Review and Recent Advances in Metal Compounds as Potential High-Performance Anodes for Sodium Ion Batteries

Abstract

Along with great attention to eco-friendly power solutions, sodium ion batteries (SIBs) have stepped into the limelight for electrical vehicles (EVs) and grid-scale energy storage systems (ESSs). SIBs have been perceived as a bright substitute for lithium ion batteries (LIBs) due to abundance on Earth along with the cost-effectiveness of Na resources compared to Li counterparts. Nevertheless, there are still inherent challenges to commercialize SIBs due to the relatively larger ionic radius and sluggish kinetics of Na⁺ ions than those of Li⁺ ions. Particularly, exploring novel anode materials is necessary because the conventional graphite anode in LIBs is less active in Na cells and hard carbon anodes exhibit a poor rate capability. Various metal compounds have been examined for high-performance anode materials in SIBs and they exhibit different electrochemical performances depending on their compositions. In this review, we summarize and discuss the correlation between cation and anion compositions of metal compound anodes and their structural features, energy storage mechanisms, working potentials, and electrochemical performances. On top of that, we also present current research progress and numerous strategies for achieving high energy density, power, and excellent cycle stability in anode materials.

1. Introduction

Over the past 50 years, the large-scale consumption of fossil fuels has accelerated, reaching a peak usage every year. Since the massive emission of carbon dioxide (CO₂) has been regarded as an urgent environmental issue, the development of sustainable and eco-friendly energy technology has emerged with primary energy sources including solar, wave, and wind power energies [1]. Toward a carbon-free economic system, the utilization of renewable energy sources is expected to increase significantly in electric vehicles (EVs) and energy storage systems (ESSs). In this regard, secondary batteries have become important power sources, providing a dominant role in EVs and large-scale ESSs. For instance, various types of secondary batteries have been developed such as lithium ion batteries (LIBs), lithium–sulfur batteries (Li-S), lithium–air batteries (Li-O₂), etc. [2,3,4].

LIBs have been considered the foremost utilized secondary batteries considered essential for EVs on account of their superior energy density, stable and high output voltage, excellent charging efficiency, and low self-discharge rate [5]. LIBs have become the most popular power sources since the commercialization by SONY Corp. in the 1990s. The increasing demand for lithium (Li) and the large-scale applications of EVs and ESSs have led to a sharp increase in price and associated fluctuations of Li resources. Given the price bubbles observed in Li markets in recent years, it is essential to explore an Earth-abundant element which can be used in secondary ion batteries replacing Li [6,7].

Sodium (Na), belonging to the alkali metal group alongside Li, has a similar standard redox potential of −2.71 V versus (vs.) the standard hydrogen electrode (SHE) against Li (−3.04 V vs. SHE), while it also has large abundance and reasonable cost (

Table 1. Comparison of lithium and sodium regarding selected physicochemical properties and cost of carbonates. Reproduced with permission [8]. Copyright 2019, Wiley-VCH.

Parameters	Lithium	Sodium
Cation radius [Å]	0.76	1.02
Relative atomic mass	6.94	22.98
E0 (vs. SHE) [V]	−3.07	−2.71
Cost, carbonates	USD 5000 ton−1	USD 150 ton−1
Theoretical capacity [mAh g−1]	3829	1165
Coordination preference	Octahedral and tetrahedral	Octahedral and prismatic
Desolvation energy in polycarbonate [kJ mol−1]	218.0	157.3

) [8]. Even though Na⁺ ion possesses a greater ionic radius and heavier mass compared to those of Li⁺ ions, which cause substantial volume expansion and slow reaction kinetics in the reaction of sodiation/desodiation, the ionic radius of Na⁺ ions is small enough to be inserted into the existing cathode and anode materials used in LIBs. The battery constituents and electrochemical energy storage mechanism of LIBs and SIBs are quite similar, and SIB technology may be amenable to facile scale-up due to compatibility with conventional LIB manufacturing processes and techniques. Furthermore, less expensive aluminum (Al) foil can serve as a current collector in SIBs for anode side instead of expensive copper (Cu) foil, which can further reduce the cost of the battery [9]. Based on these reasonable considerations, SIBs have been seen as one of the most ideal post-LIB systems for grid-scale ESS applications. Nevertheless, in order to implement SIBs into large-scale applications, there remain several factors that need to be addressed regarding energy density and fast charging capabilities [10,11,12]. With respect to lithium iron phosphate (LiFePO₄, LFP) batteries, a promising competitive system pursuing a participant opportunity in grid-scale battery markets, SIBs need to be further developed to take advantage not only in terms of manufacturing costs but also battery performances. Even though the working principles of LIBs and SIBs are similar to a “rocking-chair” mechanism, the greater ionic size and molar weight of Na⁺ compared to Li⁺ lead to the inferior specific capacity of Na (1165 mAh g⁻¹ vs. 3829 mAh g⁻¹ for Li), coupled with slower diffusion kinetics and structural instabilities [13]. Those fundamental differences cause the lower working voltage, specific capacity, and cyclability of SIBs compared to those of LIBs and, thus, high-performance electrode materials are highly required in SIBs [6,11,13,14].

Specifically, exploring suitable cathode materials has been considered as the bottleneck for boosting the power output of SIBs, so major efforts have been widely performed on develo** high-performance cathode materials for SIBs. Generally, most research on cathode materials for SIBs has prominently featured layered oxides, polyanionic compounds, and Prussian-blue analogues (PBAs) [15,16].

In the meantime, exploring commercially available anode materials is a pressing matter to address another bottleneck in SIBs. When the conventional anode materials in LIBs were applied to SIBs, such as graphite, hard carbon, and silicon (Si), poor electrochemical activity and performance were observed (Figure 1) [17,18]. Similar to LIBs, highly reversible reactions need to be based on insertion or intercalation reactions, which involve interstitial ion accommodation of Na⁺ ions. In this regard, the cheap graphite anode, a conventional anode material used in LIBs, is the most promising candidate with its highly reversible intercalation/deintercalation reactions (Figure 2a) [19]. The graphite anode, however, exhibits lower electrochemical activity in Na cells and several studies have recently been performed on the origins of the inactivity (Figure 1a) [17,20]. As shown in Figure 2b [21], the formation energies were compared with graphite intercalation compounds (GICs) such as MC₆ and MC₈, where M represents a variety of alkali metals such as Li, Na, K, Rb, and Cs to examine the thermodynamic stability of each compound. Density functional theory (DFT) calculations verified that the generation for binary Na-GICs is energetically unfavorable, while GICs with other alkali metals are stable. Since it is revealed that the unfavorable interaction of Na-graphene results instability in the formation of Na-GICs, modulating the solvation structure of Na⁺ ions is a feasible strategy to generate specific conditions for reversible intercalation of Na⁺ ions into graphite. Jache et al. discovered that a small portion of Na⁺ ions can be inserted into graphite interlayers. However, at room temperature, the amounts of Na⁺ ions are limited as they form NaC₆₄, presenting a low reversible capacity of 35 mAh g⁻¹ (Figure 2c) [22,23]. This number is about one-tenth of the theoretical capacity of graphite in LIBs of 372 mAh g⁻¹ by forming GICs of LiC₆ phase [22,24]. Recently, improved reversible capacity of graphite has been achieved by modifying the solvation structure of Na⁺ ions and applying the Na⁺-solvent co-intercalation concept with ether-based electrolytes. In a full cell configuration and tested at 1.0 A g⁻¹, this approach exhibits a reversible capacity reaching 110 mAh g⁻¹ with an improved cycle retention (Figure 2d) [17,25]. Despite the improved reversible capacity is a lot greater than before, the achieved capacity is still lower than that achieved in LIBs. Moreover, the essential use of ether-based electrolytes poses challenges for commercialization due to their expensive costs. Consequently, extensive studies have been performed on Na⁺ ion insertion into hard carbon anodes since the 2000s due to its high reversible capacity of 300 mAh g⁻¹ and the low redox potential of 0.1 V vs. Na/Na⁺ (Figure 1b) [26]. Nevertheless, there are still challenges in SIBs to be commercialized for high-energy-density applications in terms of a low initial coulombic efficiency (ICE), rapid capacity degradation, and limited rate performance, resulting in inferior energy and power densities [25,27,28].

