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Synergistic Dual-Confinement Effect: The Merit of Hollowly Metallic Co9S8 in Packaging Enhancement of Electrochemical Performance of Li-S Batteries Xiaofei Liu, Dong Wang, Xinzhe Yang, Zhenzhen Zhao, He Yang, Ming Feng, Wei Zhang, and Weitao Zheng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01997 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Energy Materials

Synergistic Dual-Confinement Effect: The Merit of Hollowly Metallic Co9S8 in Packaging Enhancement of Electrochemical Performance of Li-S Batteries Xiaofei Liu,1,# Dong Wang,2,# Xinzhe Yang,1 Zhenzhen Zhao,1 He Yang,1 Ming Feng,3 Wei Zhang,1,4,* Weitao Zheng1,* 1

State Key Laboratory of Automotive Simulation and Control, and School of Materials Science & Engineering, and Electron Microscopy Center, and International Center of Future Science, Jilin University, Changchun 130012, China 2 State Key Laboratory of Chem/Bio-Sensing and Chemometrics, Provincial Hunan Key Laboratory for Graphene Materials and Devices, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China 3 Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, China 4 CIC Energigune, Albert Einstein 48, Miñano 01510, and IKERBASQUE, Basque Foundation for Science, Bilbao 48013, Spain *E-mail: [email protected] ; [email protected] # These authors contributed equally to this work.

Abstract

Lithium-sulfur (Li-S) batteries have become a powerful alternative for

lithium-ion batteries due to their high energy density and high specific capacity. However, several problems still hinder its practical application, such as poor electron conductivity, large volumetric expansion and severe shuttle effect. To overcome these obstacles, a metallic and polar sphere-like Co9S8 with high electron conductivity is designed and prepared. Merited from a large chemical affinity and improved integration with polar polysulfides (LiPSs), Co9S8 enables chemically immobilizing LiPSs effectively with an improved cycle life of Li-S batteries. Besides, the hollow sphere structure can physically block the diffusion of LiPSs into electrolyte. Sufficient internal space can accommodate more active materials and mitigate the volume expansion. The high electron conductivity of Co9S8 facilitates improving of electrochemical reaction kinetics. Thus, serving as host material for Li-S batteries, the Co9S8 electrode exhibits a high initial specific capacity ~1200 mAh/g at 0.1 C and the discharge specific capacity retains 570 mAh/g after 100 cycles at 0.5 C. Keywords Sphere-like Co9S8, dual-confinement effect, lithium sulfur batteries, redox reaction kinetics，shuttle effect，chemical adsorption. 1

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1. Introduction Lithium ion batteries have been commercially successful since practical application in 1990s.1 Because of low energy density and specific capacity,2-5 however, it cannot meet the great demands of electric vehicles. Li-S batteries, with multielectron redox recation, have become a promising candidate because of its ultrahigh energy density (2600 Wh/kg) and specific capacity (1672 mAh/g) in theory.6-9 Nonetheless, Li-S batteries are still commercially limited due to several significant limitations: poor conductivity of sulfur, large volumetric expansion (≈80%) and critical shuttle effect.10-13 Many efforts including physical or chemical confinement have been explored to address these issues. On one hand, Carbon materials such as carbon hollow spheres, 14-16

carbon nanotubes17-20 and porous graphene21-23 have been used as encapsulation

host materials for sulfur, because they enable forming of a physical obstacle to confine the shuttle of LiPSs. But they cannot efficiently suppress the shuttle effect due to a weak interaction between polar LiPSs and nonpolar carbonaceous materials (Figure 1a). On the other hand, polar materials such as SiO2,22 MnO2,24 Fe3O4,25 TiS2,26 and MOFs27-30 have been applied into Li-S batteries as host materials. They afford retarding shuttle effect via binding with LiPSs. However, as most of them are semiconductors, their poor electrical conductivity limits electron transfer; consequently, sluggish interface redox reaction kinetics occurs (Figure 1b). Thus, materials which can not only integrate physical and chemical confinement to suppress the shuttle effect of LiPSs, but also with high conductivity to promote interfacial redox reaction kinetics should be supposed to be the host for Li-S batteries. For proof-of-concept, we successfully designed and fabricated a sphere-like Co9S8 material as the sulfur host via a simple one-step hydrothermal method. The sphere-like Co9S8 can efficiently immobilize LiPSs through physical and chemical dual-confinement. The highly polar Co9S8 enables efficiently and chemically constraining the shuttling of LiPSs by interacting and binding with polar LiPSs.29, 31-33 Moreover, our unique three dimension structure of Co9S8, acting as a solid physical obstacle, can efficiently encapsulate sulfur and LiPSs within space of internal void. 2


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Moreover, its high electrical conductivity (0.29×103 S cm-1 at room temperature) contributes to the transport of electrons and accelerates the interfacial redox reaction kinetics (Figure 1c).34-37 As a result, our designed batteries exhibit high discharge specific capacity, excellent rate performance and stable cycling performance, by applying Co9S8 as sulfur host.

