Facile synthesis of BiSb/C composite anodes for high‐performance and long‐life lithium‐ion batteries

Alloy‐type antimony (Sb) is considered as an attractive candidate anode for high‐energy lithium‐ion batteries (LIBs) because of its high theoretical specific capacity and volumetric capacity. However, Sb suffers from enormous volume variation during cycling, which causes electrode cracking and pulverization, and hence the fast capacity decay and poor cyclability, limiting its practical applications as a LIB anode. Herein, we report a facile, scalable, low‐cost, and efficient route to successfully fabricate BiSb/C composites via a two‐step high‐energy mechanical milling (HEMM) process. The as‐prepared BiSb/C composites consist of nanosized BiSb totally embedded in a conductive carbon matrix. As LIB anodes, BiSb/C‐73 (with 30 wt% carbon) electrodes exhibit excellent Li‐storage properties in terms of stable high reversible capacities, long‐cycle life, and high‐rate performance. Reversible capacities of ∼583, ∼466, ∼433, and ∼425 mAh g−1 at a current density of 500 mA g−1 after 100, 300, 500, and 1000 cycles, respectively, were achieved. In addition, a high capacity of ∼380 mAh g−1 can still be retained at a high rate of 5 A g−1. Such outstanding cycling stability and rate capability could be mainly attributed to the synergistic effects between the ability of nanosized BiSb particles to withstand electrode fracture during Li insertion/extraction and the buffering effect of the carbon matrix. The as‐prepared BiSb/C composites are based on commercially available and low‐cost Bi, Sb, and graphite materials. Interestingly, HEMM is a more convenient, efficient, scalable, green, and mass‐production route, making as‐prepared materials attractive for high‐energy LIBs.

5][6][7] Among them, antimony (Sb) is considered as an attractive candidate anode for LIBs since it can offer a high theoretical storage capacity of 660 mAh g −1 (volumetric capacity of ∼1890 mAh cm −3 , 8-10 corresponding to the formation of Li 3 Sb upon full lithiation. 2,11,128][19][20][21][22][23][24] This has severely limited its practical applications as LIB anodes. Several strategies consisting of nanostructured Sb composited with carbonaceous matrixes, like, graphite, graphene, and so forth, have been investigated to relieve the enormous volume changes and improve the electrochemical properties of Sb-based anodes for LIBs.Carbonaceous materials can not only buffer the large volume changes of Sb, but also improve the electrode conductivity. 25Explored nanostructured Sb includes nanoparticles, 12,13,17,26,27 hollow nanospheres, 28,29 nanotubes, 8 nanorods, 30,31 nanowires, 32 yolk-shell, 15,33,34 and so forth.6][37][38][39][40][41][42] For example, Nguyen et al. reported Fe-Sb-based hybrid oxides nanocomposites comprising of Sb, Sb 2 O 3 , and Fe 3 O 4 as anodes for LIBs, which were synthesized through a galvanic replacement process. 37The as-prepared electrodes exhibited high capacity, excellent cycle stability as well as rate performance.A high stable specific capacity of 434 mAh g −1 at 100 mA g −1 after 500 cycles was achieved.Xu et al. reported Zn 4 Sb 3 nanotubes directly grown on a Cu foil via a chemical vapor deposition (CVD) process. 41As-grown Zn 4 Sb 3 nanotubes demonstrated enhanced electrochemical performance as LIB anodes.A high discharge capacity of 1160 mAh g −1 in the first cycle at 100 mA g −1 as well as a reversible capacity of 450 mAh g −1 over 100 cycles, were achieved.Sengupta et al. reported Sn-Sb-Ni alloy anode for LIBs, which was electrodeposited on a 3D interconnected microporous Ni foam for the first time, displaying remarkable rate capability and stable cycling performance. 36Zhao and Manthiram reported BiSb alloy anodes for LIBs with enhanced electrochemical performance. 14In addition, Wen et al. prepared Sb-based anodes for LIBs via the dual modification method of Fe doping combined with double carbon coatings. 10Fe doping not only effectively enhanced the electrical conductivity, but also regulated the dispersion of Sb particles, and acted as a buffer layer to prevent Sb aggregation during the cycling process.Double carbon layers can reduce electrolytic corrosion and enhance electrical conductivity, providing a robust shelter layer to limit the volume change and Sb-based materials coarsening.The as-prepared anodes exhibited improved electrochemical performance with high reversible capacity, excellent long-term stability (434.6 mAh g −1 with a high-capacity retention of 92.2% at 1.0 A g −1 over 300 cycles) and structural stability.From the above results, it is noteworthy that Sb-based intermetallics as anodes have been widely explored and found to exhibit enhanced electrochemical performance in LIBs.
