Under high-voltage operation, the continuous extraction of Na⁺ from the cathode lattice can trigger detrimental phase transitions, such as P2-O2 or O3-P3 transformations^[58]. In P2-type layered oxides, Na⁺ ions occupy trigonal prismatic sites, while functioning as a pillar to reduce electrostatic repulsion across adjacent oxygen layers^[52]. Upon Na⁺ extraction, this shielding effect weakens, resulting in an increase in O-O repulsion. This forces the TM layers to slide along specific crystallographic directions [e.g., (1/3, 2/3, 0)], altering the stacking sequence from “AB-BA-AB” (P2) to “AB-AC-AB” (O2)^[98]. Similarly, in O3-type structures where Na⁺ are located in octahedral sites, their extraction modifies the interlayer spacing. When the Na⁺ extraction reaches a certain level (typically ~25%), the sliding energy barrier for the TMO₂ layers decreases^[99]. This causes the TMO₂ layers to glide and transform the stacking order from O3 to P3^[100]. Critically, these phase transitions are accompanied by significant volume changes. The resulting structural inhomogeneity generates localized stress concentrations, which in turn can induce further phase transitions. This creates a vicious cycle of “phase transition → stress concentration → accelerated phase transition”, continuously degrading the material’s structural integrity. After numerous electrochemical cycles, this leads to lattice distortion and micro-cracking. The long-term cycling stability is severely compromised by capacity/voltage degradation, inferior rate performance, and growing voltage hysteresis. To address this, Xie et al.^[101] recently focused on the interplay between phase transitions in O3-Na_1-δNi_1/3Fe_1/3Mn_1/3O₂ and the cut-off voltage. Specifically, their findings indicated that limiting the voltage to 4.0 V induces an O3-to-P3 phase transition upon charging, and the material reverts to the O3 phase during the subsequent discharge. However, as the voltage window is extended to 4.3 V, monoclinic phases O3’ and P3’ emerge, and the discharge process involves a reverse phase transition sequence of O3’-P3’-O3, ultimately reverting to the O3 phase. Additionally, Yuan et al.^[102] documented how iron doping modifies the phase transition mechanism in NaNi_0.4Fe_0.2Mn_0.4O₂. Specifically, the pathway evolved from O3-P3-O’3 in the Fe-free NaNi_0.5Mn_0.5O₂ to O3-P3-OP2 upon Fe incorporation. Such intricate structural rearrangements during phase transitions cause pronounced volume changes and intense internal stress. These mechanical failures drive TM cation migration and dissolution, foster micro-crack formation, and ultimately degrade the long-term cycling performance.

The primary factors inducing microcrack formation are volume changes stemming from phase transitions and anisotropic lattice strain. Furthermore, interfacial reactions with ambient air, oxygen evolution, parasitic side reactions, and the diffusion of transition metal cations collectively compromise the structural integrity of the material during prolonged electrochemical cycling. This degradation accelerates the extension of microcracks from the surface into the bulk crystal^[103]. Microcracks damage the internal structure of materials, resulting in poor contact between some active substances and electrolytes, preventing them from participating in electrochemical reactions and directly causing reversible capacity decay. In addition, microcracks provide more permeation channels for the electrolyte, thereby enlarging the interfacial zone between the cathode material and the electrolyte, which readily triggers parasitic reactions^[104]. This not only consumes active substances but also generates harmful products such as gases or insulation layers, hindering ion transport and reducing safety. Cracks can also damage the crystal structure and sodium ion transport channels, leading to tortuous and obstructed ion diffusion paths, decreased charge and discharge rates, and degraded rate performance, making it difficult to meet high-power application requirements. Finally, the presence of microcracks weakens the mechanical strength of the material, making the cathode more prone to pulverization due to external forces during assembly or use. This not only damages the structural integrity but may also lead to poor contact between the electrode and the current collector, further exacerbating performance degradation. According to Ding et al.^[105], O3-NaNi_0.4Fe_0.2Mn_0.4O₂ undergoes distinct microstructural evolution during cycling. While the inner structure remains layered, the surface suffers from severe lattice changes, leading to microcracks and Na⁺/TM cation mixing after 200 cycles. This surface degradation is linked to TM dissolution and results in the reconstruction of the surface into a rock-salt phase. The resulting particle pulverization increases the electrode-electrolyte contact area, triggering detrimental side reactions and capacity fading [Figure 4A-H]. In contrast, Gao et al.^[106] demonstrated that a high-entropy design in Na_xTMO₂ cathodes can mitigate these problems. The strategy utilizes multicomponent TMO₂ slabs to widen the interlayer distance, which stabilizes the crystal structure against Jahn-Teller effects and lattice variations. This stabilized skeleton not only improves Na⁺ diffusion and prevents intragranular cracking but also inhibits TM dissolution. Furthermore, the high-entropy configuration increases TM-O bond strength, preventing oxygen evolution and improving the material’s resistance to thermal runaway [Figure 4I-Q].

