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Novel cathode structure based on spiral carbon nanotubes for lithium–sulfur batteries
Journal of Electroanalytical Chemistry 851 (2019) 113477
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Novel cathode structure based on spiral carbon nanotubes for lithium–sulfur batteries Zengren Tao a, Jianrong Xiao a, b, *, Heng Wang a, Fuwen Zhang a, ** a b
College of Science, Guilin University of Technology, Guilin, China Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials, Guilin University of Technology, Guilin, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Plane double-layer structure Spiral carbon nanotubes Active material Polysulfide adsorption Lithium–sulfur battery
Conductive carbon materials have been widely used as framework materials for electrodes. Spiral carbon nanotubes (SCNTs) with excellent physical and chemical properties can be used in lithium–sulfur (Li–S) batteries. Here, a novel type of planar double-layer structure cathode was designed by introducing SCNTs. Sulfur, activated carbon (AC), and SCNTs were used to fabricate the S/AC/SCNT and S/AC/SCNT–AC planar double-layer structure cathodes. The initial discharge capacity of the batteries with the S/AC/SCNT and S/AC/SCNT–AC cathodes reached 1016 mAh g1 and 946 mAh g1 at 0.2 C, respectively. This result indicates that SCNTs play a major role given their excellent conductivity and insertion throughout the cathodes, which can provide an efficient charge transfer path for active materials. Moreover, the S/AC/SCNT–AC cathode exhibits long cycle stability with a capacity decay rate of only 0.11% per cycle at 0.5 C. Therefore, the introduction of SCNTs to a planar double-layer cathode is an efficient method for increasing the conductivity and bound polysulfide amount of Li–S batteries.
1. Introduction Lithium–sulfur (Li–S) batteries demonstrate considerable potential as next-generation energy storage devices because they not only exhibit high theoretical specific capacity (1675 mAh g1), but are also economical and environmentally friendly [1,2]. However, given the intrinsic insulating nature of elemental sulfur and lithium sulfide/disulfide, Li–S batteries suffer from low sulfur utilization, and consequently, low energy density [2–4], which limits their practical applications. To solve the aforementioned problems, conductive carbon is typically selected to prepare a sulfur–carbon composite cathode and used as a conductive framework [5–8]. Composite materials with different types of carbon have been prepared to improve the utilization rate of active substances and the cycling performance of Li–S batteries [9–11]. However, by simply compounding conductive carbon materials with sulfur, intermediate products polysulfides can be easily dissolved in an electrolyte during the battery cycle, thereby resulting in low coulomb efficiency and poor cycling stability. Therefore, considerable effort has been exerted in designing the structure of cathodes. In recent years, a special structure of cathode composites has been reported to be crucial for the utilization and retention of active materials. The coaxial carbon
nanotube [12–17], dual core–shell [18–20], and hierarchical plane [21–25] structures have been proposed to improve the electrochemical performance of Li–S batteries. Among these, the planar layered structure exhibits high potential in increasing the loading and utilization of active materials through a simple positive electrode preparation process. This hierarchical architecture provides three functions: (i) continuous electron and ion diffusion channels to improve the electrochemical utilization rate of sulfur, (ii) a layered space that inhibits the diffusion of polysulfides, and (iii) a layer-by-layer cathode with a significantly larger loading of active substances [24]. Spiral carbon nanotubes (SCNTs) have elicited considerable attention since their discovery by Amelinckx [26,27] in 1994 because of their excellent physical and chemical properties and potential applications [28–30]. SCNTs exhibit unique electrical and mechanical properties and can be used in nanomaterials engineering [31]. Carbon nanotubes (CNTs) not only display excellent physical and chemical properties, but also novel linear and nonlinear optical properties. The electronic properties of CNTs are strongly related to their size, structure, and impurity degree [32]. As a type of CNTs with a special spiral structure, SCNTs also present many excellent properties of CNTs. Due to the excellent energy absorption capacity of these nanomaterials, the mechanical strength of
* Correspondence to: JR Xiao, Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials, Guilin University of Technology, Guilin, China. ** Corresponding author. E-mail addresses: [email protected] (J. Xiao), [email protected] (F. Zhang). https://doi.org/10.1016/j.jelechem.2019.113477 Received 22 May 2019; Received in revised form 11 July 2019; Accepted 9 September 2019 Available online 10 September 2019 1572-6657/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. a) Nitrogen physisorption isotherms and b) pore size distributions of AC.
2. Experimental 2.1. Preparation of SCNT materials Deionized water was added to a mixture of nickel nitrate and citric acid with a molar ratio of 2:1 followed by magnetic stirring for 6 h. Then, the mixture was dried at 60 C in an oven for 12 h and moved to the tube furnace. The temperature of the reduction reaction was set as 600 C for 1 h. Lastly, acetylene was added and the mixture was kept at 650 C for 1 h. The SCNT materials were obtained after cooling. 2.2. Preparation of S/AC/SCNT materials Sulfur, AC, and SCNTs were placed in an agate mortar with a mass ratio of sulfur:AC:SCNTs ¼ 7:2:1 and grounded for 1 h. Then, the mixture was transferred to a polytetrafluoroethylene reaction vessel. The vessel was kept open in an argon-filled glovebox for 0.5 h to exclude air and avoid the oxidation of sulfur at high temperature during the reaction. Subsequently, the reaction vessel was heated at 155 C for 10 h. Sulfur exhibits the smallest viscosity and the best flowability under such condition. Molten sulfur can then easily penetrate into the pores of AC. The composite material was also filled with SCNTs. Finally, S/AC/SCNT composites were obtained after cooling the mixture to room temperature.
