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A bimodal porous carbon with high surface area supported selenium cathode for advanced Li–Se batteries
Solid State Ionics 274 (2015) 71–76
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
A bimodal porous carbon with high surface area supported selenium cathode for advanced Li–Se batteries Yaohui Qu a, Zhian Zhang a,b,⁎, Yanqing Lai a, Yexiang Liu a, Jie Li a a b
School of Metallurgy and Environment, Central South University, Changsha 410083, China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 15 November 2014 Received in revised form 1 March 2015 Accepted 6 March 2015 Available online 18 March 2015 Keywords: Bimodal porous carbon High surface area High rate capability Lithium–selenium batteries
a b s t r a c t A novel bimodal porous carbon (BPC) with high surface area was prepared by a simple hydrothermal route and KOH activation process, and the Se–BPC composite was synthesized for lithium–selenium batteries by the meltdiffusion method. It is found that the elemental selenium was dispersed inside the pores of BPC based on the analyses. Electrochemical tests reveal that the Se–BPC composite has a large reversible capacity and high rate performance as cathode materials. The Se–BPC (45.1 wt.% Se) composite displays an initial discharge capacity of 552 mAh g−1 and a reversible discharge capacity of 264 mAh g−1 after 80 cycles at 1 C charge/discharge rate. In particular, the Se–BPC composite presents a durable cycling performance at high rate of 2 C. These outstanding electrochemical features of Se–BPC composite cathode should be attributed to the uniform BPC with high surface area and bimodal porous structure. The bimodal porous carbon would be a promising carbon matrix to develop high performance lithium–selenium batteries. © 2015 Elsevier B.V. All rights reserved.
1. Introduction High energy density rechargeable batteries have received great attention in recent years because of their potential applications, such as the power source for electric vehicles, energy storage devices and smart grids [1]. Among various types of rechargeable batteries, Li–S batteries have been studied as one of the most promising systems for the next generation high-energy rechargeable lithium batteries due to their high theoretical specific capacity (1672 mAh g−1) and energy density (2600 Wh kg− 1) [2–4]. However, Li–S batteries still face tremendous challenges, such as the low conductivity of S and the solubility of intermediary polysulfide species during cycling [5–7]. The somewhat similar lithium–selenium (Li–Se) system in nonaqueous media was recently found to offer promising performances [8]. Although the theoretical gravimetric capacity of the selenium (675 mAh g−1) is lower than that of sulfur (1672 mAh g−1), the theoretical volumetric capacity of selenium (3253 mAh cm− 3 based on 4.82 g cm−3) is comparable to that of sulfur (3467 mAh cm−3 based on 2.07 g cm−3). It is known that for applications in portable devices and EV, volumetric energy density is more important than gravimetric energy density because of the limited battery packing space [9]. In addition, selenium has 20 orders of magnitude higher electrical conductivity than sulfur [10–12]. These features of selenium make it a prospective candidate for cathode material in high energy density rechargeable batteries for specific applications. However, at present, research on ⁎ Corresponding author. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.ssi.2015.03.007 0167-2738/© 2015 Elsevier B.V. All rights reserved.
lithium–selenium (Li–Se) batteries is still at a very early stage. Similar to sulfur, the selenium cathodes also face the dissolution issue of highorder polyselenides, resulting in fast capacity fading and low Coulombic efficiency [10,13]. To circumvent these issues, significant effects have been achieved to enhance the electrochemical performance of Li–Se batteries by confining selenium within various forms of porous carbon matrixes. Guo et al. [11] synthesized the selenium/ordered mesoporous carbon composite (49.0 wt.% selenium) through a facile melt-diffusion process from a ball-milled mixture of ordered mesoporous carbon and Se. The Se/C cathode exhibited excellent cycling stability with a discharge capacity of 600 mAh g−1 at current density of 67 mA g−1 (0.1 C) after 50 cycles. Li et al. [14] used a typical microporous carbon to encapsulate Se molecules to improve the Se utilization and restrain the shuttle effect from the polyselenide species. The Se/C cathode showed high volumetric capacity density of 3150 mAh cm−3 and excellent rate capability (retains 57% of the theoretical capacity at 20 C). Moreover, there is a new kind of Se/C composite, in which Se molecules are confined in the hierarchically micro-mesoporous carbon, also exhibited good cycling stability and high rate performance [15]. These encouraging results suggest the benefits of porous carbon matrixes (single-porous structured carbon or dual-porous structured carbon), which might have several advantages including superior incorporation and dispersion of Se and a large contact area with Se and greatly facilitating electron transport [3]. Herein, we were motivated to prepare a novel bimodal porous carbon with high surface area (2100.4 m2 g−1), large pore volume and interconnected micro@mesoporous structure to improve the utilization of selenium and the
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electrochemical performance of Li–Se batteries. The bimodal porous carbon (BPC) with high surface area was fabricated via a facile hydrothermal route and KOH activation process. The selenium–BPC (Se–BPC) composite was prepared by a melt-diffusion method. Used as the cathode material in Li–Se batteries, showing the promising electrochemical behavior, the as-prepared Se–BPC composite (45.1 wt.% Se) retained 264 mAh g−1 after 80 cycles at 1 C charge– discharge rate.
