1 Highlights

  • Bi2Se3 was investigated as a novel sodium-ion battery anode material.

  • Sodiation/desodiation mechanism of Bi2Se3 has been carefully investigated.

  • Bi2Se3/C electrode demonstrates high cycling stability.

2 Introduction

Sodium-ion batteries (SIBs) have recently regained extensive research interest as alternatives to lithium-ion batteries (LIBs) for energy storage owing to the low cost and abundance of Na [1,2,3,4,5]. The lack of high energy density anode materials has impeded the progress of SIBs for a long time [6]. Developing suitable anode materials for SIBs with both high capacity and long cycle life is highly desired. Among anode materials, alloying-type materials [7] have attracted much attention. For example, Sn, Sb, and Bi can reversibly alloy with Na+ and provide high theoretical gravimetric capacities (> 300 mAh g−1), which far exceed the capacities of carbonaceous materials and Ti-based materials. The accompanying challenge for alloying-type materials is the large volume expansion when alloying with Na+. Bi displays a relatively small volume expansion (ca. 250% expansion from Bi to Na3Bi), compared to Sn (ca. 420% expansion from Sn to Na3.75Sn) and Sb (ca. 293% expansion from Sb to Na3Sb) [8], which is beneficial for a stable anode [9]. The voltage plateau is also an important criterion in evaluating an electrode material. A low operating voltage for anode materials can endow a cell with a high operation voltage. However, Na plating, dendrite formation, and electrolyte decomposition occur on the anode side when the discharge voltage approaches 0 V, as is often the case for hard carbon anodes [10,11,12]. The plateaus of Bi between 0.3 and 0.9 V versus Na+/Na are favorable for maintaining a high operation voltage and avoiding the aforementioned detrimental effects [13, 14].

Sulfides and selenides have been actively investigated because their conversion reactions offer high capacities for ion storage [15,16,17,18]. Recently, the Bi-based compound Bi2S3 has been synthesized and displayed a high Na storage capacity [19, 20]. However, the rate capacity was unsatisfactory, limited by the low intrinsic conductivity of sulfides [15]. Bi2Se3 displays an electrical conductivity two orders of magnitude higher than that of Bi2S3 [21], which can improve the electron transport. In addition, the shuttle effect is relieved for selenides compared to sulfides [22]. Moreover, Bi2Se3 has a high density of 7.47 g cm−3 [21], permitting the opportunity to fabricate small-sized devices with high volumetric capacities (theoretically 3667 mAh cm−3). Bi2Se3 has been applied in LIBs and exhibited excellent electrochemical storage ability for Li+. Several Bi2Se3 nanostructures, such as nanosheets and microrods, have been designed for Li+ storage [23, 24]. Furthermore, high free electron densities can effectively improve the rate capability; thus, doping strategies have been employed to create S-doped and In-doped Bi2Se3 [25,26,27]. Despite the good electrochemical performance in Li+ storage, Bi2Se3 has not been reported as an anode material for SIBs.

Downsizing the bulk material to nanoscale and integrating carbon with it can improve the electrochemical performance, including the rate capability and cyclability, by the shorter diffusion distances, more abundant reaction sites on the large surface area, and additional space for expansion [28,29,30,31]. Carbon can stabilize the nanomaterial and provide an interconnected network for electron transport as well, and the voids in the carbon can accommodate volume expansion and allow permeation of the electrolyte for fast Na+ transport [32,33,34].

In our study, a simple high-energy ball milling (HEBM) method was adopted to synthesize Bi2Se3 and Bi2Se3/C nanocomposite. The Bi2Se3/C nanocomposite delivers an initial reversible capacity of 527 mAh g−1 at 0.1 A g−1 with 89% retention over 100 cycles. The phase changes during cycling were investigated by ex situ X-ray diffraction (XRD) to reveal the Na storage mechanism. The rational material design combined with effective synthetic protocol is important and this work is expected to shed light on future work on developing excellent anode materials for SIBs.

3 Experimental

3.1 Synthesis Process

The synthesis of Bi2Se3 and Bi2Se3/C was performed by HEBM. Bi (Alfa Aesar, 99.999%) and Se (Alfa Aesar, 99.999%) in a molar ratio of 2:3 were sealed in an Ar-filled stainless steel jar and then ball milled for 10 h at 1200 rpm (Spex 8000 M) to form phase-pure Bi2Se3 powder. Graphite powders were milled for 48 h beforehand. Then, the milled graphite was added to Bi2Se3 powders in the weight ratio of 2:8 and ball milled for another 6 h to form the carbon-integrated Bi2Se3 nanocomposite.