Fig. 2. XRD patterns of S/AC/SCNT-AC, S/AC/SCNT, S/AC and S.
polymer was remarkably improved by introducing SCNTs [30]. However, many experimental studies on SCNTs have focused on their electrical, mechanical, and microwave absorption properties. By contrast, minimal work has been conducted in the field of electrochemistry. In the current study, SCNTs were synthesized and used as conductive framework materials with active carbon (AC) and sulfur, i.e., S/AC/ SCNT. Then, S/AC/SCNT–AC was prepared by coating a layer of AC on the S/AC/SCNT composite material. The excellent conductivity and mechanical properties of SCNTs, along with the strong adsorption capacity of AC, lead to the good performance, cycle stability, and safety of Li–S batteries.
2.3. Preparation of cathodes Sulfur, S/AC, and S/AC/SCNTs were used to prepare sulfur, S/AC, and S/AC/SCNT-based cathode composites were mixed with a conductive agent (Super-P) and a binder (polyvinylidene fluoride or PVDF) in a solution of N-methyl-2-pyrrolidone with a mass ratio of 8:1:1, respectively. Four kinds of cathode were prepared by coating the mixture active material on aluminum foil: (1) pure sulfur monolayer cathode, (2) S/AC monolayer cathode, (3) S/AC/SCNT monolayer cathode, and (4) S/AC/
Fig. 3. The schematic illustration of S/AC/SCNT and S/AC/SCNT-AC cathode. 2
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Journal of Electroanalytical Chemistry 851 (2019) 113477
Fig. 4. High-magnification SEM images of the S/AC/SCNT cathode a, b) before and c) after 200 cycles and S/AC/SCNT-AC cathode d, e) before and f) after 200 cycles at 0.1 C.
Fig. 5. Low-magnification SEM images and elemental mapping of the S/AC/SCNT cathode a) before and b) after 200 cycles at 0.1 C.
SCNT–AC planar double-layer structure cathode (i.e., a S/AC/SCNT monolayer cathode coated with AC). Then, these cathode plates were dried in an oven at 60 C for 10 h. Subsequently, the plates were cut into
small pieces with the diameter of 16 mm. According to the weighing calculation, the sulfur loading of cathode is about 3.7 mg/cm2.
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Journal of Electroanalytical Chemistry 851 (2019) 113477
Fig. 6. Low-magnification SEM images and elemental mapping of the S/AC/SCNT-AC cathode a) before and b) after 200 cycles at 0.1 C.
voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of a battery. Discharge/charge curves and cyclability performance were estimated under galvanostatic conditions between 1.5 V and 2.8 V at 25 C. The CV tests were performed at a scan rate of 0.01 mVs1 within the voltage range of 1.5–2.8 V at 25 C. The EIS tests were performed within a frequency range of 0.01–100 kHz with a perturbation amplitude of 5 mV at 25 C.
2.4. Cell assembly The electrochemical performance of the sulfur-based cathodes was tested via CR-2025 button batteries. The batteries were composed of a cathode (S/AC/SCNT–AC, S/AC/SCNT, S/AC, or S) and a lithium metal anode separated by a separator (polypropylene, Celgard 2400). These components were assembled in an argon-filled glovebox with oxygen and water contents below 0.1 ppm. The electrolyte consisted of 1.0 wt% LiNO3 and 1.0 M LiTFSI. The assembled batteries were sealed with an aluminum soft packaging film. The cells were allowed to rest at 25 C for 12 h before conducting electrochemical tests.
3. Results and discussion 3.1. Configuration and morphology The N2 adsorption–desorption isotherms of the AC materials used in the cathodes were measured using a nitrogen adsorption apparatus at p⋅p1 0 ¼ 0.05–0.2. As shown in Fig. 1a, when the relative pressure is low (i.e., p⋅p1 0 < 0.1), the adsorption capacity of N2 increases sharply and the adsorption rate is fast. There is completely coincident and no hysteresis ring in the adsorption-desorption curve. However, a hysteresis ring is evident at higher relative pressure (i.e., p⋅p1 0 > 0. 6). A conclusion can be drawn that the AC material exhibits a micro/mesoporous dual-pore structure [33], which not only helps limit the shuttle effect but also acts as a buffer to alleviate the volume expansion of sulfur in the reaction process. The total specific surface area calculated using the BET method was 1386 m2 g1, and the total pore volume measured at a relative 3 1 pressure of p⋅p1 0 ¼ 0.97 was 0.21 cm g . The pore size distribution of the microspores and mesoporous of AC was measured using the Barrett–Joyner–Halenda and Horvath–Kawazoe models, respectively. As shown in Fig. 1b, the microspore size of AC is mostly distributed around 0.5 nm, whereas the mesoporous size is mostly distributed around
2.5. Material characterization The multipoint Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas of AC at a relative pressure p⋅p1 0 ¼ 0.05–0.2 with Micromeritics ASAP 2020, and the total number of pores was ascertained at a relative pressure of p⋅p1 0 ¼ 0.97. A field emission scanning electron microscope (Hitachi S-4800) was used to characterize the morphology of the S/AC/SCNT–AC and S/AC/SCNT cathodes before and after 200 cycles. An energy-dispersive spectrometer was utilized to determine the distribution of elements on the surface of the S/AC/SCNT-AC and S/AC/SCNT cathodes. An X-ray diffractometer (X’Pert PRO) was used to characterize the phase structure and crystallinity of S/AC/SCNT–AC, S/AC/SCNT, S/AC, and sulfur. 2.6. Electrochemical measurement An electrochemical workstation (CHI750E) was used to test the cyclic 4
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Journal of Electroanalytical Chemistry 851 (2019) 113477
Fig. 7. High-magnification SEM images and elemental mapping of the S/AC/SCNT-AC cathode after 200 cycles at 0.1 C.