For these reasons, the successful development of SIBs faces the critical issue of exploring suitable high-performance anode materials derived from electrochemical alloying or conversion reactions. In LIBs, Si anode exhibits the superior specific capacity of 4200 mAh g⁻¹ by reacting with 4.4 mole of Li⁺ and 1 mole of Si to form Li-rich Li_4.4Si alloying phase (Figure 1c) [29]. In comparison, a mole of Si anode can only react with 1 mole of Na⁺ ion for SIBs by forming NaSi phase possessing a theoretical capacity of 960 mAh g⁻¹, just a quarter of that observed in LIBs (Figure 1c and Figure 2a). Furthermore, NaSi is a thermodynamically unstable phase with several drawbacks such as low Na⁺ ion diffusivities in the phase and poor electrical conductivity. Therefore, dramatic improvements in reversible capacities and diffusion kinetics need to be achieved in the cases of conventional LIB anodes of graphite and Si [30]. As an element in group 14 with Si, Tin (Sn) can form a Na-rich Na₁₅Sn₄ alloy phase, while it leads to a theoretical capacity of 847 mAh g⁻¹ due to its heavy molecular weight of 89 g mol⁻¹ [31]. Elements of group 15 such as nitrogen (N) and phosphorus (P) can react with 3 moles of Na⁺ ion with a higher specific capacity than that found in group 14. P boasts a specific capacity reaching 2596 mAh g⁻¹ and holds a relatively low redox potential reaching 0.4 V vs. Na/Na⁺ by forming Na-rich Na₃P alloy phase [32]. However, the representative P polymorphs of red P have a flammable nature, low electrical conductivity (~10⁻¹⁴ S cm⁻¹), and high volume expansion rate of 400%, mitigating its practical applications [33]. Thus, there is need for various approaches, such as forming alloys or compounds, and forming composites with inclusion of conductive carbon-based materials. Lastly, compared to Li metal anode, which is considered as a next-generation high-capacity anode, the adoption of Na metal as an anode presents a relatively unstable solid electrolyte interphase (SEI) layer deriving from the high reactivity of electrolytes and low resistance toward metal dendrite formation during the electrochemical plating process. Hence, there is no need to say that enhancement of high-efficiency anode materials is necessary for progressive SIB systems [13,34].

Electrochemical properties of electrode materials rely on their energy storage mechanisms that can be divided into insertion, alloying, and conversion reactions (Figure 3) [35]. Similar to LIBs, electrode materials with three types of energy storage mechanism have been investigated in SIBs. The term “insertion” is used to describe reactions involving the transfer of guest species (alkali ions for LIBs and SIBs) into empty spaces in host materials (Figure 3) with the reaction equation of M_aX_b + yNa⁺ + ye⁻ → Na_yM_aX_b (M = metal cation, X = anion). The insertion mechanism exhibits typically low capacities limited by the amount of Na⁺ ion insertion (mostly limited to 1 mole of Na⁺ per 1 mole of M_aX_b) but excellent cycle retention by a negligible volume expansion during charge and discharge processes. Beyond the insertion-based materials, alloying reaction materials have been developed based on a low operating voltage and high specific capacity with the reaction equation of M + yNa⁺ + ye⁻ → Na_yM. Typically, an alloying reaction accompanies multiple transfers of Na⁺ ions and electrons, exhibiting higher capacity than that of insertion-based compounds by an order of magnitude. However, the large amounts of Na⁺ ions accompanied by a sodiation process cause a huge volume change, pulverization, and agglomeration of alloying materials, resulting a bad structural integrity of the electrode and a rapid fading of capacity. The conversion reaction mechanism offers another avenue to achieve high specific capacities and the general reaction can be written as M_aX_b + yNa⁺ + ye⁻ → aM⁰ + bNa_yX. Conversion reaction materials also undergo a large structural change by reacting with more than one mole of Na⁺ ions per mole of host compounds (y = 2 for X = O, S, and Se, and y = 3 for X = N, and P) and suffer from significant volume changes and polarization effects, resulting in the typical voltage hysteresis [36,37,38].

Consequently, understanding the correlation between crystal structure, composition of host materials, and electrochemical energy storage mechanism can be an important key to gain a deep insight into the achieved electrochemical performance of each material and to give guidelines for designing next-generation materials. In this review, we will introduce promising anode material groups classified not only by their compositions but also energy storage mechanisms to clarify the correlations between them. The primary goal of this review is to provide readers a perspective on the key parameters including theoretical capacity, operating voltage, volume change, and physicochemical properties of each material group for advanced SIB systems.

2. Metal Compounds as Anode Materials for Sodium Ion Batteries

The energy storage mechanisms illustrated in Figure 3 are determined by the components, compositions, and crystal structure of anode materials [27,35]. As an example of cation-dependent electrochemical reactions in oxide systems, early transition metal (TM, Ti or V)-containing oxides (e.g., TiO₂, VO₂, V₂O₃, etc.) exhibit an insertion reaction mechanism with redox reactions of Ti³⁺/Ti⁴⁺ or V²⁺/V³⁺/V⁴⁺ rather than full reduction to 0 valence of a metallic state (e.g., Ti⁰ or V⁰) due to the relatively lower electronegativity of early TM (Figure 4a) [39,40,41]. Meanwhile, the higher electronegativity of later TMs (e.g., Fe, Co, Ni, etc.) makes it facile to be fully reduced to 0 valence of a metallic state (e.g., Fe⁰, Co⁰, and Ni⁰), leading to a conversion reaction mechanism (Figure 4a) in layered TM-based oxides (e.g., Fe₂O₃, Fe₃O₄, CoO, Co₃O₄, NiO, etc.) [8,27].

In addition to the observed cation-dependent electrochemical reactions, anion species in metal compound (M_aX_b) anodes affect electrochemical reaction potentials. As the electronegativity difference between M and X becomes larger, conversion reaction potential where reduction of metal ions occurs shifts to more positive values (Figure 4b) [10,36]. For example, the conversion reaction anodes in metal compounds with similar TMs but different anions of phosphorus (P), oxygen (O), sulfur (S), and fluorine (F) will exhibit the order of electrochemical reaction potentials as fluoride > sulfide > oxide > phosphide as shown in Figure 4b [10,36]. By following the trend, nitrides and phosphides exhibit low reaction potentials based on the strong M-X covalency when X is nitrogen (N) or P consisting of group 15 anions. Conversely, oxides, sulfides, and selenides consisting of group 16 anions of O, S, and Se exhibit high reaction potentials with their higher ionic bonding characters. F-containing fluoride is considered as a conversion reaction type cathode material due to its high ionicity and reaction potential at 1.5–3 V vs. Na/Na⁺ [36,42].

Alongside the reaction potentials, anion species in compounds determine their theoretical capacities based on their molar weight and the number of electrons that can be stored by conversion reaction and the corresponding volume expansion rate (Figure 4c) [7,10]. In the case of group 15 elements N and P, they can react with three Na⁺ ions during conversion reaction with (3−) negative charges (N³⁻ and P³⁻) by forming Na₃N or Na₃P phases [10]. On the other hand, group 16 elements including O, S, and Se present (2−) negative charges (O²⁻, S²⁻, and Se²⁻) and react with two Na⁺ ions to form Na₂O, Na₂S, and Na₂Se phases, respectively. Meanwhile, group 17 element F forms NaF, with (1−) negative charge. As shown above, the species and amounts of anion in M_aX_b compounds determine molar weight and the number of reacted Na⁺ ions and thus their theoretical capacities. In addition, the volume expansion rate also depends on the anion compositions, representing an important role of anion composition design.

As a result, the cation and anion composition in M_aX_b compounds determines the ion storage mechanism, working potential, theoretical capacity, and volume expansion rate [36]. So, here, we will introduce various metal compounds used as anode materials in SIBs based on the sort of anion for easier comprehension, such as oxide, sulfide, selenide, phosphide, and nitride compounds.

2.1. Metal Oxides

Transition metal oxides (TMOs) have emerged as the most significant anodes for SIBs attributed by their high abundance, easy synthesis, chemical stability, and cost-effectiveness, all contributing to their huge potential for commercialization. Since the most attractive conventional graphite anode is less active in Na cells, insertion/intercalation reaction anodes in TMOs have been intensively studied to replace the graphite. Especially, TMOs with early TM groups including Ti- and V-oxides have drawn considerable attention based on their high sodiation activity and excellent cycle stability.

Ti-based oxides can be typically regarded as insertion/intercalation-type electrodes and attractive anodes for SIBs in light of their advantages, such as the light molar weight of Ti, non-toxicity, high chemical stability, facile synthesis procedure, and cycle stability. In addition, the modest insertion potential of Na⁺ ions at about 0.6 V (vs. Na/Na⁺) helps prevent Na metal plating issues [43]. The most common anodes studied in Ti-based oxides can be divided into three types: TiO₂, Li₄Ti₅O₁₂, and Na₂Ti₃O₇ phases (Figure 5a) [30,44].

TiO₂ phases have several polymorphs in crystalline form such as anatase, brookite, rutile, and bronze phases, whereas only anatase, rutile, and bronze phases exhibit electrochemical activity for SIBs [45]. The TiO₂ polymorphs are composed of TiO₆ octahedra and three-dimensional (3D) arrangement variations of the TiO₆ octahedra build different crystal structures and the corresponding electronic structures. In the case of the anatase structure, it has a space group of I4₁/amd with a tetragonal crystal system in which the TiO₆ octahedra share four edges organized in zig-zag chains as shown in Figure 5a. The rutile TiO₂ structure, with the same tetragonal crystal system as anatase but a different space group of P4₂/mmm, shares only two edges of TiO₆ octahedra and forms linear chains parallel to the (001) planes. In contrast, bronze TiO₂ phase exhibits a monoclinic structure, featuring wrinkled sheets composed of edge- and corner-sharing TiO₆ octahedra (Figure 5a).

