- Email: [email protected]
A long-life Li–S battery enabled by a cathode made of well-distributed B4C nanoparticles decorated activated cotton fibers
Journal of Power Sources 451 (2020) 227751
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A long-life Li–S battery enabled by a cathode made of well-distributed B4C nanoparticles decorated activated cotton fibers Ruihan Zhang, Cheng Chi, Maochun Wu, Ke Liu, Tianshou Zhao * HKUST Energy Institute, Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
H I G H L I G H T S
� B4C-ACF cathode substrate is successfully developed for Li–S batteries. � B4C-ACF substrate shows good catalytic activity to polysulfides conversion. � B4C-ACF substrate hold strong binding ability to polysulfides. � The Li–S battery with B4C-ACF substrate is operated for 3000 cycles at 1 C. A R T I C L E I N F O
A B S T R A C T
Keywords: Li-S batteries Shuttle effect Boron carbide nanoparticles Long-term cycling lifespan Chemical interactions
The commercialization of Li–S batteries is impeded by their short lifespan and poor rate capability resulting from the shuttle effect and sluggish reaction kinetics. Herein, we develop a bi-functional cathode substrate for Li–S batteries made of well-distributed boron carbide nanoparticles decorated activated cotton fiber (B4C-ACF), in which B4C nanoparticles serve as not only robust chemically anchoring sites to trap the polysulfides, but also afford abundant active sites for efficient sulfur conversion reactions. Meanwhile, the ACF network provides fast electron pathways for the conversion reactions and acts as the current collector. As a result, the novel cathode substrate enables a Li–S battery to deliver an initial capacity of as high as 1415 mAh g 1 at 0.1 C, and an extrahigh reversible capacity of 928 mAh g 1 at 3 C with an areal sulfur loading of 3.0 mg cm 2. More strikingly, the B4C-ACF substrate-based battery could be stably operated for 3000 cycles with a high coulombic efficiency of 99.24% and a capacity decay rate of as low as 0.012% per cycle at 1 C, demonstrating that the B4C-ACF substrate holds great potential in realizing the mass production of advanced Li–S batteries.
1. Introduction High-energy storage technologies have been attracting extensive interests in recent decades due to the skyrocketing demand for efficient and affordable energy storage systems in stationary electricity storage, portable electronic devices and transportation (such as cars, aircrafts and ships) [1–3]. However, the state-of-the-art lithium-ion (Li-ion) batteries are unable to meet the ever-increasing needs in these markets owing to the limited potential in further increase of their energy den sities [4,5]. Hence, considerable efforts have been devoted to developing advanced battery systems with higher theoretical energy density, such as zinc-air (Zn-air) batteries [6–10], lithium-air (Li-air) batteries [11–16] and lithium-sulfur (Li–S) batteries [17–20]. Among them, the Li–S batteries have drawn much attention because of their extra-high
theoretical energy density (2567 Wh kg 1), cost-effectiveness and eco-friendliness [21,22]. In a typical Li–S battery, the lithium metal acts as the anode while the sulfur (S8) is chosen as the cathode, which are separated by a porous membrane. The redox reactions between lithium and sulfur could be mainly depicted as 16Liþ þ S8 þ 16e ↔ 8Li2S (E� ¼ 2.2 V, vs Li/Liþ), in which sulfur delivers a high theoretical specific capacity of 1675 mAh g 1, five times higher than that of the cathodes (300 mAh g 1) in the commercial LIBs [23–25]. However, before the commercialization of this technology, there are several challenging is sues need to be addressed. For example, the insulator nature of S8 and the discharge product (Li2S) lead to a low utilization of active materials and poor reaction kinetics. The shuttle effect resulting from soluble long-chain lithium polysulfides (LiPSs) in organic electrolyte results in low coulombic efficiencies and severe capacity decay. Worse still, the
* Corresponding author. E-mail address: [email protected] (T. Zhao). https://doi.org/10.1016/j.jpowsour.2020.227751 Received 5 November 2019; Received in revised form 25 December 2019; Accepted 12 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
relatively large volume expansion of sulfur after lithiation easily causes the continual volume variation in cathode during discharge/charge process and eventually pulverizes the whole cathode architecture [26–28]. To tackle these challenges and realize the practical application of Li–S batteries, numerous pioneering contributions have been made during the last several years. Carbon-based materials such as porous carbon with micro-/meso-/macro-pores [29–31], carbon nanotubes (CNTs) [32,33], graphene [34–36] and carbon fibers [37,38] are firstly applied as physical barriers to encapsulate sulfur as well as to provide efficient electron transport due to their excellent electrical conductivity, low weight density and various porous structures. However, the non-polar inherent feature makes the carbonaceous hosts insufficient to entrap the polar LiPSs during the long-term cycling operation, especially for high-loading Li–S batteries [1,2,20,39]. In order to improve the cycling stability as well as obtain the high capacity retention rate simultaneously, numerous novel polar substrates, including metal car bides [40–44], sulfides [45–47], and oxides [17,18,48], are introduced for Li–S batteries, which exhibit high binding energy toward the LiPSs and form strongly Lewis acid-based interactions or other chemical bonds with LiPSs. Among these reported inorganic polar materials, the metal carbides such as WxC [49,50], Mo2C [49] and TixC [42–44,49] show great potential to improve the electrochemical performance of Li–S batteries due to their excellent metallic electrical conductivity, promi nent positive activities toward sluggish conversion reactions between sulfur species, and superb ability to anchor the LiPSs. However, the weight densities of these metal carbides are much heavier than that of sulfur, leading to a higher weight fraction occupied by the metal car bides and a relative lower sulfur content in the prepared cathodes. In this regard, the lightweight boron carbide (B4C) becomes an attractive candidate for Li–S batteries due to its low density of 2.50–2.51 g cm 3, which is comparable to that of the octasulfur (2.07 g cm 3) [51]. Moreover, B4C exhibits an excellent conductivity of 1.25–3.33 S cm 1, superior chemical stability and outstanding positive activity [52–54], which has been widely applied in Li-ion batteries [55], Li-air batteries [56] and fuel cells [57]. Very recently, B4C nanowires as part of cathode substrates for Li–S batteries were reported by Manthiram’s and Li’s groups [53,54], with which the Li–S batteries showed enhanced elec trochemical performance. In addition, the pouch-cell and flexible type Li–S batteries were proposed and tested in their works, respectively. However, the reported Li–S batteries still suffer from a severe capacity loss and thus a low cycling life. Herein, we fabricate a novel positive electrode substrate made of uniform B4C nanoparticles decorated activated cotton fibers (B4C-ACF) for Li–S batteries by a catalyst-assisted fabrication method. The B4C-ACF substrate exhibits an ultrahigh Brunauer-Emmett-Teller (BET) specific surface area of 224.51 m2 g 1 with plentiful mesopores. With this novel substrate, the Li–S batteries deliver a highly reversible capacity of 1415 mAh g 1 at 0.1 C, and an outstanding rate capability with the current density increasing from 0.1 C to 3 C with an areal sulfur loading of 3.0 mg cm 2. In addition, the B4C-ACF substrate enables the Li–S battery to stably cycle for 500 times with a retention capacity of as high as 988 mAh g 1 at 0.5 C, which is four times higher than that of the battery with pure ACF substrate (192 mAh g 1 after 500 cycles). For long-term cycling test, the B4C-ACF based Li–S battery with the areal sulfur loading of 3.0 mg cm 2 displays a remarkable cycling stability after 3000 cycles at the current density of 1 C with an average coulombic efficiency of 99.24%. More remarkably, the capacity decay rate is as low as of 0.012% per cycle, which demonstrates the great promise of B4CACF as electrode substrates for the commercially viable Li–S batteries.
catalyst-assisted method [58,59], in which the nickel nitrate (Ni (NO3)2⋅6H2O, Sigma-Aldrich, 99.99%) acts as the catalyst while the amorphous boron powders (Sigma-Aldrich, 95%) and commercially cotton fibers are chosen as the boron and carbon sources, respectively. For the typical procedure, the cotton fibers are firstly washed by acetone (Sigma-Aldrich, 99.9%), absolute alcohol (Sigma-Aldrich) and distilled water (DI water) alternately for several times, and dried under a vacuum oven at 100 � C for 24 h. Meantime, 0.5 g Ni(NO3)2⋅6H2O and 0.4 g boron powders are dispersed in 100 mL of absolute alcohol under continuously ultrasonication until the Ni–B emulsion is formed. Then, the 1.2 g pre pared cotton fibers are soaked in the emulsion under ultrasonic stirring for 4 h and dried in a preheated vacuum oven at 100 � C for 4 h. After wards, the dried samples are placed in a horizontal tube furnace and sintered at 1180 � C for 6 h with a heating rate of 2 � C min 1 under argon. For the control group, the ACF sample is prepared by directly curing cotton fibers after the first step in tube furnace under the same heating procedure without the immersing process in the Ni–B emulsion. The sulfur powders are successfully loaded on the prepared substrates by a conventional melt-diffusion method, in which sulfur powders and the B4C-ACF or ACF substrates are sealed in a 50 mL Teflon-lined stainless autoclave under argon atmosphere and heated at 155 � C for 12 h. The obtained samples are depicted as S/B4C-ACF and S/ACF electrodes, respectively. The sulfur contents in the prepared electrodes are controlled by the proper mole ratio of sulfur powders and the substrates, and the areal sulfur loading of the as-prepared cathode is around 3.0 mg cm 2. The Micro-Raman spectrophotometer (Renishaw RM 3000) and Power X-ray diffraction system (XRD, model PW 1825) measurements are carried out to inspect the crystal structure and composition of these prepared samples and electrodes. And the morphology and crystallinity are investigated by the scanning electron microscopy (SEM, JEOL7100F), high-resolution transmission electron microscopy (HRTEM, JEOL 2010) and energy-dispersive X-ray spectroscopy (EDX) elemental mapping. The sulfur contents in the prepared electrodes are examined by the thermogravimetric analysis (TGA), which was accomplished on a TGA Q5000 (TA instruments) under nitrogen atmosphere from 30 to 500 � C with a heating rate of 5 � C min 1. The BET specific surface areas of these substrates are analyzed by the nitrogen adsorption-desorption test. X-ray photoelectron spectroscopy (XPS, PHI 5600) measurement is conducted to clarify the chemical interactions formed between the B4C and LiPSs. 2.2. Battery assembly and electrochemical measurements CR2032-type coin cells are assembled in an argon-filled glovebox (Etelux, Lab 2000, H2O � 1 ppm, O2 � 1 ppm) to appraise the electro chemical properties of the as-prepared electrodes. The S/B4C-ACF and S/ACF electrodes are directly cut into discs with a diameter of 14 mm to serve as sulfur cathode, while the lithium metal foil with a diameter of 16 mm and the Celgard 2400 membrane with a diameter of 19 mm are chosen as the anode and separator, respectively. During the assembling process, 10 μL of the organic electrolyte is added into each cell, which is composed with 1 M lithium bis(trifluoromethane)sulfonamide (LiTFSI, Sigma-Aldrich, 99.95%) salt dissolved into a mixture solvent of dime thoxy methane (DME, Sigma-Aldrich, 99.9%) and 1,3-dioxolane (DOL, Sigma-Aldrich, 99.9%) by a 1:1 vol ratio. Cyclic voltammetry (CV) measurements of the coin cells are carried out by a potentiostat elec trochemical station (PARSTAT 2273) at a scan rate of 0.1 mV s 1 within the electrochemical window of 1.70–2.8 V (vs. Li/Liþ) to examine the positive effects of the as-synthesized substrates towards the sulfur con version reactions. The galvanostatic discharge/charge profiles and the cycling performance of these prepared coin cells are conducted in an 8channel battery tester (Neware, CT-3008W) at different current den sities. During the long-term cycling test, the electrochemical impedance spectrum (EIS) results of the testing cells are performed using the PARSTAT 2273 station in a frequency of 100 kHz to 10 mHz with an
2. Experimental section 2.1. Electrodes preparation and materials characterization In this work, the B4C-ACF is fabricated by a previously reported 2
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
alternating current (AC) perturbation of 5 mV at the open-circuit voltage. To detect the absorption ability of B4C towards the LiPSs, the as-prepared B4C-ACF substrate is directly cut into 14 mm discs and then immersed into the 0.025 M Li2S6 solution prepared by dissolving the solid S8 and Li2S with a proper mole ratio in the DOL/DME (1:1, by volume) cosolvent in the argon-filled glovebox.