3.2 Material Characterization

The phases were investigated by XRD on a Rigaku SmartLab diffractometer with a Cu Kα source at the scan rate of 5 deg. min−1. The morphology was studied under scanning electron microscopy (SEM, LEO 1525). The nanostructures and the diffraction patterns were characterized by transmission electron microscopy (TEM, JEOL 2010F, operated under 200 kV). The elemental mapping was collected by energy-dispersive X-ray spectroscopy (EDS) (attached to the TEM). X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantera XPS instrument. To confirm the carbon content, the samples were heated at 10 °C min−1 from room temperature to 600 °C in thermogravimetric analysis (TGA, Q500).

3.3 Electrochemical Measurements

Coin cells (CR 2025) with Bi2Se3 or Bi2Se3/C as the active material were assembled for battery tests. A slurry was made by mixing 70 wt% active material, 20 wt% carbon black, and 10 wt% polyacrylic acid (PAA) and then coated on a Cu foil to form the working electrodes, followed by drying at 60 °C under vacuum overnight. To prepare the electrolyte, 1 mol L−1 NaClO4 was dissolved in propylene carbonate/ethylene carbonate (1:1 in volume) with 5 wt% fluoroethylene carbonate (FEC) as an additive. The loading of the active materials was 1.4 ± 0.2 mg cm−2 for the Bi2Se3/C electrode and 1.5 ± 0.3 mg cm−2 for the Bi2Se3 electrode. Homemade Na lumps and glass fibers were applied as the reference/counter electrodes and the separators, respectively. The electrochemical measurements of the cells were performed galvanostatically between 0.01 and 2.5 V versus Na/Na+ on a Land CT2001A battery tester. Cyclic voltammetry (CV) curves were swept at 0.1 mV s−1 on a BioLogic SP-200 electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was measured from 100 kHz to 100 mHz with a voltage amplitude of 5 mV.

4 Results and Discussion

Figure 1 displays the structural and morphological characterization details of Bi2Se3 and Bi2Se3/C. The XRD patterns of Bi2Se3 and Bi2Se3/C are shown in Fig. 1a. The ball milled Bi2Se3 and Bi2Se3/C display the same XRD patterns, which match well with the pure rhombohedral phase (space group \( R\bar{3}m \) (166), JCPDS card No. 33-0214). After ball milling with carbon for another 6 h, the peaks of Bi2Se3/C become broader, indicating that smaller nanocrystals are produced. The crystal structure is again confirmed in the electron diffraction patterns of Fig. 1b, with the rings well indexed as the planes (0 0 6), (1 0 1), (0 1 5), (0 1 8), and (1 0 10) of rhombohedral-phase Bi2Se3. The TEM image in Fig. 1c and high-resolution TEM image in Fig. 1d show that the secondary Bi2Se3 particles are composed of well-developed nanocrystals with sizes ranging from a few nanometers to tens of nanometers. Figure 1e, f demonstrate that the Bi2Se3 nanocrystals are well encapsulated and uniformly distributed in the carbon matrix after integration with carbon. The primary nanocrystal sizes are approximately 5–20 nm, much smaller than those of as-synthesized Bi2Se3 because the carbon matrix can well separate and stabilize Bi2Se3 nanocrystals [32]. To reflect nanocrystal sizes across the samples, additional high-resolution TEM images are provided in Fig. S1. The particle sizes of the Bi2Se3/C nanocomposite also grow finer due to the separation of carbon compared to those of bare Bi2Se3, as observed in the SEM images (Fig. S2). Clear fringes of the crystal planes of Bi2Se3 can be found in Fig. 1f, indicating that the Bi2Se3 maintains good crystallinity in the carbon composite. In Fig. 1g, the uniform distribution of the elements Bi, Se, and C is confirmed by the EDS mapping. The carbon content of the composite is further confirmed to be 20.7 wt% by the TGA test (Fig. S3).

