3 Ni 1/6 Mn 2/3 Cu 1/9 Mg 1/18 O 2 (NaNMCM) electrode with a similar morphology not only shows the above-mentioned enhanced storage behaviors but also realizes solid-solution reaction mechanism, as confirmed by the in situ XRD measurement (Figure 7C). 117Onedimensional fibrous nanostructure is also a significant structure that can effectively suppress the self-aggregation of nanoparticles resulted by their high surface energy, maintain good electrode-electrolyte contact, and stable crystal structure.Meanwhile, electrospinning extensively used in electrode material preparation is a viable and credible technique to produce consecutive porous nanofibers, and this low-cost craft can be appropriate for mass manufacturing. 119,120Liu and coworkers prepared the representative Na 2/3 Ni 1/3 Mn 2/3 O 2 cathode with the hierarchical fibrous structure by this technique.The distinctive porous nanofibers with diameters ranging from 200 to 600 nm are composed of abundant primary nanoparticles, and it efficiently accelerates Na + transfer because the electrolyte is in direct contact with the inner structure (Figure 7D,E). 118Moreover, the pulverization and aggregation of the electrode material are also suppressed so that it shows outstanding rate performance (73.4 mA h/g at 20 C) and prolonged structural stability.

| Superlattice and vacancy structures
Cation migration during repeated electrochemical cycles leads to the gliding of the TM layers and the collapse of the host structure.Then, the LTMO cathodes go through slow kinetics and fast capacity decay. 67,101In other words, the arrangement of Na + and TM ions plays an important role in determining structural stability.TM/TM-ordered superstructures can supply a stable layer to tolerate long-term Na + migration and retard the adverse migration of TM ions, which are distinct from the other two breakable ordered superstructures containing mobilizable Na + ions (Na + /TM, Na + /vacancy). 121,122Ma et al. found that the superlattice in NaMn 0.6 Al 0.4 O 2 (NMA) cathode formed by the ordered arrangement of MnO 6 and AlO 6 octahedrons greatly reinforces the crystal structure, mitigates the Jahn-Teller effect, and hinders the TM-ions migration, thus the NMA maintains its original structure after 100 cycles compared with the NaMnO 2 (NMO) electrode without superstructure (Figure 8A). 35Anion redox at high voltage provides extra capacity to enhance the energy density.However, different from the Na 2 Mn 3 O 7 cathode with small-voltage hysteresis, the irreversible oxygen activity in LTMO cathodes is always accompanied by large voltage hysteresis and oxygen release, further leading to the deterioration of host structure and fast capacity decay. 10,11The superlattice can not only activate the oxygen redox reaction to provide outstanding specific capacity up to 285.9 mA h/g but also effectively suppress these negative effects. 125,126Liu and coworkers introduced Mg@Mn 6 superstructure into the Na 0.73 Li 0.11 Mg 0.12 Mn 0.77 O 2 (NLMMO-1/2) cathode to offer a pinning effect to stabilize the crystal structure and restrain the oxygen loss.As confirmed by the high-resolution transmission electron microscope (HR-TEM), the Na 0.73 Li 0.23 Mn 0.77 O 2 (NLMO) counterpart without superstructure shows severe cracks and crystal distortion after 10 cycles, while the NLMMO-1/2 still shows the intact layered structure (Figure 8B,C). 123Meanwhile, the introduction of vacancies also plays a similar role with superlattice on oxygen activity.More specifically, this special structure facilitates anionic reaction by inducing nonbonding O 2p orbitals while vacancy-containing TMO 6 octahedrons with asymmetry and flexibility characters enable improved cycle stability. 62,127,128Yang's group verified the existence of vacancies via advanced scanning transmission electron microscopic (STEM) measurement and revealed that vacancies in the as-prepared electrode can regulate the crystal structure to achieve better structural stability and reversible reaction. 129In addition, Yuan et al. found that vacancies in Na 0.93 Li 0.12 Ni 0.25 Fe 0.15 Mn 0.48 O 2 (Na 0.93 LNFM) also reduce the charge density of TM ions and boost the antioxidative capability.Thus this electrode shows less alkaline residue when exposed to air and ameliorated cycle stability (Figure 8D,E). 124

