Ultrafine Sb nanoparticles embedded in an amorphous carbon matrix for high-performance sodium ion anode materials

Abstract

Sodium-ion batteries (SIBs) have drawn growing interest as alternative energy-storage devices for lithium ion batteries owing to sodium's natural abundance, low cost and environmental benignity. To date, exploitation of suitable anode materials with high capacities and fast sodium ion uptake/removal for SIBs is still a great challenge. Herein, a sea-island-like SbNPs@C nanocomposite composed of Sb nanoparticles (NPs) uniformly embedded in a carbon matrix was synthesized via a facile, low cost, and high yield hydrothermal method followed by an annealing process. When used as anode materials for SIBs, the as-obtained SbNPs@C nanocomposite electrodes exhibited superior electrochemical performances (a high reversible capacity of 622 mA h g⁻¹, 91% capacity retention over 110 cycles at 100 mA g⁻¹ and a good rate capability of 444 mA h g⁻¹ at 1000 mA g⁻¹) and offered a good choice for advanced SIB anode materials.

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Introduction

Commercialized lithium ion batteries (LIBs) are currently in widespread use because of their advantages such as large energy density and excellent cycling stability compared to other available energy storage devices.^1,2 However, considering the limited lithium resources, the corresponding high cost is gradually becoming a problem that is exigent to be resolved.^3–6 Sodium is the second lightest and smallest alkali metal⁷ with a great abundance in the Earth's crust (2.8%), low cost and environmental benignity; sodium ion batteries (SIBs) have attracted growing interest as an attractive candidate for LIB.⁸ Furthermore, the Na⁺/Na redox potential (−2.70 V vs. SHE) is only about 0.34 V higher than that of Li⁺/Li,⁵ and both raise the issue of seeking appropriate Na-storage anode materials with the low redox potentials needed to increase the energy density.^8,9

Graphite, an anode material that is now extensively utilized for commercial LIBs,⁷ exhibits insignificant Na-ion storage capacities of 30–35 mA h g⁻¹.¹⁰ Because of the ca. 40% larger radius of the Na ion compared to that of the Li ion, the number of Na ions that can reversibly and rapidly be inserted/extracted from graphite (NaC₆₄) is considerably lower compared to Li ions (LiC₆).^11,12 Fortunately several other carbonaceous materials have relatively high specific capacities of ca. 300 mA h g⁻¹, such as hard carbon,^13,14 carbon nanowires,¹⁵ and hollow carbon nanospheres.¹⁶ Nevertheless, the potential of sodium inserted in a carbonaceous material is very close to the sodium deposition potential, and a severe polarization process occurs, which may lead to security issues.^17,18

Recently, metallic and intermetallic anodic host materials as alternative substitutes for the graphite anode have been used to develop Na-storage anode materials, which are universally accepted as safe and high capacity Na-storage anode materials.¹⁹ Among several promising anode materials, metals that can form alloys with Na, such as Sn (Na₁₅Sn₄: 847 mA h g⁻¹), Sb (Na₃Sb: 660 mA h g⁻¹), Pb (Na₁₅Pb₄: 484 mA h g⁻¹) and Ge (Na₃Ge: 1108 mA h g⁻¹), have drawn considerable attention.²⁰ These alloy-based anodes have large theoretical storage capacities and operate at more secure potentials.¹⁸ Unfortunately, the applications of these alloy electrodes are primarily hampered by their structural instabilities due to the drastic expansion/contraction during the alloying/dealloying reaction with Na, e.g., 525% for Sn and 390% for Sb,^21,22 which causes structural pulverization of the anode materials, continuous depletion of the electrolyte because of the regrowth of the solid electrolyte interface (SEI) film, loss of contact with the current collector and low conductivity,^6,23 resulting in a sudden capacity loss and rapid deterioration of cycling performance.^5,24,25