Although some of the above-mentioned strategies to relieve the large volume change of Sb-based anodes have shown promising electrochemical properties, they are still far away to be commercialized for LIBs practical applications in various systems, such as electric vehicles and advanced energy storage devices.This could be related to their complicated and expensive fabrication routes, low yield as well as limited cycle life.Therefore, it is necessary to develop a proper simple, scalable, efficient, cost-effective, and mass-production route to satisfy the requirements of commercial use.
Herein, we have successfully synthesized BiSb/C composites from relatively cheap commercially available microsized Bi, Sb, and graphite via simple, low-cost, and efficient high-energy mechanical milling (HEMM) process for high-capacity and long-cycle life LIBs.4][45][46] Compositing Sb with Bi (specific capacity of ∼385 mAh g -1 : Li 3 Bi, high volumetric capacity of ∼3800 mAh cm -3 , and volume change of ∼215%) [47][48][49] through the ball-milling could be an effective way to partly address the issues associated with the large volume changes of pristine Sb and Bi.Composite electrode materials containing multiple active components have been reported to be beneficial for LIBs practical applications since their performance can be improved by combining the advantages and eliminating the disadvantages of multiple active components. 50Given that both Sb and Bi metals are electrochemically active in LIBs, they can act as mutual cushions to defeat the huge volume change associated with them, 1 resulting in improved electrochemical performance.The as-prepared BiSb/C electrodes demonstrated an outstanding longcycle life up to 1000 cycles (high and stable reversible capacity of ∼425 mAh g −1 at 500 mA g −1 ) and superior rate capability (∼380 mAh g −1 at 5 A g −1 ).These remarkable electrochemical characteristics can be ascribed to both the nanosized BiSb nanoparticles, which can offer shortened ion/electron diffusion paths, and the carbon matrix (graphite) which can not only ameliorate the electrode conductivity but also buffer the volume variation during cycling.

| Synthesis of BiSb/C composites
BiSb/C composites were prepared from commercially available microsized Bi, Sb, and graphite powders using a two-step ball-milling treatment.Bismuth (≥99.9%,Sinopharm Chemical Reagent Co., Ltd.), antimony (99.5%, Aladdin), and graphite (Asahi Kasei) powders were used as received.First, Bi and Sb powders were mixed and milled by a Fritsch Pulverisette 6 Planetary Mill for 12 h under Ar atmosphere, to form BiSb alloy.Two samples with weight ratios of 3:7 and 7:3 were prepared, denoted as BiSb-37 and BiSb-73, respectively.Then, the obtained BiSb-37 alloy (optimized sample) and graphite powders in 7:3 weight ratio were mixed and further ball-milled for 6 h to prepare BiSb/C-73 composites.For comparison, BiSb/C-37 and BiSb/C-11 with different carbon contents were also prepared in the same way with weight ratios of 3:7 and 1:1, respectively.The HEMM treatment was performed with a rotation speed of 400 rpm at room temperature with a 30-min break each 1 h, using unchanged milling balls to powder weight ratio of 30:1.

| Materials characterization
The morphologies and microstructures of as-synthesized materials were examined by scanning electron microscopy (SEM, SU3500, Hitachi) and transmission electron microscopy (TEM, JEM-2100HR).The phases and crystal structures of materials were obtained via X-ray diffraction patterns (XRD, PANalytical, B.V.) analyses.The elemental compositions of as-prepared samples were examined using an energy-dispersive X-ray spectroscopy system that was attached to the TEM equipment (Nova NanoSEM 450, FEI).