Systematic studies on electrochemical reaction behavior of anatase TiO₂ exhibit that 0.25 mole of Na⁺ ions can be intercalated into TiO₂ at 0.3 V vs. Na/Na⁺ and 0.69 mole of Na⁺ ion can be further inserted with an amorphization reaction during the first sodiation process (Figure 5b) [46]. However, an irreversible plateau at about 0.2 V vs. Na/Na⁺ appears, where Na⁺ ions are inserted into interstitial sites of anatase and form rhombohedral active phases [30]. This irreversible reaction results in only 0.41 mole of Na⁺ ions inserted into each TiO₂ reversibly during the following process, which causes a low reversible capacity of ~140 mAh g⁻¹ [46].

In the case of rutile, it is the most chemically stable polymorph in TiO₂ and has a good cyclability. However, due to its poor electronic conductivity caused by its large band gap structure (about 3.0 eV) [30], inferior rate capability was observed. Additionally, DFT calculation indicates that rutile TiO₂ has a relatively elevated energy barrier showing 20.65 eV for Na⁺ ion diffusion compared to that of anatase TiO₂ (11.1 eV), and the Na⁺ ion has a solitary diffusion path along the c-axis. Conversely, anatase TiO₂ has two-dimensional pathways along the a- and b-axes, which mean theoretically easier Na⁺ ion diffusions in the anatase structure compared to that of rutile TiO₂ [45].

Bronze TiO₂ has a C2/m space group and three distinct intercalation sites with an open structure by edge-sharing oxygen of neighboring layers. Liu et al. compared the Na⁺ ion storage mechanism of anatase and bronze TiO₂ through X-ray absorption spectroscopy (XAS). Since bronze TiO₂ has lower crystallinity and density compared to those of the anatase structure, the sodiation process can be made easier by facilitating Na⁺ ion diffusion. That means stability of bronze TiO is much better than that of anatase. However, the still unknown mechanism of irreversible phase transformation to Na_xTi_yO_z phase in bronze TiO₂ needs to be further studied to clarify the exact sodiation/desodiation process [30,44,47,48].

Recently, binary phase of anatase/bronze TiO₂ was studied for its application as a superior Na⁺ ion storage electrode in SIBs [45,49,50]. One-dimensional nanostructured nanofibers and binary phase heterojunctions of TiO₂ with a phosphated surface were introduced to improve ionic transportation kinetics and electronic conductivities. Meanwhile, charge/discharge, well-crystallized internal structure, and disordered amorphous surface structure were formed by phosphorylation resulting ionic/electronic fields and improved Na⁺ ion conductivity. Simultaneously, the 1D nanofiber structure increased the contact area between the anode/electrolyte and thus reduced Na⁺ ion diffusion distance (Figure 5c) [50]. As a result, anatase/bronze TiO₂ nanofiber showed a reversible capacity of 224.7 mAh g⁻¹ tested at 0.2 A g⁻¹ and prolonged cycle performance with a reversible capacity of 108.9 mAh g⁻¹ tested at 2 A g⁻¹ after 1000 cycles (Figure 5d) [50].

Figure 5. (a) Crystal structures of Ti-based oxides for SIB anode materials. Reproduced with permission [30]. Copyright 2017, WILEY-VCH. (b) Voltage profiles of anatase TiO₂ anode for SIBs tested at 3.35 mA g⁻¹. Reproduced with permission [46]. Copyright 2014, WILEY-VCH. (c) Schematic illustration of working mechanism of surface phosphorylation of TiO₂ anode for SIBs. (d) Cycle performance of anatase/bronze TiO₂ nanofiber tested at 0.2 and 2.0 A g⁻¹, respectively. Reproduced with permission [50]. Copyright 2020, Elsevier B.V.

Figure 5. (a) Crystal structures of Ti-based oxides for SIB anode materials. Reproduced with permission [30]. Copyright 2017, WILEY-VCH. (b) Voltage profiles of anatase TiO₂ anode for SIBs tested at 3.35 mA g⁻¹. Reproduced with permission [46]. Copyright 2014, WILEY-VCH. (c) Schematic illustration of working mechanism of surface phosphorylation of TiO₂ anode for SIBs. (d) Cycle performance of anatase/bronze TiO₂ nanofiber tested at 0.2 and 2.0 A g⁻¹, respectively. Reproduced with permission [50]. Copyright 2020, Elsevier B.V.

As a “zero strain” anode in SIBs, Li₄Ti₅O₁₂ (LTO) was applied for reversible sodium storage behavior, accommodating three Na⁺ ions per LTO with a reversible capacity of 145 mAh g⁻¹ and an operating potential at around 0.9 V vs. Na/Na⁺ [30]. The electrochemical reaction mechanism of LTO observed in SIBs is three-phase transformation, which is different from the two-phase reaction mechanism observed in LIBs. As for the reaction equation, it is expressed as 2[Li₃]^8aV^16c[Ti₅Li]^16dO₁₂ + 6 Na⁺ + 6 e⁻¹ ↔ V^8a[Li₆]^16c[Ti₅Li]^16dO₁₂ (Li7) + V^8a[Na₆]^16c[Ti₅Li]^16dO₁₂ (Na6Li) (V = vacancy) (Figure 6a) [44,50]. The final product after the discharge process is a mixture of Na6Li and Li7. Through an annular-bright-field (ABF) image (Figure 6b), interphases between Li₇/Li₄ and Li₇/Na₆Li were confirmed after the discharge. During the long-term cycle process, the changed lattice parameters were negligible, implying that LTO can exhibit an excellent electrochemical reversibility for long-term cyclability. However, Na⁺ ion diffusivity of LTO (10⁻¹⁶ cm² s⁻¹) is not as high as that of observed in LIBs (10⁻¹¹ cm² s⁻¹) [45]. Therefore, it is necessary to introduce different strategies including nanostructure engineering, heteroion or material do** or coating methods, surface modifications, etc. [44,48]. Chen et al. synthesized porous nanofibers of LTO interconnected with conductive 3D graphene structures and it raised interfacial sites and decreased Na⁺ ion diffusion distance (Figure 6c). This resulted in a reversible capacity of 195 mAh g⁻¹ at a 0.2 C rate (35 mA g⁻¹) for 115 cycles, coupled with long-term cycling durability, maintaining a capacity of 120 mAh g⁻¹ at a 3 C rate (525 mA g⁻¹) over 12,000 cycles (Figure 6d) [51].

Na₂Ti₃O₇ (NTO) is a layered structure comprising three TiO₆ octahedra, which share two edges and corners, forming (Ti₃O₇)²⁻ zigzag layers. NTO is a monoclinic crystal system with a P2₁/m space group. The two Na⁺ ions can reversibly intercalate into layered structure of stable NTO and the reaction equation is Na₂Ti₃O₇ + 2 Na⁺ + 2 e⁻ ↔ Na₄Ti₃O₇ [45]. NTO is one of the most encouraging insertion-type anodes with a theoretical capacity reaching 177 mAh g⁻¹ on account of its innovative lowest working potential at 0.3 V vs. Na/Na⁺ [44,45,52,53]. However, NTO has a high band gap energy of 2.75 eV, indicating poor ionic and electronic conductivities and cycling stability [45]. In addition, since the calculated activation energy for Na⁺ ion migration through TiO₆ octahedral layers is too high, Na⁺ ions cannot diffuse through interlayers but just along layers [44,45]. Therefore, various strategies have been applied such as morphological modification, heteroatom do** or coating, and formation of layered NTO and tunnel Na₂Ti₆O₁₃ hybrid material (NNTO). The tunnel-type structure of Na₂Ti₆O₁₃, which has a bigger channel than the radius of Na⁺ ions, exhibits high ionic conductivity and long-term cycling stability but a lower specific capacity. As a hybrid structure formation of NNTO, solid solution reactions occurred in the tunnel-type Na₂Ti₆O₁₃ and phase transformation occurred in the layered-type NTO, where the synergies of two phases increase reaction kinetics, reversible capacity, and consistent cycle performance. NNTO delivered a higher 1st discharge capacity of 212.5 mAh g⁻¹ tested at 20 mA g⁻¹, surpassing the theoretical capacity of NTO (177 mAh g⁻¹). While the long-term cycle test exhibited an excellent cycle retention during 4000 cycles even examined at 2 A g⁻¹, the reversible capacity was only ~20 mAh g⁻¹, which requires further improvements in rate capabilities (Figure 6e) [27,54]. So far, Ti-based TMOs have been broadly applied as stable anode materials with the insertion mechanism, exhibiting superior long-term cycle performance compared to other types of anodes. Even though these types of insertion mechanism have benefits of less volumetric change and structural stability during cycling, the improvements in ion diffusion kinetics and specific capacity still need to be improved [7].