Among the alloy anode materials mentioned above, Sb is regarded as a promising candidate for SIB anode materials because of its fast reaction kinetics,^9,25,26 high theoretical capacity (660 mA h g⁻¹), improved cycling performance and excellent rate capability when integrated with conductive carbons.^4,25 Several types of Sb-based anode materials for SIBs have been studied and classified into four categories as follows: (a) pure Sb with special morphologies, such as bulk Sb,^10,25 cathodically induced Sb,⁴ Sb porous hollow microspheres,²⁷ monodisperse Sb nanocrystals²² and hollow Sb nanospheres;⁶ (b) Sb oxides (e.g., Sb₂O₃ nanowires,²⁸ Sb₂O₄ thin film²⁹), and Sb oxide–carbon composites (e.g., Sb₂O₃/Sb@graphene nanocomposites,¹¹ SbO_x/reduced graphene oxide composite³⁰); (c) Sb-alloy compounds, such as FeSb₂,^31,32 Zn₄Sb₃,³³ SnSb,^{19,20,34–38} Cu₂Sb,^9,39 Sb/Cu₂Sb,⁴⁰ Mo₃Sb₇,⁴¹ AlSb⁴² and Sn–Ge–Sb;⁴³ (d) Sb/carbon-based composites including Sb/C fibers,^17,44 mechanically milled Sb/C nanocomposites,¹⁹ Sb nanocrystals embedded in carbon microspheres,⁴⁵ Sb nanoparticle decorated N-rich carbon nanosheets,⁵ wet milled Sb/MWCNT nanocomposites,⁴⁶ rod-like Sb–C composite,⁴⁷ pitaya-like Sb@C microspheres,⁴⁸ peapod-like Sb@C submicron-structures,²⁴ graphene/nano Sb,¹² rGO/nano Sb composite,⁴⁹ Sb/acetylene black composite,²³ SiC–Sb–C nanocomposites¹⁸ etc. Some of the above mentioned materials exhibit good Na-storage properties; however, a considerable proportion of them suffer from several drawbacks in various respects, such as low specific capacity, poor cycling stability, high cost, and compatibility.⁶ Taking this into account, it is evident that carbon materials play a prominent part in improving the electrochemical performance of SIBs.¹² Their Sb-based composites fully utilize the potential of the active material because of the enhancement of the electrical conductivity compared to pure Sb. Since carbonaceous materials can not only act as a buffering matrix to alleviated the large volumetric stress and separator to restrain the aggregation or coarsening of collapsed nanograins, but also serve as conductive media for electron transfer and provide more active sites for Na-storage; they bring significant improvements in higher rate capability and better cycling stability.²³ Furthermore, reducing the particle size allows shorter electron and Na ionic diffusion paths and promotes good charge transportation; a facile approach is to fabricate Sb/carbon nanocomposites and nanostructured Sb as anode materials for SIBs.

Very recently, W. Luo et al. revealed a novel synthetic route for the fabrication of 1D peapod-like Sb@C structures by thermally reducing carbon-coated Sb₂O₃ submicron-wires composites.²⁴ In this paper, mixed-valence antimony oxide (Sb₆O₁₃) hollow nanospheres are first fabricated via a facile hydrothermal method using sodium stibogluconate powder as a precursor. Subsequently, we imitated Luo's method and successfully obtained a sea-island-like nanocomposites consisting of Sb nanoparticles (NPs) uniformly distributed in a carbon matrix, and the size of the Sb NPs varied from 5 to 20 nm. The SbNPs@C nanocomposites displayed excellent cycle stability (540 mA h g⁻¹ in the 145^th cycle) and rate performance (443 mA h g⁻¹ at a current density of 1000 mA g⁻¹) for sodium insertion. Compared with previous Sb–carbons composites for SIBs, the hybrid has a lower cost and comparable electrochemical performance, indicating that it is a promising anode material for high performance SIBs.

