Co3O4 hollow microspheres for the long-term cyclability of lithium-sulfur batteries

Journal of Alloys and Compounds 823 (2020) 153912

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Encapsulation of sulfur cathodes by sericin-derived carbon/Co3O4 hollow microspheres for the long-term cyclability of lithium-sulfur batteries Jun Wu a, *, Zhijie Pan a, Yang Dai a, Ting Wang b, Haiping Zhang b, Sheng Yan a, Junming Xu a, Kaixin Song a a b

College of Electronics and Information, Hangzhou Dianzi University, Hangzhou, 310018, China Hangzhou Dianzi University Information Engineering School, Hangzhou, 311305, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 July 2019 Received in revised form 25 October 2019 Accepted 17 January 2020 Available online 18 January 2020

For cathodes in lithium-sulfur (LieS) batteries, increasing research effort has been devoted to hollowstructured transition metal oxides to utilize their morphologies more efficiently and take advantage of their strong polar chemisorption of polysulfides. In this study, we synthesized a novel sulfur@carbon/ Co3O4 (S@C/Co3O4) composite via encapsulation of sulfur by sericin-derived carbon/Co3O4 hollow microspheres. With a sulfur content of 67.3%, the synthesized S@C/Co3O4 composite cathode delivered a high initial specific discharge capacity of 1171.6 mAh g
1 at 0.2 C and excellent rate performance at current densities up to 4 C. It also exhibited excellent long-term cyclability over roughly 1000 cycles at 1 C and 2 C with low decay rates of 0.076% and 0.062%, respectively. The improved electrochemical performance of the S@C/Co3O4 composite mainly stems from strong polar chemisorption of polysulfides by Co3O4, the physical encapsulation sulfur by hollow spheres, and high electrical conductivity of sericinderived carbon. © 2020 Elsevier B.V. All rights reserved.

Keywords: Lithium-sulfur batteries Composite cathodes Co3O4 microspheres Sericin

1. Introduction Increasing energy demands for portable electronic devices, electric vehicles, and grid storage, require that batteries be developed into novel systems with high energy density and reliability. Conventional intercalation lithium-ion technology has faced severe challenges based on its limited energy density (<300 Wh kg
1). Lithium-sulfur (LieS) batteries have received increased attention in recent years based on the high theoretical specific capacity (1675 mAh g
1) of sulfur cathodes and the high energy density (2600 Wh kg
1) achievable with lithium-metal anodes [1,2]. Additionally, sulfur has abundant natural reserves and low environmental impact that enables it to be applied to the sustainable development of energy storage systems. However, the commercialization of LieS batteries is hindered by several significant challenges, including large volumetric expansion upon lithiation (approximately 80% for all reduction products of Li2S), poor electronic conductivity (only approximately 5  10
30 S cm
1 at 25  C),

* Corresponding author. E-mail address: [email protected] (J. Wu). https://doi.org/10.1016/j.jallcom.2020.153912 0925-8388/© 2020 Elsevier B.V. All rights reserved.

and the shuttle effect of intermediate polysulfide [3,4]. In particular, long-chain polysulfides (Li2Sn, 4  n  8) formed during cycling process are easily dissolved in organic electrolytes and readily shuttle between electrodes, resulting in low coulombic efficiency and severe capacity fading. Over the past several years, extensive efforts have been devoted to solving the aforementioned problems by developing cathodes based on conductive porous or hollow materials. Such nonpolar carbon materials (porous carbon [5], carbon nanotubes [6], carbon nanofibers [7], graphene [8], etc.) are ideal hosts for sulfur particles based on their excellent electrical conductivity and easy control in structure. However, carbon-based materials fail to inhibit sufficient polysulfide dissolution based on simple encapsulation and weak physical adsorption. Therefore, various transition metal oxides with polar-polar bonds have been explored as chemisorption hosts for sulfur particles. Ti4O7/S composites were first proposed as cathodes for LieS batteries by Nazar et al. [9]. These composites exhibited initial specific capacities as high as 1200 mAh g
1 at 0.05 C and retained a value of 850 mAh g
1 after 100 cycles. Since then, several metal oxides and sulfides, including TiO2 [10], MnO2 [11], Co3O4 [12,13], SnO2 [14,15], MoS2 [16], and ZnMn2O4 [17], have been

