Self-Assembly of Bi₂Te₃-Nanoplate/Graphene-Nanosheet Hybrid by One-Pot Route and Its Improved Li-Storage Properties

Abstract

A sandwich structured Bi₂Te₃-nanoplates/graphene-nanosheet (Bi₂Te₃/G) hybrid has been synthesized by a facile in situ solvothermal route and has been investigated as a potential anode material for Li-ion batteries. Bi₂Te₃ grows during the solvothermal process with the simultaneous reduction of graphite oxide into graphene. The in situ formation process of the hybrid has been investigated by X-ray diffraction and X-ray photoelectron spectra. The Li-storage mechanism and performance of Bi₂Te₃/G and bare Bi₂Te₃ have been studied by galvanostatic cycling and cyclic voltammetry. The Bi₂Te₃/G sandwich exhibits an obviously improved cycling stability compared to bare Bi₂Te₃. The enhancement in electrochemical performance can be attributed to the combined conducting, confining and dispersing effects of graphene for Bi₂Te₃ nanoplates and to the self-assembled sandwich structure.

1. Introduction

Although carbon based materials are presently the dominant anodes in commercial Li-ion batteries, there is still a great challenge to develop alternative anode materials in order to meet the requirement of high-performance Li-ion batteries [1,2]. Alloy materials have been regarded as one of the most promising anodes for the next-generation Li-ion batteries because of their safe operating voltage and the large energy density, especially the large volumetric energy density due to the high density [3,4]. Among various elements that can form Li-alloys, Sb has received a special interest because of its appropriate alloying/de-alloying voltage (around 0.8 V vs. Li/Li⁺) and relatively large Li-storage capacity (660 mAh g⁻¹) with the formation of Li₃Sb composition [5]. However, the volume change is over 200% during the conversion from Sb to Li₃Sb [6]. The volume change causes the cracking and crumbling of the particles, resulting in the loss of physical contact between the active particles and the current collector and the consequent capacity loss upon cycling.

Many measures have been taken to alleviate the volume changes for Sb during alloying/de-alloying processes. Previous work [7,8,9,10] showed that the cycling stability of metallic Sb can be improved by using Sb-based intermetallic compound MSb_x, where M is an electrochemically inert element and acts as the buffering matrix. The improvement in electrochemical performance could also be realized by using nanostructured materials [11,12,13,14] due the large surface area, short Li-ion diffusion path and increased mechanical strength. However, the nanoparticles tend to aggregate, also leading to the disconnection of active material with the current collector. An effective approach to overcome this problem is to immobilize the nanoparticles on a matrix. Carbon-based materials [15,16,17,18] are considered as the ideal matrices since they are electrically conductive and electrochemically active in addition to the buffering effect for the volume change.

Graphene, a flat monolayer of sp²-bonded carbon atoms [19], was also the ideal matrix because of its appealing properties such as high electronic conductivity [20], high specific surface area [21] and high mechanical strength [22]. Our previous work showed that the cycling stability of some Sb-based intermetallic compounds could be improved by forming nanocomposites with graphene [23,24]. Similar to Sb, Bi in the same group can also electrochemically store Li by forming a Li₃Bi composition [5,25,26,27]. So far, the effect of graphene on the electrochemical performance of Bi-based anodes has not been reported yet. Herein, an in situ solvothermal route was used to prepare the Bi₂Te₃-nanoplate/graphene-nanosheet (Bi₂Te₃/G), where Te can also reversibly store Li by forming Li₂Te [27]. The results showed that an obvious improvement in cycling stability of Bi₂Te₃ could be realized by constructing a Bi₂Te₃/G hybrid with a sandwich structure.

2. Results and Discussion

Figure 1a shows the XRD patterns of Bi ₂Te₃/G and bare Bi₂Te₃. For comparison, the standard diffraction peaks of Bi₂Te₃ are also given. All the diffraction peaks can be indexed to hexagonal Bi₂Te₃ (JCPDS No. 89-2009) for both Bi₂Te₃/G and bare Bi₂Te₃. The diffraction peaks of graphene that should appear at around 2θ = 25 °C cannot be detected, suggesting that the restacking of graphene sheets after reduction was inhibited by uniformly loading Bi₂Te₃ plates between the graphene sheets. The graphene content is roughly estimated to be 11.5 wt% by the carbon content analysis.