because the cotton fibers act as not only the substrate but also the carbon source, which would react with boron powders to form the B4C nano particles during the sintering process, leading to the finally porous and rough surface morphology in the B4C-ACF sample. The compositions of these as-prepared samples are verified by XRD measurement, and the results are displayed in Fig. 1g. For the pure ACF sample, two diffraction peaks located at 26.2 and 44.5� are observed in the XRD spectra from 10 to 80� , which are attributed to the (002) and (100) characteristic peaks of graphite, respectively [53,60]. These two peaks are also detected in the XRD patterns of B4C-ACF sample, indi cating the high degree of graphitization of cotton fibers in these as-prepared samples after calcination, which can provide the high electrical conductivity for sulfur conversion reactions in Li–S batteries. In the XRD spectra of B4C-ACF sample, seven additional peaks are also identified, which are ascribed to the (101), (003), (012), (110), (104), (021) and (211) diffraction peaks of B4C (JCPDS #75–0424), respec tively [53,61]. No other peaks of impurities in the B4C-ACF sample are detected, confirming that the pure and well crystallized B4C sample has been successfully synthesized in this work. The nitrogen adsorption-desorption isotherm and the pore size distribution of the B4C-ACF sample are displayed in Fig. 1h and i. The BET surface area of B4C-ACF sample is calculated to be ~224.51 m2 g 1, which is seven times higher than that of the pure ACF sample (~27.86 m2 g 1 as pre sented in Fig. S3). In addition, the percentage of mesopores in the
3. Results and discussion 3.1. Characterization of the as-prepared substrates and electrodes The SEM measurement is conducted to inspect the morphologies of the as-prepared ACF and B4C-ACF samples (Fig. 1 and Fig. S1). From the SEM images in Fig. 1a–c, there are plentiful nanoparticles covered on the porous and rough surface of B4C-ACF sample, of which the diameters range from 20 to 50 nm as observed in the HRTEM image (Fig. 1d). The EDX mapping results in Fig. S2 show that the B4C nanoparticles are well distributed on the surface of the cotton fibers, demonstrating that the B4C-ACF sample has been successfully obtained. The well crystallinity of the as-prepared B4C nanoparticles is proved by the clear lattice fringes and SAED results observed in Fig. 1e and f, in which the orderly inter planar spacing of 4.48 Å is corresponding to the characteristic (101) plane of B4C (JCPDS #75–0424) [53,54]. In contrast, the cotton fibers of the pure ACF sample (Fig. S1) show a clean and smooth surface. This is
Fig. 1. (a)–(c) SEM image of as-prepared B4C-ACF sample; (d) HRTEM, (e) lattice fringes and (f) SAED image of as-prepared B4C-ACF sample; (g) XRD spectra of asprepared B4C-ACF and ACF samples; (h) nitrogen adsorption-desorption isotherm and (i) pore size distribution of the as-prepared B4C-ACF sample. 3
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
B4C-ACF sample is much higher than that in the ACF sample, which could provide much more active sites for sulfur conversion reactions in Li–S batteries. The content of B4C in the B4C-ACF sample is examined by heating this sample in air from 30 to 1000 � C on the TA instruments (Fig. S4). During the heating process, the cotton fiber substrate is oxidized to carbon dioxide, while the B4C would react with oxygen to produce the B2O3 and carbon dioxide. Finally, only the B2O3 is left in the residues, based on which the content of B4C in our sample is calculated to be ~13.93 wt%, much lower than that reported in the previous works [53,54]. The absorption tests of the B4C-ACF and ACF substrates towards the polysulfides are carried out by soaking the substrate discs into the diluted Li2S6 solution prepared as mentioned above, and the results are shown in Fig. S5. After static placed for 3 h, the solution with B4C-ACF sample becomes transparent while the solution with pure ACF substrate remains the same as the pristine one, showing that the pure ACF sample holds poor absorption ability towards LiPSs but B4C nanoparticles could strongly anchor the polysulfides during a short period, which is consistent with the previous works. To prepare the S/B4C-ACF and S/ACF electrodes, the sulfur powders are loaded onto the B4C-ACF and ACF substrates by the melt-diffusion method, and their morphologies are shown in Fig. 2 and Fig. S6, respectively. For S/B4C-ACF electrode, the porous and rough surface of pristine B4C-ACF sample is replaced by a relatively smooth one accompanied with some particles after sulfur loading (Fig. 2a and b), demonstrating that the sulfur has fully covered the surface of the B4CACF substrate. Raman measurement is carried out to verify the composition of the prepared S/B4C-ACF electrode, and the results are
shown in Fig. 2c. In the Raman spectra of S/B4C-ACF electrode, five scattering peaks located at 151, 216, 246, 437 and 470 cm 1 are observed, which are assigned to sulfur nanoparticles [18]. Additionally, another five peaks are detected at 533, 726, 818, 1004 and 1088 cm 1, corresponding to the characteristic peaks of crystalized B4C nano particles [61,62]. The two peaks at 1358 and 1597 cm 1 appear in both B4C-ACF substrate and final electrode composite, which are ascribed to the D and G peaks of the graphite [18,53,54]. The EDX mapping result of S element in Fig. 2f confirms that the sulfur is uniformly distributed on the substrate. XPS measurement is performed to investigate the chemical bonds and the elemental composition of S/B4C-ACF electrode. In the spectra of C 1s (Fig. 2g), there are two peaks located at 284.6 and 283.2 eV, which are assigned to C–C and C–B bonds, respectively. The B 1s spectrum (Fig. 2h) reveals the existence of B–B (188.3 eV) and B–C (187.2 eV) bonds, which again confirms the successfully synthesis of B4C nanoparticles [63]. As shown in Fig. 2i, three peaks centered at 163.6, 164.8 and 168.8 eV are detected in the spectra of S 2p, which are ascribed to S 2p3/2, S 2p1/2 and sulfate, respectively. The energy sepa ration between S 2p3/2 and S 2p1/2 peaks is about 0.8 eV, indicating that the sulfur powders have been successfully loaded on the substrate [54]. The sulfate mainly arises from the reaction between the sulfur powders and the oxygen-containing functional groups on the surface of the B4C-ACF substrate during the melt-diffusion process. The sulfur contents in the S/B4C-ACF and S/ACF electrodes are separately calculated to be 74.5 and 73.6 wt% based on the TGA measurement under nitrogen (Fig. S7).
Fig. 2. (a) and (b) SEM images of the S/B4C-ACF sample; (c) Raman spectra of the as-prepared samples; (d) to (f) EDX mapping results of S/B4C-ACF sample; (g) to (i) the XPS results of C 1s, B 1s and S 2p in the S/B4C-ACF electrode. 4
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
3.2. Electrochemical performance
more positive activities of the B4C-ACF electrode for the polysulfide transformation reactions. With the current densities increasing from 0.5 to 3 C, the battery with the S/B4C-ACF electrode displays high initial capacities of 1274, 1165, 1071 and 928 mAh g 1; while relatively low capacities of 1023, 757, 483 and 346 mAh g 1 can only be obtained for the control battery. Furthermore, when the current density reduces back to 0.5 C, the capacity of the battery with the S/B4C-ACF electrode re covers to a high capacity of 1270 mAh g 1, which is 37.8% higher than that of the control group (~790 mAh g 1). These results demonstrate the superior rate capability of the S/B4C-ACF electrode. In order to test the cycling stability of the prepared S/B4C-ACF and S/ACF electrodes, the Li–S batteries are operated from 1.7 to 2.8 V at the current density of 0.5 C with the areal sulfur loading of 3.0 mg cm 1 for 500 cycles. As shown in Fig. 3d, the battery with the S/B4C-ACF electrode still displays an ultrahigh retention capacity of 988 mAh g 1 after 500 cycles, which is about 4.14 times higher than that of the control battery (only ~192 mAh g 1) and even is comparable to the initial capacity (~1023 mAh g 1) of the control group. In the meantime, the average coulombic efficiency of S/B4C-ACF electrode-based battery is calculated to be as high as 99.36%, and the capacity decay rate is only about 0.04% per cycle, indicating that the S/B4C-ACF electrode significantly improves the cycling stability of the Li–S battery. These enhanced electrochemical performances in rate capability and cycling stability of the Li–S battery with the S/B4CACF electrode mainly benefit from the strongly anchoring ability and the highly positive activity of the well-distributed B4C nanoparticles on the porous ACF substrate. The B4C nanoparticles could not only effectively suppress the shuttle effect, but also facilitate the reaction kinetics during high current densities and long-term cycling test. To verify the chemical interactions formed between the B4C nano particles and polysulfides, the XPS and Raman spectroscopy of the S/ B4C-ACF electrode in the battery after cycling are measured and the
CV tests of the Li–S batteries with the S/B4C-ACF and S/ACF elec trodes are conducted at a scan rate of 0.1 mV s 1 within the electro chemical window of 1.7–2.8 V (vs. Li/Liþ), and the profiles are displayed in Fig. 3a. For the S/ACF electrode-based battery, there are two peaks centered at 1.95 and 2.21 V during the cathodic scan, which represent the reduction of S8 to long-chain polysulfides followed by their reduc tion to solid lithium sulfides (Li2S2/Li2S), respectively. In the subsequent anodic scan, only one broad peak located at 2.48 V is observed, which is ascribed to the reverse oxidation reaction of lithium sulfides species to the soluble intermediates and the final elemental sulfur. Two cathodic peaks are also observed for the battery with the S/B4C-ACF electrode, which locate at 2.03 and 2.22 V, respectively, both higher than those of the control group. During the anodic scan, two noticeable peaks are detected with the location of 2.33 and 2.38 V, respectively, which are both lower than that in the battery with S/ACF electrode, leading to a much smaller peak separation between the anodic and cathodic peaks [53]. Furthermore, the peak current densities of the S/B4C-ACF electrode-based battery are higher than those in the control group, indicating that the B4C-ACF substrate exhibits a high positive activity to polysulfide transformation reactions, consistent with the previous works [53,54]. The improved redox kinetics of sulfur conversion reactions are also validated by the excellent rate capability of the S/B4C-ACF electrodebased battery, as shown in Fig. 3b and c. At the current density of 0.1 C, the battery with S/B4C-ACF electrode displays an initial capacity of as high as 1415 mAh g 1, which is 20.5% higher than that in the control group (~1120 mAh g 1). Moreover, the overpotential of the S/B4C-ACF electrode-based battery is only about 180 mV, while the control group displays a much larger overpotential of 300 mV, indicating the much
Fig. 3. (a) CV curves of Li–S batteries with S/B4C-ACF and S/ACF electrodes; (b) the specific capacities of Li–S batteries with S/B4C-ACF electrode from 0.1 to 3 C; (c) the rate capability of Li–S batteries with S/B4C-ACF and S/ACF electrodes; (d) the cycling test of Li–S batteries with S/B4C-ACF and S/ACF electrodes at 0.5 C. 5
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