Under high voltage conditions, the oxygen element in the material is prone to being released in the form of O₂, which is essentially related to the oxygen binding energy of the crystal structure, the oxygen activity of the electronic structure, and surface defects^[107]. The detrimental effects of oxygen release are twofold. Firstly, it results in the loss of reactive oxygen species, which directly reduces the material’s capacity. Secondly, it triggers a chain reaction at the interface. The released O₂ reacts with organic solvents and sodium salts within the electrolyte, resulting in electrolyte degradation and the evolution of gaseous byproducts and detrimental species. These byproducts deposit to create a compact and thick passivation layer on the electrode surface. Such a film not only elevates the internal resistance of the cell but also induces corrosion of the electrode’s surface structure. Notably, structural instability accelerates O₂ release^[108]. Phase transitions and crack formation expose more internal oxygen-active sites, facilitating further O₂ release. Conversely, O₂ release damages the structure. The passivation film hinders ion transport, resulting in uneven local stress distribution. This uneven stress, in turn, induces more phase transitions and cracks. As a result, a vicious cycle is formed: “structural damage → oxygen release → interface deterioration → exacerbating structural damage”. The long-term stability and performance of the battery are detrimentally affected by this cycle. In a systematic study by Liu et al.^[109], gas evolution in layered oxide cathodes was examined across various compositions, voltage limits, dopants, and morphologies. Notably, NaNiO₂ demonstrates superior gas emission compared to LiNiO₂ (LNO), even at low states of charge. This behavior stems from the inductive effect of the ionic Na-O bond, which enhances Ni-O covalency. Screening of dopants (Co, Mn, Al, Mg) revealed that Mn and Mg are superior in curbing gas release via enhanced metal-oxygen bonding. While NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathodes synthesized by co-precipitation (CP-NFM) evolve more gas than those made by solid-state routes, the hierarchical secondary structure of CP-NFM mitigates this through varied reactivity of primary particles. Among Li, Ti, Mg, and Cu dopants for NFM, Li is the most potent, achieving gas levels on par with LNO. Advanced characterization shows that solvent decomposition mainly forms an organic-rich CEI instead of soluble species, while NaPF₆ salt worsens interfacial reactions by forming Na₂O. Collectively, these findings provide a roadmap for designing safer SIBs.

TM dissolution is primarily driven by the formation of soluble low-valence ions. The high solubility of low-valence oxides/fluorides in the electrolyte triggers the release of TM ions from the positive electrode. This process is further aggravated by the Jahn-Teller effect in ions like Mn³⁺, whose unique electronic structure (strong electron delocalization, weak ionicity, and strong Lewis basicity) promotes reactions with acidic electrolytes. The consequences of this dissolution are twofold: first, it causes a depletion of cathode active material, directly lowering battery capacity^[110]. Second, dissolved ions migrate to the anode, forming harmful precipitates (NiO, MnCO₃) that damage the solid electrolyte interphase (SEI) film and increase internal resistance^[111]. Ultimately, excessive TM leaching poses severe safety risks, potentially causing internal shorts and thermal runaway.

Because of the considerable radius of Na⁺ (~1.02 Å), its diffusion in layered lattices is hindered by structural and electrostatic factors, yielding a diffusion coefficient inferior to that of Li⁺ in lithium-ion systems^[112-114]. For instance, in standard layered oxide cathodes, the Na⁺ diffusion coefficient usually lies between 10^-12 and 10^-10 cm² s^-1, which restricts the battery’s rate performance^[115]. Transport limitations are closely linked to other challenges: structural instability (phase transitions, cracks) can disrupt the “continuous transport channel” of ions and electrons; the interface passivation film caused by O₂ release will directly block the surface transport path. Restricted transport can also lead to local overcharging/overdischarging inside the material (such as incomplete sodium ion deintercalation inside the particles and surface side reactions), causing more severe stress heterogeneity and phase transition, forming a vicious cycle of “poor transport → structural damage → worse transport”. Therefore, employing a simple yet effective strategy to regulate the transport channels of Na⁺ is of great significance for accelerating the transport rate of Na⁺ and improving its kinetic characteristics. Liu et al.^[116] constructed a high-rate O3-type cathode by modulating the migration pathways of Na⁺. By introducing highly electronegative fluoride ions into the oxygen sites, they altered the local chemical environment of the transition metal layers, thereby strengthening the bonding between transition metals and oxygen. This, in turn, expanded the transport channels for Na⁺, ensuring rapid reaction kinetics during the transport process. The authors verified via DFT computations that F^- doping effectively decreases the energy barrier associated with Na⁺ migration. As a result, the presence of F^- promotes faster Na⁺ diffusion, which significantly boosts the battery’s ability to handle high currents. However, a single doping or coating strategy struggles to simultaneously enhance kinetic performance and structural stability. To address this challenge, Li et al.^[117] fabricated a copper-rich spinel@O3-type composite oxide cathode with a surface-modified structure (Na_0.9Mn_0.5Ni_0.5Cu_0.1O_2+x). The surface-bound copper-rich spinel phase acts as a protective barrier for the active material: it reduces side reactions and prevents the accumulation of by-products at the electrolyte/transition metal interface, consequently boosting the material’s chemical stability. Meanwhile, its exceptional ionic and electronic conductivity significantly enhances the cathode’s capacitive energy storage capacity, enabling rapid Na⁺ intercalation/deintercalation within the framework and demonstrating outstanding rate discharge performance and high-rate capacity retention. Even under extreme conditions, the surface spinel structure effectively functions as a protective layer: it blocks the interlayer insertion of water molecules, decelerates the de-sodiation process at the cathode surface, and suppresses the emergence of Na-deficient phases such as O1 and P3. Beyond surface protection, the spinel phase is crucial for managing bulk structural changes; it buffers the significant volume expansion during the O-P phase transition, mitigating internal cracking and preserving the continuity of Na⁺ transport channels. These synergistic effects collectively guarantee the material’s structural integrity and performance durability.

