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An Entangled Cobalt-Nitrogen-Carbon Nanotubes Array Electrode with Synergetic Confinement and Electrocatalysis of Polysulfides for Stable Li-S Batteries Cejun Hu, Chengkai Yang, Jijin Yang, Nana Han, Rongyu Yuan, Yifan Chen, Hai Liu, Tianhui Xie, Ruida Chen, Heng-Hui Zhou, Wen Liu, and Xiaoming Sun ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00243 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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ACS Applied Energy Materials

An

Entangled

Cobalt-Nitrogen-Carbon

Nanotubes

Array

Electrode with Synergetic Confinement and Electrocatalysis of Polysulfides for Stable Li-S Batteries Cejun Hu†, Chengkai Yang§, Jijin Yang†, Nana Han†, Rongyu Yuan†, Yifan Chen†, Hai Liu†, Tianhui Xie†, Ruida Chen†, Henghui Zhou§, Wen Liu*,† and Xiaoming Sun*,†

†: State Key Laboratory of Chemical Resource Engineering, College of Energy, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. §: College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China

*: E-mail: [email protected]; [email protected]

KEYWORDS: Li-S battery, high sulfur loading, confinement, catalysis, carbon nanotubes array

ABSTRACT:

ACS Paragon Plus Environment

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Lithium-sulfur (Li-S) battery is considered as a promising battery chemistry for the next generation energy storage on the merits of low cost and high energy density. However, the sluggish redox kinetics and shuttling effect of lithium polysulfides (LiPSs), especially at high sulfur loading, result in low sulfur utilization, low coulombic efficiency, and short cycle life. Herein, an entangled N doped carbon nanotubes array with encapsulated Co nanoparticles has been constructed and used as an efficient host for sulfur cathode featuring both physical and chemical trapping ability for soluble LiPSs. Besides, the encapsulated Co nanoparticles combined with N species can accelerate polysulfides redox, which has been proved by electrochemical analysis, in-situ spectroscopy, and theoretical calculations. The effective confinement and fast conversion of polysulfides ensure Li-S battery with an high capacity of 1045 mAh g-1 at 1 C rate and 77.89% capacity retention even after 1000 cycles, and these advantages even can be extended to sulfur loading as high as 15 mg cm-2. More significantly, pouch cell assembled with the composite sulfur cathode can deliver a stable cycling for more than 150 cycles, further demonstrating great potential of Li-S batteries for the practical applications.


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Introduction In modern society, people rely heavily on reliable and durable batteries to drive their electronic devices such as mobile phones, laptops, and electronic vehicles.1 Therefore, there is an increasing demand for advanced battery technology with high-energy density, long cycling life and environmental benignity.2 To this end, Li-S battery, a kind of lithium metal batteries which use metallic lithium as anode and elemental sulfur as cathode, has attracted great attention and been considered as one of the most promising battery systems owing to the merits of high theoretical energy density (2600 Wh kg-1),3 low cost and abundant resource of active materials.4, 5 However, there are still several issues need to be addressed both in cathode and anode side before further deployment of Li-S battery in practical application. In sulfur cathode, problems exist in 1) the insulating nature of active S/Li2S and the volume change (~80%) during charge and discharge,6 2) the sluggish redox kinetics of LiPSs7 and 3) the dissolution and diffusion of long-chain polysulfides (Li2Sx 4≤x≤8) and the consequent shuttling effect.8 These inherent and painful problems result in the low elemental sulfur utilization, diminished capacity and short cycle life, which limit the practical application of Li-S batteries.


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To address these issues, great efforts have been devoted to sulfur trapping and cathode design. Carbon-based materials, owing to the high conductivity, low density, and structural stability, have been considered as one of the most feasible hosts of sulfur cathode. For instance, the mesoporous CMK-39 was constructed by Nazar’s group in 2009 to encapsulate sulfur, and after that, graphene,10 carbon nanotubes11, 12 (CNTs) and hollow carbon spheres13 have also demonstrated as effective host materials. However, the nonpolar feature of carbon materials is reported to have poor interaction with polar LiPSs,14 and it is hard to suppress the migration of long-chain polysulfides that results in rapid capacity fading.3 Thus further amendment of carbon materials is mainly focused on enhancing the interaction with LiPSs.15, 16 Functionalized carbon frameworks doping with N/S elements, which have been reported to grapple and immobilize LiPSs.17,