2.2. Material characterization Field emission scanning electron microscopy (SEM, Nova NanoSEM 230) and transmission electron microscopy (TEM, Tecnai G2 20ST) were applied to characterize the materials. X-ray diffraction (XRD, Rigaku3014) measurements were made with Cu Кα radiation. Thermogravimetric analysis (TGA, SDTQ600) was conducted in determining the selenium content in the composite. N2 adsorption/desorption measurements were performed by using Quantachrome instrument (Quadrasorb SI-3MP) at 77 K.
2. Experimental 2.1. Material synthesis
2.3. Electrochemical measurements
The bimodal porous carbon (BPC) with high surface area was fabricated via a facile hydrothermal route and KOH activation process. First, 6.0 g glucose was dissolved in 40 ml distilled–deionized water to form a clear solution, which was placed in a 60 ml teflon-sealed autoclave and maintained at 160 °C for 16 h. Second, the puce products were collected by centrifugation, successively washed with water and ethanol, and finally dried at 60 °C in an oven overnight. Third, a mixture of the dried carbon precursor and KOH in the weight ratio of 1:3 was heated in a tube furnace in nitrogen atmosphere at 850 °C for 3 h, the product was then washed with 1 mol L−1 HCl solution and deionized water till the filtrate became neutral. Finally, the product was dried overnight at 80 °C in an oven, and the resulting bimodal porous carbon (BPC) with high surface area was obtained. To prepare selenium–BPC (Se–BPC) composite, elemental selenium (AR, Aladdin, China) and the as-prepared BPC with a weight ratio of 5:5 were mixed together and placed in a sealed vessel, and the mixture was heated to 260 °C for 20 h under an argon atmosphere with the heating rate of 5 °C min− 1 , then the Se–BPC composite was obtained.
The selenium cathode was prepared by mixing 80 wt.% active material (Se–BPC composite), 10 wt.% acetylene black, and 10 wt.% sodium alginate (SA) binder in deionized water solvent. The slurry was spread onto aluminum foil, and then dried at 60 °C overnight, then the cathodes were cut into pellets with a diameter of 1.0 cm and dried for 12 h in a vacuum oven at 60 °C. The typical mass loading of active material was 0.7–1.0 mg cm−2. The electrochemical performance was performed using a CR2025 coin-type cell. CR2025-type coin cells were assembled in an argon-filled glove box (Universal 2440/750) in which oxygen and water contents were less than 1 ppm. The electrolyte used was 2 M bis(trifluoromethane)sulfonamide lithium salt (LiTFSI, Sigma-Aldrich) in a solvent mixture of 1,3-dioxolane and 1,2dimethoxyethane (1:1, v/v) (Acros Organics). Lithium metal was used as counter electrode and reference electrode and Celgard 2400 was used as a separator. Cyclic voltammetry (CV) and measurements were conducted using PARSTAT 2273 electrochemical measurement system. CV tests were performed at a scan rate of 0.2 mV s−1 in the voltage range of 1.2 to 3.0 V. Galvanostatic charge/discharge tests were performed in the potential range of 1.2 to 3.0 V at 25 °C by using a
Fig. 1. SEM images of the BPC (a and b), TEM images of the BPC (c, d and e), and HRTEM image of the BPC (f).
Y. Qu et al. / Solid State Ionics 274 (2015) 71–76
LAND CT2001A battery-testing instrument. The cells were first discharged to 1.2 V and then the cycle number was counted.
3. Results and discussion Field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the morphology and microstructure of the as-prepared BPC. As shown in Fig. 1a and b, it can be observed that the BPC is composed of large fragment-like particles, ranging from ~ 5 to 50 μm. The typical TEM images shown in Fig. 1c and d reveal that the BPC has an advanced highly porous structure. The TEM image (Fig. 1e) and HRTEM image (Fig. 1f) indicate that the meso- and micro-porous texture of the BPC. The BPC has a unique micro- and meso-porous structure, the special micro- and meso-porous structure could favor the elemental selenium confined within the carbon matrix, and facilitate the transport of electrolyte ions [10,11]. The specific surface area analysis of the as-prepared BPC was performed by nitrogen BET adsorption measurements. The pore size distribution of carbon was calculated by the BJH method. As shown in Fig. 2a and b, a combined I/IV type isotherm is observed, indicating the assembly of micro- and mesopore structures of the BPC. The specific BET surface area and total pore volume are about 2100.4 m2 g−1 and 1.28 cm3 g− 1, respectively. As shown in Fig. 2b, the BPC exhibits a bimodal pore-size distribution (one large peak centered at 1.9 nm and
Fig. 2. The nitrogen adsorption/desorption isotherm of the BPC (a), and the pore size distribution of the BPC (b).
