Ion Doping of P2-Type Cathodes for Sodium-Ion Batteries

Xuantian Feng; Bowen Xu; Da Zhang; Feng Liang

doi:10.7536/PC20251105

Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (6) : 1038-1058 DOI: 10.7536/PC20251105

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Ion Doping of P2-Type Cathodes for Sodium-Ion Batteries

Xuantian Feng ¹^,²^,³
, Bowen Xu ¹^,²^,³
, Da Zhang ¹^,²^,³^,^*
, Feng Liang ¹^,²^,³^,^*

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Abstract

Sodium-ion batteries（SIBs）have attracted increasing attention as a promising alternative to conventional lithium-ion batteries owing to their abundant resources，low cost，and improved safety. Among various cathode systems for SIBs，P2-type layered transition metal oxides stand out due to their relatively simple crystal structures，mature synthesis processes，and fast Na⁺ diffusion kinetics，rendering them highly attractive for practical applications. Nevertheless，their commercialization is still hindered by several intrinsic challenges，including frequent structural phase transitions during cycling，insufficient air stability，and limited reversible capacity. To address these issues，extensive efforts have been devoted to performance optimization strategies such as ionic doping，composite structure design，surface coating，and morphology regulation，among which ionic doping has emerged as an effective and versatile approach. Despite numerous reports，existing studies are predominantly organized according to dopant species or material systems，while a systematic understanding from the perspective of coordination environment regulation remains lacking. In this review，recent advances in ionic doping strategies for P2-type layered oxide cathodes are comprehensively summarized with an emphasis on coordination environments，and the roles of dopant ions located in the alkali metal layers，transition metal layers，and anionic frameworks are systematically discussed，with particular attention to their mechanisms in stabilizing crystal structures，suppressing unfavorable phase transitions，and enhancing electrochemical performance. Furthermore，the key challenges and limitations associated with ionic doping strategies are critically analyzed，their practical applicability is evaluated，and perspectives on future research directions and development opportunities for P2-type layered cathode materials are provided.

Contents

1 Introduction

2 The challenges of P2-type cathode materials

2.1 Irreversible transition

2.2 Low air stability

2.3 Low reversible capacity

3 Alkali metal layer doping modification

4 Transition metal layer doping modification

4.1 Single metal P2 cathode material

4.2 Binary metal P2 cathode material

4.3 Ternary and polymetallic P2 cathode materials

5 Anion oxygen layer doping modification

6 Conclusion and outlook

Graphical abstract

Keywords

sodium ion battery / P2-type cathode materials / coordination environment / ion doping

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Xuantian Feng, Bowen Xu, Da Zhang, Feng Liang. Ion Doping of P2-Type Cathodes for Sodium-Ion Batteries. Progress in Chemistry, 2026, 38(6): 1038-1058 DOI:10.7536/PC20251105

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1 Introduction

With the continuous increase in human society's demand for various types of energy, higher requirements have also been placed on energy storage technologies. As early as the late 1970s, sodium-ion batteries and lithium-ion batteries were extensively studied by scientists^[1-4]. Since Whittingham proposed a titanium disulfide material capable of intercalating Li⁺ as the cathode material for lithium-ion batteries, through continuous development and improvement, lithium-ion batteries have achieved breakthroughs in both quality and quantity, becoming secondary rechargeable batteries widely used in the market^[5-7]. However, with the large-scale exploitation of lithium resources, their shortcomings such as scarce reserves, high prices, and uneven distribution have gradually been exposed^[8].

Compared to the scarcity of lithium resources in lithium-ion batteries, sodium-ion batteries benefit from abundant sodium resources^[9-10], and have become a promising technology due to their similar working principle to lithium-ion batteries. However, since the radius of sodium ions (0.95 Å) is larger than that of lithium ions (0.76 Å), this makes the deintercalation of sodium ions more difficult than that of lithium ions during charge and discharge processes, resulting in slow sodium ion migration kinetics. Furthermore, the deintercalation of sodium ions during charge and discharge easily induces irreversible structural phase transitions, leading to changes in the material structure and reducing the electrochemical performance of the material^[11]. Therefore, developing efficient, fast, and stable electrode materials for sodium-ion batteries is crucial.

Cathode materials are the key factor determining the energy density of sodium-ion batteries. Among numerous cathode materials, layered oxides have attracted significant attention due to their simple structure, ease of synthesis, and high specific capacity. Layered oxides are divided into P-phase and O-phase, such asFigure 1shown, specifically classified into P2, P3, O2, and O3 phases. In P-phase materials, alkali metal atoms occupy prismatic coordination sites, while in O-phase materials, they occupy octahedral coordination sites. The numbers indicate the repeating arrangement units of different oxide layers.^[12-13]. In layered oxide cathode materials, as the sodium ion content in the alkali metal layer increases, the material tends to form octahedral coordination; conversely, it easily forms prismatic coordination. Since the P3 phase cannot be directly synthesized and has a higher sodium ion diffusion barrier than the P2 phase, and the preparation process for the O2 phase is relatively difficult, the P2 and O3 phases are common layered oxide cathode materials. Among them, P2-type layered oxide cathode materials possess greater commercial potential due to their simple structure, fewer phase transitions, and ease of synthesis. However, their poor air stability and low sodium content lead to lower reversible capacity, thereby hindering the commercialization process of P2-type layered oxides.^[14]. Currently, optimization strategies for P2-type layered materials include ion doping, composite structure design, surface coating, and morphology design. Ion doping, as the simplest and most efficient strategy, can suppress phase transitions and improve air stability, thereby enhancing reversible capacity and cycling stability. Ion doping in different coordination environments affects material performance differently; for instance, ion doping in the transition metal layer can improve the electrochemical performance of the material but cannot effectively reduce the electrostatic repulsion between O⁺ when the material is charged to a low Na^2‒-O^2‒ state, leading to severe cracking along thea-b plane, causing active material detachment; whereas ion doping in the alkali metal layer can reduce interlayer electrostatic repulsion, alleviate cracking along thea-b plane, thereby facilitating stable cycling at high voltages and achieving higher energy/power densities.^[15-16]. Therefore, in the design of ion doping strategies, it is necessary to systematically study the influence mechanisms of different dopant ions on material structural evolution and electrochemical performance from the perspective of coordination environment regulation. Based on this, this paper provides a systematic review of research progress on ion doping in P2-type layered oxide cathode materials for sodium-ion batteries, starting from the coordination environment.

2 Challenges Facing P2-Type Cathode Materials

2.1 Irreversible phase transition

During the charge-discharge process, due to sodium ion deintercalation, the P2-type structure transforms into the O2-type structure. For example, in Na_x[Ni_1/3Mn_2/3]O₂ layered metal oxides, whenxis between 1/3 and 2/3, it is the P2 phase; whenxis below 1/3, it is the O2 phase^[17]. During the charging process, sodium ion extraction leads to a reduction in sodium content within the material; the P2-type structure is gradually replaced by the O2-type structure. Accompanied by massive sodium ion deintercalation, its crystal structure further undergoes distortion and contraction, ultimately resulting in interlayer sliding and volume changes, which lead to the collapse of the crystal structure. This irreversible phase transition causes rapid decay of the material's battery capacity.^[18-19].

In Fe-Mn-based layered metal oxide cathode materials, the diffusion capability of sodium ions is suppressed under conditions of high sodium content and low operating voltage. Consequently, a large number of sodium ions remain trapped in the alkali metal layers at the end of charging, leading to the preferential formation of prismatic coordination sites in the Z phase. This is accompanied by the evolution from the P2 phase to the O2 phase, resulting in a structural state where the Z phase coexists with the P2/O2 phases.^[20-21]. Furthermore, although Mn-Co-based layered metal oxide cathode materials can relatively maintain the P2 phase structure during charge-discharge cycles, their capacity is less than ideal due to limited sodium ion diffusion kinetics. To address this issue, current research primarily adopts two strategies: first, lowering the cutoff voltage to prevent the material from entering the phase transition region during charging. For instance, limiting the charging voltage to below 4.0 V can effectively suppress the transformation of P2-type cathode materials from the P2 phase to the O2 phase, thereby stabilizing the lattice structure.^[22-23]. Second, introducing other ions (such as Li⁺, Mg²⁺) for doping. This approach utilizes different ions to regulate the electrochemical reaction pathways and interlayer potential differences during the charge-discharge process, thereby delaying or suppressing the phase transition process. The goal of these two methods is to alleviate the contradiction between structural stability and capacity in the material; that is, as the amount of sodium deintercalation increases, irreversible phase transitions and lattice distortions become more prone to occur, leading to structural instability and capacity fading. Through ion doping, it is possible to improve structural stability and extend material life to a certain extent, while achieving greater reversible sodium ion deintercalation, thus balancing high energy density and structural stability.

2.2 Low air stability

Layered cathode materials for sodium-ion batteries generally suffer from poor air stability. When air-sensitive layered cathode materials are exposed to air, they undergo various chemical and physical reactions, including but not limited to hydrate phase formation, layered phase decomposition, de-sodiation of the layered phase, and intercalation of water and carbon dioxide. The speed and sequence of these reactions depend on the material structure and storage environment, such as atmosphere, humidity, and temperature. In this process, oxygen, water, and carbon dioxide all participate in the reactions, but water is the key component promoting degradation. For P2-phase cathode materials, the prismatic sites occupied by sodium have larger interlayer spacing compared to the O3 phase; therefore, they can accommodate not only alkali elements but also foreign groups such as water and carbonates.^[24-25]. Dahn et al.^[26]first reported P2-type cathode materials, using Na_2/3Co_1/3Mn_2/3O₂and Na_2/3Co_1/6Ni_1/6Mn_2/3O₂as examples, which react with water upon exposure to air to form hydrate phases. Asshown in Figure 2a, a new phase with an interlayer distance greater than that of the (002) plane is clearly observed in the XRD patterns after Na_2/3Co_1/3Mn_2/3O₂is exposed to air, and it was found that water molecules occupy the 2c sites while the sites of other atoms remain unchanged. Furthermore, asshown in Figure 2b, after Na_2/3Co_1/6Ni_1/6Mn_2/3O₂is exposed to air and reacts with water to form a hydrate phase, the water can be removed by heating, restoring the material to the P2 phase.Figure 2cshows the crystal structure schematic of the water-intercalated P2-type material.

