A review of advanced Ni-based cathode materials with layered structures for sodium batteries

Structural stability degradation

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.

Micro-cracking and electrolyte penetration

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].

Figure 4. Morphological evolution and Na⁺ storage kinetics of HEO424 and NFM424 cathodes after 200 cycles. (A-D) SEM and HAADF-STEM images of the cycled electrodes. (E and F) Intensity profiles along the designated lines in (C) and (D). (G and H) Schematic diagrams illustrating the structural changes in HEO424 and NFM424. (A-H) Reproduced with permission^[105]. Copyright 2022, American Chemical Society. HT-XRD patterns (30 °C to 400 °C) and contour maps of (I and K) charged HENM and (J and L) charged NM electrodes. Hy. = hydrated phase, L. = layered structure, and R. = rock salt phase. HAADF-STEM images of (M and O) NM and (N and P) HENM after 50 cycles at different magnifications. (Q) GPA analysis of the HENM cathode. (I-Q) Reproduced with permission^[106]. Copyright 2025, American Chemical Society. SEM: Scanning electron microscope; HAADF-STEM: high-angle annular dark-field scanning transmission electron microscopy; HT-XRD: high-temperature X-ray diffraction; HENM: high-entropy Na_xTMO₂; NM: Na_xTMO₂; GPA: geometric phase analysis.

O₂ release and interfacial side reactions

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.

Transition metal dissolution

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.

Slow ion diffusion

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.

Poor air stability

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.

Figure 5. (A) Visualization of water vapor corrosion on NFM cathodes. (B) Kinetic evolution of short-/long-term reactions between layered oxide cathodes and water vapor. (A and B) Reproduced with permission^[119]. Copyright 2024, Wiley-VCH GmbH; Visuals of the slurry obtained through the stirring of Super P carbon, PVDF binder, and NMP solvent, incorporating (C) NMO-NC or (D) NMO-SC. Reproduced with permission^[122]. Copyright 2024, Wiley-VCH GmbH. NFM: NaNi_1/3Fe_1/3Mn_1/3O₂; PVDF: polyvinylidene fluoride; NMP: N-methyl-2-pyrrolidone; NMO-NC: naturally cooled slowly cooled O-type Na_0.9Ni_0.25Mn_0.4Fe_0.2Mg_0.1Ti_0.05O₂; NMO-SC: slowly cooled O3-type Na_0.9Ni_0.25Mn_0.4Fe_0.2Mg_0.1Ti_0.05O₂.

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.

Heteroatom doping

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.