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Since

then, N-doped graphene,19 rGO-VS2 compounds,10 MOFs and polymer transferred carbon materials,20, 21 have been proved as the effective conductive matrix in confining LiPSs due to the formation of chemical bonds. Nevertheless, the intricacy and sluggish reaction kinetics of polysulfides is still hampering the blossom of Li-S battery.22 When the nonpolar sulfur reduced to soluble and polar polysulfides, the weak affinity between the


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conductive substrate and active species give rise to the higher charge transfer resistance and the result of slow redox kinetics.23, 24 Therefore, endowing sulfur host with the effects of entrapping LiPSs and accelerated redox kinetics are unavoidable to advance Li-S batteries. Recently, cobalt disulfide (CoS2)/carbon composites, MoS2 nanoflakes, and WO3-x nanoplates are reported to accelerate the redox kinetics of sulfur species and mediate their deposition.7,

22, 25

Withstanding the substantial progress, the effective

conversion and catalysis of sulfur to lithium sulfides are still far from realizing due to the uncoordinated synergistic between confinement and catalysis of LiPSs. In this respect, the advanced sulfur cathodes need to consist the following traits: 1) high electronic conductivity and void space to mitigate the volume change; 2) effective polar species to provide strong chemisorption toward LiPSs to suppress shuttling effect; and 3) durable catalytic sites to accelerate sulfur redox while avoiding side reactions.

Based on above comprehension, we designed an entangled N doped carbon nanotubes array with encapsulated Co nanoparticles (Co-N-CNTA) which acting as not only a reliable cathode host but also an efficient accelerator for sulfur conversion in Li-S


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batteries. The strategy of our materials design is multifold. First, the high electrical conductivity and flexibility of the CNTs guarantee fast electron transfer and adequate space to accommodate the volume changes in sulfur cathode. Secondly, the enhanced chemical adsorption, which comes from polar N species, limit the diffusion of LiPSs. Lastly, benefiting from the catalytic effect of the encapsulated Co nanoparticles, the redox kinetics could be accelerated efficiently and therefore decrease the accumulation of the soluble LiPSs and prevent the shuttling effect from happening. With these advanced features, the well-designed composite sulfur cathode could realize high sulfur loading and deliver an extra stable cycling for 1000 cycles. Moreover, the assembled pouch cells can easily light up 80 light emitting diodes and even stay overnight without noticeable fading, indicating the feasibility of rechargeable Li-S batteries for practical applications.

Result and discussion The synthetic procedure for the entangled Co-N-CNT array (Co-N-CNTA) involved two steps, as shown in Figure 1a. Firstly, Co(OH)2 nanowires array was synthesized on carbon fiber paper (CFP) by a hydrothermal method.26 Then, after removing H2O from


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Co(OH)2, the treated CoO array was used as template and pre-catalyst for CNTs growth. The CNTs were synthesized by a chemical vapour deposition (CVD) method using ethanediamine as both C and N element source. In order to investigate the sulfur confining effect of the designed nanoarray structure and the catalytic capability of Co-NC species. We also prepared CNTs arrays without heteroatoms doping using ethanol as the carbon source (noted as Co-CNTA) and the N doped carbon nanotubes without array structure using dicyandiamide as C/N source (noted as Co-N-CNT) as the control (Figure S1, 2).

From the SEM image, we can see that the smooth CFP substrate (Figure 1b, e) was covered with vertically aligned Co(OH)2 nanowires (Figure 1c, f) after hydrothermal reaction, which was then converted into CoO array as the template for CNTs growth. Thereafter, during the heat treatment at 800 oC, the CoO nanowires was consumed because of on-site nucleation and growth of carbon nanotubes, leaving micro-sized carbonaceous wires array composed of entangled CNTs by interweaving growth (Figure 1d, g). The N doping influences the CNTs growth and consequently results in a twisty and


