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MWCNTs composite cathode host for lithium–sulfur batteries
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1Three-dimensionally interconnected Co9 S8 /MWCNTs composite cathode host for lithium–sulfur batteries Shengyu Zhao , Xiaohui Tian , Yingke Zhou , Ben Ma , Angulakshmi Natarajan PII: DOI: Reference:
S2095-4956(19)30859-9 https://doi.org/10.1016/j.jechem.2019.10.011 JECHEM 980
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
25 April 2019 16 October 2019 17 October 2019
Please cite this article as: Shengyu Zhao , Xiaohui Tian , Yingke Zhou , Ben Ma , Angulakshmi Natarajan , 1Three-dimensionally interconnected Co9 S8 /MWCNTs composite cathode host for lithium–sulfur batteries, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.10.011
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Highlights
3D interconnected Co9S8/MWCNTs is prepared by a solvothermal method.
The polar Co9S8 displays strong adsorbent and catalytic effect for polysulfides.
The porous CNT networks improve the overall conductivity and electrolyte contact.
The composite presents excellent specific capacity, rate capability and cycling stability.
1
Three-dimensionally interconnected Co9S8/MWCNTs composite cathode host for lithium–sulfur batteries
Shengyu Zhao1, Xiaohui Tian1, Yingke Zhou, Ben Ma, Angulakshmi Natarajan*
The State Key Laboratory of Refractories and Metallurgy, Institute of Advanced Materials and Nanotechnology, College of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China
1
These authors contributed equally to this work.
Corresponding authors. Tel: +86 2768 862928, Fax: +86 2768 862928. E-mail addresses: [email protected] (Y. Zhou); [email protected] (A. Natarajan).
2
Graphical Abstract
Three-dimensionally interconnected porous Co9S8/MWCNTs composite has been facilely synthesized and demonstrates promising performance as a host material for lithium–sulfur batteries, due to the synergistic effect of polar Co9S8 and conductive MWCNT network.
3
Abstract Several challenging issues, such as the poor conductivity of sulfur, shuttle effects, large volume change of cathode, and the dendritic lithium in anode, have led to the low utilization of sulfur and hampered the commercialization of lithium–sulfur batteries. In this study, a novel three-dimensionally interconnected network structure comprising Co9S8 and multiwalled carbon nanotubes (MWCNTs) was synthesized by a solvothermal route and used as the sulfur host. The assembled batteries delivered a specific capacity of 1154 mA h g−1 at 0.1 C, and the retention was 64% after 400 cycles at 0.5 C. The polar and catalytic Co9S8 nanoparticles have a strong adsorbent effect for polysulfide, which can effectively reduce the shuttling effect. Meanwhile, the three-dimensionally interconnected CNT networks improve the overall conductivity and increase the contact with the electrolyte, thus enhancing the transport of electrons and Li ions. Polysulfide adsorption is greatly increased with the synergistic effect of polar Co9S8 and MWCNTs in the three-dimensionally interconnected composites, which contributes to their promising performance for the lithium–sulfur batteries.
Keywords: Three-dimensional network structure; MWCNTs; Polar and catalytic Co9S8; Lithium–sulfur batteries
4
1. Introduction Conventional Li-ion batteries have been extensively developed and applied in energy storage systems and portable electronic devices for decades [1–5]. However, their applications for electric vehicles are currently plagued by their limited theoretical power/energy densities [6–9]. Lithium–sulfur batteries have been identified as an ultimate system by virtue of their unique properties, such as high theoretical energy density, environmental benignity, low cost, better safety, and abundant resources [10,11]. Despite these advantages, the insulating nature of elemental sulfur, the huge volume change (~80%), and the shuttling effects accompanying the polysulfide dissolution in the electrolyte lead to low sulfur utilization, poor rate and cycling performances, and impede the commercialization of this system [12–18]. To overcome these drawbacks, many types of cathode host materials have been developed to enhance the electrochemical performance of lithium–sulfur batteries, such as nanocarbon materials, conductive polymer, and metal compounds [19–21]. Various nanostructural carbon materials, such as carbon nanotubes (nanofibers) [22– 24], porous carbon [25,26], graphene [27], and carbon aerogel [28] have been widely used as host materials, owing to the advantages of high electrical conductivity and surface area and adjustable pore size [29]. Nevertheless, the polysulfide confinement 5
of the carbon materials is mainly due to the physical interaction of van der Waals forces, and the shuttle effect cannot be efficiently suppressed. Recently, polar materials have been shown to effectively increase the adsorption of polysulfides through the formation of strong chemical bonding, to decrease the polysulfide dissolution and shuttling. Many studies have been carried out on nanosized polar materials, such as TiO2 [30], MnO2 [31], SiO2 [32,33], V2O5 [34], NiS2 [35], and SnS2 [36]. However, the conductivity of the polar compound is usually lower than that of the nanocarbon host material, resulting in the poor utilization of sulfur and a low rate capability [37]. Therefore, it is imperative to develop a host material with high conductivity and strong adsorption, and a combination of a polar compound with a nanocarbon is beneficial to increase the conductivity, improve the electrolyte contact, and provide more effective chemical and physical confinements of the polysulfides, and thereby to greatly enhance the battery performance [38–41]. Very recently, polar Co9S8 nanostructures have demonstrated strong chemical adsorption and intrinsic catalytic activity to accelerate the redox reaction and polysulfide conversion [39], and performance improvements of lithium–sulfur batteries through combination of polar Co9S8 and conductive nanocarbon have been reported, such as composites of Co9S8 and three-dimensional graphene foam [21], Co9S8 embedded in carbon hollo nanopolyhedra [40], and hierarchical double-shelled Co9S8@CNT
[41].