Cation doping

The classification of cation doping encompasses TM ions, AM ions, alkaline earth metal ions, and other cationic doping modalities, which typically function by substituting ions at TM or Na lattice sites^[126-131]. Among them, metal cation doping can be classified into two categories based on the electrochemical activity of the doping elements: electrochemically active elements and electrochemically inactive elements. TM cation doping stands out as the most extensively investigated system, primarily due to its relative ease of incorporation into TM sites within Na_1-δNi_xMyO₂ cathodes and its potential to enhance performance in certain scenarios. It is crucial to emphasize that, despite the extensive body of research documenting advancements in LIBs, the comprehensive investigation of doping additional metals into Na_1-δNi_xM_yO₂ cathodes for SIBs remains significantly underexplored. Incorporating inactive elements into Na_1-δNi_xM_yO₂ cathodes is an effective approach to enhance capacity and cycling stability by suppressing phase transitions. While further investigation is needed, Zr⁴⁺, Ti⁴⁺, and Mg²⁺ have been identified as highly promising dopants^[132-134]. These inactive elements not only inhibit Na/vacancy ordering and harmful phase transitions but also significantly improve Na⁺ diffusion kinetics. For example, a Zr-doped O3-type cathode material, namely NaNi_1/3Fe_1/3Mn_1/3O₂, was successfully fabricated by Jiang et al.^[132]. Their research demonstrated that the incorporation of Zr⁴⁺ could induce microscopic adjustments to the crystal structure. The robust Zr-O bond effectively contracted the octahedral configuration within the transition metal layer (TMO₆) to inhibit TM layer slipping and preserve the lattice framework, thereby preventing TM layer slippage and stabilizing the overall crystal structure, which finally boosted cyclic stability [Figure 6A-E]. Furthermore, the introduction of Zr⁴⁺ also expanded the interlayer spacing of Na⁺ diffusion, facilitating smoother Na⁺ insertion and extraction during charge and discharge, and consequently improving the material’s rate capability. Na_1-δNi_xM_yO₂ is also frequently subjected to doping modification with Ti⁴⁺, owing to its valence similarity with Zr⁴⁺. For example, Yu et al.^[133] examined the role of partial Ti⁴⁺ doping in O3-type NaNi_0.6Co_0.2Mn_0.2O₂ cathodes. Through systematic variation of Ti⁴⁺ content (0-0.03 mol%), they observed via scanning electron microscope (SEM) that Ti⁴⁺ incorporation induced the formation of densely packed secondary particles from primary ones. This densification not only enhanced mechanical strength and tap density but also minimized internal voids. Consequently, electrolyte infiltration and associated side reactions were effectively mitigated. Additionally, the presence of Ti⁴⁺ in the transition metal layer was shown to significantly enhance the structural stability of the host material. The O3-type Na[Ti_0.03(Ni_0.6Co_0.2Mn_0.2)_0.97]O₂ cathode delivers substantial battery performance enhancements - including increased capacity, cycle retention, rate capability, and thermal stability - by combining morphological and structural optimizations. Wu et al.^[134] proposed an innovative strategy involving Mg anti-site doping to stabilize the layered structure. Unlike conventional Mg-TM substitution, neutron diffraction confirms that Mg²⁺ prefers Na sites, inducing specific structural distortions (contraction of the Na layer and expansion of the TM layer). These Mg pillars in the Na slabs are directly visualized via atomic-resolution imaging. Furthermore, density functional theory (DFT) indicates that anti-site Mg does not migrate during desodiation, acting as a structural scaffold that stabilizes the Na layer, facilitates ion transport, and enhances Ni/Fe redox reversibility through optimized charge distribution. While doping with non-inert elements can enhance structural stability, their limited redox activity often leads to a reduction in specific capacity. To address this trade-off, the introduction of electrochemically active dopants offers a promising strategy to achieve structural stabilization without compromising reversible capacity. Considering the minimal disparity in ionic size between Ca²⁺ (≈1.00 Å) and Na⁺ (≈1.02 Å), when Ca²⁺ is employed as a doping element, Ca²⁺ is more prone to occupy the AM sites within the Na_1-δNi_xM_yO₂ cathodes. Such doping at AM sites, in turn, enhances the structural stability of the layered oxide. Matsui et al.^[135] synthesized a Ca-doped P3-Na_xNi_1/3Mn_1/3Co_1/3O₂, demonstrating its potential for high-performance applications. Through precise doping, Ca²⁺ was successfully incorporated into the AM layer while preserving the material’s original morphology and layered structure. Ca²⁺ incorporation leads to c-axis expansion due to electrostatic repulsion with TM ions, thereby reducing charge/discharge overpotential. This strategy significantly promotes Na⁺ transport and prevents the irreversible O’3-O1 phase transition during deep cycling. Overall, Ca-doping serves as an effective approach to enhance both structural stability and ionic conductivity. As a result, the Ca-doped Na_xNi₁/₃Mn₁/₃Co₁/₃O₂ exhibited superior capacity retention during galvanostatic cycling. In addition, the combination of transition metals and oxygen bonds forms stronger TM-O bonds, which can increase the formation energy of oxygen vacancies, regulate lattice parameters, enhance bond energy, and suppress oxygen release. However, the effect varies depending on the type of element and doping concentration.