Experimental

Preparation of Sb₆O₁₃ nanospheres (NSs)

Sb₆O₁₃NS samples were synthesized via a hydrothermal method. In a typical synthesis, 810 mg (25 mmol L⁻¹) of sodium stibogluconate (Xiya Reagent, 99%) was dissolved into 35 mL of deionized water under vigorous agitation at room temperature. After stirring to achieve a colorless solution, dilute hydrochloric acid was added drop by drop into the above solution until the final pH of the solution was about 1.0–1.5 with stirring. A homogeneous solution formed immediately, and the total volume was controlled at 35 mL. Afterwards, the resulting colorless precursor solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 190 °C for 24 h. After being cooled to ambient temperature naturally, the dark brown Sb₆O₁₃NSs in the autoclave were collected by centrifugation, washed with deionized water followed by absolute ethanol several times and dried at 80 °C over night in air.

Pretreatment of Sb₆O₁₃NSs

Typically, 400 mg of poly(vinyl pyrrolidone) (PVP, Aladdin, M_w = 10 000 g mol⁻¹) was first dissolved in a mixture of 0.6 mL of ethylenediamine (EDA, Guangzhou Jinhuada Chemical Co., Ltd.) and 74 mL of deionized water. Then, 608 mg of as-prepared Sb₆O₁₃NSs powder was added into the PVP/EDA/deionized water mixture under sonication for 30 min and then continuously stirred for 4 h at room temperature. Subsequently, the mixture solution was centrifuged and washed with deionized water followed by absolute ethanol repeatedly. Thus, pretreated Sb₆O₁₃NSs was collected after it was dried at 80 °C over night in air.

Synthesis of Sb₆O₁₃NSs@C

Pretreated Sb₆O₁₃NSs were coated with a layer of glucose-derived carbon via a hydrothermal reaction, which gave an intermediate product (referred to as Sb₆O₁₃NSs@C). Pretreated Sb₆O₁₃NSs (100 mg) were dispersed into an aqueous solution of glucose (35 mL, 0.01 M) under sonication and then transferred into a 50 mL Teflon-lined stainless steel autoclave and kept in an oven at 200 °C for 2 h. Sb₆O₁₃NSs@C was collected by centrifugation and washed with deionized water followed by absolute ethanol several times and dried at 80 °C over night in air.

Synthesis of SbNPs@C

SbNPs@C was prepared through a thermal reduction by converting Sb₆O₁₃NSs@C to the sea-island structure SbNPs@C nanocomposite with Sb NPs uniformly distributed in the carbon coating layer. The intermediate product (Sb₆O₁₃NSs@C) was then heated in a tube furnace at 500 °C for 2 h in H₂ (30 vol%)/Ar (70 vol%) atmosphere with a heating rate of 6 °C min⁻¹ to obtain the final product.

Materials characterization

The morphology of the Sb₆O₁₃NSs and SbNPs@C was characterized using a transmission electron microscope (TEM, FEI Tecnai G2F30) under 300 kV. Structure, composition and elemental analyses were based on X-ray powder diffraction (XRD, D/MAX 2200 VPC) with Cu Kα as the radiation source scanning at 5° min⁻¹ from 10° to 90° (2θ), thermogravimetric analysis (TGA, NETZSCH TG209) heating from room temperature to 800 °C with a ramp rate of 10 °C min⁻¹ in static-air conditions, energy dispersive X-ray spectrometry (EDS), scanning transmission electron microscopy (STEM) and EDS mapping.

Electrochemical measurements

The anode material was fabricated by mixing SbNPs@C, a conductive agent (Super-P, Sigma Aldrich) and sodium alginate (SA, Sigma Aldrich) in a weight ratio of 70 : 20 : 10. Then, the mixture was coated on a copper foil (d = 18 μm). The mass loading of the active materials on our electrode was about 0.25 mg cm⁻², which was measured by an ultramicro analytical balance (Mettler Toledo XP2U, Switzerland, 0.1 μg resolution). After drying in air at 90 °C for 6 h, the electrodes were assembled into coin-like cells (CR2032) with a pure home-made sodium slice as both the counter electrode and the reference electrode and a glass fiber (GF/D, Whatman) as the separator in an argon (99.999%)-filled glovebox (Mikrouna Co., Ltd. Universal). The electrolyte used was 1.0 M NaClO₄ in a 50 : 50 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) solution with 5 wt% fluoroethylene carbonate (FEC) additive. The electrochemical performance of the half-cell, including cyclic voltammetry (0.01–2.50 V, 0.1 mV s⁻¹) and electrochemical impedance spectrometry (5 mV perturbation amplitudes, 10⁵ to 10⁻¹ Hz) were conducted using an electrochemical workstation (Solartron 1280B). The galvanostatic charge/discharge test and the cycle stability at constant current were carried out using a multi-channel battery tester (Shenzhen Newware Technology Limited Co. China). All electrochemical tests were carried out at room temperature.