AC/SCNT–AC cathode due to the decreasing content of sulfur. Moreover, the reflection peaks of sulfur at 2θ ¼ 23.083 is hardly observed in the S/ AC/SCNT–AC cathode due to the surficial coating layer of AC. Fig. 3 shows the schematic illustration of S/AC/SCNT and S/AC/ SCNT-AC cathode. Fig. 4 illustrates the scanning electron microscopy (SEM) of the S/AC/SCNT and S/AC/SCNT–AC cathodes before and after 200 cycles. Before cycling, from Figs. 3 and 4, the S/AC/SCNT cathode was covered with well-distributed granular AC [33,36]. The SCNTs were
12.8 nm. The phase structure and crystallinity of the four cathode materials were detected using the X-ray diffractometer. As shown in Fig. 2, the strong and sharp reflection peaks located at 2θ ¼ 23.083 and 27.769 originated from the skew square-type structure of S8 polysulfide [34,35]. The “steamed bread peak” at 2θ ¼ 20 –30 in all the four cathode materials indicates the amorphous state of AC. The relative intensity of the reflection peaks of sulfur decreases gradually from sulfur to that of the S/ 5
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Journal of Electroanalytical Chemistry 851 (2019) 113477
Fig. 8. Cycling performance of Li–S cells with S/AC/SCNT-AC, S/AC/SCNT, S/AC and S monolayer cathodes at a) 0.1, b) 0.2, c) 0.5, and d) Long-term cycling performance at 0.5 C.
(sulfur, carbon, oxygen, and fluorine). A cloud-like substance can be clearly seen, and the corresponding element signal diagram shows that the signal of sulfur is strong in that area, but relatively weak in other areas. The signal of carbon is weak in cloud-like area, but relatively strong in other areas. This finding further confirms that the cloud-like substances on the surface of the positive electrode are active substance.
bonded together by PVDF and inserted throughout the cathode, thereby providing an efficient charge transfer paths for active substances (Fig. 4a and b). No SCNT can be observed due to the coating layer of AC on S/AC/ SCNT (Fig. 4d and e). However, the coating layer of AC can provide more pores for active substance migration to the upper layer, thereby expanding storage space and enhancing the adsorption performance of polysulfides. Consequently, the shuttle effect can be alleviated further. Fig. 4c and f show the SEM images of the S/AC/SCNT and S/AC/ SCNT–AC cathodes after 200 cycles. Granular AC evidently disappears, and instead, many cloud-like substances are observed. These cloud-like substances may be the polysulfide Li2Sx (4 n 8), which is generated during cycling. These substances are evenly attached to the surface of AC, and the loss of active substances can be limited effectively. To analysis of active substances distribution in the cathode, we performed an energy-dispersive X-ray spectroscopy (EDS) surface scan (sulfur, carbon, oxygen, and fluorine) of the S/AC/SCNT and S/AC/ SCNT–AC cathodes before and after cycling, as shown in Figs. 5 and 6. The distribution of sulfur is uneven and clearly exhibits the cluster phenomenon, whereas the signal of carbon is stronger. After cycling (Fig. 5b), sulfur is distributed homogeneously and its signal is also enhanced. Meanwhile, the signal of carbon is weakened. In Fig. 6a, the signal of sulfur is extremely weak due to the coating layer of AC. By contrast, the signal of carbon is strong and unevenly distributed, and an evident agglomeration phenomenon can be observed. After cycling, the signal of sulfur is noticeably enhanced and distributed more homogeneously because polysulfides continue to migrate to the upper layer from the first layer of the S/AC/SCNT–AC cathode during cycling and are adsorbed by AC. Fig. 7 shows the SEM image of the S/AC/SCNT–AC cathode at a higher magnification rate and the EDS surface scanning of four elements
3.2. Electrochemical stability Fig. 8 shows the cycling performance curves of the four batteries at 0.1 C, 0.2 C, and 0.5 C rates. The initial discharge specific capacity of S/ AC/SCNT and S/AC/SCNT–AC at 0.1 C is 1037 mAh g1 and 1002 mAh g1, respectively. The reversible discharge capacity after 200 cycles is 654 mAh g1 and 735 mAh g1, respectively. The specific discharge capacity of S/AC/SCNT–AC is slightly lower than that of S/AC/ SCNT in a few times ago, which can be attributed to the fact that the S/ AC/SCNT–AC battery requires an activation process because of the coating of AC layer on the surface. After 200 cycles, however, the capacity retention of S/AC/SCNT–AC and S/AC/SCNT is 73.4% and 63.1%, respectively, thereby indicating better cycling stability for the S/AC/ SCNT–AC cathode. A similar conclusion is drawn at 0.2 C and 0.5 C. Therefore, the planar double-layer structure of a cathode plays an important role in improving the cycling performance of battery. The initial specific capacity and cycling stability of the S/AC/SCNT and S/ AC/SCNT–AC cathodes were also considerably improved compared with those of the S/AC cathode, with an initial discharge capacity of 766 mAh g1 and 492 mAh g1, respectively, after 200 cycles. This finding can be attributed to the excellent electrical conductivity of SCNTs and their insertion throughout the cathode, which provide an efficient electron transfer path for active substances. Therefore, the rate 6
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Journal of Electroanalytical Chemistry 851 (2019) 113477
Fig. 9. a) Rate performance of S/AC/SCNT-AC, S/AC/SCNT, S/AC and S monolayers, b) average Coulombic efficiency of S/AC/SCNT-AC and S/AC/SCNT at different rates, and c) initial discharge/charge curves of the S/AC/SCNT-AC cathode at various C rates from 0.1 C to 1 C.