Figure 6. (a) Phase transformation mechanism of Li₄Ti₅O₁₂ (LTO) anodes during sodiation and desodiation process in SIBs. (b) ABF image for the sodiated LTO particle. Reproduced with permission [44]. Copyright 2019, WILEY-VCH. (c) Graphical illustration of the structural advantages and merits in Na⁺ ion diffusion kinetics of 3D graphene interconnected porous LTO (G-PLTO) electrode. (d) Cycle performance of G-PLTO anode for SIBs tested at 0.2 C and 3 C rates, respectively. Reproduced with permission [51]. Copyright 2016, WILEY-VCH. (e) Long-term cycle performance of NNTO anodes for SIBs tested at 2.0 A g⁻¹. Reproduced with permission [54]. Copyright 2018, WILEY-VCH.

Figure 6. (a) Phase transformation mechanism of Li₄Ti₅O₁₂ (LTO) anodes during sodiation and desodiation process in SIBs. (b) ABF image for the sodiated LTO particle. Reproduced with permission [44]. Copyright 2019, WILEY-VCH. (c) Graphical illustration of the structural advantages and merits in Na⁺ ion diffusion kinetics of 3D graphene interconnected porous LTO (G-PLTO) electrode. (d) Cycle performance of G-PLTO anode for SIBs tested at 0.2 C and 3 C rates, respectively. Reproduced with permission [51]. Copyright 2016, WILEY-VCH. (e) Long-term cycle performance of NNTO anodes for SIBs tested at 2.0 A g⁻¹. Reproduced with permission [54]. Copyright 2018, WILEY-VCH.

Beyond the insertion reaction anodes with excellent cyclabilities, conversion-reaction-type anodes have been extensively studied for SIBs with later TM oxides (Fe-O, Co-O, Ni-O, etc.) due to their high theoretical capacities. Specifically, iron oxide anodes have received huge attention as cost-effective anode materials due to the abundance of resources and easy synthesis methods. Both hematite (α-Fe₂O₃) and maghemite (γ-Fe₂O₃) exhibit a substantial theoretical capacity of 1005 mAh g⁻¹, though magnetite (Fe₃O₄) shows a significant theoretical capacity of 926 mAh g⁻¹ based on the reaction equations of Fe₂O₃ + 6 Na⁺ + 6 e⁻ → 2 Fe + 3 Na₂O and Fe₃O₄ + 8 Na⁺ + 8 e⁻ → 3 Fe + 4 Na₂O. Basically, later TMs including Fe, Co, Ni, Cu, etc. in oxides are inactive with Na⁺ ions, so they undergo the one-step conversion reaction [36]. Despite the high theoretical and practical capacities of Fe-based oxides, they suffer from their poor electronic conductivities, large volume changes, and severe agglomerations occurring during the cycling process. Fabricating nanocomposites with iron oxide particles and conductive carbon-based materials can be a facile and effective strategy to form conformal carbon coating or carbon matrix with iron oxide anodes [55].

Chen et al., for instance, synthesized a porous 3D structure of γ-Fe₂O₃@C nanocomposite using an aerosol-assisted method (Figure 7a) [56]. The TEM image of the nanocomposite shows that spherical γ-Fe₂O₃, with a mean particle diameter of 5 nm, is evenly distributed within the carbon matrix (Figure 7b). This composite shows reversible capacity of 740 mAh g⁻¹ after 200 cycles tested at 0.2 A g⁻¹ (Figure 7c) and 358 mAh g⁻¹ is maintained even after 1400 cycles tested at 2 A g⁻¹. Additionally, it has superior rate capability with a capacity of 317 mAh g⁻¹ even tested at 8A g⁻¹ [56]. With a more stable and conductive α-Fe₂O₃ phase compared to γ-Fe₂O₃, **a et al. reported the peapod-like structure of α-Fe₂O₃@N-PCNF composites, consisting of Fe₂O₃ nanoparticles encapsulated in uniformly N-doped porous carbon fibers to ensure structural stability and high electrical conductivity. α-Fe₂O₃@N-PCNFs demonstrate a remarkable discharge capacity of 1434 mAh g⁻¹ tested at 0.1 A g⁻¹ (Figure 7d) and maintain 396 mAh g⁻¹ tested at 2 A g⁻¹, maintaining 90% of their capacity after 1500 cycles [55,57].

In the case of Fe₃O₄, it indicates a theoretical capacity of 926 mAh g⁻¹ by reacting with 8 Na⁺ ions and electrons and also suffers from reaction irreversibility, poor electric conductivity, and fast capacity fading. Recently, Liu’s group synthesized Fe₃O₄/3D graphene hybrid materials where graphene matrix alleviates aggregation of nanosized Fe₃O₄ particles and improves cycling stabilities. After 200 cycles, a high discharge capacity of 518.7 mAh g⁻¹ was maintained when tested at 0.1 C (92.6 mA g⁻¹) with coulombic efficiencies (CEs) higher than 99% (Figure 7e) [55,58]. Qin et al. developed chain-like Fe₃O₄/C/red P by employing magnetic fields, which shows an adequate cycle life with a discharge capacity of 1390 mAh g⁻¹ up to 200 cycles tested at 0.2 A g⁻¹ and superior rate capability with a capacity of 692 mAh g⁻¹ tested at 2A g⁻¹ [8,59].

Besides iron oxides, other conversion reaction types of later TM oxides for SIBs have also been widely studied such as MnO₂, Mn₃O₄, Co₃O₄, NiO, CuO, etc. Among them, MnO₂ in SIBs has attracted a lot of attention due to its low price, polarization, reaction potential, and a significant theoretical capacity of 1232 mAh g⁻¹ [60]. Weng et al. synthesized ultrafine MnO₂ nanoparticles (~4 nm), surrounded by heterogenous interfacial mesoporous SiO₂ bonds, and it displays high rate capability and stable cyclability, exhibiting a notable discharge capacity of 1000 mAh g⁻¹ tested at 37.5 mA g⁻¹. It delivers an increased initial reversible capacity of 567 mAh g⁻¹ compared to that of pure MnO₂ particles (19 mAh g⁻¹), while maintaining 70% of its capacity following 500 cycles tested at 0.15 A g⁻¹ [61]. To sum up, different engineering strategies have been applied to various compositions of TM oxides as insertion- or conversion-reaction-type anodes for SIBs. Moreover, greater attention needs to be paid to achieve reversible capacities close to their theoretical capacities with a high ICE regarding stable solid electrolyte interphase (SEI) layers covering the electrode surface [62].

2.2. Metal Sulfides

Transition metal sulfides (TMSs) are representative chalcogenides, which have high theoretical capacities as anode materials for SIBs with the conversion reaction mechanism and higher reaction potentials compared to those of TMOs (Figure 4b). TMSs have attracted great attention not only as anode materials for SIBs but also for other energy storage applications of supercapacitors or energy conversion applications with electrocatalysts and photocatalysts [63,64]. TMSs, S-containing compounds, generally exhibit lower specific capacities than those of TMOs because they also react with two Na⁺ ions at the end of the discharge process by forming Na₂S or Na₂O phases, while the molar weight of S (32 g mol⁻¹) is higher than that of O (16 g mol⁻¹). However, TMSs are classified into two groups according to their composition as metal-rich sulfides (TM_xS_y, x > y) or S-rich sulfides (x < y), and some S-rich sulfides can show higher specific capacity than TMOs due to the high contents of S in the compounds. For example, the Fe-S binary system has various stoichiometric compounds from metal-rich to S-rich phases such as Fe₃S, FeS, Fe₃S₄, Fe₇S₈, and FeS₂ with theoretical capacity ranging from 269 to 894 mAh g⁻¹ [55]. There are additional advantages of TMSs such as the strength of M-S bonds in TMSs being lower than that of M-O bonds in TMOs, meaning kinetic favorability of conversion reactions in TMSs [7]. Some studies have also depicted that TMSs have better electrical conductivity and higher electrochemical activity than their corresponding TMO counterparts. Taking NiCo₂S₄ as an example, it shows much lower band gap energy of 2.4 eV (vs. 3.6 eV) and a higher sweep coefficient of I-V curves compared to those of NiCo₂O₄ [65,66].

