Engineering Nanostructured Antimony-Based Anode Materials for Sodium Ion Batteries

: Sodium-ion batteries (SIBs) are considered a potential alternative to lithium-ion batteries (LIBs) for energy storage due to their low cost and the large abundance of sodium resources. The search for new anode materials for SIBs has become a vital approach to satisfying the ever-growing demands for better performance with higher energy/power densities, improved safety and a longer cycle life. Recently, antimony (Sb) has been extensively researched as a promising candidate due to its high speciﬁc capacity through an alloying/dealloying process. In this review article, we will focus on different categories of the emerging Sb based anode materials with distinct sodium storage mechanisms including Sb, two-dimensional antimonene and antimony chalcogenide (Sb 2 S 3 and Sb 2 Se 3 ). For each part, we emphasize that the novel construction of an advanced nanostructured anode with unique structures could effectively improve sodium storage properties. We also highlight that sodium storage capability can be enhanced through designing advanced nanocomposite materials containing Sb based materials and other carbonaceous modiﬁcation or metal supports. Moreover, the recent advances in operando/in-situ investigation of its sodium storage mechanism are also summarized. By providing such a systematic probe, we aim to stress the signiﬁcance of novel nanostructures and advanced compositing that would contribute to enhanced sodium storage performance, thus making Sb based materials as promising anodes for next-generation high-performance SIBs.


Introduction
Currently, lithium-ion batteries (LIBs) have emerged as one of the most important dominant power sources for portable electronic devices, electrical vehicles (EVs) and largescale electrical grids. However, with the great concerns about limited lithium resources available on Earth, the next-generation rechargeable batteries are urgently needed to be based on another non-lithium source [1,2]. Recently, sodium-ion batteries (SIB) have been becoming one potential viable alternative to LIBs, in view of their low cost, environmental benignity, and the natural abundance of sodium resources [3,4]. In order to more intuitively reflect the differences between batteries of different systems, the five properties of batteries are presented in the form of a radar diagram in Figure 1a (LMBs and Li-S stand for lithium metal batteries and lithium sulfur batteries, respectively). Research has been, and is being, intensively carried out worldwide to explore reliable electrode materials for highperformance SIBs systems. Compared with the cathodes [5][6][7][8], the anodes of SIB exhibit an unstable cycling performance and limited capacities. One of the frustrating drawbacks of anodes is that the well-established anode materials for LIBs, such as graphite and silicon, cannot be operated and used in SIBs. To end this, much effort has been put into developing new anode materials including carbonaceous materials, alloying, conversion/alloying materials, and organic compounds. Among various alloying type anode materials for SIBs, antimony (Sb)-based materials with high theoretical capacities and appropriate reaction potential are promising anode materials for SIBs [9,10], as evidenced by the increasing number of publications shown in Figure 1b. Sb is categorized as one of the semimetallic pnictogens and displays a high theoretical capacity of 660 mAh g −1 (corresponding to the formation of Na 3 Sb as the final product of alloying reaction). Among the mostly alloying-based anodes, though the phosphorus anode defines the highest gravimetric energy density (Figure 1c), phosphorus is a nonconductive anode with huge volume expansion and a final flammable Na 3 P discharge product [11], which severely limits its electrochemical performance and widespread practical application. While a bright and optimistic future is expected for Sb anodes whose volumetric capacity is very competitive towards high energy density SIBs (Figure 1d). In 2012, Monconduit's group first compared the electrochemical performance of Li-Sb and Na-Sb batteries using pure micrometric Sb particles as anodes [12]. Surprisingly, in the case of Na, the intermediate phases are mostly amorphous and could not be precisely identified. Besides, they showed that a competition takes place at the end of the discharge of the Sb/Na cell between the formation of the hexagonal and the cubic polymorphs of Na 3 Sb. This might be partially due to decreased volume expansion upon going from Sb (181.1 Å) to hexagonal Na 3 Sb (237 Å) compared to rock salt Li 3 Sb (283.8 Å). Afterwards, Baggetto et al. rationalized the amorphization of Na x Sb phases by the long ranged strain propagation due to Na-vacancy co-mpared to Li-Sb [13]. In these regards, Sb stands out as one of the most attractive anode materials for SIBs. Meanwhile, other Sb-based chalcogenides, namely Sb 2 S 3 and Sb 2 Se 3 , hold similar advantages and a typical conversion/alloying reaction involving Na x -Sb alloying systems [14]; therefore in this review we consider both as Sb-based composite anodes.