Figure 1. Schematic illustration of the merits of metallic and polar sphere-like Co9S8 as sulfur hosts. a) Nonpolar porous carbon materials cannot efficiently suppress the diffusion of LiPSs due to their weak interaction. b) Polar porous semiconductor materials results in sluggish interface redox reaction kinetics due to their poor electrical conductivity. c) Polar and hollowly metallic sphere-like Co9S8 as sulfur hosts can not only combine physical/chemical confinement to suppress the shuttle effect of LiPSs but also to promote interfacial redox reaction kinetics via high conductivity.

2. Experimental Section Reagents and Materials The sulfur power was commercially available from Damao Chemical Reagent Factory. Hexahydrate cobalt chloride, ketjen black (KJB) and anhydrous ethanol was from Sino-pharm Chemical Reagent Co.Ltd without further purification.

Preparation of sphere-like Co9S8 and Li2S4 solution The sphere-like Co9S8 was prepared by a simple one-step hydrothermal method. Typically, hexahydrate cobalt chloride (475 mg) was dissolved into 70 ml anhydrous ethanol in a beaker with 10 min ultrasonic treatment. Then 128 mg sulfur power was added into the beaker with 20 min ultrasonic treatment to form homogeneous solution. 3



The solution was poured into Teflon autoclave, then it was sealed and maintained at 220 ℃ for 24 h. After the autoclave was cooled to room temperature naturally, the obtained black precipitates were washed with anhydrous ethanol and distilled water for three times each, and then dried in vacuum at 60 ℃ for 24 h. Sulfur powder and Li2S with a mole ratio of 3:1 were added into the DME solution, then the yellow Li2S4 solution was obtained after stirring at 80 ℃ overnight in a glove box.

Preparation of sphere-like Co9S8/S and KJB/S composites The as-prepared sphere-like Co9S8 (KJB) and sublimed sulfur with the weight ratio 1:2 were fully mixed and maintained at 155 ℃ for 12 h in a tube under an Ar atmosphere.

Characterizations The crystalline structures were invested by X-ray diffraction (Rigaku 2500) equipped with Cu Kα radiation (λ= 0.154 nm). The surface morphology and microstructure of as-prepared samples were analysised by a scanning electron microscope (SEM, Hitachi SU8010) and a transmission electron microscope (TEM, JEM-2010F) with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) of pure Co9S8 and Co9S8/S were conducted with a monochromatic Al Ka radiation source (ESCALAB-250).

Electrochemical Measurements The cathode was prepared by coating slurry on carbon fiber paper. Typically, the electrode was fabricated by mixing active materials (Co9S8/S or KJB/S), super P and PVDF with a weight ratio of 6:1:3, then it was dissolved in NMP to form homogenous slurry. The slurry was casted on clean and polished carbon fiber paper current collector and dried at 60 ℃ overnight at the vacuum ambiance. The sulfur ~1 mg/cm2 was loaded on the current collector for each battery. The coin batteries were assembled in a glove box using Celgard 2325 as membrane. The electrolyte was 1M 4


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bis(trifluoromethane) sulfonamidelithium salt (LiTFSI)

in a mixture of

1,3-dioxolane and DME (volume ratio of 1:1) with 1% LiNO3 as the electrolyte additive in this work. All the batteries were aged for several hours before examining to ensure adequate penetration of the electrolyte into the electrode. EIS of each battery was invested by PARSTAT 2273 with the frequency from 1 MHz to 0.1 Hz at open circuit voltage. The battery cycling performance and rate performance were characterized by Neware battery testing system. The CV test was conducted by CHI 660e electrochemical workstation at a scan rate of 1 mV/s. The specific capacities were calculated based on the sulfur mass.

