Acidified Multiwall Carbon

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Novel "Bird-Nest" Structured Co3O4/Acidified Multi-Wall Carbon Nanotubes (ACNTs) Hosting Materials for Lithium-Sulfur Batteries Ruiping Liu, Fei Guo, Xiaofan Zhang, Jinlin Yang, Mingyang Li, Wu Miao Miao, Hang Liu, Ming Feng, and Lei Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01914 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Applied Energy Materials

Novel "Bird-Nest" Structured Co3O4/Acidified Multi-Wall Carbon

Nanotubes

(ACNTs)

Hosting

Materials

for

Lithium-Sulfur Batteries

Ruiping Liu a*, Fei Guo a, Xiaofan Zhang a, Jinlin Yang a, Mingyang Li a, Wu Miaomiao a*, Hang Liu a, Ming Feng b, Lei Zhang c

a

Department of Materials Science and Engineering, China University of Mining &

Technology (Beijing), Beijing 100083, China b

Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of

Education, Jilin Normal University, Changchun 130103, P.R. China c

Department of Mechanical Engineering, PO Box 755905, University of Alaska

Fairbanks, Fairbanks, AK 99775, United States Ruiping Liu: 0000-0003-2277-785X Fei Guo: 0000-0001-7319-5118 Xiaofan Zhang: 0000-0002-0974-9187 Jinlin Yang: 0000-0003-0343-8641 Mingyang Li: 0000-0001-5550-1118 Wu Miaomiao: 0000-0002-1955-7701 Hang Liu: 0000-0002-5766-9405 Ming Feng: 0000-0001-5921-0190 Lei Zhang: 0000-0002-4157-8568

*Corresponding authors. Ruiping Liu, Tel.: +86 10-62339175, Fax: +86 10-62339081, E-mail address: [email protected]. Wu Miaomiao: [email protected];

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ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Lithium-sulfur battery with a much higher specific capacity and energy density than the commercial Li-ion battery is the most promising energy storage system. However, the shuttle effect of the dissolved sulfide polyanions and the insulating sulfur materials during charge/discharge of Li-S batteries reduce the coulomibic efficiency and the cycle life of the batteries. Herein, we prepared a novel Co3O4/acidified multi-wall carbon nanotubes (ACNTs) hosting materials with a stable "bird-nest" structure, which integrates metal oxide with a conductive framework effectively. The double barrier of the "bird-nest" hosting materials suppress the shuttle effect and the carbon nanotubes provide a high electronic conductivity in the sulfur cathode. The composite Co3O4/S/ACNTs cathode exhibited an initial discharge capacity of 1285 mAh∙g-1 at 0.1 C and 693.7 mAh∙g-1 was retained after 550 cycles at 0.5 C. The cross-linked “bird-nest” structure not only encapsulates polysulfide effectively, but also promotes the transformation and utilization of polysulfides thanks to the catalytic properties of Co3O4.

Keywords: lithium-sulfur batteries, shuttle effect, Co3O4, acidified carbon nanotubes, bird nest, polysulfide intermediates, catalytic

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1 INTRODUCTION The development of portable technology and the widespread use of energy storage device accelerate the exploration of next-generation battery system to promote the related industrial upgrading and restructuring1-6. Among the rechargeable battery technologies, lithium-sulfur batteries (LSBs) have a high mass-energy density (2600 Wh kg-1) and volume-energy density (2200 Wh L-1), which can meet the requirements for electric vehicles (EV)7-8. Compared with the traditional lithium-ion batteries (LIBs) which rely on the movement of lithium-ion between two electrodes, element sulfur can react with metal Li through two electrons, and thus it can achieve high specific capacity (1672 mAh•g-1). The abundance, low toxicity and low cost of sulfur made it a promising cathode material9-10. However, some inherent problems such as the low electronic conductivity of sulfur and the shuttle effect of the dissolved sulfide polyanions still hinder the commercialization of LSBs significantly11. Moreover, the insoluble insulating Li2S2/Li2S cover on the surface of the electrodes, which results in the capacity fade and an extremely low coulombic efficiency12-13.

