Recent Advances in Enhancing Air Stability of Layered Oxide Cathodes for Sodium-Ion Batteries via High-Entropy Strategies

Abstract

Layered transition metal oxide (LTMO) cathode materials for sodium-ion batteries (SIBs) have attracted extensive attention due to their unique structural stability and excellent electrochemical performance. However, their poor stability in air has significantly impeded their practical application, as exposure to moisture and carbon dioxide can lead to Na⁺ loss, phase transitions, and decreased electrochemical performance. This paper reviews the application of high-entropy strategies in sodium-ion LTMO cathode materials, focusing on the optimization of air stability and electrochemical performance through approaches including high-entropy cation regulation, P2/O3 dual-phase synergistic structures, and fluorine ion doping. Studies have shown that high-entropy design can effectively inhibit phase transitions, alleviate Jahn–Teller distortion, enhance oxygen framework stability, and markedly enhance the cycle life and rate performance of materials. Furthermore, future research directions are proposed, including the use of advanced characterization techniques to reveal failure mechanisms, the integration of machine learning to optimize material design, and the development of high-performance mixed-phase structures. High-entropy strategies provide new perspectives for the development of SIBs cathode materials with enhanced air stability, potentially promoting the practical application of SIBs in large-scale energy storage systems.

1. Introduction

The rapid development of science and technology has created new demands for energy resources. In order to adapt to changes in the global energy structure, low-cost and environmentally friendly energy storage technologies are gaining increasing attention. Sodium-ion batteries (SIBs) have emerged as a promising alternative due to their abundant elemental reserves and substantially lower cost compared to lithium-ion batteries (LIBs). They are particularly suitable for large-scale energy storage applications such as grid peak regulation, the integration of renewable energy sources, and emergency backup power supply [1,2,3]. Simultaneously, SIBs have demonstrated considerable application potential in small energy storage devices, including smart meters and small electric vehicles that do not impose strict requirements on energy density, thereby providing essential support for the construction of multi-level and multi-scenario energy storage systems. In recent years, with the development of new electrode materials and the ongoing optimization of electrolyte systems, SIBs have achieved significant advancements in energy density, cycle stability, and safety [4,5,6,7]. The cathode material is a critical factor influencing the performance of SIBs and remains a focus of extensive current research. Among them, Na_XTMO₂ is particularly promising; the value of x, representing the sodium ion content, determines the phase composition and structural stability. When x > 0.8, the material predominantly adopts the O3 phase structure. As x approaches 1, sodium insertion becomes more complete, electrostatic repulsion decreases, and structural stability improves. When 0.5 < x < 0.8, the sodium-deficient P2 phase is formed. Due to structural differences, the sodium ion diffusion path in the P2 phase is a columnar two-dimensional channel, which facilitates faster Na⁺ diffusion compared to the narrower interlayer channels in the O3 phase. In addition, P2-Na_XTMO₂ exhibits good structural stability but is limited by its inherently low sodium content. O3-Na_XTMO₂ contains a higher sodium content and thus has a higher specific capacity [8,9,10]. However, after repeatedly undergoing complex phase changes and irreversible redox reactions, it becomes difficult for layered transition metal oxides (LTMOs) to retain their original structure and sustain high capacity, particularly when the cutoff voltage reaches 4.2 V. In addition, poor air stability remains a major obstacle limiting the development of layered oxide cathode materials. LTMOs are highly sensitive to moisture and carbon dioxide in the air (resulting in the formation of sodium residues such as NaOH, NaHCO₃, and Na₂CO₃), which lead to irreversible structural changes and severe performance degradation. Moreover, the increased storage costs have significantly hindered commercialization efforts [11,12]. In order to address the challenges associated with LTMOs, traditional strategies primarily focus on element selection, surface modification, structural design, and process control, respectively, targeting issues such as moisture absorption, structural instability, and performance degradation of layer sodium-ion cathode materials [13,14,15]. However, there remains considerable room for enhancement in cathode materials; most of the existing methods represent local optimizations and face challenges in simultaneously improving structural stability, electrochemical performance, and environmental adaptability. This is where the high-entropy strategy distinguishes itself among various modification approaches. By increasing the configurational entropy within the crystal and reducing the Gibbs free energy through synergistic interactions among multiple elements, it enhances the structural stability, ion diffusion kinetics, and air stability of the material while maintaining its capacity advantage at the crystal structure level [16,17].

The concept of high-entropy alloys was first proposed by Yeh et al. [18] in 2004; their research demonstrated that alloys composed of five or more principal elements in near-equiatomic proportions can form simple solid solution structures and nanostructures. As shown in Figure 1, the unique structure of high-entropy alloys CuCoNiCrAlFeTiV endows them with superior thermodynamic and kinetic properties, as well as outstanding mechanical performance. This innovative work challenged the traditional alloy design paradigm centered on a single principal element, pioneered a new research direction in the field of materials science, and the promoted extensive research and application of high-entropy materials. Subsequently, in 2015, Rost et al. [19] synthesized oxides containing five metal cations in equimolar proportions and introduced the concept of “entropy-stable oxides.” Experimental results confirmed that Mg, Ni, Co, Cu, Zn, and O not only possess high configurational entropy but also exhibit entropy-driven stability. High-entropy systems provide unique opportunities for exploring thermodynamics and structure–performance relationships in materials exhibiting extreme configurational disorder. High-entropy materials were first applied as cathode materials in SIBs in 2019; Zhao et al. [17] prepared an O3-phase NaNi_0.12Cu_0.12Mg_0.12Fe_0.15Co_0.15Mn_0.1Ti_0.1Sn_0.1Sb_0.04O₂ cathode incorporating nine transition metal oxides. Owing to the slow kinetic diffusion characteristics of high-entropy materials, these materials effectively suppress intermediate phase transitions in metal oxides and demonstrate excellent rate capability and cycling stability, achieving approximately 83% capacity retention after 500 cycles. This study stimulated the emergence of new concepts and further research on high-entropy layered oxides. In 2022, Yao et al. [20] combined high-entropy design with superlattice stabilization to develop an O3-phase Na_2/3Li_1/6Fe_1/6Co_1/6Ni_1/6Mn_1/3O₂ layered cathode material featuring a Li/TM ordered superlattice structure. This structure effectively suppresses phase transitions and reversible oxygen redox reactions in the high-voltage region, thereby providing a high reversible capacity and excellent stability even under high charge cutoff voltages and high cycling rates, thus ensuring excellent electrochemical performance resilient to phase transitions and lattice oxygen redox. This strategy offers a promising approach for improving secondary batteries susceptible to unstable phase changes and redox reactions. In 2023 and 2024, Wang et al. and Pang et al. [21,22] employed entropy control strategies to develop high-entropy sodium-deficient O3-type Na_0.83Li_0.1Ni_0.25Co_0.2Mn_xTi_xSn_0.45–2xO_2−δ and high-entropy O3/P2 dual-phase Na_7/9Ni_2/9Mn_4/9Fe_1/9Cu_1/9Ti_1/9O₂, respectively. The former adjusts the relationship between configurational entropy and phase transition behavior, charge compensation mechanism, and sodium storage performance, while the latter inhibits interlayer slippage, thus achieving excellent cycling and rate performance, alongside enhanced air stability.