To check the reduction status of GO during the solvothermal reactions, C1s XPS spectra of GO and Bi₂Te₃/G are analyzed as shown in Figure 1b. The spectra can be fitted by four peaks for different forms of carbons: non-oxygenated carbon (C-C, 285.6 eV and C=C, 284.8 eV), carbon in C-O bonds (286.3 eV), carbonyl carbon (C=O, 287.6 eV) and carboxylate carbon (O-C=O, 289.0 eV) [28]. After the solvothermal reactions, the peak intensity of the oxygenated carbons (C-O, C=O, O-C=O) shows an obvious decrease, indicating the reduction of GO into graphene. Figure 1c gives the O1s XPS spectra of the hybrid. The broad peak at 532.2 eV is assigned to the residual oxygen-containing groups such as -COOH and -OH [29], which agrees with the result of C1s spectra. From the XRD and XPS analyses, it can be concluded that a Bi₂Te₃/G hybrid has formed with the simultaneous formation Bi₂Te₃ and reduction of GO during the one-step solvothermal process.

Figure 1. (a) XRD of Bi₂Te₃/G and bare Bi₂Te₃; (b) C1s XPS of GO and Bi₂Te₃/G; (c) O1s XPS of Bi₂Te₃/G.

Figure 1. (a) XRD of Bi₂Te₃/G and bare Bi₂Te₃; (b) C1s XPS of GO and Bi₂Te₃/G; (c) O1s XPS of Bi₂Te₃/G.

Figure 2a shows the SEM image of bare Bi ₂Te₃ prepared by the solvothermal route without adding GO in the precursors. Generally, the sample exhibits a hexagonal plate shape with a size of around 0.5–3 μm. However, smaller sized broken plates with an irregular shape can also be observed. The thickness of the plates is roughly estimated to be 50–100 nm from some vertically (or partially vertically) aligned plates denoted by the arrows. Figure 2b shows the SEM image of the Bi₂Te₃/G hybrid. It is obvious from the transparent graphene that Bi₂Te₃ plates are located between the graphene sheets. The size of the Bi₂Te₃ plates is below 2 μm. A three-dimensional (3D) sandwich structure is thus constructed by alternatively arranged graphene layer and Bi₂Te₃ layer as clearly indicated by the arrows. The transparent nature of the graphene layer suggests that it is rather thin, composed probably of single or few layers of graphene sheets.

The microstructure of Bi₂Te₃/G was further characterized by TEM as shown in 2c. Note that nearly all of the Bi ₂Te₃ plates are anchoring on graphene without forming free Bi₂Te₃ plates even after vigorous ultrasonication. The wrinkles and folded edges of graphene indicate that the graphene layer is rather thin, which agrees with the SEM observation. The enlarged view of the circled part in Figure 2c is presented in Figure 2d. The overlapping of the Bi₂Te₃ plates can be observed from Figure 2d, from which the transparent nature of the Bi₂Te₃ plate is evident. From a vertically aligned plate, the thickness of the plate is estimated to be 15 nm, which is much smaller compared with that of bare Bi₂Te₃. The thickness of the Bi₂Te₃ plates is in the range of 10−20 nm after observing some plates at the edges or defect sites of graphene. It is worth noting that the number of the Bi₂Te₃ plates with a regular hexagon is much smaller compared with that of bare Bi₂Te₃ plates. It is believed that the irregular shape and small thickness of the Bi₂Te₃ plates in Bi₂Te₃/G are caused by the inhibited diffusion and re-crystallization of small sized Bi₂Te₃ particles due to the pinning effect of the defects and the oxygen-containing groups in graphene [30]. The presence of the residual oxygen-containing groups can be confirmed by XPS shown in Figure 1b,c.

Figure 2. (a) SEM image of bare Bi₂Te₃; (b) SEM image of Bi₂Te₃/G hybrid; (c) TEM image of Bi₂Te₃/G hybrid; (d) the enlarged view of the circled part in (c).

Figure 2. (a) SEM image of bare Bi₂Te₃; (b) SEM image of Bi₂Te₃/G hybrid; (c) TEM image of Bi₂Te₃/G hybrid; (d) the enlarged view of the circled part in (c).

In Figure 3, CV plots of Bi₂Te₃/G and Bi₂Te₃ were measured to clarify the electrochemical reaction mechanism of Bi₂Te₃ with Li. As seen in Figure 3a the reaction Bi ₂Te₃ with Li exhibits a multi-step mechanism, evidenced by the multi-peak feature of the CV plots. During the first cathodic scan, three reduction peaks, located at 2.0, 1.25, and 0.6 V, respectively, appear as indicated by the black line in 3a. The small peak at 2.0 V is related possibly to the insertion reaction of Li ion into layered Bi ₂Te₃ without significantly destroying its crystal structure [31]. The peak at 1.2 V corresponds to the lithiation reaction of Te by forming Li₂Te (2Li + Te ° Li₂Te) [27], and the peak at 0.6 V is associated with the reaction of Bi with Li to form Li₃Bi (3Li + Bi ° Li₃Bi) [5,26,27]. During the second cathodic scan, the peak at 2.0 V almost disappears, the peak at 0.6 V is shifted to a higher value of 0.7 V (A’), while the peak at 1.2 V is divided into two peaks, B’ and C’, located at 1.35 and 1.65 V, respectively. The division of the peak means that the reaction of Te with Li occurs via a two-step mechanism by forming LiTe₃ and Li₂Te successively, both of which are stable phases at room temperature [32]. The lower peak potential in the first cathodic scan compared with those in the subsequent scans is due to the additional energy required for the displacement reactions of Bi₂Te₃ upon Li uptake [4]. This also means that structure of Bi₂Te₃ cannot be recovered after the anodic scan [27].