Selecting proper host materials with micro/nano-structure to encapsulate sulfur is an effective strategy to improve the cycling performance of LSBs14-15. Since the mesoporous carbon was used to constrain sulfur within its channels for the first time by Nazal in 200916, many other carboneous materials including carbon fibers, carbon spheres (CS), carbon nanotubes (CNTs), graphene and their hybrids were developed 3



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to be the host materials for sulfur cathode 17-19. It is worth noting that it is ineffective to reduce the dissolution of LiPSs because of the weak intermolecular interactions between the nonpolar carbon and polar LiPSs. The adsorption ability of carboneous materials can be enhanced to some extent via forming a valence bond with LiPSs by doping with heteroatoms (e.g., nitrogen, phosphorus, oxygen)20-24. To further trap LiPSs and increase electrode density, researchers pay more attention to the inorganic compounds with nanostructure and polar surface, such as transitional-metal oxides (TiO2, MnO2, NiFe2O4, SiO2 and Ti4O7), sulfides (NiS, WS2, TiS2 and CoS2), which have a controllable exposed surface and 3D nano-architectures, and thus can anchor LiPSs via forming a chemical bond 25-29. Among them, spinel oxides with controllable compositions and nanostructures are considered as high-efficiency LiPSs immobilizer 30-32.

However, many studies have been focused on building a reservoir to minimize

polysulfide dissolution with complex synthesis process and expensive raw materials. Moreover, the cycling improvement of Li-S battery by the interaction between LiPSs and spinel phases has not been fully investigated33-35.

Here we introduce a speical “bird-nest” structure with Co3O4 spheres as a hosting material

and

acidified

multi-wall

carbon

nanotubes

(ACNTs)

as

the

electronic-conducting framework to improve the coulombic efficiency and suppress the shuttle effect of Li-S batteries. The Li-S cell with the Co3O4/S/ACNTs electrode demonstrates a good electrochemical performance with an initial capacity of 1285 mAh g-1 at 0.1 C, and it delivers a high capacity of 693.7 mAh g-1 after 550 cycles at 4


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0.5 C. 2 RESULTS AND DISCUSSION Figure 1 and Figure S1 show the schematic fabrication process of the "bird nest" structured Co3O4/S/ACNTs hybrids. The formation mechanism of hollow Co3O4 microspheres synthesized by one-pot hydrothermal method is as follows. Firstly, the dehydration reaction of glucose molecule was conducted at low temperature to generate oligosaccharides and aromatic compounds. The oligosaccharides were dehydrated at a higher temperature, intermolecular cross-linking occurred to generate nucleus, and finally carbon spheres template was obtained (Figure S1(a)). Secondly, cobalt ions were dissolved in the suspension and bonded to the hydroxyl groups located on the surface of carbon spheres, and the coordination compounds were formed immediately (Figure S1(b)). The hollow Co3O4 microspheres (third panel in Figure 1 and Figure S1(c)) were obtained after high-temperature calcination process to remove the residual reactants and carbon sphere templates. Then, the Co3O4/S hybrid spheres were prepared by immersing Co3O4 hollow spheres into an aqueous solution containing sodium thiosulfate, hydrochloric acid and polyvinyl pyrrolidone (PVP) with a moderate stirring rate at room temperature (fourth panel in Figure 1). To further enhance electron transfer rate and increase the contact area, ACNTs were introduced to enclose the Co3O4 hollow spheres via hydrothermal treatment at 100 °C for 10h, and finally the Co3O4/S/ACNTs composites were synthesized successfully (fifth panel in Figure 1).

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Figure 1 Synthesis process of the Co3O4/S/ACNTs hybrid spheres

The Co3O4 samples possessed a perfectly spherical and monodisperse structure with a particle size of 2 µm (Figure 2(a)). Co3O4 spheres had a porous surface, and the nanopores were uniformly distributed on the shell of the Co3O4 spheres, which is due to the CO2 bubble generation during the calcination progress (Figure 2(b)). The hollow structure and soft shells could reduce the stress from the volume change of sulfur during charge/discharge and mitigate polysulfide dissolution. After sulfur loading, ACNTs were introduced to twine around the Co3O4 spheres. The dispersion and morphology of Co3O4/S/ACNTs are no longer as good as Co3O4/S composites, which can be ascribed to that the carbon nanotubes intertwine around the hollow Co3O4 spheres, forming a conductive “bird nest” structure. The Co3O4/S/ACNTs hybrids retain the spherical morphology, with ACNTs uniformly intertwined on the outer surface of Co3O4 (Figure 2(c)). A hollow Co3O4 microspheres resulted from thermal decomposition of carbon spheres had the shell thickness of 125 nm (Figure 2(d)). The EDX mapping of Co3O4/S/ACNTs microspheres revealed that C, O, Co and S elements were uniformly distributed within the microspheres (Figure 2(d)). The lattice fringes in Figure 2(i) corresponded to the (311) and (111) planes of Co3O4, respectively36. 6