In
this
work,
a
three-dimensionally
interconnected
Co9S8/MWCNTs composite was synthesized by a facile solvothermal method and 6
subsequent annealing. The synthesis process is convenient, and forms a three-dimensionally interconnected network structure with the Co9S8 nanoparticles uniformly and tightly attached to the MWCNTs. Benefiting from the highly conductive MWCNTs network, the strong adsorption and catalysis of the polar Co9S8 nanoparticles, and the firm connection between the MWCNTs and Co9S8, the Co9S8/MWCNTs/S
composite
cathode
displays
outstanding
electrochemical
properties and shows great potential to be used in high-performance lithium–sulfur batteries. 2. Experimental 2.1 Material preparation 2.1.1 Synthesis of Co9S8/MWCNTs A
three-dimensionally
interconnected
Co9S8/MWCNTs
composite
was
synthesized using a solvothermal route. Co(NO3)2 (1.46 g) and thiourea (0.76 g) were dissolved in a mixed solution of 20 mL of ethanol and 20 mL of ethylene glycol, and stirred at 25 °C for 60 min. MWCNTs (50 mg, Chengdu Organic Chemical Co., Ltd.) were then added, and the dispersion was ultrasonicated for 60 min, which then underwent a solvothermal reaction at 180 °C for 6 h in an autoclave. After the reaction, the collected precursor was washed and dried, before calcination in a mixed H2/Ar atmosphere at 700 °C for 2 h. The final material was named Co9S8/MWCNTs. For comparison, pristine Co9S8 was also synthesized using the same procedure, but without MWCNTs. 7
2.1.2 Synthesis of Co9S8/MWCNTs/S The Co9S8/MWCNTs/S composite was synthesized by a melting and diffusion process [38]. Co9S8/MWCNTs and sublimed S (1:4) were thoroughly ground and then held at 155 °C for 12 h in a closed glass bottle under an Ar atmosphere. For comparison, the Co9S8/S and MWCNTs/S composites were also synthesized using the same procedures. 2.2 Characterizations of material The phase compositions of the materials were tested by X-ray diffraction (XRD) using an Xpert Pro MPD diffractometer with Cu Kα radiation. The microstructures were observed by transmission electron microscopy (TEM, FEI Titan G2 60-300) and scanning electron microscopy (SEM, FEI NanoSEM 400). The S content and specific surface area were tested by a thermal analyzer (STA449, NETZSCH) in a N2 atmosphere and a specific surface analyzer (ASAP 2460, Micromeritics). The elemental and chemical state information was tested with X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). For the visual comparison of polysulfide absorption ability, S powder and Li2S with a molar ratio of 5:1 were added into 1,3-dioxolane (DOL) and dimethoxyethane (DME) (1:1 by volume) with continuous stirring at 80 °C for 48 h to form the Li2S6 solution, and then 30 mg of Co9S8 and Co9S8/MWCNTs were respectively added to the Li2S6 solution [42]. 2.3 Electrochemical measurements
8
The prepared active material, Super P, and PVDF were mixed (7:2:1) using NMP as the dispersing agent, and the formed slurry was smeared on an Al foil before drying at 70 °C for 12 h in an oven. The negative electrode was a Li foil, the separator was a Celgard 2400 film, and the electrolyte was 1 M LITFSI/DME+DOL (1:1) with LiNO3 (0.1 M). A 2032 button cell was then assembled in a glove box (Universal 2400, MIKROUNA) under Ar. The area loading of S was approximately 1.5 mg cm−2, the area of the cathode was 1.54 cm2, and the electrolyte volume added in per cell was 100 μL. Charge/discharge curves were obtained using a NEWARE BTS test system. Cyclic voltammetry (CV) and AC impedance were measured on a Shanghai Chenhua CHI660D electrochemical workstation. The frequency of the impedance test ranged from 10−2 to 105 Hz with an amplitude of 5 mV. All the test voltage ranges were 1.7– 2.8 V. 3. Results and discussion 3.1 Morphology and structure analysis A
three-dimensionally
interconnected
Co9S8/MWCNTs
composite
was
synthesized by a solvothermal method, as shown in Fig. 1. First, the CNTs, thiourea, and cobalt nitrate were uniformly dispersed in a solvent mixture of ethanol and ethylene glycol (Fig. 1a), and then transferred into an autoclave for the solvothermal reaction (Fig. 1b). The precursor was further annealed to obtain the Co9S8/MWCNTs composite (Fig. 1c), before loading with S to form the Co9S8/MWCNTs/S cathode material by a melting-diffusion method (Fig. 1d). Fig. 2(a) displays the XRD patterns 9
of the samples. The diffraction peaks of both samples are attributable to the cubic Co9S8 structure (JCPDS No. 65-1765, space group Fm3m) [38,43], indicating the successful formation of a well crystallized pure-phase product by the hydrothermal method. No features of CNTs were found for Co9S8/MWCNTs, indicating that the CNTs did not affect the structure of the Co9S8, which may be due to the small amount of CNTs. The XRD patterns of S, Co9S8/S, and Co9S8/MWCNTs/S are displayed in Fig. 2(b). The sharp characteristic peaks of S are observed in the patterns of both Co9S8/S and Co9S8/MWCNTs/S, and two characteristic peaks at 29.5° and 52° are ascribed to the (311) and (440) crystal planes of Co9S8, indicating that the sublimed sulfur has been well melted internally with the melt-diffusion process. The morphologies of the samples were characterized by SEM and TEM (Fig. 3). Fig. 3(a and b) displays that the Co9S8 nanoparticles with an average size of 300–500 nm are evenly dispersed. When the MWCNTs are incorporated, the Co9S8 nanoparticles are uniformly and tightly attached to the MWCNTs (Fig. 3c and d), and a three-dimensionally interconnected network structure is formed for the Co9S8/MWCNTs composite. Owing to the high aspect ratio and random orientation of the MWCNTs, the Co9S8 nanoparticles are evenly distributed, which greatly increases the conductivity among the Co9S8 nanoparticles and accelerates the transport of ions and electrons. Fig. 3(e and f) shows the TEM images of Co9S8 and Co9S8/MWCNTs. The pristine Co9S8 consists of primary particles with a diameter of 10–20 nm, which agglomerate to form the secondary particles of 300–500 nm (Figs. 3e and S1). Clear 10
interplanar spacings of 0.195, 0.250, and 0.303 nm related to the crystal planes of (511), (400), and (311) of Co9S8 are observed (Fig. 3f). For the Co9S8/MWCNTs composite, the interconnected three-dimensional interconnected network structure of the MWCNTs is also observed. Meanwhile, the HRTEM observations show that the MWCNTs and the Co9S8 particles are tightly connected, and the interplanar spacings of 0.301 nm matches the (311) planes of the Co9S8 phase (Fig. 3g and h). Such a combination of Co9S8 nanoparticles and CNTs may effectively enhance the polysulfide adsorption and the composite conductivity as a cathode host. As shown in Fig. 4, typical SEM images of Co9S8/S (Fig. 4a and b) and Co9S8/MWCNTs/S (Fig. 4c and d), the S is uniformly dispersed on both materials. The uniformly dispersed CNT networks may buffer the volume change of the cathode and increase the stability. The S content has been tested by the thermogravimetric analysis, as shown in Fig. 5(a). The weight losses of both samples are mainly between 200–300 °C. The S contents of the Co9S8/S and Co9S8/MWCNTs/S samples were determined to be 76.08% and 76.02%, respectively, which are similar to the initial ratio during mixing, indicating that the S is tightly incorporated. Fig. 5(b) displays the nitrogen adsorption–desorption isothermals with the inset of the pore distribution curves of Co9S8 and Co9S8/MWCNTs. The specific surface areas of Co9S8 and Co9S8/MWCNTs are calculated to be 16.2 and 20.1 m2 g−1, respectively, and Co9S8/MWCNTs displays a higher pore volume (0.1 cm3 g−1) than Co9S8 (0.074 cm3 g−1). These characteristics of the Co9S8/MWCNTs composite may increase the contact with the electrolyte, 11