Note that three oxidation peaks (A, B, and C), at 1.0, 1.8 and 1.9 V, respectively, appear during the anodic scans, corresponding to the successive de-lithiation reactions of Li₃Bi (peak A) and Li₂Te (peak B and peak C). The CV plots of bare Bi₂Te₃ are also given for comparison as seen in Figure 3b. The plots of bare Bi₂Te₃ display a similar feature as those of Bi₂Te₃/G except that the peak intensity decreases more rapidly with scans, indicative of its lower reversibility. The relatively high reversibility of Bi₂Te₃/G can be attributed to the incorporation of graphene that improves the electrode kinetics and keeps the electrode integrity.

Figure 3. CV plots of (a) Bi₂Te₃/G; and (b) Bi₂Te₃ scanned at 0.1 mV s⁻¹.

Figure 3. CV plots of (a) Bi₂Te₃/G; and (b) Bi₂Te₃ scanned at 0.1 mV s⁻¹.

Figure 4a shows the chare-discharge curves of Bi ₂Te₃ and Bi₂Te₃/G at 50 mA g⁻¹ at various cycles. Bi₂Te₃/G hybrid gives a first discharge (Li-uptake) capacity of 731 mAh g⁻¹ and a first charge (Li-extraction) capacity of 481 mAh g⁻¹. The first irreversible capacity can be attributed to the reduction decomposition of the electrolyte and the formation of the solid state interface (SEI) layer. Successive potential plateaus can be seen during the charge-discharge cycling, which correspond to the current peaks in the CV plots. The appearance of the potential plateaus is due to the two-phase coexistence of original phase (Te or Bi) and the Li-alloys (Li-Te or Li-Bi alloys). As shown in the figure, bare Bi₂Te₃ exhibits lower charge-discharge capacities compared with Bi₂Te₃/G. The theoretical maximum capacity of Bi₂Te₃ is 401 mAh g⁻¹, related to the formation of Li₂Te and Li₃Bi. Considering the fact that graphene itself shows a relatively low capacity of around 300 mAh g⁻¹, as reported in our previous work [33], the theoretical capacity of Bi₂Te₃/G should be lower than that of bare Bi₂Te₃. The high yieldable of Bi₂Te₃/G is possibly due to the synergistic effect between Bi₂Te₃ and graphene. On one hand, as discussed above, the presence of graphene can restrain the crystal growth of Bi₂Te₃, which is favorable for rapid Li ion diffusion within the active material. On the other hand, the separator effect of Bi₂Te₃ plates for graphene sheets maximizes the exposure of the graphene surface to the electrolyte, especially for the edges and vacancies on graphene. The edges and vacancies can supply additional sites for Li storage [34]. Note that Bi₂Te₃/G shows a progressive polarization upon cycling, but the extent is smaller than bare Bi₂Te₃ owing to the presence of the conductive graphene.

Figure 4b compares the cycling stability between bare Bi₂Te₃ and Bi₂Te₃/G charged at 50 and 200 mA g⁻¹ and discharged at 50 mA g⁻¹. Note that Bi₂Te₃/G demonstrates an obviously slower capacity fade compared to bare Bi₂Te₃. After 50 cycles at 200 mA g⁻¹ (about 0.5 C), a charge capacity of 158 mAh g⁻¹ can be retained for Bi₂Te₃/G. In contrast, the charge capacity of bare Bi₂Te₃ drops rapidly to 33 mAh g⁻¹. The Bi₂Te₃/G hybrid also shows improved cycling stability compared with Bi₂Te₃-graphite composite [27] and other Bi-based anodes [25,26]. The enhanced cycling stability is attributed to the buffering effect of graphene that alleviates the large volume changes and the confinement effect of graphene that restrains the aggregation of the Bi₂Te₃ plates, in addition to offering effective conducting networks. The 3D sandwich structure also facilitates the Li-ion diffusion within the free space and across the electrode/electrolyte interface. In addition, the free space within the sandwich also affords additional room to buffer the volume changes. It should be stressed that the long-term cycling stability of the Bi₂Te₃/G hybrid is not satisfactory yet owing to the intrinsic large volume changes of the Bi₂Te₃ during alloying/de-alloying processes. However, it is clear that graphene plays an important role in enhancing the electrochemical properties of Bi₂Te₃. It also should be noted that the large-scaled of Bi₂Te₃/G is not favored due to the toxicity, price, and resource scarcity of Bi and Te. Nevertheless, it shows promising application in micro-batteries due to the possibility of micro processing of graphene and Bi₂Te₃.