2

J. Wu et al. / Journal of Alloys and Compounds 823 (2020) 153912

explored. Among these options, Co3O4 is particularly effective at improving the cyclability of batteries, which Zhong et al. attributed to the existence of an additional chemical CoeS bond in combination with the LieO bonds between Co3O4 and polysulfides [18]. This concept was demonstrated through the fabrication of a Co3O4eC/S composite with Co3O4 nanotubes and carbon fiber cloth, which possessed an initial discharge capacity as high as 1231 mAh g
1 at 0.5 C and an average capacity attenuation rate of only 0.049% over 500 cycles at 2 C, exhibiting excellent overall electrochemical performance [19]. For Li-S batteries, very few additional studies on Co3O4-based additives, especially additives with hollow structures, have been reported. In this study, Co3O4 hollow microspheres were synthesized through a template-free solvothermal method, and then utilized as hosts for sulfur. Before adding sulfur, sericin, which is a soluble residue remaining after extracting silk from cocoons, was calcined as a carbon source to modify the surfaces of the Co3O4 spheres. As illustrated in Fig. 1, encapsulation of sublimed sulfur by C/Co3O4 spheres can not only accommodate and relieve stress generated by the volumetric expansion of sulfur, but can also increase the conductivity of sulfur and Co3O4. Furthermore, the strong chemisorption of LieO and CoeS bonds can effectively impede the dissolution of polysulfides and deliver high coulombic efficiency, as well as excellent long-term cyclability. 2. Experimental section 2.1. Synthesis of Co3O4 hollow spheres Uniform Co3O4 hollow microspheres were prepared through a two-step template-free solvothermal method. First, 1 g of cobalt acetate (Co(CH3COO)2$4H2O) and 0.03 g polyvinyl pyrrolidone (PVP, Mw ¼ 40000 g mol
1) were dissolved into 80 ml of ethylene glycol to form a uniform solution. Following magnetic stirring for 0.5 h, the mixture was sealed in a Teflon autoclave reactor. Following solvothermal reaction at 180  C for 12 h, the mixture was centrifuged, rinsed, and dried in sequence. Finally, the dried product was heated in air at 400  C for 2 h to obtain Co3O4 hollow

microspheres [20]. 2.2. Surface modification of Co3O4 spheres by sericin First, 40 mg of Co3O4 was immersed in a sericin aqueous solution at a concentration of 2 mmol/l and stirred for 1 h Co3O4 spheres coated with sericin were obtained via centrifugation. The spheres were then calcined at 550  C for 2 h in a tube furnace in a nitrogen atmosphere. Finally, heat treatment was applied at 250  C for 0.5 h in air to obtain C/Co3O4 composites. 2.3. Synthesis of S@C/Co3O4 composites The C/Co3O4 composite and sublimed sulfur were mechanically mixed at a weight ratio of 3:7 corresponding to carbon disulfide (CS2). The mixture was then treated ultrasonically to achieve the complete volatilization. The resulting solid residue was transferred into a Teflon autoclave at 155  C and S@C/Co3O4 composites were formed over 12 h. As a control sample, a S@Co3O4 composite was also synthesized simultaneously. 2.4. Material characterization Field emission scanning election microscopy (FESEM; Hitachi S4800) and transmission electron microscope (TEM, JEOL-2010) analyses were performed to investigate the morphologies and hollow structures of the composites, respectively. SEM mapping was carried out by utilizing a Sigma HD FESEM to observe elemental distributions. X-ray diffraction (XRD, Bruker/AXS D8 Advance) analysis was performed with a Cu Ka radiation source at a wavelength of 0.154 nm and 2q angle in the range of 10 e80 . Raman spectra were measured at a laser wavelength of 532 nm utilizing a Renishaw 2000/inVia Raman spectrometer. The sulfur contents of the composites were determined via thermogravimetric analysis (TGA, SDT Q600) in the range of 30e500  C in a nitrogen atmosphere. N2 adsorption/desorption tests were conducted based on the Brunauer-Emmett-Teller method. The compositions and chemical bonds of the composites were investigated via X-ray photoelectron spectroscopy (XPS, Thermo Fisher 250Xi Xray photoelectron spectrometer). In a visual adsorption experiment, the samples were immersed into polysulfide solutions, which were prepared by mixing Li2S and sublimed sulfur at a molar ratio of 1:3 into a dimethoxyethane/dioxolane (DME/DOL, v/v ¼ 1/ 1) solvent. 2.5. Electrochemical measurements

Fig. 1. Scheme of synthesis involving the surface coating of Co3O4 microspheres with sericin-derived carbon, followed by encapsulation of sulfur to form a S@C/Co3O4 composite.

Test cells (CR 2032) were assembled in an argon-filled glove box (Mikrouna, Super). The cathode slurry consisted of 70 wt% of the asprepared S@C/Co3O4, S@Co3O4 or S@C composites, 20 wt% of conductive Ketjen black, and 10 wt% of binder (polyvinylidene fluoride) in a N-Methyl-2-pyrrolidone solvent. The slurry was then uniformly deposited onto round aluminum foil current collectors with a diameter of 16 mm. Pure lithium metal was used as an anode material. A Celgard 2500 separator was saturated in an electrolyte containing 1.0 M of lithium bis-(trifluoromethanesulfonyl) imide (LiTFSI, CJC 99%) and 1.0 wt% of LiNO3 in 1,3-dioxolane/1,2-dimethoxyethane (v/v ¼ 1/1 by volume). Galvanostatic chargingdischarging experiments with a voltage window of 1.5e3.0 V (vs. Liþ/Li) were conducted utilizing a NEWARE battery system. Cyclic voltammetry (CV) data were collected by a CHI660E electrochemical workstation (CH Instrument, China) at a scan rate of 0.1 mV s
1. Electrochemical impedance spectroscopy (EIS) in a frequency range of 0.01 Hze10 kHz was also performed utilizing the CHI660E electrochemical workstation [21,22].

