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Review  |  Open Access  |  14 May 2024

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

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Yu Huang1
, 
Weixiong Zeng2
, ... 
Xiaobo Zhu1
Microstructures 2024;4:2024027.
10.20517/microstructures.2023.102 |  © The Author(s) 2024.
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Abstract

Sodium-ion batteries (SIBs) have attracted enormous attention as candidates in stationary energy storage systems, because of the decent electrochemical performance based on cheap and abundant Na-ion intercalation chemistry. Layered oxides, the workhorses of modern lithium-ion batteries, have regained interest for replicating their success in enabling SIBs. A unique feature of sodium layered oxides is their ability to crystallize into a thermodynamically stable P2-type layered structure with under-stoichiometric Na content. This structure provides highly open trigonal prismatic environments for Na ions, permitting high Na+ mobility and excellent structural stability. This review delves into the intrinsic characteristics and key challenges faced by P2-type cathodes and then comprehensively summarizes the up-to-date advances in modification strategies from compositional design, elemental doping, phase mixing, morphological control, and surface modification to sodium compensation. The updated understanding presented in this review is anticipated to guide and expedite the development of P2-type layered oxide cathodes for practical SIB applications.

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Keywords

Sodium-ion batteries, layered structure, Na deficiency, P2-type, modification strategies, sodium compensation
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INTRODUCTION

Electrochemical energy storage is critical in mitigating climate change by accelerating the integration of renewable energy sources and transportation electrification[1]. As a representative, the lithium-ion battery (LIB), first commercialized in 1991 by Sony, has dominated the market of portable electronics and electric vehicles ever since. LIBs are now extending to grid energy storage applications, maximizing their role in combating climate change. However, their large-scale applications raise concerns about the availability and affordability of LIB-related critical minerals, including Li, Co, and Ni[2]. Therefore, low-cost battery chemistries are in high demand.

Among the alternative battery technologies, sodium-ion batteries (SIBs) have attracted great interest because of their appreciable performance afforded by the similar “rock-chair” intercalation chemistry, as shown in Figure 1A, utilizing extremely abundant and low-cost Na+ as the charge carrier. The charge carrier is in high concentrations in both electrolytes and cathodes. Moreover, unlike the reliance of costly Ni and Co in the state-of-the-art LIB cathodes, the types of active Na-containing cathode compounds are much more diverse in structures and compositions[3]. Transition metals (TMs), such as Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, all can serve as the active redox media[4]. As presented in Figure 1B, according to the earth’s abundances[5] and world reserves of these metals[6], the shift to SIBs can dramatically mitigate the concerns of resource shortage. Compared to LIBs, the SIB chemistry promises reduced cost for not only electrolytes and cathodes but also other battery components such as the anode current collector (expensive copper foil is replaced by aluminum, as Na does not alloy with aluminum) and even shipping/storing because of the overdischarge tolerance of SIBs[7]. Apart from the cost advantages, the weak solvation capability of Na+ and higher thermal stability of the electrolyte salt afford additional all-climate potential for SIBs, which is highly demanded for large-scale energy storage applications[8]. However, due to the heavier weight (22.9 g mol-1vs. 6.9 g mol-1 for Li+) and higher reduction potential of Na+ [-2.71 V vs. SHE (standard hydrogen electrode) compared to -3.01 V vs. SHE for Li+/Li], SIBs commonly show lower specific energy. Also, from the perspective of intercalation reaction, the much larger size of Na+ (1.02 Å vs. 0.76 Å for Li+) tends to cause sluggish solid-state diffusion and larger dimensional changes of the host structures[9,10]. Therefore, the development of SIBs with high specific capacity, high operating voltage, high rate capability, and long lifespan remains challenging.

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Figure 1. (A) Schematic illustration of a typical SIB using a layered oxide cathode and a hard carbon anode; (B) Abundances of the key metals involved in LIBs and/or SIBs in earth’s crust and their world reserves as of 2023 (Mt = million metric tons). Data acquired from[5,6]; (C) The redox couples in NaxTMO2 compounds with their corresponding potential range. Data acquired from[14]; (D) Accumulated publications from the Web of Science with the topic of "sodium ion batteries" and "sodium ion batteries P2" in the year range of 2013 to 2023.

Akin to the situation in LIBs, the performance and cost of SIBs are also largely dependent on the cathode materials. Currently, the most common cathode materials for SIBs include layered oxides, polyanionic compounds, Prussian blue analogs, and organic compounds. Each type has its own unique set of advantages and disadvantages. For example, polyanionic compounds generally have open three-dimensional skeletons, affording excellent structural stability. The inductive effect of polyanions also contributes to high redox voltage. However, they also suffer from low capacity and complex synthesis. Structural stability poses a challenge for Prussian blue analogs, despite the high ion conductivity. Organic compounds promise high flexibility for molecular design and environmental friendliness. However, their electronic conductivity is normally poor, and there is also the problem of easy dissolution in organic electrolytes[11]. Due to the enlarged size mismatch between Na+ and the first-row TM ions, almost all TMs can readily form sodium layered oxides with a general formula of NaxTMO2[12]. Compositional variety, easy synthesis, and other merits, such as high specific capacity and high volumetric density, make layered oxides very attractive for SIBs[13]. The corresponding redox potential ranges of NaxTMO2 based on these TMs are presented in Figure 1C[14]. Sodium layered oxides mainly involve two thermodynamic polymorphs, namely P2- and O3-type phases. Although P2-type cathodes are deficient in Na content (typically ranging from 0.6 to 0.7) that requires additional Na supplement to achieve full-cell application, the intrinsically lower Na+ diffusion barrier and better moisture tolerance of P2-type cathodes still make them very competitive for enabling high-performance SIBs. The intensive research activities on SIBs and P2-type layered oxides are reflected by the fast-growing publications found on the Web of Science with the topics of “sodium ion batteries” and “sodium ion batteries P2”, as shown in Figure 1D. Recently, Wang et al. demonstrated a 1.3 Ah pack cell using a P2-type cathode and a hard carbon anode[15]. The pouch cell showed a cell-level energy density of 165 Wh kg-1 with reasonable cycling stability (over 92% capacity retention after 100 cycles). This stimulates further interest in researching and developing P2 cathode materials towards commercialization. To chart a course for future development of P2-type cathode materials, here we start with an overview of P2-type materials in terms of their structural characteristics, then discuss the key challenges hindering their application and the up-to-date progress in combating these issues, and eventually offer perspectives and recommendations for future research.

STRUCTURAL FEATURES OF P2-TYPE OXIDES

The research on layered oxides can date back to 1958 when LiCoO2 was first synthesized[16]. Its Li intercalation capability was initially investigated by Mizushima et al.[17] in 1980, leading to the remarkable success of LIBs. On the other hand, its sodium analog, NaxCoO2, was first reported by Fouassier et al.[18]. In 1981, Delmas et al.[19] studied the Na intercalation behavior of NaxCoO2, which was extended to other sodium layered oxides such as NaxCrO2[20], NaxNiO2[20], and NaxTiO2[21]. With a general formula of AxTMO2 (A = Li or Na), the term “layered structure” vividly describes their crystallography, in which alkali metals and TMs distribute orderly in the oxygen close packing lattices, forming TMO2 slabs separated by Li+/Na+ layers. When the size difference between alkali metal ions and TM ions becomes significant, such ordered atomic arrangement is energetically favored as it reduces lattice constraints. Consequently, layered structures are more prevalent in the crystal chemistry of NaxMO2 due to the larger differences between Na+ and trivalent TM ions. In fact, all sodium NaTMO2 (TM = Ti, V, Cr, Mn, Fe, Co, and Ni) materials can crystallize into layer structures, while LiTMO2 compounds, including LiTiO2, LiMnO2, and LiFeO2, tend to form metal-mixing rock-salt structures.