increase the adsorption of polysulfide, and increase the discharge specific capacity and cycle stability [43]. XPS was applied to further characterize the elemental composition and chemical state. Fig. 5(c and d) shows the XPS spectra and fitting curves for Co9S8 and Co9S8/MWCNTs, and Table S1 lists the corresponding assignments. The survey spectra present four elements of Co, S, C, and O (Fig. S2). The O element may result from the environment or a small amount of oxidation of the sample. Fig. 5(c) displays the core level Co 2p spectra, which are fitted with six peaks. The peaks at 802.9, 797.4, and 793.8 eV correspond to the vibrating satellite peaks, Co2+ and Co3+, for Co 2p1/2, and the other three peaks at 785.2, 781.2, and 778.7 eV correspond to the vibrating satellite peaks, Co2+ and Co3+, for Co 2p3/2 [44,45]. In the core level S 2p spectra (Fig. 5d), a fitted peak at 168.8 eV corresponds to SO42－, which might be derived from the partial oxidation of S on the surface of Co9S8. The peaks corresponding to S 2p1/2 and S 2p3/2 of S2－ in Co9S8 are respectively located at 163.1 and 162.4 eV. The peak at 161.5 eV corresponds to the terminal S-Co bond [46]. Compared to Co9S8, Co9S8/MWCNTs displays a new peak at 164.9 eV, which matches well with the C–S bond according to the literature [44,47]. From the results in Table S1, the contents of SO42−, S 2p1/2, S 2p3/2 and S-Co bond of Co9S8 are 48.3%, 9.0%, 26.6% and 16.1%, respectively, whereas the corresponding contents of Co9S8/MWCNTs are 38.6%, 12.3%, 20.1% and 11.8%, and the ratio of the additional C–S bond is approximately 17.2%. The increased S 2p1/2 content and the appeared C– 12
S bond of the Co9S8/MWCNTs composite indicate the possible interaction and bonding between Co9S8 and the incorporated MWCNTs network, which may be advantageous to enhance the electrical conductivity, structural and cycle stability as a S cathode host. In order to compare the polysulfide adsorptivity, the Co9S8 and Co9S8/MWCNTs were separately added into a Li2S6 solution, and digital photographs of the process are shown in Fig. S3. At the beginning, both solutions show the same yellow color. With the addition of Co9S8, the solution still displays slight yellow color after several hours. In contrast, with the addition of Co9S8/MWCNTs, the yellow solution becomes almost colorless,
indicating
the
strong
polysulfide
chemisorption
curves
MWCNT/S,
capacity
of
Co9S8/MWCNTs [38,42]. 3.2 Electrochemical performance The
initial
charge/discharge
for
Co9S8/S,
and
Co9S8/MWCNT/S at 0.1 C are shown in Fig. 6(a). The discharge specific capacities of MWCNT/S, Co9S8/S, and Co9S8/MWCNT/S are respectively 794, 1018, and 1154 mA h g−1, indicating the lowest specific capacity of the MWCNTs/S electrode and that the incorporated CNT network can efficiently increase the reaction sites and improve the specific capacity of Co9S8 [39]. Fig. 6(b) displays the rate performance for MWCNT/S, Co9S8/S, and Co9S8/MWCNT/S. The discharge specific capacities of Co9S8/MWCNT/S at 0.1, 0.2, 0.5, 1, and 2 C are 1154, 980, 845, 770, and 695 mA h g−1, respectively. In contrast, the MWCNTs/S electrode displays specific discharge 13
capacities of 794, 563, 454, 391, and 326 mA h g−1, and the Co9S8/S electrode displays discharge specific capacities of 1018, 870, 791, 682, and 535 mA h g−1, at 0.1, 0.2, 0.5, 1, and 2 C, respectively. The MWCNTs/S composite shows the lowest specific capacities at any tested current density, and compared to Co9S8/S, there is a remarkable increase in the discharge specific capacity of Co9S8/MWCNT/S at all the rates up to 2 C. The enhancement in the specific capacity and rate capability might be due to the tight bonding between the polar Co9S8 and the CNTs and the overall three-dimensional porous composite structure, which effectively accelerate the Li-ion and electron transport and provide more efficient adsorption of the polysulfide [38]. When returned to 0.1 C, a specific discharge capacity of 1050 mA h g−1 is retained by Co9S8/MWCNT/S, indicating the relatively stable internal structure of the material. Fig. 6(c) shows the cycle performances of Co9S8/S and Co9S8/MWCNT/S tested at 0.5 C. The initial discharge specific capacity of Co9S8/MWCNT/S is 845 mA h g−1, and the remaining capacity is 549 mA h g−1 after 400 cycles, corresponding to a capacity retention of 64%. However, the Co9S8/S electrode displays an initial capacity of 791 mA h g−1, and the retention is only 57% after 400 cycles. To prove the potential of the Co9S8/MWCNT/S composite for practical application in Li-S batteries, the electrodes with different sulfur loadings were prepared and the electrochemical performances were measured, as shown in Fig. S4. When the sulfur loadings are respectively 1.5, 2.8 and 4.2 mg cm-2, the Co9S8/MWCNTs/S composite cathode can release high specific discharge capacities of 1154, 981 and 802 mAh g-1 at 0.1 C. The batteries 14
with different E/S ratios were also assembled and measured, and the results are shown in Fig. S5. With the E/S ratios of 43.3, 29.5 and 12.5 μL mg-1, the Co9S8/MWCNTs/S composite cathode delivered high discharge specific capacities of 1154, 1057 and 760 mAh g-1 at 0.1 C. Further optimizations of the composition and structure of the Co9S8/MWCNTs host material may lead to even higher overall performance for actual applications. Fig. 6(d) shows the CV curves of Co9S8/S and Co9S8/MWCNT/S at 0.1 mV s−1 within 1.7–2.8 V. Both of the curves display an anodic oxidation peak and two cathodic reduction peaks. The reduction peak at approximately 2.32 V corresponds to the transition of S8 to the long-chain polysulfides (Li2Sx, 4 ≤ x ≤ 8, peak 1), and the other one at 2.05 V corresponds to the conversion of the long-chain polysulfides to short-chain ones before the final formation of Li2S and Li2S2 (peak 2). The oxidation peak corresponds to the reverse oxidation of Li2S/Li2S2 until the formation of S8 (peak 3) [48]. Compared to Co9S8/S, the Co9S8/MWCNTs/S composite displays a positive shift of peak 1, and a negative shift of peak 3. Moreover, the onset potentials of peaks 1, 2, and 3 of Co9S8/MWCNTs/S are, respectively, 2.42, 2.09, and 2.20 V, whereas those of the Co9S8/S electrode are, respectively, 2.40, 2.09, and 2.23 V, as determined by the tangent method [49]. Compared to Co9S8/S, the Co9S8/MWCNTs/S electrode displays a lower anodic onset potential and higher first cathodic onset potential, indicating that the three-dimensionally interconnected conductive network reduces the electrode polarization, improves the electrochemical activity and reversibility, and 15
effectively promotes the catalytic conversion of polysulfides [38,50-53]. The lithium-ion diffusion coefficient can be calculated by using the Randles–Sevcik equation [54–56]: I P = 2.69 105n1.5 AD0.5Cv 0.5
(1)
