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Article

CuCo2S4 Nanoparticles Embedded in Carbon Nanotube Networks as Sulfur Hosts for High Performance Lithium-Sulfur Batteries

by
Hongying Wang
1,2,†,
Yanli Song
1,†,
Yanming Zhao
1,
Yan Zhao
1,* and
Zhifeng Wang
1,2,*
1
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300401, China
2
Key Laboratory for New Type of Functional Materials in Hebei Province, Hebei University of Technology, Tianjin 300401, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Nanomaterials 2022, 12(18), 3104; https://doi.org/10.3390/nano12183104
Submission received: 13 August 2022 / Revised: 3 September 2022 / Accepted: 4 September 2022 / Published: 7 September 2022
(This article belongs to the Special Issue Nanostructured Materials for Energy Applications)
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Abstract

:
Rational design of sulfur hosts for lithium-sulfur (Li-S) batteries is essential to address the shuttle effect and accelerate reaction kinetics. Herein, the composites of bimetallic sulfide CuCo2S4 loaded on carbon nanotubes (CNTs) are prepared by hydrothermal method. By regulating the loading of CuCo2S4 nanoparticles, it is found that when Cu2+ and CNT are prepared in a 10:1 ratio, the CuCo2S4 nanoparticles loaded on the CNT are relatively uniformly distributed, avoiding the occurrence of agglomeration, which improves the electrical conductivity and number of active sites. Through a series of electrochemical performance tests, the S/CuCo2S4-1/CNT presents a discharge specific capacity of 1021 mAh g−1 at 0.2 C after 100 cycles, showing good cycling stability. Even at 1 C, the S/CuCo2S4-1/CNT cathode delivers a discharge capacity of 627 mAh g−1 after 500 cycles. This study offers a promising strategy for the design of bimetallic sulfide-based sulfur hosts in Li-S batteries.

1. Introduction

Lithium-sulfur (Li-S) batteries are the up-and-coming next-generation rechargeable batteries because of the merits of being environment-friendly, their high energy density (2600 Wh kg−1) and theoretical capacity (1675 mAh g−1) [1,2,3]. However, soluble lithium polysulfides (LiPSs) are dissolved into the electrolyte during the charge–discharge process, which induces the shuttle effect and rapid capacity decay, limiting the exploitation of high-performance Li-S batteries [4,5,6,7,8]. Therefore, various solutions, including the design of sulfur host, separator and electrolyte modification, are committed to solving the above problem. Among them, the design and preparation of suitable sulfur carriers play an essential role in boosting the performance of Li-S batteries.
In previous studies, various carbon materials including carbon spheres, carbon nanofibers and carbon nanotubes (CNTs) were used as sulfur hosts in Li-S batteries by virtue of physical adsorbing LiPSs. This method presents the effect of sulfur fixation to a certain extent; however, it still has some limitations [9,10,11,12]. Some studies reported that polar materials including metal oxides, metal sulfides and metal phosphides, etc., could mitigate the shuttle effect effectively by chemical adsorption and catalysis [11,12,13,14], such as SiO2 [15], MnO2/TiO2 [16], nickel-plated [17] and CoP-CNT@C [18]. Among them, transition metal sulfides not only interact strongly with LiPSs but also show excellent catalytic activity in the electrochemical reaction. In addition, it can stabilize the electrochemical performance and enhance the energy efficiency of Li-S batteries [19,20]. For example, CoS2 [21,22], NiS [23,24] and Co3S4 [25,26] were reported to improve the electrochemical performance by a synergistic role of adsorption and catalysis. Compared to monometallic sulfides, bimetallic sulfides possess lower band gap energy and improved electrical conductivity [27]. Simultaneously, bimetallic sulfides can provide more reactive sites than monometallic sulfides. Therefore, extensive research has been devoted to the development of new bimetallic sulfides catalyst. Huang et al. prepared Co-Fe bimetallic sulfides with robust chemical adsorption and catalytic activity, it exhibited a high reversible capacity of 1126.5 mAh g−1 at 0.2 C [28]. Lu et al. fabricated the NiCo2S4@CNTs/S for Li-S batteries. CNTs were found to promote the electronic transportation capacity and conductivity of the cathode material effectively, while NiCo2S4 showed strong adsorption toward the LiPSs, effectively suppressing the diffusion of LiPSs [29]. Previous work has proved that bimetallic sulfide/carbon composite can show a strong effect in inhibiting the shuttle effect. However, the development of different polysulfide/carbon composites is still lacking at present, and the conductivity, electrochemical stability, and conversion kinetics need to be further improved.
In this work, CNTs loaded with CuCo2S4 bimetallic sulfides (CuCo2S4/CNT) were prepared and used as the sulfur host for Li-S batteries. By further regulating the loading amount of CuCo2S4 nanoparticles on CNT materials, it is explored that the appropriate loading amount of CuCo2S4 nanoparticles can effectively improve the kinetics of LiPSs conversion, inducing a good electrochemical performance. The as-obtained S/CuCo2S4-1/CNT can sustain a specific capacity of 627 mAh g−1 after 500 cycles, with a capacity decay rate of only 0.08% per cycle.

