Letter pubs.acs.org/journal/ascecg
Natural Integrated Carbon Architecture for Rechargeable Lithium− Sulfur Batteries Jiaqi Xu,† Kuan Zhou,† Fang Chen,† Wei Chen,† Xiangfeng Wei,†,‡ Xue-Wei Liu,*,§ and Jiehua Liu*,† †
Future Energy Laboratory, School of Materials Science and Engineering, Hefei University of Technology, Tunxi Road No.193, Hefei, Anhui 230009, China ‡ School of Chemistry and Chemical Engineering, Hefei University of Technology, Tunxi Road No.193, Hefei, Anhui 230009, China § School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore S Supporting Information *
ABSTRACT: Natural integrated carbon architecture cathodes without any additives, which were derived from the bark of plane trees, have highly loaded sulfur of 3.2−4.2 mg cm−2. The as-obtained carbon slice has an integrated architecture with micropore-to-macrospore distribution and a large surface area of 528 m2 g−1. As a result, the integrated carbon−sulfur cathode exhibited an initial discharge capacity of 1159 mA h g−1 at 0.2 A g−1 for a lithium−sulfur battery. Even after 60 cycles, a high specific capacity of 608 mA h g−1 with a high Coulombic efficiency (>98%) was retained, much better than MWCNTsbased electrodes and macropore-destroyed carbon slices.
KEYWORDS: Natural nanoarchitecture, Biomass, Integrated electrode, Hierarchical pores, Lithium−sulfur batteries
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INTRODUCTION Li−S batteries are regarded as a preferred device in nextgeneration batteries due to some unique properties. The cells have a theoretical specific capacity of 1675 mA h g−1 and specific energy of 2600 Wh kg−1, up to five times higher than that of commercial batteries (such as LiFePO4/graphite, LiMn2O4/graphite, LiCoO2/graphite system). Another important feature of Li−S batteries is low cost due to abundant sulfur.1−4 Nevertheless, Li−S batteries have not been commercialized on a large scale because of their main disadvantages in nature, including electrical and ionic insulation, volume swelling, and shuttling effect. The inherent poor electrical conductivity of sulfur (5 × 10−30 S cm−1 at 25 °C) is one of the biggest barriers, which leads to a low utilization of sulfur in the electrode. Another key problem is the volume change of about 80% between sulfur and lithium sulfide during cycling, which may degrade the cathode structure, leading to fast capacity fading.5−7 Confining sulfur with conducting materials or nanoporous materials are the main approaches to improve the performance of cathode hybrids.8−13 With the rapid development of electronic devices and electric vehicles, high-performance Li− S batteries are welcomed due to the increasing demands in a huge market.14−16 In particular, carbon-based materials (e.g., carbon nanotubes, GO, and graphene) have been widely studied based on their superior conductivity and novel structures, which not only improve the conductivity of the cathode but also confine the shuttle of polysulfides.17−19 © XXXX American Chemical Society
Fabrication of low-cost and stable carbon electrodes obtained from biomass may provide a possible route to develop highperformance Li−S batteries. Biomass from plants is a kind of natural biomacromolecule that is composed of hydrocarbon compounds. The natural organizational structures may be welcomed for synthesizing novel energy materials as an attractive carbon source due to its characteristics of renewability, abundance, and low cost. In an earlier example for biomass, bamboo is used to prepare active carbon for excellent green decontamination. Recently, olive stones and pomelo peels have been chosen to obtain activated carbon with microporous or macroporous textures for lithium− sulfur batteries.20 The most natural structures, such as leaves, barks, and shells, are very complicated organizations, which provide one or more specific integrated functions from the view of bionics. However, these reasonable organizations, which are hard and artificially synthesized by the mimic method, are often discarded in garbage cans without further usage. Therefore, the natural integrated architectures inspired us to develop a high-performance electrode combining the synergetic effects from their all components. As the electrodes for Li−S cells, leaves and coconut shells are so thin or thick that they are only used to produce carbon powders and then are applied as conducting additives rather than used directly as a whole. As Received: October 8, 2015 Revised: January 8, 2016
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DOI: 10.1021/acssuschemeng.5b01258 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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to remove the trace inorganic mineral. Disc-like slices were obtained by punching with a diameter of 12 mm and drying for 6 h at 60 °C in an oven (Figure 2b). The dried slices were then annealed for carbonization at 750 °C for 5 h under N2 flow. The obtained black slices have a diameter of about 8 mm and a thickness of less than 0.6 mm (Figure 2c). More information is provided in the experiment section in the Supporting Information. FESEM is employed to analyze the structure of carbon slices. After annealing at 750 °C, as shown in Figure S1, the sandwichlike structure of carbonized bark was confirmed including two cell layers (outer layer and inner layer) and inter tube-array layer. Figure 3a and b clearly shows the outer cell layer and
known, the bark of plane trees often has complex natural structures, including a cell outer layer, vascular cambium interlayer, and cell inner layer shown in Figure 1, which provide an
Figure 1. Schematic microstructures of plane tree bark from (a) front and (b) cross section, respectively.
