Recent advances in Sb-based anodes for Li/Na/K-ion batteries and all-solid-state Li-ion batteries

Sb-anode reaction mechanism in LIBs

Various studies have examined the electrochemical reaction mechanisms associated with Sb anodes in LIBs^[60-62]. Park et al. demonstrated the reaction mechanism of an Sb anode using ex-situ X-ray diffraction (XRD)^[60]. During discharging (lithiation), rhombohedral Sb alloys with Li form an intermediate crystalline hexagonal Li₂Sb phase and then transform into a fully discharged cubic Li₃Sb phase. In contrast, during charging (delithiation), cubic Li₃Sb is directly converted into rhombohedral Sb without involving an intermediate phase. Shin et al. demonstrated the same electrochemical reaction mechanism using in-situ time-domain thermoreflectance analysis^[61]. The asymmetric behavior of the Sb anode during discharging and charging was also elucidated by Chang et al. using first-principles calculations and nuclear magnetic resonance (NMR) spectroscopy^[62]. These researchers insisted that the higher nucleation driving force associated with Sb results in a single-step reaction pathway in which Li₃Sb is directly recovered to Sb without forming a thermodynamically stable Li₂Sb intermediate phase during the charging process. Therefore, the suggested reaction mechanism for the Sb anode in LIBs during discharging/charging is summarized by equations (1) and (2), along with the crystallographic schematic shown in Figure 2.

During discharging:

During charging

Sb-anode reaction mechanism in SIBs

The Sb anode in SIBs exhibits a symmetric reaction mechanism during discharging (sodiation) and charging (desodiation), unlike the behavior observed in LIBs^[63-67]. Darwiche et al. reported that rhombohedral Sb alloyed with Na forms an amorphous intermediate Na_xSb (x < 3) phase during the discharge reaction^[63] owing to the sluggish kinetics associated with the crystallization of NaSb into the monoclinic structure. Subsequently, amorphous Na_xSb is transformed into hexagonal Na₃Sb. Conversely, during charging, Na₃Sb is transformed back into amorphous Na_xSb, which is recovered as rhombohedral Sb. Although XRD clearly identified Na₃Sb, the Na_xSb phase was not detected owing to its amorphous nature. Recent first-principles calculations performed by Caputo and Yu et al. demonstrated that the Na_xSb phase corresponds to monoclinic NaSb, which is the most thermodynamically stable phase^[64,65]. This theoretical prediction was further supported by the experimental results, which are demonstrated using extended X-ray absorption fine structure analysis by Yu et al., and in-situ XRD by Tian et al.^[66,67]. Therefore, the suggested reaction mechanism for the Sb anode in SIBs during discharging/charging is summarized by equations (3) and (4), along with the crystallographic schematic shown in Figure 3.

During discharging:

During charging:

Sb-anode reaction mechanism in PIBs

Several studies have reported the K-ion reaction pathway for an Sb anode in PIBs during discharging (potassiation) and charging (depotassiation)^[68-70]. The Sb reaction mechanism was investigated by Gabaudan et al. using operando XRD^[68]. During discharging, rhombohedral Sb is alloyed with K to form an intermediate amorphous K_xSb (x < 3) phase, which then transforms into cubic K₃Sb and hexagonal K₃Sb. In contrast, during charging, both cubic and hexagonal K₃Sb transform back into amorphous K_xSb, which is then recovered as rhombohedral Sb. To identify the amorphous K_xSb phases, Zheng et al. conducted first-principles calculations based on the K-Sb binary phase diagram and cyclic voltammetry^[69], while Ko et al. performed first-principles calculations along with in-situ XRD^[70]. These studies demonstrated that the KSb phase is the most stable among the various intermediate K_xSb phases (KSb₂, KSb, and K₅Sb₄); however, all the three phases are difficult to crystallize due to their low phase stabilities. Notably, in-situ XRD revealed only rhombohedral Sb and cubic K₃Sb, without any peaks corresponding to K_xSb observed. However, the identification of the K_xSb phase remains uncertain. Additionally, although the hexagonal K₃Sb phase is thermodynamically stable under ambient conditions, Sb is transformed into cubic K₃Sb during electrochemical testing. The proposed reaction mechanism for the Sb anode in PIBs during discharging and charging is summarized by equations (5) and (6), along with the crystallographic schematic shown in Figure 4.

During discharging:

During charging:

Sb-based anodes for LIBs

Sb is considered a competitive anode owing to its abundance, metallic properties, high theoretical capacity (Li₃Sb: 660 mAh g^-1), and moderate potential (0.5-0.8 V vs. Li⁺/Li). However, Sb-based anodes undergo excessive volume changes (~134%), and the continuous formation and destruction of the SEI layer result in capacity degradation and electrolyte starvation. To address these issues, recent advances in SEI layer control, structural control, and composite/alloy formation of Sb-based anode for LIB applications have been reported.