J. Wu et al. / Journal of Alloys and Compounds 823 (2020) 153912

3. Results and discussion In this study, hierarchical Co3O4 hollow microspheres were synthesized successfully utilizing a solvothermal method. Fig. 2a presents a high-magnification SEM image of Co3O4 spheres with uniform diameters ranging from 1.95 to 2.05 mm. The shell of spheres consists of numerous two-dimensional nanosheets, indicating high specific surface areas. The hollow structures can be clearly observed based on the openings of individual spheres. To improve the conductivity of the cathode materials further, a layer of carbon film was deposited onto the surfaces of the Co3O4 microspheres by immersing them in a sericin solution. As shown in Fig. 2b, the C/Co3O4 spheres have denser surfaces and a slightly larger diameter of approximately 2.11 mm following carbon film coating. In the TEM image in the inset of Fig. 2b, one can see that the C/Co3O4 spheres still have a hollow structure. In Fig. 2c, the hollow spheres are filled with sulfur to form S@C/Co3O4 composites based on a thermal melting technique. The darker interior of the sphere in the TEM image (inset in Fig. 2d) reveals that a significant amount of sulfur was successfully encapsulated in the spheres. Only a small amount of sulfur remains outside, making the sphere appear larger. To highlight the elemental composition and scattering the of S@C/ Co3O4 composite, the SEM mapping results for Co, O, C, and S are presented in Fig. 2(eeh), respectively. Sulfur is centralized within the sphere, indicating successful encapsulation by the C/Co3O4 spheres, which should ensure the relaxation of sulfur’s volumetric expansion and retardation of polysulfide’s dissolution. The homogenous distribution of carbon around the spheres should provide excellent conductivity and superior reutilization of the active material in cathode, even for long-term cycling. The XRD patterns of C/Co3O4, S@Co3O4, and S@C/Co3O4 composite samples are presented in Fig. 3a. The diffraction peaks at 19.0 , 31.3 , 36.8 , 38.5 , 44.7, 55.8 , 59.3 , 65.2 , and 77.3 match perfectly with the (111), (220), (311), (222), (400), (422), (511), (440), and (533) planes of cubic spinel Co3O4 (JCPDS 43e1003), indicating excellent crystallinity for all three samples. In the S@Co3O4 and S@C/Co3O4 composites, some strong and sharp characteristic peaks at approximately 23.0 , 25.8 , and 27.7 can be observed. These peaks are typically related to sublimed sulfur (JCPDS 08e0247). No obvious peaks related to carbon can be observed for either the C/Co3O4 or S@C/Co3O4 samples, which could be a result of the amorphous phase of carbon derived from sericin [23]. To confirm the existence of carbon in the composites, Raman

3

analysis was performed on the as-synthesized C/Co3O4 sample. As shown in Fig. 3b, two typical broad peaks at 1352 and 1585 cm
1 are attributed to the D and G bands of carbon materials, which are generated by sp3 and sp2 hybridization vibrations, respectively. The relative intensity ratio of ID/IG is 1.13, demonstrating the formation of amorphous graphite with a significant number of defects and edges [24,25]. Other peaks in the range of 100e800 cm
1 at 194, 478, 522, 616, and 679 cm
1 are attributed to the F12g, Eg, F22g, F32g, and A1g modes of crystalline Co3O4, respectively, which also suggests the successful synthesis of a C/Co3O4 composite [26]. The TGA curves of pure Co3O4, pure carbon from sericin, and the S@C/Co3O4 composite are presented in Fig. 3c to illustrate sulfur content in the composite. A nearly flat line for pure Co3O4 indicates its thermal stability under 500  C. For pure carbon, a small weight loss of 9.4 wt% can be observed when the temperature increases to 150  C, which can be attributed to the removal of adsorbed water on the surface of the sericin [23,27]. Therefore, the total weight loss for the S@C/Co3O4 composite between 150 and 286  C can be attributed to the evaporation of sulfur, which is roughly 67.3 wt% based on the sulfur content in the entire composite. N2 adsorption and desorption measurements were conducted to evaluate the porosity of the hollow Co3O4 spheres and C/Co3O4 composite. As shown in Fig. 3d, both Co3O4 and C/Co3O4 composites exhibit typeIV isotherm curves with type-H1 hysteresis loops, indicating their mesoporous structures. The specific surface area and total pore volume of pure Co3O4 spheres are approximately 39.69 m2 g
1 and 0.30 cm3 g
1 (pore sizes ranging from 4 to 30 nm), providing many reaction sites for sulfur and the electrolyte, thereby increasing the utilization of active materials. Following surface modification with sericin-derived carbon, the C/Co3O4 composite exhibits a decreased surface area and pore volume of 18.89 m2 g
1 and 0.21 cm3 g
1, respectively, based on the coverage of carbon on surface pores. Fig. 4 presents XPS analysis results for the as-prepared S@C/ Co3O4 composite. The high-resolution Co 2p spectrum (Fig. 4a) is fitted by two spin-orbits and two shake-up satellites. The two orbits, which correspond to Co2þ and Co3þ valence states, respectively, each consist of two peaks. The peaks at 779.5 and 794.5 eV are attributed to Co2þ 2p3/2 and 2p1/2, respectively, while the binding energies at approximately 780.9 and 796.7 eV are characteristic peaks of the Co3þ state. The strong shake-up satellite located at 802.6 eV is also a distinct peak for Co3O4, confirming its presence. The weak shake-up satellite at approximately 786.0 eV is attributed to a surface hydroxyl species (i.e., CoeOH) [28e30]. The