2. Materials and Methods

Fabrication of CuCo2S4/CNT and CuCo2S4

A total of 15 mg slightly oxidized carbon nanotubes were ultrasonically dispersed into 30 mL ethylene glycol, and the suspension was sonicated for 2 h with stirring. Then, 0.15 g Cu(CH3COO)2-H2O (A reagent) and 0.0265 g Co(CH3COO)2-4H2O (B reagent) were dissolved in the mixture and stirred magnetically for 1 h. Afterwards, 0.117 g thiourea was added and stirred for 40 min. The mixture was poured into a 50 mL autoclave, sealed and reacted at 180 °C for 24 h. After cooling, the mixture was cleaned by centrifugation with anhydrous ethanol four times. The product was gathered and dried under vacuum at 70 °C to obtain CuCo2S4-1/CNT. Holding all other parameters constant, CuCo2S4-2/CNT was also obtained by adding 0.3 g A reagent and 0.053 g B reagent, while CuCo2S4-0.5/CNT can be obtained by adding 0.075 g A reagent and 0.01325 g B reagent. CuCo2S4 nanoparticles were obtained without adding slightly oxidized carbon nanotubes and ethylene glycol under the same fabrication conditions as CuCo2S4/CNT above.
Further details about the fabrication of the S/CuCo2S4/CNT and S/CuCo2S4 composites, preparation of Li2S6 solution, material characterization, electrochemical measurements and symmetric cells measurement, can be obtained from Supporting S0.

