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Preparing a composite including SnS2, carbon nanotubes and S and using as cathode material of lithium-sulfur battery
Scripta Materialia 177 (2020) 208–213
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Preparing a composite including SnS2 , carbon nanotubes and S and using as cathode material of lithium-sulfur battery Wu Jun, Chen Bing, Liu Qingqing, Hu Ailin, Lu Xiaoying∗, Jiang Qi∗ Superconductivity and New Energy R&D Centre, Southwest Jiaotong University, Key Laboratory of Advanced Technologies of Materials (Ministry of Education of China), Chengdu 610031, China
a r t i c l e
i n f o
Article history: Received 17 July 2019 Revised 1 September 2019 Accepted 23 October 2019
Keywords: Sns2 /CNTs/S composite Lithium-sulfur battery Cathode material
a b s t r a c t SnS2 is used to inhibit the “shuttle effect” and carbon nanotubes (CNTs) are used to improve the conductivity, thus a new Li-S battery cathode material (SnS2 /CNTs/S composite) is prepared in this paper. Morphological structure and electrochemical properties of the samples were characterized. The results show that among the composite, the CNTs are uniformly wrapped on the surface of SnS2 and S is uniformly loaded on the SnS2 /CNTs. The composite has a promising electrochemical performance: the first discharge capacity is 1308.6 mAh g − 1 at 0.1 C and there is a reversible capacity of 1002.3 mAh g−1 after 100 cycles. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
With the rapid development of human society, low-energy density lithium-ion batteries have become increasingly unable to meet the current needs of people. Lithium-sulfur (Li-S) battery has been focused on for its low cost, environmental friendliness, high theoretical capacity (1675 mAh g−1 ) and energy density (2600 Wh kg−1 ) [1–3], which is much higher than that of the conventional lithium-ion battery. However, the low conductivity of sulfur and Li2 S/Li2 S2 (discharge final products) and the “shuttle effect” of the lithium polysulfides have restricted the practical electrochemical performance of Li-S battery [4,5]. In order to address these problems, carbon materials with excellent electrical conductivity, for example, porous carbon [6], reduced graphene oxide (rGO) [7] and carbon nanotubes (CNTs) [8] are used to improve the electrical conductivity of the whole electrode. Metal oxides or metal sulfides with strong chemisorption, for example, SnO2 [9], MnO2 [10] and CoS2 [11], are also used to inhibit the dissolution of lithium polysulfides. In order to enhance the electrochemical performance of Li-S batteries, carbon materials and metal compounds are used together [12–14]. For example, a SnO2 @rGO/CNTs/S composite prepared by Liu etc. [15] and a MoS2 @CNTs/S composite prepared by Jeong et al. [16]. In recent years, SnS2 has been widely used as a cathode material in the field of lithium ion batteries and shows good results [17,18]. At present, SnS2 has begun to be applied to the field of Li-S battery for its strong chemical adsorption for lithium polysulfides. However, due to its poor conductivity, the improvement of Li-S battery performance is limited. Therefore, some researchers ∗
Corresponding authors. E-mail addresses: [email protected] (L. Xiaoying), [email protected] (J. Qi).
https://doi.org/10.1016/j.scriptamat.2019.10.038 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
began to use carbon materials with good conductivity to improve its conductivity. For example, Li et al. [19] used hollow carbon nanosphere and SnS2 to prepare S/AHCNS-SnS2 cathode material. Li et al. [20,21] prepared S/C-SnS2 cathode material by using porous carbon and Ketjen Black to improve the conductivity of SnS2 . The results showed that the energy storage performance of the prepared composites is improved to a certain extent after the introduction of carbon materials. One-dimensional CNTs are widely used in energy storage field because of their excellent mechanical properties, excellent conductivity and abundant nano-reaction sites [22]. The introduction of CNTs into the cathode material of Li-S battery can improve not only the conductivity of the composite, but also their mechanical properties, which is helpful to further improve their energy storage performance. So in this paper, the authors design a synergistic effect by using the excellent conductivity and mechanical properties of CNTs and the strong chemical adsorption of SnS2 to improve the electrochemical performance of Li-S battery. Four materials (S, SnS2 /S, CNTs/S and SnS2 /CNTs/S) were designed and prepared by a melting-diffusion method. Their structure and electrochemical performance were researched including SnS2 and CNTs. The results show that the CNTs and SnS2 in the composite behave a good synergistic effect. The three-dimensional conductive network composed of the CNTs improves the conductivity of the whole electrode. At the same time, the CNTs can refine the size of SnS2 , which is beneficial to enhance the adsorption of lithium polysulfides by SnS2 . SnS2 inhibits the dissolution of lithium polysulfides in the electrolyte by strong chemical adsorption and avoids the rapid loss of active material. The SnS2 /CNTs/S composite exhibits a promising electrochemical performance: the
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Fig. 1. XRD patterns (a) and TG curves (b) of the samples.