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Figure 2. The structure of hollow Co3O4 spheres (a-b) and Co3O4/S/ACNTs hybrids (c), Transmission Electron Microscope (TEM) images of Co3O4/S/ACNTs (d), EDS elemental mappings of Co3O4/S/ACNTs (e-h) and HRTEM images of the Co3O4 (i)

The diffraction peaks of the samples in Figure 3(a) confirmed the existence of S (JCPDS No.08-0247) and Co3O4 (JCPDS No.42-1467), and the carbon nanotubes were amorphous37. Compared to the pristine crystalline sulfur, which can be indexed as orthorhombic, the peak intensity of the Co3O4/S/ACNTs composites was largely weakened. It indicates that the sulfur after combination with host material had a 7



relatively poor crystallinity, which is owing to the ring cleavage of S8 molecule and it was mainly distributed inside of the hollow structures of Co3O4 spheres. The XRD peaks intensities of Co3O4/S/ACNTs are weaker than that of Co3O4/S, which can be ascribed to the tightly twined-ACNTs on Co3O4/S.

The reversible type IV isotherm of hollow Co3O4 spheres in Figure 3b corresponded to the characteristics of porous materials. Co3O4 spheres with an average pore size of 3.5 nm had a high surface area and pore volume of 176.4 m2g-1 and 0.177 cc g-1, respectively. After sulfur loading, the specific surface area and pore volume of Co3O4/S hybrid spheres were decreased to 26.4 m2g-1 and 0.079 cc g-1, respectively, verifying that S has been incorporated to the porous Co3O4 spheres.

Figure 3 XRD patterns (a) and Nitrogen adsorption curve and pore size distribution (the inset) (b) of the samples

To demonstrate the ability of Co3O4 to absorb polysulfides, Li2S6 solution was prepared by dissolving Li2S and S (1:5 by molar raito) in 1, 2-dimethoxyethane 8


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(DME)/1, 3-dioxolane (DOL) to form an orange-red solution. The color of the Li2S6 solution gradually faded away after adding 50 mg of Co3O4/ACNTs powders into solution (the insert in Figure S2). The blank Li2S6 solution exhibits two range of spectrum in 250-350 nm and 400-500 nm in ultraviolet-visible (UV-Vis) spectroscopy39. After absorption by Co3O4 powder, the characteristic absorption peaks of the Li2S6 intermediates vanished along with increasing the contact time.

Binding configuration between Co3O4 and LiPSs are further elucidated by the X-ray photoelectron spectroscopy (XPS) of Co3O4/Li2S6 solid powders extracted from the Li2S6 solution after 2 h. Two peaks at 779.8 and 782.3 eV (Figure 4) are associated with two types of cobalt, which occupied the octahedral and tetrahedral sites of Co3O4, respectively (Figure 4(a)). After contacting with Li2S6, the peaks shifted to lower binding energy (779.5 and 781.3 eV) with the peak intensity of Co3+ decreased and Co2+ increased apparently, demonstrating that electron transfer from Li2S6 to the Co atoms resulted in the reduction of Co3+ to Co2+ by Li2S6 on the surface of the Co3O4 sphere (Figure 4(c))40-41. In addition, two peaks located at 161.8 and 163.4 eV corresponded to the terminal and bridging sulfur, respectively (Figure 4(b)). Besides, three other peaks at 167.2, 168.4 and 170 eV were associated with the generation of S-O bond, including thiosulfate, polythionate and sulfate (Figure 4(d)). No obvious impurity peak is observed from the S 2p region of Co3O4/Li2S6 composites, indicating that there were no other side effects happened during the testing process. Overall, the XPS analysis of Co3O4/Li2S6 composites demonstrates the chemical adsorption of 9



LiPSs with Co3O4, which are completely consistent with the adsorption experiments.

Figure 4 XPS spectra of Co3O4/S/ACNTs (a-b) and Co3O4/S/ACNTs-Li2S6 composites (c-d)

Thermogravimetric analysis was performed in N2 to determine the loading of sulfur in Co3O4 spheres (Figure S3). The weight reduced rapidly from 150 to 280 °C because of the evaporation of sulfur on the surface or trapped in the pores whose sizes were big enough for sulfur to escape readily. The sulfur content can be tuned using different proportions of the sodium thiosulfate and hydrochloric acid during the synthesis process. The hybrids with a sulfur content of 58.73% can be obtained.

