microporous carbon host for sulfur loading toward applications in lithium-sulfur batteries

Journal of Energy Chemistry 23(2014)308–314

Core-shell meso/microporous carbon host for sulfur loading toward applications in lithium-sulfur batteries Juan Zhang, Huan Ye, Yaxia Yin, Yuguo Guo∗ Key Laboratory of Molecular Nanostructure and Nanotechnology, and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China [ Manuscript received November 4, 2013; revised December 5, 2013 ]

Abstract Lithium-sulfur (Li-S) batteries belong to one of the promising technologies for high-energy-density rechargeable batteries. However, sulfur cathodes suffer from inherent problems of its poor electronic conductivity and the shuttling of highly dissoluble lithium polysulfides generated during the cycles. Loading sulfur into porous carbons has been proved to be an effective approach to alleviate these issues. Mesoporous and microporous carbons have been widely used for sulfur accommodation, but mesoporous carbons have poor sulfur confinement, whereas microporous carbons are impeded by low sulfur loading rates. Here, a core-shell carbon, combining both the merits of mesoporous carbon with large pore volume and microporous carbon with effective sulfur confinement, was prepared by coating the mesoporous CMK-3 with a microporous carbon (MPC) shell and served as the carbon host (CMK-3@MPC) to accommodate sulfur. After sulfur infusion, the as-obtained S/(CMK-3@MPC) cathode delivered a high initial capacity of up to 1422 mAh·g−1 and sustained 654 mAh·g−1 reversible specific capacity after 36 cycles at 0.1 C. The good performance is ascribed to the unique core-shell structure of the CMK-3@MPC matrix, in which sulfur can be effectively confined within the meso/microporous carbon host, thus achieving simultaneously high electrochemical utilization. Key words core-shell structure; microporous carbon coating; mesoporous carbon; lithium-sulfur batteries; sulfur cathode

1. Introduction Rechargeable batteries with high energy densities and long-lasting life have elicited considerable research attentions because of the urgent demand for energy storage devices for electric vehicles, as well as interest in smart power grids [1−5]. Among the various types of rechargeable batteries, lithium-sulfur batteries are promising owing to their high theoretical energy density of 2600 Wh·kg−1 calculated on a Li anode (approximately 3860 mAh·g−1) and a sulfur cathode (approximately 1675 mAh·g−1) [6−8]. Despite extensive investigations on the Li-S batteries for several decades, the Li-S batteries are still fatigued with a variety of problems that hinder their commercial utilization. The problems include poor electronic/ionic conductivity of elemental sulfur, causing low utilization of the active material, as well as the high solubility of polysulfides (Li2 Sn , 46n68), which are sulfur reduction intermediates in liquid organic electrolytes. The dissolved

polysulfide ions shuttle between the sulfur cathode and the lithium anode, causing precipitation of insoluble and insulating Li2 S2 /Li2 S on the surface of the electrodes. These issues result in low utilization of the active material, poor cycle life, and low Coulombic efficiency [9−11]. Porous carbon materials have been proved to be effective and facile candidates to solve the above problems because of their excellent electrical conductivity and the electrochemical affinity with sulfur [12−15]. Among them, ordered mesoporous carbons (OMCs) are promising carbon matrixes for sulfur accommodation, based on their large pore volume and highly ordered pore structure. The former facilitates a high sulfur loading rate and the latter is favorable for homogenous sulfur infusion. CMK-3, which has been successfully utilized to host sulfur in Li-S batteries by Nazar and colleagues [16], possesses a large pore volume (2.1 cm3 ·g−1 ), with a uniform pore size of 3.3 nm, and consequently accommodates sulfur with a high loading rate of up to 70%. The as-obtained sulfur-CMK-3 composite (S/CMK-3) manifests high sulfur