first discharge capacity is 1308.6 mAh g−1 at 0.1 C and there is a reversible capacity of 1002.3 mAh g−1 after 100 cycles. The CNTs was synthesized by our group via the chemical vapor deposition [23]. CNTs and polyvinylpyrrolidone (10 wt% of CNTs) were dispersed in a certain amount of deionized water, and the mixed solution was ultrasonically dispersed for 4 h. Then, suitable amount of SnCl4 ·5H2 O (mCNTs :mSnCl4 · 5H2O =1:10) and lcysteine (nSnCl4 · 5H2O :nL-cysteine =1:4) were added into the mixture. After stirring for 30 min, the mixture was transferred to a 100 ml autoclave to react at 180 °C for 24 h to prepare the SnS2 /CNTs composite. Pure SnS2 was prepared as the same operations without adding CNTs. SnS2 /CNTs/S composite was prepared with SnS2 /CNTs and S by a melting-diffusion treatment at 155 °C for 12 h in Argon atmosphere. SnS2 /S and CNTs/S were prepared by the similar operations. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TG, N2 atmosphere with 10 °C min−1 heating rate) were used to characterize the obtained samples. In order to research the electrochemical performance of the obtained samples, the obtained samples were assembled into CR2032 button batteries using the lithium metal as the anode. The electrolyte was the 1.0 mol l−1 lithium bis-trifluoromethanesulphonyphonylimide (LiTFSI, in 1, 3-dioxolane and 1, 2 - dimethoxyethane solution with 1:1 vol ratio) with 0.5 wt% LiNO3 additive. The cathodes were prepared by pressing the mixture of 80 wt% sample powders, 10 wt% conductive acetylene black and 10 wt% polyvinylidene fluoride on an aluminum foil. The membrane was the microporous separator (Celgard 2400). The Li-S batteries were assembled in an argon-filled glove box without water and oxygen. An automatic battery tester and CHI660 electrochemical workstation were used to characterize the electrochemical performance at 25 °C, including cyclic voltammogram (CV) with a scan rate of 0.1 mV s−1 , charge discharge curves, rate performance, cycle life curves at 0.1 C and electrochemical impedance spectroscopy (EIS, tested before the initial discharging process) from 100 kHz to 0.01 Hz with the amplitude of 5 mV. Fig. 1 shows the XRD patterns (a) and TG curves (b) of the obtained samples. As shown in Fig. 1a, the diffraction peak of the prepared SnS2 is consistent with that of the standard comparison card (JCDPS 01–089–2028), indicating that the obtained SnS2 is a single phase [24]. Its diffraction peaks are located at 15.0°, 28.1°, 32.2°, 41.8°, and 50.0°, respectively, corresponding to the 0 01, 10 0, 101, 102, and 110 crystal faces of the hexagonal phase [25]. And the same time, SnS2 /CNTs exhibits all the diffraction peaks of SnS2 and a broad diffraction peak appears at around 26°, which is the char-
acteristic peak of CNTs [26]. After loading S, the diffraction peak of SnS2 in the SnS2 /CNTs/S composite becomes weak, the peak of CNTs is covered, and the characteristic diffraction peak of S appears at around 23°, whose peak shape is sharp, indicating that S exists in the elemental form and has good crystallinity [27]. It can be seen from Fig. 1b that the sulfur contents of the samples (SnS2 /S, CNTs/S and SnS2 /CNTs/S) are 61.2%, 62.3% and 66.6%, respectively. And the areal loading amount of sulfur is about 1.2 mg cm−2 . Fig. 2 is the SEM images of the obtained samples (a, CNTs; b, SnS2 ; c and d, SnS2 /CNTs; e, SnS2 /CNTs/S; f, EDS elemental mapping images of Sn, S and C in the left selected regions of SnS2 /CNTs/S). As shown in Fig. 2a, the diameters of the CNTs are about 30–60 nm and the surface is smooth. The SnS2 shows a petal-like nanosheet structure in Fig. 2b, and the sheets are irregularly stacked between the sheets to form a large number of holes, which is favorable for uniform dispersion of elemental sulfur. It can be seen from Fig. 2c that the SnS2 is clustered in SnS2 /CNTs composite, and the size of SnS2 is greatly reduced compared with Fig. 2b, indicating that the introduction of CNTs can refine the size of SnS2 . In addition, the CNTs are evenly wrapped on the surface of the SnS2 , and some CNTs are interspersed in the SnS2 , which is advantageous for improving the conductivity of the SnS2 . Moreover, it can be seen from Fig. 2d (the enlarged view of the black square in Fig. 2c) that the CNTs are interlaced into a network on