However, Sb-based anodes showed capacity fading due to the mechanical failure of the active material caused by huge volume expansion/contraction during the discharge/charge processes. In order to overcome the above problems, various nanostructuring approaches have been applied and have elicited great research interest. The results of experiments and theoretical calculations prove that the synergistic effects of different sizes, pores, morphologies and phases, along with a conductive matrix of the rational designed nanostructured electrodes, contribute to the enhanced electrochemical performance. In this review paper, recent progress in developing nanostructured Sb-based composites as high-performance anodes for SIBs are summarized. The preparation of various Sb nanostructures with different dimensions is briefly introduced, and the effects of the nanostructures on the electrochemical performance are discussed. Moreover, the role of advanced in situ/operando techniques in revealing the electrochemical mechanisms of representative Sb-based anodes is highlighted. At last, the challenges and prospects for Sb-based anodes with nanostructures are also proposed.

Pure Sb Anode
Antimony metal is one of the most interesting materials that can alloy electrochemically with Na ions. For pure Sb anodes, monodisperse nanoparticulate active materials are ideally suited for studying the effects of size and electrode morphology on the electrochemical performance. A colloidal synthesis of pure Sb nanocrystals with a mean size tunable in the 10-20 nm range was established ( Figure 2a) [15]. Another example is the fabrication of highly ordered pure Sb nanorod arrays with uniform large interval spacing (190 nm) (Figure 2b) [16]. In return for this electrode design, a superior rate capability for delivering reversible capacities of 557.7 mAh g −1 at 20 A g −1 was achieved. Notably, a morphology-controlled chemical dealloying synthesis of a nanoporous pure Sb anode confirmed that, even without conductive modification, the innovative electrode design would ensure high sodium ion accessibility and strong structural integrity [17]. Hollow materials have been explored for energy storage applications because of their special structure (high surface area, low density, high loading capacity, and shell permeability), which exhibits excellent properties such as excellent cycle stability and outstanding electronic transport properties [18]. For example, Ji's group fabricated pure Sb porous hollow microspheres, which were prepared by a galvanic replacement reaction employing Zn microspheres [19] or Ni microspheres [20] as a template. Impressively, the same group also extended the replacement reaction to fabricate cypress leaf-like Sb for SIBs anode [21] (Figure 2c), which give a superior electrochemical performance with a high reversible capacity of 629 mAh g −1 after 120 cycles, close to its theoretical capacity. More preparation strategies and advances in hollow structures will be discussed in the following parts concerning Sb/carbonaceous composites anodes.
For better understanding morphological and dimensional changes as well as interior microstructure evolution during reduction/oxidation reactions, Selvaraj et al. conducted transmission X-ray microscopy (TXM) analysis to provide sufficient information [22]. Further work is needed to broaden new routes toward green and convenient methods to synthesize pure Sb materials with higher chemical and mechanical stability, such as the replacement of toxic organic reagents and/or the development of room-temperature large-scale fabrication of stable alloy electrode materials.

Sb/Carbenecous Composite
Usually, the specific resistivity of untreated pure Sb bulk is 102~103 Ω·cm [23], which is still far from satisfactory to ensure fast electron transport when used as electrode materials in a battery. Due to large volume expansion and relative low conductivity of bulk Sb anodes, Sb-based carbonaceous composites have been proposed to maintain the structural integrity as well as to increase overall conductivity. We categorise various Sb/carbonaceous composites anodes based on their different morphological dimensional configurations.
Zero-dimensional (0D): 0D nanostructured Sb/C anodes, including quantum dots, nanodots, nanoparticles and hollow spheres and so forth. In the commercial SIBs aspect, 0D nanostructured material holds promise for high tap density electrode fabrication through multiscale assembly [24]. An aerosol spray pyrolysis technique is used to synthesize a spherical nano-Sb@C composite (Figure 2d) [25]. In this regard, nanoscale Sb alloy-based anodes were highlighted by researchers and were intensively investigated concerning its size [26,27], component design [28], composition ratio [29,30], and solid electrolyte interphase (SEI) [31,32]. For example, Ruiz et al. fabricated a macroporous Sb/magnesium fluoride (MgF 2 ) active/inactive composite material as a SIB anode. This new concept of "active/inactive composites" with nanostructure design enables the Sb/MgF 2 anode to deliver a capacity of 551 mAh g −1 after 300 cycles at a C-rate of C/2 [33].