Figure 6. (A) The crystal structure types of NaNi_1/3Fe_1/3Mn_1/3O₂ and NaNi_1/3Fe_1/3Mn_1/3-xZr_xO₂ (x = 0, 0.01, 0.02, 0.04). High-resolution transmission electron microscopy images of (B) NFM and (C) NFMZ2. (D) Cycling performance of all electrodes at 5 C. (E) The rate capability of all electrodes. (A-E) Reproduced with permission^[132]. Copyright 2024, Wiley. NFM: NaNi_1/3Fe_1/3Mn_1/3O₂.

Multi-ion doping

While single-ion doping demonstrates partial efficacy in suppressing phase transitions and enhancing performance, its application remains constrained by significant limitations. Specifically, doping with electrochemically inert ions often leads to diminished reversible capacity, whereas incorporating active ions may induce a decline in the voltage plateau. Moreover, single doping strategies provide only marginal improvements in structural stability. In contrast, multi-ion doping approaches enable the integration of diverse ionic advantages, facilitating comprehensive material optimization through synergistic interactions. For example, Yu et al.^[137] synthesized the Fe³⁺/Ti⁴⁺ co-doping O3-Na_0.95Ni_0.40Fe_0.15Mn_0.3Ti_0.15O₂ (NFMT) cathode via a high-temperature solid-state method. Anchored by electrochemically inactive Ti⁴⁺, which underpins structural integrity and cycling retention, the cathode design further utilizes high-redox-potential Fe³⁺ to boost oxidation resistance and capacity through charge compensation mechanisms. The synergistic interaction between Fe/Ti co-doping effectively elevates the redox potential, thereby optimizing both electrochemical kinetics and structural robustness. Additionally, Fe/Ti co-doping significantly stabilizes the cathode by suppressing complex phase changes. The reversible O3 → P3 → OP2 transition occurs with negligible volume change, guaranteeing structural integrity over prolonged cycles [Figure 7A-F]. However, current multi-cation doping strategies predominantly focus on substituting TM sites to suppress irreversible phase transitions. Emerging studies indicate that rational substitution at Na sites plays a critical role in optimizing structural integrity and Na⁺ diffusion kinetics. Therefore, developing a synergistic dual-site (Na/TM) substitution strategy represents a promising approach to advancing the performance of Na_1-δNi_xM_yO₂ cathodes. Therefore, Wu et al.^[138] proposed a Cu²⁺/Ca²⁺ dual-doping strategy and designed a Na_0.98Ca_0.01Ni_0.35Cu_0.05Fe_0.2Mn_0.4O₂ (CuCa-NFM) cathode. O3-type oxides underwent targeted interlayer modification by doping Cu²⁺ (TM layer) and Ca²⁺ (AM layer). Cu²⁺ incorporation facilitates Na⁺ transport by widening diffusion channels and reducing oxygen-related distortions, effectively inhibiting cation ordering. Simultaneously, Ca²⁺ stabilizes the lattice by mitigating volumetric expansion, thereby lowering internal stress and retarding phase transitions during cycling. Wu et al.^[139] synthesized Ti⁴⁺/K⁺ dual-site co-doped Na_0.99K_0.01Ni_0.5Mn_0.4Ti_0.1O₂ cathodes, which enable a reversible O3-P3, significantly reduce the formation energy of Na⁺ vacancies, and contribute to the establishment of a dense, protective CEI film. These advancements collectively enhance the material’s oxidation resistance. Specifically, Ti⁴⁺ doping effectively decreases the charge density around Ni ions, thereby improving oxidative stability. The synergistic interaction between K⁺ and Ti⁴⁺ further lowers the formation energy of Na⁺ vacancies and minimizes lattice strain during Na⁺ deintercalation. The assembled Na⁺ half-cell achieves an impressive specific capacity of 97 mAh g^-1 at a high rate of 5 C, alongside excellent stability with 81% capacity retention after 400 cycles. Furthermore, the full cell assembled with a hard carbon anode showcases robust long-term cycling and high-capacity characteristics [Figure 7G-I], confirming its promise for practical applications. To boost the cycle life and kinetic properties of Na_1-δNi_xM_yO₂, engineering the TM and O sublattices via doping is essential. For instance, Mao et al.^[140] synergistically improved the O3-type cathode by a dual-doping strategy involving B³⁺ and Al³⁺. In this approach, Al³⁺ was introduced into the TM layer, while B³⁺ was positioned at the TM-O interstitial sites, resulting in the concurrent stabilization of the bulk and the interface. Al alleviated the strain induced by phase transitions by enhancing the TM-O bonding interaction, whereas B suppressed the activation of lattice oxygen through strong covalent interactions. After 200 cycles, Al³⁺/B³⁺ co-doped NFM retained 63.1% capacity (31.3% for pristine NFM) and maintained 87.6 mAh g^-1 at 10 C. This dual-doping strategy synergistically resolves high-voltage structural and interfacial degradation in O3-type oxides, offering a promising route for high-energy-density SIBs.