Fig. 10. CV curves at a scan rate of 0.1 mV s1 a) S/AC/SCNT and b) S/AC/SCNT-AC.
capacity decline rate of 0.11% per cycle and a high coulombic efficiency of 98%. SCNTs can play dual roles in improving the cycling performance of a battery: (i) they can increase the conductivity of a composite to facilitate the transfer of electrons, and (ii) they can limit the diffusion of
performance and coulomb efficiency (close to 100%) of a battery are considerably improved. Fig. 8d shows the cycling performance of the S/AC/SCNT–AC cathode in 500 cycles. The capacity is approximately 463 mAh g1, with a low
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Journal of Electroanalytical Chemistry 851 (2019) 113477
Fig. 11. EIS of the cells with S/AC/SCNT-AC, S/AC/SCNT, S/AC and S monolayer cathodes a) before and b) after 100 cycles at 0.1 C.
electrochemical performance, the electrochemical impedance spectra of the four batteries were tested before and after 100 cycles at 0.2 C. In the equivalent circuit (Fig. 11), R1 indicates the resistance of the electrolyte, R2 denotes the charge transfer resistance, CPE1 represents the constant phase elements, and W1 is the Warburg diffusion impedance. As shown in Fig. 11a and b, the electrochemical impedance curve consists of two parts: (1) a semicircle in the middle with a high-frequency band, which corresponds to the R2 of a battery; and (2) the slope in the low-frequency band, which represents W1 [43–48]. The batteries with the S/AC/SCNT and S/AC/SCNT–AC cathodes have a considerably smaller diameter for the semicircle, which indicates that charge transfer resistance is significantly reduced through the introduction of SCNTs. After 100 cycles, the two batteries display a noticeable decrease in R2 value due to the redistribution of active materials through improved contact with the electrolyte. In conclusion, the introduction of SCNTs can effectively reduce the impedance and improve the conductivity of batteries.
polysulfides. Fig. 9a shows the rate performance of the batteries with the four cathodes (sulfur, S/AC, S/AC/SCNT, and S/AC/SCNT–AC) at different rates (0.1 C, 0.2 C, 0.3 C, 0.5 C, and 1 C). The capacity of the S/AC/SCNT and S/AC/SCNT–AC cells is stable within each rate range, thereby indicating that active substances can be fully oxidized and reduced. The capacity of the S/AC/SCNT and S/AC/SCNT–AC cells decreases slowly from 1038 mAh g1 and 1010 mAh g1 at 0.1 C to 883 mAh g1 and 919 mAh g1, 768 mAh g1 and 792 mAh g1, and 571 mAh g1 and 672 mAh g1 at 0.2 C, 0.3 C, and 0.5 C, respectively. The capacities are considerably higher than those of the sulfur and S/AC cathodes. Meanwhile, the capacity of the S/AC/SCNT battery is higher than that of the S/ AC/SCNT–AC battery at 0.1 C. By contrast, the capacity of the S/AC/ SCNT–AC battery is higher when the current is larger than 0.2 C, which is consistent with the results of a previous analysis. These findings show that the S/AC/SCNT–AC battery exhibits better performance. Moreover, the S/AC/SCNT and S/AC/SCNT–AC batteries are evidently superior to the cells with the sulfur and S/AC cathodes; therefore, SCNTs can remarkably improve the performance of cells. Both cells with SCNTs maintain high coulombic efficiency at even higher rates (Fig. 9b). Fig. 9c shows the charge–discharge curves of the S/AC/SCNT–AC cathode cells at various charge–discharge rates. A long charge/discharge platform can be observed at 0.1 C, which indicates lower polarization. Therefore, sulfur can be completely converted into insoluble Li2S2/Li2S, thereby increasing the utilization ratio of active materials. The first three CV curves of the S/AC/SCNT and S/AC/SCNT–AC batteries with a voltage range of 1.5–2.8 V and a scanning rate of 0.1 mV s1 are shown in Fig. 10. Each CV curve exhibits a reduction peak at voltages of 2.3 V and 2.0 V, which correspond to the transformation from S8 to Li2Sx (4 x 8) and from Li2Sx (4 x 8) to Li2S2/Li2S, respectively [37,38]. In addition, the CV curve also presents an oxidation peak at 2.5 V, which corresponds to the conversion reaction of Li2S2/Li2S to Li2Sx (4 x 8) and S8 [39–42]. The position of the reduction peak of the S/AC/SCNT battery is slightly higher than that of the S/AC/SCNT–AC battery, it due to the better conductivity of the S/AC/SCNT cathode. However, Fig. 10b shows that the first three CV curves of the S/AC/ SCNT–AC batteries are highly coincident, thereby indicating more stable electrochemical reactions, better cycling performance, and lower polarization. The stable CV curve also indicates that sulfur has reacted sufficiently in the cell. To explain why batteries with SCNTs exhibit considerably better
4. Conclusions SCNTs are applied to cathode materials, and a planar double-layer structure cathode is reasonably designed for Li–S batteries. An S/AC/ SCNT–AC planar double-layer cathode material was prepared using SCNTs as the conductive skeleton material, AC, and sulfur. Lastly, a coating of AC material layer on the basis of the composite material, the layer can improve the safety and rate performance of Li–S batteries. Before and after cycling, the SEM and EDS images show that an active substance can be effectively intercepted by AC during the cycling process, which improves the cycling performance of a battery. In the electrochemical tests of the cells, the S/AC/SCNT and S/AC/SCNT-AC cathode cells obtain a higher initial discharge capacity at 0.2 C (1027 mAh g1 and 932 mAh g1). After 200 cycles, the reversible capacities of 650 mAh g1 and 563 mAh g1 are maintained. In particular, the S/AC/ SCNT–AC cell can retain 46% capacity after 500 cycles at 0.5 C, and the capacity decay rate is only 0.11% per cycle. Therefore, SCNTs and the planar double-layer structure provide considerable advantages in the application of Li–S batteries. Declaration of competing interest The authors declare that there is no conflict of interests regarding the publication of this paper. 8
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Acknowledgments