TM disulfides (TMS₂), known as 2D layered materials (TM = Ti, V, Mo, W, etc.), have high contents of S in the structure with TM planes sandwiched by two-hexagonal S planes per unit of layer and exhibit high specific capacities (Figure 8a) [67,68]. During the sodiation process, the reaction mechanism is expressed as TMS₂ + x Na⁺ + x e⁻ → Na_xTMS₂ (x < 1.0) and Na_xTMS₂ + (4 − x) Na⁺ + (4 − x) e⁻ → TM⁰ + 2 Na₂S. At the initial stage of discharge, the insertion of Na⁺ ions into the layered structure occurs followed by the conversion reaction with phase separation (Figure 8b) [69]. Since TMSs have relatively higher reaction potentials than anodes for SIBs, most TMSs undergo a conversion reaction at the end of the discharge process when the potential reaches close to the redox potential of Na/Na⁺. Previous studies on exploring electrochemical properties of anodes for SIBs have mainly been performed on early TMS₂ of TiS₂ and VS₂, which have the lightest molar weight of TMs and are the most common 2D layered materials of MoS₂ phase. As an example, the MoS₂ structure undergoes an initial intercalation of Na⁺ ions followed by a conversion reaction, sharing an analogous structure of graphite with covalently bonded layers separated by weak van der Waals bonding with common interlayer spacing of 0.62 nm. This facilitates the faster ion diffusion and sustains reversible capacities and volumetric changes during cycles [67]. Specifically, structural changes in MoS₂ during the sodiation process can be classified into three groups: those with a two-layer stacked hexagonal structure (2H-MoS₂), one-layer stacked trigonal structure (1T-MoS₂), and three-layer stacked rhombohedral structure (3R-MoS₂) (Figure 8c) [70]. The total reaction equations are expressed as 2H-MoS₂ + 0.5 Na⁺ + 0.5 e⁻ → 2H-Na_0.5MoS₂ (at 0.85 V vs. Na/Na⁺), 2H-Na_0.5MoS₂ + (x − 0.5) Na⁺ + (x − 0.5) e⁻ → 1T-Na_xMoS₂ (at 0.75 V vs. Na/Na⁺, x < 1.5), and 1T-Na_xMoS₂ + (4 − x) Na⁺ + (4 − x) e⁻ → Mo⁰ + Na₂S (<0.2 V vs. Na/Na⁺) [69]. Based on the above reaction mechanisms, MoS₂ exhibits an inferior cyclability and significant voltage hysteresis arising from irreversible phase transformation and the shuttle effect in polysulfides.

The 2D layered structure of MoS₂ also shows intrinsically low electronic conductivity, resulting a high interfacial charge transfer impedance during cycling [71]. Normally, nanostructured particles in the form as nanosheets, nanotubes, or nanoflowers are combined with conductive carbonaceous materials, including amorphous carbon particles, carbon nanotubes (CNTs), or reduced graphene oxide (rGO), to facilitate Na⁺ ion diffusion and electron transport [72,73,74]. While the 2D layered structure of MoS₂ has a high theoretical capacity of 670 mAh g⁻¹, the reported monolayer MoS₂/graphene in SIBs suffered from a low reversible capacity of 230 mAh g⁻¹ when tested at 25 mA g⁻¹, along with a poor cyclability [75,76]. Therefore, to enhance an electrochemical performance of MoS₂, advanced nanostructure fabrication is essential. Shi et al. synthesized 2D MoS₂-carbon hybrid sandwiched nanosheets into 3D hierarchical hollow nanotubes. The carbon hybrid structure enlarges the space of the 2H-MoS₂ (002) interlayer from 0.615 to 0.986 nm for fast Na⁺ ion insertion/extraction kinetics. Simultaneously, the 3D hollow structure can alleviate aggregation of particles and cope with volume change for electrode structure integrity. They showed an initial discharge capacity of 620 mAh g⁻¹ with an accompanying ICE of 84% tested at 0.2 A g⁻¹ and maintained 477 mAh g⁻¹ after 200 cycles (Figure 8d). Furthermore, a high reversible capacity of 415 mAh g⁻¹ could be kept through 200 cycles tested at 1 A g⁻¹ and 187 mAh g⁻¹ is attained at a higher current density of 20 A g⁻¹ [75].

Figure 8. (a) Crystal structure illustration of TMS₂ structure (TM = Mo or W). Reproduced with permission [68]. Copyright 2014, Elsevier Ltd. (b) Crystal structures of MoS₂ structure with 2H, 1T, and 3R structures. Reproduced with permission [69]. Copyright 2017, WILEY-VCH. (c) Voltage profiles and phase transformation sequences of MoS₂ structure during sodiation and desodiation processes. Reproduced with permission [70]. Copyright 2022, Elsevier B.V. (d) Voltage profiles of MoS₂/C nanotube electrode tested at 0.2 A g⁻¹. Reproduced with permission [75]. Copyright 2016, Elsevier Ltd.

Figure 8. (a) Crystal structure illustration of TMS₂ structure (TM = Mo or W). Reproduced with permission [68]. Copyright 2014, Elsevier Ltd. (b) Crystal structures of MoS₂ structure with 2H, 1T, and 3R structures. Reproduced with permission [69]. Copyright 2017, WILEY-VCH. (c) Voltage profiles and phase transformation sequences of MoS₂ structure during sodiation and desodiation processes. Reproduced with permission [70]. Copyright 2022, Elsevier B.V. (d) Voltage profiles of MoS₂/C nanotube electrode tested at 0.2 A g⁻¹. Reproduced with permission [75]. Copyright 2016, Elsevier Ltd.

In the case of FeS₂ with a non-layered cubic structure, it features a high theoretical capacity of 894 mAh g⁻¹ and several advantages of cost-effectiveness, natural abundance, and environmental friendliness [69]. Nevertheless, in terms of sodium storage, it also shows an inferior cycle performance stemming from the large volume changes occurring during conversion reactions below 0.8 V vs. Na/Na⁺ after irreversible intercalation reactions [77]. Several studies exhibited better cycle retentions by using a higher discharge cut-off potential at ~0.5 V vs. Na/Na⁺ to avoid severe structural changes by conversion reactions, while this approach is not an ideal solution due to the substantially decreased reversible capacities [55].

2.3. Metal Selenides

One of the representative materials in chalcogenides commonly studied as anodes for SIBs is transition metal selenide (TMSe). TMSes have been applied in numerous applications of electrochemical capacitors, optoelectronics, photodetectors, and secondary batteries of LIBs, SIBs, and magnesium ion batteries (MIBs) [78,79]. Obviously, selenium (Se), belonging to group 16 with S, can store two Na⁺ ions and then form Na₂Se phase after the sodiation process. Compared to TMSs, it is hard to achieve higher specific capacities in TMSes due to the 2.5 times higher molar weight of Se than S. Additionally, Se has received less attention as anodes for SIBs compared to S because S is more Earth abundant and cheaper in cost [80]. Nonetheless, the much higher electronic conductivity (~10⁻³ S m⁻¹) of Se than S has led many studies on TMSes as anodes for SIBs to explore their own advantages [77,78,81,82]. The TM di-selenide (TMSe₂, where TM = Sn, Mo, W, V, Bi, Ti) structure is the same layered structure as TMS₂ with sandwiched Se-TM-Se layers with covalent bonds connected by weak van der Waals bonds to the c-axis. Considering a Se atom has a larger atomic radius and stronger metallicity, TMSes can have a larger interlayer distance and a higher intrinsic ionic conductivity [78].

As shown in Figure 9a, since Morales et al. first discovered MoSe₂ as an anode for SIBs in 1996s, diverse compositions have been researched [78]. MoSe₂ has merits of higher electronic conductivity and broader interlayer spacing (0.64–0.65 nm) relative to those of MoS₂ (0.62 nm), whereas MoSe₂ initiates a conversion reaction at ~0.56 V vs. Na/Na⁺, surpassing the potential of 0.4 V observed in MoS₂ phase. In order to further improve electronic conductivity while withstanding large volume changes (up to 300%) [80], Liu et al. synthesized MoSe₂ nanosheets encapsulated by hollow carbon nanospheres (MoSe₂@HCNs) via a hydrothermal method. They showed an enlarged interlayer spacing of 1.02 nm and excellent CEs higher than 98.3%, while maintaining high discharge and charge capacities of 502 and 471 mAh g⁻¹ following 1000 cycles tested at 1 A g⁻¹ and 3 A g⁻¹, respectively. Additionally, a reversible capacity of 382 mAh g⁻¹ can be attained even when tested at a high current density of 10 A g⁻¹ [83].

Chen et al. synthesized a hybrid mixture of 1T and 2H phases of MoS₂/MoSe₂ with the 2D structure of rGO (MoSSe@rGO) as a conductive matrix to create 3D pathways for electron transport through a high-temperature hydrothermal method (Figure 9b) [84]. Based on the improvements of its electrical conductivity and reactivity through metallic 1T phase, MoSSe features the high energy density of MoS₂ and stable stability of MoSe₂, with improved Na⁺ ion storage kinetics by achieving a high pseudocapacitance of MoSSe@rGO. It has a few-layered structure (<6 layers) and an interlayer spacing of 0.8 nm, which can alleviate lattice strain and enhance Na⁺ ion migration. The MoSSe@rGO electrode showed an initial capacity of 634.1 mAh g⁻¹ with an improved ICE of 73.6%, originated by its low Na⁺ ion diffusion barrier of 0.087 eV and its high electrical conductivity of 5.6 × 10⁻⁴ S cm⁻¹ (Figure 9c). With increasing the current density, the applied polarization is 0.7 V even when tested at 6.4 A g⁻¹ and cycle retention is excellent at 90% during 600 cycles tested at 1 A g⁻¹ (Figure 9d). In addition, the MoSSe@rGO electrode exhibited fast reaction kinetics at the discharge capacity 533.9 mAh g⁻¹ even at a low temperature of around 0 °C, which is 87.8% of the capacity obtained at room temperature [84]. Beyond MoSe₂ phase, layered and non-layered TMSe phases have been studied in early and later TMSes, specifically including cost-effective Fe-containing FeSe, Fe₃Se₄, and FeSe₂ phases (Figure 9a).