Researchers have explored the use of various solvents and salts to improve the SEI layer properties of Sb-based anodes^[72-74]. Bian et al. reported incorporating fluoroethylene carbonate (FEC) into a propylene carbonate (PC) electrolyte in the LIB system^[72]. This approach facilitates the construction of a stable SEI layer on a microsized Sb anode during Li cycling [Figure 6A]. Analysis using first-principles calculations, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) revealed that FEC has a lower LUMO (the lowest unoccupied molecular orbital) energy level (-0.11 eV) compared to PC (0 eV). The FEC additive decomposes ahead of PC to create a LiF-rich SEI layer on the Sb surface that suppresses continuous electrolyte decomposition and contributes to facile ion/electron transfer and structural stability during repeated cycling. Furthermore, the microsized Sb exhibited a high reversible capacity of 575 mAh g^-1 and a high ICE of 81% after 70 cycles at a high current rate of 5 A g^-1. Sun et al. developed a non-flammable triethyl phosphate (TEP)/1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) electrolyte to improve the stability of a bulk Sb anode [Figure 6B]^[73]. The solvation structures formed by lithium bis(fluoroslufonyl)imide (LiFSI)-TEP/HFE exhibited distinct dipole-dipole interactions that provided excellent kinetics and compatibility with the anode material. Therefore, the bulk Sb anode showed a highly reversible capacity of 604 mAh g^-1 with a substantial capacity retention of 92% after 100 cycles at a current rate of 500 mA g^-1 when evaluated using 1.2 M LiFSI in TEP/HFE as the electrolyte. Cai et al. investigated the effect of LiNO₃, as an electrolyte additive, on ether-based electrolytes with the aim of stabilizing the Sb anode [Figure 6C]^[74]. To determine the effectiveness of the LiNO₃ electrolyte additive, the Sb anode and SEI layers on the particles were investigated using SEM, XPS, and electrochemical impedance spectroscopy (EIS), which revealed that NO^3- weakens the Li⁺-DME (dimethoxyethane) interaction, resulting in Li⁺-2DME-NO^3- located away from the surface of the Sb anode, thereby suppressing electrolyte decomposition. The LiNO₃ additive decreased the strength of the interaction between the Li ion and the DME solvent molecule and affected the Li-ion solvation/desolvation process. The pristine Sb anode exhibited an ICE of 82.5% and a reversible capacity of 624 mAh g^-1 after 100 cycles at a current density of 66 mA g^-1. Furthermore, a full cell with a LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) cathode maintained a capacity of 140.8 mAh g^-1 over 100 cycles without any apparent capacity loss. Therefore, selecting appropriate solvents and salts is critical for establishing stable SEI layers on Sb-based anodes in LIBs.

Figure 6. (A) Schematic illustration and rate capabilities of microsized Sb using FEC additives^[72]. (B) Schematic illustration and cyclabilities of bulk Sb using LiFSI-based electrolytes^[73]. (C) Schematic illustration, rate capabilities, and cyclabilities of a pristine Sb anode using LiNO₃ additives^[74]. This figure is reproduced with permission from Bian et al.^[72], Sun et al.^[73], and Cai et al.^[74].

Porous and multidimensional structural materials with high surface areas have been proposed as high-performance Sb-based anodes for LIBs^[75-77]. These structured Sb anodes effectively accommodate volume changes during cycling. Liu et al. synthesized spherical Sb/C composites to address volume change during the charge/discharge process [Figure 7A]^[75]. The mesoporous Sb/C structure provided more Li-active sites and faster kinetics, which were attributed to the higher surface area that effectively accommodated volume changes due to the buffer effect. Consequently, the spherical Sb/C composite anode exhibited an impressive ICE of 86.7% and maintained a reversible capacity of 590 mAh g^-1 after 80 cycles at a current rate of 100 mA g^-1. Schulze et al. prepared an Sb/carbon nanotube (CNT) composite film anode without any conductive additive or binder to improve mechanical/electrical connectivity [Figure 7B]^[76]. SEM revealed that the initial morphology of the prepared film consisted of a porous bead-on-string structure. The film thickness increased by 500% during more than 100 cycles owing to volume expansion and continuous SEI layer formation. However, the Sb/CNT composite film retained mechanical and electrical connections without delamination and exhibited a stable cycling performance of 340 mAh g^-1 after 100 cycles at a current rate of 100 mA g^-1 without any binder or conductive additives. Luo et al. fabricated a durable LIB anode using Sb/N-C with a unique nanorod-in-nanotube structure [Figure 7C]^[77] with an internal void capable of accommodating volume changes upon cycling. In addition, the combination of N-doped 1D conductive carbon coating improved electronic conductivity and facilitated Li-ion diffusion. Consequently, the nanorod-in-nanotube Sb/N-C anode delivered an impressive rate capability of 343 mAh g^-1 after 45 cycles at a rapid current rate of 20 A g^-1. The porous structure provided additional space to accommodate volume changes during discharging and charging, whereas the robust multidimensional structure maintains both mechanical and electrical connections to improve cycling stability.

Figure 7. (A) Schematic illustration and cyclabilities of spherical Sb/C composite anodes^[75]. (B) Schematic illustration and cyclabilities of Sb/CNT composite film anodes^[76]. (C) Schematic illustration and rate capabilities of a nanorod-in-nanotube-structured Sb/N-C anode^[77]. This figure is reproduced with permission from Liu et al.^[75], Schulze et al.^[76], and Luo et al.^[77]_.