| Electrochemical measurements
The working electrodes were made by coating a homogeneous slurry consisting of 60 wt% of active material (Bi, Sb, BiSb, and BiSb/C), 20 wt% of carbon black (super-P) as a conductive additive, and 20 wt% of sodium alginate as the binder on a Cu foil.The electrodes were then dried in a vacuum oven at 80°C overnight.CR 2032-type coin cells were finally assembled in an Arfilled glovebox using a lithium metal foil as the counter/ reference electrode and Celgard 2500 membrane as a separator.LiPF 6 (1 M) dissolved in a mixture of ethylene carbonate and diethyl carbonate with a 1:1 volume ratio, containing 5 wt% fluoroethylene carbonate additive was applied as the electrolyte.The average mass loading of active materials is around 0.45 mg cm −2 .The cycling performance and rate capability tests were carried out in the potential range of 0.01-2.00V using a LAND battery testing system at room temperature.Cyclic voltammetry tests were performed using a CHI760E electrochemical workstation between 0.01 and 2 V at a rate of 0.1 mV s −1 .All the specific capacities reported here were determined based on the active material total mass.gets broken because of the kinetic energy exerted by rolling balls, which results in fragmented particles, and the formation of nanostructured materials with new chemical properties. 7,51The ball-milling could not only alter the crystallite and particle size of milled powders, but could also impact the mechanical deformation and surface modification, inducing chemical reactions that might not commonly occur at room temperature. 52The obtained BiSb alloy (Figure 2D) and graphite (with several micrometers Figure 2C) powders were then mixed in three different ratios of 3:7, 1:1, and 7:3, and once again ball-milled for 6 h to manufacture BiSb/C composites.It is clear that the particle size of BiSb alloy is further decreased during the second ball-milling process, as can be seen from SEM images of BiSb/C composites in Figure 2F-I.One can see that as-milled BiSb/C powders consist mainly of some large particles of few micrometers composed of aggregates of nanosized BiSb particles well connected with each other, embedded into the carbon matrix with a sheet-like structure.This connection of nanosized BiSb particles is believed to improve the cycling stability of as-prepared materials.Remarkably, prolonging the milling time resulted in large particles due to the formation of BiSb aggregates, and this could negatively impact the cyclic stability.Similar results were observed and discussed in our previous works. 7,53,54The HEMM technique is a simple and highly efficient way to decompose coarse/bulk particles into very small grains for the fabrication of nanostructured materials. 55s-prepared BiSb/C composites consist principally of nanosized and large BiSb particles uniformly distributed and embedded in the carbon matrix.Large particles result from the aggregates of connected nanosized particles together.This connection between nanosized BiSb particles could enhance cycling stability.Nanosized particles offer shortened Li-ions diffusion and electrons pathways, which can relieve the volume change over cycling, enhancing the cycle stability and rate capability. 13,56It is noticeable that after the second ball-milling with graphite, some sheet-like structures were developed on the BiSb surface (Figure 2F-I).The above SEM results demonstrate that BiSb/C composites consist of BiSb particles wrapped by carbon (graphite) sheet-like structures, highlighting that the BiSb particles are well encapsulated by carbon matrix after the second ball-milling.As shown in Figure 1, the synthesis route of BiSb/C composites is cost-effective, does not involve complex steps and expensive chemicals.This makes the as-prepared BiSb/C attractive as highenergy LIBs anodes.
