Graphene-Based Hierarchically Micro/Mesoporous Nanocomposites ...

Article pubs.acs.org/cm

Graphene-Based Hierarchically Micro/Mesoporous Nanocomposites as Sulfur Immobilizers for High-Performance Lithium−Sulfur Batteries Junxiang Jia,†,‡ Kai Wang,*,† Xiong Zhang,† Xianzhong Sun,† Hailei Zhao,*,‡ and Yanwei Ma*,† †

Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

‡

ABSTRACT: Herein, a hierarchically micro/mesoporous nanocomposite of graphene and carbon nanospheres (HGC) is used as an immobilizer for a lithium−sulfur (Li−S) battery with enhanced performance. HGC derived from graphene oxides and polyvinylidene fluoride polymers, combined with the advantages of graphene and porous carbon nanospheres, exhibits a hierarchically micro/ mesoporous structure with an ultralarge specific surface area of up to 3182 m2 g−1 and a large pore volume of 1.91 cm3 g−1. Graphene as a conducting network can enhance electronic conductivity, while porous nanospheres like a reservoir can effectively store and immobilize sulfur particles. HGC/sulfur electrode material obtained via a melting infusion process exhibits high reversible specific capacity of 1250 mA h g−1 with a high sulfur content of 74.5 wt %, and it still has a capacity of 916 mA h g−1 after 100 cycles, which is better than that of pristine porous graphene and carbon nanospheres. Furthermore, the relative capacity decay of the HGC/sulfur electrode is only 0.005 and 0.004% per cycle at 2 C and 4 C, respectively, after 450 charge/discharge cycles, exhibiting remarkable performance in terms of long-term electrochemical stability.

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INTRODUCTION

Because of its excellent electronic conductivity and large theoretical specific surface area (SSA) of 2600 m2 g−1, graphene has been widely used as a substrate to load sulfur.19,20 In addition, graphene can also form a three-dimensional conducting network to improve the electronic conductivity of the sulfur electrode.21 Porous graphene obtained via a postactivation process can effectively improve the SSA of graphene by producing large amounts of micro/mesopores,22−24 which improves the immobilization of the active sulfur and polysulfides to depress their rate of dissolution in the liquid electrolyte. However, the low yields of porous graphene that result from the postactivation process limit its scalable application as a sulfur host material.25 On the other hand, a nanosphere with micro/mesoporous structure can effectively immobilize sulfur and be used to prepare a high-performance sulfur−carbon cathode.26−28 In addition, the abundant pores are beneficial for achieving high sulfur loading in a sulfur cathode. However, the preparation of a micro/mesoporous nanosphere usually requires complex procedures, normally involving the synthesis of a spherical template, coating and carbonization, and removal of a template, which restrict its practical utilization as a carbon host for Li−S batteries.29,30 Herein, a hierarchically micro/mesoporous nanocomposite of graphene and carbon nanospheres (HGC) is used as an

With the growing need for electric vehicles (EVs) and advanced energy storage devices, the lithium−sulfur (Li−S) batteries have attracted a great deal of attention because of their high theoretical specific capacity (1675 mA h g−1) and energy density (2600 W h kg−1).1,2 Moreover, the low cost, natural abundance, and environmental compatibility of sulfur make it very promising as a cathode in lithium ion batteries (LIBs).3 The practical application of Li−S batteries is still hampered by several issues, which mainly originates from the sulfur cathode. First, the insulating nature of sulfur (S8) and its discharge products (Li2S) limits the utilization of sulfur. Second, the increase in the volume of sulfur upon lithiation (80% volume change) and the dissolution of polysulfides (Li2Sx, where 4 ≤ x ≤ 8) intermediates in the electrolyte cause an irreversible decrease in the capacity of the sulfur cathode.4 Substantial progress has been made in past few years to address these problems.5−8 One of the most effective methods is injecting element sulfur into a conductive host, such as various carbon nanomaterials,9−13 conductive polymers,14−16 and metal oxides (nitrides/sulfides, etc.).17,18 This method not only can improve the electrical conductivity but also can capture the soluble polysulfides. Nevertheless, obtaining a high sulfur loading (>70 wt %) remains a great challenge with this strategy. Generally, increasing the sulfur content would decrease the conductivity of the sulfur composite cathode and also cause severe polysulfide intermediate dissolution and a shuttle effect. © 2016 American Chemical Society

Received: August 13, 2016 Revised: October 12, 2016 Published: October 14, 2016 7864