| MIXED STRUCTURES
Though the hexagonal P2 phase provides direct diffusion pathways for Na + and hexagonal O3 possesses sufficient Na + in the host structure, it is still hard to obtain high specific capacity, eminent rate performance, and pre-eminent cycle stability simultaneously for single-phase structures due to their intrinsic drawbacks. 48,57For instance, pure P2-type cathodes undergo deleterious P2-O2 or P2-OP4 phase transition at the deep desodiation state due to the gliding of TM layers, and it is even more complicated in a single O3 structure. 16Designing a sample with mixed phases combines the merits of single phase and changes the spatial structure of pure phases.[132][133][134] 9A). 138Benefitting from the modulation strategy and synergistic effect of the heterostructure, the as-prepared composite electrode delivers enhanced rate performance (Figure 9B) accompanied by excellent long-cycle life when cycled at 2 C (capacity retention of 82.16% over 600 cycles).The boosted structural robustness was further verified by in situ XRD measurement.All the diffraction peaks return to their original position at the end of discharging, though they go through a reversible O3-P3 phase transition (Figure 9C).Encouraged by the good effects of composite structure, Chen's group reported another P2/O3 Na 0.67 Li 0.11 Fe 0.36 Mn 0.36 Ti 0.17 O 2 (NLFMTO) electrode in which the structure was demonstrated by HR-TEM test (Figure 9D). 139Compared with the P2-type Na-Fe-Mn oxide (NFMO), the biphasic structure effectively inhibits the P2-OP4 phase transition to mitigate the stress, thus achieving improved structural stability and prolonged cycle life (Figure 9E).In addition, as  124 mentioned before, the LTMO cathodes easily react with air to produce insulated Na 2 CO 3 , leading to Na + loss and deteriorating the electronic and ionic conductivities. 45,46,140Thermal activation of Na 2 CO 3 on the surface of Na x TMO 2 provides a possible way to solve this issue, which not only makes reuse of the active Na + but also generates mixed phase to promote the electrochemical properties (Figure 9F).

| P2/P3 structures
In addition to the P2/O3 mixed-phase heterostructure, P2/P3 composite structure has also been extensively explored to pursue better performance because the P3type cathode can deliver high specific capacity though it suffers from severe structural degradation.2][143] Zhou et al. reported a P2/P3-Na 0.7 Li 0.06 Mg 0.06 Ni 0.22 Mn 0.67 O 2 (P2/P3-NLMNM) cathode material via a simple sol-gel reaction using citric and nitrates as raw materials, in which the structure was corroborated by XRD and HR-TEM tests (Figure 10A,B). 144hough reversible OP4/P3 phase transition is observed during the sodiation/desodiation process, it causes reduced stress than P2/P3-O2/P″3 direct phase transition by suppressing the gliding of TM layers to some extent, thereby boosting more stable structure (Figure 10C).Sometimes, the intergrowth structure can even display improvement in humidity resistance induced by element doping, providing a solution to solve one of the most serious issues for the commercial application of SIBs. 147

| Layer-tunnel and multiphase structures
Though the insufficient Na + reserves in the host structure greatly limit its theoretical capacity, orthorhombic tunnel-type TM oxide cathode such as Na 0.44 M-nO 2 has garnered much attention on account of its distinct 3D structure and ample S tunnels, which are beneficial to achieve prolonged cycle stability and excellent rate performance.What is even more interesting is that the tunnel-type material is quite steady in water solution owing to its special structural frame composed of MnO 6 octahedra and MnO 5 squarepyramids, unlike the LTMO cathodes that are unstable when exposed to moisture.Hence, they can be applied to both aqueous and nonaqueous SIBs systems. 148,149ntergrowth layer-tunnel structure optimizes the electrochemical performance by integrating the strengths of single phases.Thus the as-synthesized Na 0.6 MnO 2 cathode with this composite structure synthesized by thermal polymerization reaction has a high specific capacity of 198.2 mA h/g and improved rate capacity (80.6 mA h/g at 5 C) compared to with the single-phase products (Figure 10D). 145Moreover, the reversible mA h/g P2/tunnel-OP4/tunnel phase transition verified by in situ XRD test suggests its stable structure because the harmful P2-O2 structural revolution which causes large volume change and gliding of TM layers is effectively restrained.The layer-spinel composite structure also has been thoroughly studied, which shows superior properties because of the high electronic conductivity of the cubic spinel phase, though no detailed description is given here. 150,151Except for these above-mentioned bi-phasic structures, in recent years, multiphase intergrowth heterostructures including P2/O3/O1, P3/P2/O3, P2/tunnel/O3′ (monoclinic structure), P2/P3/spinel, and so on have drawn much attention and opened a new field for promoting the development of LTMO cathodes.It is worth studying that the ratio of different structures can be tuned by adjusting the elemental stoichiometry to boost better electrochemical performance, such as superior energy efficiency and high Coulombic efficiency.10E,F) and introduced a strain engineering strategy which could triumphally tune the physical and chemical properties of this electrode material. 146This technology, on the basis of local chemistry, greatly inhibits adverse structural revolution transition and contributes to low intrinsic stress.Thus the as-prepared material shows good structural stability though highly reversible P2/P3″/spinel phase transition is observed and displays superior rate performance (Figure 10G).