Zhang et al.^[27]based on Na_0.7Ni_0.35Sn_0.65O₂ investigated the degradation mechanism of layered oxides in air. Due to H⁺/Na⁺ exchange, Na⁺ is extracted from the Na layer; the reduced sodium ion content in the layer facilitates interlayer sliding, leading to structural transformation, increasing repulsion between adjacent oxide layers, and reducing the shielding effect of Na⁺ on O^2‒-O^2‒ electrostatic repulsion, resulting in increased interlayer spacing^[28-29], while CO₃^2‒, after intercalating into the transition metal layer, requires the transition metals to oxidize to higher valence states to balance the charge^[30]; this charge compensation mechanism forces the transition metals to remain continuously in high valence states, significantly reducing the reversibility of their redox reactions and causing a decline in active material utilization. This structure-electrochemistry coupling failure leads to specific capacity decay and increased impedance. In summary, exploring layered transition metal oxides with sufficient air stability is of great significance for their application^[31-33]. Currently, strategies to improve the air stability of materials include surface coating, washing, and ion doping. Surface coating acts as a physical barrier, preventing contact between the cathode material and the surrounding air^[34-36], but this method only suppresses air sensitivity to a certain extent; washing is a common method to remove excess salt residues, similar to surface coating^[37-38]; washing only improves the air stability of materials to a certain extent. Furthermore, while washing removes excess salt residues, it may also remove some sodium ions, leading to capacity reduction or even damaging the material structure. Through ion doping, the interlayer spacing structure can be modified without destroying the structure, inhibiting the intrusion of H⁺ and water into the alkali metal layer, thereby improving the air stability of P2-type cathode materials^[31,33].

2.3 Low irreversible capacity

In P2-type layered cathode materials, Na_2/3Fe_1/3Mn_2/3O₂is representative; its electrochemical capacity essentially originates from the reversible deintercalation of Na⁺, while the transition metals Fe and Mn participate in charge compensation through valence changes, thereby maintaining the electrical neutrality of the system. Ideally, if Na⁺could be completely deintercalated, this class of materials could achieve a high theoretical capacity. However, limited by factors such as Na⁺/vacancy ordering, structural phase transitions, and kinetic barriers, Na⁺is often difficult to fully deintercalate during actual charge-discharge processes, resulting in a usable capacity significantly lower than the theoretical value^[39]. To address this capacity limitation issue, researchers have attempted to regulate the electronic structure and local coordination environment of the materials through ion doping strategies to improve Na⁺deintercalation behavior and enhance reversible capacity.

Among numerous doping elements, compared to those undergoing only a single valence change or lacking redox activity themselves, Ni possesses higher electrochemical activity, as it can experience Ni²⁺/Ni³⁺/Ni⁴⁺ multi-valence evolution during charge-discharge processes, thereby expanding the charge compensation range and significantly improving the specific capacity of the material^[40]. In contrast, the advantages of Co doping are mainly reflected in structural regulation. Due to the high crystal field stability of Co ions, their introduction helps enhance the structural integrity of the transition metal layer, suppressing unfavorable phase transitions and interlayer sliding^[41]. However, Co primarily participates in the Co³⁺/Co⁴⁺ redox couple during charge-discharge processes; its adjustable valence range is relatively limited, contributing less directly to capacity improvement. Furthermore, compared to Ni, Co resources are scarcer and costs are higher; therefore, developing high-nickel low-cobalt or even cobalt-free cathode materials has become one of the important research directions for balancing energy density enhancement with cost control. Introducing inactive or low-activity elements usually comes at the expense of reducing the content of other high-activity elements, which may negatively affect the overall specific capacity of the material. Besides Ni and Co, doping with other variable-valence elements such as Cu can regulate capacity to a certain extent, while doping with inert or electrochemically less active elements like Ca, Zn, and Mg mainly enhances structural stability by providing a "pillaring effect" or regulating charge distribution, maintaining structural stability during deep charge-discharge cycles.

On the other hand, due to the low initial sodium content in P2-type materials, vacancies exist in the alkali metal layer. During discharge, sodium ions migrate back to the alkali metal layer to occupy these vacancies, resulting in a first-cycle efficiency greater than 100% in half-cells. To compensate for Na loss and adjust the Na stoichiometry of the material, sodium supplementation strategies have been widely used in the synthesis and performance optimization of P2-layered cathode materials. Related studies indicate that various Na-containing precursors, including N-substituted organic compounds^[42], Na₃PS₃O^[43], and others, can serve as sodium supplements to effectively compensate for Na loss during charge-discharge cycles and enhance the reversible capacity of the material. However, different sodium supplements vary in environmental stability and process compatibility; therefore, the most suitable sodium supplementation method must be selected based on the specific material system and process requirements.

3 Alkali metal layer doping modification

In P2-type layered oxides under high-voltage deep desodiation states, the interlayer O^2‒-O^2‒electrostatic repulsion significantly increases, very easily inducing irreversible phase transitions such as P2→O2/OP4 and structural instability, leading to capacity fading and degraded cycling performance^[17,41-45]. To overcome this bottleneck, besides limiting the voltage window to avoid the phase transition region^[46], ion doping of the alkali metal layer has become an effective intrinsic modification strategy. This strategy aims to introduce heteroions with different ionic radii and valence states (such as Li⁺, K⁺, Mg²⁺, Zn²⁺, Ca²⁺, etc.) into the alkali metal layer or occupy its vacancies, expanding the interlayer spacing and enhancing the mechanical stability of the layered framework through the "pillar effect", while regulating Na⁺/vacancy ordering, thereby suppressing phase transitions, improving Na⁺diffusion kinetics and enhancing the structural tolerance of the material under high voltage. This section will systematically review the doping mechanisms of different ions in the alkali metal layer and their regulatory effects on the electrochemical performance of the materials.

Hu et al.^[47]prepared P2-Na_0.66Li_xMn_0.5Ti_0.5O₂(x = 0.1, 0.2, 0.3) layered metal oxides via a molten salt method. Whenx = 0.2, Na_0.66Li_0.2Mn_0.5Ti_0.5O₂as a SIB cathode exhibited a reversible discharge capacity of 128 mAh/g after 100 cycles at 0.1 C within a voltage range of 2–4.3 V, and a capacity of 72 mAh/g at 5 C. A full cell based on Na_0.66Li_0.2Mn_0.5Ti_0.5O₂demonstrated an initial discharge capacity of 65 mAh/g at a current density of 20 mA/g. Li et al.^[48]referenced the Li/Ni mixing strategy in Ni-rich lithium-ion battery cathodes to design a P2-Na_2/3Li_1/9[Ni_2/9Li_1/9Mn_2/3]O₂(Li-NNMO) material featuring interlayer Li-Na mixing. Neutron diffraction confirmed that the introduced Li⁺can simultaneously occupy Ni²⁺sites in the transition metal layer and intrinsic Na⁺vacancies in the alkali metal layer, wherein Li⁺entering the alkali metal layer and occupying Na⁺vacancies is the core mechanism playing a key regulatory role. This process not only directly enhances the mechanical stability of the layered framework through a "pillaring effect," but also effectively disrupts Na⁺/vacancy ordering, significantly suppressing the P2-O2 phase transition at high voltages.Figure 3ashows the crystal structure and ion occupancy of the material after Li doping.Figure 3bpresents in situ XRD results indicating that only a solid-solution reaction of the P2 phase was observed during charge/discharge, with no harmful phase transitions occurring. Furthermore, benefiting from the aforementioned regulation of the alkali metal layer structure by Li⁺, the material exhibits exceptional cycling stability; as shown inFigure 3c, the capacity retention reached 74.5% after 1500 cycles at 5 C within a voltage range of 2.0–4.2 V. This study elucidates from the perspective of alkali metal layer doping that occupying interlayer vacancies with Li⁺can simultaneously achieve structural pillaring reinforcement and disruption of Na⁺/vacancy ordering, providing important insights for designing highly stable P2-type cathodes.

Due to having the same electron shell structure, potassium and sodium exhibit similar chemical properties. However, since the mass and ionic radius of potassium ions are both larger than those of sodium ions, their lattice parameters differ from the sodium ion system, thereby affecting ion deintercalation behavior and electrochemical performance during charge-discharge processes. Nevertheless, doping with larger-radius potassium ions to induce lattice distortion can effectively reduce the ion migration energy barrier, consequently improving diffusion kinetics. For example, Li et al.^[49]by preparing K⁺-doped lithium-rich layered oxide materials, demonstrated for the first time that potassium doping stabilizes the host layered structure by preventing the formation of spinel structures. Liu et al.^[50]employed a solid-state method to achieve Na⁺and K⁺co-doped Li₄Ti₅O₁₂. The results indicate that Na⁺and K⁺co-doping can increase lattice parameters and unit cell volume; the doped materials exhibit greater reversible capacity and rate capability, indicating that Na⁺and K⁺co-doping has a significant effect on improving the electrochemical performance of the materials.