Among them, the design of metallic Sb with hollow structures or a yolk-shell Sb@C structure is a reasonable and significant strategy to improve electrochemical performance for SIBs [19,34,35]. Hollow structures refer to materials with well-defined boundaries and interior cavities, which show broad applications in batteries and other energy related devices [18]. Cation exchange reaction based on Kirkendall's effect is an effective way to manipulate the core-shell hollow heterostructures. Benefiting from the unique stable architecture, a hollow core-shell Sb/ZnS@C heterostructure nanosphere anode shows a high reversible capacity, good rate capability and excellent cycling stability [36]. Particularly, Song et al. employed TEM tomography to visualize the architecture and quantify details of the Sb@C yolk-shell structure (Figure 3a) [37]. In this way, the accurate structural information on the core-particle size, the void space and the complex structure can be systemically established (Figure 3b  Recently, metal-organic-framework (MOF) derived carbon has shown great capability to realize precise carbon-confined nanostructures with diverse compositions and delicate morphologies [38,39]. For example, Li et al. reported an ultrafine Sb embedded in a porous carbon nanocomposite (Sb@PC) synthesized via a facile in situ substitution of the Cu nanoparticles in an MOF-derived octahedron carbon framework for sodium storage ( Figure 3f).
Apart from the aforementioned routines, high-energy ball milling has also been intensively used to realize well-dispersive distribution or formation of intermetallic alloying compounds [40][41][42]. Besides, the spraying method is an effective method for synthesizing 0D nanostructures, for example, pitaya-like Sb@C microspheres are prepared successfully by facile aerosol spray drying synthesis [43].
One-dimensional(1D): 1D nanomaterials have great capability towards applications in the fields of energy storage since they could ensure electron transport along a long axis and maintain a confinement effect across radial direction [44]. In 2013, Zhu et al. first reported a Sb/C electrode with~30 nm Sb nanoparticles uniformly encapsulated in interconnecting 1D 400 nm carbon fibers by electrospinning method (Figure 2e) [45]. Similar work involving of electrospun Sb/C as an SIBs anode was accomplished by Cao et al. almost at the same time [46]. Afterwards, a variety of 1D Sb/C composite anodes have been established including Sb/CNT composite [38], peapod-like Sb@C [47], Sb@C coaxial nanotubes ( Figure 2f) [48], N-doped carbon nanonecklaces with encapsulated Sb [49] and 1D yolk-shell Sb@Ti-O-P nanostructures [50]. Impressively, the full battery of Sb@Ti-O-P//Na 3 V 2 (PO 4 ) 3 -C presents a high output voltage (~2.7 V) and a capacity of 392 mAh g −1 after 150 cycles at 1 A g −1 .
Galvanic replacement synthesis of the 1D nanostructure is a novel mediated growth strategy to fabricate highly uniform Sb nanotubes [51]. Impressively, a full cell with assynthesized Sb nanotubes anode and Na 3 (VOPO 4 ) 2 F cathode, which exhibits a high energy density (252 Wh kg −1 ) and high output voltage (2.7 V). With respect to monitoring the structural evolution of 1D Sb@C, Luo et al. also demonstrated a novel synthetic route to fabricate nanorod-in-nanotube Sb@nitrogen-doped carbon composites [52]. The key observation is that an operando high-temperature XRD was conducted to monitor the in-situ structural evolution from Sb 2 S 3 sacrifice template to crystalline Sb. Additionally, other Sb composite anodes based on CNT [53,54] or carbon nanofibers [55] were also proven to demonstrate improved conductivity and enhanced electrochemical performance.
Two-dimensional: In the family of 2D carbonaceous functional materials, carbon nanosheets, graphene or reduced graphene oxide (rGO) sheets and newly emerging 2D Mxene materials have been widely employed to construct Sb/C composites anodes [26,[56][57][58]. Graphene incorporation should be one effective strategy for developing advanced electrode materials for SIBs [59][60][61][62][63][64]. rGO has been improved to interact with SbO 2 via C-O-Sb bonds, which effectively improves the conductivity and stability of Sb/SbO 2 @rGO composites [65]. In a very recent work, the amorphous feature of Sb nanoparticles, as well as the synergistic effect between amorphous Sb and layered nitrogen-doped layered carbon nanosheet, endow the composite with improved electron/ion transport [66].
Notably, graphene modification often introduces heteroatoms doping to further adjust charge transport and adsorption ability. For instance, a promising Sb adsorbent, nitrogen-doped reduced graphene oxide (NGO), was synthesized by a simple hydrothermal method [67]. Benefiting from the structure of graphene, researchers show that the electrophoretic deposition of Sb nanoparticles embedded in rGO sheets on Cu foil is a viable technique to fabricate binder-free electrodes [68]. Finally, graphene matrix is an ideal candidate to fabricate a binder-free, flexible electrode for SIBs due to its high mechanical strength and high conductivity. For instance, the Sb nanoparticles embedded homogenously in the interconnected framework of rGO nanosheets (Figure 2g), which give an excellent rate capacity of up to 30 C and enable SIB full cell to run up 100 charge/discharge cycles [69].
