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A Novel Sulphur Host Comprised of Cobalt and Porous Graphitic Carbon Derived from MOFs for the High-Performance Li-S Battery Yanqiu Lu, Yijin Wu, Tian Sheng, Xinxing Peng, Zhenguang Gao, Shaojian Zhang, Li Deng, Rui Nie, Jolanta Swiatowska, Jun-Tao Li, Yao Zhou, Ling Huang, Xiaodong Zhou, and Shi-Gang Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00915 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Materials & Interfaces

A Novel Sulphur Host Comprised of Cobalt and Porous Graphitic Carbon Derived from MOFs for the High-Performance Li-S Battery Yan-Qiu Lu †, Yi-Jin Wu †, Tian Sheng ‡, Xin-Xing Peng §, Zhen-Guang Gao †, Shao-Jian Zhang †, Li Deng †, Rui Nie †, Jolanta Światowska //, Jun-Tao Li †*, Yao Zhou †, Ling Huang §, Xiao-Dong Zhou ⊥, and Shi-Gang Sun †§ † ‡

College of Energy, Xiamen University, Xiamen, 361005, P. R. China

College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241000, P. R. China

§

State Key Lab of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of

Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China. //

PSL Research University, Chimie ParisTech-CNRS, Institut de Recherche de Chimie Paris (IRCP), 11 rue Pierre et Marie Curie, 75005 Paris, France ⊥

Institute for Materials Research and Innovation, Department of Chemical Engineering, University of Louisiana at Lafayette, Lafayette, LA 70503, USA *Correspondence to: [email protected]

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ABSTRACT: A composite consisting of cobalt and graphitic porous carbon (Co@GC-PC) is synthesized from bimetallic-metal-organic frameworks (MOFs) and employed as the sulfur host for high-performance Li-S batteries. Due to the presence of a large surface area (724 m2 g-1) and an abundance of macro/meso pores, the Co@GC-PC electrode is able to alleviate the debilitating effect originating from the volume expansion/contraction of sulfur species during the cycling process. Our In situ UV/Vis analysis indicates that the existence of Co@GC-PC promotes the adsorption of polysulfides during the discharge process. Density functional theory (DFT) calculations show a strong interaction between Co and Li2S and a low decomposition barrier of Li2S on Co (111), which is beneficial to the following Li2S oxidation in the charge process. As a result, at 0.2 C, the discharge capacity of the S/Co@GC-PC cathode is stabilized at 790 mAh g-1 after 220 cycles, much higher than that of a carbon-based cathode which delivers a discharge capacity of 188 mAh g-1. KEYWORDS: lithium-sulfur batteries; bimetallic-metal-organic frameworks; in situ UV/vis analysis; cycle stability; catalysis


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INTRODUCTION The development of future energy storage technology is driven by the recognition of a widespread use of electric vehicles and a growing integration of the industrial applications with green energy sources.1 The conventional rechargeable batteries based on the nickel metal hydride, magnesium ion, cadmium-nickel, and lead-acid may not satisfy the demand of these emerging markets because of their low energy density.2 Although the cost of Na-, Mg-, and Al-ion batteries is low, more efforts are needed for them to mature as a technology for practical applications.3-5 Rechargeable lithium-sulfur (Li-S) battery is attractive due to the nontoxicity, natural abundance, and low cost of sulfur, and more importantly its remarkably high theoretical energy density (~ 2,600 Wh kg-1).6, 7 During the past two decades, numerous advancements have been reported to design Li-S batteries; however, the commercialization of Li-S batteries is still restrained by a few technological barriers.8

Major issues for the Li-S batteries are the incomplete use of S, poor cycleability, and low coulombic efficiency due to the polysulfide shuttling.9, 10 During discharge, S experiences the reduction via long-chain polysulfides to short-chain ones. When the soluble long-chain polysulfides pass through the separator and reach the surface of lithium anode, a so-called polysulfide shuttling process, they are reduced to short-chain polysulfides, as well as insoluble Li2S2 and Li2S, leading to the formation of a passivation layer on the lithium metal. During the charging process, the oxidation of Li2S results in the formation of long-chain polysulfides and subsequent polysulfide shuttling.11 This recurrent crossover of polysulfides during cycling exacerbates the loss of active material (S) and causes a rapid capacity degradation.12 Other issues include a lack of control on and a poor understanding of the formation of Li2S in Li-S battery. The deposition of Li2S2 and Li2S in the pores of sulfur host will increase internal resistances,