Due to K⁺'s large ionic radius (1.38 Å) inducing structural distortion, research on K⁺ doping in P2-type layered cathodes for sodium-ion batteries is far less than that for Li⁺ (0.76 Å) and Mg²⁺ (0.72 Å). Zhou et al.^[51]introduced K₊into the alkali metal layer of the P2-Na_0.67Ni_0.11Cu_0.22Mn_0.67O² (NNCM) cathode material and synthesized Na_0.62K_0.03Ni_0.11Cu_0.22Mn_0.67O₂ (NKNCM). Studies show that K⁺ with a large ionic radius acts as a structural pillar occupying the alkali metal layer, effectively widening the interlayer spacing and enhancing the stability of the transition metal-oxygen octahedral layers. Asshown in Figure 3d, K⁺ doping increases the interlayer spacing d(O-Na-O) while compressing the TM layer; this structural modification significantly suppresses the P2-OP4 phase transition of the material at high voltages. Benefiting from this stable framework and expanded ion transport channels, the doped material exhibits excellent sodium-ion diffusion kinetics and rate performance. Asshown in Figure 3e, NKNCM delivers reversible capacities of 68.6 and 51.3 mAh/g at high rates of 5 and 10 C, respectively, far superior to the undoped material. More critically, this study reveals the inhibitory effect of K⁺ doping on the oxygen redox reaction from the perspective of electronic structure. Asshown in Figures 3f,g, density functional theory calculations indicate that the introduction of K⁺ enhances the hybridization between Cu 3d and O 2p orbitals, significantly lowering the energy of non-bonding O 2p states, thereby reducing the oxidation activity of lattice oxygen. This work clarifies that K⁺ doping in the alkali metal layer can synergistically achieve structural stabilization, kinetic enhancement, and suppression of detrimental oxygen reactions, providing new insights for designing high-performance layered oxide cathodes for sodium-ion batteries.

P2-type cathode material Na_0.67Ni_0.33Mn_0.67O₂possesses high theoretical capacity and energy density; however, when charged above 4.2 V, it is prone to structural phase transitions, leading to capacity degradation. When charged to a low-sodium state, it cannot effectively reduce the electrostatic repulsion between O^2‒-O^2‒, causing the active material to crack along thea-bplane, resulting in the detachment of active material. Peng et al.^[52]doped the alkali metal layer with Zn²⁺via the sol-gel method to prepare [Na_0.67Zn_0.05]Ni_0.18Cu_0.1Mn_0.67O₂cathode material. The introduction of the inert element Zn²⁺formed an O^2‒-Zn²⁺-O^2‒stable structure with oxygen anions, enhancing the electrostatic cohesion between adjacent transition metal layers, preventing cracking of the active material along thea-bplane, and suppressing the P2-O2 phase transition. In situ XRD characterization, studies on ion diffusion and charge transfer kinetics, as well as microstructural analysis after deep cycling indicate that Zn²⁺doping can effectively reduce the Na⁺diffusion activation energy, increase the Na⁺diffusion rate, and suppress the formation of the O2 phase at the end of charging. After 100 cycles at 0.1 C, 200 cycles at 1 C, and 2000 cycles at 10 C, the half-cell based on [Na_0.67Zn_0.05]Ni_0.18Cu_0.1Mn_0.67O₂exhibited high capacity retention rates of 97.2%, 90.9%, and 80.6%, respectively. The full cell assembled with the [Na_0.67Zn_0.05]Ni_0.18Cu_0.1Mn_0.67O₂cathode and a commercial hard carbon anode achieved a high energy density of 217.9 Wh/kg at 60.3 W/kg and demonstrated excellent cycle life exceeding 1000 cycles.

Traditional views consider crystal defects the root cause of material performance degradation, but modern materials design indicates that specific defect types can regulate electronic structures and optimize ion transport pathways, endowing materials with special functions such as doubled catalytic active sites and enhanced ion sieving selectivity. In the field of battery material research, the appropriate introduction of defects has been proven to be an effective strategy for optimizing battery material performance. Li et al. first introduced Mg²⁺into the alkali metal layer to prepare P2-Na_0.7Mg_0.2[Fe_0.2Mn_0.6□0.2]O₂^[53]and P2-Na_0.8Mg_0.13[Mn_0.6Co_0.2Mg_0.07□0.13]O₂^[54]cathode materials, achieving synergy between structural stability and enhanced oxygen activity. Due to charge compensation effects, transition metal vacancies (denoted by □) were generated in the transition metal layer. Na-site Mg²⁺acted as "pillars" to suppress interlayer collapse and phase transitions. Vacancies in the transition metal layer existed in the forms of "□-O-□", "Na-O-□", "Mg-O-□", and "Na-O-Mg", enhancing structural flexibility and inducing the formation of highly active oxygen species.Figure 4aintuitively displays this unique atomic arrangement. Benefiting from this, the material exhibits a high-voltage plateau based on oxygen redox and excellent electrochemical stability. As shown in Figure 4b, after 150 cycles at a high rate of 10 C, the capacity retention remains as high as 81.7%. This work provides important insights for stabilizing oxygen redox reactions through precise regulation of local structures. Liu et al.^[55-57]systematically studied the multiple regulatory effects of Na-site Ca²⁺doping on P2-type layered oxide cathodes. In a series of materials (such as Na_0.76Ca_0.05[Ni_0.23□0.08Mn_0.69]O₂, Na_0.75Ca_0.05[Li_0.15Fe_0.2Mn_0.6□0.05]O₂and Na_0.75Ca_0.04[_□0.03Li_0.1Ni_0.2Mn_0.67]O₂, etc.), Ca²⁺acted as a "rigid pillar" to effectively regulate Na⁺/vacancy ordering and buffer O—O electrostatic repulsion through strong Ca—O bonds, thereby suppressing the P2-O2 phase transition and mitigating lattice oxygen release^[55-57]. Among these, breakthrough progress was achieved through in-depth research on Na_0.75Ca_0.04[_□0.03Li_0.1Ni_0.2Mn_0.67]O₂(NCLNM4)^[57]. As shown in the atomic-scale imaging in Figure 4c, Ca²⁺clearly occupies the sodium site, laying the foundation for its structural stabilization role. The study transcended the simple "suppression" strategy for the first time, revealing an innovative "complementary manganese-oxygen redox" mechanism: moderate irreversible oxygen release in the early cycling stages reduced the average valence state of manganese, thereby continuously activating more Mn³⁺/Mn⁴⁺redox activity in subsequent cycles. As shown in the differential capacity curves (dFigure 4dQ/dV)in , with cycling, the intensity of the Mn reduction peak at ~2.4 V significantly increased, while the oxygen reduction peak at ~4.2 V synchronously weakened, visually confirming the dynamic capacity compensation of manganese activity for oxygen loss. This ingenious complementary effect enabled the material to achieve extraordinary cycling stability while utilizing oxygen redox to enhance capacity. As shown in Figure 4e, NCLNM4 maintained a capacity retention rate as high as 95.2% after 200 cycles at a high rate of 5 C. This series of works demonstrates that alkali metal layer doping not only stabilizes the crystal structure through the "pillar effect" but also actively manages oxygen activity by establishing a dynamic balance between redox couples, providing a novel and profound perspective for designing high-energy-density, long-life cathode materials.

ComprehensiveTable 1As seen in the listed systems, alkali metal layer doping is a key strategy for enhancing the structural stability and cycling performance of P2-type cathodes. After different ions enter the alkali metal layer, besides regulating interlayer spacing, suppressing phase transitions, and widening Na⁺channels, they also exert synergistic effects at the electronic and lattice levels: In terms of electronic structure, some divalent ions (such as Mg²⁺, Ca²⁺) induce transition metal vacancies or regulate anionic redox activity through charge compensation, achieving a synergistic improvement in capacity and stability; Regarding lattice stress, the tensile strain generated by large-radius ions (such as K⁺) combined with the local compression effect of small-radius ions (such as Li⁺) can establish a gradient stress field, effectively inhibitinga-bplane cracking and alleviating the accumulation of cycling strain. Although this type of doping significantly improves high-voltage stability, its impact on initial sodium content and ionic conductivity still needs to be comprehensively balanced in material design. Overall, local structural optimization of the alkali metal layer is an important pathway to enhance the stability and kinetic performance of P2-type cathodes.

4 Transition metal layer doping modification

The transition metal layer serves as the redox reaction center for P2-type layered oxides, directly determining the material's specific capacity, operating voltage, and electronic conductivity. However, this layer also faces challenges such as Mn³⁺/Ni³⁺-induced Jahn-Teller (J-T) lattice distortion, irreversible phase transitions during charge-discharge cycles, dissolution of transition metal ions, and insufficient intrinsic electronic conductivity. Ion doping of the transition metal layer is the most direct approach to modulating the material's intrinsic electrochemical performance. By introducing metal ions with different valence states, ionic radii, and electronegativities, one can precisely regulate the average valence state of the transition metal layer, suppress J-T distortion, stabilize the crystal structure, enhance electronic conductivity, and effectively control the redox behavior of anionic oxygen. This section will systematically elucidate the modification mechanisms and performance evolution patterns of transition metal layer doping, ranging from unary and binary to ternary and even multi-component systems.

4.1 Monovalent metal P2-type cathode material

As the main constituent elements of layered oxides for sodium-ion batteries, only Co, Mn, and V among the 3d transition metals can synthesize single-metal P2-type cathode materials. These cathode materials suffer from complex phase transitions, poor cycling performance, and severe capacity decay during charge-discharge processes. To address these issues, researchers often employ elemental doping to enhance material performance.

Due to advantages such as abundant Mn resources, high theoretical capacity, and environmental friendliness, Na_xMnO₂has become a highly promising cathode material for sodium-ion batteries. Na_xMnO₂exists in tunnel structures^[58](x = 0.4, 0.44) and layered structures (x = 0.7, 1), with the layered structure divided intoα-NaMnO₂(space groupC2/m), β-NaMnO₂(space groupPmnm), and P2-Na_0.7MnO₂(space groupP6₃/mmc).

P2-Na_xMnO₂was first discovered by Mendiboure in 1985^[59]. When Mn is in the +3 oxidation state, it forms a high-spin complex with a single e_gelectron in its 3d orbitals. Since the d_x_2-_y₂orbital lacks one electron, the repulsion from ligands along thex,yaxes decreases, causing the four ligands at ±x, ±yto move inward, forming four short bonds. Furthermore, because the ligands on these four short bonds exert increased repulsion on the d_x_2-_y₂orbital, the energy level of d_x_2-_y₂rises while that of d_z₂falls, resulting in four short bonds and two long bonds, which produces the J-T effect^[60]. Since Mn⁴⁺does not exhibit the J-T effect, whereas Mn³⁺readily exhibits the J-T effect, leading to a rapid decline in the capacity of Na_xMnO₂. By introducing other elements to substitute for part of the Mn³⁺, the average oxidation state of Mn is increased, reducing the impact of the J-T effect on the material's capacity and enhancing structural stability.