3. Results and Discussion

The schematic of the synthesis process and structure of S/CuCo2S4/CNT is shown in Figure 1. In brief, CuCo2S4/CNT is first synthesized by the hydrothermal method. Then, S/CuCo2S4/CNT can be obtained by heating of S and CuCo2S4/CNT mixture. The detailed process can be found in Supporting S0. The final product S/CuCo2S4/CNT was used as a cathode in this work for Li-S batteries application. By adjusting the content of Cu(CH3COO)2-H2O and Co(CH3COO)2-4H2O, the ratio of CuCo2S4 particles loaded on CNTs can be regulated. The products are marked as CuCo2S4-0.5/CNT, CuCo2S4-1/CNT and CuCo2S4-2/CNT, respectively, with the increase in contents of raw materials. As shown in Figure 2a, when CuCo2S4 particles were synthesized by hydrothermal method, the particle size was about 30–55 nm. However, severe particle agglomeration occurs which reduces the specific surface area of the material. As shown in Figure S1, although the loading of CuCo2S4 on CNT (CuCo2S4-0.5/CNT) inhibits CuCo2S4 agglomeration, the loading is too sparse (Figure S1a), which limits the adsorption ability toward polysulfides. While the loading of CuCo2S4 on CNTs is too dense for CuCo2S4-2/CNT (Figure S1c), restraining the exposure of active sites. The scanning electron microscope (SEM) images of CuCo2S4-1/CNT (Figure 2b,c and Figure S1b) exhibits uniform loading of CuCo2S4 particles on the CNTs’ surface, which most possibly enhances the performance of Li-S batteries. Transmission electron microscope (TEM) images of CuCo2S4-1/CNT in Figure 2d also confirm that CNTs are closely covered by CuCo2S4 with a granular diameter of 8–15 nm. Furthermore, it can be found from the above images that the CNTs are multi-walled. The average diameter and lengths of CNTs are 34 nm and 2 μm, respectively. In addition, when CNTs are exposed to air, they are inevitably oxidized. Some oxygen-containing groups, such as epoxide (C−O−C), hydroxyl (−OH), carboxyl (−COOH), and carbonyl (C=O), may be produced on the CNTs’ surface [30]. The presence of these oxygen-containing groups may affect the loading of CuCo2S4, as well as the electrochemical performance of Li-S batteries. Therefore, related tests need to be further explored in the future. The corresponding element mapping demonstrates the uniform distribution of S, Co, Cu (CuCo2S4 particle) on CNTs (Figure 2e–i).
The crystal structures of CuCo2S4, CuCo2S4-0.5/CNT, CuCo2S4-1/CNT and CuCo2S4-2/CNT materials were characterized by X-ray diffraction (XRD) (Figure 3a). The XRD patterns of four samples exhibit five characteristic diffraction peaks at 26.4°, 31.3°, 38.0°, 50.2° and 54.9°, matching with (220), (311), (400), (511) and (440) planes of CuCo2S4 (JCPDS 42–1450), respectively. The Raman spectra of CuCo2S4-0.5/CNT, CuCo2S4-1/CNT and CuCo2S4-2/CNT samples are shown in Figure 3b. The obvious peak near 1353 cm−1 can be marked as the D peak reflecting disordered and defective carbon, while the peak at 1587 cm−1 is attributed to the G peak of carbon, relating to the presence of sp2-hybridized carbon. The intensity ratio of D peak to G peak of CuCo2S4-1/CNT (ID/IG, 0.69) is lowest in the experimental materials, indicating that the graphitization degree and electric conductivity of CuCo2S4-1/CNT are higher than that of CuCo2S4-0.5/CNT (0.81), CuCo2S4-2/CNT (0.77) and CNT (0.85) (Figure S2) [31,32]. In addition, Figure 3c displays the thermogravimetric analysis (TGA) plots of different composites. It could be seen that S/CuCo2S4-1/CNT presents higher sulfur loading up to 76.3%. The specific surface area and pore size characteristics of CuCo2S4-1/CNT, CuCo2S4-2/CNT, CuCo2S4-0.5/CNT and CuCo2S4 were studied by N2 adsorption-desorption experiments (Figure 3d and Figure S3a,c). It displays typical type III isotherms with H3 type hysteresis loop, indicating the existence of mesopores. CuCo2S4-1/CNT (152.7 m2 g−1) shows a higher surface area than CuCo2S4-2/CNT (138.6 m2 g−1), CuCo2S4-0.5/CNT (102.5 m2 g−1) and CuCo2S4 (85.4 m2 g−1). Pore distribution reveals that there exists a large proportion of micropores in CuCo2S4-1/CNT compared with the other three materials (Figure 3e,f and Figure S3b,d). This is beneficial to enhance the sulfur limitation by physical role. Higher surface area also facilitates the exposure of active sites and provides a rich electrode/electrolyte interface for LiPSs conversion.