3. Results and Discussion Scanning electron microscopy (SEM) images of the as-prepared Co9S8 in Figure 2a suggested that homogeneous and monodisperse sphere-like Co9S8 was successfully synthesized by simple hydrothermal method.38 Its detailed structure was presented in Figure 2a (inset). The diameter of sphere-like Co9S8 was ~ 2.5 μm and it displayed rough surface with decoration of holes. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were conducted for analyzing the structure of Co9S8 in Figure 2b. As one can see, the sphere-like Co9S8 was hollow and three-dimensional with abundance of internally void space (inset of Figure 2b). Such architecture enabled accommodating more active materials, consistent with our SEM observation. Nevertheless, the HRTEM images confirmed that the lattice was ~ 0.35 nm, corresponding to (220) plane of Co9S8. X-ray diffraction (XRD) pattern in Figure 2c was indexed as Co9S8 (No.86-2273 in JCPDS) with several sharp diffraction peaks, indicative of high crystallization.29,

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Besides, the EDS elemental mapping of pure

Co9S8 in Figure S1 suggested homogeneously dispersed cobalt and sulfur elements, in agreement with our XRD analysis. In addition, the specific surface of Co9S8 had been investigated by using nitrogen adsorption desorption isotherms in Figure S2. The BET surface area of Co9S8 sample was 44.389 m2/g. 5



Figure 2. Morphological and structural characterization and XRD analysis of pure Co9S8. (a) SEM image; (b) HRTEM image (TEM insets); (c) XRD pattern.

Visual discrimination and X-ray photoelectron spectroscopy (XPS) analysis were used to probe the interaction between Co9S8 and LiPSs in Figures 3/S3. LiPSs were synthesized by interacting sulfur with Li2S for several hours in a glove box, then the prepared LiPSs was dissolved into dimethoxymethane (DME) with offering a yellow solution. As shown in Figure S3, when the nonpolar carbon material ketjen black (KJB) was added into LiPSs solution, it turned to dark yellow. After several hours of interaction with LiPSs, the solution was still yellow but slightly bleached from the pristine one. In contrast, as displayed in Figures 3a/b, it was dark yellow when we added pure Co9S8 into the LiPSs solution; but the solution turned colorless after an interaction with LiPSs for an equal period. SEM image of Co9S8 after examination of adsorption ability was displayed in Figure S4. The 3D sphere-like structure of Co9S8 was still completely preserved, indicating that Li2S4 was just adopted on the surface of Co9S8 material without reacting. Consequently, it was in favor of maintaining a stable structure in cycling. All aforementioned results suggested that compared with nonpolar KJB carbon materials, the chemical adsorption ability between LiPSs and polar Co9S8 became stronger, which greatly improved the confinement of intermediate LiPSs. To probe the interaction between polar Co9S8 and Li2S4, and the underlying mechanism of visual examination, XPS measurements of pure Co9S8 and Co9S8+Li2S4 composite after adsorption measurement were conducted. As displayed in Figure 3a, two peaks due to the spin-orbit doublet were present at 781.20 and 778.40 eV, and 6


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another border peak was attributed to shake-up satellite.39-42 These two peaks represented different types of cobalt of Co9S8. The former at 781.20 eV represented Co2+ that occupied tetrahedral site, and the latter located at 778.40 eV corresponding to Co3+ occupying octahedral site.31, 43 Such analysis was consistent with the previous report for Co9S8.31, 44-45 In contrast, the peaks of Co 2p3/2 shifted to 780.30 and 778.38 eV after adsorption examination with Li2S4 in Figure 3b, the tetrahedral and octahedral sites shifted by 0.80 and 0.02 eV to lower energy. Besides, the relative content of Co2+ in polar Co9S8 and Co9S8+Li2S4 composite was calculated in Table S1. The proportion of Co2+ in initial pure Co9S8 was ~77.72%. However, the content of Co2+ rose up to ~81.08% after contacting with LiPSs of Li2S4. There was more Co2+ in Co9S8+Li2S4 composite than pure Co9S8. It indicated that Co3+ gaining electrons was reduced to Co2+, which was due to electrons transferred from Li2S4 to Co.37 These results suggested that metallic polar Co9S8 interacted with LiPSs after contacting with the solution of Li2S4. Li atoms of LiPSs can bond with S in the Co9S8, and S of LiPSs bonds with Co in the Co9S8 due to the coulomb interactions.41 It was well consistent with a strong chemical affinity between Co9S8 and LiPSs. This feature, strong chemical affinity, favored to restrain the shuttle effect for improving the electrochemical performance once Co9S8 was applied in Li-S batteries. The Co9S8/S composite was fabricated, as active materials sulfur was filled into Co9S8 by using simple metal-diffusion method. Figure 3c showed the XRD pattern of sulfur and Co9S8/S composite. As one can see, the sharp diffraction peaks of S (No.08-0247 in JCPDS) and Co9S8 (No. 08-2273 in JCPDS) were observed. Thermo-gravimetric analysis (TGA) was used to determine the content of sulfur in Co9S8/S composite, ~ 63 wt% as shown in Figure 3d. The SEM and TEM images of Co9S8/S composite were given in Figures 3e/f. There were little bulk material adsorption on the surface of Co9S8. However, the sphere structure of Co9S8 was still perfectly preserved, and the inner void space disappeared. It suggested that lots of elemental sulfur existed in the composite and the sulfur powder was just physically filled into the internal space and hole of Co9S8 with no interaction. In addition, the surface area of Co9S8/S composite was 4.36 m2/g, displayed in Figure S2. The EDS 7