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bamboo-like morphology with wrapped nanoparticles, as shown in the transmission electron microscopy (TEM) image (Figure 1h). Elemental mapping was also carried out by scanning transmission electron microscopy (STEM). As we can see in Figure 1i, the carbon and nitrogen elements were distributed throughout the CNTs evenly. Besides, the Co element can be clearly observed at the end of CNTs even after acid etching, indicating the protective capability of carbon shell layers. The typical XRD patterns of CoO array, unwashed carbonization product, and the final Co-N-CNTA sample are presented in Figure S3. The characteristic peaks of CoO (JCPDS No.74-2392) and the converted product of Co nanoparticle (JCPDS No. 15-0806) indicate the catalyst-assisted synthesis of CNTs. The sharp diffraction peaks located at 26.4o and 54.5o were indexed to graphitic carbon (JCPDS No. 75-2078) which reflected the CFP substrate and as-synthesized CNTs. The high-resolution transmission electron microscopy (HRTEM) image (Figure 1j) reveals Co nanoparticles wrapped with several graphene layers, in which the lattice fringes with the d-spacing of 0.204 nm corresponding to the (111) crystal plane of metallic Co. The chemical composition and surface electronic structure were further investigated by X-ray photoelectron spectroscopy (XPS) analysis. In Figure S4, C 1s, O 1s, N 1s, and


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Co 2p signals are exhibited in the XPS survey spectrum, and the weak and illegible Co 2p peak is attributed to the good shielding effect of carbon shells. The high-resolution N 1s spectrum shows two peaks located at 401.1 and 398.8 eV, which corresponding to the graphite N and pyridinic N.27 The Co 2p spectrum can be assigned to the Co 2p3/2 and 2p1/2 doublet, located at 781.6 and 796.7eV, respectively.28 In the Co-N-CNTA sample, the N content is as high as 8.6 at. %. According to previous research, N doping can significantly change the electron distribution in carbon materials and increase their interaction with lithium polysulfides.14, 29, 30 On the other hand, Co nanoparticles with the N doped carbon can catalysis of O2 reduction reaction (ORR), thus, it may also have a catalytic effect on sulfur redox and avoid soluble lithium polysulfides accumulation during the charge-discharge process.

To confirm the catalytic effect of Co-N-CNTA on the polysulfide redox, symmetric cells were assembled using the identical working and counter electrode of CNTs based electrodes and Li2S6 in the electrolyte as a redox active material. Cyclic voltammetry (CV) tests were performed with the scanning rate of 5 mV s-1. As show in Figure 2a, a pair of


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redox peaks with large overpotential at -0.31 V and 0.29 V can be observed on the CFP electrode, which corresponding to the redox of Li2S6 and Li2S4. As for the Co-CNTA electrode, which is CNTs array without N doping, shows a similar couple of redox peaks with reduced overpotential. However, for N doped CNTs without array structure (Co-NCNT), there are two new pairs of redox peaks emerging with a very drawn-out feature, indicating the further conversion of LiPSs. In contrast, the cell with Co-N-CNTA electrodes, which is CNTs array with N doping and encapsulated Co NPs, exhibit three pairs of redox peaks: a, a’:-0.059 V, 0.071 V; b, b’: -0.229 V, 0.295 V; and c, c’: 0.118 V, -0.129 V, respectively. The a and a’ peak are similar to the peaks appeared in Co-CNTA electrodes, corresponding to the first reduction stage of Li2S6 and oxidation stages of Li2S4. In the further transformation process, the stretched peaks (b, b’) show unfavorable reaction kinetics which matching well with the phase transformation reaction. In addition, the c and c’ peak can be regarded as oxidation of Li2S6 and reduction of Li2S8, respectively.7,

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Considering the

substantially decreased over-potential of redox between Li2S6 and Li2S4 with Co-CNTA, it is obvious that the array structure which owing high conductivity and the excellent confinement of soluble lithium polysulfides, thus decreasing the reactive over-potential of liquid sulfur species. However, the array structure only could not influence the reaction