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another peak centered at 3.4 nm), revealing the presence of abundant micropores and mesopores, which is similar to the result reported by Zhang et al. [16]. The abundant micropores and mesopores are attributed from the KOH activation process. Those results are consistent with the SEM/TEM measurements. On the basis of these results, it can be concluded that the BPC has a high surface area (2100.4 m2 g− 1) and bimodal porous structure (micro@meso-pores). The high surface area (2100.4 m2 g−1) can provide large contact area with selenium, and favor the selenium homogeneously dispersed in BPC. The bimodal porous structure (micro@meso-pores) can provide room for selenium loading and act as pathways for the electrolyte diffusion. The selenium–bimodal porous carbon (Se–BPC) composite was prepared via a simple melt-diffusion strategy [11]. Fig. 3a and b is the SEM of the Se–BPC composite. There is no distinguishable morphological difference between the Se–BPC composite and the BPC (Fig. 1a and b), which suggests that the selenium is successfully distributed within the pores of BPC. Moreover, the SEM elemental mapping images of carbon and selenium (Fig. 3c1 and c2) are found to have similar intensity across the Se–BPC composite, indicating that selenium has a highly dispersed state in BPC, which corroborates well with the SEM and TEM observation. Furthermore, the TEM image of the Se–BPC composite (Fig. 3d and e) illustrates that no large bulk selenium could be observed in the composite, indicating that selenium homogeneously disperses in BPC. This can be further confirmed by the BET analysis (Fig. 3f and g) that the specific surface area and pore volume of the Se–BPC (45.1 wt.% Se) composite have reduced to 858.9 m2 g−1 and 0.61 cm3 g− 1 from initial 2100.4 m2 g−1 and 1.28 cm3 g−1 of BPC, respectively. The results indicate that the selenium is filled inside the porous structure of BPC, which is similar to the reported literatures [16]. X-ray diffraction (XRD) patterns of elemental selenium (AR, Aladdin, China), BPC and the Se–BPC composite are given in Fig. 4a. For selenium, there are several peaks at 23.5° (100), 29.7° (101), 41.3° (110), 43.6° (102), 45.4° (111), 51.8° (201), 55.7° (112), and 61.5° (202), which are in good accordance with the diffraction peaks of the trigonal phase of selenium (JCPDS 06-0362) [17,18]. The XRD pattern of BPC shows the broad reflection at 2θ of about 24°, which can be attributed to the amorphous characteristic of the as-prepared BPC. After encapsulating selenium into the BPC, as shown in the XRD curve of Se–BPC composite, the sharp diffraction peaks of bulk crystalline selenium disappear entirely, which may be due to the dispersion of amorphous selenium at a molecular level confined in the pores of BPC [11], which agrees well with the results of the SEM and TEM. To determine the selenium content in the Se–BPC composite, this material was analyzed by thermogravimetric analysis (TGA) under a nitrogen atmosphere, as shown in Fig. 4b. The TGA result shows that the selenium content is 45.1 wt.% in the Se–BPC composite, which is consistent with the proportions of the added amount. On the basis of these results, it can be concluded that the melt-diffusion method can successfully trapped the selenium inside the pores of the BPC, which is similar with the result of Guo et al. [11]. To study the electrochemical properties of the Se–BPC composite, the CR2025 coin cells with metallic lithium counter electrode were assembled and evaluated. Fig. 5a shows the typical cyclic voltammogram (CV) curves of the Se–BPC composite cathode (45.1 wt.% selenium) in the voltage range of 1.2–3.0 V with a constant scan rate of 0.2 mV s−1. In the first cycle, two obvious cathodic peaks and one anodic peak were observed, which are consistent with the previous report [13, 19]. The two remarkable reduction peaks for Se–BPC composite cathode are about 1.95 V and 2.1 V, respectively, corresponding to the reduction of elemental selenium to soluble polyselenides and then to the insoluble Li2Se2 and Li2Se [13]. In the anodic scan, only one sharp oxidation peak can be observed at about 2.25 V for Se–BPC composite cathode, which corresponds to the conversion of Li2Se into high-order soluble polyselenides [13]. In the subsequent scans, the main reduction peaks are shifted to higher potentials and the oxidation peaks to lower potentials, indicating an improvement of reversibility of the cathode. This can
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Fig. 3. SEM images of the Se–BPC composite (a, b and c). (c1) and (c2) are the corresponding elemental mapping of carbon and selenium in the Se–BPC composite (c), respectively. TEM images of the Se–BPC composite (d and e). N2 adsorption/desorption isotherm of the Se–BPC composite (f), and pore size distribution of the Se–BPC composite (g).
be explained by the reduced polarization of the cathode after the first cycle [13]. In addition, the peak current is decreased slowly during the subsequent scans, indicating that the Se–BPC composite cathode may have good electrochemical stability. In order to investigate the cycling performance and rate capability of the Se–BPC composite (45.1 wt.% selenium), galvanostatic charge/discharge cycling was performed. Fig. 5b depicts the first cycle charge and discharge curves of the Se–BPC composite cathode at different rates (0.5 C, 1 C, 2 C, 1 C is 675 mA g− 1), shows the typical
two-plateau behavior of a selenium cathode, consistent with the result of cyclic voltammetry measurement, which can be ascribed to the two step reaction of elemental selenium with metallic lithium during the discharge process [13]. With the increase of the discharge rate from 0.5 C to 2 C, the first discharge capacity slightly fades, and the charge/discharge plateaus gradually rise/drop, but the typical two plateaus in the discharge curves still maintain during all the cycles even at very high current rates (2 C), suggesting little kinetic barrier in the electrode process and high rate capability. This could
Y. Qu et al. / Solid State Ionics 274 (2015) 71–76
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Fig. 4. XRD patterns of elemental selenium, BPC and Se–BPC composite (a), and TGA curve of BPC and Se–BPC composite (b).