elemental mapping of Co9S8/S composite was displayed in Figure S5 and Table S2. There were more percentages of sulfur in the Co9S8/S composite than pure Co9S8. These results indicated sulfur was successfully filled into the Co9S8 structure.

Figure 3. (a) and (b) High-resolution XPS spectra of Co 2p3/2 of the pure Co9S8 and Co9S8+Li2S4 composite after adsorption ability examining (pure Co9S8 (insets at Figures 3a/b) within the solution of Li2S4 as a representative polysulfide); (c) and (d) XRD pattern and TGA analysis of Co9S8/S composite; (e) and (f) SEM and TEM images of Co9S8/S composite.

To evaluate the electrochemical performance of Co9S8/S composite for Li-S batteries, coin cells were fabricated in glove box. In addition, the KJB/S composite was prepared with the same metal-diffusion method for control sample. The sulfur content of KJB/S composite was 66 wt% in Figure S6. Figure 4a displayed the cycle voltammeters (CV) curves of Co9S8/S composite cathode at a scan rate of 1 mV/s. Two cathodic peaks can be found in the CV curves at 2.28 and 1.92 V, and the cathodic peak at ~ 2.28 V was due to the reduction of cycl-S8 molecules to the long-chain LiPSs of Li2Sx (4≤x≤8).43 The other cathodic peak at ~ 1.92 V corresponded to the reduction of long-chain LiPSs Li2Sx (4≤x≤8) to the short-chain Li2S or Li2S2. However, only one broad anodic peak could be observed at 2.53 V in 8


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the CV curves, which represented the reverse conversion of short-chain Li2S or Li2S2 to S8.46-48 In addition, all the CV curves of subsequent cycles were almost overlapped with sharp cathodic and anodic peaks, which suggested the high reversibility of the Co9S8/S composite cathode. Conducted at a current rate from 0.1 to 1 C, the rate performance was obtained for both Co9S8 and KJB cathodes (Figure 4b). As one can see, the Co9S8/S composite cathode showed initial discharge specific capacity ~1200 mAh/g. In addition, cycled at current rate of 0.1/0.3/0.5/1 C, the individual discharge specific capacity was recorded as 1100/882/836/781 mAh/g, respectively. What’s more, if the current rate was back again to 0.1 C, the Co9S8/S composite cathode still exhibited discharge specific capacity ~ 980 mAh/g. It suggested a high rate capacity of Co9S8/S composite cathode. In contrast, the KJB cathode displayed initial discharge specific capacity ~1100 mAh/g, however, the discharge specific capacity urgently dropped down to 800 mAh/g after cycling several cycles. The discharge specific capacity was 600, 520, and 440 mAh/g, respectively, at the current rates of 0.3, 0.5, and 1 C. Besides, the discharge specific capacity only retained 612 mAh/g, once the current rate decreased back to 0.1 C. These results showed that the rate performance of Co9S8/S composite cathode was superior to the KJB cathode because of the sphere-like structure and high affinity between Co9S8 and LiPSs. It was beneficial for the transfer of electron to enhance the redox reaction kinetics of LiPSs. The electrochemical performance of Co9S8 and KJB cathodes for long cycling stability was investigated at a current rate of 0.5 C in Figure 4c. The Co9S8/S cathode displayed a reversible initial discharge specific capacity ~ 890 mAh/g after several cycles of active process at 0.1 C. In addition, the discharge specific capacity still retained 570 mAh/g after 100 cycles at 0.5 C. In contrast, the KJB cathode delivered an initial discharge specific capacity ~ 550 mAh/g, which was well below the Co9S8/S composite cathode. The KJB cathode only displayed a discharge specific capacity of 330 mAh/g after 100 cycles at 0.5 C. The discharge middle voltage of Co9S8 and KJB cathodes during cycles was studied as Figure 4d. The discharge middle voltage of Co9S8/S composite cathode was clearly higher than the KJB 9