Figure 3 displays XRD patterns of graphite, pristine Bi, pristine Sb, and all as-prepared materials.Both pristine Bi and Sb exhibit sharp and intense diffraction peaks.Due to the identical crystal structure, the Bi and Sb exhibit very comparable diffraction patterns with very small dissimilarities in their diffraction angles. 14In contrast, as-milled BiSb alloys show broadened and weakened peaks because of the ball-milling process, indicating the decrease of the particle and crystallite sizes of BiSb.These peaks are similar to those of pristine Bi and Sb, shifting to higher angles from those of pristine Bi with increasing the Sb content, as a consequence of the substitution of bigger Bi atoms by smaller Sb ones. 57ther typical peaks for impurities were not noticed in the XRD of as-milled BiSb powders, indicating the formation of a complete alloy.Furthermore, aside from the characteristic peaks of BiSb, a strong diffraction peak at 2θ = ∼26.2°andvery weak one at ∼54.4°(marked with the "*" symbol in Figure 3) in the XRD patterns of BiSb/ C composites, can be clearly observed.These diffraction peaks at ∼26.2°and ∼54.4°belong to the graphite (002) and (004) planes, 56,58 highlighting that the crystal structure of graphite was not completely destroyed during the ball-milling process.Additionally, the peaks become weaker as the graphite content in BiSb/C composites decreases.Other diffraction peaks of graphite are not visible, suggesting that amorphous carbon is also present even if no visible diffraction peak of amorphous carbon was detected in the XRD patterns, possibly because of its low content in as-prepared BiSb/C composites.As can be clearly seen from Supporting Information: Figure S1, BiSb-11 exhibits different diffraction peaks compared with those of BiSb-37 and BiSb-73.Maybe the weight ratio 1:1 is not suitable for the synthesis of BiSb alloy using HEMM.
To further analyze the microstructure of the asprepared BiSb/C composites, TEM tests were carried out as shown in Figure 4. Notably, many BiSb small particles with the size of several tens to hundreds of nanometers can be evidently observed in Figure 4A.As demonstrated in our previous works, 53,54 the ball-milling technique is an efficient way to reduce the particle size to the nanometer scale, which is beneficial to achieve improved electrochemical performances.It is noteworthy that the nanosized BiSb particles are well distributed and embedded into the carbon matrix (graphite) with a sheet-like structure, which is well consistent with SEM results.From Figure 4B spacing of 0.230 nm could be assigned to the (104) plane of BiSb.The result highlights the fact that as-prepared materials maintained their crystalline structure even after the second ball-milling.In addition, it is revealed in Figure 4C that nanosized BiSb is completely embedded in the carbon matrix.The core-shell (BiSb/C) structure is believed to achieve enhanced electrochemical performances due to the benefits of each component of the composite.Furthermore, as shown in Supporting Information: Figure S2C, the high-resolution transmission electron microscope image of BiSb-37 before the carbon coating exhibits the crystal lattices of both BiSb, Bi, and Sb.The lattice fringes with the spacings of 0.324, 0.32, and 0.223 nm could be assigned to the (102), (012), and (014) planes of BiSb, Bi, and Sb, respectively.
The high-angle annular dark-field scanning transmission electron microscopy image of the BiSb/C-73 composite as well as the corresponding energy dispersive spectroscopy (EDS) mappings in Figure 5 highlight the existence of Bi (yellow), Sb (red), and C (green) in asprepared materials.Nanosized BiSb is uniformly distributed and quite encapsulated within the carbon matrix, consistent with SEM and TEM results.Besides, the EDS analysis results confirm that the BiSb/C-73 composite mainly consists of Bi, Sb, and C elements with mass fractions of 14.09%, 41.57%, and 44.33%, respectively (Table 1).HEMM could be the simplest method to disperse successfully BiSb particles into the carbon matrix.The carbon matrix (graphite) could not only restrain the BiSb volume changes but also preserve the electrode structural stability during cycling, which improves the overall electrode electrical conductivity, resulting in enhanced cycle stability and high-rate performance. 56he above SEM, XRD, TEM, and EDS results indicate that BiSb/C composites consist of nanosized BiSb completely embedded in carbon matrix (graphite) with nanosheet-like structures.The carbon matrix not only ameliorates the electrode electrical conductivity but also alleviates the enormous BiSb volume variations during cycling, maintaining the electrode structural stability.These unique advantages could certainly result in remarkable electrochemical properties in LIBs.In addition, the results also demonstrate that the cost-effective HEMM is an efficient technique to reduce the particle

| Electrochemical properties
The cyclic voltammogram (CV) curves of BiSb/C-73 at a scan rate of 0.1 mV s -1 within the potential range of 0.01-2 V for the first three cycles are shown in Figure 6A.
During the first cathodic scan, a broad and weak peak between around 1.0 and 1.4 V absent in the following cycles can be assigned to the irreversible reduction of antimony oxides, 59,60 and irreversible solid electrolyte interface (SEI) layer formation on the electrode surface.