Fig. 1
figure 1

a XRD patterns of as-synthesized Bi2Se3 and Bi2Se3/C. b The diffraction pattern of Bi2Se3. c Low- and d high-resolution TEM images of Bi2Se3. e Low- and f high-resolution TEM images of Bi2Se3/C. g Scanning TEM (STEM) image and its corresponding elemental (Bi, Se, and C) mappings

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The half-cell of Bi2Se3/C was cycled at a scan rate of 0.1 mV s−1 within 0.01–2.5 V versus Na+/Na and the I–V curves are shown in Fig. 2a. Three cathodic peaks at 1.04, 0.52, and 0.27 V and four anodic peaks at 1.88, 1.7, 0.79, and 0.67 V are depicted in the first cycle. The peak positions are analogous to those in Bi2S3 anode because of the similar properties between S and Se as chalcogens [14, 19, 20]. In the cathodic scan, Bi and Na2Se form at 1.04 V [19, 20], followed by the sodiation of Bi at lower voltages of 0.52 and 0.27 V [14]. In the reverse scan, desodiation of the Na–Bi alloy occurs at 0.67 and 0.79 V [14], then NaBiSe2 is formed at 1.7 and 1.88 V [19, 20, 35]. The peak at 1.04 V in the first cycle is slightly shifted to 1.14 V in the following cycle. Other than this shift, the CV curves overlap very well, which indicates a highly reversible Na storage kinetics. Figure S4 also displays the CV curve of Bi2Se3. The same characteristics are observed in the CV curves of Bi2Se3 and Bi2Se3/C, which indicate that integrating carbon does not affect the sodiation process of Bi2Se3. However, integrating carbon does improve the stability of the electrode, which is evidenced by the obvious decrease in the peak intensities of bare Bi2Se3 over CV cycling.

Fig. 2
figure 2

Studies of electrochemical properties of the Bi2Se3/C anode for SIBs. a CV curves of the Bi2Se3/C anode at 0.1 mV s−1. b Cyclic performance of Bi2Se3/C and Bi2Se3 anodes at 0.1 A g−1 and the related Coulombic efficiency of Bi2Se3/C anode. c Discharge/charge profiles and d rate performance of Bi2Se3/C anode at different current densities

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The cyclic performances of Bi2Se3 and Bi2Se3/C at 0.1 A g−1 and the related Coulombic efficiency of the Bi2Se3/C anode are shown in Fig. 2b. Alloying and conversion anodes often show lower Coulombic efficiencies than intercalation anodes. At the first cycle, the Bi2Se3 and Bi2Se3/C anodes both display reasonably high Coulombic efficiencies (> 75%), indicating higher utilization of Na+ than most alloying anodes. With carbon integrated, the reversible capacity of Bi2Se3/C anode (527 mAh g−1) is somewhat comprised compared to the capacity of 557 mAh g−1 for the Bi2Se3 anode at the first cycle. In the following cycles, however, the Bi2Se3/C anode exhibits much improved stability, reaching a steady value of 510 mAh g−1 within five cycles and retaining 89% of the initial capacity over 100 cycles, while the Bi2Se3 anode displays a fast decay in capacity to below 200 mAh g−1 within 20 cycles. At a higher current density of 0.5 A g−1, the Bi2Se3/C anode still shows high stability with an initial capacity of 445 mAh g−1 after the first two cycles at 0.1 A g−1 and that of 383 mAh g−1 over 180 cycles (Fig. S5). The cyclic performance of Bi2Se3/C is superior to those of other Bi-based materials and competitive with many typical anode materials (Table S1) [36,37,38,39,40,41]. Although the initial capacity of Bi2Se3/C is not extremely high compared to those of similar materials reported, the unique advantage of this composite is its stability in long-term cycling. For example, at 0.1 A g−1, the capacity of 470 mAh g−1 for Bi2Se3/C composite at the 100th cycle is more than triple that of Bi@C microspheres [42] and ca. 50% higher than that of Bi2S3 nanorods at the 40th cycle [19]. Figure 2c shows the voltage profiles of the Bi2Se3/C anode for a wide range of discharge/charge rates between 0.01 and 2.5 V versus Na+/Na. At the low current density of 0.1 A g−1, the plateaus can be clearly identified with three discharge plateaus and four charge plateaus, corresponding to the peaks in the CV curves. The discharge/charge profiles maintain analogous shapes and plateaus even at very high current densities, indicating the fast reaction kinetics of the Na storage process. The details of the fast reaction kinetics may be ascribed to the fast capacitive contribution, as discussed later. Figure 2d shows the excellent rate capability of Bi2Se3/C as an anode material for SIBs. Remarkably, it delivers the high capacities of 500, 445, 415, 384, 332, 298, 255, and 186 mAh g−1 at 0.1, 0.3, 0.5, 1, 3, 5, 7, and 10 A g−1, respectively. To confirm the high reversibility, 0.1 A g−1 is applied again after cycling at 10 A g−1, and the capacity returns to its previous level as expected. The rate capacities of Bi2Se3/C are competitive with those of typical anode materials listed in Table S2 and the performance is better at high current densities. The volumetric capacity is also an important consideration for practical application; that of the Bi2Se3/C electrode reaches 1064 mAh cm−3, calculated by multiplying the volumetric density of Bi2Se3/C (2.02 g cm−3) with the gravimetric capacity (527 mAh g−1) at 0.1 A g−1.