Based on the above results, the Co3O4/S/ACNTs hybrids with 58.73 % of sulfur were selected as cathode materials of Li-S cells. The CV curves of the first three cycles of 10


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Li-S cells with Co3O4/S/ACNTs hybrids at 0.1 mV s-1 were shown in Figure 5(a). Two peaks at 2.0 and 2.3 V were assigned to the S8-ring reduction to soluble lithium polysulfides (Li2Sn, n ≥ 4) and the evolution of insoluble Li2S2 and Li2S, respectively. A relatively wide peak at 2.42 V is from the oxidation of Li2S2 and Li2S to S8 molecule. The double barrier of the metal oxide and conductive frameworks play a very critical role in restricting lithium polysulfides from diffusing towards the metallic lithium anode .

Figure 5(b) shows the rate performance of Li-S cells with Co3O4/S/ACNTs and Co3O4/S cathodes from 0.1 to 2 C. The discharge/charge capacities gradually decrease as the current density increases for both electrodes. The Co3O4/S/ACNTs hybrids materials deliver a reversible capacity of around 1286, 1017, 788, 643 and 589 mAh g−1 at 0.1, 0.5, 1, 1.5, and 2 C, respectively. By comparison, the Co3O4/S/ACNTs electrode exhibits better rate capability than Co3O4/S electrode, which benefits from unique conductive structure and fast reaction kinetics. Figure S4 shows the charge/discharge profiles of Li-S cells with Co3O4/S/ACNTs; the cell showed two discharge plateaus at 2.1 and 2.3 V, corresponding to a multi-step chemical reaction (S8 → Li2S4 → Li2S2/Li2S). The charge/discharge voltages of the cell were consistent with the CV. The cathodes delivered a high discharge capacity of 589 mAh g−1 at 2 C. A reversible capacity of 1005 mAh g−1 was restored at 0.2 C, indicating the excellent rate performance of the Co3O4/S/ACNTs hybrids.

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Figure 5(c-d) showed the charge/discharge profiles of Co3O4/S and Co3O4/S/ACNTs cathodes. Both of them display an integrity voltage plateau and curves, illustrating that sulfur has been fully encapsulated in Co3O4 hollow spheres. In contrast to Co3O4/S, the Co3O4/S/ACNTs cathodes demonstrated higher specific capacity and lower polarization phenomenon, and which can be attributed to the double protection barrier of “bird nest” structure and hollow Co3O4.

Figure S5 and Figure 5(e) display the cycle ability of Co3O4/S and Co3O4/S/ACNTs cathodes. The Co3O4/S/ACNTs hybrids exhibited a high capacity of 1230 mAh g−1 at 0.1 C, and it maintained a stable cycling performance with 76.6 % capacity retention (942 mAh g−1) after 50 cycles. In contrast, the Co3O4/S discharge capacity fades faster from 838.5 mAh g−1 to 703 mAh g−1 with capacity retention of 86.2% after 50 cycles. The discharge capacity of Co3O4/S/ACNTs cathodes with different areal loadings of sulfur at 0.5 C increased firstly, and then gradually stabilized to a certain level with increasing the cycling number. This originated from the activation process of active sulfur during the first few cycles at the large current density17, 42. Compared to the cycling performance at the lower current density (Figure S6), long-term testing of lithium-sulfur batteries at high rates exhibit a long capacity increase process (i.e. activation process) (Figure 5(e)) because there is not enough time for the Li+ to be fully combined with the sulfur on the surface of the pole pieces. Future work will pay attention to setting a small current in advance to do the activation process.

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For the cathode with 1.1 mg cm-2 areal loading of active sulfur, the highest discharge capacity of 966.5 mAh g−1 was obtained after 48 cycles and a discharge capacity of 693.7 mAh g−1 was obtained after 550 cycles with 0.056 % capacity decay per cycle. While for the cathode with 1.5 mg cm-2 areal loading of active sulfur, the highest discharge capacity of 748.7 mAh g−1 was obtained after 23 cycles, and the stable discharge capacity of 496.5 mAh g−1 was reserved after 550 cycles with 0.064% capacity decay per cycle. The Co3O4/S/ACNTs hybrids can effectively overcome the disadvantages of Co3O4/S, indicating that the unique double barrier of metal oxide and conductive framework can greatly improve Li+ diffusion efficiency and electron conduction efficiency.