Fig. 2. SEM images of (a) Co3O4 hollow microspheres synthesized through a template-free solvothermal method; (b) C/Co3O4 composite prepared by immersing Co3O4 spheres in a sericin solution (inset is a TEM image); (c) S@C/Co3O4 composite fabricated by encapsulating sulfur in C/Co3O4 spheres (inset is a TEM image); and (eeh) elemental SEM mapping results for Co, O, C, and S, respectively, and the corresponding SEM pattern (d) for the S@C/Co3O4 composite.

4

J. Wu et al. / Journal of Alloys and Compounds 823 (2020) 153912

Fig. 3. (a) XRD patterns of Co3O4, C/Co3O4, and S@C/Co3O4 composites; (b) Raman spectrum of C/Co3O4 composite; (c) TGA curves of pure Co3O4, pure sericin-derived carbon, and the S@C/Co3O4 composite; and (d) N2 adsorption-desorption isotherm curve of Co3O4 hollow spheres and the C/Co3O4 composite with their corresponding pore size distributions (inset).

O 1s spectrum (Fig. 4b) indicates the presence of two oxygen species labelled as O1 and O2 at 531.6 and 532.4 eV, respectively, which correspond to the O
2 states associated with CoO and Co2O3, respectively. The binding energies centered at 529.6 and 533.4 eV are related to OeH and C]O bonds, respectively [30e32]. The C 1s spectrum (Fig. 4c) is deconvoluted into three peaks at 288.1, 285.7, and 284.6 eV, corresponding to C]O, CeC, and C]C bonds, respectively [22,33]. In the S 2p spectrum (Fig. 4d), the peak at approximately 162.6 eV is attributed to CoeS bonds, which will play a vital role in retarding the shuttle of polysulfides based on strong polar chemical chemisorption. The binding peaks at 163.7 and 164.9 eV in the S 2p spectra indicate the existence of SeS bonds in a S8 ring. The distinct broad peaks at 168.3 eV in the S 2p spectra can be attributed to sulphate species, which may be generated by sulfur oxidation in air [34e39]. To verify the chemical functions of Co3O4 with respect to polysulfides, a visual adsorption experiment was conducted by immersing Co3O4 and C/Co3O4 composites in polysulfide solutions (Li2Sn in DOL/DME, 4  n  6). The adsorption effects for polysulfides can be deduced from the color differences recorded in digital photographs (Fig. 5). After 12 h, as shown in Fig. 5a, the addition of both Co3O4 and C/Co3O4 causes the polysulfide solutions to become lighter, indicating clear adsorption interactions with the polysulfides. The solution with the C/Co3O4 composite is clearer than the solution with pure Co3O4 after the same duration, which could be a result of the extra adsorption on nitrogen atoms contained in sericin-derived carbon. As shown in Fig. S1, the fitted

peaks of pyrrolic N (399.6 eV) and pyridinic N (397.7 eV) in N-doped sericin strengthen the adsorption of sericin-derived carbon for polysulfides by forming LiSnLiþ … N bonds [40,41]. After 24 h of rest, both the Co3O4 and C/Co3O4 solutions became nearly transparent (Fig. 5b), demonstrating their strong chemisorption of polysulfides, which is beneficial for enhancing the cyclability of batteries. To confirm the structural advantages of S@C/Co3O4 for LieS batteries, S@C and S@Co3O4 cathodes were also prepared as control samples under the same conditions as S@C/Co3O4. Fig. 6 presents the electrochemical performances of S@Co3O4 and S@C/Co3O4 composites for cut-off voltages in the range 1.5e3.0 V. At a constant current density of 0.2 C (1 C ¼ 1675 mA g
1) in Fig. 6a, S@C/Co3O4, S@Co3O4, and S@C electrodes deliver high initial capacities of 1171.6, 1074.8, and 1162.8 mAh g
1, respectively. However, after more than 400 cycles, the capacities of S@Co3O4 and S@C drop rapidly to only 382.3 and 295.6 mAh g
1. In contrast, S@C/Co3O4 retains a superior capacity of nearly 500 mAh g
1 over 700 cycles (decay rate of 0.049% per cycle between cycles 100 and 700), displaying excellent long-term cyclability. In terms of rate capability, S@C/Co3O4 also delivers on average higher capacities than S@Co3O4 and S@C at 0.2, 0.5, 1, 2, 3, and 4 C, as shown in Fig. 6b. In general, the electrochemical performance of the S@C/Co3O4 composite is superior to those of S@Co3O4 and S@C, which can be attributed to the synergistic effects of conductive carbon and Co3O4 hollow spheres. As shown in Fig. 6c, EIS was conducted for the S@C, S@Co3O4, and S@C/Co3O4 electrodes prior to their first cycles. S@C