To explore the insights of sodiation/desodiation mechanism of Bi2Se3, ex situ XRD was conducted. After charging/discharging, the electrodes were removed from the cells in a glove box and covered with Kapton tapes to avoid oxidation. The sampling points were chosen in reference to the dQ/dV curves in Fig. 3a. When the anode is sodiated to 0.86 V from the open-circuit voltage, the Bi2Se3 characteristic peak disappears while Bi peaks appear with Na2Se [14]. The XRD patterns of Bi and Na2Se are maintained when the material is discharged to a low voltage of 0.47 V. In this process, the intercalation of Na+ into Bi may occur. The phase of NaBi appears at 0.01 V, indicating that alloying reaction occurs at the complete sodiation state [8, 43]. In the desodiation process, Bi dealloys with Na+, evidenced by the appearance of the Bi phase at 0.88 V. However, even at the highest potential of 2.5 V, the Bi2Se3 phase does not recover; instead, NaBiSe2 with Bi phases are formed [19]. The irreversible Bi2Se3 change can also explain the peak shifting from 1.04 V in the first cycle to 1.14 V in the following cycles in the I–V curves of Fig. 2a. When the electrode is again sodiated to 1.05 V at the second cycle, diffraction patterns corresponding to Bi and Na2Se appear again. In summary, the phase changes during cycling can be listed as the following four steps:

Fig. 3: a
figure 3

dQ/dV plots for the first two cycles and b ex-situ XRD results of the Bi2Se3/C anode

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Sodiation process:

$$ {\text{conversion}}\;{\text{reaction: Bi}}_{2} {\text{Se}}_{3} + \, 6{\text{Na}}^{ + } + \, 6{\text{e}}^{ - } \to 2{\text{Bi}} + 3{\text{Na}}_{2} {\text{Se }}\left( {\text{irreversible}} \right) $$
(1)
$$ {\text{alloying}}\;{\text{reaction}}{:}{\text{ Bi}} + x{\text{Na}}^{ + } + \, x{\text{e}}^{ - } \to {\text{Na}}_{x} {\text{Bi}} $$
(2)

Desodiation process

$$ {\text{dealloying}}\;{\text{reaction}}{:}{\text{ Na}}_{x} {\text{Bi}} \to {\text{Bi }} + \, x{\text{Na}}^{ + } + \, x{\text{e}}^{ - } $$
(3)
$$ {\text{conversion}}\;{\text{ reaction}}{:}{\text{ Bi}} + 2{\text{Na}}_{2} {\text{Se}} \to {\text{NaBiSe}}_{2} + 3{\text{Na}}^{ + } + \, 3{\text{e}}^{ - } $$
(4)