Fig. 5. Cyclic voltammograms of the Se–BPC composite cathode at a scanning rate of 0.2 mV s−1 (a), charge/discharge curves of the Se–BPC composite cathode at different current density between 1.2 V and 3.0 V (b).
be attributed to the high quality of BPC and the bimodal porous structure (micro@meso-pores), which significantly improved electronic and ionic transport at the selenium cathode. Cycling performance of the Se–BPC (45.1 wt.% selenium) composite cathode at different discharge rates is presented in Fig. 6a. All capacity values in this study were calculated based on selenium mass. As shown in Fig. 6a, the initial discharge capacities of the Se–BPC composite cathode at discharge rates of 0.5, 1, and 2 C are 560, 552, and 539 mAh g− 1, the cathode maintains an irreversible capacity of 246 mAh g−1 after 80 cycles at 0.5 C, and still maintains a capacity of 264 mAh g−1 after 80 cycles at 1 C. It is demonstrate that the Se–BPC composite cathode shows good cycling performance at 0.5 C and 1 C. Moreover, the cycling performance of Se–BPC composite cathode at a high constant rate (1 C) is better than the cycling performance at a low rate (0.5 C). This can probably be attributed to the relatively rapid charge/discharge process at a high constant rate, where polyselenides are not immediately dissolved into the electrolyte with the adsorption effect of the bimodal pores (micro@meso-pores) in BPC. The shuttle effect is suppressed so that the cycling performance of Se–BPC composite cathode can improve [8,10]. More importantly, even at discharge rate as high as 2 C, the Se–BPC (45.1 wt.% selenium) composite cathode still exhibits a durable cycling performance, with its discharge capacity always stabilizing around 216 mAh g−1 after 80 cycles, suggesting a high rate capability and high capacity retention. The cycling performance of the Se–BPC composite cathode is better than the Se–C
composite [20], and the nanoporous-Se cathode [12]. From Fig. 6a, we can also see that the Se–BPC composite cathode shows high Columbic efficiency. However, as shown in Fig. 6a, all three samples (0.5 C, 1 C and 2 C) exhibit fast capacity fading at the beginning 10 cycles. The reasons of this phenomenon may be as follows: Firstly, the electronic conductivity of selenium (1 × 10−3 S m−1 at 25 °C) is not good enough, leading to a low utilization of the active material [12]. Secondly, the intermediate polyselenides are very soluble in organic electrolytes, and the shuttle effect takes place during the continuous charge– discharge processes [8,13], which results in significant decline of the discharge capacity of selenium cathode at the first 10 cycles. The rate capability of the Se–BPC (45.1 wt.% selenium) composite cathode is shown in Fig. 6b. The discharge capacity gradually decreased as the current rate increased from 0.2 C to 2 C, 1 C = 675 mA g−1. A satisfactory capacity of 272 mAh g−1 is obtained for Se–BPC at 2 C, and the material recovered most of the capacity when the current rate was reduced back to 0.2 C, indicating an excellent rate performance, likely because of the facile electronic/ionic transport and improved reaction kinetics in the BPC. Wang et al. [10] prepared Se/C composite as a cathode material for Li–Se batteries, which also shows excellent highrate capability, but the content of selenium is lower (30.0 wt.% selenium). The bimodal porous carbon structure exhibits a similar high-rate performance in the lithium-sulfur batteries [21,22]. The excellent high rate capability and high capacity retention of the Se–BPC composite cathode can be attributed to the high surface area
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KOH activation process, the Se–BPC composite was synthesized for lithium–selenium batteries by the melt-diffusion method. As a cathode material for Li–Se batteries, the Se–BPC composite cathode exhibits a high initial discharge capacity of 552 mAh g− 1 and retains 264 mAh g− 1 after 80 cycles at high current rate of 1 C. Moreover, the Se–BPC composite presents a durable cycling performance at high rate of 2 C. The high rate capability and high capacity retention of Se–BPC composite cathode can be attributed to the uniform BPC with high surface area and bimodal porous structure. Such BPC, combining unique high surface area, large pore volume and bimodal porous (micro@meso-porous) framework can effectively disperse and confine selenium, and suppress the diffusion of dissolved polyselenides. Moreover, the bimodal porous framework can supply facile transport channels for Li + ions during the electrochemical process. Consequently, the BPC would be a promising carbon matrix to develop high performance lithium–selenium batteries. Acknowledgment The authors thank the financial support of the Teacher Research Fund of Central South University (2013JSJJ027), Scientific Research Innovation Program for Postgraduate of Hunan Province (CX2014B084) and the Strategic Emerging Industries Program of Shenzhen, China (JCYJ20140509142357195). We also thank the support of the Engineering Research Center of Advanced Battery Materials, the Ministry of Education, China. References
Fig. 6. Cycling performance of the Se–BPC composite cathode at different current density (a). The rate capability of the Se–BPC composite cathode (b).