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Novel cathode structure based on spiral carbon nanotubes for lithium–sulfur batteries Zengren Tao a, Jianrong Xiao a, b, *, Heng Wang a, Fuwen Zhang a, ** a b
College of Science, Guilin University of Technology, Guilin, China Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials, Guilin University of Technology, Guilin, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Plane double-layer structure Spiral carbon nanotubes Active material Polysulfide adsorption Lithium–sulfur battery
Conductive carbon materials have been widely used as framework materials for electrodes. Spiral carbon nanotubes (SCNTs) with excellent physical and chemical properties can be used in lithium–sulfur (Li–S) batteries. Here, a novel type of planar double-layer structure cathode was designed by introducing SCNTs. Sulfur, activated carbon (AC), and SCNTs were used to fabricate the S/AC/SCNT and S/AC/SCNT–AC planar double-layer structure cathodes. The initial discharge capacity of the batteries with the S/AC/SCNT and S/AC/SCNT–AC cathodes reached 1016 mAh g1 and 946 mAh g1 at 0.2 C, respectively. This result indicates that SCNTs play a major role given their excellent conductivity and insertion throughout the cathodes, which can provide an efficient charge transfer path for active materials. Moreover, the S/AC/SCNT–AC cathode exhibits long cycle stability with a capacity decay rate of only 0.11% per cycle at 0.5 C. Therefore, the introduction of SCNTs to a planar double-layer cathode is an efficient method for increasing the conductivity and bound polysulfide amount of Li–S batteries.
1. Introduction Lithium–sulfur (Li–S) batteries demonstrate considerable potential as next-generation energy storage devices because they not only exhibit high theoretical specific capacity (1675 mAh g1), but are also economical and environmentally friendly [1,2]. However, given the intrinsic insulating nature of elemental sulfur and lithium sulfide/disulfide, Li–S batteries suffer from low sulfur utilization, and consequently, low energy density [2–4], which limits their practical applications. To solve the aforementioned problems, conductive carbon is typically selected to prepare a sulfur–carbon composite cathode and used as a conductive framework [5–8]. Composite materials with different types of carbon have been prepared to improve the utilization rate of active substances and the cycling performance of Li–S batteries [9–11]. However, by simply compounding conductive carbon materials with sulfur, intermediate products polysulfides can be easily dissolved in an electrolyte during the battery cycle, thereby resulting in low coulomb efficiency and poor cycling stability. Therefore, considerable effort has been exerted in designing the structure of cathodes. In recent years, a special structure of cathode composites has been reported to be crucial for the utilization and retention of active materials. The coaxial carbon
nanotube [12–17], dual core–shell [18–20], and hierarchical plane [21–25] structures have been proposed to improve the electrochemical performance of Li–S batteries. Among these, the planar layered structure exhibits high potential in increasing the loading and utilization of active materials through a simple positive electrode preparation process. This hierarchical architecture provides three functions: (i) continuous electron and ion diffusion channels to improve the electrochemical utilization rate of sulfur, (ii) a layered space that inhibits the diffusion of polysulfides, and (iii) a layer-by-layer cathode with a significantly larger loading of active substances [24]. Spiral carbon nanotubes (SCNTs) have elicited considerable attention since their discovery by Amelinckx [26,27] in 1994 because of their excellent physical and chemical properties and potential applications [28–30]. SCNTs exhibit unique electrical and mechanical properties and can be used in nanomaterials engineering [31]. Carbon nanotubes (CNTs) not only display excellent physical and chemical properties, but also novel linear and nonlinear optical properties. The electronic properties of CNTs are strongly related to their size, structure, and impurity degree [32]. As a type of CNTs with a special spiral structure, SCNTs also present many excellent properties of CNTs. Due to the excellent energy absorption capacity of these nanomaterials, the mechanical strength of
* Correspondence to: JR Xiao, Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials, Guilin University of Technology, Guilin, China. ** Corresponding author. E-mail addresses: [email protected] (J. Xiao), [email protected] (F. Zhang). https://doi.org/10.1016/j.jelechem.2019.113477 Received 22 May 2019; Received in revised form 11 July 2019; Accepted 9 September 2019 Available online 10 September 2019 1572-6657/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. a) Nitrogen physisorption isotherms and b) pore size distributions of AC.
2. Experimental 2.1. Preparation of SCNT materials Deionized water was added to a mixture of nickel nitrate and citric acid with a molar ratio of 2:1 followed by magnetic stirring for 6 h. Then, the mixture was dried at 60 C in an oven for 12 h and moved to the tube furnace. The temperature of the reduction reaction was set as 600 C for 1 h. Lastly, acetylene was added and the mixture was kept at 650 C for 1 h. The SCNT materials were obtained after cooling. 2.2. Preparation of S/AC/SCNT materials Sulfur, AC, and SCNTs were placed in an agate mortar with a mass ratio of sulfur:AC:SCNTs ¼ 7:2:1 and grounded for 1 h. Then, the mixture was transferred to a polytetrafluoroethylene reaction vessel. The vessel was kept open in an argon-filled glovebox for 0.5 h to exclude air and avoid the oxidation of sulfur at high temperature during the reaction. Subsequently, the reaction vessel was heated at 155 C for 10 h. Sulfur exhibits the smallest viscosity and the best flowability under such condition. Molten sulfur can then easily penetrate into the pores of AC. The composite material was also filled with SCNTs. Finally, S/AC/SCNT composites were obtained after cooling the mixture to room temperature.