Different from the stoichiometric Li in LiTMO2, the enhanced covalency of the TM-O bond attributed to the stronger ionic character of the Na-O stabilizes the tetravalent TM ions. Therefore, Na-deficient layered oxides can be thermodynamically stable. This leads to use of the more general formation of NaxTMO2 instead of NaTMO2 and richer polymorphism along with the variation in Na content. Figure 2A compares the crystal structure of Na-deficient P2 structure (typically 0.6 ≤ x ≤ 0.7) with another common polymorph of O3 phase that contains higher Na content (normally 0.8 ≤ x ≤ 1). Such structural classification was introduced by Delmas et al. based on the Na coordination environment and oxygen stacking rules[22]. The letters “P” and “O” represent the trigonal prismatic coordination and octahedral coordination environments of Na ions, respectively. Furthermore, a prime symbol (′) can be added to indicate a monoclinic/orthorhombic distortion. Three possible dense stacking positions of O atoms in a MeO2 layer are donated as A, B and C. The number “2” or “3” refers to the quantity of oxygen stacking layers in one unit cell. Specifically, P2 represents prismatic coordination of Na+ between TMO2 layers with an oxygen stacking sequence of “ABBA......”. In contrast, O3 structure accommodates Na+ with octahedral sits and has an oxygen layer stacking in a manner of “ABCABC”. The connection between the alkali content and the polymorph (oxygen stacking sequence) can be explained by the electrostatic interaction. Lower Na contents increase the electrostatic repulsion between oxygen layers, favoring the direct facing of oxygen layers (AA, BB) with larger interlayer distance in the case of P2 structure. Conversely, Na-rich layered oxides tend to crystallize into an O3 polymorph[23]. The Na+ content is not the only parameter to determine the polymorphs. Zhao et al. proposed a “cationic potential” rule to rationalize the prediction of P2 and O3 structures [Figure 2B][24]. According to the theory, a larger cationic potential indicated stronger TM electron cloud extension and interlayer electrostatic repulsion resulting in the P2-type structure, with more covalent TM-O bonds and an increased d(O-Na-O) distance. Alternatively, a larger mean Na ionic potential, achieved by increasing Na content, increased the shielding of the electrostatic repulsion between the TMO2 slabs, promoting the O3-type structure.

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Figure 2. Comparisons of O3 and P2 cathode materials from different aspects. (A) Crystal structure of P2 and O3 layered oxides; (B) Cationic potential rule to determine the formation of P2 and O3 structures. Reproduced with permission, Copyright 2020[24], American Association for the Advancement of Science; (C) Na diffusion pathways and barriers in P2 and O3 cathodes. Reproduced with permission, Copyright 2017[23], Wiley-VCH; (D) Structural evolutions of P2 and O3 cathodes under electrochemical Na extraction/insertion. Reproduced with permission, Copyright 2022[26], Wiley-VCH. Reproduced with permission, Copyright 2018[29], Wiley-VCH; (E) Air storage properties of typical P2 and O3 cathode materials. Reproduced with permission, Copyright 2018[35], American Chemical Society. Reproduced under terms of the CC-BY license, Copyright 2020[37], The Authors, Nature Publishing Group.

Compared to O3 structure, P2 is privileged for fast and reversible Na+ (de)intercalation. This is not only related to the expanded interlayer accommodating Na+ but, more importantly, the direct Na+ migration pathway in P2 structure. Na+ migrates from prismatic to neighboring sites through open square bottlenecks surrounded by four oxygen ions, which are much less repulsive than the interstitial tetrahedral sites in the O3 structure. Katcho et al. experimentally and theoretically compared the Na+ mobility in P2 and O3 structures with an identical composition of Na2/3Fe2/3Mn1/3O2[23]. The computed activation energies (Ea) are 127 and 201 meV for P2 and O3 phases, respectively [Figure 2C]. Correspondingly, the diffusion coefficients are 1.28 × 10-12 and 6.43 × 10-14 cm2 s-1. The more open framework of P2 phase not only enables a lower diffusion barrier but is also more easily preserved after sodium extraction[25], although a partial P2-O2/OP4 phase transition was reported[4,26-28]. Whereas the O3 phase always transforms to the P3 phase via gliding of the MeO2 slabs that results from the presence of intermediate tetrahedral sites between two octahedral sites [Figure 2D][29,30]. As a result, P2-type layered oxides generally show better rate performance and cyclic stability[31,32].

In addition, the higher Na content in O3 structures corresponds to stronger basicity, leading to much more severe hygroscopicity than not only P2 counterparts but even the well-known air-sensitive Ni-rich LIB cathodes[33,34]. For example, more carbonates were formed on a typical O3 cathode of NaNi1/3Fe1/3Mn1/3O2 in a much shorter time (4.64 wt.% after 1,000 h) compared with Li-NMC811 (0.39 wt.% after one year) [Figure 2E][35,36]. The consequences of generating sodium residues on the cathode are detrimental in many aspects. First, the spontaneous desodiation from the layered structure leads directly to the loss of active Na inventory. Second, the insulating sodium compounds on the surface block the sodium diffusion and electron pathway. Additionally, the basic sodium residues defluorinate polyvinylidene difluoride (PVDF) binders, damaging the electrode-coating process. Therefore, the high air-sensibility of O3 cathodes is one of the biggest challenges hindering their practical applications, which will dramatically increase the difficulty and cost in production, handling, transportation, storage, etc. Although the concern of air instability still exists in P2 cathodes, the problem is much less significant [Figure 2E] and P2 cathodes can be water-processable with the design of proper TM components[37-39].

CHALLENGES OF P2-TYPE OXIDES

Despite the instinct advantages of P2-type cathodes over O3 counterparts in terms of smaller Na+ diffusion barrier, better structural reversibility, and higher water stability, P2-type layered oxides still face some critical challenges that need to be solved prior to their practical application:

(1) Structural deformation. Although the more open crystal framework offers P2-type cathodes better structural stability, the nature of the layered structure still poses a risk of irreversible structural transformations due to slab gliding and lattice distortion upon the removal/insertion of Na+[40,41]. The structural stability of the P2 host relies significantly on the content of Na+, which plays a crucial role in mitigating electrostatic repulsions between the TMO2 slabs. Along with the desodiation in the charging process, the reduced shielding effect of Na+ triggers the slab gliding. This phenomenon leads to a structural transition from P-type to O-type stackings[42]. It was found that at deep desodiation (x ≈ 0.25), the cathode undergoes a P2-O2 and/or P2-OP4 phase transition with various Na+/vacancy ordering structures[43-45]. In the case of deep discharge to 1.5 V, an orthogonal phase was found to be produced and co-existed with P2 phase[46]. The repeated phase transition and separation processes can break down electrode particles by inducing numerous intragranular cracks due to the massive strain at phase boundaries[47-49], plaguing the electrochemical performance.

(2) Interfacial instability. An unstable electrode/electrolyte interface often interplays with the structural degradation, causing rapid electrochemical deterioration. First, the structural deformation is generally more severe at the surface of a cathode due to the higher state of charge and direct contact with the electrolyte. As a result, severe atomic rearrangement drives irreversible phase reconstruction, forming ion-blockingrock-salt/spinel phases. Furthermore, the chemical/electrochemical oxidation of the electrolyte produces acidic species, which attack the metal oxide cathode, causing TM dissolution and O loss. TM dissolution is a well-known aging mechanism in LIBs[50,51], which is serious for cathodes containing Jahn-Teller active TM ions, such as Mn3+, Ni3+, and Fe4+. At the same time, these ions are extensively involved in sodium layered oxides, including P2 ones.

(3) Insufficient ambient stability. P2 cathodes are less hygroscopic than O3 materials because of their lower basicity. This allows better ambient stability, especially in humid conditions. However, these cathodes still suffer undesirable side reactions upon prolonged ambient exposure. Apart from Na+/H+ exchange process being less severe than that of O3 materials, a prevailing explanation to the air instability of P2 cathodes is the large interlayers that allow the insertion of foreign species such as water[52] and carbonate groups[53]. Recently, Zuo et al. elaborated on the structural/chemical transformations of P2 cathodes in moist air[37]. They confirmed that Na+/H+ exchange dominated the compensation of Na+ extraction and hydration. At the same time, CO2 was found to increase the acidity at the particle surface and accelerate the Na+ loss rather than being inserted into the P2 structure. The authors further offered an empirical rule for estimating the air stability of P2 oxides, where the involvement of higher electrochemical-potential TM ions makes Na+ extraction difficult, leading to better air stability.