Corresponding author. Tel: +86-10-82617069; Fax: +86-10-82617069; E-mail: [email protected] This work was supported by the National Natural Science Foundation of China (Grant No. 51225204, 91127044, U1301244 and 21121063), the National Key Project on Basic Research (Grant No. 2011CB935700, 2013AA050903 and 2012CB932900), and the “Strategic Priority Research Program” of CAS (Grant No. XDA09010300). ∗

Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60152-2

Journal of Energy Chemistry Vol. 23 No. 3 2014

utilization. However, the problem of polysulfide dissolution still exists, arising from the open pore architecture of CMK-3, which leads to an obvious capacity decay of approximately 10% for the first 20 cycles. To overcome this issue, various coating shells, such as PEO (polyethylene oxide), conducting polymers and SiOx , were attached to OMCs [16−21]. Although these coating shells have confinement on polysulfides, they usually have poor electronic conductivities. As a result, these coated S/CMK-3 composites show an increased interfacial charge transfer resistance [22]. Therefore, the ideal coating material should not only function as a block layer, but also be ionically and electronically conductive. Apart from mesoporous carbons, microporous carbon materials have also been investigated as the loading hosts for sulfur in Li-S batteries. Compared with mesoporous carbons, microporous carbons show much improved confinement for polysulfides. It is worth to note that chain-like sulfur molecules stored in microporous carbons with 0.5 nm micropores can completely avoid the formation of soluble polysufide intermediates and thus deliver impressive electrochemical properties with high capacity, good cycle stability, and superior rate capability [23]. However, the implementation of microporous carbons in Li-S batteries is limited by their small pore volume, which will decrease the loading amount of sulfur, thus reducing the specific capacity of the entire S/C composite. Herein, we report a meso/microporous carbon material with a core-shell structure by coating CMK-3 with a thin microporous carbon layer, which acts as an ideal carbon host to contain sulfur for Li-S batteries. On the one hand, the inner large mesopores provide sufficient space to gain a high sulfur loading rate, and on the other hand, the outer microporous carbon shell can effectively constrain polysulfide dissolution towards the electrolyte. Owing to the unique core-shell structure of S/(CMK-3@MPC), the cathode can achieve high reversible capacity and stable cycle performance. 2. Experimental 2.1. Synthesis of CMK-3@MPC carbon host The carbon host of CMK-3@MPC was prepared for subsequent sulfur accommodation. Typically, silica template of rod-like SBA-15 was first prepared as described in the literature [16]. Then, a carbon precursor (CP) was filled inside the mesopores of SBA-15 via a solution-evaporation impregnation to obtain the SBA-15@CP. Thereafter, the carbon-coating process was performed on the SBA-15@CP. In a typical synthesis, 200 mg of SBA-15@CP was ultrasonically dispersed in 45 mL of aqueous solution containing 400 mg of D-glucose for 4 h to form a homogenous brown suspension. Then, the suspension was sealed in a 70 mL Parr autoclave with a quartz tube and heated at 180 ◦ C for 15 h. After the hydrothermal reaction, the dark-brown precursor was collected by centrifu-