To achieve high-performance Sb-based anodes for LIBs, various strategies involving Sb-based alloys and carbon composites have been proposed for use as high-performance Sb-based anodes in LIBs^[78-88]. Zhang et al. developed an Sb/C nanosheet anode to improve cycling and rate performance [Figure 8A]^[78]. Encapsulating Sb nanoparticles within carbon nanosheets effectively mitigated volume expansion and particle agglomeration during cycling while simultaneously minimizing direct electrolyte exposure. The Sb/C nanosheet anode demonstrated a highly reversible capacity of 598 mAh g^-1 after 100 cycles at a current rate of 200 mA g^-1. Pan et al. synthesized NiSb/N-C nanosheets that delivered outstanding rates and long-life cycling performance [Figure 8B]^[79]. The NiSb alloy nanoparticles inserted into a matrix of N-doped carbon nanosheets provided structural stability by inhibiting direct contact between the alloy nanoparticles and the electrolyte and preventing alloy nanoparticle agglomeration during cycling. The NiSb/N-C nanosheets exhibited improved cycling and rate performance, delivering a stable capacity of 401 mAh g^-1 after 1,000 cycles at a current rate of 1.0 A g^-1. Yu et al. encapsulated Sb nanoparticles derived from MOFs in hollow carbon and titanium dioxide nanotubes (Sb/CTHNs) to suppress volume expansion and enhance Li⁺ diffusion [Figure 8C]^[80]. The Sb/CTHNs provide a large surface area and pathways for Li-ion diffusion and electron transport. The robust hollow structure accommodated volume expansion during the alloying/dealloying process, resulting in a stable SEI layer and a high rate capability of 374.1 mAh g^-1 at a current rate of 5.0 A g^-1. Yang et al. fabricated a CoSb nanocomposite anchored on Swiss-cheese-like nitrogen-doped porous carbon (CS/NPC), which delivered stability and a high rate capability [Figure 8D]^[81]. The specific structure contributed to enhanced electronic conductivity, a shorter ion-diffusion distance, and suppressed Sb volume changes during repeated cycling. The CS/NPC anode exhibited a high rate capability of 343 mAh g^-1 at a current rate of 10 A g^-1. The strong metal-N-C bonds formed by the doped heteroatoms provided sufficient active sites for Li ions and strengthened interfacial adhesion between the active materials and the current collector. Coquil et al. reported ternary ZnSnSb₂ as a new LIB anode material [Figure 8E]^[82]. The ZnSnSb₂ anode exhibited a reversible capacity of 615 mAh g^-1 with a capacity retention of 84.2% after 200 cycles when evaluated galvanostatically at a current rate of 252 mA g^-1; this high electrochemical performance is attributable to the distinctive quasi-topotactic reaction mechanism associated with ZnSnSb₂ during lithiation/delithiation. Su et al. synthesized a NiSb alloy embedded in N-doped carbon (NiSb/C) to enhance the electrochemical performance of Sb-based anodes for LIBs^[83]. The NiSb/C anode exhibited good cycling stability and high rate performance, with the elemental Ni suppressing volume expansion and the N-doped carbon serving as a conductive network with sufficient active sites. Consequently, the NiSb/C anode demonstrated a reversible capacity of 426 mAh g^-1 after 450 cycles at a current rate of 2 A g^-1. In short, carbon-based composite materials effectively suppress volume expansion and improve electron/ion diffusion characteristics, resulting in enhanced cycling stability and rate capabilities. Alloy materials have also been reported to exhibit less volume expansion during charging and discharging owing to their unique insertion or conversion reaction mechanisms. Regarding the electrolyte, the FEC additive was found to be essential. An unprecedented LiFSI salt was applied to an Sb anode, and its performance was evaluated. Recent LIB studies have transcended simple approaches. Instead of focusing on a single breakthrough, researchers have reported significant performance enhancements by combining multiple effective improvements. This trend highlights the need to engineer complex and advanced anode materials for next-generation LIBs. Regarding the electrolyte, the FEC additive was found to be essential, and a promising LiFSI salt was evaluated with the Sb-based anode. Table 2 summarizes recent advancements in Sb-based LIB anodes.

Figure 8. (A) Schematic illustration and cyclability of an Sb/C nanosheet anode^[78]. (B) Schematic illustration and cyclability of a NiSb/N-C nanosheet anode^[79]. (C) Schematic illustration and rate capability of an Sb/CTHN anode^[80]. (D) Schematic illustration and rate capability of a CS/NPC anode^[81]. (E) Schematic illustration and rate capability of a ZnSnSb₂ anode^[82]. This figure is reproduced with permission from Zhang et al.^[78], Pan et al.^[79], Yu et al.^[80], Yang et al.^[81], and Coquil et al.^[82].

Sb-based SIB anodes

Sb has been extensively researched as a high energy density SIB anode material due to its high Na-storage capacity (Na₃Sb: 660 mAh g^-1) and appropriate operating voltage (0.3-0.7 V vs. Na⁺/Na). However, Sb anodes exhibit drawbacks similar to those of LIBs, including the formation of unstable SEI layers, significant volume changes (~291%), and sluggish charge transfer during discharging and charging, resulting in poor cycling stability. To address these issues, recent advances in SEI layer control, structural control, and composite/alloy formation have been proposed for Sb-based anodes for SIBs.

Electrolyte additives significantly affect the formation of a stable SEI layer, which is closely related to the electrochemical performance of the Sb anode^[72,89,90]. Lu et al. investigated the surface structure, composition, and electrochemical performance of Sb-based anodes (SiC-Sb-C) in FEC-free and FEC-containing electrolytes using XPS, Fourier transform infrared spectroscopy (FTIR), EIS, and electrochemical characterization techniques [Figure 9A]^[89]. Carbonate solvents (EC and DEC) gradually decompose on the SiC-Sb-C particle surface in the FEC-free electrolyte to form an SEI layer composed of Na₂CO₃, ROCO₂Na, and RONa units. The precipitate layers formed from these salts are generally loose, forming thick SEI layers. FEC-containing electrolyte is more reactive than EC or DEC-containing electrolyte; it decomposes first on the particle surface to form a dense and thin SEI layer composed of fluorine (F)-containing salts, such as NaF, F-ROCO₂Na, and F-RONa. Although a F-containing SEI film can inhibit the decomposition of EC and DEC to a certain extent, EC and DEC still decompose along with FEC at low potentials (below 0.5 V vs. Na⁺/Na), resulting in the formation of a double-layer SEI film. The formation of a dense and thin double-layer SEI film improves the electrochemical performance of the Sb anode. Bian et al. investigated the effect of the FEC additive on the Sb anode for SIBs [Figure 9B]^[72]. The optimal concentration of FEC (10 vol%) provides a stable and NaF-rich SEI layer that alleviates Sb volume changes and inhibits continuous electrolyte decomposition. Consequently, the microsized Sb anode with an FEC-containing electrolyte exhibited a reversible capacity of 540 mAh g^-1 with a retention of 85.3% after 150 cycles at 200 mA g^-1. Bodenes et al. revealed the role that binders play in determining the thickness, homogeneity, and chemical composition of the SEI layer formed on the surface of the Sb anode in a SIB system using XPS and electrochemical testing^[90]. Sb was detected in the Sb 3d XPS spectrum of the Sb-carboxymethyl cellulose (CMC) anode in the fully desodiated state, consistent with the partial redissolution of the SEI layer (dissolution of Na₂O/NaOH). In contrast, Sb was not detected on the Sb-PVDF anode in the fully desodiated state because the SEI layer covering the Sb anode was more than 5 nm thick. Therefore, a more uniform and thinner SEI layer was formed in the case of the Sb-CMC anode compared to the Sb-PVDF anode, resulting in superior electrochemical performance [Figure 9C].

