Recent progress in high-voltage P2-Na_xTMO₂ materials and their future perspectives

Abstract

P2-type layered materials (Na_xTMO₂) have become attractive cathode electrodes owing to their high theoretical energy density and simple preparation. However, they still face severe phase transition and low conductivity. Current research on Na_xTMO₂ is mostly focused on the modification of bulk materials, and the application performances have been infrequently addressed. This review summarizes the information on current common P2-Na_xTMO₂ materials and discusses their sodium-storage mechanisms. Furthermore, modification strategies to improve their performance are addressed for practical applications based on a range of key parameters (output voltage, specific capacity, and lifespan). We also discuss the future development trends and application prospects for P2 cathode materials.

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Manni Li

Manni Li is an undergraduate student majoring in Applied Chemistry at the College of Chemistry and Materials Science, Fujian Normal University. Her research interests are high-voltage P2-NaxTMO2 materials for sodium-ion batteries.

Xiang Ding

Xiang Ding is a professor of the College of Chemistry and Materials Science at Fujian Normal University. He received his bachelor's degree from Jilin University (JLU) in 2015 and PhD from University of Science and Technology of China (USTC) in 2020. His research interests focus on the materials and systems for secondary batteries. He has published more than 30 research papers.

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1. Introduction

To date, lithium-ion batteries (LIBs) have been extensively developed and applied in many areas, thus playing a key role in supporting the development of society. However, the low abundance (20 ppm), difficult exploitation and low recovery rates of lithium resources limit LIBs from meeting the requirements of wearable equipment, electric cars (ECs), smart grids and plant-scale energy-storage devices. SIBs have emerged as an alternative and have attracted widespread attention owing to the abundance of Na resources. SIBs are expected to replace LIBs in many fields, and research on them is growing rapidly.

Cathode materials profoundly affect the prime costs and capability of SIBs; therefore, researching and developing low-cost and long-life cathode materials is crucial for the development of SIBs. Such cathode materials include layered transition metal oxides (LTMOs),^1–10 tunnel-type oxides,^11–20 iron–fluorine-based Prussian blue analogues (PBAs)^9,10,21–29 and polyanionic compounds.^10,30–40 Among these, LTMOs have a higher specific capacity and energy density (Table 1).

Table 1 Comparison of some selected cathodes

Materials	Voltage (V)	1st capacity (mA h g^−1)	Lifespan (%@cycles)	Ref.
Na0.67Ni0.33Mn0.67O2	2.5–4.35	150	90%@100	41
Na0.44MnO2	2.0–3.8	113	82.3%@200	42
Na2Mn[Fe(CN)6]	2.0–4.2	154	43.2% @100	43
Na3V2(PO4)3	2.3–4.1	85.6	65.5%@100	44

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Layered Na_xTMO₂ (Co, Fe, Mn, Ni, Ti, and Cr) is an embedded or intercalated compound. Delmas et al. first proposed the arrangement of Na⁺ between TMO₆ layers and divided it into P phase and O phase.⁴⁵ As shown in Fig. 1, the number following O or P represents the stacking arrangement of oxygen elements, where Na⁺ in P-phase Na_xTMO₂ occupies the triangular prism gap position, while Na⁺ in O-phase Na_xTMO₂ occupies the octahedral gap position between TMO₆ layers.⁴⁶ Compared with other materials, P2 layered materials have a higher discharge capacity,^47,48 better cycling stability, and superior ionic conductivity at low Na⁺ concentrations.^49,50 Na_xTMO₂ is commonly prepared via solid-phase, sol–gel and hydrothermal methods. The most common method is the high-temperature solid-phase method, through which powder particles are prepared without agglomeration via a good filling and simple preparation process. However, the powder is not fine enough and is easily mixed with impurities. When constructing the P2 phase, this may cause a slippage of the layer interface. Compared to the solid-phase method, the sol–gel method allows for an easier chemical reaction and requires a lower synthesis temperature. Further, homogeneous mixing between reactant molecules during the formation of a gel leads to the better air stability of Na_xTMO₂. The hydrothermal method is less common as it requires higher humidity, temperature and pressure. However, it yields a product with high purity, which is favorable for the cyclic stability of Na_xTMO₂. In operation, Ni-based P2 phase cathodes have a Ni²⁺/Ni⁴⁺ redox couple with high voltage plateaus, such as P2-Na_2/3Ni_1/3Mn_2/3O₂ with an average voltage of 3.6 V. Besides, an unfavorable P2–O2 phase transition will occur at a 4.22 V high voltage, which can lead to volume shrinkage and particle cracks appearing during repeated cycling. Electrochemically active (Co³⁺ and Fe³⁺) and inactive (Li⁺/Zn²⁺/Mg²⁺/Al³⁺/Ti⁴⁺) cationic substitutions have been adopted to tackle these issues. This paper summarizes the information on P2-Na_xTMO₂ and the modification of such materials containing unitary, binary, ternary, and multi-components, with an aim to outline and clarify the current research and look forward to their further development trends and future prospects (Fig. 2).

Fig. 1 Classification of Na–TM–O electrodes with TMO₆-octahedra and phase transformation. Adapted with permission from ref. 46, Copyright {2014} American Chemical Society.⁴⁶

Fig. 2 Schematic of the modification strategies for P2-Na_xTMO₂ cathode materials for SIBs.