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path of sulfur species, resulting in no effect on accelerating sulfur transformation between liquid-solid phases. For comparison, the catalysis effect of the additional Co-N-C species could significantly change the sulfur reaction kinetics, thus result in further transformations of sulfur species with additional redox peaks in CV curves. Therefore, combining the advantages of confinement and catalysis, the Co-N-CNTA electrode exhibits the feature of enhanced sulfur redox kinetics and small polarization. To make a thorough and systematic quantitative comparison, we analyzed the current densities and overpotentials of peaks correlated with the reduction of Li2S6 to Li2S4 on different electrodes, and then summarized in Figure 2b. It is obvious that Co-N-CNTA displays the smallest polarization of ~0.06 V, which means the lowest energy barrier for Li2S6 conversion. Besides, the highest current density of Co-N-CNTA sample also indicates the fastest reaction kinetics for LiPSs conversion thus decreases the diffusion and possible shuttling of LiPSs intermediates. We tested the symmetric cells with scan rates from 5 mV/s to 100 mV/s and the results are shown in Figure S5. It can be seen that the CFP electrode show unsymmetric broad peaks while the N-CNTA electrodes display redox peaks with the smallest polarizations. Meanwhile, the largest CV area also corresponds


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to the highest electrochemical reaction amount. In addition, the current density of N-CNTA electrode is almost doubled than that of CFP electrode, indicating the catalytic effect and favorable reaction kinetics. Moreover, decreased charge transfer resistance was also verified by electrochemical impedance spectra (EIS) of symmetrical cells (Figure 2c). The CNTs based electrodes exhibit considerably lower charge transfer resistance than that of CFP electrode, in which Co-N-CNTA with both catalytic effect and sulfur confinement shows the smallest ohm resistance (3.19 ohms) and charge transfer resistance (1.89 ohms), which is in good agreement with the CV test.

To further confirm the trapping ability and accelerated LiPSs redox kinetics, we carried out an in-situ UV-vis spectroscopy measurement to observe the dissolved sulfur species in the electrolyte during the discharge process (Figure 2d). For which, a light transmittable cell was constructed with quartz tube used as the reaction vessel while sulfur loaded (2mg cm-2) materials (Co-CNTA/S, Co-N-CNT/S and Co-N-CNTA/S) as the cathode, lithium foil as the anode and filled with 2 mL of ether electrolyte. In the discharging process, sulfur trapping and catalysis ability can be evaluated by the wavelength and intensity of


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adsorption peaks. It is put forward by previous researchers that the adsorption peak with higher wavelength attributes to the longer chain polysulfides.32-35 Based on this understanding, the species and quantity of dissolved LiPSs could be collected and analyzed. The derivative peaks of dissolved species from different electrodes at various potentials were performed in Figure S6 and Figure 2e. Apparently, Co-CNTA/S electrode exhibited the maximal absorbance, corresponding to the highest dissolution amount of LiPSs (Figure S6b). As for the Co-N-CNT/S electrode, it shows the lowest dissolution amount before 2.2 V, however, flooded with soluble sulfur species at 2.0 V as reflected by the increased absorbance intensity (Figure S6c). In contrast, during the discharge process, Co-N-CNTA/S electrode exhibited an admirable confinement for LiPSs, even after the voltage drops to 2.0 V, the dissolution of LiPSs didn’t increase noticeably (Figure S6d). Based on the results of these two experiments, we concluded the mechanism of accelerated redox in two parts: 1) the confinement effect could suppress the dissolution and diffusion of polysulfides, thus leading to the high utilization of sulfur species but barely help to reduce the reaction barrier while 2) the Co-N-C species could significantly reduce the reaction barrier which induces the further conversion of LiPSs.


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DFT calculations were performed to further reveal the intrinsic inter-facial confinement and catalysis of LiPSs at the atomic level. 13-atoms Co cluster with bilayer graphene or N-doped graphene was designed to mimic the actual environment of the encapsulated Co clusters (Figure 3). The optimized adsorption configurations of different kinds of LiPSs on the surface of Co-graphene (Co-G) and Co-N-doped-graphene (Co-NG) fabric are shown in Figure 3a-b. It is observed that the Co cluster with graphene substrate exhibit undesirable chemical interaction with LiPSs while the adsorption ability of LiPSs on Co-N-G were conspicuously enhanced: Li2S8 (-1.02 eV), Li2S6 (-1.22 eV), Li2S4 (-1.30 eV), Li2S2 (-1.79 eV) and Li2S (-1.85 eV), respectively. Apparently, the appropriate absorb-ability of Co-N-G for LiPSs not only provides a better anchoring effect but also influences the breaking of Li-S bonds. The conversion processes from S to Li2S are shown in Figure 3c, demonstrating the free energy of reaction intermediates on CoG and Co-NG substrate. At the liquid phase transformation (Li2S8 ↔ Li2S4), inconspicuous difference in free energy were exhibited on this two substrate, denoting this transformation process are merely influenced by the incorporation of N species. However, the consequent phase conversion procedure on the two different substrates exhibit