In Li-S batteries, X-ray photoelectron spectrometry (XPS) is usually used to determine the composition, structure and element content of the material. Therefore, in order to identify the composition and valence of the CuCo2S4-1/CNT, we conducted XPS measurement. It can be concluded that Cu, Co, S, C, and O elements exist in CuCo2S4-1/CNT (Figure 4a). The Co XPS spectrum (Figure 4b) shows six peaks at 794.8 eV for Co3+ 2p1/2, 779.5 eV for Co3+ 2p3/2, 798.8 eV for Co2+ 2p1/2, 781.4 eV for Co2+ 2p3/2, 805.1 and 785.2 eV for satellite peaks [33]. In the Cu XPS spectrum (Figure 4c), the binding energy values at 952.5 eV and 932.5 eV correspond to Cu+ 2p1/2 and Cu+ 2p3/2, respectively. While 954.0 eV and 933.5 eV can be contributed to Cu2+ 2p1/2 and Cu2+ 2p3/2, and 943.7 eV and 963.2 eV for satellite peaks [34]. In addition, two characteristic peaks in the S 2p XPS spectra at 163.9 eV (2p1/2) and 162.2 eV (2p3/2) correspond to S2−species (Figure 4d) [35,36]. The lower intensity characteristic peak at 168.8 eV suggests the presence of small amounts of sulphate or sulfite species and the presence of thin oxide layers on the surface. The peak at 165.1 eV probably corresponds to an M-S bond (M = Cu or Co), where the sulfur presents in the form of polysulfides (Sn2−, 2 ≤ n < 8) [37]. In addition, the present type of polysulfides in different charge–discharge states can be detected by XPS, which can provide a better understanding of the charge–discharge mechanism of lithium-sulfur batteries. These in-depth analyses and discussions will be carried out and published in the future.
In order to investigate the feasibility of S/CuCo2S4/CNT composites as Li-S batteries cathodes, a series of electrochemical performance tests were carried out. As shown in Figure S4, the red lines and blue lines correspond to the standard PDF cards of sulfur (JCPDS 08-0247) and CuCo2S4 (JCPDS 42-1450), respectively. The XRD results of S/CuCo2S4/CNT composites also show characteristic diffraction peaks of S and CuCo2S4, indicating a successful sulfur loading. The final mass ratios of CuCo2S4 to CNT in S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT, and S/CuCo2S4-2/CNT composites are calculated by combining XPS, EDS and inductively coupled plasma mass spectrometry (ICP-MS) results, showing 4.92:1, 9.81:1 and 18.53:1, respectively, which are close to the theoretical materials input ratios of 5:1, 10:1 and 20:1.
Figure 5a shows the Nyquist plots of Li-S batteries of different cathodes. The electrochemical impedance spectroscopy (EIS) curves contain a semicircle and a slope line, in line with the charge transfer resistance and the Warburg bulk impedance, respectively. The charge-transfer resistance of S/CuCo2S4-1/CNT is smaller than other electrodes, indicating it has the smallest charge-transfer resistance [38]. As shown in Figure 5b, the cyclic voltammetry (CV) curves at 0.1 mV s−1 show two distinct reduction peaks during discharge at 2.02 V and 2.31 V. The reduction peak at 2.31 V represents the reduction of S8 to soluble LiPSs (Li2Sn, n = 4, 6, 8). The peak at 2.02 V represents the conversion reaction of LiPSs to Li2S2/Li2S. During charging, the oxidation peak splits into two peaks, which are attributed to the oxidation from solid Li2S to LiPSs and eventually to S8 [39,40]. Furthermore, the first three cycles of CV curves of the S/CuCo2S4-1/CNT composite are well overlapped, reflecting excellent cycle reversibility. In addition, the first cycle CV curves of S/CuCo2S4-1/CNT, S/CuCo2S4-2/CNT and S/CuCo2S4-0.5/CNT cathodes at the scan rate of 0.1 mV s−1 are shown in Figure S5. It is obvious that S/CuCo2S4-1/CNT has the largest current response, indicating that it has superior catalytic performance. At a low current, the charging–discharging process of Li-S battery is relatively slow, it tends to produce more LiPSs, which dissolve in the electrolyte, causing the shuttle effect. In this way, we can verify the limitation of the shuttle effect by different types of CuCo2S4 and CNT composites [41]. Moreover, a lot of works have also examined electrochemical performance at 0.2 C so that we can fully compare the electrochemical data of this work with previously published works. Therefore, we perform measurements at 0.2 C based on the above considerations. Figure 5c shows the cycling property of different materials at 0.2 C. The S/CuCo2S4-1/CNT cathode shows the best electrochemical performance with a first discharge capacity of 1104.5 mAh g−1 and a very low cycle decay rate. In fact, each type was prepared for three samples. One battery of S/CuCo2S4-1/CNT presents an initial capacity of 1364.5 mAh g−1. While the other two samples of S/CuCo2S4-1/CNT cathode show the first discharge capacity of 1100.3 mAh g−1 and 1108.9 mAh g−1 at 0.2 C (Figure S6). Considering that one of the values is abnormally high, we conservatively choose the other two similar values to report. Therefore, the average discharge capacity with an error is 1104.6 ± 4.3 mAh g−1. After 100 cycles, it can maintain a high cycle capacity (1021 mAh g−1) and its coulomb efficiency closes to 100%, demonstrating