and bimodal porous structure. The high surface area (2100.4 m2 g−1) can provide large contact area with selenium, and favor the selenium highly dispersed in BPC. The bimodal porous structure can provide channels to facilitate selenium infiltration into the pores to improve the conductivity of selenium, absorb the formed polyselenides, and retard the shuttle effect during the charge–discharge process. Moreover, the bimodal porous structure can supply facile transport channels for Li+ ions during the electrochemical process. Therefore, the high rate capability and high capacity retention of selenium cathode can be enhanced significantly by confining selenium in the bimodal porous carbon with high surface area. 4. Conclusions In conclusion, a novel bimodal porous carbon (BPC) with high surface area was prepared by a simple hydrothermal route and
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Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
A bimodal porous carbon with high surface area supported selenium cathode for advanced Li–Se batteries Yaohui Qu a, Zhian Zhang a,b,⁎, Yanqing Lai a, Yexiang Liu a, Jie Li a a b
School of Metallurgy and Environment, Central South University, Changsha 410083, China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 15 November 2014 Received in revised form 1 March 2015 Accepted 6 March 2015 Available online 18 March 2015 Keywords: Bimodal porous carbon High surface area High rate capability Lithium–selenium batteries
a b s t r a c t A novel bimodal porous carbon (BPC) with high surface area was prepared by a simple hydrothermal route and KOH activation process, and the Se–BPC composite was synthesized for lithium–selenium batteries by the meltdiffusion method. It is found that the elemental selenium was dispersed inside the pores of BPC based on the analyses. Electrochemical tests reveal that the Se–BPC composite has a large reversible capacity and high rate performance as cathode materials. The Se–BPC (45.1 wt.% Se) composite displays an initial discharge capacity of 552 mAh g−1 and a reversible discharge capacity of 264 mAh g−1 after 80 cycles at 1 C charge/discharge rate. In particular, the Se–BPC composite presents a durable cycling performance at high rate of 2 C. These outstanding electrochemical features of Se–BPC composite cathode should be attributed to the uniform BPC with high surface area and bimodal porous structure. The bimodal porous carbon would be a promising carbon matrix to develop high performance lithium–selenium batteries. © 2015 Elsevier B.V. All rights reserved.
1. Introduction High energy density rechargeable batteries have received great attention in recent years because of their potential applications, such as the power source for electric vehicles, energy storage devices and smart grids [1]. Among various types of rechargeable batteries, Li–S batteries have been studied as one of the most promising systems for the next generation high-energy rechargeable lithium batteries due to their high theoretical specific capacity (1672 mAh g−1) and energy density (2600 Wh kg− 1) [2–4]. However, Li–S batteries still face tremendous challenges, such as the low conductivity of S and the solubility of intermediary polysulfide species during cycling [5–7]. The somewhat similar lithium–selenium (Li–Se) system in nonaqueous media was recently found to offer promising performances [8]. Although the theoretical gravimetric capacity of the selenium (675 mAh g−1) is lower than that of sulfur (1672 mAh g−1), the theoretical volumetric capacity of selenium (3253 mAh cm− 3 based on 4.82 g cm−3) is comparable to that of sulfur (3467 mAh cm−3 based on 2.07 g cm−3). It is known that for applications in portable devices and EV, volumetric energy density is more important than gravimetric energy density because of the limited battery packing space [9]. In addition, selenium has 20 orders of magnitude higher electrical conductivity than sulfur [10–12]. These features of selenium make it a prospective candidate for cathode material in high energy density rechargeable batteries for specific applications. However, at present, research on ⁎ Corresponding author. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.ssi.2015.03.007 0167-2738/© 2015 Elsevier B.V. All rights reserved.
lithium–selenium (Li–Se) batteries is still at a very early stage. Similar to sulfur, the selenium cathodes also face the dissolution issue of highorder polyselenides, resulting in fast capacity fading and low Coulombic efficiency [10,13]. To circumvent these issues, significant effects have been achieved to enhance the electrochemical performance of Li–Se batteries by confining selenium within various forms of porous carbon matrixes. Guo et al. [11] synthesized the selenium/ordered mesoporous carbon composite (49.0 wt.% selenium) through a facile melt-diffusion process from a ball-milled mixture of ordered mesoporous carbon and Se. The Se/C cathode exhibited excellent cycling stability with a discharge capacity of 600 mAh g−1 at current density of 67 mA g−1 (0.1 C) after 50 cycles. Li et al. [14] used a typical microporous carbon to encapsulate Se molecules to improve the Se utilization and restrain the shuttle effect from the polyselenide species. The Se/C cathode showed high volumetric capacity density of 3150 mAh cm−3 and excellent rate capability (retains 57% of the theoretical capacity at 20 C). Moreover, there is a new kind of Se/C composite, in which Se molecules are confined in the hierarchically micro-mesoporous carbon, also exhibited good cycling stability and high rate performance [15]. These encouraging results suggest the benefits of porous carbon matrixes (single-porous structured carbon or dual-porous structured carbon), which might have several advantages including superior incorporation and dispersion of Se and a large contact area with Se and greatly facilitating electron transport [3]. Herein, we were motivated to prepare a novel bimodal porous carbon with high surface area (2100.4 m2 g−1), large pore volume and interconnected micro@mesoporous structure to improve the utilization of selenium and the
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electrochemical performance of Li–Se batteries. The bimodal porous carbon (BPC) with high surface area was fabricated via a facile hydrothermal route and KOH activation process. The selenium–BPC (Se–BPC) composite was prepared by a melt-diffusion method. Used as the cathode material in Li–Se batteries, showing the promising electrochemical behavior, the as-prepared Se–BPC composite (45.1 wt.% Se) retained 264 mAh g−1 after 80 cycles at 1 C charge– discharge rate.