Layered oxides with high nickel content are highly susceptible to chemical degradation upon exposure to atmospheric moisture and CO₂ (such as surface hydroxylation and carbonate formation), leading to crystal structure damage (such as increased interlayer spacing) and electrochemical performance degradation (such as decreased capacity and increased polarization)^[63]. The impact of poor air stability permeates the entire process of preparation, storage, and use: the additional protective measures required for poor air stability (such as encapsulation and inert atmosphere) will also increase production costs, forming a dual burden with the “cost and resource” issue. Even brief contact with moist air initiates a degradation cascade in layered oxide cathodes. Surface-migrated Na⁺ reacts with atmospheric H₂O/CO₂, producing alkaline salts (e.g., NaOH, NaHCO₃, Na₂CO₃)^[63]. The resulting active Na loss, combined with TM ion oxidation, destabilizes the lattice, causing irreversible TMO₂ layer slippage and severe capacity decay. This instability is particularly acute in Ni-rich NaNi_0.7Mn_0.15Co_0.15O₂, which undergoes drastic morphological alterations in air^[118]. Spectroscopic evidence confirms the formation of surface species like NaOH, Na₂CO₃, and Na₂CO₃·H₂O on these materials. Surface reaction products act as insulating barriers, severely degrading the performance of NaNi_0.7Mn_0.15Co_0.15O₂. Ji et al.^[119] reported that Na⁺/H⁺ exchange causes severe lattice expansion along the c-axis, resulting in particle rupture. Prolonged water vapor exposure further decomposes NFM into Na₂O and TM oxides (FeO, NiO, MnO) with oxygen release [Figure 5A and B]. Sun et al.^[120] noted that ambient storage causes Na⁺ loss and the growth of Na₂CO₃ fibers, which harm performance. Yang et al.^[121] recently demonstrated that the simultaneous presence of moisture with either CO₂ or O₂ is required for significant degradation. The H₂O-CO₂ system generates protons for ion exchange, causing acid corrosion, cracks, and lattice distortion. In contrast, the H₂O-O₂ system drives oxidation of TM ions to form NaOH. Both pathways generate surface residues that induce parasitic reactions with the electrolyte.

From a manufacturability perspective, these degradation mechanisms directly dictate dry-room requirements, electrode slurry stability, storage, transport, and cost. First, the high alkalinity (pH > 12) of the electrode slurry decomposes the Polyvinylidene fluoride (PVDF) binder during mixing, causing gelation in N-Methyl-2-pyrrolidone (NMP) and irreversible performance loss [Figure 5C and D], necessitating strict dry-room control throughout slurry preparation and subsequent processing^[122]. Second, material exposure to moisture and CO₂ during ambient storage and transport accelerates Na⁺ loss and Na₂CO₃ fiber growth, requiring inert-atmosphere encapsulation or specialized moisture-proof packaging, which directly increases logistics and warehousing costs. Furthermore, the synthesis stage likewise demands a controlled dry environment to prevent precursor degradation. All these additional protective measures - from raw material storage, dry-room operations, to finished product encapsulation - significantly increase both capital and operational costs, creating a compounding effect with the inherent “cost and resource” challenges of sodium-ion batteries, and becoming a critical manufacturability bottleneck that limits the scalable production of Ni-rich layered oxide cathodes.

Given that most phase transitions occur within the high-voltage range, constraining the upper cut-off voltage can effectively suppress these phase transitions. However, this approach sacrifices the capacity of the cathode material, thereby reducing overall performance. Another effective strategy is element doping, which can modulate complex phase transitions in the cathodes and eliminate Na⁺/vacancy ordered structures, thereby improving electrochemical performance, such as cycle stability and rate capability.

To improve performance, aliovalent dopants are introduced in trace amounts into the original Na_1-δNi_xM_yO₂ cathodes^[123,124]. Once embedded in the cathode lattice, these elements modify the material’s physical and chemical nature, thereby boosting its electrochemical behavior^[125]. The host structure possesses three distinct substitution sites: TM, AM, and oxygen (O). To combat layered structural degradation, cationic and anionic substitutions are widely adopted. This broad spectrum of dopants and sites offers substantial versatility for material engineering. The review initiates with single-site doping within the bulk phase and progresses to multi-element co-doping. Analysis underscores the benefits of these strategies, such as lowering energy barriers, strengthening M-F bonds, and exploiting pillar/pinning effects. For systematic presentation, ion doping approaches are categorized into three primary groups: cationic, anionic, and multi-ion doping.

Surface coating has established itself as a critical technique to boost the performance of sodium-ion layered cathodes. By establishing physical isolation, ensuring structural stability, bolstering air stability, and optimizing ion transport pathways, it synergistically resolves a range of issues, including side reactions, structural degradation, and sluggish ion transport that cathode materials encounter during cycling. This, in turn, lays a solid foundation for achieving high energy density, extended cycle life, and outstanding rate performance in sodium-ion batteries^[144]. An optimal coating should uniformly and thoroughly encapsulate cathode particles, effectively minimizing electrolyte decomposition, gas evolution, and transition metal dissolution, thereby significantly enhancing the cycle life of the battery. A critical observation is that the optimal coating thickness is generally below 10 nm^[145]. Excessively thick coatings can hinder Na⁺ diffusion and elevate interfacial resistance, leading to a deterioration in rate capability. Conversely, coatings that are too thin may not provide adequate protection to the cathode surface, resulting in electrolyte degradation and accelerated capacity loss due to transition metal dissolution. Recent advancements have explored diverse coating materials to optimize the performance of Na_1-δNi_xM_yO₂ cathodes, including metal oxides, polyanionic composites, and organic polymers. This section summarizes the effects of these coating types on the electrochemical properties of Na_1-δNi_xM_yO₂ cathodes. The surface coating type can be selected or combined according to specific needs to address the issues of sodium ion layered positive electrode materials, thereby improving the overall performance and cycle life of sodium ion batteries.