2 Progress of P2-type materials

2.1 Unitary Na_xTMO₂

Initially, researchers studied single transition metal oxide cathode materials, such as NaCoO₂,^51–54 NaCrO₂,^54–57 and NaNiO₂,^54,58 drawing on such cathode materials that are widely used in LIBs (LiCoO₂,^59–61 LiCrO₂,⁶² and LiNiO₂,^63–65 etc.). Because of the larger radius of sodium ions (Na⁺ : Li⁺ = 108 : 76 pm), Na⁺ diffusion is harder and can result in structure collapse. Therefore, designing a more suitable structure (e.g., larger lattice parameters) for Na⁺ (de)intercalation would be desirable.⁶⁶

P2-type Na_xCoO₂ is endowed with a simple structure and competitive capacity.⁶⁷ More importantly, compared to commercial LiCoO₂, the Na⁺ diffusion (D_Na⁺) for the Na_xCoO₂ electrode (0.5–1.5 × 10⁻¹¹ cm⁻² s⁻¹) is better than Li⁺ diffusion in the LiCoO₂ electrode (1 × 10⁻¹¹ cm⁻² s⁻¹).⁶⁸ Therefore, increasing D_Na⁺ in this type of material has become a research hotspot. Microspherical P2-Na_xCoO₂ (S-NCO) possesses an inferior specific surface (∼2.82 m² g⁻¹) area and layered structure,⁶⁹ and also exhibits competitive electrochemical stability (82.2 mA h g⁻¹ @300 cycles@720 mA g⁻¹). For obtaining single-phase domains in P2-NaCrO₂, Gan et al. constructed Na⁺ vacancy ordering by a deiodination method, demonstrating the system's multiple voltage mechanism.⁷⁰ They also proved that structural relaxation as well as electron transfer were responsible for the de-anodization energy. Na_xMnO₂ materials possess the great advantage of the high abundance of sodium resources. Recently, Zuo's team designed an efficient water-mediated system to synthesize P2-Na_0.67MnO₂ (S-NMO) with a shale structure (Fig. 3),⁷¹ and reported it could regulate the D_Na⁺ effectively. Further, the S-NMO electrodes displayed high cycling (>3000 cycles) and rate capabilities (100 mA h g⁻¹@960 mA g⁻¹); proving that the superior D_Na⁺ has a great influence on the performances of electrodes.

Fig. 3 Synthetic route for preparing S-NMO. Adapted with permission from ref. 71, Copyright {2021} Springer Nature.

2.2 Binary Na_xTMO₂

To address the problems of unitary Na_xTMO₂ (e.g., inferior cycling, poor structural stability, unsatisfactory specific capacity),⁷² doping a new TM ion to construct a binary-TM system is an effective strategy. The resulting synergistic effect of the ions can improve the electrochemical/structural stabilities.⁷³ The Ni²⁺/Ni⁴⁺ redox couple can provide a higher working voltage. However, P2-Na_0.67Ni_0.33Mn_0.67O₂ usually exhibits disappointing cycle/rate abilities, because of drastic phase transformation and its vulnerable D_Na⁺.⁷⁴ When charged to 4.2 V, Na_0.67Ni_0.33Mn_0.67O₂ can undergo P2 → O2 phase transition, which could be observed by the appearance of a new (002′) diffraction peak in the XRD analysis (Fig. 4a).⁷⁵ For P2-Na_0.67Ni_0.33Mn_0.67O₂, Yang's group systematically studied the optimal synthesis parameters via orthogonal experiments,⁷⁷ and reported that an excess Na content (3%) is able to effectively improve the capacity (159.3 mA h g⁻¹). Another research study found that the cut-off voltage (up to 4.5 V or low to 1.5 V) could influence the P2−O2 transition and Mn⁴⁺/Mn³⁺ redox reaction.⁷⁸ Yet another study reported a one-pot method to obtain porous P2-Na_0.67Ni_0.33Mn_0.67O₂ microcuboids with the {010} plane exposed,⁷⁹ which exhibited good performance (94.6%@1500 cycles@850 mA g⁻¹). Moreover, it was also reported that the design of porous hierarchical P2-Na_2/3Ni_1/3Mn_2/3O₂ nanofibers could primarily stabilize the structure as well as stimulate electrochemical reactions,⁸⁰ thereby facilitating superb rate abilities (73.4 and 166.7 mA h g ⁻¹ at 3.4 and 17 mA g⁻¹, respectively) and significantly improved cycling performance (81%@500 cycles). The highly reversible changes in the structure and Ni/Mn redox during cycling were studied by in situ XRD and XPS, and it was found that the improved capacity was derived from the Ni^2+/3+ as well as Mn^4+/3+ redox reactions (1.5–4.0 V).

Fig. 4 (a) Ex situ XRD of Na_0.67Ni_0.33Mn_0.67O₂ in the initial cycle.⁷⁵ (b) Calculated lattice parameters from in situ XRD; (c) GITT and D_Na⁺ of P2-Na_0.67MnO₂ and P2-Na_0.67Cu_0.1Mn_0.9O₂ electrodes; (d) ex situ XPS of P2-Na_0.67Cu_0.1Mn_0.9O₂ electrodes in pristine, fully charged and discharged states. Adapted with permission from ref. 76, Copyright {2021} American Chemical Society.