(4) Na deficiency. The Na deficiency in P2 oxides contributes to the open crystal framework and low hygroscopicity, but it also poses a challenge for their practical application. As the common anode materials (e.g., hard carbon) are Na-free, the insufficient Na inventory greatly limits the capacity and energy density of P2 cathode-based full batteries. This is further aggravated due to the consumption of active Na in forming passivating solid electrolyte interphase (SEI) layers on the anode. As a result, P2 cathodes rely on sodium compensation in assembling practical full cells, which can be realized through anode pre-sodiation[54,55], cathode oversodiation[56,57], introduction of self-sacrificial sodiation reagents[58-63], etc. Cost, safety, and effectiveness are important from the perspective of industrialization.

ADVANCES IN MODIFICATION STRATEGIES

P2-type layered oxides exhibit compelling advantages but encounter persistent constraints, as discussed above. To propel the advancement beyond current bottlenecks, researchers have proposed various modification strategies focusing on compositional design, elemental doping, phase mixing, morphological control, surface modification, and sodium compensation [Figure 3]. These efforts have yielded promising results with the potential to unlock the full potential of P2-type materials for high-performance SIBs.

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Figure 3. Key challenges and improvement strategies of P2-type layered cathode materials for SIBs.

Compositional design

Although Na+ can easily form layered structure with all the first-row TMs due to the significant size difference between Na+ and TM ions, only a few elements such as Co[64], Mn[65], and V[66] were demonstrated to be synthesized into P2 structure via solid-state reactions. This could be explained by the instability of their tetravalent states. Single TM P2-type oxides typically show low capacities and multiple plateaus due to complex Na+/vacancy ordering. For example, an in situ X-ray diffraction (XRD) experiment conducted by Berthelot et al. revealed nine potential drops separated by either potential plateaus or sloping curves in the Na content range of 0.45 ≤ x ≤ 0.90 for P2-NaxCoO2, corresponding to nine ordered phases[67]. P2-NaxMnO2 was reported to have a high specific capacity (ca. 140 mAh g-1)[68]. However, strains and distortions caused by Jahn-Teller active Mn3+ rapidly destroyed the crystal framework, leading to severe capacity drop.

In the effort to remove multiple plateaus and improve the average voltage and capacity, P2-type layered oxides have evolved to binary and multiple TM systems[69]. For example, Voronina et al. synthesized Na0.6Co0.78Ru0.22O2 using a combustion method [Figure 4A], and the specific capacity of the first discharge reached 163 mAh g-1 [Figure 4B] with no Na+/vacancy ordering superstructures detected within the P2 phase, followed by a P2-OP4 phase transition under deep desodiation[70]. The increased capacity was attributed to additional Ru4+/Ru5+ and O2-/(O2n-) redox reactions. However, Co and Ru are both costly and scarce, undermining the sustainability benefits of SIBs. In this regard, Co-free or low-Co formulas have gained much interest[71], leading to a range of representative binary/ternary P2-phase compositions such as Na2/3Ni1/3Mn2/3O2[72-74], Na2/3Fe1/2Mn1/2O2[4], Na2/3Mn1/2Fe1/4Co1/4O2[75], and Na2/3MnxNix-1/3Co4/3-2xO2[76]. Lee et al. found that P2 phase can stably exist with the sodium content range of from 1/3 to 2/3 for P2-phase NaxNi1/3Mn2/3O2, which transferred to a more stable O2 phase when x approaches 0[27]. However, the cut-off voltage had to be set at 4.1 V (vs. Na+/Na) to ensure stable cycling, delivering a reversible capacity below 90 mAh g-1.Lu et al. detailed the structural evolution of P2-Na2/3Ni1/3Mn2/3O2 using in situ XRD[43]. It was found that P2-O2 phase transition occurred at a Na content of less than 1/3 (at a voltage of about 4.1 V), which was accompanied by a large volume change (about 23%). This answered the rapid capacity decay. The high-voltage operation of P2-NaxNi1/3Mn2/3O2 can be stabilized by metal substitutions. Liu et al. suggested that phase transition was controlled by the amount and distribution of Na ions retained in the lattice[77]. As shown in Figure 4C, a P2-Na2/3Mn1/2Ni1/6Co1/3O2 was synthesized from Na2/3Mn2/3Ni1/3O2 and Na2/3Mn1/3Co2/3O2. The ternary P2 structure was found to remain stable with 0.17 Na ions, resulting in an ultra-low strain of 1.9% in volume, guaranteeing excellent cyclability [Figure 4D]. Moreover, the authors demonstrated the universality of this strategy, where P2-Na2/3Mn1/2Ni1/6Fe1/3O2 and P2-Na2/3Mn7/12Ni1/4Fe1/6O2 were also synthesized and did not transfer to O2 or OP4 phase when charging to 4.4 V [Figure 4E]. The involvement of non-active metals such as Ti, Mg, Zn, Al, and Li was also proven to alleviate structural evolutions[78-81]. For example, Na2/3Ni1/3Mn1/2Ti1/6O2 permitted a much higher reversible capacity of 127 mA h g-1 within the voltage range of 4.5-2.5 V at a current density of 12.1 mA g-1[81]. Recently, Zou et al. reported a zero-strain (0.53% volume change) P2-Na2/3Li1/6Co1/6Mn2/3O2 cathode, which showed an excellent capacity retention of 95.8% after 250 cycles at 1 C[82]. The inactive monovalent Li+ and active trivalent Co3+ were suggested to help stabilize the Mn-O bond by reducing Mn3+/Mn4+ redox. Liu et al. proposed the co-substitution of inactive Mg and Ti elements to activate anionic redox reaction while maintaining excellent structural stability with a minor volume change of only 0.3%[83].

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Figure 4. (A) Schematic illustration of structure, phase transitions, and corresponding reaction mechanisms for P2-Na0.67CoO2 and P2-Na0.6Co0.78Ru0.22O2; (B) Electrochemical performance of P2-Na0.67CoO2 and P2-Na0.6Co0.78Ru0.22O2. Reproduced with permission, Copyright 2023[70], Wiley-VCH; (C) Schematic illustration of the synthesis of P2-Na2/3Mn1/2Ni1/6Co1/3O2; (D) Cycling stability of P2-Na2/3Mn1/2Ni1/6Co1/3O2; (E) Schematic illustration of the phase transitions for P2-Na2/3MnxNix−1/3Co4/3−2xO2 (1/3 ≤ x ≤ 2/3). Reproduced with permission, Copyright 2021[77], Wiley-VCH; (F) Voltage profiles and electrochemical performance of Ti-substituted and Ti-Zn-substituted P2 cathodes; (G) Selected operando XRD patterns and schematic illustrations of structural change mechanisms for these cathodes. Reproduced with permission, Copyright 2023[86], Wiley-VCH.

Further multi-elementalization leads to the creation of high-entropy (HE) layered oxides, which have attracted increasing attention in recent studies[84]. The entropy effect that promotes the metal disordering in the TM slabs greatly alleviates the phase segregation, permitting sloping voltage curves and excellent electrochemical properties. Wang et al. revealed the entropy effect on the electrochemical stability by comparing three different P2 cathodes: Na0.67Mn0.55Ni0.21Co0.24O2, Na0.67Mn0.45Ni0.18Co0.24Ti0.1Mg0.03O2, and Na0.67Mn0.45Ni0.18Co0.18Ti0.1Mg0.03Al0.04Fe0.02O2 with low, medium, and high configurational entropy, respectively[85]. The HE one showed less structural transformation and Mn dissolution upon cycling in an expanded voltage range (1.5-4.6 V). Similarly, Cheng et al. reported a multiple metal-substituted P2-Na0.7Li0.03Mg0.03Ni0.27Mn0.6Ti0.07O2, which delivered a reversible capacity (134 mAh g-1), a high working voltage (3.57 V), and excellent cycling and rate performance[26]. However, without a precise understanding of the atomic arrangements and functionalities, the involvement of multiple elements could increase the unpredictability of the electrode materials. Kubota et al. challenged the necessity of HE by proposing a selective dual-substitution of Ti4+ for Mn4+ and Zn2+ (Mg2+, Cu2+) for Ni2+ in P2-Na2/3Ni1/3Mn2/3O2[86]. As presented in Figure 4F, the Ti-sub cathode still exhibited stepped voltage curves, whereas the further dual-substitution with divalent metals effectively smoothed the voltage curves. Correspondingly, in situ XRD analysis [Figure 4G] revealed suppressed phase separation from pristine P2 cathode to Ti-substituted cathode and almost monotonous solid solution for dual-substituted Na2/3Ni1/4Mn1/2Ti1/6Zn1/12O2. It should be noted that the high-proportional metal substitution also reduces the specific capacities of P2 cathodes. In short, compositional design normally involves various elements that have rather high concentrations, which not only greatly affect both the crystal/electronic structure of the material and, hence, the reaction mechanism, but the selection of the elementals also needs to consider the cost efficiency and sustainability, since the high usage of costly elementals such as Co and Ru will sacrifice the key advantage of SIBs.