Simple element doping can delay phase transition and enhance structural stability, but there are limitations in the modification effect and an inability to balance bulk and interfacial stability. To bolster the robustness of layered oxide cathodes, it is imperative to establish a unified modification framework involving concurrent doping and surface coating. Li et al.^[154] reported a Ti-doped and Al₂O₃-coated Na[Ni_0.6Co_0.2Mn_0.2]O₂ cathode with a core-shell structure. The Ni-rich core ensures high capacity, while the Mn-rich shell, augmented by Ti doping and Al₂O₃ coating, dramatically improves structural stability. This design simultaneously mitigates volume expansion and interfacial cracking while boosting Na⁺ diffusion kinetics. This structure not only effectively mitigates lattice volume changes and surface/interface structural fractures during cycling but also markedly improves the kinetics of desodiation/sodiation processes. Consequently, it achieved remarkable chemical-mechanical stability and facilitated efficient electronic/ionic transport, enabling effective sodium storage across a broad temperature spectrum ranging from -20 °C to 50 °C [Figure 9]. This research comprehensively underscores the immense advantages of employing multi-level modification strategies in the field of battery materials. Currently, research endeavors predominantly concentrate on metal oxide coatings, with relatively scarce reports on non-metallic oxide-coated cathode materials for SIBs. For instance, Guo et al.^[155] fabricated synergistic modified cathode materials through solid-state ball milling, incorporating B³⁺ doping and B₂O₃ coating, by combining boric acid powder and NaNi_1/3Fe_1/3Mn_1/3O₂. The introduction of B₂O₃ not only prevents electrolyte corrosion but also stabilizes the transition metal layer via strong B-O bonds, thereby enhancing structural integrity. As a result, capacity retention was significantly boosted from 78% to 87% after 200 cycles at 1 C. This efficacious modification approach offers a dependable pathway to improve the cycling stability of SIB layered oxide cathodes. A detailed performance comparison of coated Ni-based cathodes is presented in Table 2.

In 2015, Rost et al. introduced high-entropy oxides (HEOs), a class of materials defined by unique compositions that revolutionized research directions in materials science^[156]. Since then, it has attracted widespread attention and in-depth research from academia and industry on this type of material. This type of material typically involves mixing five or more elements in equal or nearly equal proportions to form a material system with higher configurational entropy. The significant increase in configurational entropy endows high-entropy oxides with unique physical and chemical properties, making them highly applicable in the field of layered cathodes. Leveraging the synergy of multiple elements, the material harnesses both entropy structures and doping effects to concurrently enhance electrochemical activity, structural stability, and average voltage. Notably, the optimized anion redox mechanism elevates both capacity and voltage, while the air tolerance of sodium-ion layered cathodes is markedly improved [Figure 10]^[157-160]. For example, a rational high-entropy strategy was adopted by Liu et al.^[161] to engineer a cobalt-free layered oxide cathode with the composition Na_0.9Ni_0.3Fe_0.2Mn_0.3Ti_0.1Cu_0.05Sn_0.05O₂. By introducing multiple metal ions, they achieved a significant increase in entropy (∆S = 1.57 R), thermodynamically favoring single solid-solution phase formation. The results revealed that the Na-O bond length in the HEO (2.31 Å) was shorter than that in NFM (2.38 Å), indicating stronger bond strength, which contributes to enhanced air stability. Additionally, the oxygen charge density distribution in the HEO was uneven and notably reduced, suggesting that high-entropy doping suppressed the covalency of TM-O bonds and stabilized the interlayer oxygen charge. This high-entropy state resulted in a highly disordered arrangement of cations within the TM layers, triggering a series of synergistic effects, known as the “cocktail effect”, which significantly improved the performance of the cathode material.

Qiu et al.^[158] substantially enhanced the high-voltage performance and air stability of the O3-Na_xTMO₂ cathode by modulating the interlayer oxygen charge. They also successfully fabricated an O3-Na_0.85Li_0.1Al_0.02Sn_0.08Cu_0.1Ti_0.1Ni_0.3Mn_0.3O₂ type HEO, enabling the stable operation of high-loading SIBs [Figure 11A]. In addition, ion-doping with larger radii can also effectively increase interlayer spacing and improve ion diffusion efficiency. For example, Shi et al.^[125] proposed a Ca/Sn dual site doping strategy and prepared Na_0.98Ca_0.01Ni_0.33Fe_0.33Mn_0.315Sn_0.015O₂ (CS-NFM) to improve the electrochemical performance of O3-type layered oxide cathode materials. The synergistic effect of Ca/Sn dual-site doping can regulate the coordination environment and chemical bonds of TM ions, leading to rapid charge transfer kinetics and highly reversible phase transitions with minimal volume changes. The improved structural stability effectively suppressed the dissolution of TM and inhibited the formation of salt rock phases and microcracks during long-term cycling, maintaining the integrity of the particles [Figure 11B-J]. Despite the superior rate capability and cycling stability of CS-NFM - attributed to its suppression of metal dissolution and cracking - research on high-entropy oxides is still nascent. Key challenges regarding inhomogeneous mixing, transition metal interactions, and optimal stoichiometric ratios remain unresolved, even as basic design rules begin to emerge.