Results and discussion

In this communication, we present a facile hydrothermal carbon-coating reduction route for the fabrication of island-like SbNPs@C with Sb nanoparticles uniformly distributed in the carbon matrix. As illustrated in Scheme 1, we first synthesized Sb₆O₁₃ nanospheres (NSs) with irregular nanopores via a hydrothermal reaction using sodium stibogluconate powder as a precursor. Later, the as-prepared Sb₆O₁₃NSs powder was soaked in a mixture solution of a poly(vinyl pyrrolidone) (PVP)–ethylenediamine (EDA)–deionized water. In our experiments, PVP was present as the surface stabilizer, and EDA possibly acted as a catalyst.⁵⁰ Subsequently, the pretreated Sb₆O₁₃NSs were coated with a layer of glucose-derived carbon via a hydrothermal reaction, which gave an intermediate product (referred to as Sb₆O₁₃NSs@C). There was no significant variation in the size and shape of the Sb₆O₁₃NSs, but the peripheries of the nanospheres were covered with carbon layers. Finally, Sb₆O₁₃NSs@C was reduced to a sea-island structure SbNPs@C through a hydrogen thermal reduction with Sb NPs uniformly distributed in the carbon coating layer.

Scheme 1 Schematic for the formation of a sea-island-like SbNPs@C nanocomposite by a simple hydrothermal-carbon coating-reduction route.

The morphology, nanoparticle size and structure features of the Sb₆O₁₃NSs were revealed by transmission electron microscope (TEM) and X-ray diffraction (XRD) tests. The typical TEM images in Fig. 1a and b show that the average size of the Sb₆O₁₃ spherical nanoparticles was ca. 20 nm with irregular nanopores. The high-resolution TEM (HRTEM) image of Sb₆O₁₃NSs, shown in the inset of Fig. 1a, exhibits a well-defined lattice spacing of 0.595 nm, corresponding to the (111) plane of cubic Sb₆O₁₃. The selected area electron diffraction (SAED, inset in Fig. 1b) pattern of Sb₆O₁₃ shows four weak diffraction rings corresponding to the (222), (400), (440) and (622) crystalline planes of the cubic Sb₆O₁₃, confirming the formation of polycrystalline cubic Sb₆O₁₃ nanocrystals. The phase and crystal structure of the Sb₆O₁₃NSs were confirmed by the XRD test. The XRD pattern shown in Fig. 1c indicates that high-purity and highly-crystalline products were obtained that could be indexed to cubic Sb₆O₁₃ (PDF#00-071-1091). As can be seen in Fig. 1c, the major diffraction peaks for Sb₆O₁₃NSs at 14.88°, 28.71°, 30.02°, 34.80°, 50.04° and 59.73° can be indexed to (111), (311), (222), (400), (440) and (622) of cubic Sb₆O₁₃.

Fig. 1 TEM images (a, b), HRTEM image (inset in (a)), the corresponding SAED pattern (inset in (b)) and XRD patterns (c) of the Sb₆O₁₃NSs.