the surface of SnS2 , forming a large number of pores signed by the red circles in Fig. 2d, which can facilitate the rapid transfer of electron and ions. As shown in Fig. 2e, the CNTs and the surface of SnS2 /CNTs/S become thicker and smoother than that of SnS2 /CNTs in Fig. 2d, indicating that S (signed by the red arrow) has been uniformly distributed in the composite. Fig. 2f shows the EDS element mapping images of S, Sn and C in the left selected regions. The element distribution maps are well overlapped, indicating that S, Sn, and C are all homogeneous distribution within the SnS2 /CNTs material. Fig. 3 shows the electrochemical performance of the Li-S batteries based the samples (a) CV curves; (b) first charge discharge curves; (c) rate performance curves; (d) cycle life curves; (e) Coulomb efficiency curves at 0.1 C, the inset is the partial enlargement; (f) EIS curves, the inset is the equivalent circuit diagram; g, Z’-ω−1/2 curves). As shown in Fig. 3a, the redox peak currents gradually increase with the addition of CNTs, SnS2 , and SnS2 /CNTs, indicating better electrochemical performance. Moreover, the number and location of peaks (two reduction peaks and one oxidation peak) are the same as that of the typical Li-S battery. The reduction peak around 2.25 V is attributed to the reduction of the S8 ring to long chain lithium polysulfides (Li2 Sn , 4 ≤ n ≤ 8). Another
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Fig. 2. SEM images of the samples (a, CNTs; b, SnS2 ; c and d, SnS2 /CNTs; e, SnS2 /CNTs/S; f, EDS elemental mapping images of Sn, S and C in the left selected regions of SnS2 /CNTs/S).
reduction peak around 2.0 V is associated with the reduction of the soluble long chain lithium polysulfides (Li2 Sn , 4 ≤ n ≤ 8) to insoluble short chain lithium polysulfides (such as Li2 S2 , Li2 S). The oxidization peak around 2.5 V is related to the conversion of the short chain and long chain lithium polysulfides into S8 [28]. With the addition of SnS2 , CNTs and SnS2 /CNTs, the reduction and oxidation peaks become sharper, indicating the increase of the charge transfer rate and electrode reaction, which is good for the enhancement of Li-S battery electrochemical performance [19]. At the same time, the peak differences between the oxidation–reduction peaks are getting smaller and smaller, indicating the reversibility of the electrode reaction is getting better and better [21]. There are obvious two discharge platforms and one charge platform in Fig. 3b, which is the typical Li-S battery phenomenon consisted with the CV results. Moreover, the charge discharge platforms are lengthening with the addition of CNTs, SnS2 , and SnS2 /CNTs, indicating the enhancement of electrochemical capacity.
Rate performance curves in Fig. 3c show that the first discharge specific capacities of the cathode materials (S, SnS2 /S, CNTs/S, and SnS2 /CNTs/S) are 593.3, 1056.7, 1157.2, and 1308.6 mAh g−1 at 0.1 C, respectively. At high rate (3C), the capacities of the cathode materials are 25.5, 284.7, 361.4 and 695.5 mAh g−1 , respectively. Changing the rate from 3C to 0.1C, the capacity recovery rates of the four samples are 66.0%, 77.9%, 73.6% and 91.9%, respectively. So the discharge capacity increases with the addition of SnS2 , CNTs and SnS2 /CNTs. And the discharge capacities of CNTs/S at each rate are all higher than those of SnS2 /S. The SnS2 /CNTs/S composite has the highest discharge specific capacity at each rate and exhibits the best capacity recovery rate among the four samples for the synergistic effect of SnS2 and CNTs. The cycle life curves in Fig. 3d show that the discharge capacity of the SnS2 /CNTs/S composite falls to 1002.3 mAh g−1 after 100 cycles, and the capacity retention rate is about 76.6% after 100 cycles. The discharge capacities of the other three samples are 367.3, 666.3 and 556.3 mAh g−1 after 100 cycles, respectively. The Coulomb efficiency curves in Fig. 3e shows that
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Fig. 3. Electrochemical performance of the Li-S batteries based the samples (a, CV curves; b, first charge discharge curves; c, rate performance curves; d, cycle life curves; e, Coulomb efficiency curves at 0.1 C, the inset is the partial enlargement; f, EIS curves, the inset is the equivalent circuit diagram; g, Z’-ω −1/2 curves).