Figure 7. (A) Na-layer interplanar spacing evolution in substituted samples. (B) Structural modification of NM host via Fe/Ti co-doping. (C) Contour plots of principal diffraction peaks. (D and E) Lattice parameter variation during charging. (F) Cycling stability of NFMT/HC full cell. (A-F) Reproduced with permission^[137]. Copyright 2023, American Chemical Society. (G) Crystal structures of NaNMO and NaK_0.01NMTi_0.1O. (H) Na⁺ vacancy formation energies and 500-cycle performance of full cells at 1 C within 1.5-4.0 V. (I) The full cell assembled with a hard carbon anode showcases robust long-term cycling and high-capacity characteristics. (G-I) Reproduced with permission^[139]. Copyright 2023, Wiley-VCH GmbH. NM: Na_xTMO₂; NFMT: O3-Na_0.95Ni_0.40Fe_0.15Mn_0.3Ti_0.15O₂; HC: hard carbon.

In contemporary research endeavors, numerous attempts have been made to optimize the outcomes of co-doping, exemplified by the co-doped system^[141], etc. These exploratory efforts have, to a certain degree, enhanced the performance of relevant materials. However, the underlying mechanisms responsible for these performance improvements remain largely uncharted territory and warrant in-depth investigation. In addition to the aforementioned double-doping approach, doping strategies involving an even broader spectrum of elements have also garnered extensive research attention. Li et al.^[142] adopted a synergistic multi-element doping strategy to fabricate the O3-NaNi_0.40Mn_0.40Ti_0.13Fe_0.06Al_0.01O₂, systematically investigating the mechanism by which multi-metal substitution suppresses complex phase transitions. The disordered doping of Ti⁴⁺, Fe³⁺, and Al³⁺ modulates the electronic structure. By activating Ni/Fe redox above 2.5 V and suppressing Mn Jahn-Teller distortion below 2.5 V, structural integrity is significantly enhanced. Ex-situ XRD reveals that the P3 phase prevents P3” phase formation, alleviating structural collapse, while DFT calculations show a lowered Na⁺ diffusion barrier via optimized electronic configuration and strengthened TM-O bonds. Additionally, Zhang et al.^[143] demonstrated that co-doping with Li⁺, Mg²⁺, Ca²⁺, and Sb⁵⁺ in O3-Na_0.8Ni_0.4Fe_0.2Mn_0.4O₂ effectively suppresses detrimental phase transitions and anisotropic strain at 4.1 V. Sb⁵⁺ doping combined with Li-O/Mg-O vacancies stabilized oxygen redox, allowing the material to retain 130 mAh g^-1 (85% retention) after 250 cycles at 4.2 V. It also showed low thermal runaway risk and good humidity resistance, greatly boosting practical viability. Table 1 summarizes the electrochemical performance of Ni-based cathodes for SIBs by doping strategies.

Surface coating

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.