Fig. 2a and b show the microstructures of SbNPs@C. A low-magnification TEM image in Fig. 2a reveals that Sb₆O₁₃ spherical nanoparticles with an average diameter of 20 nm have been reduced to Sb nanoparticles that homogeneously disperse in the irregular amorphous carbon matrix to obtain a “sea-island”-like structure. An HRTEM image (Fig. 2b) of the SbNPs@C composite shows the crystalline, clear lattice fringes of the Sb nanoparticles and the amorphous structure of the carbon matrix. The HRTEM image (Fig. 2c) attached to the SAED pattern (Fig. 2d) revealed the polycrystallinity of SbNPs@C. Five clear rings made up of discrete spots in the SAED pattern further confirm the presence of nanocrystalline Sb. The XRD patterns of the as-synthesized Sb₆O₁₃NSs, Sb₆O₁₃NSs@C and SbNPs@C are depicted in Fig. 2e. All of the diffraction peaks can be indexed to the standard diffraction data of Sb₆O₁₃ (PDF#00-071-1091) and Sb (PDF#00-071-1173). The as-prepared SbNPs@C with polycrystalline texture was identified and was in good agreement with the HRTEM and SAED results (Fig. 2c and d).

Fig. 2 (a) Low-resolution TEM image of SbNPs@C; (b) high-resolution TEM image of SbNPs@C: Sb nanocrystals are marked in red regions; (c) the (110) and (012) lattice fringes; (d) the SAED pattern of the SbNPs@C: major diffraction circles are labelled with their respective hkl notation; (e) XRD patterns of the pretreated Sb₆O₁₃NSs, Sb₆O₁₃NSs@C and SbNPs@C, including the reference pattern for Sb₆O₁₃ and Sb (purple line – pretreated Sb₆O₁₃NSs, green line – Sb₆O₁₃NSs@C, blue line – SbNPs@C).

Furthermore, TEM, STEM and EDX mapping characterizations were conducted to investigate the microstructure of SbNPs@C. Since the reduction temperature was set to 500 °C, which is lower than the melting point of Sb (630 °C), we speculated that it will effectively prevent the aggregation of SbNPs during the annealing process. As shown in Fig. 3a and b, it can be clearly seen that the SbNPs, with particle size ranging from 5 to 20 nm, are uniformly embedded in the carbon matrix with no evident agglomeration of the Sb nanoparticles. The magnified TEM observation of the selected area marked in Fig. 3a further reveals the amorphous carbon structure of SbNPs@C. The STEM image of SbNPs@C (Fig. 3c) and corresponding EDS mapping images (Fig. 3d–f) confirm that the SbNPs are homogeneously dispersed on the carbon matrix. Moreover, an EDS measurement was performed to analyze the chemical composition of SbNPs@C (Fig. 3g). Strong Sb and C peaks confirm that the elemental contents of Sb and C were 27.3 wt% and 27.0 wt%, respectively. The signals of the Cu and O peaks originate from the carbon-coated copper grid, which is in good agreement with the mapping results mentioned above.

Fig. 3 (a) Bright field image of SbNPs@C; (b) HRTEM image of the area marked by the red circle in (a); (c) dark field image of SbNPs@C used for EDS mapping; (d–f) the EDS elemental mapping of the area marked by the red rectangle in (c), for C (d), O (e) and Sb (f); (g) result of EDS analysis of SbNPs@C; (h) TGA curve of the SbNPs@C composite in flowing air; (i) XRD pattern of the combustion product of SbNPs@C composite.

The weight percentage of Sb and carbon in the SbNPs@C was measured by thermogravimetric analysis (TGA) in flowing air. The TGA curve is presented in Fig. 3h, showing a 3.4% weight loss around 100 °C, which is ascribed to adsorbed-water evaporation. Almost no weight loss was observed from 100 °C to 420 °C, demonstrating that the composite is stable up to 420 °C in air, i.e., no oxidation reaction occurs at a temperature lower than 420 °C. The weight of the SbNPs@C composite increased at 420 °C, which is attributed to the oxidation of Sb to Sb₂O₄, and the following weight loss from 480 °C to 580 °C was mainly due to carbon combustion. After heating to a temperature higher than 580 °C, the weight was stable with increasing temperature because all the Sb nanoparticles were oxidized and the amorphous carbon was burned off. Fig. 3i is the XRD pattern of the combustion product, and all the diffraction peaks can be indexed to the standard diffraction data of Sb₂O₄ (PDF#00-071-0143). Based on the TGA and XRD results, the SbNPs@C composite was calculated to be approximately 56.6 wt% Sb and 43.4 wt% carbon.