results are presented in Fig. 4. For the spectra of C 1s (Fig. 4a), the two peaks of C–C and C–B bonds are observed on the same locations with those detected in the electrode before cycling, while one additional peak located at 285.2 eV appears, which is ascribed to the C–S bond [54]. In the spectrum of B 1s (Fig. 4b), an extra peak located at 191.30 eV cor responding to the B–S bond is detected [63], and the original B–B and B–C bonds still exist but their locations shift negatively from 187.8 to 186.9 eV. The S 2p spectra (Fig. 4c) could be divided into three peaks, ascribed to S 2p3/2 (164.3 eV), S 2p1/2 (165.7 eV) and sulfate (169.0 eV), respectively, all of which with a slight positive shift. The appearance of the new C–S and B–S bonds indicates that the chemical interactions have been formed between B4C nanoparticles and LiPSs, proving the strongly anchoring ability of B4C towards soluble polysulfides. The formation of chemical bonds is also confirmed by the Raman measurement (Fig. 4d) from 200 to 1200 cm 1. Before cycling, there are only peaks corre sponding to sulfur and B4C in the S/B4C-ACF electrode. After 500 cycles, four additional peaks located at 393, 443, 772 and 1042 cm 1 are detected, which are assigned to the characteristic peaks of boron sulfide (B2S3) as reported in the previous works [64], further confirming the existence of strong chemical interactions formed between B4C and polysulfides. Due to the excellent performance of the S/B4C-ACF electrode in rate capability and cycling stability, we operate the Li–S battery with this electrode with the areal sulfur loading of 3.0 mg cm 2 at 1 C for ultralong cycling test to examine the potential of this electrode for real izing the practical application of Li–S battery. For the selected cycles (the 1000th, and 2000th cycles, Fig. 5a) during cycling, the battery displays the discharge capacities of as high as 997 and 901 mAh g 1, respectively. Even at the end of the 3000th cycle, the battery could still deliver a high capacity of 740 mAh g 1, which is even much higher than
that of the control group after 500 cycles at 0.5 C. EIS plots of the Li–S battery with the S/B4C-ACF electrode for the selected cycles are pre sented in Fig. 5b. For the fresh cell, the bulk resistance (Rb) and the charge transfer resistance (Rct) are only around 3.32 and 10.40 Ω, respectively, which are both smaller than those of the battery in the previous works [18,53,54], indicating the high electronic conductivity and good charge transfer capability of the S/B4C-ACF electrode. For the selected cycles, the values of Rb are calculated to be 10.36 Ω (1000th cycle), 16.23 Ω (2000th cycle) and 20.12 Ω (3000th cycle), while the values of Rct increase from 21.74, 32.24–45.14 Ω for the selected cycles during cycling, which are much lower than that reported in previous woks. These results demonstrate the S/B4C-ACF electrode could provide efficient electronic conductivity and stable charge transfer for sulfur redox reactions during the long-term cycling operation, which mainly benefit from the much more positive effect of B4C nanoparticles on immobilizing LiPSs and facilitating the redox kinetics of polysulfides conversion reactions during cycling. As shown in Fig. 5c, the S/B4C-ACF electrode enables the battery to stably operate for 3000 cycles with an average coulombic efficiency of as high as 99.24% at the current density of 1 C. More strikingly, the capacity decay rate is calculated to be only about 0.012% per cycle, which is quite lower than that reported in the previous works. The remarkable improvements in cycling stability and capacity retention capability of the Li–S battery under ultra-long life- span indicate that the S/B4C-ACF electrode could be regarded as one of the promising cathode materials for realizing the practical application of Li–S batteries. The morphology changes of the S/B4C-ACF electrode after 2000 and 3000 cycles are examined by the SEM measurement, and the results are shown in Fig. S9 and Fig. 6, respectively. Compared with the pristine S/ B4C-ACF electrode before cycling, the surface of this electrode after
Fig. 4. (a)–(c) the XPS results of C 1s, B 1s and S 2p in the S/B4C-ACF electrode after cycling; (d) Raman spectra of the S/B4C-ACF electrode before and after chargedischarge cycling. 6
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
Fig. 5. (a) The specific capacities of Li–S battery with S/B4C-ACF electrode at 1 C for the selected cycles; (b) EIS plots of the Li–S battery with S/B4C-ACF electrode after the selected charge-discharge cycles; (c) the long-term cycling test of Li–S battery with S/B4C-ACF electrode at 1 C.
2000 and 3000 cycles becomes more smoothly during cycling without any large particles. Instead, abundant nanoparticles connecting with each other are well distributed on the surface after cycling, which are confirmed to be the sulfur nanoparticles by the EDX mapping result of S element in Fig. 6b accompanying with the Raman spectra in Fig. 4d. The morphology change of the sulfur mainly arises from two factors: the strongly binding ability of B4C nanoparticles towards polysulfides that could anchor the intermediates tightly on the ACF substrate and sup press the shuttle effect efficiently, and the highly positive activity of B4C for polysulfides transformation which facilitates the redox kinetics to obtain the uniform participations at the end of charge. The EDX mapping result of B in Fig. 6e and the existence of B4C in the Raman spectra in Fig. 4d demonstrate the high stability of the S/B4C-ACF electrode during long-term cycling, suggesting the great potential of this electrode for realizing the practical application of Li–S batteries.
4. Conclusions In summary, a B4C-ACF cathode substrate is successfully developed for Li–S batteries, in which the ACF textile acts as the electron pathway as well as the current collector, while the B4C nanoparticles serve as anchoring sites to trap polysulfides and positive sites to accelerate sulfur conversion reactions. The B4C-ACF substrate is proved to hold higher absorption ability towards polysulfides and more positive activities for polysulfides transformation than the pure ACF sample. As a result, a Li–S battery with the S/B4C-ACF electrode exhibits an ultrahigh capacity of 1415 mAh g 1 at 0.1 C with a low overpotential of 180 mV and an extrahigh reversible capacity of 928 mAh g 1 at 3 C with an areal sulfur loading of 3.0 mg cm 2. More strikingly, the S/B4C-ACF electrode en ables the battery to be stably operated for 3000 cycles with a high average coulombic efficiency of 99.24% at the current density of 1 C and
Fig. 6. Morphology of the S/B4C-ACF electrode after 3000 cycles: (a) SEM image (inset: the surface morphology of the fiber), (b) to (e) EDX mapping results of S, F, C and B elements. 7
Journal of Power Sources 451 (2020) 227751
R. Zhang et al.
a relatively low capacity decay rate of 0.012% per cycle. The ultra-longterm cycling test demonstrates that the S/B4C-ACF electrode exhibits great potential to realize the commercial application of the Li–S batteries.
[22] S. Urbonaite, T. Poux, P. Nov� ak, Progress towards commercially viable Li-S battery cells, Adv. Energy Mater. 5 (2015) 1–20. [23] S. Evers, L.F. Nazar, New approaches for high energy density lithium–sulfur battery cathodes, Acc. Chem. Res. 46 (2012) 1135–1143. [24] Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Lithium-sulfur batteries: electrochemistry, materials, and prospects, Angew. Chem. Int. Ed. 52 (2013) 13186–13200. [25] J.Q. Huang, Q. Zhang, F. Wei, Multi-functional separator/interlayer system for high-stable lithium-sulfur batteries: progress and prospects, Energy Storage Mater. 1 (2015) 127–145. [26] Y.V. Mikhaylik, J.R. Akridge, Polysulfide shuttle study in the Li/S battery system, J. Electrochem. Soc. 151 (2004), A1969. [27] X. Ji, L.F. Nazar, Advances in Li-S batteries, J. Mater. Chem. 20 (2010) 9821–9826. [28] A. Manthiram, Y. Fu, Y.S. Su, Challenges and prospects of lithium-sulfur batteries, Acc. Chem. Res. 46 (2013) 1125–1134. [29] J. Song, T. Xu, M.L. Gordin, P. Zhu, D. Lv, Y.B. Jiang, Y. Chen, Y. Duan, D. Wang, Nitrogen-doped Mesoporous carbon promoted chemical adsorption of sulfur and fabrication of high-Areal-capacity sulfur cathode with exceptional cycling stability for lithium-sulfur batteries, Adv. Funct. Mater. 24 (2014) 1243–1250. [30] S. Zheng, F. Yi, Z. Li, Y. Zhu, Y. Xu, C. Luo, J. Yang, C. Wang, Copper-stabilized sulfur-microporous carbon cathodes for Li-S batteries, Adv. Funct. Mater. 24 (2014) 4156–4163. [31] Y. Xu, Y. Wen, Y. Zhu, K. Gaskell, K.A. Cychosz, B. Eichhorn, K. Xu, C. Wang, Confined sulfur in microporous carbon renders superior cycling stability in Li/S batteries, Adv. Funct. Mater. 25 (2015) 4312–4320. [32] G. Zheng, Y. Yang, J.J. Cha, S.S. Hong, Y. Cui, Hollow carbon nanofiberencapsulated sulfur cathodes for high specific capacity rechargeable lithium batteries, Nano Lett. 11 (2011) 4462–4467. [33] J. Guo, Y. Xu, C. Wang, Sulfur-impregnated disordered carbon nanotubes cathode for lithium-sulfur batteries, Nano Lett. 11 (2011) 4288–4294. [34] M.R. Kaiser, Z. Ma, X. Wang, F. Han, T. Gao, X. Fan, J.Z. Wang, H.K. Liu, S. Dou, C. Wang, Reverse microemulsion synthesis of sulfur/graphene composite for lithium/sulfur batteries, ACS Nano 11 (2017) 9048–9056. [35] Z. Li, C. Li, X. Ge, J. Ma, Z. Zhang, Q. Li, C. Wang, L. Yin, Reduced graphene oxide wrapped MOFs-derived cobalt-doped porous carbon polyhedrons as sulfur immobilizers as cathodes for high performance lithium sulfur batteries, Nano Energy 23 (2016) 15–26. [36] S. Zheng, Y. Wen, Y. Zhu, Z. Han, J. Wang, J. Yang, C. Wang, In situ sulfur reduction and intercalation of graphite oxides for Li-S battery cathodes, Adv. Energy Mater. 4 (2014), 1400482. [37] C. Luo, H. Zhu, W. Luo, F. Shen, X. Fan, J. Dai, Y. Liang, C. Wang, L. Hu, Atomiclayer-deposition functionalized carbonized mesoporous wood fiber for high sulfur loading lithium sulfur batteries, ACS Appl. Mater. Interfaces 9 (2017) 14801–14807. [38] Z. Zhang, Z. Li, F. Hao, X. Wang, Q. Li, Y. Qi, R. Fan, L. Yin, 3D interconnected porous carbon aerogels as sulfur immobilizers for sulfur impregnation for lithiumsulfur batteries with high rate capability and cycling stability, Adv. Funct. Mater. 24 (2014) 2500–2509. [39] Q. Pang, X. Liang, C.Y. Kwok, L.F. Nazar, Review—the importance of chemical interactions between sulfur host materials and lithium polysulfides for advanced lithium-sulfur batteries, J. Electrochem. Soc. 162 (2015) A2567–A2576. [40] W. Cai, G. Li, K. Zhang, G. Xiao, C. Wang, K. Ye, Z. Chen, Y. Zhu, Y. Qian, Conductive nanocrystalline niobium carbide as high-efficiency polysulfides tamer for lithium-sulfur batteries, Adv. Funct. Mater. 28 (2018) 1–11. [41] H. Tang, W. Li, L. Pan, C.P. Cullen, Y. Liu, A. Pakdel, D. Long, J. Yang, N. McEvoy, G.S. Duesberg, V. Nicolosi, C. Zhang, In situ formed protective barrier enabled by Sulfur@Titanium carbide (MXene) ink for achieving high-capacity, long lifetime Li-S batteries, Adv. Sci. 5 (2018) 1–9. [42] F. Zhou, L.T. Song, L.L. Lu, H. Bin Yao, S.H. Yu, Titanium-carbide-decorated carbon nanofibers as hybrid electrodes for high performance Li-S batteries, ChemNanoMat 2 (2016) 937–941. [43] H.J. Peng, G. Zhang, X. Chen, Z.W. Zhang, W.T. Xu, J.Q. Huang, Q. Zhang, Enhanced electrochemical kinetics on conductive polar mediators for lithium–sulfur batteries, Angew. Chem. Int. Ed. 55 (2016) 12990–12995. [44] X. Zhao, M. Liu, Y. Chen, B. Hou, N. Zhang, B. Chen, N. Yang, K. Chen, J. Li, L. An, Fabrication of layered Ti3C2 with an accordion-like structure as a potential cathode material for high performance lithium-sulfur batteries, J. Mater. Chem. A. 3 (2015) 7870–7876. [45] X. Li, L. Chu, Y. Wang, L. Pan, Anchoring function for polysulfide ions of ultrasmall SnS2 in hollow carbon nanospheres for high performance lithium-sulfur batteries, Mater. Sci. Eng. B Solid-State Mater. Adv. Technol. 205 (2016) 46–54. [46] S.S. Zhang, D.T. Tran, Pyrite FeS2 as an efficient adsorbent of lithium polysulphide for improved lithium-sulphur batteries, J. Mater. Chem. A. 4 (2016) 4371–4374. [47] Z. Yuan, H.J. Peng, T.Z. Hou, J.Q. Huang, C.M. Chen, D.W. Wang, X.B. Cheng, F. Wei, Q. Zhang, Powering lithium-sulfur battery performance by propelling polysulfide redox at sulfiphilic hosts, Nano Lett. 16 (2016) 519–527. [48] X. Liu, J.-Q. Huang, Q. Zhang, L. Mai, Nanostructured metal oxides and sulfides for lithium-sulfur batteries, Adv. Mater. 29 (2017) 1601759. [49] F. Zhou, Z. Li, X. Luo, T. Wu, B. Jiang, L.L. Lu, H. Bin Yao, M. Antonietti, S.H. Yu, Low cost metal carbide nanocrystals as binding and electrocatalytic sites for high performance Li-S batteries, Nano Lett. 18 (2018) 1035–1043. [50] H. Al Salem, V.R. Chitturi, G. Babu, J.A. Santana, D. Gopalakrishnan, L.M. Reddy Arava, Stabilizing polysulfide-shuttle in a Li-S battery using transition metal carbide nanostructures, RSC Adv. 6 (2016) 110301–110306. [51] Y. Gu, L. Chen, Y. Qian, W. Zhang, J. Ma, Synthesis of nanocrystalline boron carbide via a solvothermal reduction of CCl4 in the presence of amorphous boron powder, J. Am. Ceram. Soc. 88 (2005) 225–227.