2.4. Metal Phosphides

Transition metal phosphides (TMPs) have gained considerable attention owing to the highest theoretical capacity of P anodes for SIBs. Since Si anodes in group 15 can only form the NaSi alloy phase, not a Na-rich phase akin to the Li_4.4Si phase formed in LIBs, P in group 16 is able to deliver the highest theoretical capacity reaching 2596 mAh g⁻¹ through generating Na-rich Na₃P alloy phase [33]. However, red P, the most commonly applied anode material for SIBs among the P polymorphs, is flammable and has a limited electronic conductivity (~10⁻¹⁴ S cm⁻¹). TMPs have been proven to address the flammability and low electronic conductivity issues by reacting with TM cations and P. Basically, TMPs have high degree of electron delocalization and high-lying mixed anion–TM bands, resulting in an overall lower reaction potential as anodes for SIBs compared to that of the above-mentioned compounds [85]. These lower reaction potentials of TMPs can provide diversity in electrochemical energy storage mechanisms of anodes for SIBs with insertion, alloying, and conversion reactions depending on compositions of TMPs.

The covalent bond character of the TM-P bonding increases across the periodic table [86] and early TM-containing phosphides (i.e., V-P) can exhibit insertion reaction mechanisms without conversion reactions occurred even at the end of discharge, close to Na/Na⁺. In addition, a strong consistency in elemental compositions and structural trends in TMPs was observed with interesting structural and compositional diversity including different electronic properties from electrical conducting metallic phase to semiconducting and insulating phases of TMPs [87]. In this regard, TMPs can be candidates for the most attractive anode materials for SIBs based on their considerable specific and volumetric capacities, low reaction potentials (0.3–0.7 V vs. Na/Na⁺), and diverse electrochemical reaction mechanisms and properties [33].

As an early-TM-containing binary V-P system, various phases are known from TM-rich to P-rich phases such as V₃P, V₂P, V₁₂P₇, V₄P₃, VP, V₄P₇, VP₂, and VP₄ and the latter four compounds have been researched as anode materials in LIBs [85,88,89,90]. Among them, VP showed a topotactic intercalation reaction with Li⁺ ions with excellent cycle retentions and favorably low insertion potentials as anodes (Figure 10a) [91]. Unfortunately, this attractive material is not electrochemically active in Na cells, exhibiting a meaningless capacity. However, Kim et al. explored a novel anode material of V₄P₇ phase, exhibiting topotactic insertion/extraction reactions with Na⁺ ions and an excellent cycle stability [92]. V₄P₇ does not undergo conversion reactions even when completely discharged to 0.01 V vs. Na/Na⁺ but shows insertion reaction mechanisms with the reaction equation of V₄P₇ + xNa⁺ + xe^- ↔ Na_xV₄P₇ (x $\approx$ 0.95) (Figure 10b). Topotactic Na⁺ ion insertion/extraction reactions of V₄P₇ were confirmed by transmission electron microscopy (TEM), ex situ X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) of V K edges (Figure 10c–e). The V₄P₇ electrode exhibits a reversible capacity of 234 mAh g⁻¹ tested at 0.1 A g⁻¹ during 100 cycles alongside an imperceptible capacity decay (cycle retention of 99.99%) (Figure 10f). When V₄P₇ is compared to the representative oxide anode of VO₂(B), which has a similar insertion reaction mechanism, the reversible capacities between the two phases are comparable, while V₄P₇ shows much lower reaction potential, leading to its higher energy density (Figure 10g,h). Although this insertion type of TMPs can compensate for the drawback of insertion-type oxides with too high reaction potential, exploring other candidates beyond V₄P₇ has been limited to date [92].

Additionally, the conversion reaction type of TMPs has been studied in P-rich compounds due to their high theoretical capacities. As shown in

Table 2. The state-of-the-art electrochemical performance achieved in P-rich phosphide anodes in SIBs.

Materials	Initial Reversible Capacity	ICE	Cycling Performance	Capacity Retention (%)	Ref.
CoP3@C	238.1 mAh g−1 @ 100 mA g−1	87	212 mAh g−1 after 80 cycles @ 100 mA g−1	77.6	[93]
CoP4/CF	902 mAh g−1 @ 0.3 A g-−1	53.3	535 mAh g−1 after 1000 cycles @ 1 A g−1	90	[94]
CrP4/C	881 mAh g−1 @ 50 mA g−1	78.3	613 mAh g−1 after 50 cycles @ 50 mA g−1	86.4	[95]
CuP2/C	396 mAh g−1 @ 50 mA g−1	65	450 mAh g−1 after 100 cycles @ 200 mA g−1	-	[96]
CuP2/C	470 mAh g−1 @ 150 mA g−1	67.1	450 mAh g−1 after 30 cycles @ 150 mA g−1	-	[97]
FeP4	1137 mAh g−1 @ 89 mA g−1	84.1	1023 mAh g−1 after 30 cycles @ 89 mA g−1	90	[98]
FeP4/CF	984 mAh g−1 @ 0.3 A g−1	58.8	711 mAh g−1 after 1000 cycles @ 1 A g−1	90	[94]
GeP5/AB/p-rGO	597.5 mAh g−1 @ 100 mA g−1	60	400 mAh g−1 after 50 cycles @ 500 mA g−1	81.6	[99]
MnP4/Graphene	718 mAh g−1 @ 50 mA g−1	66.5	627 mAh g−1 after 100 cycles @ 50 mA g−1	87.3	[100]
NiP1.5S0.5	608 mAh g−1 @ 50 mA g−1	75.8	299 mAh g−1 after 200 cycles @ 500 mA g−1	-	[101]
NiP2/C	489 mAh g−1 @ 50 mA g−1	75	231 mAh g−1 after 250 cycles @ 500 mA g−1	93.6	[102]
NiP3/CNT	868.4 mAh g−1 @ 100 mA g−1	-	363.8 mAh g−1 after 200 cycles @ 1600 mA g−1	65	[103]
SiP2/C	501 mAh g−1 @ 50 mA g−1	76	410 mAh g−1 after 100 cycles @ 50 mA g−1	-	[104]
SnP3/C	805 mAh g−1 @ 150 mA g−1	71.2	810 mAh g−1 after 150 cycles @ 150 mA g−1	-	[105]
Sn3P4-C	721 mAh g−1 @ 200 mA g−1	60	420 mAh g−1 after 2000 cycles @ 2 A g−1	-	[106]
ZnP2/C	704 mAh g−1 @ 50 mA g−1	65.8	883 mAh g−1 after 130 cycles @ 50 mA g−1	-	[107]

, various compositions of P-rich phosphides have been studied as anodes for SIBs and they exhibit reversible capacities ranging from 250~1100 mAh g⁻¹ depending on their compositions (Table 2) [93,94,95,96,97,98,99,100,101,102,103,104,105,106,107].

Specifically, TM tetraphosphides (TMP₄s) contain four P elements and exhibit a high theoretical capacity over 1500 mAh g⁻¹. TMP₄s consisting of Earth-abundant elements of Mn or Fe can be promising candidates for economically efficient and high-performing anode materials for SIBs. Kim et al. reported a triclinic structure of 6-MnP₄ phase, which is one of the MnP₄ polymorphs and classified according to the arrangement of MnP₆ octahedra. This phase features high capacity for an anode material for SIBs, showing an initial reversible storage capacity of 1028 mAh g⁻¹ tested at 50 mA g⁻¹ (Figure 11a) [100]. The reaction mechanism of MnP₄ for SIBs was examined through ex situ XRD, TEM, and XANES spectra exhibiting a direct conversion reaction as MnP₄ + 12 Na⁺ + 12 e⁻ → Mn⁰ + 4 Na₃P, which is different to the two-step reactions of alloying and conversion observed in LIBs as MnP₄ + 7 Li⁺ + 7 e⁻ → Li₇MnP₄ + 5 Li⁺ + 5 e⁻ → Mn⁰ + 4 Li₃P (Figure 11b,c). The MnP₄ phase was combined with 20 wt.% graphene nanosheets (G20) to form MnP₄/G20 nanocomposite and it showed reversible capacity of 446 mAh g⁻¹ tested at 0.5 A g⁻¹ after 250 cycles, maintaining 78% of its initial capacity (Figure 11d) [100].