Figure 9. (A) Schematic illustration of the SEI layer formation mechanism and the cyclabilities of Sb-based anodes in FEC-free and FEC-containing electrolytes^[89]. (B) Schematic illustration, cyclabilities, and rate capabilities associated with morphological changes experienced by micro-sized Sb anodes in FEC-free and 10 vol% FEC-containing electrolytes^[72]. (C) XPS results and cyclabilities of Sb-CMC anodes^[90]. This figure is reproduced with permission from Bian et al.^[72], Lu et al.^[89], and Bodenes et al.^[90].

Nanostructural design and low-dimensional engineering aimed at accelerating reaction kinetics are considered the most efficient strategies for addressing the abovementioned drawbacks, including the sluggish rate capability and rapid capacity decay associated with the excessive volume change experienced by Sb^[67,91-96]. Liu et al. fabricated highly uniform Sb nanotubes (NTs) using a galvanic replacement approach using a Cu₂Sb-mediated formation mechanism^[91]. The Sb NT anode delivered a reversible capacity of 546 mAh g^-1 after 100 cycles with a capacity retention of 98.7% at 0.1 A g^-1 and excellent long-term cycling stability at 1.0 A g^-1 (342 mAh g^-1 after 6,000 cycles with a capacity retention of 74%) [Figure 10A]. The excellent Na-storage performance of the Sb NTs was attributed to the one-dimensional hollow structure, which effectively relieves structural deformation and shortens the ion-diffusion path, thereby improving the Na-ion diffusion kinetics. Tian et al. synthesized two-dimensional few-layer antimonene (2D FLA) nanosheets via liquid-phase exfoliation of β-antimony in a 1:1 mixture of N-methyl pyrrolidone (NMP) and ethanol^[67]. The 2D FLA anode maintained a stable capacity of 620 mAh g^-1 after 150 cycles at 0.5 C (330 mA g^-1) along with a capacity retention of 99.7% from cycles 10 to 150 [Figure 10B]. Fast Na-ion diffusion in 2D FLA was attributed to the small diffusion barrier of 0.14 eV, and its ability to efficiently accommodate anisotropic volume expansion along the a/b plane during cycling, thereby achieving high structural stability. Liu et al. synthesized a yolk-shell Sb/graphdiyne (GDY) nanobox with an inner void space as an anode material using a galvanic replacement reaction [Figure 10C]^[92]. The voids accommodate Sb volume changes, and the GDY shell, with its intrinsic in-plane cavities, facilitates Na-ion diffusion. This anode delivered a capacity of 593 mAh g^-1 at 100 mA g^-1, with little loss of capacity observed after 200 cycles. The Sb/GDY nanobox anode was subjected to full-cell testing with Na₃V₂(PO₄)₃. The full cell delivered a capacity retention of 75% and a capacity of 354 mAh g^-1_anode after 500 cycles at 1 A g^-1_anode. The stable cycling performance of the spherical Sb/C anode is a result of its finely organized nanostructure that effectively accommodates volume changes and inhibits Sb-nanoparticle agglomeration. Li et al. designed a 3D porous carbon matrix containing Sb nanoparticles (Sb/3DPC) through polymer blowing and the use of a galvanic replacement method^[93]; this architecture provided an enlarged electrode/electrolyte interface owing to its large pore volume and surface area, which shortened the Na-ion transfer path and inhibited volume expansion. The Sb/3DPC anode showed superior cycling stability, namely, 461 mAh g^-1 over 200 cycles at 100 mA g^-1 with a capacity retention of ~66% and an excellent rate capability of 346 mAh g^-1 at 5 A g^-1 [Figure 10D]. Chen et al. prepared yolk-shell-structured Sb@C using a continuous one-pot multistep strategy, with the Sb nanoparticles confined to the N and S co-doped 3D porous carbon microspheres (Sb/NS-3DPCMSs)^[94]. Remarkably, the Sb/NS-3DPCMS anode exhibited a specific capacity of 331 mAh g^-1 after 10,000 cycles at 20 A g^-1 with almost 100% capacity retention [Figure 10E]. The robust yolk-shell structure provided sufficient space to effectively relieve the volume expansion experienced by Sb and stabilized the 3D architecture during long-term cycling. Furthermore, empty carbon boxes with rich hierarchical pores and high conductivities exhibited excellent rate performance by promoting fast Na-ion/electron transfer. Li et al. synthesized various multidimensional Sb nanostructures as SIB anode materials using a chemical dealloying approach^[95]. The 0D Sb nanoparticles (Sb-NPs), 2D Sb nanosheets (Sb-NSs), and 3D nanoporous (NP) Sb were synthesized by modifying the dealloying reaction kinetics using different etching solvents. Among the various nanostructures, the Sb-NS anode showed a high reversible capacity of 620 mAh g^-1 after 100 cycles, with a capacity retention of 90.2% at 100 mA g^-1. Furthermore, the Sb-NS anode delivered high rate capabilities at various current densities in the 100-6,400 mA g^-1 range. The excellent electrochemical performance of the 2D nanostructure is ascribable to its high Na-ion availability, robust structural unity, and fast reaction kinetics. Yang et al. also fabricated 0D Sb nanoparticles, 2D antimonene nanosheets, and 3D porous Sb networks by electrochemically delaminating bulk Sb and exploiting varying reaction mechanisms in distinct electrolytes^[96]. The 2D antimonene nanosheet anode delivered a high reversible capacity of 572.5 mAh g^-1 after 200 cycles at 0.2 A g^-1 (capacity retention: 91.5%) and a superior rate capability of 553.6 mAh g^-1 at 5 A g^-1. Among these multidimensional strategies, the use of 2D shapes is particularly well-suited for accommodating the volume expansion experienced by Sb anodes in SIBs.