Declaration of competing interest The authors declare that there is no conflict of interest. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227751. References [1] Z.W. Seh, Y. Sun, Q. Zhang, Y. Cui, Designing high-energy lithium-sulfur batteries, Chem. Soc. Rev. 45 (2016) 5605–5634. [2] S. Zhao, R. Fang, Z. Sun, F. Li, D.-W. Wang, H.-M. Cheng, More reliable lithiumsulfur batteries: status, solutions and prospects, Adv. Mater. 29 (2017), 1606823. [3] J. Zhang, H. Huang, J. Bae, S.-H. Chung, W. Zhang, A. Manthiram, G. Yu, Nanostructured host materials for trapping sulfur in rechargeable Li-S batteries: structure design and interfacial chemistry, Small Methods 2 (2018), 1700279. [4] P.G. Bruce, S.a. Freunberger, L.J. Hardwick, J.-M. Tarascon, Li–O2 and Li–S batteries with high energy storage, Nat. Mater. 11 (2011), 172–172. [5] J.W. Choi, D. Aurbach, Promise and reality of post-lithium-ion batteries with high energy densities, Nat. Rev. Mater. 1 (2016), 16013. [6] M.C. Wu, T.S. Zhao, H.R. Jiang, L. Wei, Z.H. Zhang, Facile preparation of highperformance MnO2/KB air cathode for Zn-air batteries, Electrochim. Acta 222 (2016) 1438–1444. [7] P. Tan, B. Chen, H. Xu, W. Cai, W. He, M. Chen, M. Ni, Synthesis of Fe2O3 nanoparticle-decorated N-doped reduced graphene oxide as an effective catalyst for Zn-air batteries, J. Electrochem. Soc. 166 (2019) A616–A622. [8] P. Tan, B. Chen, H. Xu, W. Cai, W. He, M. Ni, In-situ growth of Co3O4 nanowireassembled clusters on nickel foam for aqueous rechargeable Zn-Co3O4 and Zn-air batteries, Appl. Catal. B Environ. 241 (2019) 104–112. [9] P. Tan, B. Chen, H. Xu, W. Cai, W. He, M. Ni, Porous Co3O4 nanoplates as the active material for rechargeable Zn-air batteries with high energy efficiency and cycling stability, Energy 166 (2019) 1241–1248. [10] P. Tan, B. Chen, H. Xu, W. Cai, W. He, M. Ni, Growth of Al and Co co-doped NiO nanosheets on carbon cloth as the air electrode for Zn-air batteries with high cycling stability, Electrochim. Acta 290 (2018) 21–29. [11] P. Tan, W. Kong, Z. Shao, M. Liu, M. Ni, Advances in modeling and simulation of Li–air batteries, Prog. Energy Combust. Sci. 62 (2017) 155–189. [12] R.H. Zhang, T.S. Zhao, H.R. Jiang, M.C. Wu, L. Zeng, V2O5-NiO composite nanowires: a novel and highly efficient carbon-free electrode for non-aqueous Liair batteries operated in ambient air, J. Power Sources 409 (2019) 76–85. [13] P. Tan, B. Chen, H. Xu, H. Zhang, W. Cai, M. Ni, M. Liu, Z. Shao, Flexible Zn- and Li-air batteries: recent advances, challenges, and future perspectives, Energy Environ. Sci. 10 (2017) 2056–2080. [14] R.H. Zhang, T.S. Zhao, M.C. Wu, H.R. Jiang, L. Zeng, Mesoporous ultrafine Ta2O5 nanoparticle with abundant oxygen vacancies as a novel and efficient catalyst for non-aqueous Li-O2 batteries, Electrochim. Acta 271 (2018) 232–241. [15] K. He, X. Bi, Y. Yuan, T. Foroozan, B. Song, K. Amine, J. Lu, R. Shahbazian-Yassar, Operando liquid cell electron microscopy of discharge and charge kinetics in lithium-oxygen batteries, Nano Energy 49 (2018) 338–345. [16] K. He, Y. Yuan, X. Bi, T. Foroozan, J. Lu, R. Shahbazian-Yassar, Study of the Lioxygen battery discharging and charging process using in-situ TEM, Microsc. Microanal. 24 (2018) 328–329. [17] M. Liu, Q. Li, X. Qin, G. Liang, W. Han, D. Zhou, Y.B. He, B. Li, F. Kang, Suppressing self-discharge and shuttle effect of lithium–sulfur batteries with V2O5-decorated carbon nanofiber interlayer, Small 13 (2017), 1602539. [18] R. Zhang, M. Wu, X. Fan, H. Jiang, T. Zhao, Superior cycling life of Li–S batteries with high sulfur loading enabled by a bifunctional layered-MoO3 cathode, J. Power Sources 436 (2019), 226840. [19] Y. Yuan, G. Tan, J. Wen, J. Lu, L. Ma, C. Liu, X. Zuo, R. Shahbazian-Yassar, T. Wu, K. Amine, Encapsulating various sulfur allotropes within graphene nanocages for long-lasting lithium storage, Adv. Funct. Mater. 28 (2018) 1–10. [20] M. Liu, X. Qin, Y.B. He, B. Li, F. Kang, Recent innovative configurations in highenergy lithium-sulfur batteries, J. Mater. Chem. A. 5 (2017) 5222–5234. [21] R. Xu, J. Lu, K. Amine, Progress in mechanistic understanding and characterization techniques of Li-S batteries, Adv. Energy Mater. 5 (2015) 1–22.
8
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751 [59] X. Tao, L. Dong, X. Wang, W. Zhang, B.J. Nelson, X. Li, B4C-nanowires/carbonmicrofiber hybrid structures and composites from cotton T-shirts, Adv. Mater. 22 (2010) 2055–2059. [60] L. Zhang, L. Xu, Y. Zhang, X. Zhou, L. Zhang, A. Yasin, L. Wang, K. Zhi, Facile synthesis of bio-based nitrogen- and oxygen-doped porous carbon derived from cotton for supercapacitors, RSC Adv. 8 (2018) 3869–3877. [61] L. Shi, Y. Gu, L. Chen, Y. Qian, Z. Yang, J. Ma, A low temperature synthesis of crystalline B4C ultrafine powders, Solid State Commun. 128 (2003) 5–7. [62] J. Guo, L. Zhang, T. Fujita, T. Goto, M. Chen, Pressure-induced depolarization and resonance in Raman scattering of single-crystalline boron carbide, Phys. Rev. B Condens. Matter Mater. Phys. 81 (2010) 1–4. [63] L.G. Jacobsohn, R.K. Schulze, M.E.H. Maia Da Costa, M. Nastasi, X-ray photoelectron spectroscopy investigation of boron carbide films deposited by sputtering, Surf. Sci. 572 (2004) 418–424. [64] M. Royle, J. Cho, S.W. Martin, Royle, Raman spectroscopy studies of xNa2S þ (1 x)B2S3 glasses and polycrystals, J. Non-Cryst. Solids 279 (2001) 97–109.
[52] F. Thevenot, State of the art boron carbide A comprehensive review, J. Eur. Ceram. Soc. 6 (1990) 205–225. [53] L. Luo, S.H. Chung, H. Yaghoobnejad Asl, A. Manthiram, Long-life lithium–sulfur batteries with a bifunctional cathode substrate configured with boron carbide nanowires, Adv. Mater. 30 (2018) 1–7. [54] N. Song, Z. Gao, Y. Zhang, X. Li, B 4 C nanoskeleton enabled, flexible lithium-sulfur batteries, Nano Energy 58 (2019) 30–39. [55] C. Kim, T. Fujino, T. Hayashi, M. Endo, M.S. Dresselhaus, Structural and electrochemical properties of pristine and B-doped materials for the anode of Li-ion secondary batteries, J. Electrochem. Soc. 147 (2002) 1265. [56] W. Bin Luo, S.L. Chou, J.Z. Wang, H.K. Liu, A B4C nanowire and carbon nanotube composite as a novel bifunctional electrocatalyst for high energy lithium oxygen batteries, J. Mater. Chem. A. 3 (2015) 18395–18399. [57] S. Mu, X. Chen, R. Sun, X. Liu, H. Wu, D. He, K. Cheng, Nano-size boron carbide intercalated graphene as high performance catalyst supports and electrodes for PEM fuel cells, Carbon N. Y. 103 (2016) 449–456. [58] L. Xu, S. Li, Y. Zhang, Y. Zhai, Synthesis, properties and applications of nanoscale nitrides, borides and carbides, Nanoscale 4 (2012) 4900–4915.