2.2. Material characterization Field emission scanning electron microscopy (SEM, Nova NanoSEM 230) and transmission electron microscopy (TEM, Tecnai G2 20ST) were applied to characterize the materials. X-ray diffraction (XRD, Rigaku3014) measurements were made with Cu Кα radiation. Thermogravimetric analysis (TGA, SDTQ600) was conducted in determining the selenium content in the composite. N2 adsorption/desorption measurements were performed by using Quantachrome instrument (Quadrasorb SI-3MP) at 77 K.
2. Experimental 2.1. Material synthesis
2.3. Electrochemical measurements
The bimodal porous carbon (BPC) with high surface area was fabricated via a facile hydrothermal route and KOH activation process. First, 6.0 g glucose was dissolved in 40 ml distilled–deionized water to form a clear solution, which was placed in a 60 ml teflon-sealed autoclave and maintained at 160 °C for 16 h. Second, the puce products were collected by centrifugation, successively washed with water and ethanol, and finally dried at 60 °C in an oven overnight. Third, a mixture of the dried carbon precursor and KOH in the weight ratio of 1:3 was heated in a tube furnace in nitrogen atmosphere at 850 °C for 3 h, the product was then washed with 1 mol L−1 HCl solution and deionized water till the filtrate became neutral. Finally, the product was dried overnight at 80 °C in an oven, and the resulting bimodal porous carbon (BPC) with high surface area was obtained. To prepare selenium–BPC (Se–BPC) composite, elemental selenium (AR, Aladdin, China) and the as-prepared BPC with a weight ratio of 5:5 were mixed together and placed in a sealed vessel, and the mixture was heated to 260 °C for 20 h under an argon atmosphere with the heating rate of 5 °C min− 1 , then the Se–BPC composite was obtained.
The selenium cathode was prepared by mixing 80 wt.% active material (Se–BPC composite), 10 wt.% acetylene black, and 10 wt.% sodium alginate (SA) binder in deionized water solvent. The slurry was spread onto aluminum foil, and then dried at 60 °C overnight, then the cathodes were cut into pellets with a diameter of 1.0 cm and dried for 12 h in a vacuum oven at 60 °C. The typical mass loading of active material was 0.7–1.0 mg cm−2. The electrochemical performance was performed using a CR2025 coin-type cell. CR2025-type coin cells were assembled in an argon-filled glove box (Universal 2440/750) in which oxygen and water contents were less than 1 ppm. The electrolyte used was 2 M bis(trifluoromethane)sulfonamide lithium salt (LiTFSI, Sigma-Aldrich) in a solvent mixture of 1,3-dioxolane and 1,2dimethoxyethane (1:1, v/v) (Acros Organics). Lithium metal was used as counter electrode and reference electrode and Celgard 2400 was used as a separator. Cyclic voltammetry (CV) and measurements were conducted using PARSTAT 2273 electrochemical measurement system. CV tests were performed at a scan rate of 0.2 mV s−1 in the voltage range of 1.2 to 3.0 V. Galvanostatic charge/discharge tests were performed in the potential range of 1.2 to 3.0 V at 25 °C by using a
Fig. 1. SEM images of the BPC (a and b), TEM images of the BPC (c, d and e), and HRTEM image of the BPC (f).
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LAND CT2001A battery-testing instrument. The cells were first discharged to 1.2 V and then the cycle number was counted.
3. Results and discussion Field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the morphology and microstructure of the as-prepared BPC. As shown in Fig. 1a and b, it can be observed that the BPC is composed of large fragment-like particles, ranging from ~ 5 to 50 μm. The typical TEM images shown in Fig. 1c and d reveal that the BPC has an advanced highly porous structure. The TEM image (Fig. 1e) and HRTEM image (Fig. 1f) indicate that the meso- and micro-porous texture of the BPC. The BPC has a unique micro- and meso-porous structure, the special micro- and meso-porous structure could favor the elemental selenium confined within the carbon matrix, and facilitate the transport of electrolyte ions [10,11]. The specific surface area analysis of the as-prepared BPC was performed by nitrogen BET adsorption measurements. The pore size distribution of carbon was calculated by the BJH method. As shown in Fig. 2a and b, a combined I/IV type isotherm is observed, indicating the assembly of micro- and mesopore structures of the BPC. The specific BET surface area and total pore volume are about 2100.4 m2 g−1 and 1.28 cm3 g− 1, respectively. As shown in Fig. 2b, the BPC exhibits a bimodal pore-size distribution (one large peak centered at 1.9 nm and
Fig. 2. The nitrogen adsorption/desorption isotherm of the BPC (a), and the pore size distribution of the BPC (b).
