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Vertically Aligned Sulfur-Graphene Nanowalls on Substrates for Ultrafast Lithium-Sulfur Batteries Bin Li, Songmei Li, Jianhua Liu, Bo Wang, and Shubin Yang Nano Lett., Just Accepted Manuscript • Publication Date (Web): 06 Apr 2015 Downloaded from http://pubs.acs.org on April 6, 2015
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Vertically Aligned Sulfur-Graphene Nanowalls on Substrates for Ultrafast Lithium-Sulfur Batteries Bin Li, Songmei Li*, Jianhua Liu, Wang Bo, Shubin Yang* Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science & Engineering, Beihang University, 100191, Beijing, China.
ABSTRACT: Although lithium-sulfur batteries have gained great interest owing to their high energy density, they lack suitable electrodes capable of rapid charging and discharging to enable a high power density critical for wide applications. Here, we demonstrate a simply electrochemical assembly strategy to achieve vertically aligned sulfur-graphene (S-G) nanowalls onto electrically conductive substrates. Remarkably, in each individual S-G nanowalls, sulfur nanoparticles are homogeneously anchored in between of graphene layers and ordered graphene arrays arrange perpendicularly to the substrates, which are favorable for the fast diffusions of both lithium and electron. Moreover, the hierarchical and porous structures facilate the effective accommodation of the volume change of sulfur. As a consequence, a high reversible capacity of 1261 mAh g-1 in the first cycle, and over 1210 mAh g-1 after 120 cycles with excellent cycleability and high-rate performance (over 400 mAh g-1 at 8C, 13.36 A g-1) are achieved with these S-G nanowalls as cathodes for lithium-sulfur batteries, providing the best reported rate performance for sulfur-graphene cathodes to date.
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KEYWORDS: Graphene, nanosheets, assembly, cathode materials, lithium-sulfur batteries
Rechargeable lithium-sulfur (Li–S) batteries have recently attracted great interest as potential energy storage devices for wide applications such as in electrical vehicles owing to their high theoretical energy density of 2567 Wh kg-1, which is more than 5 times that of commercial lithium ion batteries.1-9 Moreover, their cathode material, elemental sulfur is abundant, non-toxic, and inexpensive.9,10 Despite of these promises, several challenges exist in cathode of Li−S batteries that hampered their practical applications.10 Such challenges include: (1) the inherent low electrical conductivities of sulfur (5 × 10-30 S cm−1), intermediate polysulfide products, and final Li2S, which result in low active material utilization efficiency and poor rate capability; (2) high solubility of polysulfide intermediates, which can shuttle between the anode and cathode, forming deposits of solid Li2S2 and Li2S on the cathode, causing large loss of the active material and severe degradation of cycle life.10-13 In order to overcome above obstacles of Li-S batteries, one effective strategy is to confine sulfur and polysulfides into carbonaceous matrix and meanwhile improve the electrical conductivity of overall electrodes.9,14 In this respect, various carbons including mesoporous carbons1, microporous carbons11,15, hollow carbon spheres16-18, carbon nanotubes19,20 and graphene5,21,22 have been investigated to produce sulfur-carbon hybrids and demonstrated the improved electrochemical performances. In particular, graphene, atomic layer of carbon, exhibits ultrahigh electrical conductivity, good flexibility, large surface area, high chemical and thermal stabilities.23-25 This provides new opportunities to design and fabricate sulfur-graphene
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composites with unique structures. For instance, sulfur nanocrystals anchored onto graphene6,26, core-shell sulfur@graphene27,28 and sandwich-like sulfur-graphene-sulfur composites29 have been achieved for high-capacity Li-S batteries. Whereas graphene layers are prone to aggregate or restack in disorder during the hybrid fabrication processes due to their high surface area, strong π-π and/or hydrophobic interactions, commonly resulting in the loss of many unique properties of graphene and obvious capacity decay especially at high current rates.27,28 Very recently, controllably self-assembly of graphene layers into three-dimensional architectures with macroporous or mesoporous structures is becoming an effective approach to harness the intriguing properties of graphene to practical applications.28,29 However, until now, how to facilely control the assembly of graphene layers or graphene-guest hybrids remains a big challenge. Herein, we demonstrate a simply electrochemical assembly strategy to achieve ordered sulfurgraphene (S-G) nanowalls, which are controllably and vertically aligned onto the surface of electrically conductive substrates during the tunable cyclic voltammetry (CV) processes. Interestingly, in each individual S-G nanowalls, sulfur nanoparticles are homogeneously anchored in between of graphene layers and graphene layers arrange in order and perpendicularly to the surface of substrates, which are favorable for the fast diffusions of both lithium and electron. Moreover, the overall S-G nanowalls possess hierarchical and macroporous structures, which facilite the easy access of electrolyte and effective accommodation of the volume change of sulfur during the cycling processes. As a consequence, the unique S-G nanowalls exhibit a high reversible capacity of 1261 mAh g-1 in the first cycle, and over 1210 mAh g-1 after 120 cycles, with excellent cycleability and high-rate performance (over 400 mAh g-1 at 8C, 13.36 A g-1).