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Figure 5 The electrochemical performance of Co3O4/S/ACNTs hybrids as cathode materials for LSBs. (a) CV curves, (b) the rate performance of Co3O4/S and Co3O4/S/ACNTs electrodes, (c-d) discharge/charge profiles of cells with Co3O4/S and Co3O4/S/ACNTs electrodes at 0.1C, (e) cyclic performance of Co3O4/S/ACNTs with different areal loading at 0.5C for 550 cycles.

Low-magnification SEM was used to examine the structural integrity after the 50th cycling. The Co3O4/S/ACNTs hybrids remain the “bird nest” structure to some extent 14


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(Figure S7). To further investigate the electrochemical kinetics of the Co3O4/S and Co3O4/S/ACNTs electrodes, the impedance of the cell at 1st and 50th cycle was tested. The semicircle at the high frequency in Figure S8 corresponded to the charge transfer resistance (Rct) at the electrode/electrolyte interface, which reflected the Li+ diffusion resistance through the interface. Co3O4/S/ACNTs electrodes showed a much smaller Rct (Figure S8(b)) than Co3O4/S, indicating the enhanced electronic conductivity by ACNTs. In addition, it is worth emphasizing that ohmic resistance (Rs) gradually increased with increasing the cycling number, illustrating that celgrad and electrolyte resistance increased due to the dissolved polysulfide in electrolytes. The diameter of the semicircle decreased after cycling, regardless of ACNTs addition, indicating that Rct decreased along with the cycling. It also can be explained that the wettability between active materials and electrolyte was improved after the first cycle, moreover, a small quantity of sulfur was reduced to LiPSs during cycling, and then the long-chain LiPSs dissolved into the electrolytes, leading to the gradient concentration distribution of LiPSs, which will finally promote the subsequent electrochemical reaction kinetics as well as polarization reduction.

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Figure 6 Schematic illustration of the mechanism of the Co3O4/S/ACNTs in a Li-S cell.

The superior electrochemical performance of Co3O4/S/ACNTs cathodes materials was from the following reasons. Firstly, the hollow Co3O4 spheres can accommodate the sulfur and alleviate the stress generated during cycling due to the volume expansion of sulfur. Secondly, the electric conductivity of cathodes can be improved by introducing the ACNTs. Thirdly, some impurities of multi-walled carbon nanotubes were oxidized under the action of strong oxidizer, and the abundant functional groups (-COOH, C=O, etc.) located on the surface of as-obtained ACNTs, which can contribute to immobilizing polysulfides (Figure S9)37. Meanwhile, the breakage not only reduced the length and the diameter of ACNTs, but also increased the number of carbon atoms that have an unsaturated dangling bond (π), and thus the ACNTs can play a more active role in limiting polysulfide diffusion process. Most importantly, the shuttle effect of LiPSs can be largely reduced by the unique “bird nest” structure of the Co3O4/S/ACNTs hybrids. The polar Co3O4 can absorb LiPSs by forming strong 16


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chemical bonding, and the cross-connected pipeline structure can trap polysulfides like fishing nets. When polysulfides attempt to migrate from the interior of the Co3O4 hollow spheres to outside, it is hard to overcome the double chemical and physical barriers, which increases the utilization of active materials (Figure 6). Furthermore, the catalytic properties of Co3O4 could promote the transformation and utilization of polysulfides43-44.

Figure 7 The charge density difference for the optimized structures of Li2S4 (a), (b), (c) and Li2S6 (d), (e), (f) adsorbed on Co3O4 (111) surface. Yellow and blue surfaces corresponded to charge gains and loss.