In this context, the high-entropy strategy, an emerging material design concept in recent years, provides a new approach for improving the comprehensive performance of layered oxide cathode materials. By introducing a variety of main group or transition metal elements into the crystal structure, high-entropy cathode materials can not only enhance the structural stability by mitigating phase transitions, Jahn–Teller distortions, and oxygen redox irreversibility but also mitigate the corrosion effects of moisture and CO₂ in the air, enhancing the material’s environmental stability at the atomic level. Therefore, the air stability regulation strategy based on high-entropy design is becoming a key development direction in sodium-ion cathode material research. This review provides the air-induced failure mechanisms of conventional LTMO materials and introduces the application of high-entropy strategies in sodium-ion LTMO cathode materials, focusing on optimizing air stability and electrochemical performance through high-entropy cation regulation, P2/O3 dual-phase synergistic structures, and fluorine ion doping. Additionally, this review further outlines potential future research directions, including the use of advanced characterization techniques to reveal failure mechanisms, the integration of machine learning to optimize material design, and the development of high-performance mixed-phase structures to further enhance the air stability of LTMO materials under high-entropy strategies.

2. Fundamentals of High-Entropy Materials

High entropy is a type of mixing entropy that represents the combined entropy of multi-component single-phase solid solutions, including configurational, vibrational, magnetic, and electronic disorder entropy [23]. Configurational entropy dominates in many research systems due to its relative simplicity, and LTMO materials are no exception. High mixing entropy helps stabilize the solid solution structure, inhibit the formation of ordered phases or multiphase separation, thereby enhancing thermal stability, structural stability, and the material’s performance tunability. In recent years, this concept has been widely applied in fields such as battery materials, catalysts, and ceramics to optimize performance and achieve breakthroughs via the synergistic effects of multiple elements [24,25].

2.1. Definition of High Entropy

Physicist Boltzmann discovered the intricate relationship between the entropy of an isolated system and the numerous microscopic states that constitute its thermodynamic characteristics [26]. This discovery also led to the renowned Boltzmann thermodynamics, a far-reaching statistical principle that allows for us to numerically quantify the configurational entropy (ΔS_confi) of a system. High entropy is defined in two ways; one definition is determined by the composition of the components: if a material contains five or more main elements and the atomic percentage of each element is between 5% and 35%, it is classified as a “high-entropy” material. The other definition uses the quantitative value of ΔS_confi for classification, as shown in Equation (1) [27]:

$$∆{\mathrm{S}}_{\mathrm{confi}}=-\mathrm{R}\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{x}}_{\mathrm{i}}\ln {\mathrm{x}}_{\mathrm{i}}$$

(1)

where R, n, and x_i represent the ideal gas constant (8.314 J mol⁻¹ K⁻¹), the number of components, and the mole fraction of component i, respectively. According to the classification of configurational entropy, materials with ΔS_confi < 1 R are classified as “low entropy”, those with 1.5 R > ΔS_confi ≥ 1 R are classified as “medium entropy”, and materials with ΔS_confi ≥ 1.5 R are classified as “high entropy” systems [28,29]. When considering a fixed number of components (n), the configurational entropy achieves its maximum value when the atomic fraction of all components is equal, which is known as an equimolar composition. Then, the configurational entropy per mole is

$$∆{\mathrm{S}}_{\max }=-\mathrm{R}\ln \mathrm{n}$$

(2)

For layered oxides, the repeated TMO₆ structure and the free arrangement of Na⁺ ions form various phases, such as the O3 and P2 phases. This structure offers a greater degree of freedom for element substitution. Although Na, transition metal (TM), and O sites all offer opportunities for chemical modification, the TM site provides greater flexibility for modification, including the types and concentrations of elements that can be introduced. This site can typically accommodate five or more elements, enabling a variety of combinations to precisely adjust the material properties [30,31]. Na and O sites also offer opportunities for substitution by other elements, but the substitution range is more limited compared to TM sites. For instance, replacing a small amount of sodium with Li facilitates Li-TM interaction, which relieves lattice stress and improves ionic conductivity. Fluorine (F) is the most commonly used anion doping element [32]. As all three sites contribute to the total configurational entropy to varying degrees, the total configurational entropy in layered oxides can be quantified by Equation (3):

$$∆{\mathrm{S}}_{\mathrm{con}\mathrm{fi}}=-\mathrm{R}\left[{\left({\displaystyle {\sum }_{\mathrm{i}=1}^{\mathrm{n}}}{\mathrm{x}}_{\mathrm{i}}\ln {\mathrm{x}}_{\mathrm{i}}\right)}_{\mathrm{cation-site}}+{\left({\displaystyle {\sum }_{\mathrm{j}=1}^{\mathrm{m}}}{\mathrm{x}}_{\mathrm{j}}\ln {\mathrm{x}}_{\mathrm{j}}\right)}_{\mathrm{anion-site}}\right]$$

(3)

where x_i and x_j represent the molar fractions of the cation and anion elements, respectively, and n and m denote the number of cation and anion species. Additionally, in high-entropy systems, since ΔS_confi ≥ 1.5 R, the compound phase generally adopts a single-phase state. The stability of the compound phase can be qualitatively analyzed using the mixing Gibbs free energy (ΔG_mix) from Equation (4), where ΔH_mix represents the mixing enthalpy, ΔS_mix denotes the mixing entropy, and T represents the absolute temperature. It is important to note that ΔS_mix is typically dominated by ΔS_confi. In this context, entropy is directly related to temperature, and high-temperature synthesis typically promotes the formation of a single-phase system with enhanced configurational entropy [33,34].

$$∆{\mathrm{G}}_{\mathrm{mix}}=∆{\mathrm{H}}_{\mathrm{mix}}-\mathrm{T}∆{\mathrm{S}}_{\mathrm{mix}}$$

(4)

2.2. Four Core Effects

Based on the four core effects of high entropy, as shown in Figure 2, improving the stability of layered oxides under the high-entropy strategy should give priority to the combination of elements with complementary electronic structures, high oxygen stability, and flexible valence states, and employ the high-entropy strategy to inhibit phase transitions, expand interlayer spacing, and enhance structural resilience.