| SUMMARY AND OUTLOOK
Developing high-performance cathode materials for practical applications of SIBs is a significant measure for achieving a sustainable future.LTMO cathodes have been considered the most promising cathode materials owing to their high operating voltage, high specific capacity, and other merits.In this review, we comprehensively summarize the recent development status of LTMO cathodes for SIBs.In the meantime, several specific materials are listed in Table 1, including the compositions, modified methods, and electrochemical performance, to show the recent progress.Some severe issues that hindered the utilization potential of LTMO cathodes, such as low energy density, sluggish kinetics, structural rearrangement and revolution, and water/air sensitivity, are effectively handled by taking reasonable steps to optimize the performance (Scheme 1).
(1) Cationic ions can mitigate the gliding of the TM layers and inhibit the adverse phase transition at the deep desodiation process as well as restrain the Jahn-Teller effect of TM ions with low valence at the end of the discharging process.Anionic ion (F − ) greatly strengthens the chemical bonds, suppresses the dissolution of TM ions, and destroys the cation ordering.Doping facilitates diffusion kinetics, induces oxygen redox activity to deliver higher specific capacity, and promotes water/air stability to some extent.In addition, multielement substitution may bring about a synergistic effect and even contribute to forming a high entropy cathode, which further boosts the cycle stability.(2) Surface coating with metal phosphate, metal oxide, metal fluoride, and so on can protect the active materials from direct contact with the electrolyte, effectively suppresses the side reactions, inhibits exfoliation of the active materials from the current collector, and mitigates HF attack.These protective layers can also mitigate the large volume change,  inhibit the harmful phase transition, retard metal dissolution, and buffer host structure against inherent strain and stress.Therefore, the structural stability and cycle life are improved.Meanwhile, TM ions with a larger radius in the coating material can dope into the crystal structure to increase the layer spacing, and materials with excellent conductivity can reduce the resistance, further promoting the rate performance.
(3) Reasonable morphology and structure design, including microsphere, nanoflake, and nanofiber, are useful to shorten the diffusion pathways for Na + and mitigate mechanical stress caused by Na + insertion/extraction.Moreover, the microspherical structure with high tap density is of great importance in increasing energy density.Superlattice and vacancy in the host structure not only stabilize the crystal structure but also activate the anionic redox to provide extra specific capacity.In the meantime, mixed structures (bi-phase/tri-phase) combining the merits of different single-phase components can achieve better cycle stability and rate performance, as well as reversible structural revolution.
These modified strategies actually do favor ameliorating the electrochemical performance because of the close structure-stoichiometry-properties relationship, leading to eminent rate capacity, cycling stability, higher specific capacity, and other improvements.However, existing LTMO cathodes face difficulty in meeting the requirements for practical applications and need to take measures to further optimize their performance.(1) Most of the doping elements are inactive, so partial substitution may cause a decrease in the specific capacity due to the reduced redox center.We need to explore lightweight element doping, such as boron, to minimize the sacrifice of specific capacity.(2) Surface coating and structure design are difficult to eliminate in the adverse phase transition intrinsically, so the improvement in cycle stability is limited, and it is necessary to integrate them with other modified methods to further promote the performance.(3) Mixed structures can greatly boost the electrochemical properties of LTMO cathodes because they successfully make good use of the merits of distinct structures, but how to precisely control the ratio of diverse structures is still a challenge.The proportion of each phase is closely related to the chemical stoichiometry and calcination temperature; thus, the ratios can be reasonably modulated by accurately adjusting these two parameters.(4) Low energy density caused by the relatively heavier and less-reducing potential of Na + is further hindered by cation-based charge compensation.Anionic redox reaction has been verified as an effective method to provide extra capacity at a high voltage range and realize high energy density of LTMO cathodes, greatly facilitating their potential for practical applications.Nevertheless, the anionic redox activated by forming some special configurations is accompanied by the release of oxygen, which can accelerate electrolyte degradation and migration of TM ions into the Na layers, further bringing about large volume stress, severe structure collapse, fast voltage decaying, irreversible structural revolution, and so on.It is difficult to realize superior capacity and excellent cycle life via a simple modified strategy in the LTMO cathodes with anionic redox activity.We need to get deep insight into the crystal structure change during the anionic redox and find a feasible solution to suppress oxygen release and TM-ion migration.The novel strain engineering strategy, on the basis of local chemistry, which tunes the physical and chemical properties, is a good selection to solve the above problems because it can effectively reduce lattice strain and restrain phase transformation.(5) Last but not least, the formation of an SEI layer on the surface of anode materials leads to irreversible Na + consumption in the first charging process while assembling sodium-ion full batteries, which could further impede the commercial application of Na-deficient P-type LTMO cathodes. 155,156Sodium compensation, including anode presodiation, cathode sodium-rich, and cathode selfsacrificial additive methods, has been widely explored to solve this critical issue. 157,158Although there are still some challenges that need to be solved to develop LTMO cathodes with low cost, high specific capacity, high energy density, and superior cycle stability.Along this line, the utilization of LTMO cathodes in commercial SIB systems will be realized in the foreseeable future through the efforts of our researchers.