9
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A long-life Li–S battery enabled by a cathode made of well-distributed B4C nanoparticles decorated activated cotton fibers Ruihan Zhang, Cheng Chi, Maochun Wu, Ke Liu, Tianshou Zhao * HKUST Energy Institute, Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
H I G H L I G H T S
� B4C-ACF cathode substrate is successfully developed for Li–S batteries. � B4C-ACF substrate shows good catalytic activity to polysulfides conversion. � B4C-ACF substrate hold strong binding ability to polysulfides. � The Li–S battery with B4C-ACF substrate is operated for 3000 cycles at 1 C. A R T I C L E I N F O
A B S T R A C T
Keywords: Li-S batteries Shuttle effect Boron carbide nanoparticles Long-term cycling lifespan Chemical interactions
The commercialization of Li–S batteries is impeded by their short lifespan and poor rate capability resulting from the shuttle effect and sluggish reaction kinetics. Herein, we develop a bi-functional cathode substrate for Li–S batteries made of well-distributed boron carbide nanoparticles decorated activated cotton fiber (B4C-ACF), in which B4C nanoparticles serve as not only robust chemically anchoring sites to trap the polysulfides, but also afford abundant active sites for efficient sulfur conversion reactions. Meanwhile, the ACF network provides fast electron pathways for the conversion reactions and acts as the current collector. As a result, the novel cathode substrate enables a Li–S battery to deliver an initial capacity of as high as 1415 mAh g 1 at 0.1 C, and an extrahigh reversible capacity of 928 mAh g 1 at 3 C with an areal sulfur loading of 3.0 mg cm 2. More strikingly, the B4C-ACF substrate-based battery could be stably operated for 3000 cycles with a high coulombic efficiency of 99.24% and a capacity decay rate of as low as 0.012% per cycle at 1 C, demonstrating that the B4C-ACF substrate holds great potential in realizing the mass production of advanced Li–S batteries.
1. Introduction High-energy storage technologies have been attracting extensive interests in recent decades due to the skyrocketing demand for efficient and affordable energy storage systems in stationary electricity storage, portable electronic devices and transportation (such as cars, aircrafts and ships) [1–3]. However, the state-of-the-art lithium-ion (Li-ion) batteries are unable to meet the ever-increasing needs in these markets owing to the limited potential in further increase of their energy den sities [4,5]. Hence, considerable efforts have been devoted to developing advanced battery systems with higher theoretical energy density, such as zinc-air (Zn-air) batteries [6–10], lithium-air (Li-air) batteries [11–16] and lithium-sulfur (Li–S) batteries [17–20]. Among them, the Li–S batteries have drawn much attention because of their extra-high
theoretical energy density (2567 Wh kg 1), cost-effectiveness and eco-friendliness [21,22]. In a typical Li–S battery, the lithium metal acts as the anode while the sulfur (S8) is chosen as the cathode, which are separated by a porous membrane. The redox reactions between lithium and sulfur could be mainly depicted as 16Liþ þ S8 þ 16e ↔ 8Li2S (E� ¼ 2.2 V, vs Li/Liþ), in which sulfur delivers a high theoretical specific capacity of 1675 mAh g 1, five times higher than that of the cathodes (300 mAh g 1) in the commercial LIBs [23–25]. However, before the commercialization of this technology, there are several challenging is sues need to be addressed. For example, the insulator nature of S8 and the discharge product (Li2S) lead to a low utilization of active materials and poor reaction kinetics. The shuttle effect resulting from soluble long-chain lithium polysulfides (LiPSs) in organic electrolyte results in low coulombic efficiencies and severe capacity decay. Worse still, the
* Corresponding author. E-mail address: [email protected] (T. Zhao). https://doi.org/10.1016/j.jpowsour.2020.227751 Received 5 November 2019; Received in revised form 25 December 2019; Accepted 12 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
relatively large volume expansion of sulfur after lithiation easily causes the continual volume variation in cathode during discharge/charge process and eventually pulverizes the whole cathode architecture [26–28]. To tackle these challenges and realize the practical application of Li–S batteries, numerous pioneering contributions have been made during the last several years. Carbon-based materials such as porous carbon with micro-/meso-/macro-pores [29–31], carbon nanotubes (CNTs) [32,33], graphene [34–36] and carbon fibers [37,38] are firstly applied as physical barriers to encapsulate sulfur as well as to provide efficient electron transport due to their excellent electrical conductivity, low weight density and various porous structures. However, the non-polar inherent feature makes the carbonaceous hosts insufficient to entrap the polar LiPSs during the long-term cycling operation, especially for high-loading Li–S batteries [1,2,20,39]. In order to improve the cycling stability as well as obtain the high capacity retention rate simultaneously, numerous novel polar substrates, including metal car bides [40–44], sulfides [45–47], and oxides [17,18,48], are introduced for Li–S batteries, which exhibit high binding energy toward the LiPSs and form strongly Lewis acid-based interactions or other chemical bonds with LiPSs. Among these reported inorganic polar materials, the metal carbides such as WxC [49,50], Mo2C [49] and TixC [42–44,49] show great potential to improve the electrochemical performance of Li–S batteries due to their excellent metallic electrical conductivity, promi nent positive activities toward sluggish conversion reactions between sulfur species, and superb ability to anchor the LiPSs. However, the weight densities of these metal carbides are much heavier than that of sulfur, leading to a higher weight fraction occupied by the metal car bides and a relative lower sulfur content in the prepared cathodes. In this regard, the lightweight boron carbide (B4C) becomes an attractive candidate for Li–S batteries due to its low density of 2.50–2.51 g cm 3, which is comparable to that of the octasulfur (2.07 g cm 3) [51]. Moreover, B4C exhibits an excellent conductivity of 1.25–3.33 S cm 1, superior chemical stability and outstanding positive activity [52–54], which has been widely applied in Li-ion batteries [55], Li-air batteries [56] and fuel cells [57]. Very recently, B4C nanowires as part of cathode substrates for Li–S batteries were reported by Manthiram’s and Li’s groups [53,54], with which the Li–S batteries showed enhanced elec trochemical performance. In addition, the pouch-cell and flexible type Li–S batteries were proposed and tested in their works, respectively. However, the reported Li–S batteries still suffer from a severe capacity loss and thus a low cycling life. Herein, we fabricate a novel positive electrode substrate made of uniform B4C nanoparticles decorated activated cotton fibers (B4C-ACF) for Li–S batteries by a catalyst-assisted fabrication method. The B4C-ACF substrate exhibits an ultrahigh Brunauer-Emmett-Teller (BET) specific surface area of 224.51 m2 g 1 with plentiful mesopores. With this novel substrate, the Li–S batteries deliver a highly reversible capacity of 1415 mAh g 1 at 0.1 C, and an outstanding rate capability with the current density increasing from 0.1 C to 3 C with an areal sulfur loading of 3.0 mg cm 2. In addition, the B4C-ACF substrate enables the Li–S battery to stably cycle for 500 times with a retention capacity of as high as 988 mAh g 1 at 0.5 C, which is four times higher than that of the battery with pure ACF substrate (192 mAh g 1 after 500 cycles). For long-term cycling test, the B4C-ACF based Li–S battery with the areal sulfur loading of 3.0 mg cm 2 displays a remarkable cycling stability after 3000 cycles at the current density of 1 C with an average coulombic efficiency of 99.24%. More remarkably, the capacity decay rate is as low as of 0.012% per cycle, which demonstrates the great promise of B4CACF as electrode substrates for the commercially viable Li–S batteries.
catalyst-assisted method [58,59], in which the nickel nitrate (Ni (NO3)2⋅6H2O, Sigma-Aldrich, 99.99%) acts as the catalyst while the amorphous boron powders (Sigma-Aldrich, 95%) and commercially cotton fibers are chosen as the boron and carbon sources, respectively. For the typical procedure, the cotton fibers are firstly washed by acetone (Sigma-Aldrich, 99.9%), absolute alcohol (Sigma-Aldrich) and distilled water (DI water) alternately for several times, and dried under a vacuum oven at 100 � C for 24 h. Meantime, 0.5 g Ni(NO3)2⋅6H2O and 0.4 g boron powders are dispersed in 100 mL of absolute alcohol under continuously ultrasonication until the Ni–B emulsion is formed. Then, the 1.2 g pre pared cotton fibers are soaked in the emulsion under ultrasonic stirring for 4 h and dried in a preheated vacuum oven at 100 � C for 4 h. After wards, the dried samples are placed in a horizontal tube furnace and sintered at 1180 � C for 6 h with a heating rate of 2 � C min 1 under argon. For the control group, the ACF sample is prepared by directly curing cotton fibers after the first step in tube furnace under the same heating procedure without the immersing process in the Ni–B emulsion. The sulfur powders are successfully loaded on the prepared substrates by a conventional melt-diffusion method, in which sulfur powders and the B4C-ACF or ACF substrates are sealed in a 50 mL Teflon-lined stainless autoclave under argon atmosphere and heated at 155 � C for 12 h. The obtained samples are depicted as S/B4C-ACF and S/ACF electrodes, respectively. The sulfur contents in the prepared electrodes are controlled by the proper mole ratio of sulfur powders and the substrates, and the areal sulfur loading of the as-prepared cathode is around 3.0 mg cm 2. The Micro-Raman spectrophotometer (Renishaw RM 3000) and Power X-ray diffraction system (XRD, model PW 1825) measurements are carried out to inspect the crystal structure and composition of these prepared samples and electrodes. And the morphology and crystallinity are investigated by the scanning electron microscopy (SEM, JEOL7100F), high-resolution transmission electron microscopy (HRTEM, JEOL 2010) and energy-dispersive X-ray spectroscopy (EDX) elemental mapping. The sulfur contents in the prepared electrodes are examined by the thermogravimetric analysis (TGA), which was accomplished on a TGA Q5000 (TA instruments) under nitrogen atmosphere from 30 to 500 � C with a heating rate of 5 � C min 1. The BET specific surface areas of these substrates are analyzed by the nitrogen adsorption-desorption test. X-ray photoelectron spectroscopy (XPS, PHI 5600) measurement is conducted to clarify the chemical interactions formed between the B4C and LiPSs. 2.2. Battery assembly and electrochemical measurements CR2032-type coin cells are assembled in an argon-filled glovebox (Etelux, Lab 2000, H2O � 1 ppm, O2 � 1 ppm) to appraise the electro chemical properties of the as-prepared electrodes. The S/B4C-ACF and S/ACF electrodes are directly cut into discs with a diameter of 14 mm to serve as sulfur cathode, while the lithium metal foil with a diameter of 16 mm and the Celgard 2400 membrane with a diameter of 19 mm are chosen as the anode and separator, respectively. During the assembling process, 10 μL of the organic electrolyte is added into each cell, which is composed with 1 M lithium bis(trifluoromethane)sulfonamide (LiTFSI, Sigma-Aldrich, 99.95%) salt dissolved into a mixture solvent of dime thoxy methane (DME, Sigma-Aldrich, 99.9%) and 1,3-dioxolane (DOL, Sigma-Aldrich, 99.9%) by a 1:1 vol ratio. Cyclic voltammetry (CV) measurements of the coin cells are carried out by a potentiostat elec trochemical station (PARSTAT 2273) at a scan rate of 0.1 mV s 1 within the electrochemical window of 1.70–2.8 V (vs. Li/Liþ) to examine the positive effects of the as-synthesized substrates towards the sulfur con version reactions. The galvanostatic discharge/charge profiles and the cycling performance of these prepared coin cells are conducted in an 8channel battery tester (Neware, CT-3008W) at different current den sities. During the long-term cycling test, the electrochemical impedance spectrum (EIS) results of the testing cells are performed using the PARSTAT 2273 station in a frequency of 100 kHz to 10 mHz with an
2. Experimental section 2.1. Electrodes preparation and materials characterization In this work, the B4C-ACF is fabricated by a previously reported 2
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
alternating current (AC) perturbation of 5 mV at the open-circuit voltage. To detect the absorption ability of B4C towards the LiPSs, the as-prepared B4C-ACF substrate is directly cut into 14 mm discs and then immersed into the 0.025 M Li2S6 solution prepared by dissolving the solid S8 and Li2S with a proper mole ratio in the DOL/DME (1:1, by volume) cosolvent in the argon-filled glovebox.