Huang et al.^[61]To suppress the phase transition of P2-type cathode materials during charge/discharge processes and address issues such as sluggish Na⁺ion diffusion, a Zn/Mg co-doping strategy was proposed to prepare the P2-Na_0.67Mn_0.7Zn_0.15Mg_0.15O₂(NMZMO) cathode material. Compared with the undoped P2-Na_0.67MnO₂(NMO), NMZMO exhibits a lower Mn³⁺/Mn⁴⁺ratio and higher lattice oxygen content, which is beneficial for suppressing the J-T effect and improving the structural stability of the cathode material. Asshown in Figure 5a, NMZMO maintains the P2 phase structure throughout the entire charge/discharge process without any shift in characteristic peaks. The sodium ion diffusion coefficients of NMO and NMZMO were obtained based on galvanostatic intermittent titration technique (GITT) analysis. Asshown in Figure 5b, due to the Zn/Mg co-doping increasing the interlayer spacing of the material, providing wide ion diffusion channels for Na⁺deintercalation, the Na⁺diffusion coefficient of NMZMO is improved. NMZMO achieves a discharge capacity of 67.2 mAh/g at 10 C and exhibits good cycling stability, with a capacity retention rate of 93.8% after 100 cycles at 1 C.

Luo et al.^[62]synthesized Cu- and Fe-doped P2-Na_0.67Mn_0.92Cu_0.04Fe_0.04O₂(NMFCO) cathode material using the sol-gel method. The results indicate that the doped cathode material Na⁺exhibits improved diffusion kinetics and structural stability. The NMFCO cathode material delivers a discharge capacity of 110.5 mAh/g at 5 C and 91.8 mAh/g at 10 C. After 100 cycles at 1 C, the capacity retention rate is 94.35%. The introduction of Cu and Fe mitigates the Jahn-Teller effect during charging and discharging, suppresses Na⁺/vacancy ordering, and demonstrates good structural stability during cycling. Co-doping with Cu and Fe effectively reduces charge transfer impedance and enhances the Na⁺diffusion coefficient.

Improving the electrochemical performance of P2-type cathode materials can be achieved by designing the crystal structure, selecting doping elements, increasing the contact area between the cathode material and the electrolyte, and reducing the sodium ion diffusion energy barrier. Li et al.^[63]employed a co-precipitation method to synthesize, as shown in Figure 5c, Co-doped P2-Na with a layered structure of directionally stacked multi-layer nanosheets_0.67Mn_1‒_xCo_xO₂, and systematically investigated the effect of Co on the crystal structure and electrochemical performance of Na_0.67MnO₂. The Na_0.67Mn_0.99Co_0.01O₂cathode material exhibited optimal electrochemical performance, delivering an initial discharge specific capacity of 154.3 mAh/g at 0.1 C within a voltage range of 2.0–4.0 V, maintaining a high capacity retention of 90.3% after 100 cycles at 1 C, and retaining a capacity of 61.2 mAh/g after 500 cycles at 5 C.

Altin et al.^[64]studied the cycling behavior of Ru-doped Na_0.67Mn_1-_xRu_xO₂cathode materials. Whenx = 0.4, the material exhibited optimal electrochemical performance, with a discharge capacity of approximately 170 mAh/g. Asshown in Figure 5d, analysis via in situ XRD revealed that the cathode material displayed a two-phase region composed of P2 and OP4 phases during charging. When the voltage increased to 4.3 V, the structure completely transitioned to the OP4 phase. After discharge, the material returned to the P2 phase, indicating that the phase transition between the P2 and OP4 phases is highly reversible throughout the charge-discharge process. Asshown in Figure 5e, analysis via in situ X-ray absorption spectroscopy (XANES) indicated that the initial XANES peak for Ru was at 22132.8 eV. When charged to 4.3 V, the Ru K-edge spectrum shifted to higher energy at 22134.4 eV. This is due to the oxidation of Ru ions from the +4 to the +5 valence state. Magnetization calculations revealed strong Ru—O bonds within the structure, suggesting that Ru doping stabilizes the MnO₆octahedral structure. Density of states results show that after Ru doping, Na_0.67Mn_0.6Ru_0.4O₂exhibits a reduced band gap, thereby enhancing electronic conductivity.

In addition to the aforementioned strategies, some studies are also attempting co-doping with other ions^[65-72], aiming to further improve the electrochemical performance of P2-type cathode materials while suppressing the phase transition process. Active elements, possessing multiple valence states, undergo dynamic valence changes during charge and discharge cycles, directly participating in the charge compensation process and contributing to capacity. In contrast, inert elements lacking redox activity, although not directly involved in electrochemical reactions themselves, can enhance the overall charge transport efficiency and cycling stability of the material by influencing the local coordination environment of the doping sites or stabilizing the oxygen framework network.

Bai et al.^[73]synthesized Na_2/3[Mn_2/3-2_y³⁺Zn_y²⁺ Mn_1/3-_y⁴⁺]O₂cathode material by doping with Zn to increase the average valence state of Mn. Combined with hard X-ray photoelectron spectroscopy (HAXPES) and in-situ XPS (In-situ XPS) characterization, it was found that at a high potential of 4.3 V, a new characteristic peak appears at 531.2 eV in the O 1s spectrum, corresponding to (O₂)ⁿ^‒(0<n<1) oxidation state. This phenomenon confirms lattice oxygen participation in charge compensation, contributing extra capacity through reversible O^2‒/(O₂)ⁿ^‒redox reactions, effectively mitigating capacity decay caused by saturation of transition metal oxidation states. In addition, doping with other inactive elements such as Li⁺^[74]can enhance Na⁺insertion and extraction capabilities; Al³⁺^[75]can increase the interlayer spacing of the Na⁺diffusion layer, improving the Na⁺diffusion coefficient; Mg²⁺ions can be doped not only in the TM layer but also in the alkali metal layer. When Mg²⁺ions are incorporated into intrinsic Na⁺vacancies in the alkali metal layer, the interlayer spacing of the Na ion diffusion layer can be increased, improving material capacity and cycling stability^[76].

In P2-type Na_xCoO₂(0.6 ≤ x ≤ 0.8) cathode materials, their relatively large interlayer spacing provides sufficient spatial buffering for the reversible deintercalation of Na⁺during charge and discharge processes, making lattice distortion during cycling relatively controllable^[77]. The rigid framework constructed by strong covalent interactions between Co and O can effectively suppress structural relaxation, ensuring the stability of the P2 phase structure under deep charge and discharge conditions. However, asFig. 6ashows^[77], this material exhibits multiple voltage plateaus during charge and discharge processes, a characteristic closely related to the ordered distribution behavior of Na⁺within thea-bplanes. When NaO₆and CoO₆polyhedra are coplanar, the resulting electrostatic repulsion drives Na⁺to tend to occupy lower-energy positions, thereby forming specific spatial arrangements; furthermore, the high mobility of Na⁺and electron localization effects can also induce local structural reorganization. Theoretical calculation results indicate^[78]that there is a significant difference in occupation energy for Na⁺at edge-sharing sites (Na_e) and face-sharing sites (Na_f), and this difference leads to different voltage responses exhibited by Na⁺during the multi-stage deintercalation process.

Although this system possesses certain advantages in structural stability, P2-Na_0.67CoO₂ still faces the problem of capacity decay after long cycling, mainly due to irreversible sodium vacancy aggregation and distortion of the oxygen coordination environment. To address this issue, Kang et al.^[79]doped 10% Ti into Na_0.67CoO₂ (NCO) to prepare Na_0.67Co_0.90Ti_0.10O₂(NCTO). Through the strong oxygen affinity of Ti⁴⁺ (Ti—O bond length 1.89 Å), the stability of the oxygen framework network is enhanced to solve the aforementioned problems. Asshown in Figure 6b, NCO exhibits multiple distinct plateaus, which originate from Na⁺/vacancy ordering and their first-order phase transitions at specific Na⁺/vacancy concentrations. However, with the introduction of Ti, the originally clear plateaus transform into sloped curves. This indicates that Ti doping effectively suppresses the Na⁺/vacancy ordering and phase transition behavior of P2-type NCO. During cycling tests within the voltage range of 2~4.5 V, asshown in Figure 6c, the reversible capacity of NCO drops sharply, becoming almost negligible after 100 cycles. In contrast, although NCTO initially shows a sharp drop in reversible capacity similar to NCO, it exhibits stable capacity retention after 10 cycles, maintaining a reversible capacity of 114 mAh/g even after 100 cycles. Sabi et al.^[80]adopted a similar strategy by introducing 5% Ti into Na_2/3CoO₂. The Ti-doped modified material Na_2/3Co_0.95Ti_0.05O₂delivers an initial capacity of 119 mAh/g within the voltage range of 2.0~4.2 V, with a capacity retention rate of 89% after 100 cycles. Compared to the undoped material, the doped modified material exhibits significantly lower polarization, reflecting its superior electrochemical reversibility.