Since nickel and cobalt are relatively expensive and toxic, the use of these elements is not conducive to reducing battery costs or for the application of such batteries for large-scale energy storage. Therefore, Chen's group first investigated the cheap and non-toxic Cu²⁺/Cu³⁺ redox couple as an alternative with good electrochemical activity.⁸¹ They synthesized P2-Na_0.68Cu_0.34Mn_0.66O₂ by a solid-state method, which showed an initial specific capacity of 74.5 mA h g ⁻¹. Although the specific capacity of this material was slightly low, their work was significant for motivating the exploration of other low-cost and high-specific-capacity cathode materials; for instance, in another study, Na_0.67Cu_0.1Mn_0.9O₂ was reported to be able to deliver a high capacity of 222.7 mA h g⁻¹@10 mA g⁻¹@1.5–4.5 V, and 76% capacity retention @1 A g⁻¹@300 cycles.⁷² Here, it was reported that the doping of Cu²⁺ could inhibit the consecutive structural transformation and alleviate Jahn–Teller distortion (Fig. 4b–d), thus improving the whole electrochemical performance. This strategy provides a new idea for the development of P2-Na_xTMO₂ materials with structural stability and high energy density.

2.3 Ternary and multi Na_xTMO₂

As a typical P2-type layered structure material, Na_0.67Ni_0.33Mn_0.67O₂ (NNMO) has the advantages of a high theoretical specific capacity (173 mA h g⁻¹) and working voltage. However, when charged to 4.2 V, the existence of P2–O3 transition leads to volume changes, resulting in a poor cycle stability.⁸² Pahari and coworkers synthesized P2-Na_0.67Ni_0.17Ti_0.16Mn_0.67O₂ via a solid-state reaction method, and reported an initial excellent discharge capacitance (167 mA h g⁻¹@16 mA g⁻¹) at 3.7 V.⁸³ Also, Na_0.67Fe_0.3Mn_0.3Co_0.4O₂(NFMC) displayed an excellent cycling performance (retaining 85.5%@100 cycles@160 mA g⁻¹), and high rate capabilities (136.7 and 81.1 mA h g⁻¹ at 34 and 850 mA g⁻¹).⁸⁴ The NFMC cathode delivered higher voltage plateaus (3.3 V) than that for NFM (2.7 V) as noted through comparing the charge–discharge curves (Fig. 5a). Further, the polarization of the NFMC electrode was greatly decreased (Fig. 5b and c) as the Co substitution increased the D_Na⁺ in the structure. In addition, this study revealed that Co substitution could not only effectively enhance the electronic conductivity, but also relieve the polarization of the electrode to some extent.⁸⁶

Fig. 5 (a) charge–discharge curves of NFM and NFMC0.4 at 1C. CV curves of (b) NFM and (c) NFMC0.4 at 0.1 mV s⁻¹. Adapted with permission from ref. 84, Copyright {2018} Elsevier B.V. (d) Powder XRD Rietveld refinement pattern; (e) SEM pattern; (f) HRTEM image, and (g) in situ XRD at first charge/discharge of P2-NRM at 0.2C. (h) GITT results and the D_Na⁺ coefficient of the NRM material in the first cycle. Adapted with permission from ref. 85, Copyright {2020} American Chemical Society.

The electrical performance of NNMO could be effectively promoted to a new and higher level by the co-substitution of two or more transition metals. Compared with NNMO, Cu/Mo co-substituted P2-type Na_0.67Ni_0.33Mn_0.57Cu_0.05Mo_0.05O₂ could effectively inhibit the P2/O2 phase transition, leading to an improvement in the electrochemical performance (142 mA h g⁻¹@2–4.5 V@34 mA g⁻¹@91.5% capacity retention).⁸⁷ Peng et al. synthesized [Na_0.67Zn_0.05]Ni_0.18Cu_0.1Mn_0.67O₂ by doping Cu⁺ at the transition metal site (2a) and the uncommon Zn⁺ at the Na site (2d), achieving stable cycling and moisture resistance for the first time.⁴¹ Significantly, in situ XRD characterization, and measurements of the charge-transfer kinetics and ion diffusion, as well as microstructural analyses after deep cycling, indicated that the specific two-site doping method could successfully reduce the activation energy of D_Na⁺ in the bulk material, and suppress the formation of O₂ at the end of charging. Doping Zn⁺ at the Na site could effectively reduce d_(O–Na–O) and enhance the ‘pillar’ effect of O²⁻–Zn²⁺–O²⁻ electrostatic cohesion, thus strengthening the layered cathode structure, and inhibiting the generation of cracks, leading to a superior cycle stability and excellent rate performance. P2-Na_0.75Ni_1/3Ru_1/6Mn_1/2O₂(NRM) presented a gratifying capacity (161.5 mA h g⁻¹), and excellent cyclic performance (79.5%@500 cycles@10C).⁸⁵ The XRD (Fig. 5d) and HRTEM (Fig. 5f) analyses indicated that the material had a layered hexagonal structure of pure P2. As shown in Fig. 5e, NRM particles with diameters in the range of 1–2 μm were uniformly distributed. It could be seen that the (002) and (004) peaks returned to their original positions (Fig. 5g) after the first charging and discharging cycle, indicating the good cycle performance. The maximum diffusion coefficient of Na⁺ was 2.05 × 10⁻¹⁰ cm² s⁻¹ (Fig. 5h), which is higher than that of most P2 materials reported previously,⁸⁸ revealing its faster (de)sodiation process and superior rate performance. A comparison of unitary, binary, and multi Na_xTMO₂ is given in Table 2.