High-entropy effect: High-entropy materials typically contain five or more main elements in near-equal molar ratios. During the solid solution formation process, the system generates substantial mixing entropy. As mentioned, when the mixing entropy ΔS_mix is large, even if the mixing enthalpy ΔH_mix is not negative, it remains possible to form a stable single-phase structure due to the favorable entropy term, thus inhibiting the precipitation of multiple phases. The high-entropy effect expands the stable region of the phase diagram and improves the thermodynamic stability of the high-temperature phase [35].

Lattice distortion effect: Due to the different atomic radii, electronic structures, and chemical bond strengths of different elements, when they occupy lattice positions together, this results in significant lattice strain. This distortion manifests as local bond length changes, fluctuations in lattice parameters, and increased stress fields within body-centered or face-centered structures. The lattice distortion effect not only changes the mechanical properties of the material but also influences charge carrier migration, ion diffusion pathways, and other related properties. In energy storage materials, it may cause alterations in the insertion/extraction potential and modulate reaction kinetics.

Sluggish diffusion effect: In high-entropy systems, the presence of multiple elements introduces varying atomic migration barriers, complex migration paths, and interference between diffusion channels, leading to a reduced overall atomic diffusion rate. This effect can be modeled using Fick’s law or the diffusion activation energy model:

$$\mathrm{D}={\mathrm{D}}_0 \exp (-{\displaystyle \frac{\mathrm{Q}}{\mathrm{RT}}})$$

(5)

where Q represents the effective diffusion activation energy. In high-entropy materials, due to the participation of multiple elements, the average Q value may be higher, resulting in a decrease in D. This slow diffusion characteristic can effectively inhibit atomic aggregation, phase separation, and grain coarsening in a high-temperature environment, which improves the thermal stability, corrosion resistance, and cycle life of the material.

“Cocktail” effect: This effect is not an isolated mechanism but instead refers to the complex enhancements resulting from the synergistic interactions of multiple elements. This includes, but is not limited to, synergistic strengthening between multiple elements (e.g., mechanical properties, electrochemical stability), new electronic structures induced by chemical environment diversity, surface active site diversity, and improved interface stability. The “cocktail” effect is essentially a nonlinear superposition of the interactions between the physical and chemical properties of each element, which contributes to high-entropy materials achieving multifunctionality and superior performance [36].

3. Progress in Research on the Air Stability of LTMO Under the High-Entropy Strategy

This section classifies the sensitivity of LTMO to air in detail, analyzes their causes, explains representative cases using chemical equations, and discusses the research progress on various modifications to the air stability of LTMO under high-entropy strategies.

3.1. The Issue of Air Stability in LTMO

Sodium near the particle surface may dissolve to form sodium salts and can also lead to the formation of alkaline and hygroscopic compounds, as shown in Figure 3a. The formation mechanism is heavily influenced by the composition of the cathode material. Sodium ions may begin to leak during the sintering and cooling stages of material synthesis, reacting with CO₂ in the air. Alkaline residues can lead to the agglomeration of electrode materials, resulting in a jelly-like texture during slurry preparation. Furthermore, alkaline residues on the surface of the layered oxide significantly increase the impedance of the cathode material, impeding the diffusion of sodium ions and electrons [37,38]. The sodium dissolution mechanism is generally accompanied by two main redox reactions: one is oxygen reacts with TM ions, accompanied by further sodium loss, as described in Equations (6) and (7):

$${4\mathrm{TM}}^{\mathrm{n}+}+{\mathrm{O}}_2+4{\mathrm{H}}^{+}\to {4\mathrm{TM}}^{\left(\mathrm{n}+1\right)+}+2{\mathrm{H}}_2\mathrm{O}$$

(6)

$${\mathrm{CO}}_2+1/2{\mathrm{O}}_2+2{\mathrm{e}}^{-}\to {\mathrm{CO}}_3^{2-}$$

(7)

Another mechanism involves the formation of Na₂O intermediates, coupled with the generation of hydrogen, as shown in Equations (8)–(10):

$${\mathrm{Na}}_x\mathrm{TM}{\mathrm{O}}_2+{\mathrm{nO}}_2\to {\mathrm{Na}}_{x-4\mathrm{n}}\mathrm{TM}{\mathrm{O}}_2+{2\mathrm{nNa}}_2\mathrm{O}$$

(8)

$${\mathrm{Na}}_x\mathrm{TM}{\mathrm{O}}_2+{\mathrm{nH}}_2\mathrm{O}\to {\mathrm{Na}}_{x-2\mathrm{n}}\mathrm{TM}{\mathrm{O}}_2+{\mathrm{n}{\mathrm{H}}_2+\mathrm{nNa}}_2\mathrm{O}$$

(9)

$${\mathrm{Na}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {2\mathrm{Na}}_2\mathrm{OH}$$

(10)

Water insertion, as illustrated in Figure 3b, can be physically absorbed and reacted, or it can directly insert into the interlayer of the cathode material, expanding the interlayer spacing and compromising the integrity of the crystal structure. This phenomenon is particularly prevalent in P2-LTMO materials and can be explained by the following equation [39,40]:

$${\mathrm{Na}}_{x-\mathrm{n}}\mathrm{TM}{\mathrm{O}}_2+{\mathrm{yH}}_2\mathrm{O}\to {\mathrm{Na}}_{x-\mathrm{n}}{\left({\mathrm{H}}_2\mathrm{O}\right)}_y\mathrm{TM}{\mathrm{O}}_2$$

(11)

$${\mathrm{Na}}_{x-\mathrm{n}}{\mathrm{H}}_{\mathrm{n}}\mathrm{TM}{\mathrm{O}}_2+{\mathrm{yH}}_2\mathrm{O}\to {\mathrm{Na}}_{x-\mathrm{n}}{{\mathrm{H}}_{\mathrm{n}}\left({\mathrm{H}}_2\mathrm{O}\right)}_y\mathrm{TM}{\mathrm{O}}_2$$

(12)

In addition to the significant loss of sodium and the formation of hydrated phases, phase transitions have consistently played a critical role in air sensitivity, notably the transformation of the O3 structure into a P3 structure to accommodate sodium vacancies. Phase transitions can also lead to the formation of spinel-like transition metal oxides, which may further decompose into rock-salt-like and spinel-like disordered structures, as shown in Figure 3c [41]. Therefore, fully understanding the strong correlation between the chemical composition of cathode materials and phase transitions is crucial for tailoring cathode materials to enhance their air stability.