309

gation, washed with deionized water thrice, and then dried at 60 ◦ C overnight. The dried precursor was calcined under argon at 800 ◦ C for 1 h, with a heating rate of 3 ◦ C·min−1 and a flow rate of 50 mL·min−1. To remove the SBA-15 template, the composite was stirred in a 5% HF solution at room temperature for 24 h to obtain the CMK-3@MPC composite. For comparison, mesoporous carbon of CMK-3 without any coating shell was prepared similarly [16]. 2.2. Synthesis of S/(CMK-3@MPC) composite Sulfur and the as-prepared CMK-3@MPC carbon host were thoroughly mixed according to a mass ratio of 7 : 3. Then, the mixture was put into a glass tube, with both sides sealed, and heated at 400 ◦ C for 10 h to obtain the S/(CMK3@MPC). For comparison, a sulfur-CMK-3 (S/CMK-3) composite was prepared through the same sulfur-infiltration process. 2.3. Materials characterization SEM (6701F, operating at 10 kV), TEM (Tecnai F20) and EDX elemental mapping (Tecnai F20) were used to investigate the morphologies, particle sizes, structure and elemental compositions of the materials. XRD analysis of the asobtained samples was carried out with a Rigaku D/max-2500 ˚ operated at 40 kV and with Cu Kα radiation (λ = 1.54056 A) 200 mA. Thermogravimetric analysis of S/(CMK-3@MPC) was applied on TG/DTA 6300 to obtain the sulfur content in the composite. Nitrogen adsorption and desorption isotherms were measured at 77.3 K on an Autosorb-1 specific surface area analyzer from Quantachrome Instruments. 2.4. Electrode fabrication and electrochemical measurements Electrochemical measurements were performed using Swagelok-type cells assembled in an argon-filled glovebox. To prepare S/(CMK-3@MPC) electrodes, a mixture of an active material, super-P acetylene black and poly-(vinyl difluoride) at a weight ratio of 70 : 20 : 10 was pasted on an Al foil. The sulfur cathode had a diameter of 1 mm and an active material load of approximately 1 mg·cm−2. Lithium foil was used as the anode. The organic electrolytes comprised solutions of Bis-(trifluoromethane) sulfonimide lithium (LiTFSI) (99.95% trace metals basis) in a mixed solvent of 1,3-dioxolane (DOL) and dimethoxyethane (DME) with a volume ratio of 1 : 1 (purchased from Zhangjiagang GuotaiHuarong New Chemical Materials Co. Ltd.). Galvanostatic cycling of the assembled cells was carried out using an Arbin BT2000 system in the voltage range of 1.0 V to 3.0 V (vs. Li+ /Li). Cyclic voltammetry (CV) measurements were performed on an Autolab PG302N with a scanning rate of 0.1 mV·s−1 in the potential range of 1.0 V to 3.0 V (vs. Li+ /Li).

310

Juan Zhang et al./ Journal of Energy Chemistry Vol. 23 No. 3 2014

3. Results and discussion 3.1. Characterization of the materials The structural design of the S/(CMK-3@MPC) is shown

in Scheme 1. The core-shell carbon host was constructed by attaching a continuous microporous carbon layer onto a mesoporous carbon core. Then, sulfur is integrated into the meso/micropores of the carbon host to obtain the S/(CMK3@MPC).

Scheme 1. Schematic for the structural design of S/(CMK-3@MPC)

Figure 1(a) shows the scanning electron microscopy (SEM) image of the rod-like SBA-15@CP composite. All particles of the SBA-15@CP have a mean length of approximately 800 nm, a diameter of approximately 500 nm, and a rough surface (Figure 1b). After the carbon coating procedure, both the mean length and the diameter increased to approximately 1.2 µm and 550 nm, respectively (Figure 1c). The surface of the carbon-coated composite became smooth and the core-shell structure can be clearly identified in Figure 1(d). In the inset of Figure 1(d), both the outer carbon layer and the inner ordered silica wall are visible, and the thickness of the carbon shell is approximately 25 nm. After the removal of

the silica template, the meso/microporous carbon host (CMK3@MPC) was obtained. As shown in Figure 1(f), the MPC carbon layer remained uniform and continuous outside the ordered channels of the CMK-3. The pore structure of the CMK-3@MPC was further characterized by the nitrogen adsorption/desorption technique. Figure 2(a) exhibits typical features of type IV isotherms with a Brunauer-Emmett-Teller (BET) surface area of 1054.33 m2 ·g−1 . Pore size distribution (DFT adsorption branch) in Figure 2(b) shows that the carbon material has mesopores of 3.24 nm, along with micropores of 1.2 nm and 0.56 nm. The combination of mesopores and micropores in

Figure 1. SEM and TEM images of SBA-15@CP (a, b), carbon-coated SBA-15@CP (c, d), and CMK-3@MPC (e, f)

Journal of Energy Chemistry Vol. 23 No. 3 2014

the carbon host suggests that sulfur can be hosted into these two types of nanopores. The cumulative pore volume for the carbon host was calculated to be 1.021 cm3 ·g−1 , which corresponded to a theoretical sulfur loading amount of 68 wt%

311

in the composite (based on the density of sulfur). Although the pore volume of mesoporous carbon is not as high as reported in literature [16], the value can be further increased by the subsequent work.