J. Wu et al. / Journal of Alloys and Compounds 823 (2020) 153912

5

Fig. 4. XPS spectra of the as-prepared S@C/Co3O4 composite: (a) Co 1s, (b) O 2p, (c) C 1s, and (d) S 2p.

Fig. 5. Digital photographs of Li2Sn solutions (in DOL/DME, 4  n  6) containing Co3O4 and C/Co3O4 composites after resting for (a) 12 h and (b) 24 h.

and S@C/Co3O4 exhibit much smaller semicircle diameters than S@Co3O4, indicating the critical role of conductive carbon in decreasing charge transfer resistance (Rct) [38,39]. Notably, S@C has a lower Rct value, but still provides worse electrochemical performance than S@C/Co3O4, indicating that simply improving the conductivity of a cathode is insufficient for improving the overall properties of batteries. The synergistic effects between conductive carbon and Co3O4 hollow spheres in sulfur cathodes are effective at increasing cycling performance. As the constant current of charge/ discharge is increased to 1 C and 2 C, the S@C/Co3O4 cathode still provides stable output capacities of 265.3 and 194.6 mAh g
1, decay rates of 0.076% and 0.062% per cycle, and high coulombic efficiencies of 99.1% and 99.7%, respectively, over approximately 1000 cycles (Fig. 6d), demonstrating its excellent long-term

electrochemical cyclability under large current densities. The galvanostatic charge/discharge curves of the S@C/Co3O4 electrode are presented in Fig. 7a for cycles 1, 2, 3, 5, 10, and 30 at 0.2 C within a potential range of 1.5e3.0 V. Two reduction reactions can be clearly observed in all of the discharge curves. These reactions are related to the transformation of elemental sulfur into soluble polysulfides and eventually into insoluble Li2S2 and Li2S [42]. Fig. 7b presents typical CV curves for the S@C/Co3O4 electrode. The first reduction peak at 2.38 V is attributed to the reduction process from S8 rings into long-chain soluble lithium polysulfides (Li2Sn, 4  n  8). The peak at 2.02 V is likely related to further reduction of high-order Li2Sn into Li2S2 and Li2S, which agrees with the charge/discharge profiles in Fig. 7a. Furthermore, there is a broad reduction peak at approximately 1.64 V in the first cycle only,

6

J. Wu et al. / Journal of Alloys and Compounds 823 (2020) 153912

Fig. 6. (a) Charge/discharge capacities and corresponding coulombic efficiency values of S@C, S@Co3O4, and S@C/Co3O4 composites versus cycle number at a current density of 0.2 C. (b) Rate capabilities of S@C, S@Co3O4, and S@C/Co3O4 at different current rates ranging from 0.2 to 4 C. (c) EIS results for LieS batteries with S@C, S@Co3O4, and S@C/Co3O4 electrodes prior to the first cycle. (d) Long-term cycling performance of a S@C/Co3O4 cathode at 1 C and 2 C over approximately 1000 cycles.

which is likely attributable to the lithiation process of LiNO3 in the electrolyte [22]. It is noteworthy that the CV peaks overlap well after the initial cycle, which indicates stable reversibility of the S@C/Co3O4 composite. To highlight the effects of additives in terms of cyclability, the morphological changes in S@Co3O4 and S@C/Co3O4 composite electrodes before and after 50 cycles at 0.2 C were investigated. The

results are presented in Fig. 8. Prior to the assembling of the test cells, both cathodes exhibit flat and glossy surfaces, as shown in Fig. 8a and c. After 50 cycles at 0.2 C, the S@Co3O4 electrode exhibits a certain degree of collapse with some cracks on the surface (Fig. 8b). Unlike the S@Co3O4 composite, S@C/Co3O4 maintains its microstructure with few voids or pores (Fig. 8d), demonstrating that both the volumetric expansion and dissolution of polysulfides

J. Wu et al. / Journal of Alloys and Compounds 823 (2020) 153912

7

Fig. 7. (a) Galvanostatic charge/discharge curves at 0.2 C for cycles 1, 2, 3, 5,10, and 30 in a range of 1.5e3.0 V for S@C/Co3O4. (b) Cyclic voltammetry curves for LieS batteries with S@C/Co3O4 electrodes at a scan rate of 0.1 mV s
1 in a voltage range of 1.5e3.0 V.