Figure 10. (A) TEM images of Cu nanowires as sacrificial templates and the performance of Sb NTs synthesized by galvanic replacement^[91]. (B) Schematic illustration and atomic structure of 2D few-layer antimonene, and the cyclability of a 2D few-layer antimonene anode^[67]. (C) Schematic illustration, cyclabilities, and rate capabilities of a full cell with a yolk-shell Sb/graphdiyne nanobox anode^[92]. (D) Schematic illustration of the fabrication of a 3D porous Sb/C anode and its rate capabilities^[93]. (E) Schematic illustration and cyclability of a yolk-shell Sb/NS-3DPCMSs anode^[94]. This figure is reproduced with permission from Tian et al.^[67], Liu et al.^[91],Liu et al.^[92], Li et al.^[93], and Chen et al.^[94].

Sb-based alloys and carbon composites have also been proposed in the SIB system to achieve high performance by effectively mitigating the significant volume changes experienced by Sb during sodiation/desodiation^[97-104]. Ma et al. synthesized an ultrafine mesoporous Sb₂O₃@Sb nanocomposite using a one-step dealloying reaction and a two-phase Mg-Sb precursor^[97]. The mesoporous Sb₂O₃@Sb anode exhibited a high specific capacity of 659 mAh g^-1 in the second cycle, long-term cycling stability (a capacity retention of 99.8% after 200 cycles at 0.2 A g^-1), and an excellent rate capability (200 mAh g^-1 at 29.7 A g^-1) [Figure 11A]. Xie et al. developed an Sb@NGA-CMP composite comprising ultrafine Sb nanoparticles uniformly anchored in the pores of a covalent organic framework (COF) using an in-situ synthetic strategy^[98]. To facilitate COF formation, Sb³⁺ was introduced as a catalyst and subsequently immobilized in the COF channels through reduction. This unique architecture provided electronic interactions between Sb nanoparticles and π-conjugated microporous polymers (CMPs) through nitrogen groups, thereby accelerating charge transfer along the COF. The Sb@NGA-CMP composite anode exhibited a high rate capability of 223 mAh g^-1 at 2 A g^-1, and 188 mAh g^-1 at 5 A g^-1, and excellent Na storage performance of 320 mAh g^-1 after 160 cycles at 0.2 A g^-1 [Figure 11B]. Zheng et al. developed a cauliflower-shaped Sb/NiSb composite by an electrodeposition process^[99]. The cauliflower-like structure enhanced electron transfer and shortened the Na-ion transport length, while the inactive Ni contributed to high conductivity and suppressed significant volume changes during cycling. These factors collectively contributed to stable cycling performance (521 mAh g^-1 after 100 cycles with a capacity retention of 96% at 100 mA g^-1) and a high rate capability (above 400 mAh g^-1 at 2,000 mA g^-1) [Figure 11C]. Ma et al. synthesized a bimetallic single-phase NP SnSb alloy by dealloying a ternary Mg-Sn-Sb precursor^[100]. The NP-SnSb-alloy anode delivered a specific capacity of 506.6 mAh g^-1 after 100 cycles with a capacity retention of 94.5% at 0.2 A g^-1 and 457.9 mAh g^-1 after 150 cycles at 1.0 A g^-1 (capacity retention: 95.5%); in addition, it exhibited a superior rate capability with a specific capacity of 458.5 mAh g^-1 at 10 A g^-1 [Figure 11D]. Gao et al. fabricated nanoporous (np) Bi-Sb alloys (Bi₂Sb₆, Bi₄Sb₄, and Bi₆Sb₂) by one-step dealloying ternary Mg-based precursors^[101]. The np-Bi₂Sb₆-alloy anode showed the best cycling stability, exhibiting a specific capacity of 257.5 mAh g^-1 after 2,000 cycles at 200 mA g^-1, which corresponds to a capacity loss of 0.027% per cycle. Notably, the np-Bi₂Sb₆ anode exhibited remarkable long-term cycling performance, even at 1 A g^-1, maintaining a specific capacity of 150 mAh g^-1 after 10,000 cycles with a capacity decay of 0.0072% per cycle [Figure 11E]. The outstanding electrochemical performance of np-Bi₂Sb₆ is attributable to its NP structure with an optimal Bi/Sb atomic ratio; this structure not only alleviated the volume expansion of the active material but also effectively promoted electrolyte permeation and the transfer of electrons and ions. Pan et al. prepared watermelon-like nanostructures composed of Sb nanocrystals dispersed in amorphous TiPO_x (c-Sb@a-TiO_x) by hydrolyzing tetrabutyl titanate in the presence of SbPO₄ nanorods, followed by calcination^[102]. The c-Sb@a-TiO_x anode delivered a specific capacity of 147 mAh g^-1 after 1,000 cycles at 1.0 A g^-1 with a capacity retention of 82%. In a similar manner to that observed for LIBs, composite materials with carbon effectively suppress volume expansion, improve electron/ion diffusion, and enhance cycling stability and rate capability. Alloy materials, with their unique reaction mechanisms (insertion or conversion reactions), also exhibit less volume expansion during charging and discharging. Various approaches that are similar to those used for LIBs have been reported for alleviating the significant volume changes experienced by Sb anodes in SIBs. Recent research revealed that structural engineering and composite/alloy approaches are also effective for SIBs. Additionally, an optimal combination of electrolyte composition and additives, including FEC, was determined to facilitate the formation of a stable SEI layer. Recent advances in Sb-based anodes for SIBs are summarized in Table 3.

Figure 11. (A) Schematic illustration of the fabrication process, cyclability, and rate capability of a mesoporous Sb₂O₃@Sb nanocomposite anode^[97]. (B) Schematic illustration of the fabrication process and rate capability of an Sb@NGA-CMP composite anode^[98]. (C) Schematic illustration, XRD patterns, cyclability, and rate capability of an electrodeposited Sb/NiSb composite anode^[99]. (D) Schematic illustration of the one-step dealloying process used to prepare the NP-SnSb alloy anode, along with its cyclability and rate capability^[100]. (E) Structure and cyclability of a ligament-channel np-Bi-Sb alloys anode^[101]. This figure is reproduced with permission from Ma et al.^[97], Xie et al.^[98], Zheng et al.^[99], Ma et al.^[100], and Gao et al.^[101].