While P2 and O3 single-phase oxides present distinct trade-offs, the design of P2/O3 biphasic materials has emerged as a key strategy for advanced sodium-ion cathodes [Figure 12A]. This composite architecture integrates the high capacity of the O3 phase with the structural stability of the P2 phase, enabling synergistic multi-channel Na⁺ migration that effectively overcomes kinetic and structural limitations.

For example, Duan et al.^[162] synthesized a thermodynamically and kinetically stable P2/O3 biphasic K_0.05Na_0.8Ni_0.5Mn_0.5O₂ via a high-temperature solid-phase method using potassium element regulation. Fe doping suppresses Mn³⁺-induced Jahn-Teller effects, yielding a P2/O3 biphasic material with high reversible capacity and excellent 200-cycle stability. Crucially, P2/O3 composites form preferentially when the constituent phases possess similar thermodynamic energies^[163]. This study reveals that owing to the comparable formation energies of P2-type and O3-type phases [Figure 12B-D], no long-range ordered structure is generated, and a P2/O3-type Na_0.8Ni_0.23Fe_0.32Mn_0.45O₂ (abbreviated as P2/O3) composite cathode material dominated by the O3 phase is constructed [Figure 12E]. Leveraging mechanisms including reversible Mn redox activated by weak hybridization, confined Jahn-Teller effect, and reduced Na⁺ migration energy barrier, the material achieves high specific capacity and ultra-long cycling stability, and meanwhile exhibits excellent air stability and processing adaptability. Chang et al.^[164] co-doped with Li⁺ and Ti⁴⁺ to equalize the thermodynamic formation energies of the P2 and O3 phases. From a thermodynamic perspective, by changing the calcination time, they prepared Na_2/3Li_1/18Ni_5/18Mn_5/18Ti_5/18Fe_2/18O₂ materials with adjustable P2/O3 phase ratios. The high-entropy strategy can enhance lattice and interlayer stability through configurational entropy, and in conjunction with composite phase structures, can further improve electrode material performance. Nevertheless, exploring high-capacity single-phase cathodes remains crucial, and in-depth research on the structure performance relationship of composite phases is of great significance for the design of advanced SIB cathodes.

The control of Ni-based sodium layered positive electrode microstructure is a key technology for improving the diffusion rate of Na⁺ and structural stability, mainly including morphology structure control (particle size, shape) and crystal structure control (crystallinity, electronic structure, defects, crystal orientation, cell parameters, etc.) [Figure 13A], representing a key breakthrough for next-generation SIBs^[165,166]. The synthesis method and element doping can effectively regulate the microstructure of Ni-based sodium electric layered cathodes. For example, Zhang et al.^[165] prepared a rare earth-modified NaNi_1/3Fe_1/3Mn_1/3O₂ cathode in one step by spray drying combined with a high-temperature solid-state sintering process. The unique physical and chemical characteristics of rare earth element Lu were utilized to effectively regulate the local chemical environment of O and TM. This resulted in spherical secondary particles with alkali residues on the surface of primary particles, significantly improving structural stability and reducing the sodium ion diffusion energy barrier, thereby improving long-cycle and rate performance. At the same time, some high-entropy doping can also synergistically regulate the Ni-based sodium-ion layered positive electrode microstructure. For example, Ding et al.^[105] used a high-entropy strategy to prepare an elliptical spherical single-crystal positive electrode Ni_0.25Mg_0.05Cu_0.1Fe_0.2Mn_0.2Ti_0.1Sn_0.1O₂ (HEO424) with a lateral size of 2 μm and a thickness of 300 nm. High entropy helps to reduce the surface energy of the material (003) and effectively regulate crystal growth. Compared to the original sample NFM424, HEO424 has more Na⁺ transport channels and more Na⁺ storage layers, indicating that the material has superior electrochemical performance [Figure 13B-D]. In addition, at the atomic scale, defects may act as barriers to prevent layer slip, so studying the influence of microstructure on phase transition behavior is of great significance^[166]. At the same time, some elements have unique electronic structural characteristics and strong M-O bonds, which can improve structural stability and stabilize lattice oxygen through strong chemical bonding, effectively enhancing the rate performance and cycling performance of Na_xTMO₂^[167]. Notably, adjusting the Sn concentration in NaNi_0.5Mn_0.5-xSn_xO₂ serves as an effective means to engineer its microstructure. By securing optimal particle size, high crystallinity, and a stable lattice framework, the resultant material exhibits markedly improved performance^[168] [Figure 13E-K].