because the cotton fibers act as not only the substrate but also the carbon source, which would react with boron powders to form the B4C nano particles during the sintering process, leading to the finally porous and rough surface morphology in the B4C-ACF sample. The compositions of these as-prepared samples are verified by XRD measurement, and the results are displayed in Fig. 1g. For the pure ACF sample, two diffraction peaks located at 26.2 and 44.5� are observed in the XRD spectra from 10 to 80� , which are attributed to the (002) and (100) characteristic peaks of graphite, respectively [53,60]. These two peaks are also detected in the XRD patterns of B4C-ACF sample, indi cating the high degree of graphitization of cotton fibers in these as-prepared samples after calcination, which can provide the high electrical conductivity for sulfur conversion reactions in Li–S batteries. In the XRD spectra of B4C-ACF sample, seven additional peaks are also identified, which are ascribed to the (101), (003), (012), (110), (104), (021) and (211) diffraction peaks of B4C (JCPDS #75–0424), respec tively [53,61]. No other peaks of impurities in the B4C-ACF sample are detected, confirming that the pure and well crystallized B4C sample has been successfully synthesized in this work. The nitrogen adsorption-desorption isotherm and the pore size distribution of the B4C-ACF sample are displayed in Fig. 1h and i. The BET surface area of B4C-ACF sample is calculated to be ~224.51 m2 g 1, which is seven times higher than that of the pure ACF sample (~27.86 m2 g 1 as pre sented in Fig. S3). In addition, the percentage of mesopores in the
3. Results and discussion 3.1. Characterization of the as-prepared substrates and electrodes The SEM measurement is conducted to inspect the morphologies of the as-prepared ACF and B4C-ACF samples (Fig. 1 and Fig. S1). From the SEM images in Fig. 1a–c, there are plentiful nanoparticles covered on the porous and rough surface of B4C-ACF sample, of which the diameters range from 20 to 50 nm as observed in the HRTEM image (Fig. 1d). The EDX mapping results in Fig. S2 show that the B4C nanoparticles are well distributed on the surface of the cotton fibers, demonstrating that the B4C-ACF sample has been successfully obtained. The well crystallinity of the as-prepared B4C nanoparticles is proved by the clear lattice fringes and SAED results observed in Fig. 1e and f, in which the orderly inter planar spacing of 4.48 Å is corresponding to the characteristic (101) plane of B4C (JCPDS #75–0424) [53,54]. In contrast, the cotton fibers of the pure ACF sample (Fig. S1) show a clean and smooth surface. This is
Fig. 1. (a)–(c) SEM image of as-prepared B4C-ACF sample; (d) HRTEM, (e) lattice fringes and (f) SAED image of as-prepared B4C-ACF sample; (g) XRD spectra of asprepared B4C-ACF and ACF samples; (h) nitrogen adsorption-desorption isotherm and (i) pore size distribution of the as-prepared B4C-ACF sample. 3
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
B4C-ACF sample is much higher than that in the ACF sample, which could provide much more active sites for sulfur conversion reactions in Li–S batteries. The content of B4C in the B4C-ACF sample is examined by heating this sample in air from 30 to 1000 � C on the TA instruments (Fig. S4). During the heating process, the cotton fiber substrate is oxidized to carbon dioxide, while the B4C would react with oxygen to produce the B2O3 and carbon dioxide. Finally, only the B2O3 is left in the residues, based on which the content of B4C in our sample is calculated to be ~13.93 wt%, much lower than that reported in the previous works [53,54]. The absorption tests of the B4C-ACF and ACF substrates towards the polysulfides are carried out by soaking the substrate discs into the diluted Li2S6 solution prepared as mentioned above, and the results are shown in Fig. S5. After static placed for 3 h, the solution with B4C-ACF sample becomes transparent while the solution with pure ACF substrate remains the same as the pristine one, showing that the pure ACF sample holds poor absorption ability towards LiPSs but B4C nanoparticles could strongly anchor the polysulfides during a short period, which is consistent with the previous works. To prepare the S/B4C-ACF and S/ACF electrodes, the sulfur powders are loaded onto the B4C-ACF and ACF substrates by the melt-diffusion method, and their morphologies are shown in Fig. 2 and Fig. S6, respectively. For S/B4C-ACF electrode, the porous and rough surface of pristine B4C-ACF sample is replaced by a relatively smooth one accompanied with some particles after sulfur loading (Fig. 2a and b), demonstrating that the sulfur has fully covered the surface of the B4CACF substrate. Raman measurement is carried out to verify the composition of the prepared S/B4C-ACF electrode, and the results are
shown in Fig. 2c. In the Raman spectra of S/B4C-ACF electrode, five scattering peaks located at 151, 216, 246, 437 and 470 cm 1 are observed, which are assigned to sulfur nanoparticles [18]. Additionally, another five peaks are detected at 533, 726, 818, 1004 and 1088 cm 1, corresponding to the characteristic peaks of crystalized B4C nano particles [61,62]. The two peaks at 1358 and 1597 cm 1 appear in both B4C-ACF substrate and final electrode composite, which are ascribed to the D and G peaks of the graphite [18,53,54]. The EDX mapping result of S element in Fig. 2f confirms that the sulfur is uniformly distributed on the substrate. XPS measurement is performed to investigate the chemical bonds and the elemental composition of S/B4C-ACF electrode. In the spectra of C 1s (Fig. 2g), there are two peaks located at 284.6 and 283.2 eV, which are assigned to C–C and C–B bonds, respectively. The B 1s spectrum (Fig. 2h) reveals the existence of B–B (188.3 eV) and B–C (187.2 eV) bonds, which again confirms the successfully synthesis of B4C nanoparticles [63]. As shown in Fig. 2i, three peaks centered at 163.6, 164.8 and 168.8 eV are detected in the spectra of S 2p, which are ascribed to S 2p3/2, S 2p1/2 and sulfate, respectively. The energy sepa ration between S 2p3/2 and S 2p1/2 peaks is about 0.8 eV, indicating that the sulfur powders have been successfully loaded on the substrate [54]. The sulfate mainly arises from the reaction between the sulfur powders and the oxygen-containing functional groups on the surface of the B4C-ACF substrate during the melt-diffusion process. The sulfur contents in the S/B4C-ACF and S/ACF electrodes are separately calculated to be 74.5 and 73.6 wt% based on the TGA measurement under nitrogen (Fig. S7).
Fig. 2. (a) and (b) SEM images of the S/B4C-ACF sample; (c) Raman spectra of the as-prepared samples; (d) to (f) EDX mapping results of S/B4C-ACF sample; (g) to (i) the XPS results of C 1s, B 1s and S 2p in the S/B4C-ACF electrode. 4
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
3.2. Electrochemical performance
more positive activities of the B4C-ACF electrode for the polysulfide transformation reactions. With the current densities increasing from 0.5 to 3 C, the battery with the S/B4C-ACF electrode displays high initial capacities of 1274, 1165, 1071 and 928 mAh g 1; while relatively low capacities of 1023, 757, 483 and 346 mAh g 1 can only be obtained for the control battery. Furthermore, when the current density reduces back to 0.5 C, the capacity of the battery with the S/B4C-ACF electrode re covers to a high capacity of 1270 mAh g 1, which is 37.8% higher than that of the control group (~790 mAh g 1). These results demonstrate the superior rate capability of the S/B4C-ACF electrode. In order to test the cycling stability of the prepared S/B4C-ACF and S/ACF electrodes, the Li–S batteries are operated from 1.7 to 2.8 V at the current density of 0.5 C with the areal sulfur loading of 3.0 mg cm 1 for 500 cycles. As shown in Fig. 3d, the battery with the S/B4C-ACF electrode still displays an ultrahigh retention capacity of 988 mAh g 1 after 500 cycles, which is about 4.14 times higher than that of the control battery (only ~192 mAh g 1) and even is comparable to the initial capacity (~1023 mAh g 1) of the control group. In the meantime, the average coulombic efficiency of S/B4C-ACF electrode-based battery is calculated to be as high as 99.36%, and the capacity decay rate is only about 0.04% per cycle, indicating that the S/B4C-ACF electrode significantly improves the cycling stability of the Li–S battery. These enhanced electrochemical performances in rate capability and cycling stability of the Li–S battery with the S/B4CACF electrode mainly benefit from the strongly anchoring ability and the highly positive activity of the well-distributed B4C nanoparticles on the porous ACF substrate. The B4C nanoparticles could not only effectively suppress the shuttle effect, but also facilitate the reaction kinetics during high current densities and long-term cycling test. To verify the chemical interactions formed between the B4C nano particles and polysulfides, the XPS and Raman spectroscopy of the S/ B4C-ACF electrode in the battery after cycling are measured and the
CV tests of the Li–S batteries with the S/B4C-ACF and S/ACF elec trodes are conducted at a scan rate of 0.1 mV s 1 within the electro chemical window of 1.7–2.8 V (vs. Li/Liþ), and the profiles are displayed in Fig. 3a. For the S/ACF electrode-based battery, there are two peaks centered at 1.95 and 2.21 V during the cathodic scan, which represent the reduction of S8 to long-chain polysulfides followed by their reduc tion to solid lithium sulfides (Li2S2/Li2S), respectively. In the subsequent anodic scan, only one broad peak located at 2.48 V is observed, which is ascribed to the reverse oxidation reaction of lithium sulfides species to the soluble intermediates and the final elemental sulfur. Two cathodic peaks are also observed for the battery with the S/B4C-ACF electrode, which locate at 2.03 and 2.22 V, respectively, both higher than those of the control group. During the anodic scan, two noticeable peaks are detected with the location of 2.33 and 2.38 V, respectively, which are both lower than that in the battery with S/ACF electrode, leading to a much smaller peak separation between the anodic and cathodic peaks [53]. Furthermore, the peak current densities of the S/B4C-ACF electrode-based battery are higher than those in the control group, indicating that the B4C-ACF substrate exhibits a high positive activity to polysulfide transformation reactions, consistent with the previous works [53,54]. The improved redox kinetics of sulfur conversion reactions are also validated by the excellent rate capability of the S/B4C-ACF electrodebased battery, as shown in Fig. 3b and c. At the current density of 0.1 C, the battery with S/B4C-ACF electrode displays an initial capacity of as high as 1415 mAh g 1, which is 20.5% higher than that in the control group (~1120 mAh g 1). Moreover, the overpotential of the S/B4C-ACF electrode-based battery is only about 180 mV, while the control group displays a much larger overpotential of 300 mV, indicating the much
Fig. 3. (a) CV curves of Li–S batteries with S/B4C-ACF and S/ACF electrodes; (b) the specific capacities of Li–S batteries with S/B4C-ACF electrode from 0.1 to 3 C; (c) the rate capability of Li–S batteries with S/B4C-ACF and S/ACF electrodes; (d) the cycling test of Li–S batteries with S/B4C-ACF and S/ACF electrodes at 0.5 C. 5
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