Antimonene, a new-type mono-elemental 2D layered materials (Figure 4a,b), was firstly predicted to exhibit good stability and remarkable physical properties by theoretical investigation [70][71][72]. Afterwards, mechanical isolation [73], liquid-phase exfoliation [74], aqueous shear exfoliation [75], molecular beam epitaxy [76], van der Waals epitaxy growth [77] and so forth have been demonstrated to obtain high-quality few-layer Sb exfoliated nanosheets. From the view of first-principles calculations, antimonene exhibits high specific capacity, a small diffusion barrier and low lattice expansion as an anode for SIBs [78,79] and shows relatively small structure deformation upon Li adsorption [80]. Notably, Su et al. proposed a trilayer graphene/antimonene/graphene heterostructure as a potential anode, and their calculation results confirmed that a heterostructure can provide strong binding with Na and a low-migration barrier for Na (Figure 4c-h) [81].
Experimentally, antimonene has been successfully synthesized and evaluated in electrochemical energy applications including hydrogen/oxygen evolution reaction (HER/ OER) [75], lithium storage [82] and supercapacitors [83,84]. Concerning the SIBs application, Yang et al. synthesized stable porous antimonene sheets with mesopores, and the as-exfoliated antimonene can deliver a high specific capacity of 569.1 mAh g −1 upon 200 cycles at 100 mA g −1 [85]. Impressively, Tian et al. fabricated few-layer 0.38-nminterspacing, 7-nm-thick antimonene anode (Figure 4i-k), which delivered a high capacity of 642 mAh g −1 at 0.1 C (Figure 4l) and a high rate capability of 429 mAh g −1 at 5 C (Figure 4m) [86]. Their results represent the highest capacity and Sb utilization ratio at the reported time. In a word, the emerging antimonene materials boost promising applications of 2D monoelemental layered materials for advanced energy storage and conversion. Three-dimensional: Compared to 1D and 2D nanostructures, unique 3D cross-linked networks provide highly conductive frameworks for fast electron/ion transfer electrons and alleviate the volume expansion of Sb nanocrystals during the cycling [87,88]. For example, Mai et al. fabricated a novel composite with Sb nanoparticles anchored in a 3D carbon network ( Figure 2h) via a NaCl template-assisted self-assembly strategy, followed by freeze-drying and one-step in-situ carbonization [89]. They found that a 3D interconnected macroporous carbon framework can not only stabilize the architecture and buffer the volume expansion for Sb nanoparticles, but also provide high electrical conductivity for the whole electrode. Self-wrapping is another facile and highly efficient way to realize even coating and homogeneous distribution. Through self-wrapping, the ultrasmall Sb nanoparticles embedded in 3D nitrogen-doped porous carbon matrix anode can deliver a reversible capacity of 138 mAh g −1 even at a very high current density of 32 A g −1 [90]. Similar work is a porous self-standing foam electrode built from core-shelled Sb@In 2 O 3 nanostructures, which displayed superior high rate capacity (348.9 mAh g −1 even at 20 A g −1 ) [91].
Large volume change and poor conductivity are major concerns for alloy anode design and development. Various efforts have been made to overcome these issues and enhance the electrochemical performance of Sb-based anode materials for SIBs. Among these approaches, the downsizing of Sb particles to 1D and 2D nanoscale level and the control of the morphology of the Sb particles have been demonstrated as effective ways to enhance cycling stability and rate capability. Another promising strategy is the dispersion of the Sb particles in carbon matrix that can accommodate volume expansions, which will improve the electronic and ionic conductivities and also the stability of the anodes. In this regard, Yu et al. employed Sb into macroporous carbon and fabricated the Sb encapsulated in S and N co-doped 3D interconnected macroporous carbon [92]. It should be further noted that S and N co-doping introduced more defects and active sites to the carbon framework, thus improving the interfacial adsorption and electrochemical behaviors. Another work on oxygen-bond-enhanced Sb-oxygen-bonding-graphene composite anode also revealed that the in-situ constructed oxygen bonds play a significant role in enhancing Na-storage properties, especially the ultrafast charge/discharge capability [93].