In addition to XRD analysis, XPS was also applied to provide a more comprehensive understanding of the materials and the electrochemical process, because XPS is sensitive to the surface within the depth of ca. 5 nm. Figure S7 displays the XPS survey spectrum and high-resolution spectra of Bi 4f and Se 3d for Bi2Se3/C electrode. The peaks at 163.7 and 158.4 eV are assigned to Bi 4f5/2 and Bi 4f7/2, respectively. The peaks at 54.3 and 53.5 eV correspond to Se 3d3/2 and Se 3d5/2 in Bi2Se3, respectively, confirming the successful synthesis of Bi2Se3 [44]. In addition, the peaks related to BiO x and SeO x are also found, indicating oxidation happens at the surface [44]. The solid electrolyte interface (SEI) compositions were also investigated by comparing the electrode before and after one cycle. Figure 4 indicates significant changes in the C 1s and F 1s spectra. The pristine electrode has a strong signal at 284.6 eV related to the carbon bonds of graphite or carbon black, and the small peaks at 285.3, 285.9, and 288.8 eV correspond to –CH2–, –CH–COONa, and R–COONa of the PAA binder [9, 45]. After one cycle, several new peaks are formed in the higher binding energy region and the strong signal at 284.6 eV related to graphite and carbon black disappears, indicating the formation of the SEI film on the surface. The signals from 286.0 to 289.5 eV are assigned to the –C–O– and –C=O– species of the SEI film and the peak at 291.1 eV arises from Na2CO3 of the SEI film [9, 46]. The signal related to F 1s appears after one cycle, indicating that the SEI film contains F from the decomposition of FEC.

Fig. 4
figure 4

High-resolution XPS spectra a C 1s and b F 1s of the Bi2Se3/C electrode before and after one cycle

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The reaction kinetics can be revealed by EIS and the EIS spectra of Bi2Se3/C electrode and Bi2Se3 electrode are displayed in Fig. 5a. The intercept with the Zreal axis at high frequency represents the electrolyte and contact resistance (Rs), while the semicircles at medium frequency are related to the SEI resistance (Rf) and electrolyte/electrode charge transfer resistance (Rct) [47]. The equivalent circuit model for the fitting is shown in the inset of Fig. 5a with the fitting results listed in Table S3. Rct of the Bi2Se3/C electrode decreases significantly from 661.4 to 81.4 Ω after cycling benefited from the reconstructed porous structure with close connections, as seen under SEM (Fig. S8) [48]. On the contrary, the EIS spectra of Bi2Se3 display a large Rct increase after cycling due to the contact loss. For the Bi2Se3 electrode after cycling, large aggregates are formed with rough surfaces and loose contact between particles. In addition, the Rf increase of 19.9 Ω for the Bi2Se3 electrode is more significant than that of 6.8 Ω for the Bi2Se3/C electrode, caused by the fracture and the continuous growth of a thick SEI layer in the Bi2Se3 electrode.

Fig. 5
figure 5

a EIS spectra for Bi2Se3 and Bi2Se3/C anodes before and after five cycles. b CV curves of the Bi2Se3/C anode at different scan rates. c, d b-value fitted by the relationship of the logarithm peak currents and logarithm scan rates

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For nanomaterials with large surface areas, surface-induced capacitive processes can have significant effects and improve the charge/discharge capability [49,50,51]. The b value is often used as an index to estimate the surface-induced capacitive contribution. According to i = aνb, where i is the current response at the scan rate ν, the b value can be readily fitted by log(i) − log(ν) linear plots. The b value can vary from 0.5 to 1. The capacitive process dominates when the b value is close to 1, while diffusion-controlled processes dominate when the b value approaches 0.5. Figure 5b shows the I–V curves at different scan rates for the Bi2Se3/C electrode; the relations of log(i) and log(ν) at the corresponding peaks derived from the I–V curves are shown in Fig. 5c, d. The fitted b values are 0.71, 0.82, 0.78, and 0.74 for the R1, R2–1, R2–2, R3 peaks and 0.85, 0.85, 0.98, and 0.86 for O1–1, O1–2, O2, and O3 peaks. These values are much higher than 0.5, which indicates that fast capacitive process occurs during Na storage, contributing to the high rate capacity for the Bi2Se3/C electrode. The current and scan rate relations are not shown for Bi2Se3 electrode because of the significant changes of the CV curves over cycling.

5 Conclusions

The application of Bi2Se3 was explored as an anode material for SIBs. Benefiting from the high theoretical capacity and high intrinsic conductivity of Bi2Se3, the positive effects of carbon, and the effective HEBM method, a high-performance anode material was achieved. The Bi2Se3/C electrode showed a high reversible capacity of 527 mAh g−1 and retains 89% of this capacity over 100 cycles at 0.1 A g−1. To obtain insights regarding the electrochemical process of Na storage, the phase changes were revealed by ex situ XRD.