Sb-based PIB anodes

Sb is accomplished prospective candidate as high-capacity PIB anodes owing to its high theoretical gravimetric and volumetric capacities (660 mAh g^-1 and 4,420 mAh cm^-3) compared to those of graphite (279 mAh g^-1 and 631 mAh cm^-3), Si (954 mAh g^-1 and 2,223 mAh cm^-3), and Sn (226 mAh g^-1 and 1,652 mAh cm^-3), as summarized in Table 1. However, Sb experiences substantial volume changes of up to 405.7% (K₃Sb) in a PIB, which is higher than those of graphite (63.1%, KC₈), Si (216.7%, KSi), and Sn (180.8%, KSn). Therefore, alleviating these volume changes is crucial for realizing high-capacity Sb-based anodes. In this section, we briefly summarize recent studies related to various strategies aimed at achieving high-capacity Sb anodes for PIBs, including SEI layer control, structural control, and composite/alloy formation.

To confirm the effects of forming an artificial SEI layer, we introduce several relevant studies based on their high electrochemical performance^[105-107]. Zhang et al. reported effective electrolytes, including potassium hexafluorophosphate (KPF₆) and potassium bis(fluorosulfonyl)imide (KFSI) salts, using Sb-, Bi-, and Sn-based PIB anodes [Figure 12A]^[105]. Generally, the KPF₆ electrolyte contributes to the formation of an unstable SEI layer, which is incapable of protecting against electrolyte decomposition during repeated cycling. In contrast, the KFSI electrolyte provides a stable SEI layer with superior mechanical and electrical properties on the surface of the active material; this stable SEI layer effectively protects against electrolyte decomposition by reducing side reactions and providing smooth K-ion pathways, which is ascribable to the layers formed on the surfaces of the active materials. Based on the effect of the KFSI electrolyte, the Sb/C anode exhibited stable cycling performance with a reversible capacity of 470 mAh g^-1 using 1 M KFSI in EC/DEC as the electrolyte after 50 cycles. Zhou et al. reported an optimal K-ion electrolyte for high-performance Sb in PIB systems, which was obtained by tuning the electrolyte composition (i.e., anion, solvent, and concentration) [Figure 12B]^[106]. The proposed electrolyte (4 M KFSI in DME) contributed to the excellent cycling performance of the Sb anode, with an extremely high reversible capacity of 628 mAh g^-1 obtained after 100 cycles at a current rate of 100 mA g^-1. This performance is ascribable to the powerful interaction between FSI^- and K⁺ at the bulk Sb electrode surface in DME. Du et al. proposed an ether-based electrolyte to accommodate the volume change experienced by the Sb anode in a PIB during cycling [Figure 12C]^[107]. The KFSI electrolyte in ethylene glycol diethyl ether (EGDE) contributed to the outstanding cycling performance owing to its enhanced maximum elastic strain. In addition, the EGDE electrolyte suppressed the pulverization of Sb particles and prevented the formation of an additional SEI layer. Consequently, the Sb anode exhibited a reversible capacity of ~573 mAh g^-1 with a capacity retention of nearly 100% after 180 cycles at a current rate of 100 mA g^-1 using the EGDE electrolyte. In short, an effective SEI layer was formed by the optimal combination of the KFSI salt and the DME and EGDE solvents, thereby contributing to the excellent cycling characteristics associated with the chemical and mechanical properties of the SEI layer.

Figure 12. (A) Schematic illustration and cyclabilities of an Sb/C anode using KPF₆- and KFSI-salt-based electrolytes^[105]. (B) Schematic illustration and cyclabilities of a pristine Sb anode using carbonate and ether-based electrolytes^[106]. (C) Schematic illustration and cyclability of a commercial Sb anode using an EGDE-based electrolyte^[107]. This figure is reproduced with permission from Zhang et al.^[105], Zhou et al.^[106], and Du et al.^[107].

Various approaches involving multidimensional structures have been reported for structural control of Sb-based anodes in PIBs^[108-112]. Shi et al. developed a flower-like Sb₄O₅Cl₂ cluster, which was prepared by a hydrothermal method, for PIB use [Figure 13A]^[108]. The flower-like structure provides diffusion paths for K ions inserted in parallel along its plane. Furthermore, a flower-like Sb₄O₅Cl₂ anode exhibited high reversible capacities of 530, 316, and 150 mAh g^-1 at rates of 50, 100, and 150 mA g^-1, respectively. Liu et al. suggested the use of honeycomb-like porous microsized layered Sb (porous-Sb) [Figure 13B]^[109]; the porous Sb was prepared using a template-free hydrothermal method with deionized water. The honeycomb-like porous structure not only provided efficient K-ion transport but also extra space for accommodating volume changes. Based on its structural benefits, porous-Sb exhibited good electrochemical cycling stability with an initial reversible capacity of 655.5 mAh g^-1 and a capacity retention of 84% after 80 cycles at a current rate of 50 mA g^-1. Imtiaz et al. reported Sb deposited on a Cu₁₅Si₄ (Sb/Cu₁₅Si₄) nanowire array as an anode, which was synthesized by high-boiling-solvent-mediated vapor-solid-solid growth, with the aim of enhancing the electrochemical performance of Sb [Figure 13C]^[110]. The Sb/Cu₁₅Si₄ nanowire contributed to the outstanding electrochemical performance owing to its mechanically robust, highly stable, and distinctive structure. Furthermore, the Sb/Cu₁₅Si₄ nanowire anode exhibited a high initial reversible capacity of 647.9 mAh g^-1 at a current rate of 50 mA g^-1 and good cycling performance, with a capacity retention of 65% over 1,250 cycles at a high 200 mA g^-1 rate. Guo et al. proposed an MXene-based aerogel containing single Sb atoms, quantum dots, and graphene oxide (Sb-SQ@MA) [Figure 13D]^[111], which was chemically synthesized using few-layered MXene (Ti₃C₂T_x), graphene oxide (GO), and SbCl₃ as precursors. The high electrochemical performance of Sb-SQ@MA was achieved through improved charge-transfer kinetics, enhanced K-storage capability, structural stability, and highly efficient electron transfer. The Sb-SQ@MA anode exhibited a stable capacity retention of 94% with a high reversible capacity of 314 mAh g^-1 after 1,000 cycles at a fast current density of 1 A g^-1. Consequently, various structural controls have contributed to the stable high rate capability and cycling performance of Sb-based PIB anodes owing to their distinctive structural characteristics that accommodate volume expansion or facilitate K-ion diffusion. He et al. fabricated a 3D macroporous Sb@C composite (Sb@C-3DP) using a simple KCl template [Figure 13E]^[112]; Sb@C-3DP showed a reversible capacity of 516 mAh g^-1 at 50 mA g^-1, an ICE of 76.2%, and delivered a capacity retention of 97% after 260 cycles at 500 mAh g^-1. A full cell with a Prussian blue cathode delivered a reversible capacity of 508 mAh g^-1 (based on anode mass) at a current rate of 0.2 A g^-1.