Multidimensional: Multidimensional self-assembled structures combine the fast kinetics of nanomaterials (like 0D nanoparticles, 1D nanowires, 2D nanoflakes) with high tap density of micromaterials. Besides, the tunable preparation of multidimensional microstructures is of great importance for improving the energy density of SIBs [94]. For example, Li et al. systemically compared the electrochemical performance of 1D, 2D and 3D Sb composites [95]. The as-prepared 2D Sb nanosheets electrodes exhibit high reversible capacity of 620 mAh g −1 and retain 90.2% of capacity after 100 cycles at 100 mA g −1 . Another example to show the superiority of multidimensional assembly is the anode composed by Sb prisms array directly grown on copper substrate via a template-free electrodeposition [58]. The resulting 3D architecture endows the Sb array with excellent sodium storage performance. Considering its low cost and scale-up capability, the template-free route may find extensive applications in designing electrode architectures. In summary, despite the huge volume expansion and relatively poor electrical conductivity of Sb anodes, through various novel nanostructure design approaches and conductive carbonaceous modifications, sodium storage performance can be greatly enhanced to meet high capacity, long life and high rate standards.

Sb/Oxide Composite
Recently, transition oxides have been widely used as functional hosts or modification coatings in various energy storage and conversion devices. In the research of better Sb-based anode, double-walled Sb@TiO 2−x nanotubes (Figure 2i) [96], two-dimensional Sb@TiO 2−x [97], Sb/TiO 2 particles [98], Sb@C@TiO 2 triple-shell nanoboxes [99] have been reported to confirm the enhancement of titanium oxide to alloy-type anode materials. The Sb@TiO 2−x structure takes the advantages of the excellent electrochemical stability of TiO 2−x and the high capacity of Sb together. Together with its nanoscale size and hollow structure, it shows outstanding electrochemical properties. Impressively, in the full cell of Sb@TiO 2−x //Na 3 V 2 (PO 4 ) 3 -C, the energy density turns out to be 151 Wh kg −1 at 21 W kg −1 , or 61 Wh kg −1 at 1.83 kW kg −1 . Similar work on Sb@C@TiO 2 triple-shell nanoboxes further demonstrates the structure's advantage in accommodating volume expansion [99].
In addition to titanium oxides, other types of oxides, such as Sb 2 O 3 [100,101] and SiOC [102], can also be combined with antimony to make a modification. Because most oxide anode materials have a higher capacity than pure Sb alloy anodes, thus the introduction of these oxides into hybrid electrodes favor the realization of initial high capacity.
Oxide can be used for modification and its high specific capacity can also make it a potential anode material for sodium ion batteries. Antimony can also be directly used as an electrode material in the form of oxides, such as antimony oxide and antimony oxychloride. Antimony oxide can undergo conversion reactions with sodium ions or alloying reactions with sodium ions. The capacities of these two reactions are 1220 mAh g −1 and 1102 mAh g −1 [103]; the capacity is higher than that of pure Sb alloy anode just like most oxide anode materials. However, some oxides intrinsically possess low electronic conductivity and large volume expansion, leading to poor rate performance and electrochemical stability. Therefore, a more advanced electrode structure design is still highly desired concerning the future development of oxides modification. A lot of optimization has been done on the material, in order to alleviate the volume expansion problem of antimony oxide. After being deposited into a micron-level thin film by electrostatic spray deposition, the Sb 2 O 3 film can be used as a negative electrode of SIBs with a capacity of 414 mAh g −1 [104]. The one-dimensional Sb 2 O 3 was also studied, but it does not have as excellent electrochemical performance as the Sb 2 O 3 film, after being synthesized by a solvothermal reaction it only has a capacity of 143 mAh g −1 under 20 mA g −1 current density after 30 cycles [105]. On the other hand, in order to improve the electrical conductivity of antimony oxide, the SbOx particle/reduced graphene oxide composite material is obtained by mechanical ball milling, which can retain a specific capacity of 409 mAh g −1 after 100 cycles [106]. Two-dimensional composite materials of layered antimony oxide and carbon nanosheets were also prepared by the template method. The composite material can still retain a specific capacity of 442 mAh g −1 after 100 cycles [107]. Similarly, the antimony oxide nanosheets grown on carbon cloth can retain a specific capacity of 514 mAh g −1 after 500 cycles [108]. Besides, improving the crystallization and purity of antimony oxide crystals also contributes to the improvement of electrochemical properties. For example, octahedral antimony oxide synthesized by a simple hydrothermal reaction and it shows a high specific capacity of 566 mAh g −1 after 200 cycles [109].