Elemental doping

The substitution of high-content active or inactive elementals in the compositional design can effectively reduce Na+ vacancy ordering arrangement, contributing to the improved electrochemical performance. However, heavy metal substitution also renders considerable capacity and/or voltage sacrifice. Also, the involvement of high-content cations such as Co3+, Fe3+, and Ti4+ is believed to undermine the moisture stability of P2 oxides[37]. Therefore, it is compelling to lower the concentration of substitutive ions. Generally, the introduction of elementals with less than approximately 5 at.% can be deemed elemental doping[87].

Cation dopants can substitute both Na and TM ions, playing different roles during charge and discharge. For example, Zhang et al.[88] doped P2-Na0.67MnO2 with Mo-doped to avoid the phase transition and Jahn-Teller effect of Mn3+. The resultant P2-Na0.67Mn0.97Mo0.03O2 exhibited a high specific capacity of 169.7 mAh g-1 and improved cycling stability with 84% capacity retention after 100 cycles at 50 mA g-1. Wang et al. investigated the effect of K+ doping in P2-Na0.612K0.056MnO2[89]. The large-sized K+ ions were preferably placed in the prismatic Na+ sites sharing edges with MnO6 octahedra, enlarging the Na-O-Na interlayer space while reducing the distances of adjacent MnO2 slabs [Figure 5A]. P2-Na0.612K0.056MnO2 exhibited a capacity of 240.5 mAh g-1 based on the high utilization of Mn3+/Mn4+ redox, retaining 98.2% capacity after 100 cycles [Figure 5B]. Detailed structural analyses revealed that the reinforced interaction in the MnO2 layers restricted the slab gliding, suppressing the OP4 phase generation even at high desodiated state [Figure 5C].

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Figure 5. (A) XRD pattern of Na0.612K0.056MnO2 with Rietveld refinement and the corresponding structural model; (B) Typical charge/discharge curves of Na0.612K0.056MnO2 and Na0.706MnO2 at 20 mA g-1 in the third cycle and their cycling performance at 50 mA g−1; (C) In situ XRD patterns of Na0.612K0.056MnO2 and schematic diagram of the phase transitions. Reproduced under terms of the CC-BY license, Copyright 2021[89], The Authors, Nature Publishing Group; (D) Computational and experiential investigations of Na0.76Ca0.05[Ni0.23□0.08Mn0.69]O2; (E) Electrochemical properties of P2 cathodes doped with different Ca2+ concentrations; (F) Structural evolution of P2-Na0.76Ca0.05[Ni0.23□0.08Mn0.69]O2 during charge and discharge. Reproduced with permission, Copyright 2021[91], Wiley-VCH; (G) Structural model and XRD pattern of P2-Na0.67Ni0.3Co0.1Mn0.6O1.94(BO3)0.02; (H) Cycling performance of pristine and BO33--modified P2 cathodes; (I) Structural evolution of BO33--modified P2 cathode. Reproduced with permission, Copyright 2022[99], Wiley-VCH; (J) A comparison of the structural evolution between undoped and HE-doped P2 cathodes, Copyright 2023[103], Elsevier B.V.

Elemental doping is also an important strategy to unlock the anionic redox. The extra capacity enabled by anionic redox reaction also remakes an opportunity for SIBs to compete with LIBs in terms of energy density. Ma et al. first identified the oxygen activity of P2-Type Na0.78Ni0.23Mn0.69O2[90]. Unlike the Li-rich cathode materials known for anionic redox reaction, the introduction of extra sodium in TM layers is not feasible due to the mismatch. Shen et al. synthesized a P2-Na0.76Ca0.05[Ni0.23□0.08Mn0.69]O2 cathode material [Figure 5D][91]. The pillar effect of Ca2+ ions in Na layers enhanced anionic redox reversibility upon charge/discharge [Figure 5E]. Correspondingly, P2-O2 phase transition was much suppressed [Figure 5F]. Fu et al. proposed a dual-site doping of P2-type Na0.67Ni0.33Mn0.67O2, where Mg2+ replaced Ni2+ in the TM layers and Ca2+ tended to pillar the Na layers[92]. A synergetic effect was discovered in co-doped P2-Na0.62Ca0.025Ni0.28Mg0.05Mn0.67O2, which showed suppressed oxygen redox and Ni4+/Ni3+ redox reactions at the high-voltage region, permitting enhanced structural stability. Despite the surge in research on anionic redox, formidable challenges such as rapid material degradation, voltage fade, and oxygen release pose significant barriers to practical implementation of anionic redox materials[93,94]. New mechanistic understandings and novel strategies progress rapidly in this area. For example, regulating anionic redox through the combination of metal ligands in layered sulfides[95], stabilizing lattice oxygen redox through spin singlet state[96], and inhibiting excess lattice oxygen oxidation through the comprehensive strengthening of the TM-O bond by the introduction of Al3+[97] have effectively improved the rechargeability of the anionic redox.

Doping species are not limited to cations; anion dopants such as F- and BO33- have also been used to overcome the drawbacks of P2 cathodes. Liu et al. investigated a charge compensation mechanism of F- doping in P2-Na2/3Ni1/3Mn2/3O2[98]. F-substitution triggered partial reduction of Mn from Mn4+ to Mn3+, which perturbed the Ni2+/Mn4+ cation ordering and activated the Mn3+/Mn4+ redox for additional capacities. Wang et al. incorporated triangular BO33- into the P2 structure to form Na0.67Ni0.3Co0.1Mn0.6O1.94(BO3)0.02 [Figure 5G], which enabled a capacity retention of 80.1% after 1,000 cycles at 2 C [Figure 5H][99]. Structural characterizations confirmed that configuration acted as a strong pillar to address the TM layer sliding and H2O insertion issues, guaranteeing reduced volume change (1.8%) and facilitating Na+ transportation [Figure 5I]. Anion dopants are often introduced together with cation dopants, such as Al3+/F-[100], Ca2+/F-[101], and Mg2+/F-[102]. For instance, Cui et al. reported ex-situ F and in-situ Mg dual doping method to obtain P2-Na0.524Mg0.146Ni0.15Fe0.20Mn0.65F0.05O1.95[102]. F doping modified the bond energy of O-O and TM-O, allowing the reduction of interatomic distance of TM and the expansion of the interlayer spacing. As a result, capacity fading caused by structural deformation and Jahn-Teller effect was mitigated. Meanwhile, electrochemically introduced Mg-Mg dimers could act as pillars to widen the interlayer gaps and enhance the mechanical strength, further inhibiting phase transition and layer sliding.

Co-doping of multiple ions also gained research interest due to the potential of the synergistic effect of multi-elements. Ma et al. proposed HE doping strategy with five equal contents (1 at.%) of cations to achieve a novel Na0.667Mn0.667Ni0.167Co0.117Ti0.01Mg0.01Cu0.01Mo0.01Nb0.01O2 (HE-NMNC)[103]. Instead of a sudden P2-O2 transition, the HE-NMNC showed a gentle and reversible phase variation in a wider voltage region [Figure 5J], delivering a capacity of 111 mAh g-1 at 5 C and retaining around 130 mAh g-1 after 100 cycles at 1 C.

Employing minor components to fundamentally modify the electrochemical properties of P2 oxides presents a very attractive and feasible strategy for commercialization, which is also widely used for the LIB industry. However, it is crucial to acknowledge that elemental doping can have both positive and negative effects. While it may enhance cycling stability, it might also compromise capacity. Therefore, a comprehensive evaluation and mechanistic understanding of both advantages and limitations is essential to identify the optimal doping strategy.