Figure 13. (A) Schematic illustration and cyclability of a flower-like Sb₄O₅Cl₂ PIB anode^[108]. (B) Schematic illustration and cyclability of a honeycomb-like porous Sb anode for PIBs^[109]. (C) Schematic illustration and cyclability of an Sb/Cu₁₅Si₄ nanowire anode for PIBs^[110]. (D) Schematic illustration and cyclability of an Sb quantum dot/MXene-based aerogel PIB anode^[111]. (E) Schematic illustration, cyclability, and rate capability of a full cell with a 3D macroporous Sb/C composite anode^[102]. This figure is reproduced with permission from Shi et al.^[108], Liu et al.^[109], Imtiaz et al.^[110], Guo et al.^[111], and He et al.^[112].

Various strategies, such as alloy or composite formation, have been extensively studied for improving the electrochemical properties of Sb anodes for PIBs^{[12,69,70,105,110-119]}. Zheng et al. designed Sb nanoparticles encapsulated in an interconnected carbon-sphere network (Sb/CSN) via an electrospray-assisted strategy [Figure 14A]^[69]. The uniformly dispersed nano-Sb within the porous spherical network mitigated the volume change experienced by Sb. In addition, the highly concentrated electrolyte formed a robust KF-rich SEI on the Sb/CSN surface. Synergy between this interesting structure and the robust SEI greatly enhanced the electrochemical performance of the Sb/CSN anode, which maintained a reversible capacity of 504 mAh g^-1 after 220 cycles at 200 mA g^-1. Shi et al. encapsulated Sb nanoparticles in N-, S-, and F-co-doped carbon skeletons (Sb/NSF-C) using a hydrothermal method, heat treatment, and etching [Figure 14B]^[113]. Doping changes the electronic configuration, resulting in defects in the carbon layers. This 3D porous structure is highly mechanically strong and contributes to enhancing the electrochemical reaction kinetics. After 200 cycles, the Sb/NSF-C composite retained a reversible capacity of 287 mAh g^-1 at a current rate of 1.0 A g^-1. Han et al. embedded Sb nanocrystals in an ultrathin carbon nanosheet (Sb/CNS) using a one-step solvothermal reaction [Figure 14C]^[114]. The mechanical stability of the nanosheet accommodated volume changes and suppressed side reactions involving the electrolytes. Moreover, the large surface area of the nanosheet contributed to fast ionic/electronic diffusion. Consequently, a reversible capacity of 247 mAh g^-1 and up to 90% capacity retention were attained after 600 cycles at 200 mA g^-1. Cao et al. created a flexible integrated anode by confining Sb nanoparticles in porous carbon nanofibers (Sb/C PNFs) using an electrospin-assisted strategy [Figure 14D]^[115]. The vessel-like channels in the 3D interconnecting carbon nanofibers promoted electrolyte flow and shortened the the K⁺ diffusion way of the uniform Sb nanoparticles. The flexible porous carbon nanofibers function as a buffer matrix that mitigates volume expansion while simultaneously providing pathways for rapid electron transfer. As a result, a reversible capacity of 264.0 mAh g^-1 was attained after 500 cycles at 2.0 A g^-1. Xiong et al. fabricated a BiSb/C composite nanosheet by embedding Bi-Sb alloy nanoparticles inside a porous carbon matrix via freeze-drying and pyrolysis [Figure 14E]^[116]. The BiSb alloy mitigated volume changes owing to the similar physicochemical properties and infinite miscibility of Bi and Sb. Furthermore, the carbon effectively buffered the volume change of the BiSb alloy nanoparticles during cycling. The BiSb/C anode exhibited a reversible capacity of 320 mAh g^-1 with a high capacity retention of 97.5% after 600 cycles at 500 mA g^-1. Diverse approaches have been explored to address the substantial volume changes experienced by Sb-based PIB anodes, including SEI layer control, structural control, and composite/alloy formation. Except for SEI layer control, these methods parallel the techniques used for LIBs and SIBs. While FEC additives are well-known for improving the performance LIBs and SIBs, their impact on PIBs remains controversial^[120,121]. Using FEC as an additive for an anode in a half-cell system dramatically reduced the ICE while increasing the chemical and cycling stabilities of the system, demonstrating a trade-off relationship. Although a few studies have validated the potential of the FEC additive in various PIB systems, its application to Sb-based anodes remains unexplored. Therefore, the role of FEC in an Sb-based PIB anode requires further investigation. Recent progress in Sb-based PIB anodes is summarized in Table 4.