Fig. 4a shows the cyclic voltammetry (CV) curves of the initial five cycles of the SbNPs@C electrode at a scan rate of 0.1 mV s⁻¹ over the potential window of 0.01–2.50 V (vs. Na⁺/Na). The pair of redox peaks observed at 0.01 and 0.14 V is due to sodium insertion/extraction into the carbonaceous materials, which was also observed in other carbon-based materials.⁴⁴ In addition, a strong broad peak centered at 0.42 V was observed in the first cathodic scan (sodiation process), which is attributed to the formation of a solid electrolyte interface (SEI) layer and amorphous Na_xSb alloy phase that is subsequently transformed into cubic and hexagonal Na₃Sb upon further sodiation.^25,26,51 The cathodic peak splits into three peaks at 0.76, 0.60 and 0.48 V in the following discharge process that corresponds to the multi-step reactions of Sb with sodium ions, resulting in a hexagonal Na₃Sb alloy phase. During the anodic process (desodiation process), a long plateau at 0.77 V and a much shorter one at 0.9 V are clearly observed during the charge scans, and they are ascribed to the extraction of Na-ion from Na₃Sb to form Sb. In addition, the CV curves of the following cycles are found to almost overlap, implying the excellent cycling stability of the SbNPs@C electrodes.

Fig. 4 Electrochemical properties of the SbNPs@C electrode: (a) the initial 5 CV curves from 2.50 to 0.01 V vs. Na⁺/Na at a scan rate of 0.1 mV s⁻¹; (b) galvanostatic charge/discharge voltage profiles of the 1^st, 2^nd, 8^th, and 20^th cycles between 0.01 to 2.50 V vs. Na⁺/Na at a current density of 100 A g⁻¹; (c) galvanostatic cycling performance and coulombic efficiency of the SbNPs@C electrode at a current density of 0.1 A g⁻¹; (d) rate capability of the SbNPs@C electrode at various current rates from 20 mA g⁻¹ to 1000 mA g⁻¹; (e) Nyquist plots of the electrochemical impedance spectra before cycling and after one cycle of SbNPs@C electrode.

The charge/discharge profiles of the SbNPs@C electrode obtained at a current density of 100 mA g⁻¹ are shown in Fig. 4b. Remarkably, the plateau regions observed in the charge/discharge curves are consistent with the peaks seen in the CV curves. The initial discharge curve presents long sloping plateaus at around 0.48 V, which can be assigned to the formation of an SEI film and the conversion of hexagonal Sb to Na₃Sb. The subsequent discharge curve has two sloping plateaus between 0.48 and 0.75 V, corresponding to the multi-step transformation of Sb into hexagonal Na₃Sb. During the first and second charge process, sloping plateaus from 0.75 to 1.00 V were observed, which can be related to the phase transition from the Na₃Sb alloy to Sb.⁵¹

The cycling performances of the SbNPs@C electrode were investigated using a current density of 100 mA g⁻¹ with a potential range of 0.01–2.50 V vs. Na⁺/Na, as shown in Fig. 4c. The initial charge and discharge capacities of the SbNPs@C electrode were 1031 and 684 mA h g⁻¹, respectively, with a coulombic efficiency (CE) of 66%. The large capacity loss of the SbNPs@C electrode is mainly attributed to the formation of the SEI layer and electrolyte decomposition during the alloying/dealloying reactions.^25,26 After the initial cycle, the CE reached 95% at the fifth cycle and maintained above 96% after the tenth cycle. Consequently, a high capacity retention (ca. 91%) after 110 cycles was measured. It is worth noting that the specific capacity values were calculated based on the total mass of the SbNPs@C nanocomposite, but the content of Sb in the hybrid is merely 56.6%.