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I G U R E 2 (A) X-ray diffraction (XRD) patterns for F-0, F-0.03, F-0.05, and F-0.07 and the corresponding magnified (002) peaks.Reproduced with permission: Copyright 2021, American Chemical Society.70(B) Mn K-edge XANES spectra of different samples.Reproduced with permission: Copyright 2020, WILEY-VCH. 71(C) Fourier maps for the transition-metal layer and oxygen and fluorine layer.Reproduced with permission: Copyright 2020, WILEY-VCH. 72(D) The ex situ XRD patterns of P2-type NM-NFMF005.Reproduced with permission: Copyright 2021, Elsevier B.V.

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I G U R E 4 (A) Schematic illustration of the ball-mill coating process for fabricating an Al 2 O 3-coated Na[Ni 0.6 Co 0.2 Mn 0.2 ]O 2 cathode.Reproduced with permission: Copyright 2017, The Royal Society of Chemistry. 91(B) High-resolution transmission electron microscopy (HRTEM) image of MgO@MF.(C) The (004) peak of the MF and MgO@MF.Reproduced with permission: Copyright 2019, Elsevier B.V. 92 (D) In situ X-ray diffraction (XRD) patterns of TiO 2 @MFN during the charge-discharge process.Reproduced with permission: Copyright 2020, American Chemical Society. 93(E) TEM images of the uncycled Al 2 O 3 atomic layer deposition (ALD)-coated Na 2/3 Ni 1/3 Mn 2/3 O 2 composite electrode.Reproduced with permission: Copyright 2017, American Chemical Society. 94(F) Cycling stabilities of five coated electrodes at 1 C. Reproduced with permission: Copyright 2021, WILEY-VCH.

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I G U R E 5 (A) Schematic of morphology-structure change in pristine AlF 3 coated and Na 2/3 Ni 1/3 Mn 2/3 O 2 (NNMO) electrodes on electrochemical cycling.Reproduced with permission: Copyright 2018, The Royal Society of Chemistry. 97(B) Schematic illustration of the preparation process of the P2@C-PDA sample.(C) High-resolution transmission electron microscopy (HRTEM) images of the carboncoated samples.(D) Rate performance of the bare and carbon-coated samples.Reproduced with permission: Copyright 2019, Elsevier Ltd.

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I G U R E 6 (A) Schematic illustration of the two-step synthesis of P2-Na 0.7 CoO 2 microspheres.(B) Cycling performance of s-NCO and i-NCO at a current rate of 0.4 C and the corresponding Coulombic efficiency.Reproduced with permission: Copyright 2017, WILEY-VCH. 108(C) Scanning electron microscope (SEM) images of Na-NMC-180.Reproduced with permission: Copyright 2018, WILEY-VCH. 109(D) Bright-field transmission electron microscopy (TEM) images of CC Na[Ni 0.61 Co 0.12 Mn 0.27 ]O 2 particle.(E) Bright-field TEM images of spoke-like nanorods (SNA) Na[Ni 0.61 Co 0.12 Mn 0.27 ]O 2 particle.(F) Capacity retention of SNA during 100 cycles at 0.5 C rate.Reproduced with permission: Copyright 2016, WILEY-VCH. 110