where Ip is the peak current, n is the number of electrons transferred during the reaction, A is the active electrode area, D is the diffusion coefficient of lithium ions, C is the concentration of lithium ions in the electrolyte, and v is the scan rate. The lithium-ion diffusion coefficients for Co9S8/S and Co9S8/MWCNTs/S are calculated to be, respectively, 4.09 × 10−9 and 6.57 × 10−9 cm2 S−1 from the oxidation peak, and 1.82 × 10−9 and 5.26 × 10−9 cm2 S−1 from the second reduction peak 2. These results corroborate that the addition of CNTs to form the Co9S8/MWCNTs composite host indeed enhances the ion transport in the electrode [57]. In order to study the electron and ion transfer ability of the electrode materials, impedance tests were carried out on the Co9S8/S and Co9S8/MWCNTs/S samples, and Fig. 6(f) shows the curves and fitting results. The impedance spectra of both samples are composed of a high-frequency semicircle representing the charge transfer impedance between the electrode and the electrolyte, and a low-frequency inclined line representing the Warburg impedance [58]. The curves can be reasonably fitted with the built-in equivalent circuit. Re represents the resistance of the electrolyte, and CPE is the double-layer capacitor between the electrode and the electrolyte. Rct represents the charge transfer impedance and Zw corresponds to the Warburg 16
impedance. The Rct values of Co9S8/S and Co9S8/MWCNTs/S are 33.3 and 21.7 Ω, respectively. The charge transfer impedance of Co9S8/MWCNTs/S is smaller than that of Co9S8/S. The
remarkable
electrochemical
energy-storage
performance
of
Co9S8/MWCNTs/S may be ascribed to the unique structure and characteristics of the composite, a mechanical schematic of which depicted in Fig. 6(g). The three-dimensionally interconnected porous network structure of Co9S8 and MWCNTs can increase the solid/liquid contact interface and accelerate the transport of electrons and Li ions, leading to high rate capability and fast reaction kinetics [59]. The dispersed polar Co9S8 can greatly increase the adsorption and catalytic conversion of polysulfides, thereby suppressing the shuttle effect [39]. The formation of the chemical bond between MWCNTs and Co9S8 may significantly increase the connection and robustness of the composite, and thus greatly improve the cycling stability [38,43]. 4. Conclusions In summary, a three-dimensionally interconnected porous Co9S8/MWCNTs composite has been successfully prepared by a solvothermal method. Benefiting from appealing properties such as high specific surface area, enhanced conductivity, strong adsorption, and fast catalytic conversion of polysulfide, as well as the firm bonding between MWCNTs and Co9S8, the electrochemical energy storage performance of the assembled batteries was synergistically improved. The Co9S8/MWCNTs/S composite 17
displays excellent performance with a discharge capacity of 1154 mA h g−1 and a stable capacity retention of 64% after 400 cycles, and thus can be considered as a potential material for the lithium–sulfur battery applications. Further optimization of the composition and structure of the Co9S8/MWCNTs host material, such as the ratio of Co9S8 and MWCNTs, the particle size of Co9S8, and the pore size and porosity of the composite, would yield even higher overall performance. The convenient synthesis method in this work could potentially be extended to prepare other three-dimensionally interconnected host materials, such as the Co9S8/graphene or Co9S8/carbon fiber.
Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (No. 51372178) and the Natural Science Foundation of Hubei Province of China (No. 2013CFA021, 2017CFB401, 2018CFA022).
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Figure Captions
Fig. 1. Schematic depiction of the synthesis procedure of Co9S8/MWCNTs/S. (a) Uniform dispersion of reactants in solution. (b) Precursor was formed after the solvothermal reaction at 180 °C for 12 h. (c) Three-dimensional Co9S8/MWCNTs composite was obtained after calcination at 700 °C for 2 h. (d) Formation of the 24
Co9S8/MWCNTs
Intensity (a.u.)
(511)
(331)
(b)
Intensity (a.u.)
(111)
(222)
(a)
(440)
(311)
Co9S8/MWCNTs/S composite by the melting-diffusion method at 155 °C for 12 h.
Co9S8
Co9S8/MWCNTs/S △
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Co9S8/S
PDF#65-1765 10
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2Theta (degree)
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Fig. 2. XRD patterns of (a) Co9S8, Co9S8/MWCNTs, (b) Co9S8/S, and Co9S8/MWCNTs/S.
25
Fig. 3. SEM images of (a, b) Co9S8 and (c, d) Co9S8/MWCNTs. TEM images of (e, f) Co9S8 and (g, h) Co9S8/MWCNTs.
26
Fig. 4. SEM images of (a, b) Co9S8/S and (c, d) Co9S8/MWCNTs/S.
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Co9S8/S Co9S8/MWCNTs/S S
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3 -1 Volume Adsorbed (cm g )
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76.02%
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2+ 3+
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Co9S8/MWCNTs
Co9S8
0
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Relative pressure (P/P0)
Co
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S 2p
Co 2p
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Co
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(b)
3+
SO4
Intensity (a.u.)
Co
Sat.
Co 2p3/2
0.000
Adsorption Desorption
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Temperature (C) Co 2p1/2
0.002
Pore diameter (nm)
30
500
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10 100
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Co9S8
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S-Co
Co9S8 C-S
Co9S8/MWCNTs
Co9S8/MWCNTs
(c) 810
805
800
795 790 785 Binding energy (eV）
780
(d) 174 172 170 168 166 164 162 160 158 156 Binding energy / eV
775
Fig. 5. (a) Thermogravimetric analysis of Co9S8/S, Co9S8/MWCNTs/S. (b) N2 isothermal adsorption/desorption curves of Co9S8/S and Co9S8/MWCNTs/S (inset: pore size distribution curve). XPS core level spectra of Co 2p (c) and S 2p (d) of Co9S8 and Co9S8/MWCNTs
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2.6
3.0
Co9S8/MWCNTs/S
2.8
2.4
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Voltage (V)
Onset potential (V)
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Fig. 6. Electrochemical performance of materials. (a) Initial charge/discharge profiles of MWCNTs/S, Co9S8/S, and Co9S8/MWCNTs/S at 0.1 C. (b) Rate capabilities of MWCNTs/S, Co9S8/S, and Co9S8/MWCNTs/S. (c) Cycling performance of Co9S8/S and Co9S8/MWCNTs/S at 0.5 C. (d) Cyclic voltammograms of Co9S8/S and Co9S8/MWCNTs/S at 0.1 mV s−1. (e) Cathodic and anodic onset potentials of Co9S8/S and Co9S8/MWCNTs/S. (f) Impedance spectra of Co9S8/S and Co9S8/MWCNTs/S. (g) Mechanisms to demonstrate the enhanced performance of the Co9S8/MWCNTs composite.
30
1Three-dimensionally interconnected Co9 S8 /MWCNTs composite cathode host for lithium–sulfur batteries Shengyu Zhao , Xiaohui Tian , Yingke Zhou , Ben Ma , Angulakshmi Natarajan PII: DOI: Reference:
S2095-4956(19)30859-9 https://doi.org/10.1016/j.jechem.2019.10.011 JECHEM 980
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
25 April 2019 16 October 2019 17 October 2019
Please cite this article as: Shengyu Zhao , Xiaohui Tian , Yingke Zhou , Ben Ma , Angulakshmi Natarajan , 1Three-dimensionally interconnected Co9 S8 /MWCNTs composite cathode host for lithium–sulfur batteries, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.10.011
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Highlights
3D interconnected Co9S8/MWCNTs is prepared by a solvothermal method.
The polar Co9S8 displays strong adsorbent and catalytic effect for polysulfides.