The electrolyte is vital for Na⁺ transport during battery operation, serving as the primary medium for charge carrier mobility. Its properties, including conductivity and ion diffusion rate, significantly impact the battery’s overall performance^[169]. From the perspective of electrolyte formulation, the choice of sodium salt significantly impacts various crucial parameters of the electrolyte, such as sodium ion transport, sodium ion solvation structure, SEI, CEI compositions, electrochemical window, among others, which are the primary determinants of the electrolyte performance. Generally, crystals with lower lattice energies demonstrate superior conductivity (NaPF₆ > NaClO₄ > NaTFSI > NaOTf > NaBF₄), while the HOMO (Highest Occupied Molecular Orbital) level dictates their redox resistance (NaOTf > NaClO₄ > NaTFSI > NaBF₄ > NaPF₆)^[170]. NaClO₄ exhibits favorable conductivity but lacks resistance to high voltage, whereas NaPF₆ and NaTFSI offer both excellent conductivity and high voltage characteristics. Nevertheless, NaTFSI is relatively costly, while NaPF₆ salt suffers from poor chemical stability (being highly sensitive to moisture at levels below 20 ppm) and inadequate thermal stability^[169]. Therefore, a reasonable combination of concentration, solvent, and additives is imperative to strike a balance between cost and performance. Among solvents, organic solvent systems, typified by carbonate esters, carboxylate esters, and ethers, are characterized by an appropriate voltage range and a wide temperature range. Among these, ethylene carbonate (EC), featuring a cyclic structure, is the most extensively utilized. Blending this compound with linear carbonates Diethyl carbonate/dimethyl carbonate (DEC/DMC) reduces electrolyte viscosity and boosts ionic conductivity. While ester-based electrolytes offer a wide operating voltage range for layered oxide cathodes, their decomposition instability generates resistive organic species (such as ROCO₂Na). These byproducts promote a thick and brittle CEI layer, ultimately inducing structural degradation and transition metal dissolution. Wang et al.^[171] reported enhanced Na⁺ coordination at the NFM/electrolyte interface in various EC-based solvents [with 5% fluoroethylene carbonate (FEC)], attributed to the cathode’s strong nucleophilicity and high electron density. Among them, the EC/DMC electrolyte, with the smallest molecular chain and the most tightly dissolved structure, exhibits stable electrochemical performance, resulting in a uniform and stable CEI, thereby displaying superior interface compatibility and cycling stability [Figure 14A-D].

Ether-based electrolytes can effectively stabilize SIBs’ negative electrodes and play a key role in improving the low-temperature and rate performance of SIBs due to their unique weak solvation ability and lower reduction potential. Ether-based electrolytes promote a unique CEI formation mechanism, yielding a thin yet robust interphase rich in inorganic components. This optimized chemical composition significantly enhances their compatibility with layered oxide cathodes^[172]. However, although traditional ether electrolytes are suitable for sodium negative electrodes, their antioxidant capacity is poor, and they are prone to decomposition at high voltages, resulting in unstable interfaces and capacity degradation. Although high concentration or localized high concentration electrolyte strategies can improve interface stability, they are accompanied by a series of problems such as high cost, high viscosity, and gas production. Although traditional ether electrolytes are compatible with sodium anodes, they have poor antioxidant properties and are prone to decomposition under high pressure, leading to interface instability and capacity decay. Although high concentration or localized high concentration strategies can improve interface stability, they come with problems such as high cost, high viscosity, and gas production. For example, increasing the cut-off voltage of the sodium layered positive electrode from 4.0 V to 4.2 V can achieve an energy density of 184 Wh/kg, comparable to commercial LiFePO₄ (LFP) batteries, but it will exacerbate positive electrode instability and electrolyte antioxidant burden [Figure 14E]. Therefore, it is urgent to develop low-cost, high-performance ether electrolytes that can form stable CEI to promote the development of high-stability and high-energy sodium ion batteries. By optimizing the solvation structure through customized solvent molecules, the oxidation stability and low-temperature kinetic performance can be synergistically improved. For example, Cui et al.^[173] proposed an innovative intermolecular-reinforced ether (IRE) design based on ethylene glycol dimethyl ether (G2) and N,N-dimethyl-trifluoromethanesulfonamide (DM) solvents, revealing their synergistic coordination with Na⁺ in the first solvation layer. According to molecular dynamics simulations, non-polar 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) in localized high concentration electrolytes (LHCE) is pushed out of the first solvent sheath composed of Na⁺- G2-FSI^-, while DM molecules in IRE coordinate with G2 and FSI^- together with Na⁺ in the first solvent sheath [Figure 14F]. The synergistic effect between DM and G2 molecules is similar to the synergistic mechanism in ceramic-reinforced carbon fiber composites, achieving performance breakthroughs by strengthening the network structure. By virtue of this mechanism, IRE suppresses electrolyte-cathode side reactions at 4.2 V, promoting a thin, inorganic-rich CEI that delays material degradation. Conversely, existing electrolytes for Na-metal batteries fail to concurrently realize ultra-high Na coulombic efficiency and stable anodes. To address this, Li et al.^[174] designed a non-fluorinated 1.0 M NaPF₆/(1,2-Diethoxyethane/1,2-Di-tert-butoxyethane: DEE-DBE) electrolyte. The resulting ion aggregates facilitate NaF-rich interface formation on both electrodes, maintaining superior coulombic efficiency and cycle life. It is worth noting that low-concentration, high-entropy electrolytes [solvent: Tetrahydrofuran (THF), salts: NaTFSI, NaPF₆, NaDFOB, NaBF₄, NaNO₃, additives: FEC], with their anionic “aggregate” solvation structure, effectively form an inorganic-rich, dense CEI, markedly boosting the cycling stability of layered oxide cathodes^[175].

In a separate study, He et al.^[176] designed a localized high-concentration electrolyte using sulfolane and a fluorinated ether diluent, which formed a robust, thin CEI with excellent oxidation resistance. This enabled O3-NaNMF cathodes to deliver outstanding high-voltage (4.2 V) performance, with capacity retentions of 79.48% (300 cycles at 1 C) and 81.15% (400 cycles at 2 C). Moreover, the electrolyte’s non-flammability improves SIB safety [Figure 14G-H]. Beyond solvents, additives play a pivotal role by leveraging differential lowest unoccupied molecular orbital/highest occupied molecular orbital (LUMO/HOMO) energy levels to preferentially decompose and tailor interfacial chemistry. Additionally, solid-state batteries have emerged as a research focus due to their remarkable energy density and cycling stability, but still face challenges such as poor solid-solid contact, unstable solid-state electrolyte/cathode interface, and thermal instability that need to be addressed. Building continuous charge transfer channels, designing “zero strain” layered oxides, and using surface coatings are classic strategies to solve the above problems, but their commercialization still faces greater challenges compared to liquid electrolytes^[20].