The preparation of nano-scale antimony oxychloride is not complex, it can be obtained by solution-phase methods by adjusting the pH of the reaction solution and the composition of the reaction solution, we can get antimony oxychloride with different morphologies such as Sb 4 O 5 Cl 2 nanoparticles, Sb 8 O 11 Cl 2 nanoribbons and nanowires [110]. However, if antimony oxychloride was used as the anode of SIBs, better morphology regulation still needs to be explored [111,112]. For example, the electrochemical stability of micron rod antimony chloride oxide is not good enough when used as the anode material of sodium ion batteries, the Sb 8 O 11 Cl 2 micron rod synthesized by the conventional solvothermal method only shows a low current density and a low cycle life. Though its stacked micron-rod structure with a diameter of about 100 nm cannot withstand the negative effect of volume expansion, 723.4 mAh g −1 specific capacity when first used as the negative electrode of sodium ion batteries can still be obtained, which shows enormous potential of antimony oxychloride [111]. The synthesis of one-dimensional and sheaf-like microcrystal antimony chloride oxide is simple and controllable [113,114], which shows the controllability of the antimony chloride oxide structure and provides the possibility for the optimization of antimony oxychloride. On the other hand, in order to improve cycle life, compounding antimony oxychloride with other functional materials can also bring significant results. For example, the composited material of pure phase Sb 4 O 5 Cl 2 particles and graphene aerogel can have a better electrochemical performance used as anode of SIBs, the specific capacity can be maintained at 400 mAh g −1 after 50 cycles [115].

Other Sb Hybrid Composites
In the above sections, many Sb-based anode materials, including pure Sb and Sb/ carbonaceous composites, have been reviewed. Additionally, conductive polymer is another promising candidate in the design of a robust nanostructure Sb-based anode [116,117]. For example, polyaniline (PANI) coating can further buffer the volume expansion and improve the diffusion rate of sodium ions in the electrode.
Besides, intermetallic multicomponent alloying was utilized as an approach to modifying the morphology and active phases in an effort to improve the cycling stability of Sb anodes [118], whereby nanostructured Sb anodes may not play a pivotal role in regulating the electrochemical performance [28,119]. Instead, the synergistic effect induced by intermetallic phase, barrier layer or phase transformation would matter [120][121][122]. Sb-based intermetallics and alloys for SIBs would not be intensively discussed in this review, some important papers or literature reviews can be referred to [123][124][125][126]. In order to more directly compare the improvement degree of antimony based materials by different optimization schemes, we compared the structural optimization and functional coating methods of different antimony based materials, and Table 1 shows the comparison results.

Sb-Based Chalcogenide Anode
Nowadays, metal chalcogenides, namely Sb 2 S 3 and Sb 2 Se 3 , have attracted considerable attention owing to their high specific capacity, semiconductivity property and intriguing layered structure for sodium storage [118][119][120][121][122][123][124][125][126][127][128][129]. Specifically, Sb 2 Se 3 as one of a V-VI binary semiconductor compound, has received a great deal of attention due to applications of its photovoltaic, thermoelectric and electrochemical properties. For example, Jiang et al. found that the Sb 2 Se 3 @C prepared via the high energy mechanical ball milling of Sb, Se, and carbon black showed a reversible capacity of 650 mAh g −1 at 100 mA g −1 .
The improved cycling stability and enhanced electrochemical utilization were attributed to the introduction of carbonaceous modification, which have been established in many Sb 2 Se 3 based anodes in SIBs [131][132][133]. Though the theoretical specific capacity is high, the large structural changes limit their cyclability. To address these issues, tremendous efforts have been dedicated to developing novel electrodes, which can be generally classified into two major categories: preparing Sb-based nanostructures and employing carbon matrix. Similar to the construction of above Sb-based anodes, common approaches such as carbonaceous modification (carbon, CNT, graphene) [64,[134][135][136][137][138][139][140][141] or conductive polymer buffer coatings [142][143][144] have been intensively studied, so these works would not be discussed in detail in this section. Advances in nanostructure construction are highlighted by novel approaches, such as ionic liquid-assisted synthesis [145], microwave solution-guided approach [146] and few-layer 2D Sb 2 S 3 prepared by Li intercalation [147].
Chalcogenides displays highly anisotropic property, which enables the growth of uniform crystalline 1D nanowires or nanorods. Briefly, for instance, 1D Sb 2 S 3 @C nanorods were synthesized via a facile solvothermal reaction [148]. Because the core-shell structure of Sb 2 S 3 @C and ultrathin carbon layers can effectively alleviate the strain caused by a large volume change and improve the conductivity of electrodes during cycling, the Sb 2 S 3 @C rods electrode shows a much more remarkable cycle stability and rate capability compared with bare Sb 2 S 3 nanorods. A series of 1D Sb 2 S 3 or Sb 2 Se 3 based anodes has been reported, such as Sb 2 S 3 /C nanofibers [137], Sb 2 S 3 @MWCNT [149], Sb 2 S 3 /CS nanofibers [150] and Sb 2 Se 3 /C nanofibers [151] and Sb 2 S 3 @N-C [152], while in some cases, the amorphous Sb 2 S 3 significantly surpass the crystalline counterpart in electrochemical performance [153][154][155]. It is notable that a Sb 2 S 3 -graphite SIB anode has a tap density of 1.3 g cm −3 , which contributes greatly to the high volumetric capacity of the electrode [156].