Fig. 8. SEM images of the S@Co3O4 electrode (a) before and (b) after 50 cycles, as well as the S@C/Co3O4 electrode (c) before and (d) after 50 cycles at 0.2 C.

were effectively alleviated by the as-prepared carbon-modified Co3O4 spheres. This indicates that the enhanced electrochemical performance exhibited by S@C/Co3O4 is primarily caused by synergistic effects between the chemical composition and architecture of the cathodes. Co3O4 hollow microspheres acting as hosts for sulfur can not only alleviate volumetric expansion via physical confinement (Fig. 8), but also heavily restrain shuttle effects based on the strong chemical adsorption for lithium polysulfides provided by CoeS bonds (see XPS results in Fig. 4d). Additionally, the sericinderived carbon deposited on the surfaces of the Co3O4 spheres results in superior adsorption interactions with polysulfides compared to pure Co3O4, as demonstrated by the adsorption experiment results in Fig. 5. Furthermore, the addition of conductive carbon, which is beneficial in terms of increasing the electronic

conductivity of insulating sulfur and Co3O4 cathodes, yields excellent electrochemical kinetics and sulfur reutilization (Fig. 6c). Therefore, the proposed S@C/Co3O4 composite cathode with sulfur encapsulated by sericin-derived carbon/Co3O4 hollow microspheres effectively solves the intrinsic problems of LieS batteries, yielding comprehensively excellent electrochemical properties with superior specific capacity, long-term cyclability, and rate capability. 4. Conclusions In summary, a novel S@C/Co3O4 composite with sulfur encapsulated by sericin-derived carbon/Co3O4 hollow spheres was developed as a cathode for LieS batteries in this study. Co3O4 hollow microspheres were synthesized utilizing a template-free

8

J. Wu et al. / Journal of Alloys and Compounds 823 (2020) 153912

solvothermal method. A C/Co3O4 composite was formed by immersing the Co3O4 spheres into a sericin solution. With a sulfur content of 67.3 wt%, the S@C/Co3O4 cathode exhibited excellent electrochemical performance with a high initial discharge specific capacity of 1171.6 mAh g
1 at a rate of 0.2 C. High reversible capacity was maintained at 495.4 mAh g
1 over 700 cycles at 0.2 C with a low decay rate of 0.049%. Under high constant currents of 1 C and 2 C, S@C/Co3O4 also provides excellent long-term cyclability over roughly 1000 cycles. The excellent electrochemical properties of the S@C/Co3O4 composite are mainly attributed to strong polar chemical bonding with polysulfides from Co3O4, the encapsulation of sulfur by hollow spheres, and the conductive characteristics of sericin-derived carbon materials. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was financially supported by the Zhejiang Province Public Welfare Projects (Grant No. 2016C31108). The authors also gratefully acknowledge research funding from the National Natural Science Foundation of China (Grant Nos. NSFC 61376005, 51672063). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153912. References [1] X. Liu, J. Huang, Q. Zhang, L. Mai, Nanostructured metal oxides and sulfides for lithium-sulfur batteries, Adv. Mater. 29 (2017) 1601759. [2] S. Evers, L.F. Nazar, New approaches for high energy density lithiumesulfur battery cathodes, Acc. Chem. Res. 46 (2012) 1135e1143. [3] B. Wang, Y. Wen, D. Ye, H. Yu, B. Sun, G. Wang, D. Hulicova-Jurcakova, L. Wang, Dual protection of sulfur by carbon nanospheres and graphene sheets for lithium-sulfur batteries, Chem. A Eur. J. 20 (2014) 5224e5230. [4] Y. Yin, S. Xin, Y. Guo, L. Wan, Lithium-sulfur batteries: electrochemistry, materials, and prospects, Angew. Chem. Int. Ed. 52 (2013) 13186e13200. [5] X. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries, Nat. Mater. 8 (2009) 500e506. [6] H. Peng, J. Huang, M. Zhao, Q. Zhang, X. Cheng, X. Liu, W. Qian, F. Wei, Nanoarchitectured graphene/CNT@porous carbon with extraordinary electrical conductivity and interconnected micro/mesopores for lithium-sulfur batteries, Adv. Funct. Mater. 24 (2014) 2772e2781. [7] L. Ji, M. Rao, S. Aloni, L. Wang, E.J. Cairns, Y. Zhang, Porous carbon nanofibersulfur composite electrodes for lithium/sulfur cells, Energy Environ. Sci. 4 (2011) 5053. [8] G. Zhou, L. Yin, D. Wang, L. Li, S. Pei, I.R. Gentle, F. Li, H. Cheng, Fibrous hybrid of graphene and sulfur nanocrystals for high-performance lithium-sulfur batteries, ACS Nano 7 (2013) 5367e5375. [9] Q. Pang, D. Kundu, M. Cuisinier, L.F. Nazar, Surface-enhanced redox chemistry of polysulphides on a metallic and polar host for lithium-sulphur batteries, Nat. Commun. 5 (2014) 4759. [10] X. Wang, T. Gao, X. Fan, F. Han, Y. Wu, Z. Zhang, J. Li, C. Wang, Tailoring surface acidity of metal oxide for better polysulfide entrapment in Li-S batteries, Adv. Funct. Mater. 26 (2016) 7164e7169. [11] Y. Li, D. Ye, W. Liu, B. Shi, R. Guo, H. Zhao, H. Pei, J. Xu, J. Xie, A MnO2/graphene oxide/multi-walled carbon nanotubes-sulfur composite with dual-efficient polysulfide adsorption for improving lithium-sulfur batteries, ACS Appl. Mater. Interfaces 8 (2016) 28566e28573. [12] F. Ma, J. Liang, T. Wang, X. Chen, Y. Fan, B. Hultman, H. Xie, J. Han, G. Wu, Q. Li, Efficient entrapment and catalytic conversion of lithium polysulfides on hollow metal oxide submicro-spheres as lithium-sulfur battery cathodes, Nanoscale 10 (2018) 5634e5641. [13] J. Yuan, Y. Hao, X. Zhang, X. Li, Sandwiched CNT@SnO2@PPy nanocomposites enhancing sodium storage, Colloid. Surf. Physicochem. Eng. Asp. 555 (2018) 795e801. [14] J. Liu, L. Yuan, K. Yuan, Z. Li, Z. Hao, J. Xiang, Y. Huang, SnO2 as a high-efficiency