the excellent reversibility of the reaction. In contrast, the S/CuCo2S4-0.5/CNT and S/CuCo2S4-2/CNT cathodes exhibited rapid capacity decay and low cycling capacity. In addition, we also compare the S/CuCo2S4 samples without CNT, which exhibit the lowest cycling performance. This can be attributed to the fact that it lacks the CNT’s hollow structure and three-dimensional conducting framework. For charge–discharge curves of different samples (Figure 5d and Figure S7), there are two obvious reductive plateaus and a slope, which are related to the reduction and oxidation of LiPSs. The voltage profiles of the S/CuCo2S4-1/CNT cathode exhibit slower capacity decay and smaller polarization, demonstrating it has excellent catalytic activity. The rate performance of different electrode materials is exhibited in Figure 5e. The specific discharge capacities of S/CuCo2S4-1/CNT at 0.2, 0.5, 1, 2 and 3 C are 1138 mAh g−1, 943 mAh g−1, 887 mAh g−1, 741 mAh g−1 and 656 mAh g−1, respectively, which is higher than the other three electrode materials. Even when the current density reverts to 0.2 C, the capacity of S/CuCo2S4-1/CNT can reach 1072 mAh g−1, demonstrating the efficient and reversible use of the active sulfur. Moreover, the charge–discharge curves of S/CuCo2S4-1/CNT at different current densities (Figure 5f) can maintain the characteristic discharge plateau of Li-S batteries compared with S/CuCo2S4-0.5/CNT, S/CuCo2S4-2/CNT and S/CuCo2S4 (Figure S8) [42].
To further investigate the effect of S/CuCo2S4-1/CNT on the electrochemical performance, we also carried out the EIS test and morphology analysis after cycling for 100 cycles. As shown in Figure 6a, the impedance diagram is composed of two semicircles and an oblique line. The first semicircle represents the formation of the Li2S2–Li2S interface (RSEI). It can be concluded that S/CuCo2S4-1/CNT has the lowest impedance, indicating its superior electrochemical kinetics [43,44]. Moreover, the morphology of CuCo2S4-1/CNT after cycling remains relatively intact. The carbon nanotubes retain their original conductive skeleton structure (Figure 6b,c). Based on the above results, long-term cycling performance at 1 C was also carried out. As exhibited in Figure 6d, the specific capacity of S/CuCo2S4-1/CNT can maintain at 627 mAh g−1 after 500 cycles, and the capacity decay rate is only 0.08%/cycle. In contrast, S/CuCo2S4-2/CNT, S/CuCo2S4-0.5/CNT and S/CuCo2S4 decayed to 441, 389 and 236 mAh g−1 after 500 cycles, respectively. This can be ascribed to the good catalytic effect of the CuCo2S4-1/CNT composite on the conversion of LiPSs.
In order to explore its potential mechanism in improving the electrochemical performance of Li-S batteries, the adsorption experiments were performed firstly by immersing the different materials in Li2S6 solution. Equal amounts of samples of CuCo2S4-1/CNT, CuCo2S4-2/CNT, CuCo2S4-0.5/CNT and CuCo2S4 were added to the same volume of Li2S6 solution, and the mixed solutions stand for 24 h. Then, as shown in Figure 7a, the Li2S6 solution with CuCo2-1/CNT material became clear, demonstrating the significant adsorption effect of CuCo2S4-1/CNT material. Simultaneously, the ultraviolet-visible (UV-Vis) spectrum also confirms the results (Figure 7b) [45,46]. In addition, to further investigate the electrocatalytic performance, symmetric cells were also assembled toward different materials. In Figure 7c, the EIS curve shows that the CuCo2S4-1/CNT electrode has the lowest resistance, confirming its excellent electrochemical reaction kinetics. The CV curves of the CuCo2S4-1/CNT electrode clearly show the sharpest redox peaks at −0.215/0.215 V and −0.454/0.454 V and the smallest polarization, proving the most excellent catalyzing behavior of the LiPSs conversion (Figure 7d). In addition, as shown in Figure S9, the first three cycles of CV curves of the CuCo2S4-1/CNT electrode have a relatively high degree of overlap, demonstrating relatively good reversibility [47,48]. Based on the above electrochemical data, CNTs improve the overall conductivity of composites and promote efficient ion/electron transport. At the same time, the highly interconnected 3D conductive network frameworks provide adequate space to buffer volume changes during the charging–discharging cycle. In addition, the uniform loading of CuCo2S4 particles on CNTs surface guarantee abundant active sites on CuCo2S4-1/CNT, which further ensures that the material possesses a high loading of active sulfur. The CuCo2S4-1/CNT composite presents strong adsorption and catalytic conversion ability for LiPSs. In conclusion, the excellent electrochemical performance of the S/CuCo2S4-1/CNT cathode can be attributed to the synergistic effect of CuCo2S4 and CNTs.