Figure 4. (a) Voltage profiles; and (b) cycling stability of Bi₂Te₃/G and Bi₂Te₃.

Figure 4. (a) Voltage profiles; and (b) cycling stability of Bi₂Te₃/G and Bi₂Te₃.

3. Experimental Section

3.1. Preparation of Bi₂Te₃-Nanoplate/Graphene-Nanosheet (Bi₂Te₃/G) Nanohybrid

Graphite oxide (GO, 80 mg), prepared by a modified Hummer’s method [35], was added into 40 mL of ethylene glycol (EG) and sonicated for 24 h to obtain a sufficiently exfoliated GO suspension. Then, 1 mmol of BiCl₃, 1.5 mmol of Na₂TeO₃ and 0.4 g of NaOH was added to the above suspension with sonication. After stirring for 12 h, the mixed solution was then transferred to a Teflon-lined stainless steel autoclave (100 mL) and heated in an electric oven at 180 °C for 24 h. The as-obtained product was collected by centrifugation, washed with deionized water and absolute ethanol for several times and dried at 40 °C under vacuum for 10 h. Bare Bi₂Te₃ was also synthesized using a similar method without adding GO.

3.2. Materials Characterization

The X-ray diffraction (XRD) patterns were obtained on a Rigaku D/Max-2550pc powder diffractometer using Cu K_α radiation (λ = 0.1541 nm). X-ray photoelectron spectra (XPS) were collected on a KRATOS AXIS ULTRA-DLD spectrometer with a monochromatic Al K_α radiation (hv = 1486.6 eV). The morphologies of the products were observed by field emission scanning electron microscopy (FE-SEM) on a FEI-sirion microscope and transmission electron microscopy (TEM) on a JEM 2100F microscope. The carbon content analysis was conducted on a Flash EA 1112 tester.

3.3. Electrochemical Measurements

The electrochemical measurements of Bi₂Te₃/G and Bi₂Te₃ were performed with a half-cell configuration using CR2025-type coin cells. The working electrodes were prepared by spreading a slurry composed of 75 wt% active material (Bi₂Te₃/G or Bi₂Te₃/G), 15 wt% poly(vinylidene fluoride) (PVDF) and 10 wt% acetylene black onto Ni foam followed by drying at 100 °C for 12 h under vacuum. The loading mass of the active material is around 2 mg. The cells were assembled in an Ar-filled glove box using Li foil as the counter electrode and Celgard 2300 membrane as the separator. The electrolyte used was 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (volume ratio 1:1). The cells were galvanostatically charged-discharged at 50 mA g⁻¹ between 0.005 and 2.5 V vs. Li/Li⁺ on a Neware battery tester (Shenzhen, China). The specific capacity of Bi₂Te₃/G was calculated based on the total weight of Bi₂Te₃ and graphene. Cyclic voltammetry (CV) tests were carried out on an Arbin BT2000 system at a scan rate of 0.1 mV s⁻¹ between 0.005 and 2.5 V vs. Li/Li⁺. All of the electrochemical measurements were carried out at room temperature.

4. Conclusions

The Bi₂Te₃/G sandwich has been prepared by an in situ route with alternatively arranged Bi₂Te₃ nanoplates and graphene nanosheets. The diffusion and re-crystallization of Bi₂Te₃ in Bi₂Te₃/G are inhibited due to presence of oxygen-containing groups and defects on graphene, leading to more irregular shape and smaller thickness of Bi₂Te₃ in Bi₂Te₃/G compared with those in bare Bi₂Te₃. Both Bi₂Te₃/G and Bi₂Te₃ exhibit a multi-step lithiation mechanism. The Bi₂Te₃/G hybrid can yield a higher-than-the-theoretical charge capacity of mAh h⁻¹ at 50 mA g⁻¹ due to the synergistic effect between Bi₂Te₃ and graphene. The Bi₂Te₃/G hybrid shows an obviously improved cycling stability compared with bare Bi₂Te₃. The improved cycling stability is attributed to the 3D sandwich structure, where graphene not only buffers the volume changes but also immobilizes Bi₂Te₃ nanoplates, in addition to offering effective conducting networks. The unique sandwich structure also facilitates Li-ion diffusion at the electrode/electrolyte interface and within the free space of the sandwich.