In terms of morphology, these factors may induce stacking deformation and surface cracking in the cathode materials, resulting in capacity loss and poor long-term cycling performance. As illustrated in Figure 3d, the Na₂CO₃ generated from reactions with CO₂ can widen the sodium interlayer spacing and cause transverse fracture of cracks [42]. The generation of gas inside the material further aggravates interlayer stress, which in turn promotes crack propagation and raises significant safety concerns for battery operation. Additionally, the migration of transition metal elements to the surface and the formation of new phases can further alter the material’s morphology. The accumulation of these morphological changes and cracks severely deteriorates the electrochemical performance of the cathode material, underscoring the critical importance of improving air stability.

3.2. High-Entropy Cation Regulation Strategy

Different layered oxides may suffer from one or more factors, leading to stability failure. To improve the air stability of layered oxides, it is essential to address the root causes, with elemental composition being a critical factor. The high-entropy doping strategy, originally derived from the concept of high-entropy alloys, introduces multiple (≥5) TM cations with differing physical and chemical properties to significantly increase the configurational entropy of the material. As shown in Figure 4a, in LTMO cathode materials, a greater diversity of elements in the TM layer leads to higher structural entropy, thereby decreasing system energy and thermodynamically enhancing the stability of the crystal structure. Zhan et al. [43] synthesized a high-entropy O3-LTMO, NaNi_0.3Cu_0.05Fe_0.1Mn_0.3Mg_0.05Ti_0.2O₂ (NCFMMT), by simultaneously doping Cu, Mg, and Ti into the TM layer to significantly boost configurational entropy. In this system, Mg improves structural reversibility during cycling and inhibits multi-level phase transitions, Ti strengthens the crystal structure, and Cu, with its high redox potential, enhances air stability. As shown in Figure 4b,c, the high-entropy NCFMMT cathode exhibits significantly improved cycling and air stability compared to low-entropy layered oxides, delivering a high reversible capacity of 88 mAh g⁻¹ at a 5 C rate and achieving a capacity retention rate of 86.8% after 500 cycles at 5C. The highly disordered distribution of TM elements within the metal layer simultaneously suppresses the ordering of Na⁺ vacancies and charges, effectively inhibiting interlayer sliding and phase transitions. Even after 7 days of air exposure or 1 h of water immersion, the crystal structure remains unchanged.

The selection and combination of transition metal elements play distinct and complementary roles in optimizing the performance of layered cathode materials; similarly, Zhao et al. [17] applied the high-entropy strategy to design an O3-type NaNi_0.12Cu_0.12Mg_0.12Fe_0.15Co_0.15Mn_0.1Ti_0.1Sn_0.1Sb_0.04O₂ sodium-ion cathode containing nine TM components, with oxidation states ranging from divalent to pentavalent. The material was engineered to realize specific functionalities based on the elemental characteristics: Ni²⁺, Cu²⁺, Fe³⁺, and Co³⁺ contribute to charge compensation and capacity enhancement, Ti⁴⁺ and Mg²⁺ help maintain structural integrity and suppress phase transitions during cycling; Mn⁴⁺ serves as a structure stabilizer, while Sn⁴⁺ and Sb⁵⁺ increase the average operating voltage. The experimental outcomes aligned well with the design expectations. The material demonstrated ultra-long cycling stability across different current densities, achieving 80% capacity retention after 500 cycles at a high rate of 5 C, while delivering 60% more capacity compared to the capacity window of traditional O3-LTMO. Fingerprint spectra techniques were employed to verify the elemental types and oxidation states. As shown in Figure 4d,e, compared to the limited elemental composition and oxidation states in conventional O3-LTMO, the high-entropy oxide (HEO) cathode exhibited a rich and randomly distributed array of redox-active elements, resulting in diverse local interactions. During sodium insertion and extraction, specific TMs undergo changes in ionic radius and oxidation state, while Na content fluctuations influence local chemical environments and phase transitions. Unlike the relatively ordered distribution of redox elements in traditional cathodes, the random distribution in high-entropy systems enables adaptability to local structural changes, effectively delaying phase transitions and enhancing structural stability.

The TM substitution sites are not fixed and can also be doped with ions that can move freely during the reaction; Yao’s team [20] focused on the functional role of lithium doping and synthesized a layered Na_2/3Li_1/6Fe_1/6Co_1/6Ni_1/6Mn_1/3O₂ HEO cathode. As shown in Figure 4f, lithium occupying the TM layer enabled the formation of a superlattice-stabilized structure and promoted anion redox activity. This configuration facilitated a rapid O3–P3 phase transition in the low-voltage region while suppressing the reverse P3–O3 phase transition at high voltages (~4.5 V), effectively addressing the issues of poor phase transition dynamics and oxygen redox instability. As a result, the capacity was predominantly derived from the P3 phase, contributing to the excellent rate performance of the material (78 mAh g⁻¹ at 10 C). X-ray absorption spectroscopy (XAS) analysis confirmed that Ni²⁺/Ni³⁺/Ni⁴⁺ and Fe³⁺/Fe⁴⁺ redox couples significantly contributed to the improvement in reversible capacity (171.2 mAh g⁻¹ at 0.1 C), while Mn⁴⁺ and Co³⁺ served critical roles in maintaining structural stability and suppressing Jahn–Teller distortion. Joshi et al. [44] incorporated lithium into a quinary HEO system to obtain Na_0.9Li_0.1Ni_0.4Fe_0.2Mn_0.44Ti_0.04Mg_0.02O_1.9F_0.1 (Na_0.9Li_0.1), promoting superstructure formation between the sodium and TM layers and thereby enhancing sodium-ion diffusion. Lithium-induced sodium vacancies improved the ionic kinetics, leading to high capacities of 109 mAh g⁻¹ (2~4 V) and 139 mAh g⁻¹ (2~4.3 V). The formation of vacancies increased the interlayer distance, facilitating easier ion diffusion during charge–discharge cycles, thus preserving cathode structural integrity and suppressing the formation of the O3′ phase during the O3–P3 transition. As a result, the Na_0.9Li_0.1 material exhibited a stable capacity retention rate of 90% after 200 cycles. As shown in Figure 4g,h, the XRD and bright-field SAED patterns of the Na_0.9Li_0.1 cathode after 30 days of air exposure remained consistent with the initial O3-phase structure. This excellent air stability is attributed to the enhanced oxidation resistance resulting from lithium-induced vacancy formation, which lowers the charge density of the TM elements.