Figure 2. N2 adsorption/desorption isotherms (a) and pore size distribution plots (b) of CMK-3@MPC and S/(CMK-3@MPC)

Figure 3. SEM (a) and TEM images (b) of S/(CMK-3@MPC); EDX spectrum (c), annular dark-field TEM (d), elemental mapping of carbon (e) and sulfur (f) in S/(CMK-3@MPC)

To achieve better encapsulation of sulfur, a sulfurimpregnation process was performed at 400 ◦ C to obtain the S/(CMK-3@MPC) composite. After sulfur infiltration, both the rod-like morphology and the core-shell structure maintained intact, and no obvious bulk sulfur appeared in the SEM

image (Figure 3a), indicating that sulfur has been fully incorporation into the CMK-3@MPC matrix. Additionally, the result of energy-dispersive X-ray spectroscopy (EDX) evidenced the presence of C and S compositions for the S/(CMK3@MPC), besides the compositions of Cu and O for the grid

312

Juan Zhang et al./ Journal of Energy Chemistry Vol. 23 No. 3 2014

holder (Figure 3c). Figure 3(d–f) shows the highly homogeneous dispersion of nanoscopic sulfur in the CMK-3@MPC host. X-ray diffraction (XRD) patterns (Figure 4a) show no typical peaks of crystalline sulfur in the resultant S/(CMK3@MPC), indicating that sulfur dispersed in the carbon host was amorphous. In addition, the significant decrease in pore volume (from 1.02 cm3 ·g−1 to 0.16 cm3 ·g−1 ) and sharp re-

duction in pore intensity at meso/micropores also confirmed good sulfur encapsulation in the carbon host. Thermogravimetric analysis (TGA) of the S/(CMK-3@MPC) was performed under nitrogen atmosphere to determine the sulfur content. Figure 4(b) shows a weight loss of approximately 54%, suggesting that the S/(CMK-3@MPC) composite hosted up to 54 wt% sulfur.

Figure 4. (a) XRD patterns of crystalline sulfur powder and S/(CMK-3@MPC) composite, (b) TGA curve of S/(CMK-3@MPC)

3.2. Electrochemical performance of the materials To evaluate the electrochemical properties of the S/(CMK-3@MPC), the composite was assembled into Li-S batteries. The S/CMK-3 composite was also tested for comparison. All capacity values were calculated on the basis of sulfur mass. Figure 5(a) shows CV curves of the S/(CMK3@MPC) cathode. Two distinct reduction peaks and an oxidation peak are clearly observed, which are consistent with the electrochemical behavior of sulfur in literature [23]. During the first cathodic scan, two reduction peaks positioned around 2.36 V and 2.0 V were observed, indicating the twostep reduction of sulfur. The reduction peak at 2.36 V can be assigned to open-ring reduction of S8 to soluble polysulfides (Li2 Sn , 46n68), whereas the following peak at 2.0 V is ascribed to the conversion of polysulfides to insoluble Li2 S2 and then to Li2 S. In the subsequent anodic scan, only one sharp oxidation peak was observed at approximately 2.4 V, which is attributed to the oxidation of Li2 S and Li2 S2 to S8 [24]. In the second cycle, no significant shift of redox peaks was observed, suggesting good cycle performance of the S/(CMK3@MPC) electrode. Figure 5(b) presents the initial galvanostatic discharge/charge (GDC) voltage profiles of S/(CMK-3@MPC) and S/CMK-3 electrodes at 0.1 C rate (1 C = 1675 mA·g−1 ) between 1.0 V and 3.0 V. Similar to the literature [16,20], the discharge profile of S/CMK-3 involves two distinct discharge plateaus at around 2.4 V and 2.0 V. The discharging profile of the S/(CMK-3@MPC) is similar to that of the S/(CMK3) except for the sloped discharge voltage curve below 2.0 V.

