# One-Dimensional Sb2Se3 Nanorods Synthesized through a Simple Polyol Process for High-Performance Lithium-Ion Batteries

Authors: Yuan Tian; Zhenghao Sun; Yan Zhao; Taizhe Tan; Hui Liu; Zhihong Chen
Journal: Journal of Nanomaterials
 (2018)

---

## Abstract

The good crystalline Sb2Se3 nanorods have been successfully synthesized through a simple polyol process. The detailed morphological and structural characterizations reveal that nanorods are composed of Sb2Se3 single crystals oriented along the [120] orientation; the tiny Sb2Se3 nanorods are found to display a higher crystallinity with respect to thick Sb2Se3 nanorods. The nanorods have been applied as anode materials for lithium-ion batteries, with tiny Sb2Se3 nanorod anodes delivering the relatively high discharge capacity of 702 mAh g−1 at 0.1 C and could maintain the capacity of 230 mAh g−1 after 100 cycles. A more stable cycling performance is also demonstrated on tiny Sb2Se3 nanorods, which is ascribed to their more pronounced one-dimensional nanostructure.


---

## Body

## 1. Introduction

With the increase of the world energy demand and the decrease of resources, Li-ion batteries (LIBs) are one of the most important electrical energy storage devices and are widely used in various portable electronic devices due to its relatively high energy density [1–7]. Nowadays, the low-cost graphite material is the most popular anode material used in commercial LIBs [8, 9]. However, graphite anodes possess a low theoretical specific capacity (372 mAh g−1), which could hardly meet the requirements for high capacity and safety in practical energy storage application such as power stations. To improve the specific capacity of anode materials for LIBs, fabricating and testing new anode materials have become the focus of a multitude of scientific researches, such as metal oxides and metal sulfides. Among them, metal chalcogenides have emerged as promising anode materials for LIBs. In particular, researchers have fabricated antimony-based chalcogenide materials such as Sb2O3 nanomaterials and Sb2Se3 thin films, which could sensibly increase capacity and cycle performance with respect to graphite electrodes [10]. In the last few years, the Sb2Se3 nanostructured materials have been synthesized by other various techniques such as solvothermal process [11]. Recently, Kwon et al. prepared one-dimensional Sb2Se3 by evaporating a stacked elemental layer (Sb/Sb–Se/Se) deposition and then an additional annealing treatment process removed excess Se elements [12]. Ma et al. adopted a template-free solvothermal route to obtain the large-scale single-crystalline Sb2Se3 nanowires [13]. Xue and Fu have researched Sb2O3 nanomaterials, which have the high reversible capacity of about 794 mAh g−1 and a relatively large irreversible capacity loss (234 mAh g−1). Besides, Xue and Fu have also reported on a large reversible discharge capacity in Sb2Se3 thin films deposited by pulsed laser deposition for Li-ion batteries cycled between 0.3 and 2.5 V. The capacity was found in the range between 530.5 and 660.7 mAh g−1 during the first 100 cycles. This indicates that the Sb2Se3 anode material has high reversible capacity in Li-ion batteries [14]. However, the loading charge in Sb2Se3 thin films is very low and its alloying combination reaction is associated with a drastic volume expansion upon lithiation, thus usually resulting in the active material pulverization and falling out from the collector, finally leading to the marked decrease of the long-term cycle performance. In addition, the pulsed laser deposition method is not compatible with low-cost fabrication. It is commonly agreed now that by using nanostructured electrode materials having particular dimension-related effects and large specific surface area can greatly relieve the volume expansion following the Li-alloying reaction. Therefore, the Sb2Se3 nanostructured composite material has been investigated showing excellent electrochemical properties in this paper, such as long cycle stability, good initial coulombic efficiency, and high energy density. In order to investigate the improved electrochemical properties of Sb2Se3 nanostructured materials, we report on a new low-cost fabrication method to dissolve the Se powder based on the mixed solvents of monoethanolamine and thioglycolic acid at room temperature in air for the first time [15].