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Fig. 4. Schematic diagram of the inhibition mechanism of SnS2 and CNTs on the “shuttle effect” of lithium-sulfur battery (a, normal cathode material; b, SnS2 /CNTs/S composite).
the Li-S batteries based on the four samples all have high Coulomb efficiency. Amongst which, SnS2 /CNTs/S has the highest Coulomb efficiency (about 100.63% after 100 cycles). Moreover, the inset of Fig. 3e also shows that the SnS2 /CNTs/S curve is the most stable among the four curves, representing the most stable Coulomb efficiency, indicating the best conductivity, which is the result of the introduction of SnS2 and CNTs. There is a synergistic effect between the SnS2 and CNTs. At the same time, the SnS2 /CNTs/S composite exhibits the highest discharge capacity and capacity retention, which is also mainly due to the synergistic effect of CNTs and SnS2 . The three-dimensional conductive network formed by the CNTs ensures the fast transfer of electron and ions, which is good for improving the utilization rate of sulfur and thus enhancing its electrochemical performance. At the same time, the refinement of the CNTs on the size of SnS2 nanosheet increases the reaction sites of SnS2 and lithium polysulfides, which can enhance the chemical adsorption chance of SnS2 on lithium polysulfides and reduce the shuttle of lithium polysulfides by forming strong chemical bonds between the SnS2 and lithium polysulfides [29]. And thus, the SnS2 works as inhibiting the shuttle effect of lithium polysulfides and enhance the electrochemical performance by the adsorption of lithium polysulfides to avoid the loss of active material (S). The EIS curves in Fig. 3f have a semicircle at high frequency region and an inclined line at low frequency region. The semicircle at high frequency region is assigned to the charge transfer resistance (Rct ) on the solid / electrolyte interface. The inclined line at low frequency region is attributed to the Warburg impedance of Li+ diffusion in the electrode materials [30,31]. The inset in Fig. 3f is the equivalent circuit diagram, which is used to match the data. Where Rs is the system ohmic resistance, Rct is charge transfer resistance, CPE is the capacity and Zw is the inductance. Moreover, the Li+ ions diffusion coefficient (DLi + ) of different samples can be obtained according to equation DLi + =R2 T2 /2A2 F4 n4 C2 σ ω 2 [32,33]. Where R is the gas constant, T is the absolute temperature, A is the area of the positive electrode, F is the Faraday constant, n is the number of charge transfer, and C is the lithium ion concentration [34]. The Warburg factor (σ ω ) can be calculated by the slopes of the plots in Fig. 3g [35]. The related data is listed in Table 1. As shown in Table 1, with the addition of SnS2 , CNTs and SnS2 /CNTs, the Rct of the corresponding system sharply decreases from 88.2 to 63.5, 59.6, and 51.5 , indicating that the synergistic effect of SnS2 and CNTs reduces the Rct of the corresponding system
Table 1 EIS data of the samples. Samples
Rct ()
σω
DLi + (cm2 s− 1 × 10−15 )
S SnS2 /S CNTs/S SnS2 /CNTs/S
88.2 63.5 59.6 51.5
78.0 52.8 39.3 24.1
1.0 2.1 3.9 10.5
and accelerates the electron and ion transfer speed. In addition, the DLi + also gradually increases from 1.0 × 10−15 to 2.1 × 10−15 , 3.9 × 10−15 , and 10.5 × 10−15 cm2 s−1 . So the obtained SnS2 /CNTs/S composite has the least Rct and the largest DLi + in the obtained samples, indicating the best electrochemical energy performance. Fig. 4 is the schematic diagram of the inhibition mechanism of SnS2 and CNTs on the “shuttle effect” of lithium-sulfur battery (a, normal cathode material; b, SnS2 /CNTs/S composite). During the reaction of S positive electrode material, the generated lithium polysulfides easily diffuse from the positive electrode through the separator to the negative electrode and react with lithium, resulting in loss of active material and degradation of the lithium negative electrode, resulting in low discharge specific capacity and poor cycle stability of the sulfur positive electrode, which is schematic shown in Fig. 4a. However, as for the SnS2 /CNTs/S composite (which is schematic shown in Fig. 4b), SnS2 can capture polysulfides by forming strong chemical bonds with polysulfides [29], thereby achieving inhibition of polysulfides’ migration behavior [36]. At the same time, this behavior promotes the transport of lithium ions [37], and ultimately improves the performance of Li-S batteries. The addition of good conductivity CNTs not only can effectively improve the conductivity of the material, improve the utilization ratio of sulfur, but also promote electron/lithium ion transfer and improve its electrochemical rate performance [38]. Therefore, under the synergistic effect of CNTs and SnS2 , the SnS2 /CNTs/S composite exhibits high utilization ratio of active material and strong fixation to lithium polysulfides, so that it has a good cycle stability and high discharge capacity. Acknowledgments The work was supported by the National Natural Science Foundation of China (51602266), Sichuan Natural Resources Research