4. Conclusions

In summary, bimetallic sulfide CuCo2S4 nanoparticles loaded with CNT composites were synthesized by the hydrothermal method in this work. By modulating the different loadings of the CuCo2S4 nanoparticles, it is found that the CuCo2S4-1/CNT composites effectively improved the property of Li-S batteries, which can be attributed to the improved overall electrical conductivity of the CNT, promoting efficient ion/electron transport. Moreover, the bimetallic sulfide CuCo2S4 nanoparticles can provide rich adsorption sites for anchoring LiPSs and improve the conversion kinetics of LiPSs. Thus, the S/CuCo2S4-1/CNT cathode can achieve a first discharge capacity of 1104.6 ± 4.3 mAh g−1 at 0.2 C with a coulombic efficiency close to 100%. After 100 cycles, the discharge specific capacity can maintain 1021 mAh g−1. In addition, a reversible capacity of 627 mAh g−1 is demonstrated at 1 C after 500 cycles. This work provides a promising strategy for the design of a bimetallic sulfide-CNT network as a sulfur host for Li-S batteries.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano12183104/s1; Figure S1: SEM images of (a) CuCo2S4-0.5/CNT, (b) CuCo2S4-1/CNT and (c) CuCo2S4-2/CNT; Figure S2: Raman spectrum of CNT; Figure S3: (a) N2 adsorption/desorption isotherms and (b) pore size distribution of CuCo2S4-2/CNT. (c) N2 adsorption/desorption isotherms and (d) pore size distribution of CuCo2S4-0.5/CNT.; Figure S4: XRD patterns of S/CuCo2S4-1/CNT; Figure S5: The first cycle CV curves of S/CuCo2S4-1/CNT, S/CuCo2S4-2/CNT and S/CuCo2S4-0.5/CNT cathodes at the scan rate of 0.1 mV s−1; Figure S6: Cycling performances of S/CuCo2S4-1/CNT cathodes at 0.2 C; Figure S7: Charge-discharge curves at 0.2 C of (a) S/CuCo2S4-2/CNT, (b) S/CuCo2S4-0.5/CNT and (c) S/CuCo2S4 cathodes; Figure S8: Charge/discharge voltage profiles at 0.2 C, 0.5 C, 1 C, 2 C and 3 C of (a) S/CuCo2S4-2/CNT, (b) S/CuCo2S4-0.5/CNT and (c) S/CuCo2S4 cathodes. Figure S9: CV curves of symmetric cells with CuCo2S4-1/CNT electrodes at 6 mV s−1.