Table 2 Comparison of unitary, binary, and multi Na_xTMO₂

Type	Materials	1st capacity (mA h g^−1)	Lifespan (%@cycles)	Ref.
Unitary	NaxCoO2	175	82%@300	70
	Na0.67MnO2	181	79%@3000	71
Binary	Na0.67Ni0.33Mn0.67O2	122	94.6%@1500	79
	Na2/3Ni1/3Mn2/3O2	166.7	81%@500	80
	Na0.67Cu0.1Mn0.9O2	222	76%@300	72
Multi	Na0.67Fe0.3Mn0.3Co0.4O2	136.7	85.5%@100	84
	Na0.67Ni0.33Mn0.57Cu0.05Mo0.05O2	142	91.5%@500	85

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3 Problems and optimization of P2-structured materials

3.1 Problems with P2-structured materials

At present, there are still some problems to be solved with P2-cathode materials. For instance, due to the large radius of Na⁺, there are obvious kinetic barriers in the migration process and possible effects of structural collapse during the deintercalation/intercalation process, resulting in a poor rate performance and rapid capacity fading in cycling. Besides, the majority of P2-phase materials produce phase transformation when they are charged above 4.2 V, resulting in structural and volume changes.⁸⁹ For example, for the P2-Na_0.67Ni_0.33Mn_0.67O₂ (P2-NNMO) material, Wang et al. found that the main reason for the performance decay was the repeated P2–O2 transitions, which produced an outstanding density of intracrystalline cracks, ultimately destroying the primary grains.⁹⁰

3.2 Lattice doping

Currently, lattice doping can address structural changes by suppressing phase transitions.^91–101 For example, Li⁺ doping can improve the Na-storage capacity of cathode materials.¹⁰² For instance, Wang et al. engineered large-sized K⁺ into the prismatic Na⁺ sites of P2-Na_0.612K_0.056MnO₂, resulting in more favorable Na⁺ vacancies,¹⁰³ which exhibited the highest specific capacity (240.5 mA h g⁻¹@20 mA g⁻¹) and energy density (654 W h kg⁻¹) based on the redox of Mn³⁺/Mn⁴⁺. Cheng's group reported an Al-doped P2-type Na_0.6Ni_0.3Mn_0.7O₂ cathode material and investigated the corresponding charge-compensation mechanism.¹⁰⁴ Compared to Na_0.6Ni_0.3Mn_0.7O₂, Al doping facilitated the reversible oxygen redox reaction through the reductive coupling reaction between the lone O 2p state in the localized configuration of Na–O–Al and Ni⁴⁺. In addition, aluminum doping increased the interlayer spacing and suppressed the disadvantageous P2 → O2 transition during the deiodination/iodination process, which greatly improved the cycling and rate performances. The Na_0.67Ni_0.31Mn_0.67Y_0.02O₂ material synthesized by Kim et al. was found to have strong Y–O bonding, leading to a very stable structure.¹⁰⁵ In addition, it was encased by Y₂O₃, which acted as a protective layer. Due to the large ionic radius of the Y ion (0.90 Å), the atomic charges of Ni, Mn, and O were altered. The Y ion was also used as a protective layer. Meanwhile, Rb²⁺ doping in Na_0.78Ni_0.32Mn_0.68O₂ enhanced the mobility of Na⁺ and induced atomic-scale surface reorganization, which prevented the transition metals from dissolving into the electrolyte during cycling (Fig. 6a). In particular, it can be seen that it exhibited superior performance with 76% capacity retention at −40 °C (1800 cycles@368 mA g⁻¹) (Fig. 6b). According to the Zn/Mg dual-doping strategy with bifunctional effects, Huang and coworkers synthesized Na_0.67Mn_0.7Zn_0.15Mg_0.15O₂ via a facile co-precipitation method.⁹⁷

Fig. 6 (a) Schematic showing the protective effect on the main structure via surface modification. (b) Long-cycling stability at rates of 92 and 368 mA g⁻¹ at −40 °C. Adapted with permission from ref. 106, Copyright {2022} Springer Nature. Charge/discharge profiles of (c) NMMT at 0.1C during the first 15 cycles in the voltage range of 2.0–4.3 V; (d) rate performances of the NMMT cathode; (e) in situ XRD tests upon charging/discharging of the NMMT cathode between 2.0 and 4.3 V versus Na⁺/Na. Adapted with permission from ref. 107, Copyright {2022} American Chemical Society. (f) In situ XRD patterns of NaNMTi_0.3OF during the first charge/discharge process. Adapted with permission from ref. 108, Copyright {2022} Elsevier B.V.