As shown in Figure 4i,j, the O3-phase Na_0.95Li_0.06Ni_0.25Cu0.₀₅Fe_0.15Mn_0.49O₂ sample synthesized by Cai et al. [32] retained its crystal structure after 7 days of air exposure. No hydrated phases or sodium carbonate were detected during prolonged exposure, as further confirmed by Raman spectroscopy. After 48 h of air exposure, the material exhibited a first discharge capacity of 138.2 mAh g⁻¹ and an initial coulombic efficiency of 96.9%. At a current density of 400 mA g⁻¹, the air-exposed electrode exhibited a capacity retention of 83.7% after 200 cycles, comparable to that of the electrode without air exposure.

Different elemental compositions and proportions can be selected according to specific design objectives. In general, their roles can be classified based on ion activity: (1) active elements (such as Ni²⁺, Co³⁺, Mn²⁺, Fe²⁺, and Cr³⁺) contribute to increased reversible capacity by providing charge compensation; (2) inactive elements (such as Zn²⁺, Mg²⁺, Mn⁴⁺, Zr⁴⁺, Sn⁴⁺, and Sb⁵⁺) enhance cycling stability by stabilizing the structure; (3) cations with mobility (such as Li⁺, Mg²⁺, and Zn²⁺) can induce anion redox reactions, thereby providing additional capacity and expanding the voltage range.

Table 1. Layered cathodes with high-entropy configurations in SIBs.

	Cathode	Voltage Range	Initial Capacity (mAh g−1)	Cycle Retention (%)	Rate Performance (mAh g−1)	Ref.
	NaNi1/4Co1/4Fe1/4Mn1/8Ti1/8O2	2.0~4.0	128 (2C)	97.72 (100)	38.6/60 C	[46]
Active elements	NaCu0.1Ni0.3Fe0.2Mn0.2Ti0.2O2	2.0~3.9	130 (0.1C)	87 (100)	85/5 C	[47]
	Na0.667Mn0.667Ni0.167Co0.117Ti0.01Mg0.01Cu0.01Mo0.01Nb0.01O2	1.5~4.5	169.8 (1C)	76.4 (100)	111/5 C	[45]
	Na0.7Mn0.4Ni0.3Cu0.1Fe0.1Ti0.1O1.95F0.1	2.0~4.3	133.5 (1C)	99.5 (500)	97.6/10 C	[48]
	NaMn0.2Fe0.2Co0.2Ni0.2Sn0.1Al0.05Mg0.05O2	1.5~4.2	152 (0.5C)	71.1 (200)	142/0.5 C	[49]
Inactive elements	Na0.75Mn0.55Ni0.25Co0.05Fe0.10Zr0.05O2	1.5~4.2	143 (0.1C)	81 (100)	22/10 C	[50]
	NaNi0.25Mg0.05Cu0.1Fe0.2Mn0.2Ti0.1Sn0.1O2	2.0~4.0	130.8 (1C)	75 (500)	91/1 C	[51]
	NaNi0.12Cu0.12Mg0.12Fe0.15Co0.15Mn0.1Ti0.1Sn0.1Sb0.04O2	2.0~3.9	110 (3C)	83 (500)	89/3 C	[17]
Mobile cation	NaNi0.1Mn0.15Co0.2Cu0.1Fe0.1Li0.1Ti0.15Sn0.1O2	2.0~4.1	115 (1.6C)	82.7 (1000)	90/1.6 C	[52]
	Na0.95Li0.06Ni0.25Cu0.05Fe0.15Mn0.49O2	2.0~4.2	141.2 (8C)	83.2 (500)	85/20 C	[32]

summarizes the electrochemical performance metrics of representative HEOs based on this classification principle. For example, the P2-phase HEO sample synthesized by Ma et al [45] and the active elements Ni²⁺, Co³⁺, and Mn²⁺ in Na_0.667Mn_0.667Ni_0.167Co_0.117Ti_0.01Mg_0.01Cu_0.01Mo_0.01Nb_0.01O₂ can provide significant charge compensation, which enhances the initial specific capacity of the material. Among them, the mobile cation Mg²⁺ triggers lattice oxygen redox activity and contributes to charge compensation. As a result, the material demonstrates stable cycling over 100 cycles (1C) at a high cutoff voltage of 4.5 V with a capacity retention of 76.4% and maintains a specific capacity of 111 mAh g⁻¹, even at a high rate of 5C. It is important to note that the modification effect induced by high-entropy doping arises from the synergistic interaction among the various elements, rather than the simple superposition of the individual characteristics of each dopant. Overall, the diversity of high-entropy doping combinations greatly expands the design flexibility and application potential of LTMOs.