Figure 14. (A) Schematic illustration, SEM image, and cycling performance of an Sb/CSN PIB anode^[69]. (B) Schematic illustration, SEM image, and cycling performance of an Sb/NSF-C PIB anode^[113]. (C) Schematic illustration and rate performance of an Sb/CNS PIB anode^[114]. (D) Schematic illustration, SEM images, and cycling performance of an Sb/C PNF PIB anode^[115]. (E) Schematic illustration, SEM image, and cycling performance of a BiSb/C PIB anode^[116]. This figure is reproduced with permission from Zheng et al.^[69], Shi et al.^[113], Han et al.^[114], Cao et al.^[115], and Xiong et al.^[116].

Sb-based ASSLIB anodes

Although solid electrolytes provide significant advantages in terms of rigidity, thermal stability, and nonflammability, the direct utilization of Li-metal anodes can induce dendrite formation resulting in short circuits, which remains a critical challenge for ASSLIB development^[122]. To address this issue, alloy-based anodes that do not form dendrites have been considered. Lewis et al. investigated the critical differences in the SEI layer formation dynamics of LIBs and ASSLIBs, emphasizing the potential benefits of alloy anodes^[123]. Due to its immobility, the solid electrolyte in an ASSLIB inhibits excessive SEI layer formation and subsequent electrolyte depletion during continuous cycling. Therefore, alloy-based anodes form mechanically and chemically denser and more stable SEI layers compared to those of LIBs.

Several studies have reported Sb-based anodes for ASSLIBs using oxide- and boride-based solid electrolytes^[124-129]. Afyon et al. fabricated an Sb composite anode by dropping a slurry of Sb, Li_6.25Al_0.25La₃Zr₂O₁₂ (LLZO), carbon black, and poly(vinylidene fluoride (PVDF) onto the surfaces of LLZO pellets (Sb/LLZO/C) [Figure 15A]^[124]. The Sb/LLZO/C composite anode exhibited a reversible capacity of 230 mAh g^-1 with 99.9% retention after 250 cycles at a rate of 240 mA g^-1 and an operating temperature of 95 °C. This study demonstrated the superior cycling stability and fast rate capability of Sb-based anodes in oxide-type ASSLIBs. Sb-based anodes have predominantly been reported for ASSLIBs that use boride-based solid electrolytes. Mo et al. demonstrated that the Sb/LiBH₄ solid interfacial contact properties gradually deteriorated owing to the excessive volume change experienced by Sb [Figure 15B]^[125]. Therefore, a GaSb/LiBH₄/C composite anode was designed by introducing liquid Ga metal into the Sb anode to increase interfacial compatibility between the electrode and the solid electrolyte, accommodate volume changes, and promote electron and ion diffusion. The composite anode exhibited a reversible capacity of 400 mAh g^-1 with a capacity retention of 98.6% after 400 cycles at a current rate of 1 A g^-1 and an operating temperature of 125 °C. Furthermore, a full-cell employing a TiS₂ cathode and a GaSb/LiBH₄/C composite anode delivered a capacity of 226 mAh g^-1 (based on the mass of GaSb) after 1,000 cycles at a current rate of 0.5 A g^-1. Long et al. reported the use of liquid Ga metal to induce capacitive behavior and improve the rate capabilities of ASSLIBs [Figure 15C]^[126]. Liquid Ga was electrochemically introduced from a GaSb anode, and the combination of capacitive behavior and enhanced solid-phase contact was found to promote fast kinetics. The GaSb/LiBH₄/C anode demonstrated remarkable cyclability (691 mAh g^-1 after 800 cycles at a current rate of 660 mA g^-1). Kumari et al. prepared an Sb/LiBH₄/acetylene black (AB) composite that delivered stable cyclability with a reversible capacity of 621 mAh g^-1 after 50 cycles at a current rate of 150 mA g^-1[127]. The buffering effect of LiBH₄ and AB on the Sb anode contributed to cycling stability. Sharma et al. reported Sb-based chalcogenides, specifically Sb₂X₃ (X = S, Se, or Te), as ASSLIB anodes that use LiBH₄ as the solid electrolyte^[128]. The prepared Sb₂X₃/LiBH₄/AB was evaluated at an operating temperature of 120 °C. While both Sb₂Se₃ and Sb₂Te₃ exhibited poor cyclability owing to excessive volume changes, Sb₂S₃ showed a capacity retention of 80% after 100 cycles. However, all Sb-based chalcogenide anodes exhibited poor ICEs of approximately 50%. Sharma et al. also identified a compatible solid electrolyte by changing the solid electrolyte mixed with the anode composite and controlling the operating temperature [Figure 15D]^[129]. Electrochemical performance was evaluated using LiBH₄ as the solid electrolyte at operating temperatures ranging from 40 to 120 °C. The Sb₂S₃/80Li₂S-20P₂S₅ (LPS)/AB composite exhibited a high capacity of 1,373 mAh g^-1 and high cycling stability (~60% after 100 cycles) at an operating temperature of 120 °C, which is superior to the performance achieved using LPS as the solid electrolyte. Sb-based anodes have been successfully adopted and exhibit high electrochemical performance. To achieve better electrochemical performance of Sb-based anodes for ASSLIBs, further studies focusing on the optimal structure of the Sb-based anode and its compatibility with various solid electrolytes are required. Recent advances in Sb-based anodes for ASSLIBs are summarized in Table 5.

Figure 15. (A) Schematic illustration of an Sb/LLZO/C composite ASSLIB anode^[124]. (B) Schematic illustration, TEM images, and cyclability of a GaSb/LiBH₄/C composite ASSLIB anode^[125]. (C) Schematic illustration and cyclabilities of Sb/LiBH₄/C and GaSb/LiBH₄/C ASSLIB anodes^[126]. (D) Schematic illustration and cyclability of an Sb₂S₃/LPS/AB composite ASSLIB anode^[127]. This figure is reproduced with permission from Afyon et al.^[124], Mo et al.^[125], Long et al.^[126], and Kumari et al.^[127].