First, we systematically evaluated six strategies - doping, surface coating, strain-relief gradient design, high-entropy doping, P2/O3 biphasic design, and electrolyte modification - across six dimensions: mechanism, advantages, drawbacks, cost, scalability, and applicability to Ni-rich cathodes [Table 3]. Beyond individual evaluation, the various modification strategies for ternary sodium-ion nickel-based cathodes exhibit complex synergistic and conflicting relationships in practical applications. Doping and coating represent the most fundamental complementary combination: doping enhances TM-O bond strength and reduces Na⁺ diffusion barriers from the bulk perspective, while coating suppresses electrolyte erosion and side reactions at the surface; their integration simultaneously improves bulk and surface stability, yet an excessively thick or poorly conductive coating layer may offset the ionic diffusion rate enhancement achieved by doping, creating a performance trade-off. High-entropy modification introduces multi-component active elements to increase Na⁺ storage sites and broaden the voltage window, and its synergy with P2/O3 biphasic design is particularly pronounced - the lattice distortion induced by high-entropy doping can be buffered at the P2/O3 interface within the biphasic structure, which itself combines the high-rate capability of the P2 phase with the high capacity of the O3 phase; their superposition effectively suppresses phase transition degradation during cycling. Nevertheless, high-entropy design significantly increases synthesis complexity and cost, and the compositional inhomogeneity inherent to multi-element distribution may compromise the uniformity of subsequent coating, leading to processing conflicts. Crystal microstructure regulation and electrolyte modification also exhibit bidirectional interactions: the distinct chemical reactivities of P2 and O3 phases at the surface of biphasic materials demand precise electrolyte matching - fluorinated salts or ionic liquid additives can reduce interfacial impedance and suppress decomposition, but if they undergo redox reactions with specific dopants or coating layers, they may instead damage interfacial integrity. Research has shown that phase coexistence and local structural regulation at the P2/O3 interface can effectively balance energy density, dynamic performance, and cycling stability, and this biphasic design paradigm is not limited to Ni-rich systems but can also be extended to sustainable low-Ni or Ni-free chemistries^[177]. In summary, these strategies are not simply additive but require systematic balancing across three dimensions - bulk, surface, and interface: matching coating thickness and conductivity with doping-enhanced diffusion, ensuring compatibility between high-entropy composition and biphasic structure, and maintaining chemical stability between electrolyte and modified surfaces. A mismatch at any stage can convert synergistic effects into performance penalties; therefore, the optimal solution must be systematically tailored to the specific material system and application scenario.

While the exploration of modification methods is still in its infancy and faces specific challenges, strong momentum is driving the development of related technologies to address these limitations. Through comparative analysis and the synergistic integration of diverse approaches, the future enhancement of these techniques appears highly promising. This review is expected to provide critical insights for the rational design of advanced electrode materials for SIBs, thereby accelerating their industrial implementation.

Despite the promise of Na_1-δNi_xM_yO₂ layered oxides as SIB cathodes, several persistent problems limit their industrial use, notably structural collapse, parasitic interface reactions, slow Na⁺ diffusion, and sensitivity to air. In response to these bottlenecks, recent research endeavors have centered on the systematic optimization of multidimensional modification strategies, which can be summarized into seven distinct directions. Below is an analysis of the advantages, drawbacks, costs, and industrial application prospects associated with each strategy:

(1) Ion doping regulation: Ion doping into the Na_1-δNi_xM_yO₂ cathode has been shown to bolster structural stability, optimize Na⁺ diffusion pathways, regulate electrochemical activity, and improve air stability. Nevertheless, this approach is not without its limitations: using a single ion often falls short in comprehensively addressing multiple issues, while multi-ion doping processes introduce complexity. Excessive doping or the inappropriate selection of dopant elements can lead to adverse effects such as lattice expansion, diminished conductivity, and exacerbated oxygen evolution. Furthermore, the heightened synthesis complexity associated with ion doping drives up production costs. The cost increase of common elements such as Mg, Al, and Zn is small, while the cost of rare elements such as Ti, Mo, and W increases significantly.

(2) Surface coating engineering: The surface coating strategy applied to the Na_1-δNi_xM_yO₂ cathode offers a multifaceted approach to enhancing performance by suppressing interface side reactions, buffering volume changes, accelerating ion transport kinetics, improving air stability, and prolonging material cycle life. However, this strategy is not without its drawbacks, including the potential for increased interface resistance, high process complexity, rising costs, and impurity introduction. The cost of materials depends on the type and purity of coating materials (such as aluminum oxide, phosphate, carbon materials, etc.), and the cost of processes (such as atomic layer deposition, sol-gel method, spraying method, etc.) is affected by equipment investment and energy consumption. Despite these challenges, the encapsulation strategy holds significant promise for widespread application in energy storage and power delivery systems. Through meticulous process optimization and cost management, it is anticipated to facilitate the large-scale deployment of SIBs, proving to be an economically feasible approach for energy storage.

(3) Doping and coating: Combining doping and coating creates a powerful synergistic effect, but this hybrid method still struggles with common technical issues.