cathode during continual cycles, which directly affected rate and cycles performance.49 The higher discharge middle voltage represented higher discharge plateaus and small electrochemical polarization in the discharge process, indicated that LiPSs Li2Sx (4≤x≤8) was more easily reduced to short-chain Li2S or Li2S2 for improving the electrochemical performance.

42

Besides, the charge/discharge voltage

profiles of Co9S8 and KJB cathodes were displayed in Figures 4e/f at 0.5 C. Two discharge plateaus could be clearly observed for all the profiles, which were consistent with the CV curves. Moreover, the Co9S8/S composite cathode displayed more discharge specific capacity than the KJB cathode, and possessed a relatively low polarization value of 180 mV between the discharge and charge plateaus. However, the KJB cathode exhibited a higher polarization value of 310 mV. The higher discharge middle voltage and reductive polarization suggested that the sphere-like Co9S8 material enabled improving of electrochemical reaction kinetics during charge and discharge processes.29,

50

Due to the dual-confinement of physically structural

confinement and chemical confinement, the sphere-like Co9S8 material can efficiently prevent the diffusion of LiPSs from cathode to electrolyte and weak the shuttle effect in cycling. In addition, the specially spherical cross-linked structure of Co9S8 material and high electron conductivity were in favor of the transfer of electron, which resulted in relative lower polarization value and improved the electrochemical reaction kinetics. As a result, Co9S8/S composite cathode was superior to the KJB cathode at cycling stability performance.

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Figure 4. (a) the CV curves of Co9S8/S composite cathode after cycles; (b) and (c) rate performance and cycle performance of Co9S8/S composite and KJB cathodes;（d）the discharge middle voltage profiles of Co9S8/S composite and KJB cathodes (Li2Sx (4≤x≤8) to Li2S or Li2S2); (e) and (f) Charge/discharge profiles of varied cycled number at 0.5 C for Co9S8/S composite and KJB cathodes.

Electrochemical impedance spectroscopy (EIS) tests were performed to investigate the electrochemical reaction kinetics of electrodes. The corresponding Nyquist plots and equivalent circuit of Co9S8 and KJB electrodes after cycles at 0.5 C were shown in Figures 5a/d and Figure S7, and the fitting data was in Tables S3/4/5. The EIS of Co9S8 and KJB electrodes after first cycle were displayed in Figures 5a/d. There were two semicircles in the high-to-medium frequency region, corresponding to the interface resistance (Ri) and the charge transfer resistance (Rct), an oblique line in the low frequency region corresponded to the mass diffusion step resistance.51-52 The resistance values of Ri and Rct before cycling were 46.33 and 24.25 Ω for Co9S8/S composite cathode and 40.04 and 1.561 Ω for KJB cathode in Table S3. It indicted that the KJB electrode had lower Ri and Rct, which were due to the high electron transfer of KJB carbon material. However, the resistance values of Ri and Rct after first cycle were 14.24 and 12.92 Ω for Co9S8/S composite cathode and 29.44 and 11