Compared with P2-Na_0.67MnO₂ and single-ion (Zn/Mg)-doped specimens, Zn/Mg dual-doping broadens the distance between the crystal planes and supplies a spacious ion-diffusion channel for rapid D_Na⁺. It also has less Mn³⁺/Mn⁴⁺ and a higher lattice oxygen content, which is instrumental for increasing the structural stability. It was proved that the Zn/Mg dual-doped electrode displayed excellent rate performance (67.2 mA h g⁻¹@1.7 A g⁻¹) and a decent cycling stability (93.8% capacity retention@170 mA g⁻¹@100 cycles). This work thus provides a promising avenue for perfecting the performance enhancement of layered cathode materials. Based on the synergistic effect of Mg and Ti co-doping,¹⁰⁷ Li's group designed and investigated Na_2/3[Ni_2/9Mg_1/9Mn_5/9Ti_1/9]O₂ (NMMT), which displayed an apparent capacity activation during the first cycles (113 mA h g⁻¹@17 mA g⁻¹) (Fig. 6c) and outstanding rate ability (50 mA h g⁻¹@850 m A g⁻¹@500 cycles) between 2.0–4.3 V (Fig. 6d). Furthermore, as shown by the in situ XRD characterization (Fig. 6e), single-phase electrochemical reactions occurred during the Na⁺ deintercalation.

Based on the Ti⁴⁺/F⁻ co-doping strategy, P2-Na_0.67Ni_0.33Mn_0.37Ti_0.3O_1.9F_0.1 showed a much strengthened sodium-storage performance within the 2.0–4.4 V range, including a certain cycling ability (77.2%@300 cycles@300 mA g⁻¹) as well as an excellent rate capability (87.7 mA h g⁻¹@1.02 A g⁻¹).¹⁰⁸ In situ XRD (Fig. 6f) analysis showed that the Ti⁴⁺/F⁻ co-doping could inhibit both P2 → O2 transitions and Na⁺/vacancy ordering, resulting in fast Na⁺ diffusion and a stable phase structure. This study offers a novel idea for the development of layered cathode materials with anion–cation synergetic contributions (Table 3).

Table 3 Typical dopants in P2-type materials

Materials	Dopant	Voltage (V)	1st capacity (mA h g^−1)	Lifespan (%@cycles)	Ref.
Na0.67[Li0.22Mn0.78]O2	Ni^2+	1.5–4.6	160	73%@300	96
Na0.67MnO2	K^+	1.8–4.3	240.5	98.2%@100	103
Na0.67Ni0.33Mn0.67O2	Al^3+	1.5–4.5	213.6	58.7%100	104
Na0.67Ni0.33Mn0.67O2	Y^2+	2.0–4.5	126.4	63.4%60	105
Na0.78Ni0.32Mn0.68O2	Rb^2+	2.4–4.15	96.6	76%@1800	106
Na0.67MnO2	Zn^2+/Mg^2+	2.0–4.5	166.2	93.8%@100	97
Na2/3Ni1/3Mn2/3O2	Mg^2+/Ti^4+	2.0–4.3	113	84.3%@500	107
Na0.67Ni0.33Mn0.67O2	Ti^4+/F^−	2.0–4.4	140.3	77.2(300)	108

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3.3 Surface modification

Currently, energy-storage systems with greatly reduced costs and higher stability and safety can be developed via doping cations or anions. However, the large structural change in the cycle process can result in rapid capacity deterioration and an inferior cycle life. In addition, the high air sensitivity of systems can have a negative impact on the electrochemical performance. Therefore, researchers have modified the electrode surface to reduce the side reactions between the cathode material and electrolyte during cycling process, so that structural stability, ion diffusion ability and electronic conductivity are improved.^109,110 For instance, a carbon coating can be used to enhance the electrochemical performance, but the inferior mechanical properties of carbon make it harder to improve the cycle stability.¹¹¹

Applying a TMO coating on the surface of a P2 cathode is another active method, which can raise the conductivity and electrochemical performance. It was reported that P2-type Na_2/3Fe_1/2Mn_1/2O₂ materials can be synthesized by ultrasonic jet pyrolysis followed by solid-state sintering.¹¹² In addition, a thin Al₂O₃ layer can be formed on the surface of Na_2/3Fe_/1/2Mn_1/2O₂, which inhibits the formation of Na₂CO₃-H₂O and avoids its exposure to air, thus improving the storage performance. Yang et al. coated ZnO, a semiconducting material with excellent electrical conductivity, on a P2 layer of Na_2/3Ni_1/3Mn_2/3O₂, which could significantly inhibit the peeling phenomenon and maintain the morphology and structure of the electrode well.¹¹³ In addition, part of the Zn²⁺ could finds its way into the transition metal oxide (TMO₂) layer, realizing in an improvement of the crystal stability. Based on the synergistic effect of the ZnO coating and Zn²⁺ doping, the material exhibited excellent cycling performance (75%@200 cycles) and rate performance. To address the defects of the P2-Na_0.67Ni_0.17Co_0.17Mn_0.66O₂ cathode, a dual modification method incorporating Mg/Ti co-doping and MgO surface coating was reported.¹¹⁴ The results showed that the P2 structure could be stabilized by Mg²⁺/Ti⁴⁺ co-substitution, and that the MgO layer could effectively prevent the surface from being corroded by HF, while promoting Na⁺ migration. It displayed a 111.6 mA h g⁻¹ initial discharge capacity and retained 90.6% of this at a high current density of 100 mA g⁻¹, which evidently surpassed the performance of Na_0.67Ni_0.17Co_0.17Mn_0.66O₂. The obvious improvement could be attributed to the synergistic effect of Mg²⁺/Ti⁴⁺ co-substitution and the MgO surface coating.