(4) High-entropy: Increasing the configuration entropy of materials can enhance their structural stability, ion diffusion kinetics, and optimize air stability. At the same time, multiple elements work together, combining electronic conductivity, ion storage capacity, and structural support, resulting in better overall performance than traditional single- or dual-element materials. However, introducing some inactive or low-active elements may dilute the proportion of active substances, resulting in a slightly lower specific volume than the ideal value, which requires precise design to balance capacity and stability. High-entropy material synthesis necessitates precise stoichiometric and parameter control, yet faces scalability and efficiency bottlenecks with conventional routes. The mixing of multiple metal elements complicates the recycling process, and existing recycling technologies are difficult to efficiently separate and reuse each element, increasing environmental pressure and costs. In addition, the specific mechanism of the high-entropy effect is not fully understood, which limits the rational design and performance optimization of materials.

(5) Composite phase: P2/O3 biphasic oxides synergistically optimize kinetics and thermodynamics at the phase interface, markedly enhancing structural stability and cycle life. However, preparing multiphase structures is complex, and stability needs to be verified; furthermore, the synthetic route for such materials is highly intricate, and although it has excellent laboratory performance, large-scale production requires solving the problem of process amplification.

(6) Microstructure control: The control of the microstructure of nickel-based cathodes is a key technology for improving the diffusion rate of sodium ions and enhancing structural stability, mainly including morphology control and crystal structure control, which plays an important role in promoting the development of sodium ion batteries. Partial microstructure control, such as nanostructure design and precise doping, requires high synthesis processes and precise control of reaction conditions, which increases production difficulty and cost.

(7) Electrolyte modification: Electrolytes play a crucial role in facilitating the transport of Na⁺ during charge/discharge processes. Their performance metrics, encompassing conductivity and ion diffusion kinetics, exert a profound influence on the overall performance of the battery. To enhance interfacial stability and ion transport efficiency, strategies such as incorporating functional additives or modulating the solvation structure are often employed. However, the development of high-performance modified electrolytes typically necessitates the use of costly additives, specialized solvents, or sophisticated synthesis protocols, which collectively contribute to escalated production expenses. Moreover, these modified electrolytes may encounter compatibility challenges with other battery components, including separators and negative electrodes, thereby necessitating re-optimization of the battery design and assembly processes to ensure seamless integration and optimal performance.

In summary, the factors that affect the degradation of Na_1-δNi_xM_yO₂ cathode structure are often not singular and have complex correlations. Some of these factors can form a vicious cycle through the conduction chain of “structural damage performance degradation further structural damage”, severely restricting its electrochemical performance and practical application process. Therefore, a single modification strategy is difficult to comprehensively solve, and multiple modification strategies need to work together to optimize material properties from multiple dimensions such as bulk structure, electronic structure, and interface characteristics, in order to achieve the application of high-performance SIBs.

For future development of Ni-rich cathode in SIBs, the focus can be directed toward the following directions while strengthening mechanistic investigations: (1) By integrating Density Functional Theory (DFT) with Machine Learning force fields (ML) force fields [Deep Potential‌/Gaussian Approximation Potential‌ (DeepMD/GAP)], nanosecond-scale multiscale simulations reveal Na⁺ diffusion kinetics, P2/O3 phase transitions, and oxygen redox evolution to clarify degradation origins under high Ni content, while Graph Neural Networks (GNNs) enable high-throughput screening of optimal multi-element doping strategies - both refined iteratively via active learning; (2) Employing in-situ synchrotron X-ray Diffraction/X-ray Absorption Spectroscopy (XRD/XAS), cryo-electron microscopy, and Raman spectroscopy to dynamically track layered structure distortion, surface reconstruction, and transition metal dissolution during charge/discharge cycling, combined with electrochemical impedance spectroscopy to analyze the dynamic correlation between interfacial side reactions and battery performance; (3) developing a high-throughput screening platform based on graph neural networks that integrates crystal structure databases and experimental data to predict the mechanisms of multi-element doping in enhancing air stability, and optimizing doping element combinations and concentration gradients through active learning; and (4) Digital Twin Implementation: Multimodal data from in-situ characterization and EIS are fused to construct a three-tier twin (“atom → particle → cell”), upon which reinforcement learning performs inverse design of gradient-structured cathodes, enabling bidirectional closed-loop optimization - predicting performance from structure and deriving optimal structure from target performance; (5) Standardized testing and full-cell validation: Establish unified air-stability testing standards for Ni-rich layered oxides, including dry-air tolerance tests (with clearly defined temperature/humidity conditions and exposure duration) and accelerated aging protocols (high-temperature/high-humidity cycling, high-rate cycling), to ensure comparability of results across studies. Concurrently, conduct full-cell validation with hard carbon anodes to evaluate the effectiveness of cathode modifications under practical full-cell conditions; (6) Engineering validation: Promote the preparation and testing of high-loading electrodes and obtain pouch-cell data to assess capacity retention, rate performance, and cycle life under realistic operating conditions, thereby bridging the data gap between laboratory-scale small electrodes and industrial-scale production; (7) Gas-evolution analysis: Systematically conduct gas-evolution analysis during charge/discharge (O₂, CO₂, H₂), and use mass spectrometry or gas chromatography to quantitatively assess the extent of parasitic reactions, providing direct evidence for the suppression strategies of interfacial side reactions; (8) Techno-economic comparison: Perform techno-economic comparisons between Ni-based layered oxides and Ni-free layered oxides, comprehensively evaluating dimensions such as raw material costs, process complexity, energy density, and cycle life, to provide data support for industrialization route selection.

In summary, the research on sodium-ion high-nickel layered oxides is still in its early stages of development, and commercialization faces multiple challenges from basic material research to industrial implementation. Therefore, interdisciplinary collaboration is needed to accelerate the practical application of high-performance sodium-ion batteries.