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leading to an increase in overpotential and a decrease in capacity during cycling.13 In addition, the practical application of Li-S batteries is also hindered by a low electrical conductance and a large volume change of S induced by electrochemical reactions.12, 14

To address these challenges, a number of studies have been carried out to develop better S host materials,15-18 new battery configurations,19,

20

electrolytes,21 binders,22,

23

and so on.

Carbon-based materials have been extensively studied because they can be synthesized with a controllable surface area with a flexible pore structure, sufficient electrical conductance and able affinity to S.24, 25 Some transition metals or alloys show a multifunctional interaction with S species. Recently disclosed chemical interactions between the elementary substance of transitional metals and S species showed an effective approach to immobilize and trap S species.26-28 For example, Zheng et al. employed a composite of S and microporous carbon, which was stabilized by copper, to enhance the electrochemical performance of Li-S batteries through the chemical bonding of S or polysulfides with Cu. This bonding led to an increase in the S content from 30 to 50% without deteriorating its electrochemical properties.29 A few previous publications showed that the shuttling effect can be suppressed by the electrocatalysis of transitional metals or alloys. For instance, Salem et al. found that the cathodes with Pt or Ni exhibited a reduction in the overpotential and a better specific capacity than that of pristine graphene electrodes.30 Therefore, using a host material consisting of multi-functional carbon and transitional metals can be an interesting solution to overcome aforementioned drawbacks of Li-S batteries.

Recently, metal-organic frameworks (MOFs), a class of porous materials consisting of metal ions and organic ligands, have attracted a widespread attention due to their versatility, such as


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high specific surface area, tailorable pore-configuration, and tunability of metal elements and organic ligands.31, 32 Ascribing to the ordered crystalline structures of MOFs, the carbon-based composites derived from MOFs show more uniform and tunable porosity than traditional carbon materials, such as carbon nanotubes and graphene.33 For example, Lou’s group used a microporous carbon polyhedrons derived from MOFs as the host for Li-S batteries to investigate the electrochemical behavior of S embedded in microporous carbon.34 Dong’s group reported a 3-D porous composite consisting of N-doped graphitic carbon and Co, derived from MOF polyhedron (ZIF-67), which has a dual-catalyzing effect on the reduction and oxidation processes of S species. The Co, N-GC matrix and N-dopant all contributed to the excellent performance of Li-S batteries.35 Hence, a well-designed MOF can be used to prepare the host and subsequently improve the electrochemical performances of Li-S batteries.

The surface area of some carbon-metal from MOFs reported by previous papers is relatively small which may not be able to trap all polysulfides during cycling. In this article, we present a new type of bimetallic ZnCo-MOF-5 microcubes obtained through a refluxing synthesis approach. ZnCo-MOF-5 microcubes were converted to Co@graphitic carbon-porous composite (Co@GC-PC) with a large surface area through the carbonization, which was adopted as the S host material in order to improve the performance of Li-S batteries. Co@GC-PC host is of large surface area even though it contains a certain amount of Co nanoparticles. The large surface area and adequate mesopores of Co@GC-PC enable the host to absorb polysulfides, while the electronic conductance is originated from the well-distributed Co and graphitic carbon. The DFT calculations show that Co promotes the decomposition of Li2S. The catalytic activity of Co enhances the charging process and minimizes the polarization. A highly reversible capacity of



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790 mAh g-1 at 0.2 C after 220 cycles was achieved for the Li-S battery when the Co@GC-PC was used as the S host.