Phase mixing

Designing mixing-phase cathode materials is a promising approach to addressing some of the limitations of single-phase materials and achieving enhanced performance. The intergrowth of thermodynamically stable P2 and O3 phases is the most investigated mixing phase system. As discussed above, O3 phase has higher Na content and high specific capacity, which can be introduced to advance P2 for better performance and practicality.

Lee et al. first reported the layered O3/P2 intergrowth cathode in Li-substituted Na1-xLixNi0.5Mn0.5O2, where x is the Li content[104]. As Li content rose from 0 to 0.5, the P2 phase fraction increased, while the O3 phase fraction decreased. This resulted in a significant improvement in both capacity and rate capability. However, the O3-P3 phase transition could not be completely avoided due to a large amount of the O3 phase. This resulted in a significant capacity loss within 20 cycles. In contrast, Guo et al. introduced minor O3 phases into the P2 host material[105]. The P2-majority Na0.66Li0.18Mn0.71Ni0.21Co0.08O2 exhibited a remarkable increase in capacity (200 compared to 125 mAh g-1 for the single P2 phase). Additionally, the composite demonstrated superior rate capability and impressive cycling stability across a wide voltage range of 1.5-4.5 V (75% capacity retention after 150 cycles). Recently, Huang et al. demonstrated the construction of Li-enriched O3 nanodomains in the P2 phase matrix (P2/Li-O3, Figure 6A), which was designed to overcome the interlayer gliding and to suppress the multiple-phase transitions in P2-Na0.67Mn0.67Ni0.33O2[Figure 6B][106]. As can be seen, Li substitution is responsible for generating biphasic materials, where Li mainly substitutes the TM ions, maintaining the P2 structure, while some Li ions occupy Na sites to tune the Na/TM ratio, generating O3 phase[107].

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Figure 6. (A) High angle annular dark field scanning transmission electron microscope (HAADF-STEM) images of P2/Li-O3 sample; (B) Structural evolution of P2/Li-O3 during the first charge/discharge process in comparison with pure P2 cathode. Reproduced with permission, Copyright 2022[106], Elsevier B.V.; (C) Normalized composition of Ni, Fe, and Mn in selected particles of P2/O3 and P2 materials; (D) Calculated cationic potentials and formation energies rationalizing the structure-composition relationship; (E) In situ XRD revealing the structural evolution of P2/O3 cathode; (F) Electrochemical performance of O3, P2, and P2/O3 cathodes. Reproduced with permission, Copyright 2022[114], Elsevier B.V.; (G) Schematic illustration of structural evolution driven by Mg substitution contents in Na0.6Mn1-xMgxO2 (x = 0.05, 0.1); (H) Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution (HR-TEM) images with corresponding fast Fourier transform (FFT) electron diffraction (ED) patterns of P2/tunnel Na0.6Mn0.95Mg0.05O2; (I) Long-cycling stability of the P2/tunnel cathode; (J) Structural evolution of the P2/tunnel cathode revealed in situ XRD patterns. Reproduced with permission, Copyright 2023[120], Wiley-VCH; (K) Schematic diagram of the formation conditions of mixed-phase materials; (L) Electrochemical performance of the P3/P2/O3 tri-phase cathode compared to biphasic P3/P2 and P2/O3 materials. Reproduced with permission, Copyright 2022[124], Wiley-VCH.

Beside Li substitution, the introduction of Mg2+[108], Ti4+[109], or multiple ions[110-115] was also demonstrated to enable P2/O3 intergrowth. For example, Liang et al. reported a Fe-Mg-Li co-substitution to prepare P2/O3 Na7/9Ni2/9Mn4/9Fe1/9Mg1/9Li1/9O2[111]. Compared to pure P2 phase, the heterostructure showed suppressed P2-O2 phase evolution at high potential and much less microcrack generation over cycling. Gao et al. investigated the formation mechanisms of biphasic layered oxides from the perspective of cationic potential theory, where the existence of P2/O3 biphasic structure was experimentally and theoretically non-uniform element distribution within a micrometer scale [Figure 6C and Figure 6D][114]. With a reversible P2-Z/O3-P3-O3′ phase transition [Figure 6E], the biphasic Na0.7Ni0.2Cu0.1Fe0.2Mn0.5O2-δoverperformed P2 and O3-type analogs in terms of rate capability and cycling stability [Figure 6F].

Apart from P2/O3 heterostructure, recent research progress has witnessed the creation of other mixed-phase systems, such as layered P2/P3/spinel[116], P2/tunnel[117-120], P2/O'3[9], P2/T[121], P2/O2[122], P2/spinel[123], and P3/P2/O3[124]. By adjusting magnesium content in Na0.6Mn1-xMgxO2, Xiao et al. realized the control of phase structure from P2/tunnel structure to P2 structure [Figure 6G][120]. Na0.6Mn0.95Mg0.05O2, with a reasonable structure component ratio (92.6% P2 and 7.4% tunnel, Figure 6H), displayed an excellent capacity retention of 71.3% after 1,000 cycles at 5 C [Figure 6I]. A gentle structural evolution was identified [Figure 6J]. Li et al. introduced an undoped method to prepare P3/P2/O3-Na0.674Ni0.319Mn0.590O2 by varying the calcination temperature and gas atmosphere [Figure 6K][124]. The tri-phase material showed improved electrochemical performance compared to biphasic P2/O3 and P2/P3 cathodes [Figure 6L]. The enhanced structural stability was explained by a structural constraint effect, where the grain boundaries suppressed the continuous slip of the TMO2 layer [Figure 6M].

The design of multiple-phase systems creates more opportunities for developing high-performance P2-related cathode materials by combining the attractive features of other phases and even achieving unexpected synergistic effects of mixed-phase structures. However, there are also associated challenges of mixing phases, such as phase segregation, possible antagonistic effects, and increased complexity, which need to be carefully explored.

Morphological control

A central challenge in current LIBs lies in the massive, often several percent, expansive strain induced by Li intercalation within the crystal structure. Materials prone to abrupt phase transitions further concentrate this strain, amplifying the lattice mismatch at the sharp interface between lithiated and delithiated phases. The interface can not only impede ion transfer but also serve as nucleation points of microcracks, given the inherent hardness and brittleness of most metal oxide battery materials[48,49,51,125]. In view of the much larger ionic size of Na+, the kinetic and stability issues become much more serious. Morphology optimization, such as the designs of nanostructures[126-128], hierarchical structures[129-133], core-shell/concentration-gradient configurations[134-138], single crystals[139], and selective faceting[140,141], has been proved to be effective in combating these problems.

Shen et al. synthesized a P2-Na0.76Cu0.22Fe0.30Mn0.48O2 cathode with a pearl necklace-like hierarchical nanostructure through electrospinning[127]. As suggested by the authors, the nano-necklace architecture involving very small secondary nanograins (50-150 nm) could expose rich active sites to electrolyte, thus shortening the ionic diffusion distance and alleviating the aggregation of nanoparticles upon repeated (de)sodiation [Figure 7A-C]. The importance of this nanostructure was demonstrated by the sensitivity of electrochemical performance toward the synthesis temperature [Figure 7D]. Peng et al. reported a hierarchical one-dimensional rod-like P2-Na0.67Ni0.23Mg0.1Mn0.67O2 composed of nanoplate subunits [Figure 7E-G][130]. Such morphology was found to be preserved after cycling, while many cracks were presented in the common monodisperse particle [Figure 7H].

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Figure 7. (A) SEM images of electrospun nanofibers; (B and C) SEM and TEM images of resultant Na0.76Cu0.22Fe0.30Mn0.48O2nano-necklaces; (D) Electrochemical performance of Na0.76Cu0.22Fe0.30Mn0.48O2 calcinated at different conditions. Reproduced under terms of the CC-BY license, Copyright 2020[127], The Authors, Wiley-VCH; (E-G) SEM, TEM, and elemental mappings of hierarchical P2-Na0.67Ni0.23Mg0.1Mn0.67O2; (H) SEM images of cycled P2 cathodes with hierarchical and bulk morphologies. Reproduced with permission, Copyright 2021[130], Elsevier B.V.; (I) Schematic illustration of the water-mediated strategy to prepare S-NMO; (J) Electrochemical properties of S-NMO and bulk P2-Na0.67MnO2. Reproduced under terms of the CC-BY license, Copyright 2021[143], The Authors, Nature Publishing Group. (K-M) SEM images of P2-Na0.66Ni0.26Zn0.07Mn0.67O2 materials prepared by solid-state, coprecipitation, and combined coprecipitation and molten-salt methods; (N-Q) TEM, HR-TEM, ED pattern, and elemental mappings of the hexagonal single crystal; (R) Schematic illustration comparing the structural stability of primary particles with different proportions of {001} planes. Reproduced with permission, Copyright 2023[140], Elsevier B.V.