integrated 2D architecture by their synergetic effect. Moreover, we discovered that the bark of plane tree has a suitable thickness of few millimeters and an integrated structure where its derivative (carbonized bark) is expected to be used directly as a cathode for Li−S batteries. Importantly, the 2D bark of plane trees, which peels off regularly each year, is a sustainable material for energy storage. In this article, we reported an integrated architecture carbon slices with interconnected porous structures including micropores, mesopores, and macropores. The carbon slices were successfully synthesized by a facile carbonization using the bark of a plane tree as the renewable carbon source. The as-prepared carbon−sulfur slice cathodes can highly load sulfur of 3.2−4.2 mg cm−2. In addition, the integrated carbon−sulfur cathodes exhibit good electrochemical performance for lithium sulfur batteries, although there is neither binder nor conducting additives.
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RESULTS AND DISCUSSION The integrated carbon slice electrodes were prepared via a simple carbonization process as displayed in Figure 2. The bark of a plane tree was first immersed into 1 M HCl solution for 6 h
Figure 3. (a) FESEM image of outer layer and inter layer in carbonized plane tree bark and (b) its scheme. (c) Cross-section FESEM image of tube array and (d) its scheme (e) FESEM image of cleaved tubes and (f) its scheme.
inter tube-array layer in carbonized plane tree bark. The cell morphologies can be observed on the outer surface of the carbon slice showing that the natural cell structure has remained to a certain extent as shown in Figure S2a and b. Nanopores were formed on the walls of the hollow carbonized cells. We also observed the interconnections between cell and cell in Figure S3b. FESEM images of the tube array are obtained from different cross sections in Figure 3c and e, and the proposed schemes are also provided in Figure 3d and f. Figure 3c shows the well-preserved tube structures stemming from vascular cambium of the plane tree bark with a diameter of about several micrometers. The cleaved tubes prove that the
Figure 2. Photographs of the fabrication process of integrated carbon slice electrodes: (a) bark of plane tree, (b) bark slices, and (c) carbonized bark slices. B
DOI: 10.1021/acssuschemeng.5b01258 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 4. (a) First three differential capacity curves of the carbon−sulfur slice. (b) Typical charge−discharge profiles of the carbon−sulfur slice in the potential range of 1.5−2.7 V at the current density of 0.1 A g−1. Cycle performance of the (c) carbon−sulfur slice and (d) carbon−sulfur powder electrode calculated on the weight of sulfur 0.2 A g−1. (e) Scheme of carbon−sulfur slice to powder electrode.
slice are consistent with the characteristics of an orthorhombic phase (JCPDS card no. 08-0247).24 Compared to the XRD pattern of the carbon slice, the curve of the carbon−sulfur slice denotes that no phase transformation occurred during the sulfur-fixed treatment. To better understand the microstructure of the carbon slice, Raman spectroscopy was used as shown in Figure S4b. Two obvious characteristic peaks at 1337 and 1595 cm−1 can be assigned to the D-bond and G-bond. The D-band is ascribed to defects, edges, and disordered carbon, whereas the G-band corresponds to the sp2-bonded graphitic carbon.25,26 Therefore, the intensity ratio of the D-bond to G-bond (ID/IG) is closed to 1, indicating the coexistence of disorder and graphitic carbon in the natural integrated carbon slices.27,28 Moreover, the Raman spectrum of the carbon slice also exhibits a graphite phase in conformity with XRD analysis. Figure S4c and d present the nitrogen adsorption− desorption isotherms and the pore size distribution of the carbon slices. The isotherm curve for the carbon slice indicates the presence of micropores and mesopores in the integrated carbon slice. The direct data of the pore size distribution is provided in Figure S4d, and the peaks at 1.7 and 5.4 nm confirm the micro/mesopores structure of carbon, which is consistent with TEM images.20,29 The BET surface area and pore volume of the carbon slice are up to 528 m2 g−1 and 0.72 cm3 g−1, which make it possible to load high-content sulfur. After the sulfur loaded, most of the micropores are filled with
tubes are the connecting macroporous structure, as shown in Figure 3e. The natural structures may not only play the role of physical “reservoir” for sulfur storage but may also have the integrated and interconnected carbon architecture for electron transport. Cross-section FESEM images of carbon−sulfur are provided in Figure S3a and b. The carbon microsheets and macropores are still obviously observed, which are a suitable physical reservoirs for storage of the sulfur and electrolyte.21,22 Figure S3b also shows that the interconnected structures constitute a whole and integrated three-dimensional structure, which is beneficial for quickening electron transfer. In addition, the micrometer-scale walls of the veins can bear the volumetric change of the sulfur and ensure the stability and conductivity of the integrated structure during the discharge−charge process. Figure S3c shows the element mapping of the sulfur and carbon in the electrode slices, demonstrating that sulfur is homogeneously loaded in the 3D framework. The HRTEM image of the carbon powder in Figure S3d clearly indicates the nanoporous structure. A graphite lattice is also shown in Figure S3e. We think the micro/mesopores system may be conducive to improving the capacities performances and stability of the sulfur cathode.9,12,23 Figure S4a shows the XRD patterns of the carbon and the asprepared carbon−sulfur slice. The main Bragg peaks (2θ) at 23.4° and 44.6° of the carbon slice can be assigned to (002) and (101), indicating a typical graphite structure but with low crystallinity. The crystal of sulfur grains in the carbon−sulfur C
DOI: 10.1021/acssuschemeng.5b01258 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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CONCLUSIONS Integrated carbon slices are obtained by carbonizing the natural architecture of plane tree bark. The integrated structure has high specific surface area (528 m2 g−1) and three-dimensional framework with hierarchical pores, which can enhance sulfur loading in the electrode, reduce diffusion of polysulfides, and suppress the shuttling effect. Importantly, the integrated carbon−sulfur cathodes with high sulfur loading are additivefree, exhibit an initial discharge capacity of 1159 mA h g−1 at 0.2 A g−1, and have good cycling performance and Coulombic efficiency. We think the integrated architecture may provide a new horizon to develop high performance lithium−sulfur batteries combining whole synergetic effects from natural biomacromolecules or organizational structures.