Fig. 2. XRD patterns of S/AC/SCNT-AC, S/AC/SCNT, S/AC and S.
polymer was remarkably improved by introducing SCNTs [30]. However, many experimental studies on SCNTs have focused on their electrical, mechanical, and microwave absorption properties. By contrast, minimal work has been conducted in the field of electrochemistry. In the current study, SCNTs were synthesized and used as conductive framework materials with active carbon (AC) and sulfur, i.e., S/AC/ SCNT. Then, S/AC/SCNT–AC was prepared by coating a layer of AC on the S/AC/SCNT composite material. The excellent conductivity and mechanical properties of SCNTs, along with the strong adsorption capacity of AC, lead to the good performance, cycle stability, and safety of Li–S batteries.
2.3. Preparation of cathodes Sulfur, S/AC, and S/AC/SCNTs were used to prepare sulfur, S/AC, and S/AC/SCNT-based cathode composites were mixed with a conductive agent (Super-P) and a binder (polyvinylidene fluoride or PVDF) in a solution of N-methyl-2-pyrrolidone with a mass ratio of 8:1:1, respectively. Four kinds of cathode were prepared by coating the mixture active material on aluminum foil: (1) pure sulfur monolayer cathode, (2) S/AC monolayer cathode, (3) S/AC/SCNT monolayer cathode, and (4) S/AC/
Fig. 3. The schematic illustration of S/AC/SCNT and S/AC/SCNT-AC cathode. 2
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Fig. 4. High-magnification SEM images of the S/AC/SCNT cathode a, b) before and c) after 200 cycles and S/AC/SCNT-AC cathode d, e) before and f) after 200 cycles at 0.1 C.
Fig. 5. Low-magnification SEM images and elemental mapping of the S/AC/SCNT cathode a) before and b) after 200 cycles at 0.1 C.
SCNT–AC planar double-layer structure cathode (i.e., a S/AC/SCNT monolayer cathode coated with AC). Then, these cathode plates were dried in an oven at 60 C for 10 h. Subsequently, the plates were cut into
small pieces with the diameter of 16 mm. According to the weighing calculation, the sulfur loading of cathode is about 3.7 mg/cm2.
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Fig. 6. Low-magnification SEM images and elemental mapping of the S/AC/SCNT-AC cathode a) before and b) after 200 cycles at 0.1 C.
voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of a battery. Discharge/charge curves and cyclability performance were estimated under galvanostatic conditions between 1.5 V and 2.8 V at 25 C. The CV tests were performed at a scan rate of 0.01 mVs1 within the voltage range of 1.5–2.8 V at 25 C. The EIS tests were performed within a frequency range of 0.01–100 kHz with a perturbation amplitude of 5 mV at 25 C.
2.4. Cell assembly The electrochemical performance of the sulfur-based cathodes was tested via CR-2025 button batteries. The batteries were composed of a cathode (S/AC/SCNT–AC, S/AC/SCNT, S/AC, or S) and a lithium metal anode separated by a separator (polypropylene, Celgard 2400). These components were assembled in an argon-filled glovebox with oxygen and water contents below 0.1 ppm. The electrolyte consisted of 1.0 wt% LiNO3 and 1.0 M LiTFSI. The assembled batteries were sealed with an aluminum soft packaging film. The cells were allowed to rest at 25 C for 12 h before conducting electrochemical tests.
3. Results and discussion 3.1. Configuration and morphology The N2 adsorption–desorption isotherms of the AC materials used in the cathodes were measured using a nitrogen adsorption apparatus at p⋅p1 0 ¼ 0.05–0.2. As shown in Fig. 1a, when the relative pressure is low (i.e., p⋅p1 0 < 0.1), the adsorption capacity of N2 increases sharply and the adsorption rate is fast. There is completely coincident and no hysteresis ring in the adsorption-desorption curve. However, a hysteresis ring is evident at higher relative pressure (i.e., p⋅p1 0 > 0. 6). A conclusion can be drawn that the AC material exhibits a micro/mesoporous dual-pore structure [33], which not only helps limit the shuttle effect but also acts as a buffer to alleviate the volume expansion of sulfur in the reaction process. The total specific surface area calculated using the BET method was 1386 m2 g1, and the total pore volume measured at a relative 3 1 pressure of p⋅p1 0 ¼ 0.97 was 0.21 cm g . The pore size distribution of the microspores and mesoporous of AC was measured using the Barrett–Joyner–Halenda and Horvath–Kawazoe models, respectively. As shown in Fig. 1b, the microspore size of AC is mostly distributed around 0.5 nm, whereas the mesoporous size is mostly distributed around
2.5. Material characterization The multipoint Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas of AC at a relative pressure p⋅p1 0 ¼ 0.05–0.2 with Micromeritics ASAP 2020, and the total number of pores was ascertained at a relative pressure of p⋅p1 0 ¼ 0.97. A field emission scanning electron microscope (Hitachi S-4800) was used to characterize the morphology of the S/AC/SCNT–AC and S/AC/SCNT cathodes before and after 200 cycles. An energy-dispersive spectrometer was utilized to determine the distribution of elements on the surface of the S/AC/SCNT-AC and S/AC/SCNT cathodes. An X-ray diffractometer (X’Pert PRO) was used to characterize the phase structure and crystallinity of S/AC/SCNT–AC, S/AC/SCNT, S/AC, and sulfur. 2.6. Electrochemical measurement An electrochemical workstation (CHI750E) was used to test the cyclic 4
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Fig. 7. High-magnification SEM images and elemental mapping of the S/AC/SCNT-AC cathode after 200 cycles at 0.1 C.