Fig. 4d exhibits the rate performance of the SbNPs@C electrode. As these current densities increase from 20 to 50, 100, 200, 500, and 1000 mA g⁻¹, the electrodes show good capacity retention and slightly decrease from 631 to 562, 525, 507, 471, and 444 mA h g⁻¹, respectively. Although the coulombic efficiencies of the first few cycles were below 90%, the efficiency steadily increased to 93% by the 9^th cycle and continued to grow at subsequent increasing current densities. These low efficiencies were mainly attributed to consistent electrolyte decomposition, which led to the irreversible formation of the SEI film on the surface of the SbNPs electrode and carbon conductive additive during the first sodiation processes. In this study, FEC was introduced as an electrolyte additive to inhibit electrolyte decomposition and to form a stable and thin SEI layer, which can promote a longer cycle life by modifying the surface passivation layer. As reported, the coulombic inefficiencies observed in the early cycles are possibly due to: (1) the large surface area of the Sb nanoparticles, which leads to ineffectively suppressed decomposition of the electrolyte and formation of a thick SEI layer on its surface;^49,52 (2) the irreversible insertion of alkali metal ions into the carbon black conductive additive;²⁵ (3) the SbNPs@C electrode contains a small amount of oxygen (according to EDS elemental mapping in Fig. 3d and EDS analysis in Fig. 3g), and an irreversible reaction of Na⁺ with oxygen (e.g., Na⁺ + O²⁻ → Na₂O) may occur.⁴⁰ Na₂O is a much poorer conductor of Na ions and, therefore, may exclude some portion of the Sb material from the reversible charge/discharge;²² (4) the electronic conductivity of the glucose-derived carbon as a coating layer is low due to the low carbonization temperature used.²⁴ Importantly, as the current density returns to 20 mA g⁻¹, the charge capacity can recover to 570 mA h g⁻¹. After 100 cycles, the SbNPs@C electrode delivered a capacity as high as 602 mA h g⁻¹, indicating an excellent rate capability of SbNPs@C hybrids.

Furthermore, electrochemical impedance spectroscopy (EIS) studies were performed before cycling and after one cycle to verify the superior electrochemical performance of the SbNPs@C hybrids, and the results are displayed in Fig. 4e. As can be seen, each Nyquist plot contains a semicircle and one slope. The semicircle across the high-frequency region represents the charge-transfer impedance (R_ct) on the electrode/electrolyte interface, and the slope in the low-frequency region corresponds to the Warburg impedance (W) associated with the Na⁺ diffusivity of the active electrode material.²³ According to the diameter of these semicircles, the charge-transfer impedance (R_ct) of the SbNPs@C electrode decreases significantly after one cycle, indicating that an activation step might occur during the charge–discharge process, which is in agreement with the excellent rate performance shown in Fig. 4d. Consequently, the amorphous carbon matrix accommodated the volume change of the Sb nanocrystals during the repeated alloying and dealloying processes. The structural stability of the SbNPs@C hybrids during cycling resulted in the superior electrochemical properties of the anode materials for sodium ion storage.

Conclusion

In summary, we synthesized a SbNPs@C nanocomposite using a simple hydrothermal method, followed by an annealing process. The as-prepared SbNPs@C nanocomposite displayed a uniform “sea-island” structure with the Sb nanoparticles embedded homogenously in the carbon matrix, which provided a structurally stable host for Na-ion alloying and dealloying. The SbNPs@C electrode shows an outstanding cycling stability, providing a capacity of 622 mA h g⁻¹ after 110 cycles at a current density of 100 mA g⁻¹. Furthermore, the SbNPs@C nanocomposite also exhibits an excellent rate-capability with a capacity of 444 mA h g⁻¹ at a current density of 1000 mA g⁻¹. These superior electrochemical performances indicate that the SbNPs@C composite is a promising candidate to be used as a high capacity and cycling-stable anode for SIBs.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (No. 51125008 and No. 11274392).

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