EXPERIMENTAL PROCEDURES Materials and chemicals. The raw materials, Zn(NO3)2·6H2O, terephthalic acid and Li2S, were obtained from Alfa Aesar (Shanghai, China). Polyvinyl Pyrrolidone, N, N-dimethyformamide, Co(NO3)2·6H2O and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The electrolyte, 1, 2-dimetyoxy ethane (DME) and 1, 3-dioxolane (DOL) (DME: DOL= 1:1 v/v) with 0.5 M LiCF3SO3 and 0.5 M LiNO3 (battery grade) and the LiNO3-free electrolyte, 1, 2-dimetyoxy ethane (DME) and 1, 3-dioxolane (DOL) (DME: DOL= 1:1 v/v) with 0.5 M LiCF3SO3 (battery grade) were supplied by Dodochem (Suzhou, China). Synthesis of ZnCo-MOF-5 microcubes and Co@GC-PC. 0.890 g Co(NO3)2·6H2O, 0.835 g Zn(NO3)2·6H2O, 3.600 g polyvinyl pyrrolidone and 0.170 g terephthalic acid were dissolved in 75 ml N, N-dimethyformamide and 45 ml anhydrous ethanol to form a homogeneous solution through magnetic stirring in a round-bottom flask. The solution was aged at 100 °C in an oil bath with magnetic stirring for 6 h in refluxing equipment. After cooling to room temperature, the precipitates were washed with anhydrous ethanol for several times and then dried at 80 °C in vacuum for 12 h. Finally, pink ZnCo-MOF-5 microcubes were obtained, shown as the insert of Figure 1a. The as-synthesized crystals were heated at 200 °C in a tube furnace flowing with Ar for 2 h to remove the residual organic solvent, then heated to 1000 °C with a heating rate of 3 °C min-1, and held at 1000 °C for 2 h. After cooling to room temperature, the black Co@GC-PC was obtained. Synthesis of Co-acetylene black (defined as Co-AB) composite. 8.025 g Co(NO3)2·6H2O, 2 g


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AB and 200 mL deionized water were mixed by magnetic stirring in a round-bottom flask for 12 h and then dried at 80 °C for 12 h via a rotary evaporator. The mixture was then heated at 80 °C in vacuum overnight and converted to a composite of Co and AB by heating at 1000 °C for 2 h with a heating rate of 3 °C·min-1 under Ar flow.

Synthesis of S composites. Melt-diffusion strategy was used to prepare S/Co@GC-PC, S/acetylene black (named as S/AB composite) and S/Co-acetylene black (named as S/Co-AB composite). For example, a mixture of Co@GC-PC and S with a weight ratio of 3:7 was grounded in an agate mortar for 30 min and then heated in a Teflon container at 155 °C for 12 h. S/AB and S/Co-AB composites were obtained via the same procedure as above excepting that Co@GC-PC was replaced by AB or Co-AB composite. Materials characterization. Co@GC-PC and S/Co@GC-PCs were characterized by X-ray diffraction (XRD, Panalytical X’pert PRO, Philip) with Cu Kɑ radiation. The morphological characterizations of materials were obtained using scanning electron microscopy (SEM, Hitachi S-4800). The high resolution transmission electron microscopy (HRTEM) images were acquired by JEM 2100 and JEM 1400. The elemental mapping was obtained using HRTEM (Tecnai F30). Nitrogen adsorption-desorption isotherms, average pore diameter, and specific surface area were obtained from TriStar II 3020 V1.03. TGA was carried out under a N2 atmosphere using NETZSCH STA 449 F5 analyzer. PHI Quantum-2000 apparatus was used for the X-ray photoelectron spectroscopy (XPS) measurements. X-ray source was Al Kɑ monochromatized radiation (hν = 1486.6 eV). High resolution spectra and survey scans were recorded with pass energy of 30 and 100 eV, respectively. The photoelectrons take-off angle was 90°.



