Enhancing Sodium Ion Battery Performance by Strongly Binding Nanostructured Sb2S3 on Sulfur-Doped Graphene Sheets Xunhui Xiong,† Guanhua Wang,† Yuwei Lin,‡ Ying Wang,† Xing Ou,† Fenghua Zheng,† Chenghao Yang,* Jeng-Han Wang,‡ and Meilin Liu*,§ †
New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, People’s Republic of China ‡ Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan 11677 § School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States S Supporting Information *
ABSTRACT: Sodium ion batteries (SIBs) have been considered a promising alternative to lithium ion batteries for large-scale energy storage. However, their inferior electrochemical performances, especially cyclability, become the major challenge for further development of SIBs. Large volume change and sluggish diffusion kinetics are generally considered to be responsible for the fast capacity degradation. Here we report the strong chemical bonding of nanostructured Sb2S3 on sulfur-doped graphene sheets (Sb2S3/SGS) that enables a stable capacity retention of 83% for 900 cycles with high capacities and excellent rate performances. To the best of our knowledge, the cycling performance of the Sb2S3/SGS composite is superior to those reported for any other Sb-based materials for SIBs. Computational calculations demonstrate that sulfur-doped graphene (SGS) has a stronger affinity for Sb2S3 and the discharge products than pure graphene, resulting in a robust composite architecture for outstanding cycling stability. Our study shows a feasible and effective way to solve the long-term cycling stability issue for SIBs. KEYWORDS: sodium ion battery, Anode, Sb2S3/graphene composite, electrochemical performance, DFT calculation
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Various carbon-based materials have been intensively studied as anode materials for SIBs.8,9,18−24 For example, Wang et al. reported that expanded graphite could deliver a capacity of 284 mAh g−1 at a current density of 20 mA g−1.9 Ji et al. studied the performance of 3D porous carbon frameworks in SIBs, and they found its capacity could reach 255.5 mA h g−1 at 0.5 A g−1.22 Sulfur-doped disordered carbon has been reported to exhibit a reversible capacity of 516 mAh g−1 at 20 mA g−1.23 Na alloybased anodes, which can provide much higher gravimetric and volumetric specific capacities, are also good candidate anode materials for SIBs. For example, antimony (Sb) and Sb-based compounds have also been considered promising anodes due to their large theoretical capacity, including Sb (660 mAh g−1),25−27 Sb2S3 (947 mAh g−1),28−31 Sb2O3 (1109 mAh g−1),32 Sb2O4 (1227 mAh g−1),33 and Sb-based alloys.34,35 Especially, Sb2S3 has
odium ion batteries (SIBs) have attracted significant attentions as a promising alternative to lithium ion batteries (LIBs) for large-scale energy storage due to their inherent safety and the low cost of sodium.1−6 As alkali metals, lithium and sodium have similar properties (e.g., similar intercalation behavior in many electrode materials); thus, many insights obtained from LIBs can be beneficial to SIBs. However, the radius of Na+ is much larger than that of Li+,7 which may alter the electrochemical reaction kinetics associated with the intercalation process.3,4,8−11 In some cases, larger sized Na+ leads to slower Na+ diffusion effciency, greater volumetric change, and more severe pulverization of the Na-host materials. It may also result in a less stabile solid−electrolyte interface layer, which further degrades the rate capacity and cycle stability.2,12,13 These features make graphite the state-of-the-art LIB anode material, unsuitable for SIBs.14 Thus, the main challenge for advancing SIBs lies in the development of an appropriate anode material with high specific capacity, high rate capability, and good reversibility.15−17 © 2016 American Chemical Society
Received: August 22, 2016 Accepted: December 8, 2016 Published: December 8, 2016 10953
DOI: 10.1021/acsnano.6b05653 ACS Nano 2016, 10, 10953−10959