In addition, coating a conductive polymer could also effectively enhance the electrochemical performances. Applying a polydopamine-derived carbon coating was reported to be a significant strategy to improve the interfacial stability of P2-type Na_0.80Ni_0.22Zn_0.06Mn_0.66O₂.¹¹⁵ The application of consecutive and homogeneous carbonized PDA (C-PDA) layers with a thickness of ∼5 nm could inhibit Na⁺ extraction from the surface of P2-Na_0.80Ni_0.22Zn_0.06Mn_0.66O₂ particles during the electrode fabrication process and the formation of electrochemically harmful Na₂CO₃/NaOH species. Compared with pristine samples, the material exhibited a higher discharge capacity (124 mA h g⁻¹@12 mA g⁻¹), superior rate capability (62 mA h g⁻¹@1536 mA g⁻¹), and excellent cycle stability (90.7% capacity retention@100 cycles). These results show that proper surface protection can prevent the formation of side products, which is crucial to improving the performance of P2 oxide materials. Without sacrificing the high-voltage performance, Yuan et al. stabilized the lattice oxygen in a composite material by applying a small amount of Sn substitution and by further protecting the particle surface with a polypyrrole (PPy) coating.¹¹⁶ The prepared Na_0.67Ni_0.33Mn_0.63Sn_0.04O₂@PPy (3.3 wt%) composite displayed superb rate performance (137.6/120.0 mA h g⁻¹@10/100 mA g⁻¹) with 82.5% capacity retention (100 mA g⁻¹@100 cycles). The surface particles of the Na_0.67Ni_0.33Mn_0.63Sn_0.04O₂@PPy (3.3 wt%) composite did not fall off, indicating that the conductive PPy coating could not only improve the cycling stability, but also played a role as a capsule shell, inhibiting particles from falling off and protecting particles from electrolyte erosion. A NASICON-type NaTi₂(PO₄)₃ (NTP) nanoshell was coated on the surface of P2-Na_0.67Co_0.2Mn_0.8O₂ (NCM) (Fig. 7a) to promote its performance as a new cathode.¹¹⁷ The NCM@NTP7 sample displayed outstanding electrochemical charge/discharge profiles, with a high capacity (152.4 mA h g⁻¹@34 mA g⁻¹) and 86.7% capacity retention (85 mA g⁻¹@150 cycles) at room temperature. The optimized coating could effectively inhibit side reactions and greatly improve the cyclic stability. Meanwhile, the NTP could accelerate the Na⁺-migration kinetics of the host material, providing perfect D_Na⁺ channels and a higher electronic conductivity (R_ct = 26.4 Ω, Fig. 7b, D_G = 4.04 × 10 ⁻¹⁰ cm² s⁻¹, Fig. 7c). It is apparent that PPy coating is a viable method for developing stable outstanding voltage transition metal oxide cathode materials. Some typical reported modifications of P2-type materials are listed in Table 4.

Fig. 7 (a) Interface model of the NCM@NTP7 material; (b) EIS plot, and the inset image shows the equivalent circuit for NCM and NCM@NTP7; GITT curves and corresponding D values of (c) NCM@NTP7. Adapted with permission from ref. 117, Copyright {2020} Elsevier B.V.

Table 4 Typical modifications in P2-type materials

Materials	Dopant	Coating	1st capacity (mA h g^−1)	Lifespan (%@cycles)	Ref.
Na2/3Fe1/2Mn1/2O2		Al2O3	146.7	84.7%@40	112
Na2/3Ni1/3Mn2/3O2	Zn^2+	ZnO	162	75%@200	113
Na0.67Ni0.17Co0.17Mn0.66O2	Mg^2+/Ti^4+	MgO	111.6	90.6%@300	114
Na0.80Ni0.28Mn0.66O2	Zn^2+	C-PDA	124	62%@100	115
Na0.67Ni0.33Mn0.67O2	Sn^4+	PPy	137.6	82.5%@100	116
Na0.67Co0.2Mn0.8O2		NaTi2(PO4)3	152.4	86.7%@150	117

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3.4 High entropy modification

Recently, the method of building high entropy oxides has been proven to be another possible strategy to improve the performance of O3-layered oxide cathodes.^118–122 The use of high entropy P2-Na_0.6(Ti_0.2Mn_0.2Co_0.2Ni_0.2Ru_0.2)O₂ can tune the entropic stabilization of the crystal structure and the diffusion activation energy barriers, leading to superior rate performance at a very high rate (68 mA h g⁻¹ at 86C).¹²³ This work demonstrated an advanced fast-charging layered oxide cathode for SIBs. The entropy-tuned P2-Na_0.62Mn_0.67Ni_0.23Cu_0.05Mg_0.07Ti_0.01O₂ could expose more {010} active facet and improve the structural stability.¹²⁴ The cathode exhibited outstanding electrochemical performance, especially cycling stability (75% capacity retention@2000 cycles@1.2 A g⁻¹). In situ HEXRD tests (Fig. 8a) were performed, revealing that no new phase formation or phase transition occurred. Also, the structure evolution was highly reversible during the charging and discharging Na⁺ (de)intercalation process (Fig. 8b), leading to a superior electrochemical performance. Therefore, high entropy modification represents a new method for advanced P2-layered cathode materials (Table 5).

Fig. 8 (a) Waterfall plot of the in situ HEXRD patterns for CuMgTi-571 within the range 2.0–4.3 V. (b) Evolution of the cell parameters and cell volume during the charge/discharge process. Adapted with permission from ref. 124, Copyright {2022}, Springer Nature.