polysulfide trap in lithium-sulfur batteries, Nanoscale 8 (2016) 13638e13645. [15] J. Yuan, C. Chen, Y. Hao, X. Zhang, R. Agrawal, W. Zhao, C. Wang, H. Yu, X. Zhu, Y. Yu, Z. Xiong, Y. Xie, Fabrication of three-dimensional porous ZnMn2O4 thin films on Ni foams through electrostatic spray deposition for highperformance lithium-ion battery anodes, J. Alloy. Comp. 696 (2017) 1174e1179. [16] M. Liu, C. Zhang, J. Su, X. Chen, T. Ma, T. Huang, A. Yu, Propelling polysulfide conversion by defect-rich MoS2 nanosheets for high-performance lithiumsulfur batteries, ACS Appl. Mater. Interfaces 11 (2019) 20788e20795.
ctor D. Abrun ~ a, Surfactant-free [17] D. Wang, Y. Yu, H. He, J. Wang, W. Zhou, He synthesis of hollow structured Co3O4 nanoparticles as high-performance anodes for lithium-ion batteries, ACS Nano 9 (2015) 1775e1781. [18] Y. Zhong, K.R. Yang, W. Liu, P. He, V. Batista, H. Wang, Mechanistic insights into surface chemical interactions between lithium polysulfides and transition metal oxides, J. Phys. Chem. C 121 (2017) 14222e14227. [19] Z. Chang, H. Dou, B. Ding, et al., Co3O4 nanoneedle array as a multifunctional “super-reservoir” electrode for long cycle life Li-S batteries, J. Mater. Chem. 5 (2017) 250e257. [20] K. Xu, J. Zou, S. Tian, Y. Yang, F. Zeng, T. Yu, Y. Zhang, X. Jie, C. Yuan, Singlecrystalline porous nanosheets assembled hierarchical Co3O4 microspheres for enhanced gas-sensing properties to trace xylene, Sens. Actuators B Chem. 246 (2017) 68e77. [21] C. Du, J. Wu, P. Yang, S. Li, J. Xu, K. Song, Embedding S@TiO2 nanospheres into mxene layers as high rate cyclability cathodes for lithium-sulfur batteries, Electrochim. Acta 295 (2019) 1067e1074. [22] J. Wu, S. Li, P. Yang, H. Zhang, C. Du, J. Xu, K. Song, S@TiO2 nanospheres loaded on PPy matrix for enhanced lithium-sulfur batteries, J. Alloy. Comp. 783 (2019) 279e285. [23] B. Zhang, M. Xiao, S. Wang, D. Han, S. Song, G. Chen, Y. Meng, Novel hierarchically porous carbon materials obtained from natural biopolymer as host matrixes for lithium-sulfur battery applications, ACS Appl. Mater. Interfaces 6 (2014) 13174e13182. [24] Y. Liu, G. Han, X. Zhang, C. Xing, C. Du, H. Cao, B. Li, Co-Co3O4@carbon coreshells derived from metal-organic framework nanocrystals as efficient hydrogen evolution catalysts, Nano Res. 10 (2017) 3035e3048. [25] Y. Meng, G. Wang, M. Xiao, C. Duan, C. Wang, F. Zhu, Y. Zhang, Ionic liquidderived Co3O4/carbon nano-onions composite and its enhanced performance as anode for lithium-ion batteries, J. Mater. Sci. 52 (2017) 13192e13202. [26] L. He, Z. Li, Z. Zhang, Rapid, low-temperature synthesis of single-crystalline Co3O4 nanorods on silicon substrates on a large scale, Nanotechnology 19 (2008) 155606. [27] J. Zhang, Y. Cai, Q. Zhong, D. Lai, J. Yao, Porous nitrogen-doped carbon derived from silk fibroin protein encapsulating sulfur as a superior cathode material for high-performance lithium-sulfur batteries, Nanoscale 7 (2015) 17791e17797. [28] D. Ge, H. Geng, J. Wang, J. Zheng, Y. Pan, X. Cao, H. Gu, Porous