The design and application of nanostructures can both reduce the size of the electrode material, increase the electrode/electrolyte contact, and provide shorter transfer paths for off/electrons. However, nanostructured electrode materials encounter significant difficulties in practical applications. First, nanomaterials in their as-synthesized form are considerably low in the mass densities, translating into unsatisfactory volumetric energy densities. Second, the high surface area and porosity of nanostructured materials tend to stimulate the parasitic reactions with electrolytes, spoiling battery lifetime[142]. Therefore, research efforts have been devoted to the design and engineering of micron-sized single-crystal cathodes. Zuo et al.[143] developed a water-mediated method to convert bulk P2-Na0.67MnO2 into shale-like NaxMnO2 (S-NMO) with enlarged interlayer spacing [Figure 7I]. S-NMO exhibited only 1.96% volume change during charge/discharge, resulting in much improved electrochemical performance [Figure 7J]. Moreover, S-NMO showed excellent humidity tolerance, and the feasibility of the water-mediated strategy was further demonstrated by extending to other sodium layered oxides. Zhang et al. synthesized hexagonal-prism-like single-crystal P2-Na0.66Ni0.26Zn0.07Mn0.67O2 featuring a high proportion of {001} facets [Figure 7K-Q] using a molten-salt-based method[140]. Such single-crystal morphology was suggested to reduce the gliding of TMO2 slabs [Figure 7R]. This helped suppress the microcracks and associated electrolyte decomposition, guaranteeing much better electrochemical stability. Facet engineering can be coupled with other strategies. For example, Fu et al. combined crystal-facet modulation with an HE design[141]. The optimized P2 cathode exhibited outstanding cycling performance (87% capacity retention after 500 cycles at 120 mA g-1 and 75% after 2,000 cycles at 1.2 Ag-1).

Surface modification

High interfacial stability of the electrode surface is a prerequisite for realizing long-lasting secondary batteries[144]. Due to the direct electrode-electrolyte contract and normally high states of charge at the cathode surface, P2 layered cathodes undergo significant surface reactivity and a sequential transformation of the surface structure during charging and discharging especially under high-voltage operation, leading to severe surface degradation[145]. As an effective strategy, surface modification can effectively shield the direct contact between P2 cathode materials and the electrolyte, thus reducing the side reactions between the electrode material and the electrolyte and inhibiting the TM dissolution[41,146]. In addition, it can protect the P2 cathode from contacting carbon dioxide and water from the air to realize easier materials storage and processing[41].

With the chemical affinity, metal oxides are the most investigated coating species for sodium layered oxides including P2 ones[147,148]. For instance, Ren et al. coated P2-Na2/3Ni1/3Mn2/3O2 with ZrO2, where Zr4+ partially doped into the layered (P2-NaNM@Zr) [Figure 8A and B][149]. The ZrO2 coating layer was proven to prevent cathode-electrolyte interfacial reactions, suppressing electrolyte decomposition and the accumulation of associated byproducts. Meanwhile, Zr4+ modification was suggested to add “impurity-vibrational entropy” to host structure, which reduced P2-O2 phase transition and endorsed a zero-strain (1.18%) feature even when charged to 4.5 V. As a result, P2-NaNM@Zr cathode retained 77% of its capacity after 200 cycles at the rate of 5 C within the voltage range of 2.0-4.5 V [Figure 8C]. Similarly, Wan et al. modified Mg2+-doped P2-Na0.67Ni0.33Mn0.67O2 with a ZrO2 surface layer[150]. ZrO2 coating was evidenced to inhibit the side reactions at high voltages, contributing to better electrochemical stability. Besides, Xia et al. proposed a dielectric CaTiO3 coating on P2-Na2/3Ni1/3Mn2/3O2 that also featured site-selective Ca/Ti co-doping, leading to a synergy in suppressing dynamic oxygen evolution [Figure 8D and E][151]. It was confirmed that the coating layer inhibited not only the surface oxygen release but also the migration of bulk oxygen due to the reverse electric field created by the dielectric polarization. Correspondingly, the reversibility of anion redox reaction was largely improved as evidenced by voltage profiles and cycling stability [Figure 8F].

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Figure 8. (A) Schematic illustration of the Zr modification process and effects on a P2 cathode; (B) TEM characterizations of the Zr-modified P2 cathode; (C) Rate and cycling performance of P2-NaNM@Zr in comparison with the bare P2-Na2/3Ni1/3Mn2/3O2. Reproduced with permission, Copyright 2022[149], Elsevier B.V.; (D) Schematic diagram of the oxygen loss suppressed by simultaneous dielectric surface coating and site-selective co-doping; (E) SEM and TEM characterizations of CaTiO3-modified P2-Na2/3Ni1/3Mn2/3O2; (F) Voltage profiles and cycling performance of bare and CaTiO3-modified P2-Na2/3Ni1/3Mn2/3O2. Reproduced with permission, Copyright 2023[151], Wiley-VCH; (G) Schematic illustration of the lab ALD facility; (H) Simulated diagram and characterizations of the ALD coating of Al2O3 on the surface of Na2/3Ni1/3Mn2/3O2; (I) cycling performance of unmodified and Al2O3-coated Na2/3Ni1/3Mn2/3O2 cathodes. Reproduced with permission, Copyright 2022[156], Wiley-VCH; (J) Schematic illustration of alucone coating introduced by MLD process on the surface of P2 oxides; (K) Nyquist spectra and charge-discharge curves of uncoated, Al2O3-coated, and alucone-coated P2-Na0.66Mn0.9Mg0.1O2; (L) Cycling and rate performance of these P2 cathodes. Reproduced with permission, Copyright 2020[158], Wiley-VCH.

In general, the thickness, type, and uniformity of the coating layers are crucial in affecting the performance of the electrode materials. If the coating layer is too thick and/or unevenly/loosely wrapped, it can be detrimental to the ionic and electronic conductivity and/or insufficient in offering protection[152]. In response, atomic layer deposition (ALD) with Å-level control has attracted much attention for the deposition of uniform and conformal metal-oxide coatings on electrode surfaces[153-155]. Ji et al. found that the failure at the cathode-electrolyte interface was the dominant cause of capacity attenuation for P3/P2 biphasic Na2/3Ni1/3Mn2/3O2[156]. To solve this problem, they explored ALD coating with four different metal oxides, including Al2O3, TiO2, SnO2, and WO3. Figure 8G shows the schematic illustration of the ALD facility. The deposition process and outcome of Al2O3 on the surface of the Na2/3Ni1/3Mn2/3O2 was theoretically and experimentally investigated [Figure 8H]. The Al2O3 protection dramatically improved the cycle life of the cathode, permitting a capacity retention of 87.00% after 300 cycles at 5 C within a 1.5-4.1 V charge-discharge window [Figure 8I]. Zuo et al. reported a Al2O3@Na0.67Zn0.1Mn0.9O2 electrode by taking the advantages of both structural stabilization and surface passivation via Zn2+ substitution and Al2O3 ALD coating[157]. The ALD coating layer effectively alleviated the electrolyte decomposition and significantly improved the cycling performance of the P2 cathode. Similar to ALD, molecular layer deposition (MLD) can also deposit ultra-thin polymer or organometallic coatings while maintaining precise control over coating thickness and consistency[158]. Kaliyappan et al. first demonstrated alucone coating of P2-Na0.66Mn0.9Mg0.1O2 by the MLD method [Figure 8J][158]. The alucone-coated P2 cathode showed smaller charge transfer resistance and lower polarization than both uncoated and Al2O3-coated P2 counterparts [Figure 8K], corresponding to higher rate performance and longer cycling stability [Figure 8L].