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I G U R E 7 (A) Scanning electron microscope (SEM) image of O3-NaLNMCM material.(B) Rate performance of O3-NaLNMCM electrode at various rates.Reproduced with permission: Copyright 2018, WILEY-VCH. 116(C) In situ X-ray diffraction (XRD) patterns of NaNMCM during the first cycle at 0.1 C in a voltage range of 2.5-4.15V. Black asterisks represent peaks from the Al window.Reproduced with permission: Copyright 2019, WILEY-VCH. 117(D) SEM images of Na 2/3 Ni 1/3 Mn 2/3 O 2 nanofibers.(E) TEM images of Na 2/3 Ni 1/3 Mn 2/3 O 2 nanofibers.Reproduced with permission: Copyright 2019, WILEY-VCH. 1185.1 | P2/O3 structures P2/O3 composite structure is one of the most common mixed phases to solve the Na-deficient issue of P2 structure and the low-rate capacity and the complicated structure revolution of O3 structure, further facilitating the advance toward practical application.Moreover, this composite structure can be finely tuned to realize high reversible capacity and excellent cycle stability by meticulously modulating the composition of TM ions and the ratio between single phases. 135-137Xiao et al. a new cathode material Na 2/3 Ni 1/3 Mn 1/3 Sn 1/3 O 2 with a P2/ O3 bi-phase structure (P2/O3-NaNMS) by local chemistry modulation, which adjusts the formation energy and structural transformation to boost the intergrowth (Figure

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I G U R E 9 (A) Powder X-ray diffraction (XRD) pattern with P2-type and O3-type crystal structures.(B) Rate performance at various rates of P2/O3-NaNMS electrode.(C) In situ XRD patterns of P2/O3-NaNMS collected during the first charge/discharge at 0.1 C in the voltage range of 2.5-4.15V.The black asterisks represent peaks from the Al window.Reproduced with permission: Copyright 2022, WILEY-VCH. 138(D) High-resolution transmission electron microscopy (HRTEM) image at the phase boundary of NLFMTO.Inset is the corresponding FFT map.(E) Schematic view of the structure changes during the charge of Na-Fe-Mn oxide (NFMO) and Na 0.67 Li 0.11 Fe 0.

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I G U R E 10 (A) Rietveld refinement patterns of the powder X-ray diffraction (XRD) data for P2/P3-NLMNM.(B) High-resolution transmission electron microscopy (HRTEM) image of P2/P3-NLNMM.(C) In situ XRD patterns of P2/P3-NLNMM collected during the 1st cycle of the cathode electrode under a 0.1 C rate.Reproduced with permission: Copyright 2018, Elsevier Ltd. 144 (D) First galvanostatic charge/discharge curves versus specific capacity and rate performance of layered, tunnel, and layered-tunnel intergrowth electrodes.Reproduced with permission: Copyright 2018, WILEY-VCH. 145(E) Powder XRD pattern and Rietveld refinement plot of LLS-NaNCMM15 cathode.(F) HR-TEM image of P2 structure, P3 structure, and spinel structure.(G) Rate performance of LLS-NaNCMM15 electrode at various rates.Reproduced with permission: Copyright 2022, WILEY-VCH.
S C H E M E 1 Summary of the merits of different modified strategies.
2,8,52 Jin et al. prepared a new high Na content P2-type cathode Na 0.85 Li 0.12 Ni 0.22 Mn 0.66 O 2 through a facile solid-state reaction by introducing Li into the Ni site, effectively suppressing the irreversible P2-O2 phase transition, which generates large volume change (23%) and mitigates Na + /vacancy ordering as well as charge ordering.
.3% after 150 cycles), lower charge transfer resistance (26.4 Ω), and better rate performance (70.7 mA h/g at 20 C) than bare NCM.Deng et al. found that the NTP not only inhibited the irreversible P2-O2 phase transition that occurred in P2type Na 0.65 [Mn 0.70 Ni 0.16 Co 0.14 ]O 2 (NMNCO) but also enhanced the specific capacity.
Hwang et al. reported a compact composition-graded spherical Na[Ni 0.61 Co 0.12 Mn 0.27 ]O 2 cathode composed of spoke-like 136,152-154Hu et al. reported a new Na 0.5 Ni 0.05 Co 0.15 Mn 0.65 Mg 0.15 O 2 (LLS-NaNCMM15) cathode with P2/P3/spinel triphasic structure (Figure A summary of current LTMO cathodes for SIBs focusing on compositions, modified methods, and electrochemical performance.
T A B L E 1