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another peak centered at 3.4 nm), revealing the presence of abundant micropores and mesopores, which is similar to the result reported by Zhang et al. [16]. The abundant micropores and mesopores are attributed from the KOH activation process. Those results are consistent with the SEM/TEM measurements. On the basis of these results, it can be concluded that the BPC has a high surface area (2100.4 m2 g− 1) and bimodal porous structure (micro@meso-pores). The high surface area (2100.4 m2 g−1) can provide large contact area with selenium, and favor the selenium homogeneously dispersed in BPC. The bimodal porous structure (micro@meso-pores) can provide room for selenium loading and act as pathways for the electrolyte diffusion. The selenium–bimodal porous carbon (Se–BPC) composite was prepared via a simple melt-diffusion strategy [11]. Fig. 3a and b is the SEM of the Se–BPC composite. There is no distinguishable morphological difference between the Se–BPC composite and the BPC (Fig. 1a and b), which suggests that the selenium is successfully distributed within the pores of BPC. Moreover, the SEM elemental mapping images of carbon and selenium (Fig. 3c1 and c2) are found to have similar intensity across the Se–BPC composite, indicating that selenium has a highly dispersed state in BPC, which corroborates well with the SEM and TEM observation. Furthermore, the TEM image of the Se–BPC composite (Fig. 3d and e) illustrates that no large bulk selenium could be observed in the composite, indicating that selenium homogeneously disperses in BPC. This can be further confirmed by the BET analysis (Fig. 3f and g) that the specific surface area and pore volume of the Se–BPC (45.1 wt.% Se) composite have reduced to 858.9 m2 g−1 and 0.61 cm3 g− 1 from initial 2100.4 m2 g−1 and 1.28 cm3 g−1 of BPC, respectively. The results indicate that the selenium is filled inside the porous structure of BPC, which is similar to the reported literatures [16]. X-ray diffraction (XRD) patterns of elemental selenium (AR, Aladdin, China), BPC and the Se–BPC composite are given in Fig. 4a. For selenium, there are several peaks at 23.5° (100), 29.7° (101), 41.3° (110), 43.6° (102), 45.4° (111), 51.8° (201), 55.7° (112), and 61.5° (202), which are in good accordance with the diffraction peaks of the trigonal phase of selenium (JCPDS 06-0362) [17,18]. The XRD pattern of BPC shows the broad reflection at 2θ of about 24°, which can be attributed to the amorphous characteristic of the as-prepared BPC. After encapsulating selenium into the BPC, as shown in the XRD curve of Se–BPC composite, the sharp diffraction peaks of bulk crystalline selenium disappear entirely, which may be due to the dispersion of amorphous selenium at a molecular level confined in the pores of BPC [11], which agrees well with the results of the SEM and TEM. To determine the selenium content in the Se–BPC composite, this material was analyzed by thermogravimetric analysis (TGA) under a nitrogen atmosphere, as shown in Fig. 4b. The TGA result shows that the selenium content is 45.1 wt.% in the Se–BPC composite, which is consistent with the proportions of the added amount. On the basis of these results, it can be concluded that the melt-diffusion method can successfully trapped the selenium inside the pores of the BPC, which is similar with the result of Guo et al. [11]. To study the electrochemical properties of the Se–BPC composite, the CR2025 coin cells with metallic lithium counter electrode were assembled and evaluated. Fig. 5a shows the typical cyclic voltammogram (CV) curves of the Se–BPC composite cathode (45.1 wt.% selenium) in the voltage range of 1.2–3.0 V with a constant scan rate of 0.2 mV s−1. In the first cycle, two obvious cathodic peaks and one anodic peak were observed, which are consistent with the previous report [13, 19]. The two remarkable reduction peaks for Se–BPC composite cathode are about 1.95 V and 2.1 V, respectively, corresponding to the reduction of elemental selenium to soluble polyselenides and then to the insoluble Li2Se2 and Li2Se [13]. In the anodic scan, only one sharp oxidation peak can be observed at about 2.25 V for Se–BPC composite cathode, which corresponds to the conversion of Li2Se into high-order soluble polyselenides [13]. In the subsequent scans, the main reduction peaks are shifted to higher potentials and the oxidation peaks to lower potentials, indicating an improvement of reversibility of the cathode. This can
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Fig. 3. SEM images of the Se–BPC composite (a, b and c). (c1) and (c2) are the corresponding elemental mapping of carbon and selenium in the Se–BPC composite (c), respectively. TEM images of the Se–BPC composite (d and e). N2 adsorption/desorption isotherm of the Se–BPC composite (f), and pore size distribution of the Se–BPC composite (g).
be explained by the reduced polarization of the cathode after the first cycle [13]. In addition, the peak current is decreased slowly during the subsequent scans, indicating that the Se–BPC composite cathode may have good electrochemical stability. In order to investigate the cycling performance and rate capability of the Se–BPC composite (45.1 wt.% selenium), galvanostatic charge/discharge cycling was performed. Fig. 5b depicts the first cycle charge and discharge curves of the Se–BPC composite cathode at different rates (0.5 C, 1 C, 2 C, 1 C is 675 mA g− 1), shows the typical
two-plateau behavior of a selenium cathode, consistent with the result of cyclic voltammetry measurement, which can be ascribed to the two step reaction of elemental selenium with metallic lithium during the discharge process [13]. With the increase of the discharge rate from 0.5 C to 2 C, the first discharge capacity slightly fades, and the charge/discharge plateaus gradually rise/drop, but the typical two plateaus in the discharge curves still maintain during all the cycles even at very high current rates (2 C), suggesting little kinetic barrier in the electrode process and high rate capability. This could
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Fig. 4. XRD patterns of elemental selenium, BPC and Se–BPC composite (a), and TGA curve of BPC and Se–BPC composite (b).