57.05 Ω for KJB cathode in Table S4, respectively. The EIS after 100 cycles at 0.5 C was shown in Figures 5a/d, and two depressed semicircles were found in the high-to-medium frequency zone. The double semicircles resulted from the changes of cathodes at charge state, as commonly agreed in the literature.52 The values of Ri and Rct were 5.09 and 11.52 Ω for Co9S8/S composite cathode and 33.17 and 194.30 Ω for KJB cathode in Table S5, respectively. These results suggested that KJB had higher Ri and Rct resistance after 100 cycles than those uncycled and after the first cycle, indicating that the Ri and Rct resistance increased with the continuing cycle process, possibly due to the accumulation of Li2S or Li2S2 in the electrode. The low conductive lithium sulfides were physically covered on the surface of KJB and electrode, which enabled slow electron transfer, increased impedance and destroyed electrode because of the shuttle effect. In contrast, the Co9S8/S composite cathode displayed lower Ri and Rct resistance after 100 cycles than those uncycled and after the first cycle, showing that the Ri and Rct resistance decreased with the continuing cycle process. The lower resistance was ascribed to the stable structure and fast charge transfer ability for Co9S8/S composite because of the large chemical adsorption.50 Consequently, the Co9S8/S composite cathode had lower resistance for both Ri and Rct after 100 cycles, which was due to the synergistic effect of the unique 3D connected sphere-like structure and great affinity. Their merits lie at improving the efficiency of electron transport and promoting electrochemical redox reaction and also enabling stabilization of the electrode structure via confining the diffusion of polyfulfides and restraining the shuttle effect. To clearly verify actual adsorption ability, the diffusion performance of Co9S8 and KJB electrodes after cycling within the solution of DME was displayed in Figure S8. The Co9S8 and KJB electrodes were immediately put into the DME solution, when electrodes were removed from the batteries in a glove box. The initial DME solution was colorless, but it turned slightly yellow after putting KJB electrode in DME solution. In contrast, the solution remained colorless when Co9S8 electrode was put into DME. It indicated that the diffusion of LiPSs in KJB electrode was more serious than Co9S8 electrode. These results suggested polar Co9S8 material could efficiently 12


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immobilize LiPSs and suppress the shuttle effect in the actual cycling process. The surface morphological characterization of Co9S8 and KJB electrodes before and after cycling was performed in Figure 5. As shown in Figures 5b/e, surface of Co9S8 and KJB electrodes displayed a coarse and fluffy structure before cycling. The surface of Co9S8 electrode was smoothly planar with little holes on the surface and remained stable after cycling. In contrast, KJB electrode changed from granular structure to a rough and disorderly needle-like structure in Figures 5c/f. Thus, the surface of KJB electrode was destroyed because of the dissolution of LiPSs and the irregular deposition of lithium sulfide during continuous cycling. The EIS and SEM of KJB/S cathode after cycling indicated the KJB was covered by low conductive lithium sulfides, which enables slow electron transfer. So the KJB/S cathode shows much lower capacity than Co9S8/S composite cathode. The 3D sphere structure of Co9S8 could maintain the stable electrode and not collapse during cycling (insets in Figure 5e). In addition, a strong affinity with LiPSs can prevent the diffusion of LiPSs to electrolyte. The synergistic effect of structure and chemical adsorption could maintain the stability of electrode for improving the electrochemical performance.

Figure 5. (a) and (d) EIS plots and equivalent circuit of Co9S8/S and KJB electrodes after first charged and 100th charged at 0.5 C; (b) and (e) SEM images of the Co9S8/S electrode surface before and after cycling (high-resolution SEM image of Co9S8/S composite electrode, insets in Figure 5e); (c) and (f) SEM images of the KJB electrode surface before and after cycling.

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4. Conclusions In summary, through a delicate design we successfully synthesized a metallic polar sphere-like Co9S8 material as a sulfur host highly efficient for Li-S batteries. This material can efficiently restrain the shuttle effect of LiPSs due to the synergistic effect of dual-confinement, i.e., great chemical confinement of polar Co9S8 and physical confinement of 3D sphere structure. Besides, the 3D sphere cross-linked structure and high electron conductivity of polar Co9S8 enable improving the redox reaction kinetics and stabilizing the electrode structure. As a result, the Co9S8/S composite cathode exhibits a high discharge specific capacity and stable cycling life.

Acknowledgements We acknowledge the support from the National Natural Science Foundation of China (51872115), National Key R&D Program of China (2016YFA0200400), Program for the Development of Science and Technology of Jilin Province (20190201309JC), the Jilin Province/Jilin University co-Construction Project-Funds for New Materials (SXGJSF2017-3, Branch-2/440050316A36), the Program for JLU Science and Technology Innovative Research Team (JLUSTIRT, 2017TD-09), and “Double-First Class” Discipline for Materials Science & Engineering.

ASSOCIATED CONTENT Supporting Information available: The EDS elemental mapping and Nitrogen adsorption-desorption isotherms of pure Co9S8 and Co9S8/S composite, adsorption ability examination of KJB, TGA analysis of KJB/S composite, EIS plots and fitting data of Co9S8/S composite and KJB electrodes before cycling, cycle performance of pure Co9S8 cathodes at 0.5C, CV curves of Co9S8/S composite cathode before cycling, the diffusion performance of Co9S8/S composite and KJB electrodes after cycling, SEM image of Co9S8 after adsorption, and additional plots and tables.

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