3.3. P2/O3 Dual-Phase Synergistic Structure

To enhance the structural stability and air resistance of LTMO, it is essential to adopt a mixed-phase design strategy. Mixed-phase design refers to the intentional construction of a composite structure comprising multiple crystal phases, rather than simply mechanically blending separately synthesized phases. This approach maximizes the synergistic advantages of different phases, thereby improving the sodium storage performance of LTMO. Traditional P2-type and O3-type LTMO each have inherent advantages and limitations: P2-type structures feature larger interlayer spacings and superior Na⁺ diffusion kinetics but suffer from poor air stability, whereas O3-type structures possess denser frameworks with strong hydrolysis resistance but longer ion migration paths. Therefore, different phases exhibit distinct functional characteristics and applications. Generally, the formation of the P2 and O3 phases is primarily influenced by precise control of the sodium source, appropriate selection of transition metal elements, synthesis temperature, and calcination atmosphere. The distinction between the P2 and O3 phases is primarily attributed to sodium content; thus, the precursor ratio and sodium source addition must be precisely regulated during synthesis [53]. Excess sodium typically promotes the formation of the O3 phase, while insufficient sodium tends to create vacancies, thereby facilitating the formation of the P2 phase. Moreover, the selection of transition metal elements plays a more critical role. High-valent or small-radius ions (e.g., Mn⁴⁺, Ni³⁺) tend to form a more ordered layered structure, thereby supporting the structural stability of the P2 phase [54]. In contrast, larger-radius or low-valent ions (e.g., Fe³⁺, Cr⁴⁺) exhibit stronger electrostatic repulsion with the sodium layers, favoring the maintenance of the octahedral interlayer arrangement characteristic of the O3 phase. Through the high-entropy strategy, adjusting the type and proportion of elements enables regulation of the P2/O3 phase ratio and simultaneous modulation of phase stability. Synthesis temperature is also crucial but exhibits a more complex influence [55]. For example, higher temperatures lead to more complete crystal growth and improved ordering of the layered structure. However, elevated temperatures may lead to sodium volatilization, thereby facilitating transformation into the sodium-deficient P2 phase. Under the high-entropy strategy, the ratio of the two phases can be finely tuned by controlling the calcination temperature and heating rate. The selection of calcination atmosphere relies on the control of metal ion valence states. In an oxygen-rich environment, metal ions tend to be oxidized to higher valence states, favoring the formation of the P2 phase. Conversely, in an inert atmosphere (e.g., N₂ or Ar), the oxidation states remain lower, favoring the stabilization of the O3 phase. Ultimately, the ratio of the P2 and O3 phases results from the combined influence of multiple factors, rather than any single condition. By inducing the coexistence of P2 and O3 phases through high-entropy doping strategies, it becomes possible to harness the benefits of both structures simultaneously, thus enhancing the overall air stability of the material. The design of dual-phase structures introduces heterogeneous interfaces formed by the different stacking arrangements of crystal lattices at the microscopic scale. These interfaces serve as stress buffers, mitigating volume expansion due to water absorption or gas reactions, and help to dissipate environmental stresses. Furthermore, the regulation of band structures and ion diffusion pathways at heterogeneous interfaces can inhibit surface side reactions. In addition, the presence of an energy gradient between different phases can promote the rapid migration of sodium ions, reducing their residence time at the surface and thus lowering the probability of undesirable reactions with active species in the environment. Zhou et al. [48] developed a novel high-entropy cathode material, Na_0.7Mn_0.4Ni_0.3Cu_0.1Fe_0.1Ti_0.1O_1.95F_0.1 (P2/O3-NaMnNiCuFeTiOF), featuring a P2/O3 dual-phase structure that facilitates reversible structural evolution, rapid Na⁺ diffusion kinetics, and a low energy barrier for ion transport. In situ structural analysis during the synthesis process demonstrated that the P2/O3 phase ratio can be finely tuned by adjusting the sintering temperature, as shown in Figure 5a. Experimental results indicated that the P2/O3 dual-phase material sintered at 900 °C achieved the optimal phase composition, delivering an initial capacity of 138.2 mAh g⁻¹ at a current density of 20 mA g⁻¹ and exhibiting a maximum initial coulombic efficiency of 97.6%. Moreover, the assembled sodium-ion full cell, comprising the P2/O3-NaMnNiCuFeTiOF cathode and a hard carbon anode, achieved a high energy density of 240.3 Wh kg⁻¹ at a power density of 1172 W kg⁻¹. The outstanding sodium storage performance of the P2/O3-NaMnNiCuFeTiOF material is attributed to the high-entropy dual-phase structure design, which not only enhances structural stability by alleviating Jahn–Teller distortion and preventing TM layer slipping across a wide temperature range but also significantly improves air stability (Figure 5b). The material demonstrated excellent cycling and rate capabilities. Importantly, a comparative analysis of the crystal structure and electrochemical performance after 90 days of air exposure revealed no cracks at the P2/O3 phase boundaries following cycling tests, further underscoring the exceptional structural stability conferred by the dual-phase high-entropy design.

Mu et al. [56] also designed a single-phase material NaLi_0.05Ni_0.25Cu_0.025Mg_0.025Fe_0.05Al_0.05Mn_0.5Ti_0.05O₂ (S-HE) and a P2/O3 biphasic material Na_0.85Li_0.05Ni_0.25Cu_0.025Mg_0.025Fe_0.05Al_0.05Mn_0.5Ti_0.05O₂ (B-HE). Although B-HE and S-HE exhibit similar initial capacities (approximately 120 mAh g⁻¹), the B-HE demonstrates superior electrochemical performance. As shown in Figure 5c, the differential capacity (dQ/dV) curves display two pairs of redox peaks. Notably, the redox peaks of B-HE appear at higher voltages and exhibit smaller polarization, indicating better kinetic behavior. In situ XRD analysis (Figure 5d) further reveals that at the initial stage of sodium extraction from B-HE, both the O3 and P2 phases are retained. Upon continued charging, an O3-to-P3 phase transition occurs. Subsequently, the P3 phase derived from O3 coexists with the original P2 phase, and the gradual shift in diffraction peaks reflects a quasi-solid-solution mechanism. The phase evolution—following a benign sequence of O3/P2 → P3/P2 → OP2/OP4—contributes to a high Na⁺ diffusion coefficient. In contrast to the more complex phase transitions observed in S-HE, the structure of B-HE remains significantly more stable. As a result, B-HE delivers a reversible capacity of 122 mAh g⁻¹ at 0.2 C and an excellent rate capacity of 81.8 mAh g⁻¹ at 10 C, benefiting from the lower sodium ion migration barriers associated with the P-phase component. Moreover, B-HE exhibits outstanding long-term cycling stability, retaining 88% of its capacity after 300 cycles at 0.2 C and 89% after 1000 cycles at 10 C, underscoring the advantages conferred by its engineered dual-phase structure.