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significant energy differences. For the conversion of Li2S4 into Li2S2, more negative free energy and lower reaction energy barrier were observed on Co-N-G substrate compare to Co-G substrate, which is in accordance with the experimental results. At the last step (Li2S2 to Li2S), it is seen that the free energy of Li2S/Co-G was even higher than that of Li2S2/Co-G, which indicates an undesirable reaction kinetics. By contrast, the incorporation of N species significantly enhances the interaction between Li2Sx and substrate. On these N-doped sites, the sulfur substance can be easily converted into Li2S following the associative mechanism showed in Figure 3c, which coincides with our experimental observations. We further investigated the diffusion properties of Li2S on these two substrates, and the results were shown in Figure 3d. The decomposition barrier of Li+ from Li2S is related to the binding energy between Li+ and substrate. The calculated barrier of Li2S decomposition on Co-N-G substrate is 1.81 eV, while the Co-G substrate showed a largely increased barrier of 2.81 eV. In addition, the barrier of Li2S decomposition on the pure N-G substrate is 1.95 eV. In all, by combining experimental data and computational simulation we verified the existence of strong chemical binding between LiPSs and Co-N-G substrate and the accelerated reduction of LiPSs on


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substrate surface. In addition, the conversion of Li2S to Li2S2 is promoted on the N doped sites of Co-NG composite as well.

The composite sulfur cathodes were prepared by heating sublimed sulfur and different CNTs based hosts at 155 oC for 20 hours. SEM images and elemental mapping of each sample are displayed in Figure S7-10. Figure S11 showed the thermogravimetric (TG) curves of sublimed sulfur and Co-N-CNTA/S electrode with sulfur loading of 5 mg cm-2 to identify the real ratio of S and C in the sulfur cathode. After comparing with the TG curve of sublimed sulfur, the sulfur loading ratio can be deduced as approximately 40% in Co-N-CNTA/S electrode. To compare the electrochemical performance, Li-S batteries were assembled and evaluated with as prepared sulfur composites as the cathode and lithium foil as the anode. First, CV test was measured in the voltage range from 1.7 to 2.8 V at the scanning rate of 0.1 mV s-1 (Figure 4a). There are two obvious cathodic peaks corresponding to the transformation from S8 to the long-chain polysulfides (~2.31V) and further conversion to the insoluble short-chain polysulfides (~2.03V).13,

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During the

following anodic scan, the divisive double peaks at 2.34 and 2.41 V are assigned to the


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reverse conversion of Li2S to the soluble polysulfides and subsequently elemental sulfur.37,

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Figure 4b illustrates the overpotential based on Figure 4a to validate the

electro-catalytic effect. Compared with CFP/S, a remarkable mitigated polarization can be noticed on Co-N-CNT/S: the raised reduction potential and reduced oxidation potential in which attributes to the excellent polysulfides redox kinetics. The Co-CNTA/S electrode shows a similar low polarization in the first cathodic peak (I) and the anodic peak (I’), but at the subsequent transformation process, due to the undesirable reaction kinetics, the higher polarization were exhibited in the peak (II) and (II’), which matching up well with the symmetric cell experiment. As for Co-N-CNTA/S electrode, with the combination of confinement effect and accelerate redox kinetics, the biggest peak area as well as the lowest polarization were exhibited in CV curves, which proved again the effectiveness of our materials design with hierarchical array structure and Co-N-G catalysis center. Figure S12 shows the EIS analysis of four electrodes before and after CV cycling, all of CNTsbased electrodes represent a lower charge transfer resistance compares to the CFP-S electrode, which indicates the fast electron and lithium ion transport and results in faster reaction kinetics.