Furthermore, hollow nanostructure design has exhibited advantages in increasing the conductivity and buffering the volume change of Sb-based chalcogenides, such as polypyrrole (PPy)-coated Sb 2 Se 3 microclips [157], multi-shell hollow Sb 2 S 3 [158], Sb 2 S 3 @FeS 2 hollow nanorods [159], ZnSe/Sb 2 Se 3 hollow microspheres [160], and ZnS-Sb 2 S 3 @C coredouble shell polyhedron [161]. A typical schematic illustration of self-templating growth of 1D hollow PPy-coated Sb 2 Se 3 is shown in Figure 5a [157]. Through a template-engaged ion-exchange method, the 1D template, ZnSe(DETA)0.5 microbelts, gradually reacted with an appropriate amount of Sb 3+ ions and converted to Sb 2 Se 3 microclips. The elemental mapping images clearly confirm the 1D hollow structure with a homogeneous distribution of the Sb, Se, C, and N elements in the microclip (Figure 5b). Chalcogenides hold promise as a substrate for the construction of high performance, flexible, foldable and twistable electrodes for energy storage [108,162]. For instance, Luo et al. first developed a free-standing membrane based on ultralong Sb 2 Se 3 nanowires synthesized via a facile hydrothermal method followed by a vacuum filtration process. They found that a free-standing membrane constructed by pure Sb 2 Se 3 nanowires exhibits good flexibility and integrity [127]. Moreover, a flexible, foldable, and twistable Sb 2 S 3 /TiO 2 /C nanofiber membrane was prepared via an electrospinning method [162]. The prepared nanofiber membranes remain intact and crease-free after four-folded manipulation (Figure 5c-e). In lithium-ion full-cells, a high discharge capacity of 261.6 mAh g −1 can be obtained after 100 cycles when cycled at 50 mA g −1 (Figure 5f).
From the molecular level, compared to pure Sb anode, chalcogenides contain an S or Se atom, which sometimes act as chemical bonding sites to fabricate high-performance Sb 2 S 3 or Sb 2 Se 3 anode. The strong chemical bonding of nanostructured Sb 2 S 3 on heteroatom-doped graphene or carbon matrix have been disclosed by electrochemical analysis as well as computational calculations [38,163]. It has been found that heteroatoms-doped graphene or carbon usually accounts for the improvement of electronic and ionic conductivity [164,165] or improve the adsorption of Sb 2 S 3 on the matrix [166,167]. Moreover, density function theory (DFT) calculation confirmed that the sulfur and Sb doped-carbon substrates increase the adsorption energies, charge transfer, specific capacities and the diffusion properties of Na + ions [168]. A similar effect can be also realized in 2D Mxene-modified chalcogenide anodes for SIB [169]. A recent interesting work revealed that graphene can be also scrolled on Sb 2 S 3 nanowires to improve the overall performance [170].
Since Sb-based chalcogenides are typical semiconductor materials, some binary metal sulfides, in view of semiconductor knowledge, have been constructed and studied. Such as Sb 2 S 3 /MoS 2 heterostructures [171], whereby the built-in electrical field induced by heterojunctions accelerates interfacial charge transportation were realized [128,[172][173][174][175][176]. One of the explanations of the synergistic effect is that the formation of heterostructures dramatically increases electron transfer in the direction from the n-type semiconductor to the p-type semiconductor due to the built-in electrical field [177,178]. Hence, unbalanced charge distribution occurs and more electrons are transferred to heterostructures than in bare SnS 2 materials (Figure 6a,b), and this electric field can facilitate the immigration of Na + ions [179]. Besides, in some work, the combination of Sb 2 S 3 and Mxene matrix promotes charge transfer and buffers the volume expansion and inhibits the agglomeration/grain growth of Sb 2 S 3 [180,181]. Similar phenomena were also confirmed in the bismuth-based Bi 2 S 3 -Bi 2 O 3 (BS-BO) heterostructure anode for SIBs [128]. BS-BO p-n heterojunctions formed at the interface can induce a built-in electric field with a direction from Bi 2 O 3 (Eg ≈ 2.8 eV) to Bi 2 S 3 (Eg ≈ 1.3 eV). It can be anticipated that, during the discharge process, this electric field can facilitate the immigration of Na + ions (Figure 6c). Furthermore, Qiao et al. fabricated multi-shell hollow structured Sb 2 S 3 anodes exhibiting much higher reversible capacity and gravimetric energy density relative to pristine Sb 2 S 3 [158]. More importantly, operando synchrotron X-ray powder diffraction (XRPD) measurements demonstrate that the multi-shell Sb 2 S 3 sample reacts efficiently with sodium ions compared to the pristine one, indicating a more complete conversion reaction in the multi-shell sample (Figure 7f,g). Meanwhile, more Sb is detected to generate in the multi-shell sample (Figure 7h,i). Very recently, cryo-TEM has revealed a thin and uniform SEI layer formed on the a-Sb 2 Se 3 /CNT anode [182]. The thickness of the SEI layer is strongly related to the stable electrochemical performance. In a word, in situ characterization techniques have been recognized as powerful tools to track electrochemical reactions of electrode materials in real time, which facilitate the fundamental research and practical applications of high-performance alloying anodes for SIBs and other energy storage systems [183][184][185][186]. nanowires during the first sodiation process. Comparison of contour plots of the operando synchrotron X-ray powder diffraction with superimposed voltage profiles shown for selected ranges of (f) multi-shell structured Sb 2 S 3 , (g) pristine Sb 2 S 3 and their corresponding discharging products (h,i) whereby Sb signals were detected.