## 2. Experimental

As shown in Scheme1, 1 mmol antimony trichloride (SbCl3, 99.9%, Aladdin) was dissolved in 20 mL triethylene glycol (TEG, 99%, Aladdin) with magnetic stirring at room temperature for 2 h to get a clear cationic solution. 3 mmol Se powder was immediately dissolved in 2.7 mL monoethanolamine (MEA, 99%, Aladdin) and 0.3 mL thioglycolic acid (TGA, 80%, Aladdin) after stirring at room temperature to get a brownish red Se precursor solution. 80 mL TEG with 0.2 g PVP in a three-necked flask was installed in a heating mantle through magnetic stirring and attached to a Schlenk line. The three-necked flask was degassing and flowing nitrogen alternately for three times at room temperature. Then, the Se precursor solution was injected into the flask and stirred for 30 min under nitrogen flow. Then, the three-necked flask was heated to 210°C. Meanwhile, 10 mL cationic solution was swiftly injected into the flask and kept for 20 min at the same temperature. Afterward, the reaction solution was cooled to room temperature and precipitated by high-speed centrifugation. The precipitation was washed ultrasonically with excess amounts of absolute ethanol (CH3CH2OH, 99.7%, Aladdin) followed by centrifugation for 4 times in order to purify the thick Sb2Se3 nanorods. In addition, we found that it is possible to control the size of nanorods by adjusting the heating temperature of the three-necked flask. The three-necked flask with a mixture of Se precursor solution and cationic solution was heated at 180°C for 20 min and then proceed on to the next steps, such as nature cooling, centrifugation, and washing. We eventually obtained the tiny Sb2Se3 nanorods. The purified products were used for further characterization. The chemical equations associated with the preparation of Sb2Se3 are as follows:
(1)2SbCl3+3H2O→Sb2O3+6HCl2Sb2O3+6Se→2Sb2Se3+3O2Scheme 1
Sketch diagram of preparing Sb2Se3 nanorods.The crystal structural characterizations were carried out by X-ray diffraction (XRD, D8 Advance, Bruker) using Cu Kα radiation (λ=0.1542nm). The surface morphologies and microstructure were characterized by scanning electron microscopy (SEM, JSM-6700F, JOEL) and transmission electron microscopy (TEM, JEM-2100F, JOEL), respectively.The electrochemical performance was investigated using coin-type cells (CR2025), in which the Sb2Se3 nanorods were used as a working electrode and lithium foil was used as the counter and reference electrode. The electrolyte was 1 mol dm−3 LiPF6 dissolved in a mixture of ethylene carbonate/diethyl carbonate/dimethyl carbonate (EC/DEC/DMC) with a volume ratio of 1 : 1 : 1, and the separator was microporous polypropylene film. The Sb2Se3 electrode was prepared by mixing 80 wt% of as-prepared dispersed Sb2Se3 nanorods, 10 wt% polyvinylidene fluoride (PVDF, Kynar, HSV900) as a binder, and 10 wt% acetylene black (MTI, 99.5% purity) conducting agent in 1-methyl-2-pyrrolidinone (NMP, Sigma-Aldrich, ≥99.5% purity). The resulting slurry was uniformly spread on a Cu foil using a doctor blade to obtain electrode material thickness about 15 μm and dried at 60°C for 12 h. The resulting electrode film was cut into circular disks with 1.5 cm diameter. The active material loading amount of each electrode sheet is approximately 3 mg cm−2. The coin cells were assembled in high-purity argon- (99.9995%, purity) filled glove box (MBraun) and tested galvanostatically on a multichannel battery tester (BT-2000, Arbin Instruments) in a cut-off potential window from 0.01 to 3.0 V vs. Li+/Li at different current densities.