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Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Preparing a composite including SnS2 , carbon nanotubes and S and using as cathode material of lithium-sulfur battery Wu Jun, Chen Bing, Liu Qingqing, Hu Ailin, Lu Xiaoying∗, Jiang Qi∗ Superconductivity and New Energy R&D Centre, Southwest Jiaotong University, Key Laboratory of Advanced Technologies of Materials (Ministry of Education of China), Chengdu 610031, China
a r t i c l e
i n f o
Article history: Received 17 July 2019 Revised 1 September 2019 Accepted 23 October 2019
Keywords: Sns2 /CNTs/S composite Lithium-sulfur battery Cathode material
a b s t r a c t SnS2 is used to inhibit the “shuttle effect” and carbon nanotubes (CNTs) are used to improve the conductivity, thus a new Li-S battery cathode material (SnS2 /CNTs/S composite) is prepared in this paper. Morphological structure and electrochemical properties of the samples were characterized. The results show that among the composite, the CNTs are uniformly wrapped on the surface of SnS2 and S is uniformly loaded on the SnS2 /CNTs. The composite has a promising electrochemical performance: the first discharge capacity is 1308.6 mAh g − 1 at 0.1 C and there is a reversible capacity of 1002.3 mAh g−1 after 100 cycles. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
With the rapid development of human society, low-energy density lithium-ion batteries have become increasingly unable to meet the current needs of people. Lithium-sulfur (Li-S) battery has been focused on for its low cost, environmental friendliness, high theoretical capacity (1675 mAh g−1 ) and energy density (2600 Wh kg−1 ) [1–3], which is much higher than that of the conventional lithium-ion battery. However, the low conductivity of sulfur and Li2 S/Li2 S2 (discharge final products) and the “shuttle effect” of the lithium polysulfides have restricted the practical electrochemical performance of Li-S battery [4,5]. In order to address these problems, carbon materials with excellent electrical conductivity, for example, porous carbon [6], reduced graphene oxide (rGO) [7] and carbon nanotubes (CNTs) [8] are used to improve the electrical conductivity of the whole electrode. Metal oxides or metal sulfides with strong chemisorption, for example, SnO2 [9], MnO2 [10] and CoS2 [11], are also used to inhibit the dissolution of lithium polysulfides. In order to enhance the electrochemical performance of Li-S batteries, carbon materials and metal compounds are used together [12–14]. For example, a SnO2 @rGO/CNTs/S composite prepared by Liu etc. [15] and a MoS2 @CNTs/S composite prepared by Jeong et al. [16]. In recent years, SnS2 has been widely used as a cathode material in the field of lithium ion batteries and shows good results [17,18]. At present, SnS2 has begun to be applied to the field of Li-S battery for its strong chemical adsorption for lithium polysulfides. However, due to its poor conductivity, the improvement of Li-S battery performance is limited. Therefore, some researchers ∗
Corresponding authors. E-mail addresses: [email protected] (L. Xiaoying), [email protected] (J. Qi).
https://doi.org/10.1016/j.scriptamat.2019.10.038 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
began to use carbon materials with good conductivity to improve its conductivity. For example, Li et al. [19] used hollow carbon nanosphere and SnS2 to prepare S/AHCNS-SnS2 cathode material. Li et al. [20,21] prepared S/C-SnS2 cathode material by using porous carbon and Ketjen Black to improve the conductivity of SnS2 . The results showed that the energy storage performance of the prepared composites is improved to a certain extent after the introduction of carbon materials. One-dimensional CNTs are widely used in energy storage field because of their excellent mechanical properties, excellent conductivity and abundant nano-reaction sites [22]. The introduction of CNTs into the cathode material of Li-S battery can improve not only the conductivity of the composite, but also their mechanical properties, which is helpful to further improve their energy storage performance. So in this paper, the authors design a synergistic effect by using the excellent conductivity and mechanical properties of CNTs and the strong chemical adsorption of SnS2 to improve the electrochemical performance of Li-S battery. Four materials (S, SnS2 /S, CNTs/S and SnS2 /CNTs/S) were designed and prepared by a melting-diffusion method. Their structure and electrochemical performance were researched including SnS2 and CNTs. The results show that the CNTs and SnS2 in the composite behave a good synergistic effect. The three-dimensional conductive network composed of the CNTs improves the conductivity of the whole electrode. At the same time, the CNTs can refine the size of SnS2 , which is beneficial to enhance the adsorption of lithium polysulfides by SnS2 . SnS2 inhibits the dissolution of lithium polysulfides in the electrolyte by strong chemical adsorption and avoids the rapid loss of active material. The SnS2 /CNTs/S composite exhibits a promising electrochemical performance: the
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Fig. 1. XRD patterns (a) and TG curves (b) of the samples.