The porous CNT networks improve the overall conductivity and electrolyte contact.
The composite presents excellent specific capacity, rate capability and cycling stability.
1
Three-dimensionally interconnected Co9S8/MWCNTs composite cathode host for lithium–sulfur batteries
Shengyu Zhao1, Xiaohui Tian1, Yingke Zhou, Ben Ma, Angulakshmi Natarajan*
The State Key Laboratory of Refractories and Metallurgy, Institute of Advanced Materials and Nanotechnology, College of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China
1
These authors contributed equally to this work.
Corresponding authors. Tel: +86 2768 862928, Fax: +86 2768 862928. E-mail addresses: [email protected] (Y. Zhou); [email protected] (A. Natarajan).
2
Graphical Abstract
Three-dimensionally interconnected porous Co9S8/MWCNTs composite has been facilely synthesized and demonstrates promising performance as a host material for lithium–sulfur batteries, due to the synergistic effect of polar Co9S8 and conductive MWCNT network.
3
Abstract Several challenging issues, such as the poor conductivity of sulfur, shuttle effects, large volume change of cathode, and the dendritic lithium in anode, have led to the low utilization of sulfur and hampered the commercialization of lithium–sulfur batteries. In this study, a novel three-dimensionally interconnected network structure comprising Co9S8 and multiwalled carbon nanotubes (MWCNTs) was synthesized by a solvothermal route and used as the sulfur host. The assembled batteries delivered a specific capacity of 1154 mA h g−1 at 0.1 C, and the retention was 64% after 400 cycles at 0.5 C. The polar and catalytic Co9S8 nanoparticles have a strong adsorbent effect for polysulfide, which can effectively reduce the shuttling effect. Meanwhile, the three-dimensionally interconnected CNT networks improve the overall conductivity and increase the contact with the electrolyte, thus enhancing the transport of electrons and Li ions. Polysulfide adsorption is greatly increased with the synergistic effect of polar Co9S8 and MWCNTs in the three-dimensionally interconnected composites, which contributes to their promising performance for the lithium–sulfur batteries.
Keywords: Three-dimensional network structure; MWCNTs; Polar and catalytic Co9S8; Lithium–sulfur batteries
4
1. Introduction Conventional Li-ion batteries have been extensively developed and applied in energy storage systems and portable electronic devices for decades [1–5]. However, their applications for electric vehicles are currently plagued by their limited theoretical power/energy densities [6–9]. Lithium–sulfur batteries have been identified as an ultimate system by virtue of their unique properties, such as high theoretical energy density, environmental benignity, low cost, better safety, and abundant resources [10,11]. Despite these advantages, the insulating nature of elemental sulfur, the huge volume change (~80%), and the shuttling effects accompanying the polysulfide dissolution in the electrolyte lead to low sulfur utilization, poor rate and cycling performances, and impede the commercialization of this system [12–18]. To overcome these drawbacks, many types of cathode host materials have been developed to enhance the electrochemical performance of lithium–sulfur batteries, such as nanocarbon materials, conductive polymer, and metal compounds [19–21]. Various nanostructural carbon materials, such as carbon nanotubes (nanofibers) [22– 24], porous carbon [25,26], graphene [27], and carbon aerogel [28] have been widely used as host materials, owing to the advantages of high electrical conductivity and surface area and adjustable pore size [29]. Nevertheless, the polysulfide confinement 5
of the carbon materials is mainly due to the physical interaction of van der Waals forces, and the shuttle effect cannot be efficiently suppressed. Recently, polar materials have been shown to effectively increase the adsorption of polysulfides through the formation of strong chemical bonding, to decrease the polysulfide dissolution and shuttling. Many studies have been carried out on nanosized polar materials, such as TiO2 [30], MnO2 [31], SiO2 [32,33], V2O5 [34], NiS2 [35], and SnS2 [36]. However, the conductivity of the polar compound is usually lower than that of the nanocarbon host material, resulting in the poor utilization of sulfur and a low rate capability [37]. Therefore, it is imperative to develop a host material with high conductivity and strong adsorption, and a combination of a polar compound with a nanocarbon is beneficial to increase the conductivity, improve the electrolyte contact, and provide more effective chemical and physical confinements of the polysulfides, and thereby to greatly enhance the battery performance [38–41]. Very recently, polar Co9S8 nanostructures have demonstrated strong chemical adsorption and intrinsic catalytic activity to accelerate the redox reaction and polysulfide conversion [39], and performance improvements of lithium–sulfur batteries through combination of polar Co9S8 and conductive nanocarbon have been reported, such as composites of Co9S8 and three-dimensional graphene foam [21], Co9S8 embedded in carbon hollo nanopolyhedra [40], and hierarchical double-shelled Co9S8@CNT
[41].
In
this
work,
a
three-dimensionally
interconnected
Co9S8/MWCNTs composite was synthesized by a facile solvothermal method and 6
subsequent annealing. The synthesis process is convenient, and forms a three-dimensionally interconnected network structure with the Co9S8 nanoparticles uniformly and tightly attached to the MWCNTs. Benefiting from the highly conductive MWCNTs network, the strong adsorption and catalysis of the polar Co9S8 nanoparticles, and the firm connection between the MWCNTs and Co9S8, the Co9S8/MWCNTs/S
composite
cathode
displays
outstanding
electrochemical
properties and shows great potential to be used in high-performance lithium–sulfur batteries. 2. Experimental 2.1 Material preparation 2.1.1 Synthesis of Co9S8/MWCNTs A
three-dimensionally
interconnected
Co9S8/MWCNTs
composite
was
synthesized using a solvothermal route. Co(NO3)2 (1.46 g) and thiourea (0.76 g) were dissolved in a mixed solution of 20 mL of ethanol and 20 mL of ethylene glycol, and stirred at 25 °C for 60 min. MWCNTs (50 mg, Chengdu Organic Chemical Co., Ltd.) were then added, and the dispersion was ultrasonicated for 60 min, which then underwent a solvothermal reaction at 180 °C for 6 h in an autoclave. After the reaction, the collected precursor was washed and dried, before calcination in a mixed H2/Ar atmosphere at 700 °C for 2 h. The final material was named Co9S8/MWCNTs. For comparison, pristine Co9S8 was also synthesized using the same procedure, but without MWCNTs. 7
2.1.2 Synthesis of Co9S8/MWCNTs/S The Co9S8/MWCNTs/S composite was synthesized by a melting and diffusion process [38]. Co9S8/MWCNTs and sublimed S (1:4) were thoroughly ground and then held at 155 °C for 12 h in a closed glass bottle under an Ar atmosphere. For comparison, the Co9S8/S and MWCNTs/S composites were also synthesized using the same procedures. 2.2 Characterizations of material The phase compositions of the materials were tested by X-ray diffraction (XRD) using an Xpert Pro MPD diffractometer with Cu Kα radiation. The microstructures were observed by transmission electron microscopy (TEM, FEI Titan G2 60-300) and scanning electron microscopy (SEM, FEI NanoSEM 400). The S content and specific surface area were tested by a thermal analyzer (STA449, NETZSCH) in a N2 atmosphere and a specific surface analyzer (ASAP 2460, Micromeritics). The elemental and chemical state information was tested with X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). For the visual comparison of polysulfide absorption ability, S powder and Li2S with a molar ratio of 5:1 were added into 1,3-dioxolane (DOL) and dimethoxyethane (DME) (1:1 by volume) with continuous stirring at 80 °C for 48 h to form the Li2S6 solution, and then 30 mg of Co9S8 and Co9S8/MWCNTs were respectively added to the Li2S6 solution [42]. 2.3 Electrochemical measurements