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The overall synthetic procedure of vertically aligned S-G nanowalls involves two steps as shown in Figure 1a. Sulfur nanoparticles were firstly anchored onto the surface of graphene (or reduced graphene oxide) via the reaction of Na2S with graphene oxide (GO) for overnight (Figure S1). It has been known that GO consists of a basal plane decorated mostly with epoxide and hydroxyl groups, in addition to carbonyl and carboxyl groups, which are located at the edges.29 These oxygen functional groups are able to immobilize sulfur and subsequently free lithium polysulfides during charge-discharge processes.32 In order to explore the aging process, the Zeta potentials of fresh mixture and aging Na2S/graphene dispersions for different times (1h, 5h, 10h, 25h) were tested and showed in Table S1. It is clear that the Zeta potentials increase negatively with increasing the aging times, and stable at -33 mV, which enables the successful operation of electrochemical assembly in principle. Hence, a typical electrochemical assembly of S-G layers in a two-electrode cell was carried out via cyclic voltammetry at a scanning rate of 50 mV s-1, as shown in Figure 1b and Figure S2b (for detail, see Supporting Information). To make clear the assembly mechanism of the S-G layers (Figure S3), we carefully studied the assembly processes at different fabrication conditions. Firstly, we changed the CV scanning method from bidrectional scanning (-0.5-2.0V, scanning forth and back) to unidirectional scanning (from -0.5 to 2.0V) during the fabricaton processes. As shown in Figure S4a, all the sulfur-graphene nanowalls are parallel to the substrate, which is in sharp contrast with the vertically aligned sulfur-graphene nanowalls fabricated by employing the bidrectional scanning mehtod, indicating that the back scanning from 2.0 to -0.5V plays an important role in the formation of vertically aligned sulfur-graphene nanowalls. To further study which part during the back scanning process is the key, we conducted an assembly experiment during the CV voltage range of 0.5-2.0V. As shown in Figure S4b, all the sulfur-graphene nanowalls are also parallel to
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the substrate, suggesting that the back scanning process at the voltage range from 0.5 to -0.5V is the key. To further investigate the assembly process of vertically aligned sulfur-graphene nanowalls, we prepared several samples after CV scanning at the voltage range of -0.5-2.0V for 2, 5, 10 and 20 cycles and tested their SEM images. As shown in Figure S5-S7, in the case of the sample fabricated after CV scanning for 2 cycles, there are rare nanosheets vertically aligned onto the substrate. With increasing the CV number to 5, more vertically aligned nanosheets are observed in one layer. With furher increasing the CV number to 20, the vertically aligned nanosheets are compact in two layers. Thus, it can be deduced that the assembly processes of vertically aligned sulfur-graphene nanowalls contain two steps: firstly, negatively charged sulfurgraphene nanosheets were migrated to the electrode and partially achored onto the substrate under the strong electric field during the fabrication processes in the CV voltage range of -0.52.0V, which is similar to the traditionally electrochemical deposition; subsequently, the partially achored and negatively charged nanosheets were pulled to perpendicular under the weak electric field during the back scanning from 0.5 to -0.5V, as illustrated in Figure S3. This explaination for the formation mechanism of vertically aligned S-G nanowalls is in good aggreement with the experimental results and analysis. Thus, it is clear that the vertical assembly of S-G nanosheets is benefited from our especially electrochemical assembly process and the appropriate Zeta potentials of S-G nanosheets. Notably, the mass areal density of S-G nanowalls could be facilely controlled by simply adjusting CV scanning numbers, as shown in Figure S5 and Figure S6. For comparison, the disordered sulfur-graphene film was also prepared by using the same approach except for substituting the aged S-G suspension by the fresh Na2S/GO dispersion (the typical cyclic voltammetry is shown in Figure S2a).