Fig. 5. Cyclic voltammograms of the Se–BPC composite cathode at a scanning rate of 0.2 mV s−1 (a), charge/discharge curves of the Se–BPC composite cathode at different current density between 1.2 V and 3.0 V (b).
be attributed to the high quality of BPC and the bimodal porous structure (micro@meso-pores), which significantly improved electronic and ionic transport at the selenium cathode. Cycling performance of the Se–BPC (45.1 wt.% selenium) composite cathode at different discharge rates is presented in Fig. 6a. All capacity values in this study were calculated based on selenium mass. As shown in Fig. 6a, the initial discharge capacities of the Se–BPC composite cathode at discharge rates of 0.5, 1, and 2 C are 560, 552, and 539 mAh g− 1, the cathode maintains an irreversible capacity of 246 mAh g−1 after 80 cycles at 0.5 C, and still maintains a capacity of 264 mAh g−1 after 80 cycles at 1 C. It is demonstrate that the Se–BPC composite cathode shows good cycling performance at 0.5 C and 1 C. Moreover, the cycling performance of Se–BPC composite cathode at a high constant rate (1 C) is better than the cycling performance at a low rate (0.5 C). This can probably be attributed to the relatively rapid charge/discharge process at a high constant rate, where polyselenides are not immediately dissolved into the electrolyte with the adsorption effect of the bimodal pores (micro@meso-pores) in BPC. The shuttle effect is suppressed so that the cycling performance of Se–BPC composite cathode can improve [8,10]. More importantly, even at discharge rate as high as 2 C, the Se–BPC (45.1 wt.% selenium) composite cathode still exhibits a durable cycling performance, with its discharge capacity always stabilizing around 216 mAh g−1 after 80 cycles, suggesting a high rate capability and high capacity retention. The cycling performance of the Se–BPC composite cathode is better than the Se–C
composite [20], and the nanoporous-Se cathode [12]. From Fig. 6a, we can also see that the Se–BPC composite cathode shows high Columbic efficiency. However, as shown in Fig. 6a, all three samples (0.5 C, 1 C and 2 C) exhibit fast capacity fading at the beginning 10 cycles. The reasons of this phenomenon may be as follows: Firstly, the electronic conductivity of selenium (1 × 10−3 S m−1 at 25 °C) is not good enough, leading to a low utilization of the active material [12]. Secondly, the intermediate polyselenides are very soluble in organic electrolytes, and the shuttle effect takes place during the continuous charge– discharge processes [8,13], which results in significant decline of the discharge capacity of selenium cathode at the first 10 cycles. The rate capability of the Se–BPC (45.1 wt.% selenium) composite cathode is shown in Fig. 6b. The discharge capacity gradually decreased as the current rate increased from 0.2 C to 2 C, 1 C = 675 mA g−1. A satisfactory capacity of 272 mAh g−1 is obtained for Se–BPC at 2 C, and the material recovered most of the capacity when the current rate was reduced back to 0.2 C, indicating an excellent rate performance, likely because of the facile electronic/ionic transport and improved reaction kinetics in the BPC. Wang et al. [10] prepared Se/C composite as a cathode material for Li–Se batteries, which also shows excellent highrate capability, but the content of selenium is lower (30.0 wt.% selenium). The bimodal porous carbon structure exhibits a similar high-rate performance in the lithium-sulfur batteries [21,22]. The excellent high rate capability and high capacity retention of the Se–BPC composite cathode can be attributed to the high surface area
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KOH activation process, the Se–BPC composite was synthesized for lithium–selenium batteries by the melt-diffusion method. As a cathode material for Li–Se batteries, the Se–BPC composite cathode exhibits a high initial discharge capacity of 552 mAh g− 1 and retains 264 mAh g− 1 after 80 cycles at high current rate of 1 C. Moreover, the Se–BPC composite presents a durable cycling performance at high rate of 2 C. The high rate capability and high capacity retention of Se–BPC composite cathode can be attributed to the uniform BPC with high surface area and bimodal porous structure. Such BPC, combining unique high surface area, large pore volume and bimodal porous (micro@meso-porous) framework can effectively disperse and confine selenium, and suppress the diffusion of dissolved polyselenides. Moreover, the bimodal porous framework can supply facile transport channels for Li + ions during the electrochemical process. Consequently, the BPC would be a promising carbon matrix to develop high performance lithium–selenium batteries. Acknowledgment The authors thank the financial support of the Teacher Research Fund of Central South University (2013JSJJ027), Scientific Research Innovation Program for Postgraduate of Hunan Province (CX2014B084) and the Strategic Emerging Industries Program of Shenzhen, China (JCYJ20140509142357195). We also thank the support of the Engineering Research Center of Advanced Battery Materials, the Ministry of Education, China. References
Fig. 6. Cycling performance of the Se–BPC composite cathode at different current density (a). The rate capability of the Se–BPC composite cathode (b).
and bimodal porous structure. The high surface area (2100.4 m2 g−1) can provide large contact area with selenium, and favor the selenium highly dispersed in BPC. The bimodal porous structure can provide channels to facilitate selenium infiltration into the pores to improve the conductivity of selenium, absorb the formed polyselenides, and retard the shuttle effect during the charge–discharge process. Moreover, the bimodal porous structure can supply facile transport channels for Li+ ions during the electrochemical process. Therefore, the high rate capability and high capacity retention of selenium cathode can be enhanced significantly by confining selenium in the bimodal porous carbon with high surface area. 4. Conclusions In conclusion, a novel bimodal porous carbon (BPC) with high surface area was prepared by a simple hydrothermal route and
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