results are presented in Fig. 4. For the spectra of C 1s (Fig. 4a), the two peaks of C–C and C–B bonds are observed on the same locations with those detected in the electrode before cycling, while one additional peak located at 285.2 eV appears, which is ascribed to the C–S bond [54]. In the spectrum of B 1s (Fig. 4b), an extra peak located at 191.30 eV cor responding to the B–S bond is detected [63], and the original B–B and B–C bonds still exist but their locations shift negatively from 187.8 to 186.9 eV. The S 2p spectra (Fig. 4c) could be divided into three peaks, ascribed to S 2p3/2 (164.3 eV), S 2p1/2 (165.7 eV) and sulfate (169.0 eV), respectively, all of which with a slight positive shift. The appearance of the new C–S and B–S bonds indicates that the chemical interactions have been formed between B4C nanoparticles and LiPSs, proving the strongly anchoring ability of B4C towards soluble polysulfides. The formation of chemical bonds is also confirmed by the Raman measurement (Fig. 4d) from 200 to 1200 cm 1. Before cycling, there are only peaks corre sponding to sulfur and B4C in the S/B4C-ACF electrode. After 500 cycles, four additional peaks located at 393, 443, 772 and 1042 cm 1 are detected, which are assigned to the characteristic peaks of boron sulfide (B2S3) as reported in the previous works [64], further confirming the existence of strong chemical interactions formed between B4C and polysulfides. Due to the excellent performance of the S/B4C-ACF electrode in rate capability and cycling stability, we operate the Li–S battery with this electrode with the areal sulfur loading of 3.0 mg cm 2 at 1 C for ultralong cycling test to examine the potential of this electrode for real izing the practical application of Li–S battery. For the selected cycles (the 1000th, and 2000th cycles, Fig. 5a) during cycling, the battery displays the discharge capacities of as high as 997 and 901 mAh g 1, respectively. Even at the end of the 3000th cycle, the battery could still deliver a high capacity of 740 mAh g 1, which is even much higher than
that of the control group after 500 cycles at 0.5 C. EIS plots of the Li–S battery with the S/B4C-ACF electrode for the selected cycles are pre sented in Fig. 5b. For the fresh cell, the bulk resistance (Rb) and the charge transfer resistance (Rct) are only around 3.32 and 10.40 Ω, respectively, which are both smaller than those of the battery in the previous works [18,53,54], indicating the high electronic conductivity and good charge transfer capability of the S/B4C-ACF electrode. For the selected cycles, the values of Rb are calculated to be 10.36 Ω (1000th cycle), 16.23 Ω (2000th cycle) and 20.12 Ω (3000th cycle), while the values of Rct increase from 21.74, 32.24–45.14 Ω for the selected cycles during cycling, which are much lower than that reported in previous woks. These results demonstrate the S/B4C-ACF electrode could provide efficient electronic conductivity and stable charge transfer for sulfur redox reactions during the long-term cycling operation, which mainly benefit from the much more positive effect of B4C nanoparticles on immobilizing LiPSs and facilitating the redox kinetics of polysulfides conversion reactions during cycling. As shown in Fig. 5c, the S/B4C-ACF electrode enables the battery to stably operate for 3000 cycles with an average coulombic efficiency of as high as 99.24% at the current density of 1 C. More strikingly, the capacity decay rate is calculated to be only about 0.012% per cycle, which is quite lower than that reported in the previous works. The remarkable improvements in cycling stability and capacity retention capability of the Li–S battery under ultra-long life- span indicate that the S/B4C-ACF electrode could be regarded as one of the promising cathode materials for realizing the practical application of Li–S batteries. The morphology changes of the S/B4C-ACF electrode after 2000 and 3000 cycles are examined by the SEM measurement, and the results are shown in Fig. S9 and Fig. 6, respectively. Compared with the pristine S/ B4C-ACF electrode before cycling, the surface of this electrode after
Fig. 4. (a)–(c) the XPS results of C 1s, B 1s and S 2p in the S/B4C-ACF electrode after cycling; (d) Raman spectra of the S/B4C-ACF electrode before and after chargedischarge cycling. 6
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751
Fig. 5. (a) The specific capacities of Li–S battery with S/B4C-ACF electrode at 1 C for the selected cycles; (b) EIS plots of the Li–S battery with S/B4C-ACF electrode after the selected charge-discharge cycles; (c) the long-term cycling test of Li–S battery with S/B4C-ACF electrode at 1 C.
2000 and 3000 cycles becomes more smoothly during cycling without any large particles. Instead, abundant nanoparticles connecting with each other are well distributed on the surface after cycling, which are confirmed to be the sulfur nanoparticles by the EDX mapping result of S element in Fig. 6b accompanying with the Raman spectra in Fig. 4d. The morphology change of the sulfur mainly arises from two factors: the strongly binding ability of B4C nanoparticles towards polysulfides that could anchor the intermediates tightly on the ACF substrate and sup press the shuttle effect efficiently, and the highly positive activity of B4C for polysulfides transformation which facilitates the redox kinetics to obtain the uniform participations at the end of charge. The EDX mapping result of B in Fig. 6e and the existence of B4C in the Raman spectra in Fig. 4d demonstrate the high stability of the S/B4C-ACF electrode during long-term cycling, suggesting the great potential of this electrode for realizing the practical application of Li–S batteries.
4. Conclusions In summary, a B4C-ACF cathode substrate is successfully developed for Li–S batteries, in which the ACF textile acts as the electron pathway as well as the current collector, while the B4C nanoparticles serve as anchoring sites to trap polysulfides and positive sites to accelerate sulfur conversion reactions. The B4C-ACF substrate is proved to hold higher absorption ability towards polysulfides and more positive activities for polysulfides transformation than the pure ACF sample. As a result, a Li–S battery with the S/B4C-ACF electrode exhibits an ultrahigh capacity of 1415 mAh g 1 at 0.1 C with a low overpotential of 180 mV and an extrahigh reversible capacity of 928 mAh g 1 at 3 C with an areal sulfur loading of 3.0 mg cm 2. More strikingly, the S/B4C-ACF electrode en ables the battery to be stably operated for 3000 cycles with a high average coulombic efficiency of 99.24% at the current density of 1 C and
Fig. 6. Morphology of the S/B4C-ACF electrode after 3000 cycles: (a) SEM image (inset: the surface morphology of the fiber), (b) to (e) EDX mapping results of S, F, C and B elements. 7
Journal of Power Sources 451 (2020) 227751
R. Zhang et al.
a relatively low capacity decay rate of 0.012% per cycle. The ultra-longterm cycling test demonstrates that the S/B4C-ACF electrode exhibits great potential to realize the commercial application of the Li–S batteries.
[22] S. Urbonaite, T. Poux, P. Nov� ak, Progress towards commercially viable Li-S battery cells, Adv. Energy Mater. 5 (2015) 1–20. [23] S. Evers, L.F. Nazar, New approaches for high energy density lithium–sulfur battery cathodes, Acc. Chem. Res. 46 (2012) 1135–1143. [24] Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Lithium-sulfur batteries: electrochemistry, materials, and prospects, Angew. Chem. Int. Ed. 52 (2013) 13186–13200. [25] J.Q. Huang, Q. Zhang, F. Wei, Multi-functional separator/interlayer system for high-stable lithium-sulfur batteries: progress and prospects, Energy Storage Mater. 1 (2015) 127–145. [26] Y.V. Mikhaylik, J.R. Akridge, Polysulfide shuttle study in the Li/S battery system, J. Electrochem. Soc. 151 (2004), A1969. [27] X. Ji, L.F. Nazar, Advances in Li-S batteries, J. Mater. Chem. 20 (2010) 9821–9826. [28] A. Manthiram, Y. Fu, Y.S. Su, Challenges and prospects of lithium-sulfur batteries, Acc. Chem. Res. 46 (2013) 1125–1134. [29] J. Song, T. Xu, M.L. Gordin, P. Zhu, D. Lv, Y.B. Jiang, Y. Chen, Y. Duan, D. Wang, Nitrogen-doped Mesoporous carbon promoted chemical adsorption of sulfur and fabrication of high-Areal-capacity sulfur cathode with exceptional cycling stability for lithium-sulfur batteries, Adv. Funct. Mater. 24 (2014) 1243–1250. [30] S. Zheng, F. Yi, Z. Li, Y. Zhu, Y. Xu, C. Luo, J. Yang, C. Wang, Copper-stabilized sulfur-microporous carbon cathodes for Li-S batteries, Adv. Funct. Mater. 24 (2014) 4156–4163. [31] Y. Xu, Y. Wen, Y. Zhu, K. Gaskell, K.A. Cychosz, B. Eichhorn, K. Xu, C. Wang, Confined sulfur in microporous carbon renders superior cycling stability in Li/S batteries, Adv. Funct. Mater. 25 (2015) 4312–4320. [32] G. Zheng, Y. Yang, J.J. Cha, S.S. Hong, Y. Cui, Hollow carbon nanofiberencapsulated sulfur cathodes for high specific capacity rechargeable lithium batteries, Nano Lett. 11 (2011) 4462–4467. [33] J. Guo, Y. Xu, C. Wang, Sulfur-impregnated disordered carbon nanotubes cathode for lithium-sulfur batteries, Nano Lett. 11 (2011) 4288–4294. [34] M.R. Kaiser, Z. Ma, X. Wang, F. Han, T. Gao, X. Fan, J.Z. Wang, H.K. Liu, S. Dou, C. Wang, Reverse microemulsion synthesis of sulfur/graphene composite for lithium/sulfur batteries, ACS Nano 11 (2017) 9048–9056. [35] Z. Li, C. Li, X. Ge, J. Ma, Z. Zhang, Q. Li, C. Wang, L. Yin, Reduced graphene oxide wrapped MOFs-derived cobalt-doped porous carbon polyhedrons as sulfur immobilizers as cathodes for high performance lithium sulfur batteries, Nano Energy 23 (2016) 15–26. [36] S. Zheng, Y. Wen, Y. Zhu, Z. Han, J. Wang, J. Yang, C. Wang, In situ sulfur reduction and intercalation of graphite oxides for Li-S battery cathodes, Adv. Energy Mater. 4 (2014), 1400482. [37] C. Luo, H. Zhu, W. Luo, F. Shen, X. Fan, J. Dai, Y. Liang, C. Wang, L. Hu, Atomiclayer-deposition functionalized carbonized mesoporous wood fiber for high sulfur loading lithium sulfur batteries, ACS Appl. Mater. Interfaces 9 (2017) 14801–14807. [38] Z. Zhang, Z. Li, F. Hao, X. Wang, Q. Li, Y. Qi, R. Fan, L. Yin, 3D interconnected porous carbon aerogels as sulfur immobilizers for sulfur impregnation for lithiumsulfur batteries with high rate capability and cycling stability, Adv. Funct. Mater. 24 (2014) 2500–2509. [39] Q. Pang, X. Liang, C.Y. Kwok, L.F. Nazar, Review—the importance of chemical interactions between sulfur host materials and lithium polysulfides for advanced lithium-sulfur batteries, J. Electrochem. Soc. 162 (2015) A2567–A2576. [40] W. Cai, G. Li, K. Zhang, G. Xiao, C. Wang, K. Ye, Z. Chen, Y. Zhu, Y. Qian, Conductive nanocrystalline niobium carbide as high-efficiency polysulfides tamer for lithium-sulfur batteries, Adv. Funct. Mater. 28 (2018) 1–11. [41] H. Tang, W. Li, L. Pan, C.P. Cullen, Y. Liu, A. Pakdel, D. Long, J. Yang, N. McEvoy, G.S. Duesberg, V. Nicolosi, C. Zhang, In situ formed protective barrier enabled by Sulfur@Titanium carbide (MXene) ink for achieving high-capacity, long lifetime Li-S batteries, Adv. Sci. 5 (2018) 1–9. [42] F. Zhou, L.T. Song, L.L. Lu, H. Bin Yao, S.H. Yu, Titanium-carbide-decorated carbon nanofibers as hybrid electrodes for high performance Li-S batteries, ChemNanoMat 2 (2016) 937–941. [43] H.J. Peng, G. Zhang, X. Chen, Z.W. Zhang, W.T. Xu, J.Q. Huang, Q. Zhang, Enhanced electrochemical kinetics on conductive polar mediators for lithium–sulfur batteries, Angew. Chem. Int. Ed. 55 (2016) 12990–12995. [44] X. Zhao, M. Liu, Y. Chen, B. Hou, N. Zhang, B. Chen, N. Yang, K. Chen, J. Li, L. An, Fabrication of layered Ti3C2 with an accordion-like structure as a potential cathode material for high performance lithium-sulfur batteries, J. Mater. Chem. A. 3 (2015) 7870–7876. [45] X. Li, L. Chu, Y. Wang, L. Pan, Anchoring function for polysulfide ions of ultrasmall SnS2 in hollow carbon nanospheres for high performance lithium-sulfur batteries, Mater. Sci. Eng. B Solid-State Mater. Adv. Technol. 205 (2016) 46–54. [46] S.S. Zhang, D.T. Tran, Pyrite FeS2 as an efficient adsorbent of lithium polysulphide for improved lithium-sulphur batteries, J. Mater. Chem. A. 4 (2016) 4371–4374. [47] Z. Yuan, H.J. Peng, T.Z. Hou, J.Q. Huang, C.M. Chen, D.W. Wang, X.B. Cheng, F. Wei, Q. Zhang, Powering lithium-sulfur battery performance by propelling polysulfide redox at sulfiphilic hosts, Nano Lett. 16 (2016) 519–527. [48] X. Liu, J.-Q. Huang, Q. Zhang, L. Mai, Nanostructured metal oxides and sulfides for lithium-sulfur batteries, Adv. Mater. 29 (2017) 1601759. [49] F. Zhou, Z. Li, X. Luo, T. Wu, B. Jiang, L.L. Lu, H. Bin Yao, M. Antonietti, S.H. Yu, Low cost metal carbide nanocrystals as binding and electrocatalytic sites for high performance Li-S batteries, Nano Lett. 18 (2018) 1035–1043. [50] H. Al Salem, V.R. Chitturi, G. Babu, J.A. Santana, D. Gopalakrishnan, L.M. Reddy Arava, Stabilizing polysulfide-shuttle in a Li-S battery using transition metal carbide nanostructures, RSC Adv. 6 (2016) 110301–110306. [51] Y. Gu, L. Chen, Y. Qian, W. Zhang, J. Ma, Synthesis of nanocrystalline boron carbide via a solvothermal reduction of CCl4 in the presence of amorphous boron powder, J. Am. Ceram. Soc. 88 (2005) 225–227.