first discharge capacity is 1308.6 mAh g−1 at 0.1 C and there is a reversible capacity of 1002.3 mAh g−1 after 100 cycles. The CNTs was synthesized by our group via the chemical vapor deposition [23]. CNTs and polyvinylpyrrolidone (10 wt% of CNTs) were dispersed in a certain amount of deionized water, and the mixed solution was ultrasonically dispersed for 4 h. Then, suitable amount of SnCl4 ·5H2 O (mCNTs :mSnCl4 · 5H2O =1:10) and lcysteine (nSnCl4 · 5H2O :nL-cysteine =1:4) were added into the mixture. After stirring for 30 min, the mixture was transferred to a 100 ml autoclave to react at 180 °C for 24 h to prepare the SnS2 /CNTs composite. Pure SnS2 was prepared as the same operations without adding CNTs. SnS2 /CNTs/S composite was prepared with SnS2 /CNTs and S by a melting-diffusion treatment at 155 °C for 12 h in Argon atmosphere. SnS2 /S and CNTs/S were prepared by the similar operations. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TG, N2 atmosphere with 10 °C min−1 heating rate) were used to characterize the obtained samples. In order to research the electrochemical performance of the obtained samples, the obtained samples were assembled into CR2032 button batteries using the lithium metal as the anode. The electrolyte was the 1.0 mol l−1 lithium bis-trifluoromethanesulphonyphonylimide (LiTFSI, in 1, 3-dioxolane and 1, 2 - dimethoxyethane solution with 1:1 vol ratio) with 0.5 wt% LiNO3 additive. The cathodes were prepared by pressing the mixture of 80 wt% sample powders, 10 wt% conductive acetylene black and 10 wt% polyvinylidene fluoride on an aluminum foil. The membrane was the microporous separator (Celgard 2400). The Li-S batteries were assembled in an argon-filled glove box without water and oxygen. An automatic battery tester and CHI660 electrochemical workstation were used to characterize the electrochemical performance at 25 °C, including cyclic voltammogram (CV) with a scan rate of 0.1 mV s−1 , charge discharge curves, rate performance, cycle life curves at 0.1 C and electrochemical impedance spectroscopy (EIS, tested before the initial discharging process) from 100 kHz to 0.01 Hz with the amplitude of 5 mV. Fig. 1 shows the XRD patterns (a) and TG curves (b) of the obtained samples. As shown in Fig. 1a, the diffraction peak of the prepared SnS2 is consistent with that of the standard comparison card (JCDPS 01–089–2028), indicating that the obtained SnS2 is a single phase [24]. Its diffraction peaks are located at 15.0°, 28.1°, 32.2°, 41.8°, and 50.0°, respectively, corresponding to the 0 01, 10 0, 101, 102, and 110 crystal faces of the hexagonal phase [25]. And the same time, SnS2 /CNTs exhibits all the diffraction peaks of SnS2 and a broad diffraction peak appears at around 26°, which is the char-
acteristic peak of CNTs [26]. After loading S, the diffraction peak of SnS2 in the SnS2 /CNTs/S composite becomes weak, the peak of CNTs is covered, and the characteristic diffraction peak of S appears at around 23°, whose peak shape is sharp, indicating that S exists in the elemental form and has good crystallinity [27]. It can be seen from Fig. 1b that the sulfur contents of the samples (SnS2 /S, CNTs/S and SnS2 /CNTs/S) are 61.2%, 62.3% and 66.6%, respectively. And the areal loading amount of sulfur is about 1.2 mg cm−2 . Fig. 2 is the SEM images of the obtained samples (a, CNTs; b, SnS2 ; c and d, SnS2 /CNTs; e, SnS2 /CNTs/S; f, EDS elemental mapping images of Sn, S and C in the left selected regions of SnS2 /CNTs/S). As shown in Fig. 2a, the diameters of the CNTs are about 30–60 nm and the surface is smooth. The SnS2 shows a petal-like nanosheet structure in Fig. 2b, and the sheets are irregularly stacked between the sheets to form a large number of holes, which is favorable for uniform dispersion of elemental sulfur. It can be seen from Fig. 2c that the SnS2 is clustered in SnS2 /CNTs composite, and the size of SnS2 is greatly reduced compared with Fig. 2b, indicating that the introduction of CNTs can refine the size of SnS2 . In addition, the CNTs are evenly wrapped on the surface of the SnS2 , and some CNTs are interspersed in the SnS2 , which is advantageous for improving the conductivity of the SnS2 . Moreover, it can be seen from Fig. 2d (the enlarged view of the black square in Fig. 2c) that the CNTs are interlaced into a network on