8
The prepared active material, Super P, and PVDF were mixed (7:2:1) using NMP as the dispersing agent, and the formed slurry was smeared on an Al foil before drying at 70 °C for 12 h in an oven. The negative electrode was a Li foil, the separator was a Celgard 2400 film, and the electrolyte was 1 M LITFSI/DME+DOL (1:1) with LiNO3 (0.1 M). A 2032 button cell was then assembled in a glove box (Universal 2400, MIKROUNA) under Ar. The area loading of S was approximately 1.5 mg cm−2, the area of the cathode was 1.54 cm2, and the electrolyte volume added in per cell was 100 μL. Charge/discharge curves were obtained using a NEWARE BTS test system. Cyclic voltammetry (CV) and AC impedance were measured on a Shanghai Chenhua CHI660D electrochemical workstation. The frequency of the impedance test ranged from 10−2 to 105 Hz with an amplitude of 5 mV. All the test voltage ranges were 1.7– 2.8 V. 3. Results and discussion 3.1 Morphology and structure analysis A
three-dimensionally
interconnected
Co9S8/MWCNTs
composite
was
synthesized by a solvothermal method, as shown in Fig. 1. First, the CNTs, thiourea, and cobalt nitrate were uniformly dispersed in a solvent mixture of ethanol and ethylene glycol (Fig. 1a), and then transferred into an autoclave for the solvothermal reaction (Fig. 1b). The precursor was further annealed to obtain the Co9S8/MWCNTs composite (Fig. 1c), before loading with S to form the Co9S8/MWCNTs/S cathode material by a melting-diffusion method (Fig. 1d). Fig. 2(a) displays the XRD patterns 9
of the samples. The diffraction peaks of both samples are attributable to the cubic Co9S8 structure (JCPDS No. 65-1765, space group Fm3m) [38,43], indicating the successful formation of a well crystallized pure-phase product by the hydrothermal method. No features of CNTs were found for Co9S8/MWCNTs, indicating that the CNTs did not affect the structure of the Co9S8, which may be due to the small amount of CNTs. The XRD patterns of S, Co9S8/S, and Co9S8/MWCNTs/S are displayed in Fig. 2(b). The sharp characteristic peaks of S are observed in the patterns of both Co9S8/S and Co9S8/MWCNTs/S, and two characteristic peaks at 29.5° and 52° are ascribed to the (311) and (440) crystal planes of Co9S8, indicating that the sublimed sulfur has been well melted internally with the melt-diffusion process. The morphologies of the samples were characterized by SEM and TEM (Fig. 3). Fig. 3(a and b) displays that the Co9S8 nanoparticles with an average size of 300–500 nm are evenly dispersed. When the MWCNTs are incorporated, the Co9S8 nanoparticles are uniformly and tightly attached to the MWCNTs (Fig. 3c and d), and a three-dimensionally interconnected network structure is formed for the Co9S8/MWCNTs composite. Owing to the high aspect ratio and random orientation of the MWCNTs, the Co9S8 nanoparticles are evenly distributed, which greatly increases the conductivity among the Co9S8 nanoparticles and accelerates the transport of ions and electrons. Fig. 3(e and f) shows the TEM images of Co9S8 and Co9S8/MWCNTs. The pristine Co9S8 consists of primary particles with a diameter of 10–20 nm, which agglomerate to form the secondary particles of 300–500 nm (Figs. 3e and S1). Clear 10
interplanar spacings of 0.195, 0.250, and 0.303 nm related to the crystal planes of (511), (400), and (311) of Co9S8 are observed (Fig. 3f). For the Co9S8/MWCNTs composite, the interconnected three-dimensional interconnected network structure of the MWCNTs is also observed. Meanwhile, the HRTEM observations show that the MWCNTs and the Co9S8 particles are tightly connected, and the interplanar spacings of 0.301 nm matches the (311) planes of the Co9S8 phase (Fig. 3g and h). Such a combination of Co9S8 nanoparticles and CNTs may effectively enhance the polysulfide adsorption and the composite conductivity as a cathode host. As shown in Fig. 4, typical SEM images of Co9S8/S (Fig. 4a and b) and Co9S8/MWCNTs/S (Fig. 4c and d), the S is uniformly dispersed on both materials. The uniformly dispersed CNT networks may buffer the volume change of the cathode and increase the stability. The S content has been tested by the thermogravimetric analysis, as shown in Fig. 5(a). The weight losses of both samples are mainly between 200–300 °C. The S contents of the Co9S8/S and Co9S8/MWCNTs/S samples were determined to be 76.08% and 76.02%, respectively, which are similar to the initial ratio during mixing, indicating that the S is tightly incorporated. Fig. 5(b) displays the nitrogen adsorption–desorption isothermals with the inset of the pore distribution curves of Co9S8 and Co9S8/MWCNTs. The specific surface areas of Co9S8 and Co9S8/MWCNTs are calculated to be 16.2 and 20.1 m2 g−1, respectively, and Co9S8/MWCNTs displays a higher pore volume (0.1 cm3 g−1) than Co9S8 (0.074 cm3 g−1). These characteristics of the Co9S8/MWCNTs composite may increase the contact with the electrolyte, 11
increase the adsorption of polysulfide, and increase the discharge specific capacity and cycle stability [43]. XPS was applied to further characterize the elemental composition and chemical state. Fig. 5(c and d) shows the XPS spectra and fitting curves for Co9S8 and Co9S8/MWCNTs, and Table S1 lists the corresponding assignments. The survey spectra present four elements of Co, S, C, and O (Fig. S2). The O element may result from the environment or a small amount of oxidation of the sample. Fig. 5(c) displays the core level Co 2p spectra, which are fitted with six peaks. The peaks at 802.9, 797.4, and 793.8 eV correspond to the vibrating satellite peaks, Co2+ and Co3+, for Co 2p1/2, and the other three peaks at 785.2, 781.2, and 778.7 eV correspond to the vibrating satellite peaks, Co2+ and Co3+, for Co 2p3/2 [44,45]. In the core level S 2p spectra (Fig. 5d), a fitted peak at 168.8 eV corresponds to SO42－, which might be derived from the partial oxidation of S on the surface of Co9S8. The peaks corresponding to S 2p1/2 and S 2p3/2 of S2－ in Co9S8 are respectively located at 163.1 and 162.4 eV. The peak at 161.5 eV corresponds to the terminal S-Co bond [46]. Compared to Co9S8, Co9S8/MWCNTs displays a new peak at 164.9 eV, which matches well with the C–S bond according to the literature [44,47]. From the results in Table S1, the contents of SO42−, S 2p1/2, S 2p3/2 and S-Co bond of Co9S8 are 48.3%, 9.0%, 26.6% and 16.1%, respectively, whereas the corresponding contents of Co9S8/MWCNTs are 38.6%, 12.3%, 20.1% and 11.8%, and the ratio of the additional C–S bond is approximately 17.2%. The increased S 2p1/2 content and the appeared C– 12
S bond of the Co9S8/MWCNTs composite indicate the possible interaction and bonding between Co9S8 and the incorporated MWCNTs network, which may be advantageous to enhance the electrical conductivity, structural and cycle stability as a S cathode host. In order to compare the polysulfide adsorptivity, the Co9S8 and Co9S8/MWCNTs were separately added into a Li2S6 solution, and digital photographs of the process are shown in Fig. S3. At the beginning, both solutions show the same yellow color. With the addition of Co9S8, the solution still displays slight yellow color after several hours. In contrast, with the addition of Co9S8/MWCNTs, the yellow solution becomes almost colorless,
indicating
the
strong
polysulfide
chemisorption
curves
MWCNT/S,
capacity
of
Co9S8/MWCNTs [38,42]. 3.2 Electrochemical performance The
initial
charge/discharge
for
Co9S8/S,
and