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The initial charge and discharge capacity of the Co-N-CNTA/S electrodes is 1408 and 1382 mAh g-1 at 0.1 C, suggesting the efficient utilization of sulfur (Figure 4c). In comparison, the Co-CNTA/S electrode exhibits a similar capacity of 1402 and 1368 mA h g-1 while the Co-N-CNT/S electrodes exhibit a much lower capacity of 1074 and 1094 mA h g-1, respectively. These contrasting results demonstrated the confinement effect of array structures thus leading to elevated sulfur utilization. The cycle performance was further investigated at 0.5 C rates with the sulfur mass loading of 5 mg cm-2. As shown in Figure 4d, the Co-N-CNTA/S cathode exhibits a reversible capacity of 882 mA h g-1 at 0.5 C after the initial activation cycle at 0.1 C, and the capacity remains nearly 100% after 200 cycles, demonstrating exceptional cycling stability. In comparison, Co-CNTA/S electrode displays a poor capacity retention of 82.2% from 827.3 to 681.2 mA h g-1 after 200 cycles. The Co-N-CNT/S electrode shows a lower sulfur utilization with only 726.4 mA h g-1 in capacity, and maintain 669 mA h g-1 after 200 cycles with 92.4% capacity retention. In addition, the sulfur cathodes after 10 cycles between 1.7-2.8 V were shown in Figure S13-16. After several cycles, the sulfur tend to accumulate as particles on the surface of CFP electrode (Figure S13). As for Co-CNTA/S and Co-N-CNT/S cathodes,


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thick sulfur film formed on the surface of electrode, which caused loss of porosity in the electrode and block Li+ transportation (Figure S14 and S15). In contrast, the Co-NCNTA/S still maintained its porosity and hierarchical structure with uniform and thin layer of sulfur deposition, which is suitable for sulfur reaction with high capacity and stability (Figure S16). Thus, the remarkable electrochemical performance of Co-N-CNTA/S electrode could be attributed to the following advantages: 1) well-designed orderly hierarchical architecture afford enough space for sulfur storage; 2) chemical adsorption of polysulfide induced by nitrogen doping and 3) encapsulated Co nanoparticles combine with the doped nitrogen which enhances the reaction efficiency and reduces the polarization during the redox processes.

To further explore the application potential of CNTs based sulfur cathode, all samples were performed an overlong cycling test for 1000 cycles at 1 C rate as showed in Figure 4e. The Co-N-CNTA/S electrode delivered the highest initial specific capacity of 1045 mA h g-1. After 1000 cycles, the capacity still remains 814 mA h g-1 with a nearly 100% coulombic efficiency, and the capacity retention was as high as 77.89% with the decay


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rate as low as 0.014% per cycle, which also ranks as one of the best sulfur cathodes till now (Table S1). As for the control samples, the CNTA/S electrode discharged 1023 mA h g-1 and 714.9 mA h g-1 after 540 cycles, showing a rapid capacity decay with about 69.8% retention and 0.036% capacity loss per cycle. A lower sulfur utilization was observed for Co-N-CNT/S electrode with an initial discharge capacity of 802 mA h g-1, further confirming the importance of the cathode architecture for the sulfur reduction. Additionally, high sulfur loading electrodes were also attempted with Co-N-CNTA sample. As showed in Figure 4f, Co-N-CNTA/S electrodes with different sulfur loading have been tested at 0.2 C rate. Reversible capacity of 1059, 975 and 797 mAh g-1 remained after 100 cycles with the sulfur loading of 5.0, 10.0, 15.0 mg cm-2. It is noted that an activation process of the sulfur cathode was observed when the sulfur loading increased to more than 10 mg cm-2. To further demonstrate the application potential of Co-N-CNTA/S electrode, pouch cells were assembled using Li foil as the anode and Co-N-CNTA/S electrode as the cathode (Figure 4g and S17). With sulfur loading of 5.0 mg cm-2, the pouch cell delivered a reversible capacity of 5.484 mAh cm-2 at the initial cycle of 0.1 C, and 4.823 mAh cm-2 remained after nearly 150 stable cycles at 0.2 C. Moreover, 80 light


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emitting diodes were connected in parallel to form a “BUCT” pattern as shown in Figure 4g, which can be lighted by the Li-S pouch cell and even stay overnight without any fade in brightness, indicating excellent voltage characteristic and high durability that is important for practical applications.