the surface of SnS2 , forming a large number of pores signed by the red circles in Fig. 2d, which can facilitate the rapid transfer of electron and ions. As shown in Fig. 2e, the CNTs and the surface of SnS2 /CNTs/S become thicker and smoother than that of SnS2 /CNTs in Fig. 2d, indicating that S (signed by the red arrow) has been uniformly distributed in the composite. Fig. 2f shows the EDS element mapping images of S, Sn and C in the left selected regions. The element distribution maps are well overlapped, indicating that S, Sn, and C are all homogeneous distribution within the SnS2 /CNTs material. Fig. 3 shows the electrochemical performance of the Li-S batteries based the samples (a) CV curves; (b) first charge discharge curves; (c) rate performance curves; (d) cycle life curves; (e) Coulomb efficiency curves at 0.1 C, the inset is the partial enlargement; (f) EIS curves, the inset is the equivalent circuit diagram; g, Z’-ω−1/2 curves). As shown in Fig. 3a, the redox peak currents gradually increase with the addition of CNTs, SnS2 , and SnS2 /CNTs, indicating better electrochemical performance. Moreover, the number and location of peaks (two reduction peaks and one oxidation peak) are the same as that of the typical Li-S battery. The reduction peak around 2.25 V is attributed to the reduction of the S8 ring to long chain lithium polysulfides (Li2 Sn , 4 ≤ n ≤ 8). Another
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Fig. 2. SEM images of the samples (a, CNTs; b, SnS2 ; c and d, SnS2 /CNTs; e, SnS2 /CNTs/S; f, EDS elemental mapping images of Sn, S and C in the left selected regions of SnS2 /CNTs/S).
reduction peak around 2.0 V is associated with the reduction of the soluble long chain lithium polysulfides (Li2 Sn , 4 ≤ n ≤ 8) to insoluble short chain lithium polysulfides (such as Li2 S2 , Li2 S). The oxidization peak around 2.5 V is related to the conversion of the short chain and long chain lithium polysulfides into S8 [28]. With the addition of SnS2 , CNTs and SnS2 /CNTs, the reduction and oxidation peaks become sharper, indicating the increase of the charge transfer rate and electrode reaction, which is good for the enhancement of Li-S battery electrochemical performance [19]. At the same time, the peak differences between the oxidation–reduction peaks are getting smaller and smaller, indicating the reversibility of the electrode reaction is getting better and better [21]. There are obvious two discharge platforms and one charge platform in Fig. 3b, which is the typical Li-S battery phenomenon consisted with the CV results. Moreover, the charge discharge platforms are lengthening with the addition of CNTs, SnS2 , and SnS2 /CNTs, indicating the enhancement of electrochemical capacity.
Rate performance curves in Fig. 3c show that the first discharge specific capacities of the cathode materials (S, SnS2 /S, CNTs/S, and SnS2 /CNTs/S) are 593.3, 1056.7, 1157.2, and 1308.6 mAh g−1 at 0.1 C, respectively. At high rate (3C), the capacities of the cathode materials are 25.5, 284.7, 361.4 and 695.5 mAh g−1 , respectively. Changing the rate from 3C to 0.1C, the capacity recovery rates of the four samples are 66.0%, 77.9%, 73.6% and 91.9%, respectively. So the discharge capacity increases with the addition of SnS2 , CNTs and SnS2 /CNTs. And the discharge capacities of CNTs/S at each rate are all higher than those of SnS2 /S. The SnS2 /CNTs/S composite has the highest discharge specific capacity at each rate and exhibits the best capacity recovery rate among the four samples for the synergistic effect of SnS2 and CNTs. The cycle life curves in Fig. 3d show that the discharge capacity of the SnS2 /CNTs/S composite falls to 1002.3 mAh g−1 after 100 cycles, and the capacity retention rate is about 76.6% after 100 cycles. The discharge capacities of the other three samples are 367.3, 666.3 and 556.3 mAh g−1 after 100 cycles, respectively. The Coulomb efficiency curves in Fig. 3e shows that
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Fig. 3. Electrochemical performance of the Li-S batteries based the samples (a, CV curves; b, first charge discharge curves; c, rate performance curves; d, cycle life curves; e, Coulomb efficiency curves at 0.1 C, the inset is the partial enlargement; f, EIS curves, the inset is the equivalent circuit diagram; g, Z’-ω −1/2 curves).