AC/SCNT–AC cathode due to the decreasing content of sulfur. Moreover, the reflection peaks of sulfur at 2θ ¼ 23.083 is hardly observed in the S/ AC/SCNT–AC cathode due to the surficial coating layer of AC. Fig. 3 shows the schematic illustration of S/AC/SCNT and S/AC/ SCNT-AC cathode. Fig. 4 illustrates the scanning electron microscopy (SEM) of the S/AC/SCNT and S/AC/SCNT–AC cathodes before and after 200 cycles. Before cycling, from Figs. 3 and 4, the S/AC/SCNT cathode was covered with well-distributed granular AC [33,36]. The SCNTs were
12.8 nm. The phase structure and crystallinity of the four cathode materials were detected using the X-ray diffractometer. As shown in Fig. 2, the strong and sharp reflection peaks located at 2θ ¼ 23.083 and 27.769 originated from the skew square-type structure of S8 polysulfide [34,35]. The “steamed bread peak” at 2θ ¼ 20 –30 in all the four cathode materials indicates the amorphous state of AC. The relative intensity of the reflection peaks of sulfur decreases gradually from sulfur to that of the S/ 5
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Fig. 8. Cycling performance of Li–S cells with S/AC/SCNT-AC, S/AC/SCNT, S/AC and S monolayer cathodes at a) 0.1, b) 0.2, c) 0.5, and d) Long-term cycling performance at 0.5 C.
(sulfur, carbon, oxygen, and fluorine). A cloud-like substance can be clearly seen, and the corresponding element signal diagram shows that the signal of sulfur is strong in that area, but relatively weak in other areas. The signal of carbon is weak in cloud-like area, but relatively strong in other areas. This finding further confirms that the cloud-like substances on the surface of the positive electrode are active substance.
bonded together by PVDF and inserted throughout the cathode, thereby providing an efficient charge transfer paths for active substances (Fig. 4a and b). No SCNT can be observed due to the coating layer of AC on S/AC/ SCNT (Fig. 4d and e). However, the coating layer of AC can provide more pores for active substance migration to the upper layer, thereby expanding storage space and enhancing the adsorption performance of polysulfides. Consequently, the shuttle effect can be alleviated further. Fig. 4c and f show the SEM images of the S/AC/SCNT and S/AC/ SCNT–AC cathodes after 200 cycles. Granular AC evidently disappears, and instead, many cloud-like substances are observed. These cloud-like substances may be the polysulfide Li2Sx (4 n 8), which is generated during cycling. These substances are evenly attached to the surface of AC, and the loss of active substances can be limited effectively. To analysis of active substances distribution in the cathode, we performed an energy-dispersive X-ray spectroscopy (EDS) surface scan (sulfur, carbon, oxygen, and fluorine) of the S/AC/SCNT and S/AC/ SCNT–AC cathodes before and after cycling, as shown in Figs. 5 and 6. The distribution of sulfur is uneven and clearly exhibits the cluster phenomenon, whereas the signal of carbon is stronger. After cycling (Fig. 5b), sulfur is distributed homogeneously and its signal is also enhanced. Meanwhile, the signal of carbon is weakened. In Fig. 6a, the signal of sulfur is extremely weak due to the coating layer of AC. By contrast, the signal of carbon is strong and unevenly distributed, and an evident agglomeration phenomenon can be observed. After cycling, the signal of sulfur is noticeably enhanced and distributed more homogeneously because polysulfides continue to migrate to the upper layer from the first layer of the S/AC/SCNT–AC cathode during cycling and are adsorbed by AC. Fig. 7 shows the SEM image of the S/AC/SCNT–AC cathode at a higher magnification rate and the EDS surface scanning of four elements
3.2. Electrochemical stability Fig. 8 shows the cycling performance curves of the four batteries at 0.1 C, 0.2 C, and 0.5 C rates. The initial discharge specific capacity of S/ AC/SCNT and S/AC/SCNT–AC at 0.1 C is 1037 mAh g1 and 1002 mAh g1, respectively. The reversible discharge capacity after 200 cycles is 654 mAh g1 and 735 mAh g1, respectively. The specific discharge capacity of S/AC/SCNT–AC is slightly lower than that of S/AC/ SCNT in a few times ago, which can be attributed to the fact that the S/ AC/SCNT–AC battery requires an activation process because of the coating of AC layer on the surface. After 200 cycles, however, the capacity retention of S/AC/SCNT–AC and S/AC/SCNT is 73.4% and 63.1%, respectively, thereby indicating better cycling stability for the S/AC/ SCNT–AC cathode. A similar conclusion is drawn at 0.2 C and 0.5 C. Therefore, the planar double-layer structure of a cathode plays an important role in improving the cycling performance of battery. The initial specific capacity and cycling stability of the S/AC/SCNT and S/ AC/SCNT–AC cathodes were also considerably improved compared with those of the S/AC cathode, with an initial discharge capacity of 766 mAh g1 and 492 mAh g1, respectively, after 200 cycles. This finding can be attributed to the excellent electrical conductivity of SCNTs and their insertion throughout the cathode, which provide an efficient electron transfer path for active substances. Therefore, the rate 6
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Fig. 9. a) Rate performance of S/AC/SCNT-AC, S/AC/SCNT, S/AC and S monolayers, b) average Coulombic efficiency of S/AC/SCNT-AC and S/AC/SCNT at different rates, and c) initial discharge/charge curves of the S/AC/SCNT-AC cathode at various C rates from 0.1 C to 1 C.