Polyanionic composite coatings

Conventional multi-step heat treatments are typically required to ensure strong adhesion between the coating layer and matrix, while the limited functionality of single-component coatings may restrict further optimization of Na⁺ transport kinetics. Given the inherent air instability of the Na_1-δNi_xM_yO₂ cathode, conventional secondary heat treatment coating methods often induce irreversible capacity loss, compromising the efficacy of surface modification. Achieving effective material modification, particularly via coating strategies, while preserving electrochemical performance remains critical.

To address this, in-situ one-step coating methods are imperative to minimize degradation pathways. Furthermore, the judicious selection of coating materials is instrumental in alleviating the performance degradation inherent to conventional two-step synthesis routes. To circumvent the air-sensitivity of NFM424, Wang et al.^[153] introduced a one-step approach to in-situ form a plastic crystal Na_3-3xAl_xPO₄ layer. High-temperature synthesis enabled the simultaneous bulk doping of Al [Figure 8A]. The 1 mol %NAP@NFM424 cathode delivers superior rate capability (~88% retention at 5 C vs. 76.2% for bare NFM424) and improved cycling stability (80% vs. 65% retention over 200 cycles) in half-cell tests (2.0-4.0 V) [Figure 8B and C]. High-resolution Scanning Transmission Electron Microscopy (STEM) images clearly reveal the fine interlayer interface structure between the sample matrix and the surface. Through local lattice fringe analysis, the bulk retains the original layered structure, corresponding to the (003) crystal plane of NFM424 with an interplanar spacing of 5.325 Å. A Na_3-3xAl_xPO₄ phase structure with a thickness of about 3-4 nm can be observed on the surface [Figure 8D, E, E1 and E2]. The Na₃-_3xAl_xPO₄ outer layer plays a dual role - enhancing air stability while serving as a barrier against electrolyte erosion and interfacial side reactions. The in-situ coating competes with the bulk for sodium sources, promoting sodium-deficient surface phases that generate fast ion-conducting pathways and sodium vacancies, thereby improving dynamic performance. This is corroborated by the faint lattice oxygen peak of 1 mol % NAP@NFM424, which confirms suppressed particle fragmentation - a direct result of the coating buffering volume changes and Al doping strengthening metal-oxygen bonds, preserving structural integrity over 200-500 cycles. In summary, despite the success of surface coatings in enhancing the rate capability and safety of Na_1-δNi_xM_yO₂, resolving interface and kinetic limitations necessitates innovations in both coating materials and synthesis methods. A more rational approach involves integrating surface coatings with complementary strategies - including heteroatom doping and crystal morphology engineering to achieve synergistic optimization of Na_1-δNi_xM_yO₂ cell performance.

Figure 8. (A) Schematic of the coating/doping modification on NFM424. (B and C) Cycling and rate performance of NFM424 and 1 mol% NAP@NFM424 (2.0-4.0 V). (D) HAADF-STEM micrograph of the layered phase. (E) STEM image showing the cross-section of a modified Na_3-3xAl_xPO₄ particle. (E1) Schematic of the Na_3-3xAl_xPO₄ crystal structure. (E2) Interfacial structure diagram. (A-E) Reproduced with permission^[153]. Copyright 2023, American Chemical Society. HAADF-STEM: High-angle annular dark-field scanning transmission electron microscopy; STEM: scanning transmission electron microscopy; NAP: Na_3-3xAl_xPO₄.

Co-modification through doping and coating

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.

Figure 9. (A) Schematic illustration for the synthetic procedure. Typical FESEM images, TEM images, and corresponding XRD patterns of N6 (B and C), and T-CSN6@A (D and E) harvested from matching post-cycled electrodes after 200th cycles, respectively. (F) Cycling performance (0.5 C) of T-CSN6@A. (G) rate behaviors at different temperatures of T-CSN6@A. This figure is reproduced with permission from Ref.^[154]. Copyright 2023, Wiley-VCH GmbH. FESEM: Field emission scanning electron microscope; TEM: transmission electron microscope; XRD: X-ray diffraction.