This segment can be ascribed to the electrochemical behavior of chain-like sulfur stored in the micropores of the MPC shell [25,26]. The S/CMK-3 composite exhibits a discharge capacity of 1290 mAh·g−1 (based on the mass of sulfur, and the same below), whereas the S/(CMK-3@MPC) delivers a higher initial discharge capacity of 1423 mAh·g−1. The result indicates that the conductive microporous carbon shell can not only host a portion of the sulfur, but also supply more electronic contact sites to the insulating sulfur. Therefore, sulfur encapsulated in CMK-3@MPC achieves high sulfur utilization, being derived from the unique core-shell carbon framework [27,28]. After the first cycle, a rapid decay of capacity was observed for the S/CMK-3 cathode. The reversible capacity was only 768 mAh·g−1 in the second cycle and decreased to 398 mAh·g−1 in the 36th cycle (Figure 5c). This result is consistent with previous reports [16,20], suggesting that the strategy to store sulfur in CMK-3, an outward open structure, is difficult to confine soluble polysulfides, thus leading to an inevitable loss of the active material. In contrast, the S/(CMK-3@MPC) composite, with an effective MPC coating, maintains a high capacity of 1200 mAh·g−1 in the second cycle. Furthermore, the good overlaps of the discharge plateaus in the subsequent cycles (Figure 5d) suggest good stability and reversibility of the S/(CMK-3@MPC) cathode. At the 36th cycle, the discharge capacity could still maintain at 654 mAh·g−1, and the Coulombic efficiency was greatly improved (inset of Figure 5c). Therefore, the much improved cycle stability of the S/(CMK-3@MPC) composite benefits from the introduction of the MPC shell, which successfully prevents the outlet of lithium polysulfides dur-

Journal of Energy Chemistry Vol. 23 No. 3 2014

ing discharge/charge processes. In addition, the microporous carbon shell can accommodate chain-like sulfur molecules, which has been proved to possess good capacity retention

313

[25,26]. Consequently, derived from the distinctive core-shell structure, the S/(CMK@MPC) composite can achieve high sulfur utilization and stable capacity retention.

Figure 5. Electrochemical performances of the sulfur-carbon cathodes: (a) the first two cyclic voltammograms of the S/(CMK-3@MPC) cathode at a scanning rate of 0.1 mV·s−1 , (b) initial GDC voltage profiles, (c) cycle performances (discharge capacity) and Columbic efficiencies (inset image) of S/(CMK-3) and S/(CMK-3@MPC) cathodes at 0.1 C, (d) GDC voltage profiles of the subsequent cycles of the S/(CMK-3@MPC) electrode at 0.1 C

4. Conclusions In summary, we have constructed a novel carbon matrix (CMK-3@MPC) with a core-shell meso/microporous structure to host sulfur for application in Li-S batteries. The asprepared S/(CMK-3@MPC) cathode delivered a much higher initial specific capacity of up to 1422 mAh·g−1 and had more stable capacity retention compared with its counterpart of no coating. The excellent performance of the S/(CMK-3@MPC) cathode can be exclusively attributed to the merits of the carbon matrix with a highly ordered mesoporous core and a continuous microporous shell as follows. First, the inner ordered mesoporous carbon enables high sulfur loading rate and homogenous sulfur dispersion. Second, the outer MPC shell not only functions as a blocked layer to confine polysulfides but also serves as a microporous host for encapsulated chainlike sulfur. Third, the conductive core-shell carbon framework provides sufficient electronic contacts to insulate sulfur, thereby facilitating high electrochemical utilization of the