Fig. 10. CV curves at a scan rate of 0.1 mV s1 a) S/AC/SCNT and b) S/AC/SCNT-AC.
capacity decline rate of 0.11% per cycle and a high coulombic efficiency of 98%. SCNTs can play dual roles in improving the cycling performance of a battery: (i) they can increase the conductivity of a composite to facilitate the transfer of electrons, and (ii) they can limit the diffusion of
performance and coulomb efficiency (close to 100%) of a battery are considerably improved. Fig. 8d shows the cycling performance of the S/AC/SCNT–AC cathode in 500 cycles. The capacity is approximately 463 mAh g1, with a low
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Fig. 11. EIS of the cells with S/AC/SCNT-AC, S/AC/SCNT, S/AC and S monolayer cathodes a) before and b) after 100 cycles at 0.1 C.
electrochemical performance, the electrochemical impedance spectra of the four batteries were tested before and after 100 cycles at 0.2 C. In the equivalent circuit (Fig. 11), R1 indicates the resistance of the electrolyte, R2 denotes the charge transfer resistance, CPE1 represents the constant phase elements, and W1 is the Warburg diffusion impedance. As shown in Fig. 11a and b, the electrochemical impedance curve consists of two parts: (1) a semicircle in the middle with a high-frequency band, which corresponds to the R2 of a battery; and (2) the slope in the low-frequency band, which represents W1 [43–48]. The batteries with the S/AC/SCNT and S/AC/SCNT–AC cathodes have a considerably smaller diameter for the semicircle, which indicates that charge transfer resistance is significantly reduced through the introduction of SCNTs. After 100 cycles, the two batteries display a noticeable decrease in R2 value due to the redistribution of active materials through improved contact with the electrolyte. In conclusion, the introduction of SCNTs can effectively reduce the impedance and improve the conductivity of batteries.
polysulfides. Fig. 9a shows the rate performance of the batteries with the four cathodes (sulfur, S/AC, S/AC/SCNT, and S/AC/SCNT–AC) at different rates (0.1 C, 0.2 C, 0.3 C, 0.5 C, and 1 C). The capacity of the S/AC/SCNT and S/AC/SCNT–AC cells is stable within each rate range, thereby indicating that active substances can be fully oxidized and reduced. The capacity of the S/AC/SCNT and S/AC/SCNT–AC cells decreases slowly from 1038 mAh g1 and 1010 mAh g1 at 0.1 C to 883 mAh g1 and 919 mAh g1, 768 mAh g1 and 792 mAh g1, and 571 mAh g1 and 672 mAh g1 at 0.2 C, 0.3 C, and 0.5 C, respectively. The capacities are considerably higher than those of the sulfur and S/AC cathodes. Meanwhile, the capacity of the S/AC/SCNT battery is higher than that of the S/ AC/SCNT–AC battery at 0.1 C. By contrast, the capacity of the S/AC/ SCNT–AC battery is higher when the current is larger than 0.2 C, which is consistent with the results of a previous analysis. These findings show that the S/AC/SCNT–AC battery exhibits better performance. Moreover, the S/AC/SCNT and S/AC/SCNT–AC batteries are evidently superior to the cells with the sulfur and S/AC cathodes; therefore, SCNTs can remarkably improve the performance of cells. Both cells with SCNTs maintain high coulombic efficiency at even higher rates (Fig. 9b). Fig. 9c shows the charge–discharge curves of the S/AC/SCNT–AC cathode cells at various charge–discharge rates. A long charge/discharge platform can be observed at 0.1 C, which indicates lower polarization. Therefore, sulfur can be completely converted into insoluble Li2S2/Li2S, thereby increasing the utilization ratio of active materials. The first three CV curves of the S/AC/SCNT and S/AC/SCNT–AC batteries with a voltage range of 1.5–2.8 V and a scanning rate of 0.1 mV s1 are shown in Fig. 10. Each CV curve exhibits a reduction peak at voltages of 2.3 V and 2.0 V, which correspond to the transformation from S8 to Li2Sx (4 x 8) and from Li2Sx (4 x 8) to Li2S2/Li2S, respectively [37,38]. In addition, the CV curve also presents an oxidation peak at 2.5 V, which corresponds to the conversion reaction of Li2S2/Li2S to Li2Sx (4 x 8) and S8 [39–42]. The position of the reduction peak of the S/AC/SCNT battery is slightly higher than that of the S/AC/SCNT–AC battery, it due to the better conductivity of the S/AC/SCNT cathode. However, Fig. 10b shows that the first three CV curves of the S/AC/ SCNT–AC batteries are highly coincident, thereby indicating more stable electrochemical reactions, better cycling performance, and lower polarization. The stable CV curve also indicates that sulfur has reacted sufficiently in the cell. To explain why batteries with SCNTs exhibit considerably better
4. Conclusions SCNTs are applied to cathode materials, and a planar double-layer structure cathode is reasonably designed for Li–S batteries. An S/AC/ SCNT–AC planar double-layer cathode material was prepared using SCNTs as the conductive skeleton material, AC, and sulfur. Lastly, a coating of AC material layer on the basis of the composite material, the layer can improve the safety and rate performance of Li–S batteries. Before and after cycling, the SEM and EDS images show that an active substance can be effectively intercepted by AC during the cycling process, which improves the cycling performance of a battery. In the electrochemical tests of the cells, the S/AC/SCNT and S/AC/SCNT-AC cathode cells obtain a higher initial discharge capacity at 0.2 C (1027 mAh g1 and 932 mAh g1). After 200 cycles, the reversible capacities of 650 mAh g1 and 563 mAh g1 are maintained. In particular, the S/AC/ SCNT–AC cell can retain 46% capacity after 500 cycles at 0.5 C, and the capacity decay rate is only 0.11% per cycle. Therefore, SCNTs and the planar double-layer structure provide considerable advantages in the application of Li–S batteries. Declaration of competing interest The authors declare that there is no conflict of interests regarding the publication of this paper. 8
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Acknowledgments
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