Co9S8/MWCNT/S at 0.1 C are shown in Fig. 6(a). The discharge specific capacities of MWCNT/S, Co9S8/S, and Co9S8/MWCNT/S are respectively 794, 1018, and 1154 mA h g−1, indicating the lowest specific capacity of the MWCNTs/S electrode and that the incorporated CNT network can efficiently increase the reaction sites and improve the specific capacity of Co9S8 [39]. Fig. 6(b) displays the rate performance for MWCNT/S, Co9S8/S, and Co9S8/MWCNT/S. The discharge specific capacities of Co9S8/MWCNT/S at 0.1, 0.2, 0.5, 1, and 2 C are 1154, 980, 845, 770, and 695 mA h g−1, respectively. In contrast, the MWCNTs/S electrode displays specific discharge 13
capacities of 794, 563, 454, 391, and 326 mA h g−1, and the Co9S8/S electrode displays discharge specific capacities of 1018, 870, 791, 682, and 535 mA h g−1, at 0.1, 0.2, 0.5, 1, and 2 C, respectively. The MWCNTs/S composite shows the lowest specific capacities at any tested current density, and compared to Co9S8/S, there is a remarkable increase in the discharge specific capacity of Co9S8/MWCNT/S at all the rates up to 2 C. The enhancement in the specific capacity and rate capability might be due to the tight bonding between the polar Co9S8 and the CNTs and the overall three-dimensional porous composite structure, which effectively accelerate the Li-ion and electron transport and provide more efficient adsorption of the polysulfide [38]. When returned to 0.1 C, a specific discharge capacity of 1050 mA h g−1 is retained by Co9S8/MWCNT/S, indicating the relatively stable internal structure of the material. Fig. 6(c) shows the cycle performances of Co9S8/S and Co9S8/MWCNT/S tested at 0.5 C. The initial discharge specific capacity of Co9S8/MWCNT/S is 845 mA h g−1, and the remaining capacity is 549 mA h g−1 after 400 cycles, corresponding to a capacity retention of 64%. However, the Co9S8/S electrode displays an initial capacity of 791 mA h g−1, and the retention is only 57% after 400 cycles. To prove the potential of the Co9S8/MWCNT/S composite for practical application in Li-S batteries, the electrodes with different sulfur loadings were prepared and the electrochemical performances were measured, as shown in Fig. S4. When the sulfur loadings are respectively 1.5, 2.8 and 4.2 mg cm-2, the Co9S8/MWCNTs/S composite cathode can release high specific discharge capacities of 1154, 981 and 802 mAh g-1 at 0.1 C. The batteries 14
with different E/S ratios were also assembled and measured, and the results are shown in Fig. S5. With the E/S ratios of 43.3, 29.5 and 12.5 μL mg-1, the Co9S8/MWCNTs/S composite cathode delivered high discharge specific capacities of 1154, 1057 and 760 mAh g-1 at 0.1 C. Further optimizations of the composition and structure of the Co9S8/MWCNTs host material may lead to even higher overall performance for actual applications. Fig. 6(d) shows the CV curves of Co9S8/S and Co9S8/MWCNT/S at 0.1 mV s−1 within 1.7–2.8 V. Both of the curves display an anodic oxidation peak and two cathodic reduction peaks. The reduction peak at approximately 2.32 V corresponds to the transition of S8 to the long-chain polysulfides (Li2Sx, 4 ≤ x ≤ 8, peak 1), and the other one at 2.05 V corresponds to the conversion of the long-chain polysulfides to short-chain ones before the final formation of Li2S and Li2S2 (peak 2). The oxidation peak corresponds to the reverse oxidation of Li2S/Li2S2 until the formation of S8 (peak 3) [48]. Compared to Co9S8/S, the Co9S8/MWCNTs/S composite displays a positive shift of peak 1, and a negative shift of peak 3. Moreover, the onset potentials of peaks 1, 2, and 3 of Co9S8/MWCNTs/S are, respectively, 2.42, 2.09, and 2.20 V, whereas those of the Co9S8/S electrode are, respectively, 2.40, 2.09, and 2.23 V, as determined by the tangent method [49]. Compared to Co9S8/S, the Co9S8/MWCNTs/S electrode displays a lower anodic onset potential and higher first cathodic onset potential, indicating that the three-dimensionally interconnected conductive network reduces the electrode polarization, improves the electrochemical activity and reversibility, and 15
effectively promotes the catalytic conversion of polysulfides [38,50-53]. The lithium-ion diffusion coefficient can be calculated by using the Randles–Sevcik equation [54–56]: I P = 2.69 105n1.5 AD0.5Cv 0.5
(1)
where Ip is the peak current, n is the number of electrons transferred during the reaction, A is the active electrode area, D is the diffusion coefficient of lithium ions, C is the concentration of lithium ions in the electrolyte, and v is the scan rate. The lithium-ion diffusion coefficients for Co9S8/S and Co9S8/MWCNTs/S are calculated to be, respectively, 4.09 × 10−9 and 6.57 × 10−9 cm2 S−1 from the oxidation peak, and 1.82 × 10−9 and 5.26 × 10−9 cm2 S−1 from the second reduction peak 2. These results corroborate that the addition of CNTs to form the Co9S8/MWCNTs composite host indeed enhances the ion transport in the electrode [57]. In order to study the electron and ion transfer ability of the electrode materials, impedance tests were carried out on the Co9S8/S and Co9S8/MWCNTs/S samples, and Fig. 6(f) shows the curves and fitting results. The impedance spectra of both samples are composed of a high-frequency semicircle representing the charge transfer impedance between the electrode and the electrolyte, and a low-frequency inclined line representing the Warburg impedance [58]. The curves can be reasonably fitted with the built-in equivalent circuit. Re represents the resistance of the electrolyte, and CPE is the double-layer capacitor between the electrode and the electrolyte. Rct represents the charge transfer impedance and Zw corresponds to the Warburg 16
impedance. The Rct values of Co9S8/S and Co9S8/MWCNTs/S are 33.3 and 21.7 Ω, respectively. The charge transfer impedance of Co9S8/MWCNTs/S is smaller than that of Co9S8/S. The
remarkable
electrochemical
energy-storage
performance
of
Co9S8/MWCNTs/S may be ascribed to the unique structure and characteristics of the composite, a mechanical schematic of which depicted in Fig. 6(g). The three-dimensionally interconnected porous network structure of Co9S8 and MWCNTs can increase the solid/liquid contact interface and accelerate the transport of electrons and Li ions, leading to high rate capability and fast reaction kinetics [59]. The dispersed polar Co9S8 can greatly increase the adsorption and catalytic conversion of polysulfides, thereby suppressing the shuttle effect [39]. The formation of the chemical bond between MWCNTs and Co9S8 may significantly increase the connection and robustness of the composite, and thus greatly improve the cycling stability [38,43]. 4. Conclusions In summary, a three-dimensionally interconnected porous Co9S8/MWCNTs composite has been successfully prepared by a solvothermal method. Benefiting from appealing properties such as high specific surface area, enhanced conductivity, strong adsorption, and fast catalytic conversion of polysulfide, as well as the firm bonding between MWCNTs and Co9S8, the electrochemical energy storage performance of the assembled batteries was synergistically improved. The Co9S8/MWCNTs/S composite 17
displays excellent performance with a discharge capacity of 1154 mA h g−1 and a stable capacity retention of 64% after 400 cycles, and thus can be considered as a potential material for the lithium–sulfur battery applications. Further optimization of the composition and structure of the Co9S8/MWCNTs host material, such as the ratio of Co9S8 and MWCNTs, the particle size of Co9S8, and the pore size and porosity of the composite, would yield even higher overall performance. The convenient synthesis method in this work could potentially be extended to prepare other three-dimensionally interconnected host materials, such as the Co9S8/graphene or Co9S8/carbon fiber.
Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (No. 51372178) and the Natural Science Foundation of Hubei Province of China (No. 2013CFA021, 2017CFB401, 2018CFA022).
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Figure Captions
Fig. 1. Schematic depiction of the synthesis procedure of Co9S8/MWCNTs/S. (a) Uniform dispersion of reactants in solution. (b) Precursor was formed after the solvothermal reaction at 180 °C for 12 h. (c) Three-dimensional Co9S8/MWCNTs composite was obtained after calcination at 700 °C for 2 h. (d) Formation of the 24
Co9S8/MWCNTs
Intensity (a.u.)
(511)
(331)
(b)
Intensity (a.u.)
(111)
(222)
(a)
(440)
(311)
Co9S8/MWCNTs/S composite by the melting-diffusion method at 155 °C for 12 h.
Co9S8
Co9S8/MWCNTs/S △
△
△
△
Co9S8/S
PDF#65-1765 10
20
30
40
50
60
70
S 80
10
2Theta (degree)
20
30
40
50
60
70
80
2Theta (degree)
Fig. 2. XRD patterns of (a) Co9S8, Co9S8/MWCNTs, (b) Co9S8/S, and Co9S8/MWCNTs/S.
25
Fig. 3. SEM images of (a, b) Co9S8 and (c, d) Co9S8/MWCNTs. TEM images of (e, f) Co9S8 and (g, h) Co9S8/MWCNTs.
26
Fig. 4. SEM images of (a, b) Co9S8/S and (c, d) Co9S8/MWCNTs/S.
27
Co9S8/S Co9S8/MWCNTs/S S
90
3 -1 Volume Adsorbed (cm g )
80
Weight (%)
70 60
76.08%
76.02%
50 40 30 20
(a)
10 0
0
50
80
0.008
70
0.006
Co9S8
Pore volume (m3g-1)
100
60 50 40
150
200
250
300
350
400
450
Co
2+ 3+
100
Co9S8/MWCNTs
Co9S8
0
0.0
0.2
0.4
0.6
Relative pressure (P/P0)
Co
0.8
1.0
S 2p
Co 2p
2+
Sat.
Intensity (a.u.)
Co
10
(b)
3+
SO4
Intensity (a.u.)
Co
Sat.
Co 2p3/2
0.000
Adsorption Desorption
20
Temperature (C) Co 2p1/2
0.002
Pore diameter (nm)
30
500
0.004
1
10 100
Co9S8/MWCNTs
Co9S8
S 2p3/2
2-
S 2p1/2
S-Co
Co9S8 C-S
Co9S8/MWCNTs
Co9S8/MWCNTs
(c) 810
805
800
795 790 785 Binding energy (eV）
780
(d) 174 172 170 168 166 164 162 160 158 156 Binding energy / eV
775
Fig. 5. (a) Thermogravimetric analysis of Co9S8/S, Co9S8/MWCNTs/S. (b) N2 isothermal adsorption/desorption curves of Co9S8/S and Co9S8/MWCNTs/S (inset: pore size distribution curve). XPS core level spectra of Co 2p (c) and S 2p (d) of Co9S8 and Co9S8/MWCNTs
28
2.6
3.0
Co9S8/MWCNTs/S
2.8
2.4
(a)
2.6
Voltage (V)
Onset potential (V)
(e)
Co9S8/S
2.2
2.4 2.2 2.0
Co9S8/S
2.0 1.8
Co9S8/MWCNTs/S MWCNTs/S
1.6 0
1.8 1
2
200
3
Peaks
(f)
40
Co9S8/MWCNTs/S
0.8
Co9S8/S
0.6
35
1000
1200
peak 3
(d)
Current (mA)
0.4
30
-Z'' (ohm)
800
1.0
45
25 20 15 10
0.2 0.0 -0.2 -0.4
peak 1
-0.6
5
Co9S8/S
-0.8
0
10
20
30
40
50
peak 2
-1.0
60
1.8
2.0
Z' (ohm) 1200
0.1 C
0.2 C 0.5 C
800
1C 2C
600 400
Co9S8/S Co9S8/MWCNTs/S
200
(b)
MWCNTs/S 0 0
5
2.2
2.4
2.6
2.8
1200
0.1 C
1000
Co9S8/MWCNTs/S
Potential (V)
10
15
20
25
1100
Specific capacity (mAhg-1)
Specific capacity (mAhg-1)
600
Specific capacity (mAhg-1)
50
0
400
1000 900 800 700 600 500 400 300 200
Co9S8/MWCNTs/S
100
Co9S8/S
0 0
30
Cycle number
(c)
50
100
150
200
250
Cycle number
29
300
350
400
Fig. 6. Electrochemical performance of materials. (a) Initial charge/discharge profiles of MWCNTs/S, Co9S8/S, and Co9S8/MWCNTs/S at 0.1 C. (b) Rate capabilities of MWCNTs/S, Co9S8/S, and Co9S8/MWCNTs/S. (c) Cycling performance of Co9S8/S and Co9S8/MWCNTs/S at 0.5 C. (d) Cyclic voltammograms of Co9S8/S and Co9S8/MWCNTs/S at 0.1 mV s−1. (e) Cathodic and anodic onset potentials of Co9S8/S and Co9S8/MWCNTs/S. (f) Impedance spectra of Co9S8/S and Co9S8/MWCNTs/S. (g) Mechanisms to demonstrate the enhanced performance of the Co9S8/MWCNTs composite.
30