sulfur (Figure S5). Moreover, the sulfur content could be loaded up to 48% by thermal gravity analysis. The carbon−sulfur slices were used to set up the Li−S battery as the cathode. Figure 4a shows the differential capacity curves of the carbon−sulfur electrode at the current density of 0.1 A g−1.30,31 The two cathode peaks at 2.29 and 2.05 V are ascribed to the formation of polysulfide chains (Li2Sn, n ≥ 4) and then the transformation of the insoluble Li2S compound at the cathode surface. During the charging process, Li2S is first oxidized to low-order polysulfides and further oxidized to highorder polysulfides, corresponding to the two anodic peaks.32 The typical charge−discharge curves of the carbon−sulfur cathode at the current density of 0.1 A g−1 between 1.5 and 2.7 V are shown in Figure 4a and b. These curves demonstrate a high discharge capacity of 1192 mA h g−1 in the first cycle. Furthermore, the discharge curves include a short higher potential plateau at about 2.3 V and a prolonged potential plateau at about 2.1 V, and the charge curves have a plateau at about 2.3 V in favor of the typical Li−S batteries. The cycle performance of the carbon−sulfur at the current density of 0.2 A g−1 is shown in Figure 4c. The discharge capacity decays rapidly in the first five cycles from 1159 to 801 mA h g−1, which could be attributed to the residual sulfur on the surface of the carbon slice. After that, the capacity decreases slightly and remains 608 mA h g−1 after 60 full charge− discharge cycles, which is much higher than 466 mA h g−1 for multi-walled carbon nanotubes electrode at 0.175 Ag1− as shown in Figure S6. The good performance may be due to partial dissolution of polysulfides and escaping from the micro/ mesopores of the carbon slice. In addition, the Coulombic efficiency of more than 98% demonstrated the good reversibility for lithium−sulfur batteries. Subsequently, the rate capabilities are performed at different current densities of 0.1 and 0.2 A g−1 as shown in Figure S7. After 20 cycles, the carbon-sulfur slice electrode can still give a capacity of 789.5 mA h g−1 at 0.1 A g−1, which is higher than the capacity of 645 mA h g−1 at 0.2 A g−1. The results indicate that the integrated carbon electrodes exhibit high capacities at low rates. As a comparison, the cycling performance of the carbon− sulfur destroyed electrode is shown in Figure 4d. The electrode was directly pressed to destroy cell’s structure by presser but kept the electrode as the integration, shown in Figure 4e. There are similar capacities during first 10 cycles that compare to an integrated carbon-based cathode. However, the retained capacity is only 410 mA h g−1 after 60 cycles, and the Coulombic efficiency drops to 85%. The poor performance may be ascribed to the damage of the integrated 3D network and “reservoir” structures to affect the electron transfer and “shuttling effect”. From the EIS plots in Figure S8, the lower resistance of the carbon sulfur slice supports our deduction. Another comparative experiment was conducted to investigate the effect of calcination temperature. The sample obtained by annealing bark at 850 °C gives lower capacities compared to the one annealed at 750 °C. After 60 full charge− discharge cycles, the discharge capacity remains only 431.9 mA h g−1 at a current drain of 0.2 A g−1 as shown in Figure S9. According to the above results, we draw a conclusion that the synergetic effect of tubular channels and cell-like hollow microstructures in natural integrated carbon slices could play an important role in high-performance Li−S batteries.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01258. Experimental section, FESEM images, TEM images, N2 adsorption−desorption isotherms, EIS plots, and capacity performance. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (X.-W. Liu). *E-mail:
[email protected] (J.H. Liu) Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Natural Science Foundation of China (21303038), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Hundred Talents Program of Anhui Province.
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