Conclusion In summary, an entangled carbon nanotubes array with N doping and encapsulated Co nanoparticles (Co-N-CNTA) was successfully constructed using the CoO nanowires array as both the template and pre-catalyst followed by chemical vapor deposition (CVD) process. With unique orderly architecture, nitrogen doping and encapsulated Co nanoparticles, the Co-N-CNTA enables not only adsorption ability to LiPSs but also an efficient electrocatalysis promoting sulfur redox kinetics. Based on both effective confinement and electrocatalysis of LiPSs, the Co-N-CNTA/S electrode exhibit high initial capacity of 1045 mAh g-1, and admirable capacity retention of 77.89% after 1000 cycles. When sulfur loading increased to 15 mg cm-2, the electrode can still deliver a capacity of 797 mAh g-1 after 100 stable cycles. More importantly, the pouch cell demonstrated the


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possibility and potential application of Li-S batteries. We believe this work could not only provide a high performance sulfur cathode, but also a new synthesis approach and strategy for the developing cathode with catalysis for other energy storae devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Experimental Sections, SEM images of Co-CNTA and Co-N-CNT, XRD patterns, XPS profiles, UV/vis measurement, SEM images after sulfur depositions of each samples, EIS spectra and Discharge/charge profiles are shown in Supporting Information.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected] Author contributions


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C. Hu, W. Liu, and X. Sun conceived the idea and designed the experiment. C, Hu did the materials synthesis. C. Yang and R. Yuan did the DFT calculation. Y. Chen, N. Han, H. Liu, R. Chen, J. Yang, and T. Xie performed physical characterizations of SEM, TEM, XRD and XPS measurements. C. Hu, W. Liu and X. Sun wrote the paper. All authors discussed and approved the submission of this manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (21771018, 21875004), Beijing University of Chemical Technology (start-up grant buctrc201901, BUCT, China), the Program for Changjiang Scholars and Innovative Research Team in the University, the Fundamental Research Funds for the Central Universities, and the Long Term Subsidy Mechanism from the Ministry of Finance and the Ministry of Education of PRC.

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Figure 1: Schematic illustration of the fabrication and morphology characterization of the CNTs array. a) Schematic illustration. The low and high magnification SEM images of (b, e) bare CFP substrate, (c, f) precursor Co(OH)2 nanowires array, and (d, g) as-synthesized Co-N-CNTs array. h) TEM image shows the bamboo-like CNTs and the encapsulated Co nanoparticles. i) HAADFSTEM image and corresponding elemental mapping images of C, Co, N. j) HRTEM image shows the Co nanoparticle wrapped with several graphene layers, in which the lattice fringe correspond to Co (111) facet.


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Figure 2: Verification of confinement and catalysis effect toward lithium polysulfides by the symmetrical cell test and in-situ UV-vis measurement. a) The CV curves of symmetric cells with identical electrodes of CFP, Co-CNTA, Co-N-CNT and Co-N-CNTA in electrolyte with Li2S6. b) The summarized overpotentials and the current densities for the peak a, which correspond to the reduction of Li2S6 to Li2S4. c) EIS spectra of the symmetric cells with different samples. d) Schematic illustration of the in situ UV/vis measurement and discharging curves. e) Representative UV/vis derivative curves at the selected discharge potentials of 2.2 and 2.0 V.


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Figure 3: DFT calculation of the trapping and electro-catalysis of polysulfides on different substrate. a) Adsorption of LiPSs on the surface of Co-G substrates. b) Adsorption of LiPSs on the surface of Co-NG. c) The conversion processes from S to Li2S on the surface of Co-G, N-G and Co-NG, and corresponding free energies. d) Energy profiles for the decomposition of Li2S cluster on Co-G and Co-NG substrates. Here, yellow, green, gray, blue, and purple represent S, Li, C, N and Co atoms, respectively.


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Figure 4: Electrochemical performance of different sulfur cathodes. a) CV curves of the different sulfur cathodes in asymmetrical Li-S cells. b) Summarization of the cathodic and anodic peak voltages and overpotentials of CVs in (a). c) Charge-discharge profiles of the different sulfur cathodes at the first cycle and d) cycling performance at 0.5 C following 5 cycles of activation at 0.1 C. e) Long-term cycling stability of as-synthesized samples at 1.0 C. f) Cycling performance


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of Co-N-CNTA/S with high-sulfur-loading. g) Pouch Cell assembled with the sulfur cathode of Co-N-CNTA/S which lightening luminous diodes, and cycling at 0.2 C for 140 cycles.


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