Table 5 Typical high-entropy P2-type materials

Materials	Voltage (V)	1st capacity (mA h g^−1)	Lifespan (%@cycles)	Ref.
Na0.6(Ti0.2Mn0.2Co0.2Ni0.2Ru0.2)O2	1.5–4.5	164	70%@40	123
Na0.62Mn0.67Ni0.23Cu0.05Mg0.07Ti0.01O2	2.0–4.3	148	75%@2000	124
Na2/3[Ni1/4Mn1/2Ti1/6Zn1/12]O2	2.5–4.5	114	100%@40	125
Na2/3Li1/6Fe1/6Co1/6Ni1/6Mn1/3O2	2.0–4.5	171	89.3%@90	126

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3.5 Composite phase modification

A composite phase strategy^127–129 (such as P2/P3, P2/O3, and P2/P3/O3) has also been proposed to enhance the electrochemical performance of P2-Na_xTMO₂ (Table 6).

Table 6 Typical multi-phase layered materials for sodium-ion batteries

Materials	Voltage (V)	1st capacity (mA h g^−1)	Lifespan (%@cycles)	Ref.
P2/P3-Na0.67Mn0.64Co0.30Al0.06O2	1.5–4.0	160	81%@200	130
P2/P3-Na0.5Mg0.2Co0.15Mn0.65O2	1.5–4.3	136	89%@100	131
P2/O3-Na0.76Ni0.33Mn0.50Fe0.10Ti0.07O2	2.2–4.3	144	82%@100	132
P2/O3-Na0.8Li0.2Ni0.33Mn0.67O2	2.0–4.3	133	80%@120	133
P2/O3-Na0.80Li0.13Ni0.20Fe0.10Mn0.57O2	2.0–4.5	172	89%@100	134
P2/O3-Na7/9Ni2/9Mn4/9Fe1/9Mg1/9Li1/9O2	2.0–4.4	170	72.1%@400	135
P3/P2/O3-Na0.674Ni0.319Mn0.590O2	2.0–4.2	100	67%@200	136

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Attributed to the fast Na⁺ diffusion and stable crystal structure, P2/P3-Na_0.67Mn_0.64Co_0.30Al_0.06O₂ displayed an outstanding rate capability (83 mA h g⁻¹ at 1700 mA g⁻¹) and distinguished cycling stability (81% @200 cycles@1000 mA g⁻¹).¹³⁰ In situ XRD tests confirmed there were no new peaks except for the P2/P3 phases and that the Jahn–Teller effect was largely relieved during the charge/discharge process, thus realizing a superior long cycling ability. Wang's group explored the sodium-storage mechanism of the P2/O3-Na_0.76Ni_0.33Mn_0.50Fe_0.10Ti_0.07O₂ cathode.¹³² In operando XRD measurements revealed the reversible structural transformation of P2/O3–P2/O3/P3–P2/P3–P2/Z/O3′–Z/O3′, attributed to the Ni²⁺/Ni^3.5+, Fe³⁺/Fe⁴⁺, and Mn^3.8+/Mn⁴⁺ redox couples during the Na⁺-(de)intercalation process. This also led to its high capacity (144 mA h g⁻¹ at 42 mA g⁻¹) and dramatic rate performance (82 mA h g⁻¹ at 210 mA g⁻¹). This work provides a novel idea for the design of high-performance layered multi-phase structures. Because of the staggered arrangement of different phase structures, the P3/P2/O3-Na_0.674Ni_0.319Mn_0.590O₂(NNMO) cathode displayed an improved rate performance (100 mA h g⁻¹ at 750 mA g⁻¹).¹³⁶ The HRTEM image of P3/P2/O3-NNMO clearly showed the co-existence of P2 and O3 phases (Fig. 9a), while ex situ XRD (Fig. 9b) revealed that P3/P2/O3-NNMO experienced a reversible conversion process of P3/P2/O3—P3/P2—P3/P2/P3′—P3/P2/O3′—P3/P2/P3′—P3/P2—P3/P2/O3 during the first cycle. Also, the P2–O2 phase transition was inhibited, leading to the improved cycling stability (80 mA h g⁻¹@200 cycles@30 mA g⁻¹). The work offers a model to investigate the independent influence of the structure of the electrode on its electrochemical performance. Therefore, the strategy of composite phase modification provides a new approach to suppress irreversible phase transitions and enhance the performance of layered oxide cathodes.¹³⁷

Fig. 9 (a) HRTEM images of P2/P3/O3-NNMO; (b) ex situ XRD patterns (right) of P3/P2/O3-NNMO at different voltages between 2.0 and 4.2 V during the first charge/discharge process. Adapted with permission from ref. 136, Copyright {2022}, Wiley-VCH.

In summary, the modification methods broadly include structural lattice doping, applying a coating on the surface of the material particles, and composite phase modification. In particular, surface coating can improve the interfacial stability of a material, mitigate the side reactions at the electrode/electrolyte interface, and improve the ionic/electronic conductance at the interface. The drawback is that cladding cannot modulate the lattice, i.e., it cannot regulate the spatial and electronic effects of the internal structure of the material. Conversely, ion doping of the lattice of the electrode material with ions with a different valence and radius can play a role in expanding the ion-diffusion channels, improving the conductivity of the bulk phase of the material, and enhancing the structural stability. Moreover, multi-doping has significant superiority over single doping, such as an enhanced effect from synergistic mechanisms, providing multiple ions with a richer electron cloud density, and the different radii of the different ions, which can endow the structure with stronger toughness.