Author Contributions

Methodology, Y.Z. (Yan Zhao) and Z.W.; formal analysis, H.W. and Y.S.; investigation, H.W. and Y.S.; writing—original draft preparation, H.W. and Y.S.; writing—review and editing, Y.Z. (Yanming Zhao), Y.Z. (Yan Zhao) and Z.W.; supervision, Y.Z. (Yanming Zhao); project administration, Y.Z. (Yanming Zhao) and Y.Z. (Yan Zhao); funding acquisition, Y.Z. (Yan Zhao) and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to thank the financial support from the Natural Science Foundation of Hebei Province, China (E2020202071, B2020202052), and Hebei Provincial Key Research Special Project “Development and Application of Key Preparation Technology of High Strength and Toughness Magnesium Alloys for Automobile Wheel Hub” (from the Hebei Development and Reform Commission and Hebei Provincial Department of Finance, China).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram showing the synthesis and structure of S/CuCo2S4/CNT.
Figure 1. Schematic diagram showing the synthesis and structure of S/CuCo2S4/CNT.
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Figure 2. SEM images of (a) CuCo2S4, (b,c) CuCo2S4-1/CNT; (d) TEM images of CuCo2S4-1/CNT. (e) TEM image of CuCo2S4-1/CNT and the corresponding elemental mappings: (f) C, (g) S, (h) Co, (i) Cu.
Figure 2. SEM images of (a) CuCo2S4, (b,c) CuCo2S4-1/CNT; (d) TEM images of CuCo2S4-1/CNT. (e) TEM image of CuCo2S4-1/CNT and the corresponding elemental mappings: (f) C, (g) S, (h) Co, (i) Cu.
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Figure 3. (a) XRD patterns of experimental materials. (b) Raman spectrum of CuCo2S4-0.5/CNT, CuCo2S4-1/CNT and CuCo2S4-2/CNT. (c) TGA plots of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT. (d) N2 adsorption/desorption isotherms of CuCo2S4 and CuCo2S4-1/CNT. Pore size distribution of (e) CuCo2S4 and (f) CuCo2S4-1/CNT.
Figure 3. (a) XRD patterns of experimental materials. (b) Raman spectrum of CuCo2S4-0.5/CNT, CuCo2S4-1/CNT and CuCo2S4-2/CNT. (c) TGA plots of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT. (d) N2 adsorption/desorption isotherms of CuCo2S4 and CuCo2S4-1/CNT. Pore size distribution of (e) CuCo2S4 and (f) CuCo2S4-1/CNT.
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Figure 4. XPS spectra of CuCo2S4-1/CNT (a) Survey; (b) Co 2p; (c) Cu 2p; and (d) S 2p.
Figure 4. XPS spectra of CuCo2S4-1/CNT (a) Survey; (b) Co 2p; (c) Cu 2p; and (d) S 2p.
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Figure 5. (a) Nyquist plots of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes before cycling. (b) CV curves at the scan rate of 0.1 mV s−1 of S/CuCo2S4-1/CNT cathodes. (c) Cycling performances of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes at 0.2 C. (d) Charge/discharge voltage profiles of S/CuCo2S4-1/CNT at 0.2 C. (e) Rate performances of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes. (f) Charge/discharge voltage profiles at 0.2 C, 0.5 C, 1 C, 2 C and 3 C of S/CuCo2S4-1/CNT.
Figure 5. (a) Nyquist plots of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes before cycling. (b) CV curves at the scan rate of 0.1 mV s−1 of S/CuCo2S4-1/CNT cathodes. (c) Cycling performances of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes at 0.2 C. (d) Charge/discharge voltage profiles of S/CuCo2S4-1/CNT at 0.2 C. (e) Rate performances of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes. (f) Charge/discharge voltage profiles at 0.2 C, 0.5 C, 1 C, 2 C and 3 C of S/CuCo2S4-1/CNT.
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Figure 6. (a) Nyquist plots of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes after 100 cycles. (b,c) SEM image of S/CuCo2S4-1/CNT cathodes after charge–discharge cycle at 0.2 C. (d) Cycling performances of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes at 1 C.
Figure 6. (a) Nyquist plots of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes after 100 cycles. (b,c) SEM image of S/CuCo2S4-1/CNT cathodes after charge–discharge cycle at 0.2 C. (d) Cycling performances of S/CuCo2S4, S/CuCo2S4-0.5/CNT, S/CuCo2S4-1/CNT and S/CuCo2S4-2/CNT cathodes at 1 C.
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Figure 7. (a) Optical images and (b) UV-vis spectra after LiPSs adsorption by S/CuCo2S4-1/CNT, S/CuCo2S4-2/CNT, S/CuCo2S4-0.5/CNT and S/CuCo2S4. (c) EIS spectra and (d) CV curves at 6 mV s−1 of symmetric cells with CuCo2S4-1/CNT, CuCo2S4-2/CNT, CuCo2S4-0.5/CNT and CuCo2S4 electrodes.
Figure 7. (a) Optical images and (b) UV-vis spectra after LiPSs adsorption by S/CuCo2S4-1/CNT, S/CuCo2S4-2/CNT, S/CuCo2S4-0.5/CNT and S/CuCo2S4. (c) EIS spectra and (d) CV curves at 6 mV s−1 of symmetric cells with CuCo2S4-1/CNT, CuCo2S4-2/CNT, CuCo2S4-0.5/CNT and CuCo2S4 electrodes.
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Wang, H.; Song, Y.; Zhao, Y.; Zhao, Y.; Wang, Z. CuCo2S4 Nanoparticles Embedded in Carbon Nanotube Networks as Sulfur Hosts for High Performance Lithium-Sulfur Batteries. Nanomaterials 2022, 12, 3104. https://doi.org/10.3390/nano12183104

AMA Style

Wang H, Song Y, Zhao Y, Zhao Y, Wang Z. CuCo2S4 Nanoparticles Embedded in Carbon Nanotube Networks as Sulfur Hosts for High Performance Lithium-Sulfur Batteries. Nanomaterials. 2022; 12(18):3104. https://doi.org/10.3390/nano12183104

Chicago/Turabian Style

Wang, Hongying, Yanli Song, Yanming Zhao, Yan Zhao, and Zhifeng Wang. 2022. "CuCo2S4 Nanoparticles Embedded in Carbon Nanotube Networks as Sulfur Hosts for High Performance Lithium-Sulfur Batteries" Nanomaterials 12, no. 18: 3104. https://doi.org/10.3390/nano12183104

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