## 3. Results and Discussion

Figure1 shows the XRD patterns of the tiny Sb2Se3 and thick Sb2Se3 nanorods. We could clearly index all the diffraction peaks with the orthorhombic phase of Sb2Se3 with space group of Pbnm (62), in good agreement with the standard JCPDS card no. 15-0861. Interestingly, no impurity phases are found in the diffraction pattern. Compared with the thick Sb2Se3, the tiny Sb2Se3 nanorods exhibit the stronger diffractions (110) and (040), which indicates the enhanced crystallinity of the tiny Sb2Se3 nanorods.Figure 1
XRD patterns of the thick Sb2Se3 and the tiny Sb2Se3 nanorods.Raman spectroscopy is used to further analyze the structure of the crystalline thick Sb2Se3 and tiny Sb2Se3 nanorods. Both of the spectrum in Figure 2 are detected at 83, 118, 189, 252, 356, 373, and 450 cm−1, respectively, implying that the existence of the pure Sb2Se3 phase [16]. Compared with the thick Sb2Se3 nanorods, the greater peak intensity with the tiny Sb2Se3 nanorods indicates the enhancement of crystallinity.Figure 2
Raman spectrums of the thick Sb2Se3 and the tiny Sb2Se3 nanorods.XPS data are carried out to verify the chemical components and the valence states of elements in the Sb2Se3 nanorods. Figure 3(a) reveals two obvious high intensity Sb peaks at around 3d3/2 (537.87 eV) and 3d5/2 (528.47 eV), which are ascribed to the charge state of Sb3+ [17]. In addition, the other two relative weak Sb peak at around 3d3/2 (539.15 eV) and 3d5/2 (529.78 eV), implying the oxidation of the surfaces for Sb2Se3 nanorods although Sb2O3 is not detected by XRD and Raman results [18]. The XPS data for the valence state of Se2− is split into 3d3/2 (53.81 eV) and 3d5/2 (53.01 eV) in Figure 3(b) [17]. Both of the thick Sb2Se3 and tiny Sb2Se3 nanorods with Sb3+ and Se2− states prove the purity of Sb2Se3 phase.Figure 3
XPS data of (a) Sb 3d and (b) Se 3d for the thick Sb2Se3 and the tiny Sb2Se3 nanorods.
(a)
(b)High-resolution transmission and scanning electronic microscopy (HR-TEM and SEM) images from the thick Sb2Se3 and tiny Sb2Se3 nanorods are shown in Figures 4 and 5, respectively. The images show the diameter distribution histograms of the Sb2Se3 nanorods obtained from SEM data. As shown in Figures 4(a)–4(c), the arrangement of thick Sb2Se3 nanorods is relatively disordered and the average diameter of nanorods has already exceeded the value of 90 nm. However, the other Sb2Se3 nanorods are well-dispersed, tiny, and smooth with an average diameter ranging from 6 to 8 nm in Figures 5(a)–5(c). The analysis on HR-TEM images provides further insights on the Sb2Se3 nanostructure: both Figures 4(d) and 5(d) show the presence of lattice fringes with a plane spacing of 0.525 nm, corresponding to the (120) lattice plane of single-crystal Sb2Se3 nanorods. The result also indicates that the preferential growth direction of Sb2Se3 nanorods is along the [120] crystal orientation. Consistent with the XRD, Raman, XPS, SEM, and TEM results, the tiny Sb2Se3 nanorods deliver a well-crystallized form. The EDS mapping of the tiny Sb2Se3 nanorods shown in Figure 6 indicates a uniform distribution of Sb and Se throughout the tiny Sb2Se3 nanorods.Figure 4
SEM image (a), diameter size histogram (b), TEM image (c), and HR-TEM image (d) of the thick Sb2Se3 nanorods.
(a)
(b)
(c)
(d)Figure 5
SEM image (a), diameter size histogram (b), TEM image (c), and HR-TEM image (d) of the tiny Sb2Se3 nanorods.
(a)
(b)
(c)
(d)Figure 6