Declaration of competing interest The authors declare that there is no conflict of interest. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227751. References [1] Z.W. Seh, Y. Sun, Q. Zhang, Y. Cui, Designing high-energy lithium-sulfur batteries, Chem. Soc. Rev. 45 (2016) 5605–5634. [2] S. Zhao, R. Fang, Z. Sun, F. Li, D.-W. Wang, H.-M. Cheng, More reliable lithiumsulfur batteries: status, solutions and prospects, Adv. Mater. 29 (2017), 1606823. [3] J. Zhang, H. Huang, J. Bae, S.-H. Chung, W. Zhang, A. Manthiram, G. Yu, Nanostructured host materials for trapping sulfur in rechargeable Li-S batteries: structure design and interfacial chemistry, Small Methods 2 (2018), 1700279. [4] P.G. Bruce, S.a. Freunberger, L.J. Hardwick, J.-M. Tarascon, Li–O2 and Li–S batteries with high energy storage, Nat. Mater. 11 (2011), 172–172. [5] J.W. Choi, D. Aurbach, Promise and reality of post-lithium-ion batteries with high energy densities, Nat. Rev. Mater. 1 (2016), 16013. [6] M.C. Wu, T.S. Zhao, H.R. Jiang, L. Wei, Z.H. Zhang, Facile preparation of highperformance MnO2/KB air cathode for Zn-air batteries, Electrochim. Acta 222 (2016) 1438–1444. [7] P. Tan, B. Chen, H. Xu, W. Cai, W. He, M. Chen, M. Ni, Synthesis of Fe2O3 nanoparticle-decorated N-doped reduced graphene oxide as an effective catalyst for Zn-air batteries, J. Electrochem. Soc. 166 (2019) A616–A622. [8] P. Tan, B. Chen, H. Xu, W. Cai, W. He, M. Ni, In-situ growth of Co3O4 nanowireassembled clusters on nickel foam for aqueous rechargeable Zn-Co3O4 and Zn-air batteries, Appl. Catal. B Environ. 241 (2019) 104–112. [9] P. Tan, B. Chen, H. Xu, W. Cai, W. He, M. Ni, Porous Co3O4 nanoplates as the active material for rechargeable Zn-air batteries with high energy efficiency and cycling stability, Energy 166 (2019) 1241–1248. [10] P. Tan, B. Chen, H. Xu, W. Cai, W. He, M. Ni, Growth of Al and Co co-doped NiO nanosheets on carbon cloth as the air electrode for Zn-air batteries with high cycling stability, Electrochim. Acta 290 (2018) 21–29. [11] P. Tan, W. Kong, Z. Shao, M. Liu, M. Ni, Advances in modeling and simulation of Li–air batteries, Prog. Energy Combust. Sci. 62 (2017) 155–189. [12] R.H. Zhang, T.S. Zhao, H.R. Jiang, M.C. Wu, L. Zeng, V2O5-NiO composite nanowires: a novel and highly efficient carbon-free electrode for non-aqueous Liair batteries operated in ambient air, J. Power Sources 409 (2019) 76–85. [13] P. Tan, B. Chen, H. Xu, H. Zhang, W. Cai, M. Ni, M. Liu, Z. Shao, Flexible Zn- and Li-air batteries: recent advances, challenges, and future perspectives, Energy Environ. Sci. 10 (2017) 2056–2080. [14] R.H. Zhang, T.S. Zhao, M.C. Wu, H.R. Jiang, L. Zeng, Mesoporous ultrafine Ta2O5 nanoparticle with abundant oxygen vacancies as a novel and efficient catalyst for non-aqueous Li-O2 batteries, Electrochim. Acta 271 (2018) 232–241. [15] K. He, X. Bi, Y. Yuan, T. Foroozan, B. Song, K. Amine, J. Lu, R. Shahbazian-Yassar, Operando liquid cell electron microscopy of discharge and charge kinetics in lithium-oxygen batteries, Nano Energy 49 (2018) 338–345. [16] K. He, Y. Yuan, X. Bi, T. Foroozan, J. Lu, R. Shahbazian-Yassar, Study of the Lioxygen battery discharging and charging process using in-situ TEM, Microsc. Microanal. 24 (2018) 328–329. [17] M. Liu, Q. Li, X. Qin, G. Liang, W. Han, D. Zhou, Y.B. He, B. Li, F. Kang, Suppressing self-discharge and shuttle effect of lithium–sulfur batteries with V2O5-decorated carbon nanofiber interlayer, Small 13 (2017), 1602539. [18] R. Zhang, M. Wu, X. Fan, H. Jiang, T. Zhao, Superior cycling life of Li–S batteries with high sulfur loading enabled by a bifunctional layered-MoO3 cathode, J. Power Sources 436 (2019), 226840. [19] Y. Yuan, G. Tan, J. Wen, J. Lu, L. Ma, C. Liu, X. Zuo, R. Shahbazian-Yassar, T. Wu, K. Amine, Encapsulating various sulfur allotropes within graphene nanocages for long-lasting lithium storage, Adv. Funct. Mater. 28 (2018) 1–10. [20] M. Liu, X. Qin, Y.B. He, B. Li, F. Kang, Recent innovative configurations in highenergy lithium-sulfur batteries, J. Mater. Chem. A. 5 (2017) 5222–5234. [21] R. Xu, J. Lu, K. Amine, Progress in mechanistic understanding and characterization techniques of Li-S batteries, Adv. Energy Mater. 5 (2015) 1–22.
8
R. Zhang et al.
Journal of Power Sources 451 (2020) 227751 [59] X. Tao, L. Dong, X. Wang, W. Zhang, B.J. Nelson, X. Li, B4C-nanowires/carbonmicrofiber hybrid structures and composites from cotton T-shirts, Adv. Mater. 22 (2010) 2055–2059. [60] L. Zhang, L. Xu, Y. Zhang, X. Zhou, L. Zhang, A. Yasin, L. Wang, K. Zhi, Facile synthesis of bio-based nitrogen- and oxygen-doped porous carbon derived from cotton for supercapacitors, RSC Adv. 8 (2018) 3869–3877. [61] L. Shi, Y. Gu, L. Chen, Y. Qian, Z. Yang, J. Ma, A low temperature synthesis of crystalline B4C ultrafine powders, Solid State Commun. 128 (2003) 5–7. [62] J. Guo, L. Zhang, T. Fujita, T. Goto, M. Chen, Pressure-induced depolarization and resonance in Raman scattering of single-crystalline boron carbide, Phys. Rev. B Condens. Matter Mater. Phys. 81 (2010) 1–4. [63] L.G. Jacobsohn, R.K. Schulze, M.E.H. Maia Da Costa, M. Nastasi, X-ray photoelectron spectroscopy investigation of boron carbide films deposited by sputtering, Surf. Sci. 572 (2004) 418–424. [64] M. Royle, J. Cho, S.W. Martin, Royle, Raman spectroscopy studies of xNa2S þ (1 x)B2S3 glasses and polycrystals, J. Non-Cryst. Solids 279 (2001) 97–109.
[52] F. Thevenot, State of the art boron carbide A comprehensive review, J. Eur. Ceram. Soc. 6 (1990) 205–225. [53] L. Luo, S.H. Chung, H. Yaghoobnejad Asl, A. Manthiram, Long-life lithium–sulfur batteries with a bifunctional cathode substrate configured with boron carbide nanowires, Adv. Mater. 30 (2018) 1–7. [54] N. Song, Z. Gao, Y. Zhang, X. Li, B 4 C nanoskeleton enabled, flexible lithium-sulfur batteries, Nano Energy 58 (2019) 30–39. [55] C. Kim, T. Fujino, T. Hayashi, M. Endo, M.S. Dresselhaus, Structural and electrochemical properties of pristine and B-doped materials for the anode of Li-ion secondary batteries, J. Electrochem. Soc. 147 (2002) 1265. [56] W. Bin Luo, S.L. Chou, J.Z. Wang, H.K. Liu, A B4C nanowire and carbon nanotube composite as a novel bifunctional electrocatalyst for high energy lithium oxygen batteries, J. Mater. Chem. A. 3 (2015) 18395–18399. [57] S. Mu, X. Chen, R. Sun, X. Liu, H. Wu, D. He, K. Cheng, Nano-size boron carbide intercalated graphene as high performance catalyst supports and electrodes for PEM fuel cells, Carbon N. Y. 103 (2016) 449–456. [58] L. Xu, S. Li, Y. Zhang, Y. Zhai, Synthesis, properties and applications of nanoscale nitrides, borides and carbides, Nanoscale 4 (2012) 4900–4915.
9