Apart from the metal oxide coatings, other species such as carbon[159], metal fluorides[160], polyanionic compounds[161-165], and conductive polymers[166,167] have been extensively investigated to improve the surface/interface stability of P2 layered oxides. Surface/interface stability plays a pivotal role in determining the performance and stability of SIBs. At its core, surface modification aims to minimize harmful side reactions between the cathode and electrolyte, thereby boosting interface stability. Moreover, depending on the chosen coating species and surface treatment methods, surface modification can facilitate Na+ diffusion, suppress detrimental phase transitions, and ultimately enhance the overall electrochemical performance. Despite its vast potential, surface modification faces challenges. For example, ex situ synthesis methods often struggle to achieve uniform coatings, compromising the effectiveness of the coating layer. Additionally, precise control of the coating thickness is crucial. While thicker oxide coatings provide enhanced stability, their inherently low electron conductivity can rapidly hinder performance if excessive. Striking this balance remains a bottleneck for large-scale implementation of this promising technology.

Sodium compensation

Most P2-type materials have low Na stoichiometry, which contributes to electrochemical and ambient stability but poses a problem to significantly reduce the accessible capacity in practical cells without additional active Na+ sources. When tested in half cell, Na-deficient cathodes present abnormal initial coulombic efficiencies (ICEs) of higher than 100% due to the discharge back of more Na ions offered by the Na metal anode. However, there is generally no Na inventory from the practical anode. This issue was deteriorated by the further trapping and/or consumption of the limited Na+ inventory in forming electrolyte-electrode interfaces, restricting the capacities and cyclability of the resultant cells[168-170]. The development of Na-rich P2-type oxides can partially relieve this problem[171-175], but critical limitations of Na content in P2 oxides persist, and it could sacrifice the air stability of P2 compounds. Therefore, it is compelling to develop feasible sodium compensation strategies useful for not only P2 oxides but also many other Na-stoichiometric materials[63,176,177].

Various methods such as electrochemical pre-sodiation[178], direct sodium foil contact[179], anode pre-sodiation[55,180], cathode oversodiation[56,57], self-sacrificial sodiation additives[58-63] have been explored to solve the Na+ inventory problem for different types of SIB cathode chemistries. Among them, the easy introduction of sacrificial reagents has gained enormous attention from the perspective of industrialization. Ideally, suitable sodium replenishers should feature high sodiation capacity, non-toxicity, moderate decomposition potential, and easy handling and synthesis. In 2013, Singh et al. proposed using NaN3 as a sodium replenishing agent, which decomposed into compensating Na source and gaseous N2 during initial charge[60]. This was also explored by Martinez De Ilarduya et al.[62]. The addition of 10 wt.% NaN3 to P2-Na0.67Fe0.5Mn0.5O2 allowed a 60% increase in reversible capacity. However, the following studies found that NaN3 decomposition caused porosity on the electrode surface. Moreover, in the selection of additives, not only the efficiency but also the sustainability must be considered. NaN3 is highly toxic and explosive, hindering its practical application. Niu et al.[63] then proposed an efficient cathodic sodium supplement agent, Na2C2O4 [Figure 9A], which had a high theoretical capacity of 400 mAh g-1 and could achieve a high capacity utilization of 99% [Figure 9B], which outperformed Na2CO3[181], Na2NiO2[57], and Na2C4O4[182] with ratios of 20%, 22%, and 70%, respectively. Furthermore, the oxidation overpotential (2.44 V) of NaC2O4 could be reduced using a conductive additive with a high specific surface area. The sodiation additive boosted the capacity retention of hard carbon||P2-Na2/3Ni1/3Mn1/3Ti1/3O2 full cell from 63% to 85% (200 cycles) and the energy density from 129.2 to 172.6 Wh kg-1. In addition, other sodium replenishing agents have been investigated, such as NaCrO2[61], Na4C6O6[183], Na2C6O6[184], Na2C6H2O6[176], EDTA-4Na[185] and DTPA-5Na[186], etc., although not all of them were applied to P2 materials.

Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries

Figure 9. (A) Structural characteristics of Na2C2O4; (B) Electrochemical properties of Na2C2O4 and its application as a cathode sodium compensator. Reproduced with permission, Copyright 2020[63], Wiley-VCH; (C) Schematic illustration of the slurry spraying method to deposit Na2O2 sacrificial layer on the P2 electrode film and corresponding characterization of the resultant composite electrode; (D) Electrochemical properties of Na2O2 and its use as a cathode sodium compensator. Reproduced with permission, Copyright 2021[187], American Chemical Society; (E) Schematic illustration of the treatment of P2-Na0.85Li0.12Ni0.22Mn0.66O2 with acetic acid and the role of the as-formed AC-Na as sodium reservoir. Reproduced with permission, Copyright 2021[188], American Chemical Society; (F) Schematic illustration of the preparation of Na3PS3O and Na3PS3O-cast P2-Na0.66Ni0.26Zn0.07Mn0.67O2 electrode; (G) Electrochemical properties of Na3PS3O and its application as a sodium sacrificing additive. Reproduced with permission, Copyright 2022[189], Elsevier B.V.

Guo et al. proposed spraying Na2O2 slurry on the surface of P2-Na2/3Ni1/3Mn1/3Ti1/3O2 electrode as a sodium supplementary method [Figure 9C][187]. Na2O2 showed good chemical and storage stability under a dry atmosphere and had no negative effect on the electrochemical performance of the cathode. Compared to common blending process, this spraying method was suggested to avoid the incomplete decomposition of additives and the generation of porous electrodes due to gas release. As a result, the capacity, rate capability, and cycling stability of the hard carbon||P2-Na2/3Ni1/3Mn1/3Ti1/3O2 full cell were considerably improved [Figure 9D]. While all the above-mentioned approaches require external Na sources, Zhang et al. reported the utilization of the residual alkali compounds of P2-Na0.85Li0.12Ni0.22Mn0.66O2 by an acetic-acid treatment, which neutralized the surface alkali substance into sodium acetate (AC-Na) [Figure 9E][188]. This strategy transformed the detrimental substances into active Na reservoirs. The energy density of the cell optimized in this way was increased from 112 to 130 Wh kg-1, and the soft-pack battery was also able to achieve capacity retention of 95.1% over 120 cycles.

Note that many additives release gases upon the supplement of active Na+. This easily causes increase in the internal pressure and problematic inner contact. The development of gas-free cathode additives with high sodium availability is attractive for practical application. Liao et al. introduced Na3PS3O in a P2-Na0.66Ni0.26Zn0.07Mn0.67O2 electrode using the solution casting method [Figure 9F][189]. Na3PS3O offered a sodiation capacity of over 300 mAh g-1 (60% capacity delivered below 4.0 V). More importantly, the decomposition products did not involve gaseous species, and the solid products were even found to be beneficial in suppressing electrolyte decomposition and oxygen release from the P2 cathode. The addition of Na3PS3O improved the energy density of the hard carbon||P2-Na0.66Ni0.26Zn0.07Mn0.67O2 cell by 29.7% [Figure 9G]. The metrics (theoretical capacity, practical capacity, and decomposition potentials) of these typical pre-sodiation additives are summarized in Table 1.

Table 1

A comparison of typical cathode pre-sodiation additives

AdditivesTheoretical capacity [mAh g-1]Practical capacity [mAh g-1]Decomposition potential [V vs. Na+/Na]Ref.
NaN34123003.55[60]
Na3P8026004.30[56]
Na2C2O4400394.64.41[63]
Na2CO3505924.00[181]
Na2NiO2392.288.72.10[57]
Na2C4O43392563.60[182]
Na3PS3O379> 3004.00[189]
Na2O2421.54084.00 [187]
AC-Na3163004.10[188]
NaCrO22512294.20[61]
Na2C6O62502003.75[184]
Na4C6O64124144.00[183]
Na2C6H2O63123024.00[176]
EDTA-4Na2824204.00[185]
DTPA-5Na2663633.97[186]

SUMMARY AND OUTLOOK

SIBs are considered to be the most promising alternative or complementing energy storage technology to current LIBs due to the extremely abundant Na resource and a range of attractive characteristics such as the allowance of using aluminum foil as the anode current collector, easy desolvation of Na+, 0 V capability, etc. These features make SIBs highly competitive especially in the scenario of large-scale stationary energy storage. P2-type layered cathodes could be a key enabler for the success of SIBs owing to their advantages of high structural reversibility, low Na+ diffusion energy barrier, compositional diversity, acceptable humid stability, high volumetric density, easy synthesis, etc. However, many issues still hinder the practical application of P2 cathodes, which have been summarized into four aspects: structural deformation, interfacial instability, insufficient ambient stability, and Na deficiency. The progress in solving these problems has also been comprehensively reviewed from six categories, i.e., compositional design, elemental doping, phase mixing, morphological control, surface modification, and sodium compensation. Table 2 compares the electrochemical properties of recently reported P2 layered cathode materials using various modification strategies.