A strong broad cathodic peak centered at ∼0.65 V can also be clearly observed.This peak becomes very strong and shifts to ∼0.75 V in the following cycles, indicating the major lithiation processes corresponding to the formation of LiBi and Li 3 Bi as well as Li 3 Sb alloys. 14In the anodic scan, peaks at ∼0.95 and ∼1.1 V can be attributed to the delithiation reactions from Li 3 Bi 61 and Li 3 Sb, 17,27,56 respectively, to BiSb alloy. 14The second and third CV curves are almost overlapped, suggesting the excellent reversibility as well as stability of BiSb/C-73 electrodes.Besides, in the first anodic scan, a broad peak at ∼1.4 V that is not present in the subsequent cycles might be largely assigned to an irreversible process.][64] The synergistic effects of nanosized BiSb and carbon matrix as well as concurrent Li-storage reaction dynamics between active Sb and Bi could effectively alleviate the electrode mechanical degradation during cycling and, thus improve the structural stability and electrochemical performance. 65,66igure 6B demonstrates the charge/discharge profiles of the BiSb/C-73 composite.As can be clearly seen from the profiles, all the voltage plateaus observed match well with the cathodic and anodic peaks in the CV curves.BiSb/C-73 composite delivers the first charge and discharge capacities of 772.4 and 1197.1 mAh g -1 , respectively, at a current density of 500 mA g -1 , corresponding to an initial Coulombic efficiency (CE) of ∼64.5%.The CE reaches >95% after only three cycles and increases to >99% after 100 cycles, which remains stable above 99.5% after 200 cycles (Figure 7A), showing stable reversible electrochemical reactions.The large first cycle irreversible capacity loss is related to the formation of solid electrolyte interphase consuming active Li.The above ICE can be improved by prelithiation, electrode structure and morphology design, interfacial design, electrolyte, binder optimization, and so forth. 67,68The cycling performances of pristine Bi, pristine Sb, BiSb-73, BiSb-37, BiSb/C-37, BiSb/C-11, and BiSb/C-73 composite electrodes at 500 mA g -1 over the potential range of 0.01-2 V are displayed in Figure 7A.As it is clearly observed from Figure 7A, the cycling performance of the BiSb-37 electrode outperforms both pristine Bi, pristine Sb, and the BiSb-73 with higher Bi content.Both the pristine Bi, pristine Sb, and BiSb-73 electrodes exhibit a drastic capacity fade as well as a very poor cycling performance compared with the BiSb-37 electrode.The severe capacity loss and poor cycling characteristics are mainly attributed to the huge volume variations associated with the electrodes cracking and serious pulverization, as well as electrical isolation from the current collector. 69In contrast, the BiSb-37 electrode demonstrates a relatively enhanced performance.BiSb-37 achieves the first cycle charge/discharge capacities of 659.2 and 1092.5 mAh g -1 , corresponding to an initial CE of ∼60.3%.After 100 cycles, the capacity becomes unstable and starts declining progressively.A capacity of only 136.2 mAh g -1 (∼19.4% capacity retention) is retained after 200 cycles.During the cycling, pure BiSb-37 may experience enormous volume expansion upon Li insertion, which results in mechanical stress and consequently the electrode pulverization and delamination, causing the fast capacity fade as well as poor cycling. 35he electrochemical properties of carbon-coated electrodes are shown in Figure 7A-C and are also summarized in Table 2. Obviously, carbon-coated electrodes exhibit excellent cycling compared with BiSb-37.Among carbon-coated electrodes, the BiSb/C-73 composite electrode demonstrates high reversible capacities and a high initial CE of ∼64.5%.As generally found in other previously reported Sb-based electrodes, 15,17,27,35,41,56,70 our asprepared electrodes also present high first-cycle irreversible capacities, which could be mainly attributed to the electrolyte decomposition during the SEI layer formation on the electrode surfaces. 41BiSb/C-73 electrode delivers higher stable reversible capacities of 583.3, 499.4,465.6, 443.4,433.5, and 424.8 mAh g -1 at 500 mA g -1 after 100, 200, 300, 400, 500, and 1000 cycles, respectively.Evidently, after 200 cycles BiSb/C-73 electrodes demonstrate a very small capacity decay.One can see that the capacity retention after 1000 cycles is >91% based on the F I G U R E 7 Electrochemical properties of the as-prepared electrodes.(A) Cycling performance of the pristine Bi, pristine Sb, and asprepared electrodes at 500 mA g −1 , (B) rate capability of carbon-coated electrodes at different current densities from 0.2 to 5 A g −1 , and (C) long-term cycling of BiSb/C-73 at 1 A g −1 .