DOI: 10.1021/acs.chemmater.6b03365 Chem. Mater. 2016, 28, 7864−7871

Article

Chemistry of Materials

the ultralarge SSA and pore volume, HGC can achieve a high sulfur loading amount of ≤74.5 wt % in the HGC/S composite. In HGC, two-dimensional (2D) graphene sheets as a conductive network can improve electronic conductivity. Zero-dimensional (0D) porous carbon nanospheres distributed on the surface of graphene serving as a polysulfide reservior can effectively confine sulfur and polysulfide during the charge/ discharge process.32,33 HGC is a 2D and 0D hierarchical nanocomposite, possessing hierarchically micro/mesoporous structure with 0.6−3 nm pores, which is quite helpful for trapping polysufides and improving the electrochemical performance of the HGC/S cathode.4 As a result, the HGC/ S cathode shows a high initial specific capacity of 1250 mA h g−1 with a high sulfur content of 74.5 wt % at 0.2 C, which is superior to that of pristine graphene/S and porous carbon/ sulfur substrates. Furthermore, the HGC/S electrode also demonstrates excellent rate performance and superior cycling stability in long-term discharge/charge cycles.

immobilizer for lithium−sulfur (Li−S) batteries with enhanced performance. HGC is elaborately synthesized via a one-step thermolysis process, and then the HGC/S cathode is prepared by impregnating HGC with sulfur. The overall process is depicted in Figure 1. HGC combined with the advantages of

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Figure 1. Schematic illustration of the synthesis of the HGC/S composite.

RESULTS AND DISCUSSION A hierarchically micro/mesoporous nanocomposite of graphene and carbon nanospheres (HGC) was synthesized by a thermolysis activation process with blend precursors of graphene oxide and polyvinylidene fluoride (PVDF) polymer. KOH activation has been widely used on carbon materials to

graphene and porous carbon nanospheres shows a hierarchically micro/mesoporous structure with an ultralarge SSA of ≤3182 m2 g−1, which is much higher than the theoretical value of graphene,31 and a large pore volume of 1.91 cm3 g−1. With

Figure 2. (a) Nitrogen adsorption/desorption isotherms (the inset is an enlarged isotherm plot for the AGO sample). (b) Pore size distribution plots of HGC, APG, and AGO. The inset shows the cumulative pore volume plots. (c) XRD patterns of HGC, APG, and AGO. (d) XRD patterns and (e) Raman patterns of HGC+S and HGC/S composite samples. (f) TGA curve of the HGC/S composite. 7865


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phenomenon does not occur in AGO because of the relatively small SSA and pore volume of AGO. The HGC/S composite was synthesized by using a melting infusion process.29 The physical mixture of sulfur and HGC (so-called HGC+S) still shows characteristic peaks of crystalline sulfur according to the XRD patterns (Figure 2d) and Raman spectroscopy results (Figure 2e). However, the sharp XRD diffraction peaks of crystalline sulfur (JCPDF Card No. 080247) and Raman vibration peaks of S8 disappear completely for the HGC/S composite, indicating that sulfur exists as an amorphous state inside the micropores of HGC.37 The large SSA and large pore volume of HGC help to increase the sulfur loading in the sulfur cathode without a decline in electrochemcial performance. On the basis of the pore volume of HGC (1.91 cm3 g−1) and the density of elemental sulfur (1.96 g cm−3), the theoretical sulfur content in HGC is up to 78.9 wt %. Figure 2f shows the thermogravimetric analysis (TGA) of HGC, elemental sulfur, and HGC/S samples. Via calculation, the S content of the HGC/S composite is around 74.5 wt %, which is higher than many values reported previously. This high sulfur loading makes Li−S batteries much closer to being used for practical applications. Figure 3 shows the scanning electron microscopy (SEM) images of HGC, HGC/S, APG, and AGO samples. On the