High entropy and nickel-based layered oxide cathodes

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.

Figure 10. Classification of the functions of high-entropy modified Ni-based cathode for SIBs. SIBs: Sodium-ion batteries.

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.

Figure 11. (A) Interlayer spacing & Na-O/TM-O bonds (HEO vs. NFM). Reproduced with permission^[158]. Copyright 2025, Wiley-VCH GmbH. AC-TEM images of (B-F) NFM and (G-J) CS-NFM after 500 cycles at 1 C. (B-J) Reproduced with permission^[125]. Copyright 2025. American Chemical Society. NFM: NaNi_1/3Fe_1/3Mn_1/3O₂; HEO: high-entropy oxides; CS-NFM: Ca/Sn dual site doping strategy and prepared Na_0.98Ca_0.01Ni_0.33Fe_0.33Mn_0.315Sn_0.015O₂; AC-TEM: spherical aberration-corrected transmission electron microscopy.

Composite

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.

Figure 12. (A) Preparation strategy, advantages and existing problems of P2/O3 composite phase. (B) Pre-evaluation of cation potential for the custom-developed materials and widely reported counterparts; (C) and (D) the corresponding structural sketches and phase formation energy characteristics of P2-type and O3-type Na_0.8Ni_0.23Fe_0.32Mn_0.45O₂. (E) TEM images of P2/O3 Na_0.8Ni_0.23Fe_0.32Mn_0.45O₂. (B-E) Reproduced with permission^[163]. Copyright 2025, Wiley-VCH GmbH. TEM: Transmission electron microscope.

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.

Microstructural modulation

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].

Figure 13. (A) Types and functions of microstructural modules schematic diagram. (B and C) SEM images of HEO424 and NFM424. (D) Schematic illustrations showing the morphological structure of HEO424 and NFM424. (B-D) Reproduced with permission^[105]. Copyright 2022. American Chemical Society; (E) Schematic showing the arrangement of the outermost electron of Sn⁴⁺. (F) Schematic of morphological evolution. (G-K) SEM images of NaNi_0.5Mn_0.5-xSn_xO₂ cathode materials. (E-K) Reproduced with permission^[168]. Copyright 2023. American Chemical Society. SEM: Scanning electron microscope.

Electrolyte modification

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].

Figure 14. (A) Na⁺ solvation and (003) plane interface chemistry of cathodes. (B) EIS plots and (C and D) R_ct and R_CEI fitted models. (A-D) Reproduced with permission^[171]. Copyright 2023. Springer; (E) Calculated battery-level energy density (E_g) as a function of cut-off voltage in the NFM||HC cell. (F) Solvation sheath structures of LHCE (NaFSI-G2-TE) and IRE (NaFSI-G2-DM), and their corresponding HOMO/LUMO. (E and F) Reproduced with permission^[173]. Copyright 2025. Wiley-VCH GmbH; (G) LSV of Na||stainless steel half cells. (H) Combustion testing of E-Control and NaTFSI/SUL:OTE:FEC. (G and H) Reproduced with permission^[176]. Copyright 2024. Springer. EIS: Electrochemical impedance spectroscopy; NFM: NaNi_1/3Mn_1/3Fe_1/3O₂; HC: hard carbon; LHCE: localized high-concentration electrolyte; IRE: intermolecular-reinforced ether; HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital; SUL: sulfone; OTE: 1H, 1H, 5H-octafluoropentyl-1, 1, 2, 2-tetrafluoroethyl ether; FEC: fluoroethylene carbonate; CEI: cathode electrolyte interphase; DEC: diethyl carbonate; EMC: ethyl methyl carbonate; PC: propylene carbonate; DMC: dimethyl carbonate.

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].

Synergy and conflict among modification strategies

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.

Summary

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.

Technical challenge analysis

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.

Possible research directions

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.