In addition to the Mn-P system, the most abundant element in TMs as Fe-containing TMPs have been applied to anode materials with FeP, FeP₂, and FeP₄ phases for SIBs. FeP has an orthorhombic crystal system with FeP₆ sharing edges and faces in the structure, holding a theoretical capacity of 926 mAh g⁻¹. Since FeP undergoes conversion reactions as FeP + 3 Na⁺ + 3 e⁻ → Fe⁰ + Na₃P accompanying large volume changes and structural instability, fabricating a composite with conductive carbon materials can be a simple approach. The hybrid 3D composite structure of FeP and P-doped graphitic N-rich graphene (FeP/NPG) was synthesized with higher ionic conductivity and structural stability of FeP particles [108]. It demonstrated high reversible capacities of 613 mAh g⁻¹ tested at 50 mA g⁻¹ and 349 mAh g⁻¹ tested at a high current density of 3 A g⁻¹. Additionally, a good long-term cyclability was achievable with a reversible capacity of 378 mAh g⁻¹ and 90% capacity retention after 700 cycles tested at 1 A g⁻¹ [108]. While orthorhombic FeP₂ phase is a high-capacity anode for LIBs [109], possessing a theoretical capacity of 1365 mAh g⁻¹, this phase has been found to be electrochemically inactive in Na cells. Monoclinic FeP₄ phase, another P-rich phosphide in the Fe-P system, undergoes a direct conversion reaction, the same as that observed in MnP₄ electrodes. The FeP₄ electrode indicates a reversible capacity of 1137 mAh g⁻¹ tested at 89 mA g⁻¹ with an ICE of 84% and the high capacity is maintained for 30 cycles (Figure 11e) [98].

Recently, the monoclinic structure of CrP₄ phase with the space group C2/c, identical to that of the metallic phase of VP₄, was introduced as a high-capacity anode for SIBs. Its narrower direct band gap, in contrast to the aforementioned 6-MnP₄ and FeP₄ phases, is a distinguishing feature [95]. The electrochemical reaction mechanism of CrP₄ was confirmed as a two-step process including insertion and conversion reactions: CrP₄ + xNa⁺ + xe⁻ → Na_xCrP₄ + (12 − x)Na⁺ + (12 − x)e⁻ → Cr⁰ + 4 Na₃P. The CrP₄ electrode shows a first discharge capacity of 1125 mAh g⁻¹ tested at 50 mA g⁻¹, accompanied by an ICE of 78.3%. For high-rate cycle retention, CrP₄ nanoparticles were mixed with 15 wt.% of acetylene black carbon to achieve a higher reversible capacity of 369 mAh g⁻¹ over 100 cycles tested at 500 mA g⁻¹ compared to CrP₄ electrodes (Figure 11f) [95].

In addition to TMPs, binary phosphide phases including electrochemically active metals or non-metals of Sn, Si, and antimony (Sb) reacting with Na⁺ ions have also been introduced as anode materials for SIBs. In particular, Sn-containing phosphide anodes have been most widely studied in compositions of Sn₄P₃, SnP, Sn₃P₄, and SnP₃ phases [105,110,111]. Binary Sn-P phases exhibit high theoretical capacity ranging from 1133 to 1616 mAh g⁻¹ since both elements in the binary phases are electrochemically active with Na⁺ ions. As an example, SnP₃ undergoes both conversion and alloying reactions and the reaction mechanism can be expressed as SnP₃ + 12.75 Na⁺ + 12.75 e⁻ → Na_3.75Sn + 3 Na₃P with a theoretical capacity of 1616 mAh g⁻¹ [105]. The SnP₃/C electrode delivers a reversible capacity close to 800 mAh g⁻¹ during 150 cycles without noticeable capacity decay tested at 150 mA g⁻¹ [105]. The high electrochemical performance of Sn-P anodes can be attributed to the dispersed Na₃P phases providing a buffer matrix preventing the aggregation of Na₁₅Sn₄ nanoparticles and ensuring the structural stability of the electrode during reversible sodiation/desodiation processes [110].

So far, it can be inferred that a variety of TM-rich and P-rich phosphides have been studied as anodes for SIBs, and the electrochemical reaction mechanisms of some of them are still unclear. In addition, the electrochemical reaction mechanisms and properties for anode candidates have not been fully explored for all of the compositions and polymorphs of TMPs. Above all, the drawbacks that can possibly occur in high-capacity TMP anodes such as severe volume changes during cycling, sluggish ionic and electronic conductivities caused by a conversion reaction, and unstable SEI layers by mechanical fractures call for solutions for high-performance anode applications for SIBs.

2.5. Metal Nitrides

Transition metal nitrides (TMNs) have received relatively little attention compared to the aforementioned compounds, however, they may be promising candidates as next-generation anode materials for SIBs owing to their superior ion diffusion kinetics and lower conversion reaction potentials, which have been observed in various TMNs [112,113]. The N-do** strategy has been widely applied in multiple types of carbonaceous materials including graphene, CNTs, carbon matrix, carbodiimide, MXene, etc. The beneficial effect of N do** has been proved for alkali ion storage performance as well as electronic conductivity enhancement [114,115]. Utilizing N as an anionic redox center in anode materials for LIBs can yield a notable specific capacity, based on the relatively low molecular weight of N, but a large number of Li⁺ ions are accompanied by a conversion reaction forming the Li₃N phase. Li₃N phase is known to have an unprecedentedly high ionic conductivity (~10⁻³ S cm⁻¹) [116] with robust mechanical stability as a promising component of solid electrolyte interface (SEI) phase [117,118,119,120]. In this regard, TMNs can generate a mechanically stable SEI layer on the surface of TMNs with a high ionic conductivity. Compared to oxygen-centered counterparts such as TMOs, the N anionic centers show structural stability and high electrochemical reactivity, exhibiting fast alkali ion diffusion kinetics [121,122,123,124]. Hence, TMNs are expected to be highly advantageous for next-generation anodes, and various compounds including TiN, VN, Mo₂N, MnN, Fe₂N, and Fe₃N have been recently introduced for SIBs.

TMNs have three representative forms, face-centered cubic (fcc), hexagonal closed packed (hcp), and simple hexagonal structures, with unusual combinations of metallic, covalent, and ionic bonding properties [123]. As for early TMNs, several candidates are TiN, VN, NbN, MoN, Mo₂N, and WN, where the N atoms are integrated into numerous interstitial sites of the parent TMs and generate a rearranged electronic structure.

The intercalation type of cubic structure TiN can bring open channels for Na⁺ ion intercalation reactions based on its low volumetric density of 5.4 g cm⁻³ and has several advantages such as high chemical stability, electrical conductivity (3.7 × 10⁶ S m⁻¹), and Young’s modulus (420 GPa) [125]. However, this material has a bottleneck of finding cost-effective and eco-friendly synthesis methods. The traditional synthesis process of TiN uses a direct reaction between Ti-based powders and N₂ or NH₃ gases under high temperature (600~1200 °C) [124], which is not desirable for safe and mass productive processes. Meanwhile, Liu et al. synthesized TiN@C composite by using MXene as the Ti source and the sacrificial templates [126]. MXenes are a class of 2D inorganic compounds such as TM carbides, nitrides, or carbonitrides. A simple method to prepare TiN was proposed as 2D layered MXene of Ti₃C₂T_x (T = F or O) which was synthesized first and converted to TiN by using an intercalation agent of hexamethylenetetramine (HMT) with heating at 800 °C under Ar conditions. The quantum-dot-sized TiN can provide shorter ionic diffusion distances, while 2D ultrathin carbon nanosheets can reinforce structural stability and enhance electron transport rate. As-synthesized TiN@C delivers a discharge capacity of 414 mAh g⁻¹ with a current density of 0.5 A g⁻¹ and high reversible capacities of 170 mAh g⁻¹ and 149 mAh g⁻¹ after 5000 cycles at 0.5 A g⁻¹ and 1 A g⁻¹, respectively [126].

One of the other early TMNs, VN, which has a significant theoretical capacity of 1238 mAh g⁻¹ and electrical conductivity of 1.49 × 10⁶ S m⁻¹ [125,127], was introduced as an anode for SIBs. Cheng et al. applied a cost-effective synthesis process involving hard carbon (HC) composites by coating 8.6 wt.% of VN on the HC surface (VN-HC). The synthesis process includes the reaction of VOCl₃ with cellulose followed by heating under N₂ at 1400 °C. The as-synthesized VN-HC exhibited a first discharge capacity of 354 mAh g⁻¹ tested at 50 mA g⁻¹ and a reversible capacity of 294 mAh g⁻¹ following 50 cycles. When the total capacity of VN-HC composite is divided by the mass of VN alone, it reaches the highest specific capacity of 605 mAh g⁻¹ in VN phases [127]. Zeng et al. synthesized ultrafine VN quantum dots encapsulated in yolk–shell N-doped carbon nanospheres (VN QDs/N-C) that can shorten ion diffusion distances, relieve volume expansions, and improve conversion reaction rates. VN QDs/N-C shows a high first discharge capacity of 1167.7 mAh g⁻¹ while a low charge capacity of 486 mAh g⁻¹ tested at 50 mA g⁻¹ with an ICE of 41.6%. However, it showed moderate reversible capacity of 275.4 mAh g⁻¹ even with a very high current density of 20 A g⁻¹ after 30,000 cycles [128].