Didier et al.^[81]first systematically studied the sodium storage behavior of P2-Na_xVO₂. This material possesses a P2/O3 mixed-phase structure, exhibiting a reversible capacity of 126.4 mAh/g, and atx = 0.5, 0.67, and 1.0, it displays single-phase solid solution characteristics, reflecting complex Na⁺rearrangement processes through multiple voltage plateaus. Although the charge-discharge curves exhibit significant voltage hysteresis, this study first confirmed the feasibility of sodium ion storage in V-based P2-phase materials, providing an important reference for expanding the system of P2 layered metal oxide cathode materials. Hamani et al.^[82]revealed the structural evolution laws of the V-based system by comparing the phase transition behaviors of P2-Na_0.7VO₂and O3-NaVO₂. As shown inFigure 6d, the P2-phase material exhibits highly reversible Na⁺deintercalation characteristics in the voltage range of 1.2–2.4 V, where the multi-plateau features within the 1.6–2.4 V range correspond to local structural ordering processes. In situ XRD results show that the material maintains theP6₃/mmcspace group unchanged throughout the entire charge-discharge process, indicating that changes in Na⁺content only slightly adjust lattice parameters without destroying the structure, and reversible deintercalation occurs within the range of 0.47 < x < 0.92.

In binary metal P2-type cathode materials, the introduction of dopant ions with different characteristics systematically optimizes their electrochemical behavior by regulating the material's electronic structure and local lattice environment. Based on ionic radius and valence state, dopant ions can be classified into three categories: small-radius high-valence ions (such as Ti⁴⁺, Ru⁴⁺) enhance TM—O bonds through strong covalency, improving structural stability; multi-valent transition metal ions (such as Co³⁺, Fe³⁺) can adjust the Mn³⁺/Mn⁴⁺ratio to suppress J-T distortion; large-radius low-activity ions (such as Zn²⁺, Mg²⁺) expand the sodium layer spacing via the "pillar effect," improving Na⁺transport kinetics. The above classification research not only systematically summarizes the mechanism of doping strategies in binary systems but also further clarifies the essence of doping revealed by such models, namely, regulating the electronic structure and coordination environment of the host metal by introducing trace heterogeneous ions, providing a clear theoretical framework and strategic basis for designing more complex multi-element doping systems.

4.2 Binary metal P2-type cathode material

P2-type layered metal oxide cathode materials with a single component struggle to achieve their theoretical electrochemical performance due to limited capacity release and significant cycling degradation. To address these issues, researchers have attempted to introduce various transition metals, leveraging their electronic synergistic effects to overcome the performance bottlenecks of single-metal systems. For instance, incorporating highly active elements such as Ni and Mn optimizes the charge transfer process, while doping with inert elements like Ti and Al constructs a rigid oxygen framework network, achieving simultaneous improvements in capacity and cycling stability.

In Fe-Mn-based P2-type layered oxides, Mn³⁺/⁴⁺^[83]is the primary capacity-contributing couple, while Fe³⁺/⁴⁺^[84]contributes to increasing the operating voltage of the material by participating in redox reactions at higher voltages. Such materials typically exhibit good rate performance and capacity output in the low-voltage range; however, due to their inherently poor electronic conductivity, they are prone to charge transport limitations under high-rate and high-voltage conditions, thereby affecting the efficiency of their electrochemical reactions. More seriously, the Fe-Mn layered structure is susceptible to lattice expansion and even structural collapse during cycling, particularly in the high-voltage range, which easily leads to rapid capacity decay. To address these issues, researchers have introduced doping strategies to improve their structural stability and electrochemical performance.

Zhang et al.^[85]employed a citric acid-assisted sol-gel method combined with liquid nitrogen quenching to prepare a series of P2-Na_0.67Fe_0.5Mn_0.5‒_xAl_xO₂(x = 0, 0.02, 0.05, 0.1) materials.Figure 7apresents the schematic structures of the materials before and after doping. It can be observed that the interlayer spacing of the sodium layers increases after doping. This change is primarily attributed to the significantly higher Al—O bond energy (512.4 kJ/mol) compared to the Mn—O bond energy (402.3 kJ/mol), while the ionic radius of Al³⁺(0.535 Å) is smaller than that of high-spin Mn³⁺(0.645 Å). After Al doping, due to the enhanced strength of the transition metal-oxygen (TM-O) bonds, the O-Na-O bonds within the O-Na-O-TM-O-Na-O configuration along the c-axis are relatively weakened, leading to an increase in the corresponding O-Na-O distance d_O-Na-O, which accelerates the migration kinetics of Na⁺. As shown in Figure 7b, Na_0.67Fe_0.5Mn_0.45Al_0.05O₂maintains a stable P2 phase structure throughout the entire charge-discharge process in the voltage range of 2.0–4.0 V, exhibiting highly reversible lattice parameter changes and excellent structural stability. This result indicates that Al doping effectively suppresses the detrimental P2-P2′ phase transition, thereby significantly enhancing the electrochemical performance of the material. Furthermore, when cycled at a 5 C rate within the 2.0–4.0 V voltage range, the material delivers a discharge capacity of 53.7 mAh/g, and the capacity retention remains as high as 92.8% after 200 cycles at a 1 C rate. Feng et al.^[86]further designed a P2-Na⁺/Mg²⁺dual-site co-doped _0.65K_0.05[Mg_0.2Fe_0.3Mn_0.5]O₂(KMFM) material, wherein Mg²⁺resides in the transition metal layer, helping to activate reversible lattice oxygen redox reactions while enhancing structural stability in the high-voltage region; K⁺acts as a "pillar ion" intercalated between layers, improving structural integrity and significantly suppressing oxygen loss under high-voltage conditions. As shown in Figure 7c,d, electrochemical tests indicate that KMFM achieves a discharge capacity of 193 mAh/g at 0.1 C within the voltage range of 1.5–4.2 V, and the capacity retention reaches 95% after 50 cycles at a 0.2 C rate in the 2.0–4.0 V voltage interval. As shown in Figure 7e, analysis of the O 1s spectra during the charge-discharge process of the material in the 1.5–4.2 V voltage range reveals that Na_0.7[Mg_0.2Fe_0.3Mn_0.5]O₂(MFM) exhibits a distinct peroxide characteristic peak (O₂^α‒) when charged to 4.0 V, accompanied by a shift in the main O 1s peak, indicating that Mg doping can prematurely induce oxygen redox reactions to achieve charge compensation. The evolution trend of the O 1s peaks in the KMFM sample is consistent with that of MFM, suggesting that the introduction of K does not suppress the reversible anionic redox process activated by Mg. When charged to 4.2 V, all samples exhibit an O₂^α‒peak. For the undoped Na_0.7Fe_0.5Mn_0.5O₂(FM), as the charging voltage increases, the lattice oxygen peak (O^2‒) gradually weakens. Although it partially recovers during the discharge process, it remains significantly lower than the initial level at 1.5 V, and a residual O₂^α‒peak persists, indicating an irreversible O^2‒→O₂^α‒→O₂transformation. In contrast, MFM and KMFM maintain a higher lattice oxygen content throughout the cycling process, demonstrating superior structural stability and electrochemical reversibility.

Ni possesses high redox activity and is often used in P2-type layered oxides as the primary capacity-contributing ion. However, Ni³⁺ with a low-spin d⁷ electron configuration (t_2g⁶e_g¹) is highly prone to triggering the J-T effect, thereby leading to lattice distortion. Such distortion during charge-discharge processes results in the appearance of multiple voltage platforms, accompanied by irreversible structural degradation phenomena^[87], and its mechanism essentially shares similar electronic structures with lattice distortions caused by Mn³⁺^[88]. To suppress the aforementioned structural distortions, researchers have proposed constructing a Ni-Mn synergistic system; under the premise of maintaining charge balance, the coexistence of Ni²⁺ (t_2g⁶e_g²) and Mn⁴⁺ (t_2g³e_g⁰) can effectively eliminate the single-electron occupancy state of e_g orbitals in the transition metal layer, fundamentally reducing the occurrence of the J-T effect, significantly enhancing the structural stability of the layered structure during charge-discharge processes, and improving its cycling performance^[89-90].

P2-type layered metal oxide cathode materials in the Ni-Mn system possess a high theoretical specific capacity (160~180 mAh/g) and exhibit good cycling stability within the operating voltage range below 3.0 V, which is mainly attributed to Ni²⁺/Ni³⁺/Ni⁴⁺ and Mn³⁺/Mn⁴⁺ synergistic redox reactions between them, as well as the enhanced charge transfer capability resulting from effective hybridization between their 3d orbitals^[89,91]. However, this system still suffers from limited fast charging and discharging performance under high rates due to relatively low electronic conductivity, and is prone to irreversible P2-to-O2 phase transitions at high voltages (> 4.0 V), accompanied by lattice oxygen loss and interlayer sliding issues^[92-93]. To address this, researchers generally employ elemental doping strategies to regulate the electronic structure of the transition metal layer and optimize the electrostatic environment of the alkali metal layer, thereby constructing more efficient electron/ion conduction networks and suppressing high-voltage phase transitions.

Kong et al.^[94]systematically investigated the regulation mechanism of redox behavior in P2-type Na^0.67Mn^0.75Ni_0.25O₂cathode materials via a Cu₂₊/Mg₂₊co-doping strategy. A series of materials were synthesized using the sol-gel method, namely Na_0.67Mn_0.75Ni_0.25O₂, Cu-doped Na_0.67Mn_0.71Cu_0.04Ni_0.25O₂(Cu_0.04), Mg-doped Na_0.67Mn_0.71Mg_0.04Ni_0.25O₂(Mg_0.04), and Cu/Mg co-doped Na_0.67Mn_0.71Cu_0.02Mg_0.02Ni_0.25O₂(Cu_0.02Mg_0.02). The study revealed that the Cu/Mg co-doped system exhibits optimal cycling stability and rate performance. Its enhancement mechanism can be attributed to three aspects: (1) The strong covalency of Cu—O and Mg—O bonds shortens the transition metal-oxygen bond length, and the strengthened bond energy effectively improves the redox reversibility of lattice oxygen and structural rigidity; (2) The co-doping system enhances the reversibility of the P2-O2 phase transition while increasing the thermal decomposition temperature, leading to a significant enhancement in anionic redox activity; (3) Cu/Mg co-doping reduces the Mn³⁺/Mn⁴⁺ratio and the e_gorbital electron occupancy, suppressing the damage to the layered structure caused by J-T distortion and enhancing layered structural stability. Furthermore, asshown in Fig. 7f, co-doping induces an expansion of the (002) interplanar spacing, increasing the sodium ion diffusion coefficient by one order of magnitude.