The interaction between Li2Sn (n=4,6) and the Co3O4 (111) surface was studied by DFT calculations. The Co3O4 cubic bulk lattice constant is computed to be 7.977 Å, which is closed to the experimental result (8.06 Å)

45,

as shown in Figure S10. We

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used the isolated cluster models of Li2Sn (Figure S11) based on the most stable Li2Sn structures reported in previous works 46-47. The adsorption energy (Ea) of Li2Sn (n=4,6) on the Co3O4 (111) surfaces is calculated using the equation48, Ea = Etotal - (EX+ESS). Etotal, EX (X= Li2Sn, n=4,6) and ESS are the energy of the system, molecular clusters, and Co3O4 (111) substrate, respectively. Based on Co3O4 (111) surface symmetry and rotating Li2Sn molecule clusters, six typical initial modeling structures are considered for adsorbed systems (Figure S12), and the optimized adsorption structures are shown in Figure 7. The molecule interacts with Co3O4 through Li atoms binding with O atoms and S atoms binding with Co atoms. It can be seen that all the Ea are very large, the strongest binding energies of Li2S4 and Li2S6 clusters on the (111) surface of Co3O4 is -5.87eV and -5.56eV respectively, which mean the chemical interaction between Li2S4 and Li2S6 clusters with Co3O4 (111) surface is quite strong. It can be mainly ascribed to the coexistence of Co-S and Li-S bonds, which is in good agreement with previous works40,

49.

The electronic properties of Li2Sn (n=4,6)

adsorbed on Co3O4 (111) surface are further analyzed to understand the large adsorption energies of Li2Sn (n=4, 6) on Co3O4 (111) surface (Figure 7). S atoms strongly interacted with the surface Co atoms with a character of the ionic interaction (Figure 7(b) and Figure 7(d)). We also investigate the electron transfer between Li2Sn (n=4,6) and Co3O4 (111) by the Bader charge analysis 46. It is found that when Co3O4 (111) surface adsorbs Li2Sn clusters, negative charges are accumulated by the Co ions of the substrate to Li2Sn clusters, at the same time electrons will flow to the surfaces through Li. It is found that the more negative Ea, the more electrons 18


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transferred from the Li2Sn clusters to the Co3O4 surface.

3 CONCLUSIONS A sulfur-immobilized host material with a novel “bird-nest” structure, which integrates the superiorities of different cathode systems (hollow metal oxide spheres and carbon materials), was successfully synthesized by a hydrothermal method. Mesoporous hollow Co3O4 spheres can decrease the migration of lithium polysulfides. Meanwhile, the ACNTs network can provide a stronger bind effect to polysulfides, preventing the active sulfur from losing simultaneously. The novel nanostructure could reduce the shuttle effect effectively by the function of both the physical and chemical adsorption, and finally exhibits outstanding electrochemical performance. The strategies are promising and can be extended to other compounds easily, and it may offer promising potential as effective host materials with high performance in LSBs.

4 EXPERIMENTAL Synthesis of hollow Co3O4 spheres The Co3O4 hollow spheres were synthesized via hydrothermal method. Typically, 9.4g of glucose was dissolved in 50mL de-ionized water containing 5mg of cetyltrimethyl ammonium bromide (CTAB). After stirring for 30 min, 7 g cobalt sulfate was added into the solution under continuously stirring for 2 h. The obtained solution was transferred into a 50 mL autoclave and heated at 180 °C for 12 h. The 19



precipitants were collected and purified by centrifugation. The obtained black powders were fired at 450 °C for 2 h under argon atmosphere firstly and finally were fired at 500°C for 2 h in air to yield hollow Co3O4 spheres.

Preparation of Co3O4/S hybrids Firstly, 1 g of Na2S2O3·5H2O and 0.02 g of PVP were dissolved in 100 mL de-ionized water to form a transparent solution. Then, 0.2 g of the as-synthesized hollow Co3O4 spheres were added to the solution with a stirring time of 4 h. Finally, the 0.8 mL of 10M HCl was dropwise added into the above solution. After stirring 2 h, the Co3O4/S hybrids were obtained after centrifugation and washing three times, followed by drying at 60 °C for 5 h in a vacuum oven.

Preparation of Co3O4/S/ACNTs 0.25 g of MWCNTs were dispersed in a mixture of 10 mL of de-ionized water and 40 mL of ethanol in a round bottom flask, 15 mL of sulfuric acid and 5 mL of nitric acid were mixed in a glass beaker and added into the above solution. After refluxing at 70 °C for 5 h, acidified MWCNTs (ACNTs) were collected by centrifugation; the ACNTs was washed for three times with deionized water and dried at 70 °C for 12 h.

0.3 g of Co3O4/S powers were dispersed in 40 mL of ethanol and 10 mL of water under continuous stirring, followed by introducing 0.03 g of ACNTs. The mixture was transferred into a 50 mL autoclave and heated at 100 °C for 10 h. 20


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Co3O4/S/ACNTs was synthesized by collecting the precipitants, washed with de-ionized water and dried at 70 °C for 12 h.