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Fig. 4. Schematic diagram of the inhibition mechanism of SnS2 and CNTs on the “shuttle effect” of lithium-sulfur battery (a, normal cathode material; b, SnS2 /CNTs/S composite).
the Li-S batteries based on the four samples all have high Coulomb efficiency. Amongst which, SnS2 /CNTs/S has the highest Coulomb efficiency (about 100.63% after 100 cycles). Moreover, the inset of Fig. 3e also shows that the SnS2 /CNTs/S curve is the most stable among the four curves, representing the most stable Coulomb efficiency, indicating the best conductivity, which is the result of the introduction of SnS2 and CNTs. There is a synergistic effect between the SnS2 and CNTs. At the same time, the SnS2 /CNTs/S composite exhibits the highest discharge capacity and capacity retention, which is also mainly due to the synergistic effect of CNTs and SnS2 . The three-dimensional conductive network formed by the CNTs ensures the fast transfer of electron and ions, which is good for improving the utilization rate of sulfur and thus enhancing its electrochemical performance. At the same time, the refinement of the CNTs on the size of SnS2 nanosheet increases the reaction sites of SnS2 and lithium polysulfides, which can enhance the chemical adsorption chance of SnS2 on lithium polysulfides and reduce the shuttle of lithium polysulfides by forming strong chemical bonds between the SnS2 and lithium polysulfides [29]. And thus, the SnS2 works as inhibiting the shuttle effect of lithium polysulfides and enhance the electrochemical performance by the adsorption of lithium polysulfides to avoid the loss of active material (S). The EIS curves in Fig. 3f have a semicircle at high frequency region and an inclined line at low frequency region. The semicircle at high frequency region is assigned to the charge transfer resistance (Rct ) on the solid / electrolyte interface. The inclined line at low frequency region is attributed to the Warburg impedance of Li+ diffusion in the electrode materials [30,31]. The inset in Fig. 3f is the equivalent circuit diagram, which is used to match the data. Where Rs is the system ohmic resistance, Rct is charge transfer resistance, CPE is the capacity and Zw is the inductance. Moreover, the Li+ ions diffusion coefficient (DLi + ) of different samples can be obtained according to equation DLi + =R2 T2 /2A2 F4 n4 C2 σ ω 2 [32,33]. Where R is the gas constant, T is the absolute temperature, A is the area of the positive electrode, F is the Faraday constant, n is the number of charge transfer, and C is the lithium ion concentration [34]. The Warburg factor (σ ω ) can be calculated by the slopes of the plots in Fig. 3g [35]. The related data is listed in Table 1. As shown in Table 1, with the addition of SnS2 , CNTs and SnS2 /CNTs, the Rct of the corresponding system sharply decreases from 88.2 to 63.5, 59.6, and 51.5 , indicating that the synergistic effect of SnS2 and CNTs reduces the Rct of the corresponding system
Table 1 EIS data of the samples. Samples
Rct ()
σω
DLi + (cm2 s− 1 × 10−15 )
S SnS2 /S CNTs/S SnS2 /CNTs/S
88.2 63.5 59.6 51.5
78.0 52.8 39.3 24.1
1.0 2.1 3.9 10.5
and accelerates the electron and ion transfer speed. In addition, the DLi + also gradually increases from 1.0 × 10−15 to 2.1 × 10−15 , 3.9 × 10−15 , and 10.5 × 10−15 cm2 s−1 . So the obtained SnS2 /CNTs/S composite has the least Rct and the largest DLi + in the obtained samples, indicating the best electrochemical energy performance. Fig. 4 is the schematic diagram of the inhibition mechanism of SnS2 and CNTs on the “shuttle effect” of lithium-sulfur battery (a, normal cathode material; b, SnS2 /CNTs/S composite). During the reaction of S positive electrode material, the generated lithium polysulfides easily diffuse from the positive electrode through the separator to the negative electrode and react with lithium, resulting in loss of active material and degradation of the lithium negative electrode, resulting in low discharge specific capacity and poor cycle stability of the sulfur positive electrode, which is schematic shown in Fig. 4a. However, as for the SnS2 /CNTs/S composite (which is schematic shown in Fig. 4b), SnS2 can capture polysulfides by forming strong chemical bonds with polysulfides [29], thereby achieving inhibition of polysulfides’ migration behavior [36]. At the same time, this behavior promotes the transport of lithium ions [37], and ultimately improves the performance of Li-S batteries. The addition of good conductivity CNTs not only can effectively improve the conductivity of the material, improve the utilization ratio of sulfur, but also promote electron/lithium ion transfer and improve its electrochemical rate performance [38]. Therefore, under the synergistic effect of CNTs and SnS2 , the SnS2 /CNTs/S composite exhibits high utilization ratio of active material and strong fixation to lithium polysulfides, so that it has a good cycle stability and high discharge capacity. Acknowledgments The work was supported by the National Natural Science Foundation of China (51602266), Sichuan Natural Resources Research
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