Bi2Se3/C Nanocomposite as a New Sodium-Ion Battery Anode Material
KeywordsBi2Se3 Sodium-ion battery High-energy ball milling Sodium storage mechanism
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.
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 . Developing suitable anode materials for SIBs with both high capacity and long cycle life is highly desired. Among anode materials, alloying-type materials  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) , which is beneficial for a stable anode . 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 . Bi2Se3 displays an electrical conductivity two orders of magnitude higher than that of Bi2S3 , which can improve the electron transport. In addition, the shuttle effect is relieved for selenides compared to sulfides . Moreover, Bi2Se3 has a high density of 7.47 g cm−3 , 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.
3.1 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
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  and ca. 50% higher than that of Bi2S3 nanorods at the 40th cycle . 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.
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 = aν b , 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.
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.
The authors thank the support from TcSUH as the TcSUH Robert A. Welch Professorships on High Temperature Superconducting (HTSg) and Chemical Materials (E-0001). H.C. acknowledges the support from the National Science Foundation under grant number DMR-1410936.
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