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ACS Nano several advantages over pure Sb or other Sb-based composites, including greater gravimetric energy density, smaller volume change, and better cycling performance because the Na2S produced during the sodiation of Sb2S3 has a higher reversibility than its equivalent oxide.36 These advantages are promising, but the long-term cyclability and rate capability of Sb2S3 need to be further improved due to the poor electrical conductivity and unavoidable volume changes, which easily induce particle aggregation and pulverization. Embedding nanostructured SIB anode materials in graphene has been regarded as a major approach to enhance the conductivity and mitigate the volume change during the sodiation/ desodiation processes. For example, the SnS2-reduced graphene oxide (RGO) nanocomposite demonstrates a capacity retention of 84% at a current density of 1 A g−1 after 400 cycles. Sb2S3/ RGO shows a capacity of about 670 mAh g−1, with a capacity retention of ∼95% after 50 cycles at 50 mA g−1.37 The SnS@ graphene hybrid nanostructured composite delivers an excellent specific capacity of 940 mAh g−1 at 30 mA g−1 and impressive cycle stability at current densities of 7290 mA g−1.38 SnS2/RGO shows a capacity of 565 mAh g−1, with a capacity retention of ∼84% after 400 cycles at 1 A g−1.37 However, the detachment of metal sulfides from graphene induced by volume change often occurs during the short-term cycling tests, owing to the planar geometry of graphene39,40 and the weak interactions between nonpolar graphene and highly polar metal sulfides. Therefore, the longer-term cyclability remains a major challenge for this kind of composite. Herein, we report a facile approach toward high-performance Sb2S3/sulfur-doped graphene anodes (Sb2S3/SGS) for SIB by strong chemical binding of Sb2S3 on sulfur-doped graphene sheets (SGS). The stronger affinity between Sb2S3 and SGS is verified by density functional theory (DFT) calculations, as characterized by a much higher adsorption energy of −2.15 eV. The composite exhibits an exceptionally stable capacity retention of 83% for 900 cycles at 2 A g−1 with high capacities and excellent high-rate response up to 5 A g−1. To the best of our knowledge, the performance of Sb2S3/SGS is superior to those reported for any other Sb-based materials for SIBs. The simple preparation process and high performances of the as-prepared Sb2S3/SGS composite electrode make it highly promising for future SIBs.
Figure 1. (a) Schematic illustration of the fabrication process of the Sb2S3/SGS composite. (b) Crystal structure of orthorhombic Sb2S3 and SGS in a porous Sb2S3/SGS composite highlighting the fast transportation of Na+ and the electron.
RESULTS AND DISCUSSION Figure 1 schematically displays the steps involved in preparation of the Sb2S3/SGS composite via a simple and scalable wet chemical process. First, commercial Sb2S3 (C-Sb2S3) was dissolved in a Na2S solution, and then it was mixed with a graphene oxide (GO) suspension under ultrasonication. Upon pouring them into a mixed Na2SO3 and H2SO4 solution, Sb2S3 and sulfur were precipitated on a GO surface. Subsequently, the precipitate was annealed at 300 °C under N2 to facilitate the crystallization of Sb2S3, the reduction of GO, and doping of sulfur into the graphene sheets, thus producing a nanostructured Sb2S3/SGS composite. X-ray diffraction (XRD) analysis (Figure 2a) suggests that the diffraction patterns of Sb2S3/SGS composite are similar to that of the reagent Sb2S3, which has an orthorhombic structure (JCPDS Card No. 42-1393). There are no observable secondary phases. Raman spectroscopy was also used to investigate the composition of the Sb2S3/SGS sample (Figure 2b). The peaks located between 100 and 400 cm−1 are characteristic Raman shifts of Sb2S3,41,42 whereas the bands at ∼1350 and ∼1580 cm−1 are attributed to the D-band and G-band of graphene
Figure 2. (a) XRD patterns of commercial Sb2S3 (C-Sb2S3), pure Sb2S3, and Sb2S3/SGS composite. (b) Raman spectra of pure Sb2S3, SGS, and Sb2S3/SGS composite. (c) TG curves of the Sb2S3/SGS composite in air with a heating rate of 2 °C min−1 and air flow of 100 mL min−1. (d) Raman spectra of the GO, SGS, and Sb2S3/SGS composite. (e) High-resolution S 2p XPS spectra of the SGS, Sb2S3, and Sb2S3/SGS composite. (f) High-resolution C 1s XPS spectra of GO, SGS, and Sb2S3/SGS composite with C1 and C2. 10954
DOI: 10.1021/acsnano.6b05653 ACS Nano 2016, 10, 10953−10959