EDS mapping showing the distribution of Sb and Se of the tiny Sb2Se3 nanorods.The electrochemical performance of thick Sb2Se3 and tiny Sb2Se3 nanorod electrodes was investigated in a LIB cell assembly with lithium metal acting as the counter and reference electrode. Figures 7(a) and 7(b) showed the galvanostatic charge and discharge behavior obtained for the tiny Sb2Se3 nanorods, and the thick Sb2Se3 nanorods as working electrode were measured at a current density of 0.1 C between 0.01 and 3 V. As can be seen from Figure 7(a), all the discharge curves at the initial three cycles have two prolonged plateaus at about 1.5 V and 0.9 V, corresponding to the two-step reaction of reduction. The upper plateau at about 1.5 V is associated to the lithiation process of Sb2Se3 directly to Sb and Li2Se [19]. And the other plateau at about 0.9 V is attributed to the alloying reaction of metallic Sb to Li3Sb [20]. These results confirm the global reactions (2) and (3) based on the simple equations, which suggests that a lithium insertion could take place [21–23].
(2)Sb2Se3+6Li++6e−→2Sb+3Li2Se(3)6Li++2Sb+6e−→2Li3SbFigure 7
Galvanostatic charge and discharge profiles of (a) tiny Sb2Se3 nanorods and (b) thick Sb2Se3 nanorods as working electrodes in LIBs.
(a)
(b)Besides, the plateaus in the charge curves are ascribed to the multistep oxidative transformation of Li3Sb associated with the formation of Sb2Se3 [24]. The discharge capacities of thick Sb2Se3 nanorod electrode during the first three cycles are 634 mAh g−1, 318 mAh g−1, and 280 mAh g−1, shown in Figure 7(b). However, the initial discharge and charge capacities of tiny Sb2Se3 nanorod electrode are around 702 mAh g−1 and 475 mAh g−1, respectively, and the initial coulombic efficiency of the LIB is 67.7%. The irreversible capacity may be attributed to the irreversible electrolyte decomposition and the inevitable formation of the SEI film on the electrode surface [25]. What is more, the well-defined voltage plateaus and potential positions remained almost unchanged during the subsequent cycles. This phenomenon suggests a good cycling stability of the tiny Sb2Se3 nanorod electrode in LIBs.The cycling performance and coulombic efficiency of the tiny Sb2Se3 and the thick Sb2Se3 nanorod electrodes are compared in Figure 8. Indeed, the thick Sb2Se3 nanorod electrode only shows a discharge capacity of 48 mAh g−1 after 50 cycles at 0.1 C. However, a reversible capacity of the tiny Sb2Se3 nanorods is still 230 mAh g−1, indicating a much higher capacity retention after 100 cycles at 0.1 C. In addition, the coulombic efficiency increases significantly during the first five cycles and stabilizes around 99% in the subsequent cycles. Compared with the tiny Sb2Se3 nanorod electrodes, the thick Sb2Se3 nanorod electrodes exhibit faster capacity fading and minimum reversible capacity because of the presence of different Sb2Se3 nanorod diameters. We can attribute the improved cycle stability to the pure one-dimensional Sb2Se3 nanostructure, which cannot only increase the contact area between the active materials and the electrolyte improving the utilization of Sb2Se3 nanorods but also decrease diffusion length of lithium ions. In addition, the one-dimensional nanostructure could relieve the intrinsic stress in Sb-based electrode materials, then slow the electrode pulverization upon cycling and enhance the capacity and cycling stability [26].Figure 8
Cycling performance and corresponding coulombic efficiency of the thick Sb2Se3 nanorods (green) and tiny Sb2Se3 nanorods (blue) as working electrodes in LIBs.Figure9 exhibits the rate performances of tiny Sb2Se3 nanorods and thick Sb2Se3 nanorods. As shown in Figure 9(a), after the first cycle at 0.1 C, the tiny Sb2Se3 nanorods have an average discharge capacity of 438 mAh g−1 at 0.1 C. With the increasing current rate, the capacity drops to 120 mAh g−1 at 1 C. More importantly, the tiny Sb2Se3 nanorods can regain the reversible capacity of 304 mAh g−1, when the current rate returns back to 0.1 C. The thick Sb2Se3 nanorods present reversible capacities of 283, 140, 79, and 37 mAh g−1 at current rates of 0.1, 0.2, 0.5, and 1 C, respectively. Hence, the tiny Sb2Se3 nanorods deliver a relative excellent rate performance.Figure 9