Unlike the biphasic structure dominated by O3, Pang et al. [22] introduced Cu and Ti into O3-NaNi_1/3Fe_1/3Mn_1/3O₂ (O3-NNFM) and P2-Na_2/3Ni_1/3Mn_2/3O₂ (P2-NM) via a high-entropy strategy to design a dual-phase composite cathode, Na_7/9Ni_2/9Mn_4/9Fe_1/9Cu_1/9Ti_1/9O₂ (NMFCT), with a P2-type structure as the dominant phase, as shown in Figure 5f. Five cathode materials—NMFCT, Na_7/9Ni_2/9Mn_5/9Fe_1/9Cu_1/9O₂ (NMFC), Na_7/9Ni_1/3Mn_5/9Fe_1/9O₂ (NMF), P2-NM, and O3-NNFM—were tested for long-term cycling at 5C, as depicted in Figure 5e. Although the O3-NNFM cathode demonstrates excellent initial capacity, it suffers from rapid capacity decay due to the unfavorable O3-P3-O3’ phase transition during cycling, which leads to significant irreversible structural damage. Similarly, the P2-NM cathode undergoes an unfavorable P2-O2 phase transition. In contrast, the NMFCT cathode, after 500 cycles at 5 C, maintains a reversible capacity of 72.85 mAh g⁻¹, with a capacity retention rate of 83.25%. This enhanced electrochemical performance can be attributed to the synergistic effect of the high-entropy strategy and the dual-phase structure, achieved by the Cu and Ti dual-substitution. The P2/O3 dual-phase structure disrupts the interlayer stacking mode inherent to single-phase structures, with the O3 phase serving as a pinning agent relative to the P2 phase, thus inhibiting the sliding of the transition metal oxide (TMO₂) layers. Cu substitution stabilizes the oxygen framework, while Ti substitution introduces chemically rigid Ti-O bonds into the TMO₂ layers, which reduces ion migration under high pressure and disrupts the Na⁺/vacancy ordering.

In fact, there are certain principles governing the optimal compositional ratio between the two phases; recently, Li et al. [57] synthesized 43 high-entropy composite structures by optimizing the Ni/Mn/Cu/Ti/Sn content and established a correlation between the weighted average ionic radius (WAIR) of TMs and the resulting phase structure. When the WAIR ranged between 0.583 and 0.637 Å, a composite phase containing both P2 and O3 was formed. As the WAIR increased, the proportion of the O3 phase increased accordingly. Electrochemical performance, charge and discharge processes, and stress comparisons of selected low-entropy P2/O3-Na_0.75Ni_0.35Mn_0.6O₂ (LE-P2/O3), high-entropy P2/O3-Na_0.75Ni_0.33Mn_0.4Cu_0.1Ti_0.13Sn_0.05O₂ (HE-P2/O3), and high-entropy O3- Na_0.67Ni_0.2Mn_0.25Cu_0.13Ti_0.2Sn_0.22O₂ (HE-O3) composites from the 43 categories were analyzed. The cycle stability of the HE-P2/O3 composite cathode after 300 cycles was 77.3 mAh g⁻¹, with a rate performance of 88.7 mAh g⁻¹ at 750 mA g⁻¹, demonstrating the superiority of the dual-phase HE strategy. The disordered distribution of TMs ions within the TMO₂ layers restricts the O3 → P3 phase transition and suppresses the irreversible P2 → O2 phase transformation. The differing lattice orientations of the P2 and O3 phases in HE-P2/O3 inhibit interlayer slip, thereby enhancing cycle stability. Additionally, the HE-P2/O3 composite exhibits improved air stability and mechanical strength. Simulations of stress evolution during Na⁺ distribution within the particles, shown in Figure 5g,h, suggest that the biphasic structure facilitates greater Na⁺ extraction compared to single-phase structures, and the stress experienced during cycling is minimized in the biphasic structure.

3.4. Fluoride Ion Doping

Fluorine, due to its strong electronegativity, has been identified as an effective dopant capable of significantly enhancing the electrochemical properties of cathode materials. Its incorporation can improve Na⁺ diffusion kinetics, enhance structural stability, and mitigate adverse phase transitions. Fluorine doping has been widely applied in the modification of traditional layered oxides and continues to serve as an effective strategy within high-entropy systems, further optimizing the performance of advanced cathode materials. Cao et al. [58] synthesized a series of high-entropy oxyfluoride (HEOF) Na_0.66Mn_0.6Li_0.1Ti_0.1(MgAlCuZn)_0.05O_2−xF_x (x = 0, 0.1, 0.2, 0.3, 0.4, denoted as HEO, HEOF-1, HEOF-2, HEOF-3, and HEOF-4) materials with varying fluorine substitution levels via a traditional solid-state reaction route and confirmed the successful incorporation of fluorine through comprehensive characterization techniques, including X-ray powder diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (Raman), and X-ray photoelectron spectroscopy (XPS) analysis. Electrochemical performance evaluations revealed that the sample with a fluorine doping level of 0.3 (HEOF-3) exhibited the best rate capability and cycling stability. Cyclic voltammetry (CV) curves demonstrated that increasing the fluorine content enhanced the reversibility of the oxygen redox reaction. After 100 cycles at a 1 C rate, HEOF-3 achieved an outstanding capacity retention of 99.32%. Additionally, Na⁺ migration barriers were calculated, as shown in Figure 6a,b. HEOF-3 exhibited a lower migration barrier compared to undoped HEO, indicating that Na⁺ could migrate more readily within the structure. This improvement is primarily attributed to the moderate fluorine doping, which significantly expanded the Na layer interspacing, thereby facilitating faster Na⁺ transport kinetics—a conclusion further supported by GITT results. As a result, HEOF-3 demonstrated excellent rate performance, delivering a specific capacity of 91 mAh g⁻¹ even at a high rate of 10 C, while maintaining 170.68 mAh g⁻¹ when the rate was reduced to 0.1 C.