nano-structured Co3O4 anode materials generated from coordination-driven self-assembled aggregates for advanced lithium ion batteries, Nanoscale 6 (2015) 9689. [29] H. Wang, Y. Zhu, C. Yuan, Y. Li, Q. Duan, Cobalt-phthalocyanine-derived ultrafine Co3O4 nanoparticles as high-performance anode materials for lithium ion batteries, Appl. Surf. Sci. 414 (2017) 398e404. [30] D. Guo, M. Zhang, Z. Chen, X. Liu, Hierarchical Co3O4@PPy core-shell composite nanowires for supercapacitors with enhanced electrochemical performance, Mater. Res. Bull. 96 (2017) 463e470. [31] X. Wu, L. Meng, Q. Wang, W. Zhang, Y. Wang, A flexible asymmetric fiberedsupercapacitor based on unique Co3O4@Ppy core-shell nanorod arrays electrode, Chem. Eng. J. 327 (2017) 193e201. [32] G. Qu, H. Geng, D. Ge, J. Zheng, H. Gu, Graphene-coated mesoporous Co3O4 fibers as an efficient anode material for Li-ion batteries, RSC Adv. 75 (2016) 71006e71011. [33] C. Sengottaiyan, R. Jayavel, P. Bairi, R.G. Shrestha, K. Ariga, L.K. Shrestha, Cobalt oxide/reduced graphene oxide composite with enhanced electrochemical supercapacitance performance, Bull. Chem. Soc. Jpn. 90 (2017) 955e962. [34] M. Khandelwal, S. Chandrasekaran, S.H. Hur, J.S. Chung, Chemically controlled in-situ growth of cobalt oxide microspheres on N,S-Co-doped reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction, J. Power Sources 407 (2018) 70e83. [35] S. Fu, C. Zhu, J. Song, S. Feng, D. Du, M.H. Engelhard, D. Xiao, D. Li, Y. Lin, Twodimensional N,S-Co doped carbon/Co9S8 catalysts derived from Co(OH)2 nanosheets for oxygen reduction reaction, ACS Appl. Mater. Interfaces 9 (2017) 36755e36761. [36] T. Jiang, N. Yin, Z. Bai, P. Dai, X. Yu, M. Wu, G. Li, Wet chemical synthesis of S doped Co3O4 nanosheets/reduced graphene oxide and their application in dye sensitized solar cells, Appl. Surf. Sci. 450 (2018) 219e227. [37] X. Zhou, F. Chen, J. Yang, Core@shell Sulfur@polypyrrole nanoparticles sandwiched in graphene sheets as cathode for lithium-sulfur batteries, J. Energy Chem. 24 (2015) 448e455. [38] J. Wang, L. Lu, D. Shi, R. Tandiono, Z. Wang, K. Konstantinov, H. Liu, A conductive polypyrrole-coated, sulfur-carbon nanotube composite for use in lithium-sulfur batteries, ChemPlusChem 78 (2013) 318e324. [39] J. Kim, D. Lee, H. Jung, Y. Sun, J. Hassoun, B. Scrosati, An advanced lithiumsulfur battery, Adv. Funct. Mater. 23 (2013) 1076e1080. [40] M. Xiang, Yan Wang, J. Wu, Y. Guo, H. Wu, Y. Zhang, H. Liu, Natural silk cocoon derived nitrogen-doped porous carbon nanosheets for high performance

J. Wu et al. / Journal of Alloys and Compounds 823 (2020) 153912 lithium-sulfur batteries, Electrochim. Acta 227 (2016) 7e16. [41] S.S. Zhang, Heteroatom-doped carbons: synthesis, chemistry and application in lithium/sulphur batteries, Inorg. Chem. Front. 12 (2015) 1059e1069.

9

[42] W. Ren, W. Ma, S. Zhang, B. Tang, Nitrogen-doped carbon fiber foam enabled sulfur vapor deposited cathode for high performance lithium sulfur batteries, Chem. Eng. J. 341 (2018) 441e449.