Rate performances of (a) tiny Sb2Se3 nanorods and (b) thick Sb2Se3 nanorods.
(a)
(b)Figure10 shows the CV curves of (a) the tiny Sb2Se3 nanorods and (b) the thick Sb2Se3 nanorods between 0.01 and 3 V for the initial cycle at the scan rate of 0.1 mV s−1. Both of the CV curves have two redox pairs. Two cathodic peaks appears at around 1.42 V and 0.83 V, which can be ascribed to the lithiation formation of Sb embedded in Sb2Se3 and the alloying transformation between metallic Sb and Li3Sb, respectively [20]. In additional, the other obvious anodic peaks are located at about 1.01 V and 2.03 V corresponding to the dealloying transformation and delithiation transmission [24]. There is a tiny peak located at around 1.72 V, which can be attributed to the formation of the SEI layers.Figure 10
CV curves of (a) the tiny Sb2Se3 nanorods and (b) the thick Sb2Se3 nanorods.
(a)
(b)Electrochemical impedance spectroscopy (EIS) plots of the tiny Sb2Se3 and the thick Sb2Se3 nanorod electrodes are at different depths of discharge (DODs). In Figure 11, D1, D2, and D3 represent relevant curves obtained at DODs of 0 mAh g−1, 200 mAh g−1, and full discharge in this study. Both of the Nyquist plots deliver a semicircle corresponding to the charge transfer resistance (Rct) in the high-to-medium frequency and an inclined line of the Warburg impedance in the low frequency. The equivalent circuit of the discharge behavior for samples is shown in Figure 12. R1 represents the Ohmic resistance, and Rct and R2 represent the charge transfer resistance and the resistance of the surface layer of Sb2Se3 in contact with the electrolyte, respectively. CPEct and CPE2 are the double-layer capacitances, and ZW is the Warburg impedance. With the increase in DOD, the diameter of the semicircle becomes increasingly larger, indicating higher charge transfer resistance. Compared with the thick Sb2Se3 nanorods, the tiny Sb2Se3 nanorods all displayed lower charge transfer resistance throughout the whole discharge process, as indicated by their semicircles, which may contribute to the increase in the discharge capacity [27].Figure 11
EIS plots of (a) the tiny Sb2Se3 nanorods and (b) the thick Sb2Se3 nanorods at different DODs.
(a)
(b)Figure 12
Equivalent circuit diagram of EIS.To further understand the mechanism of Li cycling, the electrode after 50 cycles was investigated by SEM images. As shown in Figure13, both of the Sb2Se3 nanorod morphologies change after cycling, and a partial agglomeration of the active material was observed. Such change can negatively affect the battery performance due to separation of active substance from the conducting agent. The morphology and structure of the one-dimensional tiny Sb2Se3 nanorod structure were retained after cycling and indicate its structural stability upon electrochemical cycling in Figure 13(a). However, the thick Sb2Se3 nanorods are almost invisible in Figure 13(b). It is clear that the tiny Sb2Se3 nanorods deliver a favorable electrochemical performance with high reversible capacity and good cycling stability in LIBs.Figure 13
SEM images of (a) the tiny Sb2Se3 nanorods and (b) the thick Sb2Se3 nanorods after 50 charge/discharge cycles.
(a)
(b)


## 4. Conclusions

In this study, a new and facile polyol process is reported preparing Sb2Se3 nanorods, with controlled sizes, applied as working electrode material for LIBs. Compared with the thick Sb2Se3 nanorods, the tiny Sb2Se3 nanorods demonstrate a favorable electrochemical performance with high reversible capacity and good cycling stability in LIBs. The high reversible capability of the Sb2Se3 nanorods is 230 mAh g−1 after 100 cycles at 0.1 C. This study provides insights and indications on how to develop Sb2Se3-based electrode materials for conversion alloying reactions for high-performance batteries.



---

*Source: 4273945-2018-11-22.xml*