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ACS Nano sheets, respectively.43 The results indicate the existence of graphene sheets in the composite, and the content of SGS was determined to be 7.82 wt % by dissolving Sb2S3/SGS in concentrated HCl and weighing the remains. Thermogravimetric analysis (TGA) (Figure 2c) was also conducted to estimate the content of SGS. Considering that Sb2S3 is converted into Sb2O3 when heated to 800 °C in air, the SGS content in the composite is calculated to be about 7.6 wt %. Meanwhile, X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical structure of the resulting SGS and Sb2S3/SGS. Obviously, sulfur signals appeared in the XPS spectra (Figure 2d), suggesting that S species have been successfully incorporated into graphene with a content of 3.1%. To probe the chemical state of sulfur, we analyzed the highresolution S 2p peaks of Sb2S3, SGS, and Sb2S3/SGS (Figure 2e). First, the two strong peaks of SGS are located at 164.51 and 163.35 eV with a splitting energy of 1.16 eV, confirming the presence of sulfur covalently bonded to graphene in a heterocyclic configuration.44 Second, two peaks at ∼161.55 and 162.37 eV for pure Sb2S3 show the existence of S2−.45 It is worthwhile to note that the S 2p peaks and Sb 3d peaks (Supporting Information, Figure S1) for Sb2S3/SGS experience a slight shift to lower binding energy when compared with pure Sb2S3, indicating an increased density of electron clouds around Sb2S3. Considering that sulfur doping makes graphene more electron-rich46 and that Sb2S3 itself is a p-type semiconductor material,47 the electron clouds are biased to Sb2S3 from SGS, leading to a stronger electronic coupling between the Sb2S3/SGS composites. The stronger binding between Sb2S3 and SGS ensures a more stable structure for superior long cycling in SIB applications. Except for the sulfur doping, an obvious reduction of GO at 300 °C in N2 also occurred. As shown in Figure 2d, a significant decrease of the oxygen content from about 32.2% for GO to 6.9% for SGS is observed based on the elemental analysis. Furthermore, the conclusion can be verified by the change of high-resolution C 1s peaks. As displayed in Figure 2f, the peak at higher binding energy in the C 1s XPS spectrum of GO sheets has almost disappeared after thermal treatment, indicating that most of the oxygen-functional groups are largely removed.48 The morphology and detailed structure of the Sb2S3/SGS composite were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 3).
within SGS and are uniformly distributed on SGS. A higher annealing temperature resulted in severe aggregation (Figure S2b), leading to poor electrochemical performances. It is noted that the morphology and detailed structure of the Sb2S3−graphene sheets (Sb2S3/GS) composite (Figure S3) appear to be similar to those of the Sb2S3/SGS composite, but quite different from those of the Sb2S3 particles without graphene (Figure S4). It should be emphasized that, even after a long period of sonication during the sample preparation, Sb2S3 crystals are still spatially confined by the SGS, suggesting the strong affinity between the Sb2S3 nanoparticles and SGS. The tightly wrapped SGS on Sb2S3 will ensure an effective buffer layer to accommodate the volumetric change during the sodiation/desodiation process, thus leading to an extraordinary cycle stability. The unique structure of the Sb2S3/SGS composite was also investigated by TEM studies. As shown in Figure 3b, Sb2S3 nanoparticles with a size of 30−80 nm (black) are encapsulated by flexible SGS. The selected area electron diffraction (SAED) patterns (inset of Figure 3c) of Sb2S3/SGS composite reveal the diffraction pattern of SGS and Sb2S3, showing Sb2S3 nanoparticles and SGS are well-crystallized and mixed together. The elemental mapping images (Figure 3d) reveal the uniform distribution of Sb, S, and C all over the single Sb2S3/SGS particle. The porous structure was further confirmed by measuring the Brunauer−Emmett−Teller (BET) specific surface area of the Sb2S3/SGS (Figure S5), which greatly influences the electrochemical properties. The isotherms exhibit a typical type-IV behavior, with a distinct hysteresis loop at the relative pressure P/P0 ranging from 0.5 to 1, implying that both samples have no or a very small fraction of micropores (pores