Zhao et al.^[95]Addressing the structural instability caused by irreversible oxygen redox in high-voltage P2-type layered oxide cathode materials, a Cu/Co bimetallic synergistic substitution strategy was innovatively proposed. By constructing a P2-Na_0.67Mn_0.6Ni_0.2Cu_0.1Co_0.1O₂system, after Co³⁺substitutes the Ni site, strongly covalent Co—O octahedra are formed, and their high charge density characteristics effectively enhance the structural stability of the material at a high cutoff voltage of 4.3 V. Meanwhile, by enhancing the delocalization of O 2p orbitals, reversible oxygen redox reactions are activated. Additionally, Cu²⁺substitution at the Mn site induces the formation of a stable Cu—O coordination structure, which not only suppresses lattice oxygen loss through Cu—O orbital hybridization but also regulates the transition metal layer bond angles to contract the TMO₂layers and expand the Na interlayer spacing, thereby improving the sodium ion diffusion coefficient. Asshown in Fig. 7g, this bimetallic complementary effect enables the material to maintain the P2 phase structure during high-voltage cycling at 4.3 V; no characteristic diffraction peaks of the P2″ phase were detected, and the (002) and (004) diffraction peaks retained high crystallinity, indicating that dual-element substitution with Cu and Co can effectively enhance the structural stability of the material and suppress unfavorable phase transitions. The optimized cathode material achieved a capacity retention rate of 82.07% after 500 cycles at a cutoff voltage of 4.3 V in half-cells, and its full cell maintained a reversible capacity of 62.6 mAh/g even at a high rate of 20 C.

In the binary system, through Al³⁺, Cu²⁺, Mg²⁺and other ion doping, synergistic optimization of multi-dimensional material properties is achieved. These ions not only enhance performance through macroscopic mechanisms such as strengthening TM—O bond covalency, suppressing J-T distortion, regulating oxygen reaction extent, and inhibiting Na⁺/vacancy ordering; the essence of their underlying synergy lies in the linked regulation of the system's electronic structure and lattice stress by multiple ions. For example, in the Cu²⁺/Mg²⁺co-doped system, the hybridization of Cu 3d and O 2p orbitals enhances electronic conductivity, while Mg²⁺enhances oxygen reduction reversibility by stabilizing high-valence ions and regulating charge distribution; meanwhile, the co-doping of large-radius K⁺and small-radius Mg²⁺constructs a complementary gradient stress field within the lattice, which both expands the sodium layer spacing and strengthens the TM-O framework rigidity, thereby synergistically suppressing phase transitions and micro-crack propagation, significantly improving the structural stability and cycle life of the material under high voltage, demonstrating the immense potential and flexibility of binary system performance optimization.

4.3 Ternary and multi-metal P2-type cathode materials

Under extreme operating conditions such as high voltage, high rate, and long cycling, P2-type layered metal oxide cathode materials generally face the dual challenges of irreversible phase transitions and declining redox kinetics. Although doping strategies based on single or dual elements have achieved certain results in improving specific capacity and cycling stability, binary systems have limited freedom in regulating the metal coordination environment. Especially in the high-voltage operating region, it is difficult to balance anionic redox activity with structural stability. During deep desodiation, binary system materials are prone to P2-O2 phase transitions, leading to stress accumulation and microcrack propagation within particles, while simultaneously exacerbating side reactions between the electrolyte and the material interface, which severely affects the structural integrity and long-term cycling performance of the electrode. Therefore, introducing ternary or even multi-element transition metals to construct a synergistic system with multiple redox couples not only helps optimize the overall electronic structure of the material but also enhances its structural stability under high-voltage conditions, providing new design concepts and development directions for achieving high-performance sodium-ion battery cathode materials.

Compared to traditional binary systems, ternary and multi-component transition metal systems demonstrate significant advantages in regulating electronic structures and achieving multi-ion synergy. By introducing a third or fourth transition metal element, multiple regulatory effects can be realized at the atomic scale: on one hand, the overlap of d-orbital energy levels among multiple metals forms a continuous distribution of electronic density of states, significantly enhancing overall electronic conductivity; on the other hand, synergistic doping with metals of different ionic radii can create a gradient stress field within the transition metal layers, optimizing Na⁺diffusion pathways in the crystal structure and improving ion migration efficiency^[96]. Furthermore, differences in redox potentials among multi-metal sites can hierarchically activate anion/cation synergistic redox reactions^[97], which not only broadens the operating voltage window but also alleviates lattice distortion caused by single-element overloading through charge compensation mechanisms. Relevant studies indicate that ternary and even multi-component systems can effectively suppress irreversible lattice oxygen loss under high-voltage cycling conditions, and can even raise the stable upper operating voltage limit of P2-type cathode materials to above 4.3 V^[97-98], providing solid theoretical support and feasible design directions for developing sodium-ion battery cathode materials that simultaneously possess high energy density and long cycle life.

Taking the Cu-Fe-Mn ternary P2-type layered oxide (Na_x[CuFeMn]O₂) as an example, it has attracted widespread attention due to its high electronic conductivity and large specific capacity; however, under harsh conditions such as high rates or long cycling, it remains prone to structural phase transitions and capacity fading. To address this, Wei et al.^[99]introduced electrochemically inactive Ti elements into Na_0.70Fe_0.20Cu_0.20Mn_0.60O₂to prepare the Na_0.70Fe_0.20Cu_0.20Mn_0.55Ti_0.05O₂(NFCMT-0.05) cathode material.Figure 8ashows the schematic structure, where Na⁺occupies two distinct sites (Na1 and Na2), while Ti⁴⁺is located at the transition metal sites. Asshown in Figure 8b, throughout the entire charging process, NFCMT-0.05 maintains a stable P2 phase structure with no new phases detected, indicating that Ti substitution can effectively suppress the unfavorable phase transition between P2 and O2 phases. In the early stage of charging, the (002) diffraction peak shifts to lower angles, while the (100) and (012) diffraction peaks shift to higher angles. The lattice parametercgradually increases during the charging process, primarily attributed to enhanced electrostatic repulsion, whereas the lattice parameteradecreases due to the oxidation of transition metals. Furthermore, NFCMT-0.05 exhibits a small magnitude of volume change during charging, further indicating that Ti substitution is beneficial for enhancing the structural stability of the material. During discharge, the (002) and (010) diffraction peaks basically return to their initial positions, indicating that the deintercalation process of Na⁺is highly reversible, and NFCMT-0.05 demonstrates excellent structural stability. The introduction of Ti not only enhances structural stability but also activates reversible oxygen anion redox reactions in the high-voltage region (> 4.1 V), providing additional capacity contribution to the material. At a 0.1 C rate, the initial discharge capacity of NFCMT-0.05 reaches 186 mAh/g; within a wide voltage window of 2.0–4.3 V, the capacity retention rate is 83.5% after 400 cycles at a 5 C rate, and the discharge capacity at 5 C remains at 60% of that at 0.1 C, demonstrating excellent rate adaptability and cycling stability. Density functional theory (DFT) calculations confirm that the introduction of Ti effectively suppresses interlayer sliding of the lattice structure and distortion of the transition metal layers during the Na⁺deintercalation process, thereby enhancing overall structural stability.

Ni-Co-Mn-based P2-type layered oxides (Na_x[NiCoMn]O₂) represent another typical ternary system, exhibiting high specific capacity during sodium ion deintercalation, especially in the low-voltage region (< 3.0 V) through synergistic redox reactions between Ni²⁺/Ni³⁺and Mn³⁺/Mn⁴⁺, achieving multi-electron transfer and thereby obtaining energy output superior to traditional binary materials. However, this system faces significant challenges under high-voltage operating conditions (> 4.2 V): the high-spin state of Mn³⁺ (t_2g³e_g¹) easily triggers Jahn-Teller distortion, while the strong oxidizing nature of Co³⁺ tends to cause breaking of transition metal-oxygen bonds; the combination of both induces irreversible phase transitions and structural degradation, ultimately leading to capacity fading and reduced cycling stability. To address these issues, Eom et al.^[100]regulated the bonding strength of the transition metal layer through Al doping and prepared P2-Na_0.7Ni_0.2Co_0.2Mn_0.6‒_xAl_xO₂ (NCMA, x = 0, 0.02, 0.05, 0.10) cathode materials. Experimental results indicate that the introduction of Al not only enhances the overall stability of the metal-oxygen framework, effectively suppressing phase transition behavior under high-voltage conditions, but also reduces the proportion of Mn³⁺, alleviating the adverse effects of Jahn-Teller distortion on the structure. In situ XRD tests further confirm that within the 1.5–4.5 V voltage window, the NCMA2 sample with an Al doping amount of 0.02 undergoes only reversible phase transitions, achieving a capacity retention rate of 80% after 50 cycles, demonstrating excellent structural reversibility and cycling performance. Furthermore, the introduction of Al effectively prevents the dissolution of transition metal ions, enhancing the stability of the electrode/electrolyte interface.

High-entropy materials (HEMs), as emerging functional materials, have demonstrated unique potential in the field of sodium-ion battery cathodes in recent years. Based on the design concept of high-entropy alloys (HEAs), high-entropy P2-type layered oxides introduce five or more transition metal elements such as Mn, Ni, Co, Cu, and Fe into a single lattice to form a multi-component solid solution structure, thereby endowing the material with special electrochemical properties. The synergistic effect of multiple metal elements not only significantly increases the density of electrochemical active sites and enhances specific capacity, but its high configurational entropy effect can also suppress element segregation, enhance crystal structure stability, and reduce volume changes during charge-discharge processes^[101-102]. However, the coexistence of multiple components in high-entropy systems leads to tortuous sodium ion diffusion pathways; their diffusion barriers are higher than those of traditional binary systems, resulting in a significant increase in charge transfer impedance at high rates^[103]. Furthermore, heterogeneous redox behaviors of different metal ions under high-voltage conditions easily induce lattice mismatch, especially the J-T distortion of Mn³⁺, as well as the strong oxidizing nature of Co³⁺and Ni³⁺/Ni⁴⁺, which triggers irreversible P2-O2 phase transitions, leading to excessive capacity decay rates after cycling. Therefore, how to balance high-entropy systems with kinetic/stability defects through component optimization remains a scientific problem urgent to be solved in this field.