4 Conclusion and future perspectives

SIBs have developed rapidly in the past decade, but their energy density is still not as good as that of LIBs. Compared to P2-Na_xTMO₂, O3-Na_xTMO₂ has a higher initial sodium content, which can provide more specific capacity in the same voltage range, making it more suitable for full-cell applications. For now, using a composite phase is optimal for a full battery. In addition to this, the dual-modification strategy can promote the electrochemical performance of the material more than a single-modification strategy. For example, the combination of elemental doping and surface capping can not only improve the structural stability, but can also further enhance the conductivity of the material and reduce energy losses during charging and discharging. It is also an effective strategy for full battery performance improvement. Another problem to solve is the problem of high cost of materials. Here, the use of abundant raw materials and simple preparation make P2 layered cathode materials one of the most competitive cathode materials.¹³⁸ Compared with other cathode materials, P2 cathode materials also have the advantages of volume and mass energy density (Fig. 10). From the initial unitary to binary, ternary, and multi metal oxides, the properties of the materials have been further optimized by lattice doping and surface modification, which can also be applied in industry.

Fig. 10 Gravimetric energy density (W h kg⁻¹) for P2-Na_xMO₂ with different numbers of transition metals in half-cell systems.

Currently, one of the most urgent problems to be solved for this kind of material is to improve the specific capacity of the first charge. The redox reaction of O²⁻ in the structure is an important consideration that could help solve this problem.¹³⁹ However, this could incur other problems, such as voltage hysteresis, and poor cycling and rate performance with anionic redox reactions.¹⁴⁰ With the help of some characterization tools (e.g., XANES, XRT, AFM¹⁴¹ and Cryo-TEM¹⁴²), the redox reaction mechanism of O²⁻ was investigated to provide theoretical guidance for further improving the energy density, cycling, and rate performance of such materials.^143,144

Although pure-phase P3 type oxides, such as Na_2/3Mg_1/3Mn_2/3O₂ and Na_2/3Ni_1/3Mn_2/3O₂, have been reported, the P3 phase is more commonly reported as an accompanying impurity for P2 and O3 types. P2-type and P-type Na_0.67MnO₂ have been shown to exhibit a high reversible capacity and good structural reversibility. However, the initial Na content of P-type layered oxides is relatively low, which is not conducive to assembling the entire battery. Both O3-type and sawtooth-type NaMnO₂ have a high initial Na content and specific capacity, but their electrochemical reversibility is poor. The sawtooth-type NaMnO₂ is generally accompanied by a mixture of O3-type NaMnO₂ phases, and pure sawtooth-type NaMnO₂ has not been reported yet.^145,146

Designing high-entropy layered oxides is another strategy to suppress the P2–O2 phase transition. A transition metal layer composed of various different metal ions can accommodate the local volume changes caused by Na⁺ (de)intercalation. Also, the phase transition of high-entropy layered cathodes is highly reversible. Therefore, scientific adjustment of the components and the structure of high entropy layered materials can also contribute to the development of new high-performance cathode materials for sodium-ion batteries. Besides, the low Na content makes it difficult for P2-Na_xTMO₂ to be used in high-energy full-battery systems. So, introducing a certain amount of sodium supplements (such as NaN₃ or Na₃P) can increase the initial sodium content. This could considerably accelerate the industrialization process of SIBs. Otherwise, their poor air stability is another barrier for layered oxide cathodes to attain greater commercialization. To solve this problem, strategies such as nanostructure design, surface coating, and lattice ion doping can be adopted to enhance the air stability and improve the competitiveness of layered cathode materials for large-scale application. Today, there are many other state-of-the-art solutions available for modification. First, sodium-rich layered transition metal oxide cathode materials can be tried out. In these materials, sodium ions occupy the octahedral position of the transition metal layer. In this way, more sodium ions can participate in the reaction and provide more capacity. Second, careful regulation of the ratio of each metal element could be an effective way to improve the structural stability of layered transition metal oxides. Finally, complementary anodes, electrolytes, diaphragms, additives and binders can be developed.

Among these optimization strategies, high-entropy doping designs have been extensively adopted in the last two years, mainly based on a cocktail of effects, entropy-increasing effects, and poly-electron effects, moreover, we believe that a combination of high-entropy doping in different lattice sites with a surface coating strategy may be a practicable solution to simultaneously address issues related to the structural and interface instability, thereby fundamentally making such layered materials have greater practical value for future SIBs.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Author contribution

Manni Li: investigation, formal analysis, writing – original draft. WeiqiLin: investigation, writing – original draft. Yurong Ji: investigation. Lianyu Guan: investigation. Linyuan Qiu: investigation. Yuhong Chen: investigation. Qiaoyu Lu: investigation. Xiang Ding: conceptualization, methodology, writing – review & editing, supervision, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 52102216), the Natural Science Foundation of Fujian Province (Grant No. 2022J01625, 2022-S-002) and the Innovation Training Program for College Students (202310394020, cxxl-2023097, cxxl-2024131, cxxl-2024136).

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