Characterization methods The morphologies of the samples were observed by field-emission scanning electron microscope (Gemini SEM 500) and high-resolution transmission electron microscope (JEM-2010). The crystal structure was analyzed by X-ray diffraction (XRD) with a Bruker diffractometer. Brunner-Emmet-Teller (BET) surface area was obtained by N2 adsorption with a NOVA4000 automated gas sorption system. The sulfur loading of samples was confirmed with thermogravimetric analysis (TG/DTA 6300) in N2. X-ray photoelectron spectra (XPS) were performed with an ESCALAB 250Xi with an Al Kα X-ray radiation.

Electrochemical measurements CR2032 coin cells were assembled in an Argon-filled glove box. Lithium metal and polypropylene microporous film (Celgard 2400) were used as the anode and separator, respectively; 1, 2-dimethoxyethane (DME) and 1, 3-dioxolane (DOL) (1:1, v/v) with 1wt% of LiNO3 containing 1 M bis(trifluoromethanesulfony)imide lithium (LiTFSI) were used as electrolyte. The cathodes were prepared with 80 wt% of Co3O4/S/ACNTs, 10 wt% of acetylene black and 10 wt% LA133 binder in water. After stirring for 12 h, the slurries were uniformly coated onto an aluminum foil and dried at 60 °C overnight in vacuum. The cell was cycled between 1.7 and 2.8 V (vs. 21



Li /Li+) from 0.1 C to 2 C with a LAND CT2001A testing system. Cyclic voltammetry (CV) and the impedance spectroscopy (EIS) were tested on a CHI660C electrochemical workstation at 0.1mV/s.

Computational method The computational simulations were performed based on Density Functional Theory with the generalized gradient approximation for the exchange-correlation energy with the Perdew-Burke-Ernzerhof exchange-correlation (PBE) form50. The interaction between the Co ions and the valence electrons of sulfur was described by the projector augmented wave (PAW) method as implemented in the Vienna Ab Initio Package51. The energy cutoff was set to 500 eV15, 48, 52-54. The structures were optimized without symmetry constrains and the convergence criteria for energy and force were 1x10-6eV and 0.05eV/Å, respectively55. The reciprocal space was presented by Monkhorst-Pack k-point scheme with 3x3x2 k-points grid mesh to determine the optimized structures of Li2Sn (n=4, 6) molecules adsorbed on the Co3O4 surface. A 22 supercell slab of the Co3O4 (111) surface involving 132 atoms was employed. The vacuum region between the slabs was set to 15Å to avoid interactions between atoms in neighbor cells. Noted that the Li2Sn (n=4,6) coverage on the Co3O4 (111) surface is around 1/12, van der Waals interactions between adsorbed molecules can be ignored56. During the structure optimization process, all atoms in the four topmost and four bottom layers of the Co3O4 structure were allowed to relax, while the remaining five middle layers of the Co3O4 structure were frozen at the bulk positions57. 22


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx. Formation mechanisms of the hollow Co3O4 spheres, UV-Vis spectrum, adsorption properties, TG curve, charge/discharge voltage profiles, cycling performance at 0.1C and 0.2C of the Co3O4/ACNTs hybrids, SEM images of Co3O4/ACNTs hybrids after cycling, EIS of Co3O4/S and Co3O4/S/ACNTs electrodes before and after 50 cycles, FTIR spectrum of the MWCNT before and after acidified treatment, geometric structures of Li2S4 and Li2S6, surface structure of Co3O4 (111) and different initial modelling structures between Co3O4 and Li2S4/Li2S6.

ACKNOWLEDGEMENTS The work is financially supported by National Natural Science Foundation of China (No.51202117), Natural Science Foundation of Beijing (No.2162037 and L182062), Beijing Nova program (No. Z171100001117077), Fundamental Research Funds for the Central Universities (No.2014QJ02), Key Laboratory of Advanced Materials of Ministry of Education (No. 2018AML03) and Yue Qi Young Scholar Project of China University of Mining & Technology (Beijing) (No. 2017QN17). L. Zhang thanks the support from the start-up fund of the University of Alaska Fairbanks (2014). 23



DECLARATION OF INTERESTS The authors declare no conflict of interests.

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Graphical Abstract

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Acidified Multiwall Carbon

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