Conclusions and Outlook
In conclusion, we summarize different categories of the emerging Sb based anode materials with distinct sodium storage mechanisms including antimony (Sb) and antimony chalcogenide (Sb 2 S 3 and Sb 2 Se 3 ), and briefly introduce some Sb-based alloys. The major issue associated with using Sb based anodes is the excessive volume and relatively low conductivity. To mitigate these issues, various nanostructure morphology designs (such as 0D hollow structure, 1D nanostructures and 3D interconnected networks are reviewed), crystal structure designs, conductive matrix modifications, heteroatom-doping carbon additive optimizations and so forth, can be used to improve the electrochemical performances of Sb-based alloying anodes. Besides, a new 2D layered monoelemental antimonene and its theoretical/experimental studies on energy storage are highlighted with regards to pure Sb anodes. Moreover, in regard to Sb-based chalcogenides, beyond their sodium storage performance, their fantastic physicochemical properties/applications such as foldable flexibility, free-standing electrodes, enhanced charge transfer induced by semiconductor heterojunctions, in-situ TEM, operando synchrotron XRD and cryo-TEM studies, are emphasized in this review.
The theoretical capacity of antimony metal as the negative electrode of a sodium ion battery is 660 mAh g −1 . Currently, the energy density of lithium-ion batteries can reach up to 300 Wh kg −1 . After structural optimization and functional combination, the energy density of a sodium ion battery with an antimony base material as an anode can reach about 200 Wh kg −1 . For example, the SIB using Sb array as an anode and Na 0 . 67 (Ni 0 . 23 Mg 0 . 1 Mn 0 . 67 )O 2 as a cathode, this full cell affords a specific energy of 197 Wh kg −1 at 0.2 C and a specific power of 1280 W kg −1 at 5 C [58]. Besides, a full cell assembled with a configuration of Sb@C@TiO 2 //Na 3 (VOPO 4 ) 2 F displays a high output voltage of 2.8 V and a high energy density of 179 Wh kg −1 [99]. A full cell coupled with a P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 cathode and an Sb nanorod array anode also shows good cycle performance up to 250 cycles, high rate capability up to 20 A g −1 , and large energy density up to 130 Wh kg −1 [16]. However, GWh-factories need good safety. Although the antimony based anode optimization method has been widely studied, the safety performance of the whole battery is determined by electrolyte and cathode materials, so the construction of GWh-factories, such as energy storage power stations, is still relatively difficult.
The electrochemical performance of antimony as the anode material of sodium ion batteries has been significantly improved by structural optimization and buffer material composite technology, but the cycle life of the optimized material still cannot meet the needs of practical application. Antimonene, as a two-dimensional material with high entropy, can exist in the environment for a long time and it also has good structural stability. Improving antimonene on the basis of guaranteeing antimonene capacity should become an important link in the process of the practical application of Sb-based electrode materials.
Although the Sb-based anode materials with a nanostructure have realized great progress, there are still some challenges. From academic research to practical commercialization, novel preparation routes with fewer steps and lower costs should be developed. Sb anodes have superior specific capacity over carbonaceous materials and a competing rate performance over metal oxide. In addition, in situ observation of Sb-based anodes for sodium ion batteries, at full cell or even pouch cell levels, is expected in future research. We hope this review provides insight into the effective design of high-capacity alloying-type anode materials for advanced secondary batteries.  Institutional Review Board Statement: Ethical review and approval were waived for this study, due to the research direction in this field is only for the anode materials of sodium ion batteries.
Informed Consent Statement: Not applicable.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.