Wei et al. presented a novel strategy to control the electrochemical reaction mechanism of a layered VN structure pillared by Al atoms, showing an intercalation reaction with Na⁺ ions rather than a conventional conversion reaction (Figure 12a) [129]. Al atoms can act like a pillar to stabilize the inside channels of layered VN and improve ion transport in the interlayers by hindering the occurrence of conversion reactions. The results showed a high reversible specific capacity of 372 mAh g⁻¹ tested at a current density of 50 mA g⁻¹ and long cycle stability with a reversible capacity of 115 mAh g⁻¹ after 7500 cycles tested at 0.5 A g⁻¹ (Figure 12b). The as-prepared VN was combined with rGO and it indicated a higher capacity of 400 mAh g⁻¹ tested at 50 mA g⁻¹ and a longer cyclability with a capacity of 155 mAh g⁻¹ during 10,000 cycles at a high current density of 1 A g⁻¹. However, the ICEs of VN and VN@rGO are extremely low, about 12%, which might be due to the mismatch of electrolyte and should be further improved in the future [129].

To date, various later TMNs such as MnN, FeN, Fe₂N, Fe₃N, CrN, and Cu₂N have also been applied in energy storage applications of supercapacitors and LIBs. MnN is one of the attractive candidates for anode materials based on the Earth-abundant Mn resources and its high theoretical capacity, while there have been limited studies on this material for SIB applications. A 3D nanostructured MnN@rGO composite has been applied in anodes for SIBs to accommodate severe volumetric changes of MnN during sodiation/desodiation [130]. When the electrode was tested at a current density of 0.25 A g⁻¹, MnN@rGO composite exhibits an ICE of 52.1%, accompanied by discharge and charge capacities of 1635 and 850 mAh g⁻¹, respectively (Figure 12c) [130]. The electrochemical reaction mechanism of MnN has not been investigated in this study, while it could exhibit a theoretical capacity of 1166 mAh g⁻¹ if MnN undergoes the reaction of conversion to form Na₃N phase. It is expected that improvements in the ICE of this material will achieve high specific capacity.

Fe-based TMNs are one of the promising candidates for cost-effective anode materials and Fe₂N showed relatively higher electrochemical performance. Recently, Li et al. developed 1D self-supporting Fe₂N nanocubes as hollow core–shell hybrid fibers (CSHN) coated by N-doped carbon matrix. The hierarchical carbon matrix can contribute to the fast electronic conductivity and protect active materials from side reactions, simultaneously leading to fast charge/discharge capability. Moreover, its porous nature can buffer the volume expansion and retain reversible capacities with a high cycle retention. This composite electrode delivers a high capacity around 400 mAh g⁻¹, maintaining 95.6% of the initial capacity after 2000 cycles tested at 2 A g⁻¹ (Figure 12d). The achieved performance is superior to those of state-of-the-art anodes of TMNs for SIBs such as Fe₃N, Sn₃N₄, VN, Mo₂N, and Ni₃N [131].

The above-mentioned TMNs present rather high energy densities due to the high specific capacities and low reaction potentials of TMNs, while a significantly high capacity has not been achieved yet. One of the bottlenecks of TMNs for realizing high-performance anodes is mainly originated by the inferior ICEs observed in most TMNs. There are several factors that influence the ICE of anode materials such as irreversible decomposition of electrolyte forming SEI layers, surface defects or functional groups, irreversible sodiation/desodiation processes, etc. [132]. Further optimization of electrolyte compositions, binders, and compatibilities with TMNs should be studied to address these issues. Furthermore, the lack of research on N-rich TMNs may be another reason for the lack of attention on TMNs as potential next-generation anodes, thus a lot more studies on exploring new compositions of TMNs are required for further improvements in this material group.

2.6. Future Perspectives Regarding the Commercialization of SIBs

So far, the five groups of transition metal compounds as anodes for SIBs have been summarized in terms of reaction mechanism with Na⁺ ions, advantages, and common issues that need to be addressed of each group (

Table 3. Na⁺ ion storage mechanism, advantages, common issues that need to be addressed, and example of each group in metal compound anodes for SIBs.

Material Group	Reaction Mechanism (Discharged to Na/Na+)	Advantages	Common Issues that Need to Be Addressed	Example
Oxides (TM-O)	Insertion	Cost-effective Excellent cycle retention	Low capacity Moderate working potential	Early TM oxides (e.g., TiO2, Li4Ti7O12, Na4Ti5O12, VO2, etc.)
	Conversion	Cost-effective High capacity	Poor cycle retention Moderate working potential Moderate voltage hysteresis	Later TM oxides (e.g., Fe2O3, Fe3O4, Co3O4, NiO, etc.)
Sulfides (TM-S)	Conversion	Moderate capacity	High working potential Large voltage hysteresis Moderate cycle retention	TM sulfides (e.g., TiS2, MoS2, FeS, FeS2, NiS2, etc.)
Selenides (TM-Se)	Conversion	High electrical conductivity	Low capacity High working potential High voltage hysteresis	TM selenides (e.g., TiSe2, MoSe2, FeSe, FeSe2, etc.)
Phosphides (TM-P)	Insertion	Low working potential Excellent cycle retention	Low capacity	Early TM phosphides (e.g., V4P7)
	Conversion	Low working potential High capacity	Low voltage hysteresis Poor cycle retention	Middle and later TM phosphides (e.g., CrP4, MnP4, CoP4, FeP4, etc.)
Nitrides (TM-N)	Insertion	Low working potential Excellent cycle retention	Moderate capacity Low ICE	Early TM nitrides (e.g., TiN, VN, etc.)
	Conversion	Low working potential Feasible high capacity	Low voltage hysteresis Low ICE	Later TM nitrides (e.g., Ni3N, Cu3N, etc.)

). The presented reaction mechanisms and corresponding electrochemical performances are important, while it is worth considering the research gap between the present and commercial-scale applications in this material group. Since a strong candidate of LFP batteries competes with SIBs targeting cost-effective and grid-scale battery markets, the cost of electrode materials and their manufacturing costs should be critical factors. On the anode side, hard carbon, a cost-effective carbonaceous material, is compelling as the first anode material applied to commercialization of SIBs like the graphite anode in LIBs. Battery manufacturer CATL Corp. recently announced it would mass-produce SIBs with the use of in-house hard carbon as the anode material, and it is obvious that hard carbon will soon be the 1st-generation commercial anode material [133]. Similar to the commercially used graphite–Si composite in LIBs using the high capacity of Si anodes, it is essential to develop a high-performance anode in SIBs that can be applied for the composite with hard carbon to compensate for the low capacity and poor rate capability of hard carbon anodes. Since the alloying reaction anodes in LIBs such as Si, P, and Sn have exhibited their own hurdles for practical applications, there would be an opportunity in transition metal compounds for commercial SIBs by further develo** their reaction mechanisms and performances. To this end, it is necessary to develop and modify the large-scale synthesis process of metal compound anodes by considering manufacturing cost and scalability as well as their electrochemical performances. Additionally, the reversible capacity, ICE, reaction potential, and interfacial interaction of anodes need to be considered in terms of compatibility with cathodes and electrolytes in order to implement full cell performance [134].

3. Conclusions

This review summarized the recent progress in transition metal compounds as potential high-performance anodes for sodium ion batteries (SIBs). Specifically, theoretical selection rules of electrode materials were found to be the critical factors important for achieving desirable electrochemical performance regarding specific capacity, volume change rate, and reaction potential. The hard carbon anode is the only available option for commercialization of SIBs at this moment, while its low reversible capacity and poor rate capability act as bottleneck problems for the commercial implementation of SIBs for high-energy-density applications. Transition metal compounds may be the most promising candidates to address the lack of potential high-performance anodes in SIB systems based on their cost-effectiveness with Earth-abundant elements, numerous phases presenting different elemental choices, and various corresponding physicochemical and electrochemical properties.

Beyond previous common strategies including particle nanosizing, carbon coating, and fabricating composites with conductive carbon-based materials, the fundamental correlations between material composition, energy storage mechanism, and electrochemical performance have not been well established but are critical to enable rational improvement of the electrochemical performance of SIBs. The choice of elemental compositions has emerged as a strategy to tune the electrochemical energy storage mechanisms, including insertion, alloying, and conversion reactions, and electrochemical reaction potentials.

We have categorized the research into potential anode candidates with transition metal compounds based on the sort of anion for easier comprehension such as group-16-element-containing oxide, sulfide, and selenide and group-15-element-containing phosphide and nitride, respectively. Each type of compound has its own intrinsic advantages and drawbacks as anode material for SIBs. We described the characteristics of their crystal structures, the resulting electrochemical reaction mechanisms, and furthermore the latest and the most significant research results and directions. Through this paper, we hope that insights on each transition metal compound can be constructed for designing high-performance anodes for SIBs, further serving as a practical alternative to LIBs in reality.