sulfur. All these advantages guarantee good electrochemical performance with high specific capacity and stable cyclability of the S/(CMK-3@MPC). Considering the unique structural characteristics of the CMK-3@MPC matrix, it would be of vital interest to extend this strategy to other cathode materials (e.g., Se) with an electrochemical reaction similar to sulfur. References [1] Bruce P G. Solid State Ionics, 2008, 179: 752 [2] Scrosati B, Garche J. J Power Sources, 2010, 195: 2419 [3] Dong S M, Chen X, Zhang K J, Gu L, Zhang L X, Zhou X H, Li L F, Liu Z H, Han P X, Xu H X, Yao J H, Zhang C J, Zhang X Y, Shang C Q, Cui G L, Chen L Q. Chem Commun, 2011, 47: 11291 [4] Ji H X, Wu X L, Fan L Z, Krien C, Fiering I, Guo Y G, Mei Y F, Schmidt O G. Adv Mater, 2010, 22: 4591 [5] Xu J J, Xu D, Wang Z L, Wang H G, Zhang L L, Zhang X B. Angew Chem-Int Ed, 2013, 52: 3887 [6] Bruce P G, Freunberger S A, Hardwick L J, Tarascon J M. Nat

314

Juan Zhang et al./ Journal of Energy Chemistry Vol. 23 No. 3 2014

Mater, 2012, 11: 19 [7] Ellis B L, Lee K T, Nazar L F. Chem Mater, 2010, 22: 691 [8] Ji X L, Nazar L F. J Mater Chem, 2010, 20: 9821 [9] Cheon S E, Ko K S, Cho J H, Kim S W, Chin E Y, Kim H T. J Electrochem Soc, 2003, 150: A800 [10] Cheon S E, Ko K S, Cho J H, Kim S W, Chin E Y, Kim H T. J Electrochem Soc, 2003, 150: A796 [11] Mikhaylik Y V, Akridge J R. J Electrochem Soc, 2004, 151: A1969 [12] Ji L W, Rao M M, Aloni S, Wang L, Cairns E J, Zhang Y G. Energy Environ Sci, 2011, 4: 5053 [13] Rao M M, Li W S, Cairns E J. Electrochem Commun, 2012, 17: 1 [14] Zhang C F, Wu H B, Yuan C Z, Guo Z P, Lou X W. Angew Chem-Int Ed, 2012, 124: 9730 [15] Zhang K, Hu Z, Chen J. J Energy Chem, 2013, 22: 214 [16] Ji X L, Lee K T, Nazar L F. Nat Mater, 2009, 8: 500 [17] Li G C, Li G R, Ye S H, Gao X P. Adv Energy Mater, 2012, 2:

1238 [18] Qiu L L, Zhang S C, Zhang L, Sun M M, Wang W K. Electrochimica Acta, 2010, 55: 4632 [19] Wu F, Chen J Z, Li L, Zhao T, Chen R J. J Phys Chem C, 2011, 115: 24411 [20] Yang Y, Yu G H, Cha J J, Wu H, Vosgueritchian M, Yao Y, Bao Z A, Cui Y. ACS Nano, 2011, 5: 9187 [21] Schuster J, He G, Mandlmeier B, Yim T, Lee K T, Bein T, Nazar L F. Angew Chem-Int Ed, 2012, 51: 3591 [22] Zhang S S. J Power Sources, 2013, 231: 153 [23] Wang X F, Fang X P, Guo X W, Wang Z X, Chen L Q. Electrochim Acta, 2013, 97: 238 [24] Liang C D, Dudney N J, Howe J Y. Chem Mater, 2009, 21: 4724 [25] Xin S, Gu L, Zhao N H, Yin Y X, Zhou L J, Guo Y G, Wan L J. J Am Chem Soc, 2012, 134: 18510 [26] Ye H, Yin Y X, Xin S, Guo Y G. J Mater Chem A, 2013, 1: 6602 [27] Cao A M, Hu J S, Wan L J. Sci China-Chem, 2012, 55: 2249 [28] Xin S, Guo Y G, Wan L J. Acc Chem Res, 2012, 134: 18510