Joshi et al. [44] synthesized Na_0.9Li_0.1 oxyfluoride cathode materials under a high-entropy strategy, selecting oxyfluorides over pure oxides due to their superior ability to suppress oxygen loss during long-term cycling; this is attributed to the more ionic nature of M–F bonds compared to M–O bonds, which also leads to an increase in working potential. After confirming successful fluorine incorporation through various characterization techniques, a control experiment was designed by synthesizing a fluorine-free counterpart, denoted as (–F) Na_0.9Li_0.1. Although the initial capacity of (–F) Na_0.9Li_0.1 was slightly higher, as shown in Figure 6c, its capacity declined sharply over extended cycling. Furthermore, ex situ XRD recorded at different discharge states and after prolonged cycling revealed that while a reversible phase transition between the O-type and P-type phases occurred during initial cycling, the full recovery of the O3 phase was not achieved in (–F) Na_0.9Li_0.1 during long-term cycling. As illustrated in Figure 6d, thermal analysis showed that the onset of thermal activity occurred at a much lower temperature for (–F) Na_0.9Li_0.1, and the total heat release was almost twice that of Na_0.9Li_0.1 cathode. These findings indicate that fluorine substitution significantly enhances the structural stability of the cathode during interaction with the electrolyte and improves thermal abuse tolerance.

Further F-ion substitution under the high-entropy strategy effectively reduces the covalency of the TM–O bond, thereby mitigating structural changes. Ding et al. [41] synthesized Na_0.95Li_0.07Cu_0.11Ni_0.11Fe_0.3Mn_0.41O_1.97F_0.03 (LCNFMF) and attributed the improved cycling performance to the suppression of Mn³⁺/Mn⁴⁺ redox activity and the enhancement of crystal structure stability facilitated by F substitution. The lattice parameter changes during the first charge and discharge cycles for LCNFMF and two comparison samples were recorded. As shown in Figure 6e, the variations in the a- and c-lattice parameters of Na_0.89Li_0.05Cu_0.11Ni_0.11Fe_0.3Mn_0.43O₂ (LCNFM) and LCNFMF were only 0.4% and 0.4%, respectively, compared with 0.9% and 1.3% observed in Na_0.9Cu_0.11Ni_0.11Fe_0.3Mn_0.48O₂ (CNFM). This indicates that F substitution suppresses Mn⁴⁺ reduction and additional Na⁺ insertion, thereby weakening the Jahn–Teller effect and improving the structural stability of the oxide cathode. Moreover, as shown in Figure 6f, the contraction of the c-lattice parameter in LCNFMF at high voltages was smaller than that in LCNFM, suggesting that the high electronegativity of fluorine alleviates the increase in TM–O bond covalency caused by the sharp rise in TM valence, thus reducing O–O repulsion and preventing lattice collapse. These results demonstrate that F doping not only optimizes the electronic structure but also stabilizes the local crystal framework, significantly enhancing the overall electrochemical capacity and lattice stability. The formation of strong TM–F bonds in the F-substituted samples also minimizes surface catalytic effects and inhibits excessive growth of the cathode–electrolyte interphase, enabling the stable cycling of the LCNFMF cathode. Remarkably, LCNFMF maintained 80% capacity retention (102.5 mAh g⁻¹) after 300 cycles at a current density of 1 C and retained a high specific capacity of 115 mAh g⁻¹ even at 10 C, demonstrating excellent rate performance.

4. Summary and Outlook

In recent years, with the continuous expansion of SIBs in large-scale energy storage applications, the improvement of cathode material performance has become a critical bottleneck restricting further development. LTMOs have attracted considerable attention due to their high capacity, abundant raw material sources, and mature synthesis processes. However, issues such as their hygroscopic nature, poor air stability, and rapid electrochemical performance degradation have severely limited their practical application. As an emerging material design strategy, the high-entropy approach significantly enhances structural stability, chemical uniformity, and air resistance by introducing multiple elements to achieve high configurational entropy, providing a promising pathway for fundamentally improving the performance of layered oxide systems. Research findings have demonstrated that rational design of elemental composition and phase structure can effectively mitigate the problems of hydrolysis and inactivation, offering both theoretical guidance and a practical foundation for the development of next-generation high-performance SIBs cathodes. Nevertheless, the current understanding of material failure mechanisms under complex environmental conditions remains incomplete and requires further in-depth exploration and refinement. Based on the findings of this work, future research could focus on the following directions:

• Employ advanced characterization techniques to elucidate the intrinsic failure mechanisms of high-entropy layered oxides in air. In general, the phase composition of high-entropy materials is identified by X-ray diffraction (XRD) or neutron powder diffraction (NPD), while elemental analysis relies on techniques such as scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) or scanning transmission electron microscopy (STEM). However, these methods are insufficient for accurately tracking real-time material changes during air exposure. Future studies should leverage real-time techniques such as in situ XRD, in situ X-ray photoelectron spectroscopy (XPS), and in situ X-ray absorption spectroscopy (XAS) to monitor microstructural evolution, charge transfer dynamics, and compositional changes at surfaces and interfaces. These insights are critical for revealing key failure pathways and developing targeted stabilization strategies.
• Integrate machine learning and high-throughput screening to accelerate rational design and optimization of high-entropy oxide systems. A comprehensive understanding of the roles of individual elements and their synergistic effects is essential. Building a large-scale materials database that incorporates parameters such as electronegativity, ionic radius, and valence stability can facilitate the establishment of predictive models correlating material composition, structure, and performance. Combining first-principles calculations (DFT) with machine learning algorithms will significantly enhance the efficiency of screening new air-stable high-entropy oxides, enabling the rapid discovery of novel materials with outstanding comprehensive performance from an expanded chemical space.
• Further refine the P2/O3 dual-phase ratio and phase interface characteristics to optimize synergistic effects in cathode materials. Dual-phase systems can substantially enhance structural stability and electrochemical activity by rationally coordinating interlayer spacing, ion diffusion pathways, and interfacial stress. Future research should focus on interface engineering strategies, including dislocation regulation, crystal plane orientation control, and nanoscale structural optimization, to minimize interfacial energy, maximize ion migration rates, and further boost high-rate capability and long-term cycling stability.

This review systematically discussed the principle of the high-entropy effect and its four fundamental characteristics, analyzed the failure mechanisms of traditional layered oxides, and highlighted recent progress in improving air stability via high-entropy strategies, particularly through cation regulation, dual-phase synergy, and fluorine ion doping. Overall, the high-entropy strategy has opened a new research avenue for enhancing the performance and air stability of layered sodium-ion cathode materials. With the continuous advancement of characterization techniques and theoretical modeling methods, it is anticipated that broader development opportunities will emerge in novel material design, performance optimization, and the practical deployment of SIBs technologies.