T A B L E 2 Electrochemical properties of as-prepared electrodes with different carbon contents at a current density of 500 mA g -1 .300th cycle, indicating outstanding cycling of the BiSb/C-73 composite electrode.On the other hand, BiSb/C-37 and BiSb/C-11 electrodes display initial discharge capacities of 620.1 and 820.0 mAh g -1 , with initial CEs of ∼53.1% and ∼56.7%, respectively.Low initial CEs in as-prepared BiSb/C electrodes could be assigned to the increased irreversible reaction of ball-milled carbon (graphite) over the first Li insertion. 71Stable reversible capacities as high as 270.5 mAh g −1 for BiSb/C-37 and 319.7 mAh g −1 for BiSb/C-11 can still be maintained after 1000 cycles.Meanwhile, BiSb/C-37 electrodes (70 wt% carbon) display the highest capacity retention among the three carboncoated electrodes.BiSb/C-37 manifests ∼87.0%capacity retention after 200 cycles compared with ∼74.6% for BiSb/ C-11 (50 wt% carbon) and ∼61.0% for BiSb/C-73 (30 wt% carbon).It is noteworthy that the capacity retention increases with increasing the carbon content in asprepared electrodes.In contrast, the specific capacities and initial CEs of carbon-coated electrodes decrease with increasing the carbon content (Figure 7A and Table 2).Apparently, the high content of carbon leads to an increase in the irreversible capacity in the first cycle and decreased initial capacities, as also pointed out in our previous work, 54 but improves the capacity retention of asprepared BiSb/C electrodes.Generally, as-prepared carboncoated electrodes exhibit low initial CEs (Table 2).Even though initial CEs are fairly low, the CEs of as-prepared electrodes increase progressively in the following cycles and reach >99% within few tens of cycles.Furthermore, the CEs in all carbon-coated electrodes keep stably and are nearly 100% for prolonged cycling, suggesting the formation of a very thin and stable SEI layer on the surface of the BiSb/C electrodes.

Samples Initial charge capacity (mAh g
To further investigate the remarkable performance of as-prepared BiSb/C-73 composites, the long cycling was explored at a high rate of 1 A g −1 (Figure 7C).High initial charge and discharge capacities of 653.4 and 1038.4 mAh g −1 were achieved, indicating an initial CE of ∼63.0%.Higher reversible capacities of 426.3 and 390.2 mAh g −1 than that of graphite (372 mAh g −1 ) were maintained after 200 and 300 cycles, respectively.Even after 2000 cycles, a high stable reversible capacity of 364.5 mAh g −1 closer to that of graphite can still be well retained, implying outstanding long-term cycle stability.In addition, the CE is closely 100% over the long cycling, highlighting the excellent cycle reversibility of BiSb/C-73 composite electrodes.As can be seen in Figure 7A,C, the capacity degrades during the first cycles but in subsequent cycles the capacity remains undamaged.This is probably due to high current densities (500 mA g -1 and 1 A g -1 ) which may contribute to the decomposition of the electrolyte in LIBs. 72As displayed in Figure 7B, the rate capabilities of BiSb/C composite electrodes are compared at different rates.Remarkably, all electrodes display excellent rate capabilities.BiSb/C-73 demonstrates high reversible capacities of ∼700, ∼620, ∼540, ∼460, ∼425, and ∼380 mAh g −1 at the rates of 0.2, 0.5, 1, 2, 3, and 5 A g −1 , respectively.On the other hand, BiSb/C-11 delivers reversible capacities of ∼525, ∼490, ∼450, ∼380, ∼335, and ∼270 mAh g −1 , while BiSb/C-37 displays capacities of ∼410, ∼375, ∼315, ∼230, ∼180, and ∼125 mAh g −1 , at the same rates.Notably, when the rate current returns to 0.2 A g −1 , high reversible capacities of ∼655, ∼510, and ∼400 mAh g −1 for BiSb/C-73, BiSb/C-11, and BiSb/C-37, respectively, can be maintained, suggesting the superior rate capabilities of as-prepared materials.These results could be probably attributed to the formation of a thin and more stable SEI layer on electrodes surfaces as well as improved electrodes electrical conductivity because of the carbon matrix (graphite).