Table 2

Summary of the properties of P2 layered cathode materials using various modification strategies

MaterialsPotential range
[V vs. Na+/Na]		Discharge capacity [mAh g-1]/IEnergy density [Wh kg-1]Capacity retention (%)/cycle numberStrategiesYear of publication
Na0.6[Co0.78Ru0.22]O2[70]1.5-4.3163/23.5499.998/50 (1.5-4 V, 23.5)Compositional design2023
P2-Na2/3MnxNix-1/3Co4/3-2xO2(1/3 ≤ x ≤ 2/3)[77]1.5-4.5156/5041087/300 (1.5-4.0 V, 1,000)Compositional design2021
Na2/3Li1/6Co1/6Mn2/3O2[82]2.0-4.5178/1553495.8/250 (150)Compositional design2023
Na0.67Mn0.53Ni0.30Mg0.085Ti0.085O2[83]2.0-4.25118/5041091.5/100 (50)Compositional design2023
Na0.67(Mn0.45Ni0.18Co0.18Ti0.1Mg0.03Al0.04Fe0.02)O2[85]1.5-4.6146/2047769/50 (100)Compositional design2023
Na2/3[Ni1/4Mn1/2Ti1/6Zn1/12]O2[86]2.5-4.5116/13-higher (33%) than pristine/40 (13)Compositional design2023
Na0.67Mn1-xMoxO2[88]1.8-4.3109.73/2,000-86.4/100 (50)Elemental doping2021
Na0.612K0.056MnO2[89]1.0-4.3240.5/2065498.2/100 (50)Elemental doping2021
Na0.76Ca0.05[Ni0.230.08Mn0.69]O2[91]2.0-4.3153.9/12257.687.1/50 (12)Elemental doping 2021
Na0.67Ni0.3Co0.1Mn0.6O1.94(BO3)0.02[99]2.0-4.198.7/30-80.1/1,000 (300)Elemental doping 2022
Na0.67Ni0.15Fe0.2Mn0.65F0.05O1.95[102]1.5-4.3229/0.1C-87.7/50 (0.1C)Elemental doping 2022
Na0.667Mn0.667Ni0.167Co0.117Ti0.01Mg0.01Cu0.01Mo0.01Nb0.01O2[103]1.5-4.5111/875-76.4/100 (175)Elemental doping 2023
P2/O'3-biphasic NaMn0.89Cu0.08Sb0.03O2[9]2.0-4.0204/10-80/150 (500)Phase mixing & Compositional design2020
P2/Li-O3 Na0.67Mn0.67Ni0.33O2[106]2.0-4.5104/850-60/200 (85)Phase mixing & Elemental doping & Morphological control2022
P2/O3 biphasic Na0.7Ni0.2Cu0.1Fe0.2Mn0.5O2-δ[114]1.5-4.162/2,40019084/500 (600)Phase mixing2022
P2@P3-Na0.5Ni0.1Co0.15Mn0.65Mg0.1O2[116]1.5-4.0153.8/16325.880.6/320 (200)Phase mixing & Compositional design2020
layered–tunnel Na0.6Mn0.95Mg0.05O2xMgxO2[120]2.0-4.0188.9/10050871.3/1,000(1,000)Phase mixing & Compositional design2023
P3/P2/O3-Na0.674Ni0.319Mn0.590O2[124]2.0-4.2155.4/15-51.5/200 (15)Phase mixing2022
Na0.76Cu0.22Fe0.30Mn0.48O2[127]2.0-4.0125.4/12177.479/300 (240)Morphological control & Elemental doping 2020
P2-Na0.67Ni0.23Mg0.1Mn0.67O2[130]2.5-4.3560/850249.990.9/1,000 (850)Morphological control & Elemental doping 2021
Na0.6Mn0.95Ti0.05O2[137]2.0-4.1106.4/20296.189.6/100Morphology & Compositional design2023
single-crystal P2-type Na0.66Ni0.26Zn0.07Mn0.67O2[140]2.0-4.4122.1/10-95.8/100 (100)Morphological control & Elemental doping 2023
Na0.62Mn0.67Ni0.23Cu0.05Mg0.07Ti0.01O2[141]2.0-4.378.6/1200-87/500 (120)Morphology & Compositional design2022
shale-like NaxMnO2[143]1.5-4.0181/24-83/3,000 (960, 2.0-4.0 V)Morphology2021
Na2/3Ni1/3Mn2/3O2 with ZrO2 coating[149]2.0-4.5157.8/87.5-86/1,000 (875)Surface modification & Elemental doping 2022
ZrO2-Na0.67Ni0.28Mg0.05Mn0.67O2[150]2.0-4.35121.9/12238.9181.5/150 (120)Surface modification & Elemental doping 2023
CaTiO3-Na2/3Ni1/3Mn2/3O2[151]1.5-4.5178/17.3-75.3/200 (346)Surface modification & Elemental doping 2022
Na2/3Ni1/3Mn2/3O2-Al2O3 ALD[156]1.5-4.1200/15-87/300 (750)Surface modification2021
Al2O3@Na0.67Zn0.1Mn0.9O2[157]2.0-4.4156/1285/400 (120)Surface modification & Compositional design2020
alucone coated P2-type Na0.66Mn0.9Mg0.1O2[158]2.0-4.5163.1/200-86/100 (200)Surface modification & Elemental doping 2020

From the table, it should be noted that the simultaneous achievements of high capacity and long cycling stability are still highly challenging. This could be explained by the formidable and interconnected degradation mechanisms that are unlikely to be eliminated by a single or even a combined approach. To develop practical P2 layered oxide materials, we believe that systematic strategies are required with precise understanding on the functionalities from atom to lattice, particle, and electrode. Also, the performances of these reports show huge divergence due to the differences in materials and the testing protocols. For example, using wider electrochemical window can contribute to higher specific capacity. However, this could cause more serious structural/interfacial instability issues that are unacceptable for practical applications. In addition, the manufacturability and cost of the improvement strategies are also critically important in determining their practical applications. While most studies focus on materials engineering, more attention should also be devoted to electrolyte engineering to realize wider electrochemical stability window and dense, stable, and high ionic conductivity electrolyte-electrode interfaces.

In summary, this review has critically examined the progress made in Na-deficient P2-type layered oxide cathodes for SIBs. The inherent advantages and challenges associated with these materials have been discussed, along with promising strategies for improvement through materials and electrode engineering strategies. Continued advancements in materials science, electrode design, and cell engineering hold significant potential to overcome the limitations of P2-type cathodes and expedite their development for practical SIB applications. This study aims to be a valuable resource for researchers and stakeholders working towards realizing high-performance, cost-effective, and sustainable SIBs.

DECLARATIONS

Authors’ contributions

Made substantial contributions to the conception and design of the study and provided administrative and technical support: Zhu X, Li K

Performed data acquisition and writing: Huang Y, Zeng W, Zhu X

Availability of data and materials

Not applicable.

Financial support and sponsorship

This work was supported by the National Natural Science Foundation of China (52202210) and the Scientific Research Fund of Hunan Provincial Education Department (22B0292).

Conflicts of interest

The other authors declared that there are no conflicts of interest.

Kui Li and Weixiong Zeng are affiliated with “Tuo Feng New Energy Co., Ltd, China”.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

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Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries
Yu Huang, ... Xiaobo ZhuXiaobo Zhu

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Huang, Y.; Zeng W.; Li K.; Zhu X. Na-deficient P2-type layered oxide cathodes for practical sodium-ion batteries. Microstructures. 2024, 4, 2024027. http://dx.doi.org/10.20517/microstructures.2023.102

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