Bao et al.^[104]successfully prepared spherical P2-Na_0.67Ni_0.17Co_0.17Mn_0.56Ti_0.05Mg_0.05O₂cathode material via a Ti/Mg co-doping strategy and systematically investigated its structural evolution and electrochemical performance. High-temperature XRD analysis indicated that the material formed a pure P2 phase structure when calcined at 850 °C, and Ti/Mg doping effectively maintained the long-range P2 structure and spherical particle morphology. Electrochemical tests showed that the optimized material exhibited significantly enhanced performance within a voltage window of 2.0–4.5 V. At 0.1 C, its initial reversible capacity reached 151 mAh/g with an average operating voltage of 3.8 V, and the capacity retention rate was 87.7% after 300 cycles at a current density of 100 mA/g. This originates from Ti⁴⁺and Mg²⁺synergistic effects; Ti⁴⁺suppresses Mn³⁺Jahn-Teller distortion by stabilizing transition metal-oxygen (TM—O) covalent bonds; Mg²⁺effectively mitigates the irreversible P2-O2 phase transition by suppressing alkali metal layer collapse and transition metal layer sliding. This study provides new insights into regulating the structural stability and redox activity of layered oxides through multi-element doping.

Wang et al.^[105]synthesized three P2-type layered oxide cathode materials with different configurational entropies via solid-state chemistry: low-entropy Na_0.67(Mn_0.55Ni_0.21Co_0.24)O₂(3-NTMO₂), medium-entropy Na_0.67(Mn_0.45 Ni_0.18Co_0.24Ti_0.1Mg_0.03)O₂(5-NTMO₂), and high-entropy Na_0.67(Mn_0.45 Ni_0.18Co_0.18Ti_0.1Mg_0.03Al_0.04Fe_0.02)O₂(7-NTMO₂), systematically investigating the effect of configurational entropy on the electrochemical performance and structural stability of sodium-ion battery cathode materials. Asshown in Figure 8c, the dissolution behavior of Mn into the electrolyte after cycling was quantitatively analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). After 50 cycles within the voltage range of 1.5~4.6 V, the Mn dissolution amounts in 3-NTMO₂, 5-NTMO₂, and 7-NTMO₂showed a gradient decreasing trend with increasing configurational entropy, meaning it significantly decreased as configurational entropy increased. Comprehensive analysis indicates that severe Mn dissolution is a major reason for the poor cycling stability of 3-NTMO₂, while 7-NTMO₂, which has the highest configurational entropy, exhibits significantly enhanced structural stability. This suggests that high-entropy design can suppress transition metal dissolution through multi-component synergistic effects.Figure 8d displays the cycling performance of the materials at a 0.5 C rate. Although 3-NTMO₂and 5-NTMO₂exhibit higher initial specific capacities, both show obvious capacity decay during the first 15 cycles, with 3-NTMO₂decaying more severely. In contrast, 7-NTMO₂demonstrates the optimal cycling stability. By the 50th cycle, the capacity retention rates for 3-NTMO₂, 5-NTMO₂, and 7-NTMO₂were 69%, 78%, and 86%, respectively. High-entropy 7-NTMO₂exhibits excellent reversibility of sodium deintercalation/intercalation, with cycling stability significantly superior to low-entropy and medium-entropy systems. Low-entropy 3-NTMO₂suffers rapid capacity decay due to severe Mn dissolution, while the decay rate of medium-entropy 5-NTMO₂lies between the two. This study reveals the correlation between configurational entropy and electrochemical stability, providing theoretical support for optimizing the interfacial stability and cycling life of layered oxide cathodes through high-entropy engineering.

Cheng et al.^[106]Based on a multi-ion synergistic doping strategy, developed P2-Na_0.7Li_0.03Mg_0.03Ni_0.27Mn_0.6Ti_0.07O₂(NaLMNMT) layered oxide cathode material, aiming to address structural phase transitions induced by sodium ion deintercalation and Na⁺/vacancy ordering issues. Studies show that doping Li⁺alleviates lattice stress accumulation by breaking the ordered arrangement of sodium vacancies, introducing Ti⁴⁺enhances the redox reaction potential and improves the reversibility of anionic oxygen redox in the high-voltage region, while Mg²⁺stabilizes the interlayer framework through a "pillaring effect," suppressing transition metal layer sliding. Through multi-component synergy, the drastic P2-O2 phase transition is transformed into a mild P2-Z phase transition, significantly reducing the degree of structural distortion. Asshown in Figure 8e, the multi-component modified material maintains excellent structural stability after 200 cycles, with its capacity decay rate significantly reduced compared to the undoped system. This work provides an important reference for regulating the phase transition pathways and interface stability of layered oxides through multi-ion synergy.

Ternary and high-entropy systems represent the cutting-edge direction of pursuing performance breakthroughs through compositional complexity. Multi-component strategies not only suppress element segregation via high configurational entropy effects but also achieve a dual enhancement of stability and capacity through synergistic regulation of electronic structures and autonomous optimization of lattice stress. At the electronic structure level, the interlacing of d-orbitals from various transition metals forms a continuous density of states distribution, significantly improving electronic conductivity and hierarchically activating redox reactions. In terms of lattice stress, the local coherent stress fields formed by random ion occupancy can effectively counteract long-range ordered distortions, diluting drastic phase transformation processes into mild solid-solution reactions. Furthermore, the introduction of inert elements such as Ti and Mg further anchors the oxygen framework, suppressing phase transitions and transition metal dissolution. Although this system exhibits significant advantages in cycling stability, its complex synthesis processes, high costs, and constrained ion diffusion kinetics remain key challenges that must be overcome for industrialization.

ComprehensiveTable 1The data show that transition metal layer doping is a core strategy for optimizing the electrochemical performance of P2-type cathode materials. By introducing metal ions with different characteristics, Mn can be effectively regulated.³⁺/Mn⁴⁺ratio, suppress J-T distortion, enhance TM—O bond stability, and influence phase transition processes and oxygen reduction behavior, thereby significantly improving the specific capacity, rate performance, and cycle life of the material. The evolution from unary to multinary systems reflects a research trajectory in this field shifting from addressing single issues to multi-objective synergistic optimization. Successful doping design requires a systematic consideration of the balance among electronic structure, crystal stability, and interfacial chemistry. Future research should leverage theoretical calculations and in-situ characterization to deeply reveal multi-ion synergistic mechanisms and develop scalable, precisely controllable synthesis strategies to promote the practical application of high-performance P2 cathode materials.

5 Anionic oxygen layer doping modification

As a core component of large-scale energy storage systems, sodium-ion batteries must be designed to meet the stringent requirements for long-cycle stability in grid-level applications. Research indicates that reducing the charging cutoff voltage can effectively suppress the formation of the OP4 phase under high desodiation states, thereby significantly improving cycle performance.^[107-110]However, while narrowing the voltage window suppresses phase transitions, it also limits the capacity range available for reaction, constraining the material's energy density. To address this, researchers have turned to intrinsic structural modulation strategies, among which anionic oxygen layer doping has emerged as a key direction to break through this bottleneck. Distinct from cationic site modulation, this strategy directly involves ion doping and substitution within the anionic oxygen layers that constitute the material framework. By reconstructing the local bonding environment and adjusting electronic structures and lattice parameters, it plays a critical role in suppressing oxygen loss, optimizing Na⁺transport pathways, and activating reversible anionic redox reactions within the material system.^[111-115]For instance, F^‒doping, leveraging its high electronegativity, can stabilize transition metal coordination and induce the partial reduction of Mn⁴⁺to Mn³⁺, thereby alleviating Jahn-Teller distortion and structural stress.^[116-117]This section will take F^‒and BO₃as representative examples to systematically analyze the mechanism of anionic oxygen layer doping and its regulatory effects on the electrochemical performance of P2-type cathodes.

Chen et al.^[118]prepared F^‒gradient-doped Na_0.6Mn_0.95Ni_0.05O_2-_xF_x(x = 0.00, 0.02, 0.05, 0.08) and Na_0.6MnO_2-_yF_y(y = 0.00, 0.02, 0.05, 0.08) systems, revealing for the first time the enhancement effect of anion doping on transition metal redox activity. F^‒doping enhances reversible capacity by stabilizing the Ni²⁺/Ni³⁺redox couple; Na_0.6Mn_0.95Ni_0.05O_1.95F_0.05achieved a capacity retention of 75.0% after 960 cycles at 2 C, significantly superior to the 58% of the undoped sample. Furthermore, it exhibited excellent high-temperature performance, maintaining a discharge capacity of 95.77 mAh/g after 200 cycles at 50 °C.