improve porosity. According to the literature, KOH can effectively etch graphene and produce hierarchically mesoand micropores.31 Therefore, the KOH activation process was employed in this work to prepare a hierarchically micro/ mesoporous nanocomposite of graphene and carbon nanospheres. The mixture of GO, PVDF, and KOH powders was first heated to 380 °C for 2 h. During this process, the oxygencontaining groups of GO were reduced. At 800 °C, the thermolysis of PVDF and the reaction of KOH with carbon materials were conducted. The melted KOH was reacted with carbon material as 6KOH + C ↔ 2K + 3H2 + 2K2CO3, followed by decomposition of K2CO3 and/or reaction of K/ K2CO3/CO2 with carbon. The KOH activation process generated nanoscale pores in the product carbon. The unique hierarchically micro/mesoporous morphologies of HGC were formed by the thermolysis of PVDF and the reaction of KOH with carbon material. For comparison, activated graphene (AGO) and activated PVDF (APG) were also prepared via a similar process. AGO was obtained via a direct thermolysis activation process of the pristine GO precursor, while APG was synthesized with a similar process based on the PVDF precursor. On the basis of nitrogen sorption measurements, HGC possesses a large Brunauer−Emmett−Teller (BET) SSA of 3182 m2 g−1 and a single-point adsorption total pore volume of 1.91 cm3 g−1 at P/P0 = 0.99, while APG and AGO show much smaller SSAs (1674 and 340 m2 g−1, respectively) and pore volumes (0.88 and 0.27 cm3 g−1, respectively). As shown in Figure 2a, the nitrogen adsorption/desorption isotherms of HGC and AGO samples are typical type IV isotherms with a H4 hysteresis loop and capillary condensation at a relative pressure (P/P0 = 0.40−0.90), which demonstrates the mesoporous structures in samples. APG exhibits a typical type I isotherm, indicating that APG has a higher ratio of micropores. The sharp increase at a low relative pressure of HGC also indicates a number of micropores in the sample.34 From Figure 2b, all three samples show similar pore size distributions. The HGC sample has two micropore peaks at 0.6 and 1.2 nm and a mesopore peak at 3.0 nm, indicating that HGC has an excellent hierarchically micro/mesoporous structure. The inset of Figure 2b shows the cumulative pore volume plots, showing that HGC has a very large pore volume compared with those of other samples. Considering that the S− S bond length in sulfur’s allotropes is between 0.189 and 2.066 nm,6 the micropores in carbon matrices are ideal for immobilizing sulfur particles and limiting polysulfide dissolution.35 Furthermore, mesopores can provide inside tunnels to transport Li ions and improve ionic conductivity.36 Therefore, a hierarchically micro/mesoporous material is very useful for showing improved performance in Li−S batteries. X-ray powder diffraction (XRD) is used to check the structure of HGC, AGO, and APG, as shown as in Figure 2c. All three samples show a diffraction peak at 2θ ≈ 44°, indicating the (101) diffraction peak of hexagonal graphite (JCPDF Card No. 65-6212). Meanwhile, there is no apparent difference in this diffraction peak among these samples. Another broad hexagonal graphite layer (002) diffraction peak also appears in all samples, which reveals that the carbon materials show an amorphous characteristic and no stacking. However, the (002) diffraction peak intensity of HGC is much lower than others, which is likely caused by the large SSA of HGC. It is noted that the diffraction intensities of the low-angle scatter of HGC and APG are rapidly increased, which may be due to the high-density pores of the samples. However, this

Figure 3. Scanning electron microscopy images of (a) HGC, (b) the HGC/S composite, (c) AGO, and (d) APG.

basis of Figure 3a, the carbon nanospheres are dispersed on graphene a surface to form a 0D and 2D nanocomposite in HGC. Because of the excellent electronic conductivity and flexibility of graphene, the high conductivity and antideformation ability of HGC are expected. According to Figure 3b, the HGC/S sample shows a structure similar to that of HGC. No observed sulfur particles on the surface of HGC confirm the results of XRD and Raman patterns described above. AGO exhibits a typical reduced graphene oxide (rGO) morphology from Figure 3c,38 indicating that GO is exfoliated to graphene sheets in the thermal expansion process. Figure 3d shows the SEM images of APG, for which the spherical structures of PVDF disappeared after the activation process, which is also observed in another different activation method report.39 The collapse of the PVDF sphere is due to the continuous annealing and activation with KOH at high temperatures, but in the case of HGC, the collapse of spherical structures is avoided because of the restraint of the reduced GO. 7866


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Figure 4. (a−c) HRTEM images of the HGC sample. Panels b and c show the enlarged morphology of the porous carbon nanosphere and porous graphene, respectively. (d−f) HRTEM images of HGC/S. (g) Dark field STEM image and the corresponding (h) C and (i) S EDX elemental mappings of the HGC/S composite.

Furthermore, high-resolution transmission electron microscopy (HRTEM) is also used to check the micro/mesoporous morphology and structure of the samples. As shown as in Figure 4a−c, it can be clearly observed that HGC are composed of porous carbon nanospheres and porous graphene. The sizes of pores on both carbon nanospheres and graphene are