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The morphology and structure of vertically aligned S-G nanowalls were first investigated by field-emission scanning electron microscopy (FE-SEM). As shown in Figure 2a and S7a, the asprepared film is constructed by numerous thin nanowalls with several micrometers in length. Interestingly, almost all the nanosheets at the bottom are perpendicular to the surface of the substrate and subsequently vertically aligned onto the initially existed nanowalls, showing hierarchical and macroporous structures. These unique structures should attribute to the layer-bylayer electrochemical assembly processes with increasing CV times during our fabrication process. Moreover, we find that the Na2S concentration has a large influence on the assembly of S-G nanowalls. With increasing the Na2S concentration in electrolyte from 0.1 M to 0.2 M, the thickness of S-G nanowalls increases significantly, and even form big sulfur particles (Figure S8). In contrast, if the fresh mixture of Na2S and GO was used as electrolyte, vertically aligned S-G nanowalls are invisible as shown in Figure S9, where the sulfur nanoparticles and graphene sheets are dispersed inhomogeneously and aggregated together (this sample is denoted as S-GFresh). To identify the structure of an individual S-G nanowalls, transmission electron microscope (TEM) and high-resolution TEM (HRTEM) measurements were carried out. As illustrated in Figure S7b, the S-G nanowalls has a large aspect ratio, similar to that of multi-layers of graphene. To further make clear the internal microstructure of a nanowalls, bio-slicing technology was employed and the corresponding TEM and HRTEM measurements of the slices were conducted. Obviously, the S-G nanowalls is constructed by numerous sulfur nanocrystals with the diameters of 5~10 nm (Figure 2b and 2c). The crystalline structure of S can be clearly identified from the typical HRTEM image insetted into Figure 2d, where the lattice spacing of 0.22 nm is assigned to {048} facet of sulfur. More interestingly, these sulfur nanocrystals seem
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to be aligned orderly along the vertical axis of the nanowalls (Figure 2b and 2c). Associated with the building blocks of S-G layers used in our fabrication process, it is reasonable to believe that these sulfur nanocrystals are isolated by graphene sheets, and thus S-G nanowalls arrange perpendicularly to the substrate or existed nanowalls as we expected. Thus, the aggregation of pure graphene and sulfur nanoparticles are invisible during our TEM measurement process. The invisible graphene in HRTEM images should ascribe to the single-layer feature. This is in contrast different from the most previously reported sulfur-graphene composites in which sulfur nanoparticles are intrinsically incompatible with graphene, except for those reported in the literatures [32,33]. X-ray diffraction (XRD) was employed to confirm the successful reduction of sulphide during the aging process. As shown in Figure 3a, the XRD patterns of S-G nanowalls are almost the same to those of elemental sulfur with preponderant peaks at 23.28, 26.06, 27.88 and 28.86o, demonstrating that elemental sulfur was successfully formed during our fabrication processes. The XRD diffraction indicated the original state of sulfur in the composites is S8, which is as the same as the element sulfur in most porous carbon/sulfur composites15-18, commonly synthesied in the liquid-phase6,33. This can be further confirmed by thermogravimetric analysis (TGA) of S-G nanowalls as presented in Figure 3b, in which the sulfur volatilizes when temperature arrives to 250℃.23 At the same time, the TGA reveals that the sulfur contents of S-G and S-G-Fresh are 66.0%, and 91.5%, respectively. To confirm the interactions between sulfur and graphene, X-ray photoelectron spectroscopy (XPS) measurements was conducted (Survey scanning spectrum is shown in Figure S10). In the high-resolution S 2p spectrum (Figure 3c), there are two peaks at around 163.65eV and 164.85eV, which should be assigned to S 2p3/2 and S 2p1/2, and other two peaks at around 164.1eV and 165.3 can be assigned to -S-O- bonding.6,27 The existence of -S-O-
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bonding is further confirmed by the high-resolution O 1S spectrum with a peak of 530.53 eV (Figure 3d). To further demonstrate the presence of -S-O- bonding, both FTIR and Raman of vertically aligned sulfur-graphene nanosheets were carried out and their spectra were shown in Figure 3e and 3f. In the FTIR spectra (Figure 3e), the peaks at 1030 and 1090 cm-1 can be assigned to S-O and C-O vibration, respectively, demonstrating that both S-O and C-O exist in the S-G nanowalls. In the Raman spectrum for S-G nanosheets (Figure 3f), it is observed that there is a peak at 630 cm-1, which is absence in that of S-G-fresh, demonstrated clearly the presence of S-O bond in our S-G nanosheets. Notably, the -S-O- bonding is rare in the reported sulfur/graphene composites except for Guo’s work6. The S-O should play an important role to anchor sulfur and intermediate polysulfide products during the cycle processes as they are used as cathode materials for Li-S batteries. The electrochemical performance of S-G nanowalls for Li-S batteries was firstly studied by CV. As shown in Figure 4a, there is only one peak at ~1.5 V during the first cathodic scanning process, which shifts to ~1.56 V during the subsequent scanning processes, indicating the elemental sulfur is one-step reduced to lithium sulphide according to literatures23. During the first anodic scanning process, there are two peaks at 2.15V and 2.4V, which can be assigned to the formation of low chain sulphide (Li2Sx, 2