Kang et al.^[119]synthesized microsphere-structured P2-Na_0.6Mn_0.7Ni_0.3O_2‒_xF_x(x = 0, 0.03, 0.05, 0.07) cathode materials. Compared with undoped Na_0.6Mn_0.7Ni_0.3O₂, F^‒doping alters the lattice parameters and redox centers of P2 cathode materials. Asshown in Figure 9a, appropriate F^‒doping can enhance the activity of the Ni²⁺/Ni⁴⁺redox reaction in the high-voltage region, achieving a high initial specific capacity (95.3 mAh/g) and an initial Coulombic efficiency close to 100%, while promoting reversible Na⁺deintercalation/intercalation and improving high-voltage reversibility. In terms of rate performance, asshown in Figure 9b, F-0.05 maintains a high specific capacity over a wide current density range of 0.05~2.0 A/g and can almost fully recover its initial capacity when the current is restored to a low value, indicating that appropriate F doping effectively suppresses structural degradation at high rates and maintains the stability of Na⁺migration channels. Long-term cycling tests at high rates (Figure 9c) show that F-0.05 retains a discharge capacity of 70.5 mAh/g after 900 cycles at 1000 mA/g, corresponding to a capacity retention rate of 78%, significantly outperforming F-0, F-0.03, and F-0.07. This indicates that appropriate F doping can improve microstructural stability and extend cycle life, whereas excessive doping disrupts lattice integrity, leading to accelerated capacity decay. Comprehensive analysis of the three figures reveals that moderate F doping not only optimizes the high-voltage capacity and rate performance of P2-type layered oxides but also achieves an optimal balance between rapid Na⁺deintercalation/intercalation and structural stability, providing an effective strategy for tuning the performance of P2 cathode materials.

Liu et al.^[120]Based on the Na_2/3Ni_1/3Mn_2/3O_2-_xF_x(x = 0, 0.03, 0.05, 0.07) system, the charge compensation mechanism of F^‒ doping was revealed through X-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS). Mn⁴⁺ is partially reduced to Mn³⁺, synergizing with Ni²⁺/Ni³⁺ redox contributions to provide extra capacity. To track the structural evolution of the Na_2/3Ni_1/3Mn_2/3O_1.95F_0.05 cathode material during Na⁺ deintercalation, in situ XRD technology was employed for testing. As shown in Figure 9d, during the charging process, the (100), (012), (110), and (112) diffraction peaks of the material continuously shift to higher angles, while the (106) peak shifts to lower angles, indicating that it undergoes a solid-solution reaction accompanied bya-b plane contraction. In the low-voltage region of the discharge process, Na_2/3Ni_1/3Mn_2/3O_1.95F_0.05 basically maintains solid-solution reaction characteristics across the entire 1.5~4.0 V voltage range; only peak broadening is observed at extremely low voltages (≈ 1.5 V), but it recovers upon charging, indicating that F^‒ doping effectively extends the solid-solution region and suppresses the low-voltage two-phase reaction. This effect can be attributed to F^‒ doping increasing the disorder at transition metal sites, thereby disrupting the cooperative J-T structural distortion triggered by Mn³⁺, which is similar to the mechanism by which cationic doping improves structural stability. Furthermore, in situ XRD also shows that in the high-voltage region (2~4.3 V), a new O2 phase forms when the material is charged to >4 V, caused by the sliding of transition metal oxide layers forming octahedral sites. The formation of the O2 phase significantly reduces cycling performance; however, over multiple cycles, the discharge capacity of Na_2/3Ni_1/3Mn_2/3O_1.95F_0.05 remains significantly higher than that of the undoped Na_2/3Ni_1/3Mn_2/3O₂, indicating that F^‒ doping plays a positive role in improving cycling capacity and structural stability.

Current research on anion doping primarily focuses on F⁻, but existing theoretical simulations and materials chemistry studies indicate that other anions with high electronegativity or ionic radii close to O^2-also possess the potential as partial substitutes for O^2-and can be used to regulate the structural stability and electrochemical performance of P2-type cathode materials. Wang et al.^[121]constructed a unique planar triangular BO₃coordination configuration by introducing the element B at anion sites, forming Na_0.67Ni_0.3Co_0.1Mn_0.6O_1.94(BO₃)_0.02cathode material. This work reveals the doping modification effect of B in the anionic oxygen layer, where the BO₃units act as structural pillars, effectively suppressing the intercalation of water molecules and the sliding of transition metal layers during Na⁺deintercalation, enabling the material to maintain good structural integrity during cycling. After 1000 cycles at a 2 C rate, the material retains 80.1% of its capacity with minimal volume change. Meanwhile, this doping promotes more Na⁺to occupy Na_esites favorable for diffusion, optimizing ion transport kinetics. This study highlights the critical role of anionic oxygen layer doping in regulating the structural stability of P2 materials, providing new insights for designing high-performance layered oxide cathodes through non-metallic anion site engineering.

CombiningTable 1with the research system and existing progress shown, anion oxygen layer doping is one of the key strategies to enhance the structural stability and high-voltage cycling reversibility of P2-type cathode materials. F^‒, BO₃, etc., significantly improve the electrochemical performance and volume stability of materials by directly regulating the local coordination environment of the anion oxygen layer, enhancing TM-O bond stability, and suppressing oxygen release and interlayer sliding under high voltage. Their mechanism of action can be further elucidated from two dimensions: electronic structure regulation and lattice stress synergy. At the electronic level, high-electronegativity F^‒can lower the O 2p orbital energy level, stabilize high-valence transition metals, and effectively suppress irreversible oxygen loss; when F^‒is co-doped with low-valence cations, it can also optimize the distribution of electronic states near the Fermi level through charge compensation, improving reaction reversibility. At the lattice level, the local compressive strain introduced by F^‒and the rigid pillar effect of BO₃groups both suppress interlayer sliding and structural distortion during charge-discharge processes. In addition, the introduction of larger-radius anions such as Cl^‒, S^2‒, etc., may induce beneficial local structural distortions, alleviate stress concentration, and enhance oxygen framework flexibility, thereby regulating transition metal redox behavior and energy storage mechanisms^[122-123]. Although such systems currently lack systematic experimental support, theoretical studies indicate their potential in improving electronic conductivity, stabilizing redox couples, and optimizing Na⁺migration pathways. Furthermore, the successful application of anions such as Cl^-in some solid-state electrolyte systems also provides important insights for anion regulation in cathode materials^[124-125]. Future research needs to combine in-situ characterization with theoretical calculations to systematically elucidate the doping mechanisms and synergistic effects of different anions, develop multi-site co-doping strategies for anions/cations, expand the anion regulation system for P2-type materials, and thereby construct sodium-ion battery cathode materials that possess both high energy density and long cycle life.

6 Conclusion and Outlook

As an important candidate for next-generation large-scale energy storage technology, sodium-ion batteries derive their development potential from the abundant crustal distribution of sodium resources and significant advantages in raw material costs. However, the current energy density of sodium-ion batteries still lags significantly behind that of commercial lithium-ion batteries, creating a bottleneck that restricts their industrial application. Breaking through this bottleneck revolves around innovative exploration of cathode material systems. Through multi-scale structural regulation, it aims to achieve synergistic optimization of energy density, cycle life, and rate performance, thereby meeting the dual demands for performance and stability in grid-level energy storage scenarios.

Among various sodium cathode materials, P2-type layered metal oxide cathode materials have become a research focus for high-energy-density sodium-ion batteries due to their open two-dimensional sodium ion transport channels and tunable transition metal redox behavior. The evolution from single-transition-metal systems to multi-component composite systems reflects a shift in material design strategies from relying on the redox reactions of a single metal to more complex multi-metal synergistic mechanisms to enhance overall performance. In recent years, through multi-dimensional optimization strategies including anion/cation co-doping, nanostructure engineering, and interface modification techniques, the reversible capacity and cycle life of P2-type materials have continuously approached practical performance metrics. Based on the above analysis, the performance improvement of P2-type materials can be attributed to three aspects: (1) Multi-ion doping in the transition metal layer suppresses Jahn-Teller distortion and irreversible phase transitions via an entropy enhancement effect; (2) Element doping in the alkali metal layer strengthens structural stability through a "pillaring effect"; (3) Regulating the oxygen coordination environment activates reversible anionic redox reactions, thereby expanding the dimensions of the charge compensation mechanism.

Despite significant progress in related research, the industrialization of P2-type cathode materials still faces several key challenges: accelerated lattice oxygen loss under high voltage leads to capacity decay; it is difficult to balance thermodynamic stability and synthetic controllability in multi-component systems; and existing laboratory strategies lack clear structure-activity relationships to guide scalable preparation. In response, future research should focus on the following directions: (1) Develop cross-site doping strategies that synergistically suppress oxygen loss and structural phase transitions, prioritizing the screening of dopant elements that possess both strong TM-O bond formation and oxygen vacancy anchoring functions. By achieving synergistic occupancy at alkali metal sites, transition metal sites, and oxygen sites, crystal stability and oxygen reversibility can be simultaneously enhanced across multiple dimensions, including electronic structure, coordination environment, and interlayer spacing; (2) Construct a research framework integrating multi-scale in-situ characterization with theoretical simulations to real-time analyze structural evolution, oxygen loss kinetics, and interfacial side reactions during cycling, elucidating the mechanism of dopants at the atomic scale to provide precise guidance for doping design; (3) Establish an industrial-oriented "performance-cost" dual-guided framework for screening dopant elements, systematically evaluating the correlation between element abundance, cost, and electrochemical performance, and leveraging machine learning and high-throughput computing to accelerate the development of material systems that combine excellent comprehensive performance with commercial feasibility. Through the aforementioned systematic research focused on doping engineering, it is expected to break through the industrialization bottlenecks of P2-type cathode materials and lay the foundation for the application of sodium-ion batteries in the field of large-scale energy storage.

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Funding

National Natural Science Foundation of China(12575270)

Applied Basic Research Programs of Yunnan Provincial Science and Technology Department(202401AV070008)

Applied Basic Research Programs of Yunnan Provincial Science and Technology Department(202301AS070051)

Applied Basic Research Programs of Yunnan Provincial Science and Technology Department(202401BE070001-060)

Applied Basic Research Programs of Yunnan Provincial Science and Technology Department(202501AU070106)

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1 Introduction

2 Challenges Facing P2-Type Cathode Materials

2.1 Irreversible phase transition

2.2 Low air stability

2.3 Low irreversible capacity

3 Alkali metal layer doping modification

4 Transition metal layer doping modification

4.1 Monovalent metal P2-type cathode material

4.2 Binary metal P2-type cathode material

4.3 Ternary and multi-metal P2-type cathode materials

5 Anionic oxygen layer doping modification

6 Conclusion and Outlook

References

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