Generally, the BiSb/C composite electrodes displayed outstanding electrochemical behaviors which could be attributed to the synergistic effects of nanosized BiSb uniformly distributed in the carbon matrix and the buffering effect of the carbon matrix (graphite) against the volume variation.Nanosized BiSb can help relieve mechanical fracture during cycling.Meanwhile, the carbon matrix could not only ease the BiSb volume changes and increase the electrodes electrical conductivity but also preserve the electrodes structural integrity and permit rapid transport of both Li-ions and electrons, and allow for stable SEI layer formation, leading to remarkable cyclability and excellent rate capability.
To further characterize the surface morphology changes of the BiSb/C-73 electrodes, an SEM test was conducted after cycling.Figure 9 shows SEM images of the BiSb/C-73 composite electrodes over 100 cycles.Noticeably, no serious cracks were found on the BiSb/C-73 surface, demonstrating the structural integrity of BiSb/C-73 materials.

| CONCLUSION
In summary, the BiSb/C composites were successfully fabricated from commercially available and cost-effective microsized Bi, Sb, and graphite powders by a two-step HEMM process.First, BiSb alloy was prepared by milling Bi and Sb powders for 12 h.Then, BiSb particles were dispersed in the carbon matrix through the second ballmilling of BiSb alloy and graphite powders for 6 h.Asprepared BiSb/C composites possess the following benefits: (1) Bi and Sb in the BiSb alloy act as a buffer for one another.(2) The carbon matrix in as-prepared BiSb/C materials could not only effectively relieve the volume variations of BiSb particles during Li insertion/extraction, but also ameliorate the electronic conductivity of the electrodes, prevent the direct contact of BiSb particles with electrolyte and preserve the electrodes structural stability, thereby improving the electrochemical performances of the as-prepared BiSb/C electrodes.BiSb/ C composites comprising 70 wt% BiSb alloy and 30 wt% graphite exhibit excellent Li-storage properties as LIBs anodes in terms of high and stable reversible capacities, long-cycle life, and high-rate performance.Reversible capacities of ∼583, ∼466, ∼433, and ∼425 mAh g -1 at a current density of 500 mA g -1 after 100, 300, 500, and 1000 cycles, respectively, were achieved.Besides, even at a high rate of 5 A g -1 , a stable and reversible capacity as high as ∼380 mAh g -1 greater than that of graphite (372 mAh g -1 ) can still be retained.It was also found that prolonging the milling time enhanced the cycling stability and capacity retention of the as-prepared carbon-coated electrodes.The simple, low-cost, scalable, and mass-production synthetic route used as well as outstanding electrochemical properties of as-prepared BiSb/C composite electrodes, make as-prepared materials attractive for high-energy LIBs applications.

3 |
RESULTS AND DISCUSSION 3.1 | Structure characterization BiSb/C composites were fabricated by a simple two-step ball-milling technique, as described in Figure 1.First, commercial microsized Bi and Sb powders with uneven sizes and irregular shapes (Figure 2A,B) were mixed in two weight ratios of 3:7 and 7:3, and then milled under an Ar atmosphere for 12 h to form BiSb alloy.During the milling process, chemical bonding in the milled powder F I G U R E 1 Schematic illustration of the preparation process of BiSb/C composites.HEMM, high-energy mechanical milling.

F
I G U R E 5 (A) HAADF-STEM image of BiSb/C-73 composite and (B-F) corresponding elemental mapping images for the distribution of C (green), Sb (red), and Bi (yellow).HAADF-STEM, high-angle annular dark-field scanning transmission electron microscopy.T A B L E 1 Elemental composition.nanometer scale and uniformly disperse nanosized BiSb particles into the carbon matrix.