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Electrochemical characterization. The electrochemical performances of electrodes were conducted by galvanostatic cycling in 2025 type coin cells at 25 °C using LANHE-CT2001A battery tester. S composites, acetylene black (AB) and La133 with a mass ratio of 7:2:1 were mixed to form homogeneous slurry using deionized water as dispersant via ball-milling. The slurry was then uniformly spread on Al foils and heated at 55 °C in vacuum overnight to form the electrodes with the areal mass loading of ~3 mg cm-2 and ~4 mg cm-2 for cycle stability test and cyclic voltammetry (CV) test. The coin cells were assembled in Ar-filled glove box using Li foil (China Energy Lithium Co., Ltd. China) and Celgard 2400 membrane as counter electrode and separator, respectively. CV of the cells were carried out on a CHI660E electrochemical working station (Chenhua, China) in the voltage range of 1.8-2.6 V vs. Li/Li+ with a scan rate from 0.1 mV s-1 to 0.4 mV s-1. Preparation of polysulfide adsorption. Li2S and S with a molar ratio of 1:5 were added into the DOL/DME solvent to obtain a solution of Li2S6 via magnetic stirring at room temperature. 50 mg of Co@GC-PC and AB were respectively added into two bottles with the same amount of Li2S6 solution. After a few minutes of vibration, the mixtures were allowed to stand until the solid precipitated to the bottom of the bottle. The above procedures were carried out in an Ar-filled glove box (H2O and O2 < 0.5ppm). In situ UV/Vis spectra tests. Four catholytes with a different ratio of lithium sulphides and S were prepared. For example, when preparing Li2S6, Li2S and S with a molar ratio of 1:5 were added into electrolyte to obtain a solution of 20 mM Li2S6 via continuous magnetic stirring at room temperature under Ar flow. The glass fiber separators wetted with a 200 µL catholyte solution were assembled in 2025 coin cells with a hole (Φ=10 mm). In situ UV/Vis reflectance spectrum was obtained from UV/Vis spectrometer (Shimadzu UV-2700, Japan). S/Co@GC-PC


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and S/AB slurries were made using the same method as described above. The areal mass loading of the electrodes were ~4 mg cm-2. The 2025 coin cell with a hole (Φ=10 mm) was used in the operando UV/Vis tests. The cell covered with a sealed glass was assembled in an Ar-filled glove box using Li foil with a hole (Φ= 12 mm) and glass fiber as counter electrode and separator, respectively. A 200 µL electrolyte was applied to wet the glass fiber separator. The cell was discharge at 0.2 C (1 C = 1675 mAh g−1); at the same time, UV/vis spectra at different voltages were recorded. Shuttle currents measurement. For the measurement of shuttle currents, LiNO3-free electrolyte was adopted to prevent the generation of the passivation on lithium metal anode. In the beginning, the cells were discharged to 1.8 V and then charged to 2.6 V. Following this, the cells were allowed to rest for 10 minutes at open circuit. Then the voltage of the cells was switched to potentiostatic mode at 2.6 V during which the current was adopted to the cell and was recorded until the current achieve to a steady-state value. Without the external electric current, the voltage of the cell would drop because of the continuous shuttle of polysulfides, so the steady-state current was considered as the shuttle current. The measurements of subsequent shuttle currents were conducted by discharging the cell to 2.5, 2.4, 2.35, 2.3, 2.2, 2.15 V and switched to potentiostatic mode and then recorded the steady-state current as previously described.36 Computational methods. The spin-polarized DFT calculations were performed with the Perdew-Burke-Ernzerhof

(PBE)

generalized

gradient

approximation

(GGA)

exchange-correlation functional and the projector-augmented-wave (PAW) pseudopotential by the Vienna Ab-initio Simulation Package (VASP) code.37-41 The vacuum layer was ~15 Å to eliminate the slab interaction between the z direction. A three-layer Co (111) surface model was



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constructed by a p(6x6) unit cell with 108 Co atoms. The cut-off energy was set as 400 eV and a 1×1×1 Monkhorst-Pack k-point sampling was used. The bottom layer atoms were fixed and the top two-layer atoms with adsorbates were fully relaxed during all the optimization process. The transition state was located by a constrained optimization approach with the force converge criteria below 0.05 eV/Å.42 The adsorption energy here was defined as: Ead = E(Li2S/Co) E(Li2S) - E(Co), where E(Li2S/Co), E(Li2S), and E(Co) are the total energies of the Li2S on the Co(111) surface, one Li2S molecule in vacuum and clean Co(111) respectively.

RESULTS AND DISCUSSION We first synthesized the bimetallic ZnCo-MOF-5 microcubes with a unique morphology as the precursor to prepare for the S host. SEM and TEM images of ZnCo-MOF-5 microcubes are shown in Figure 1. As shown in Figure 1a, the edge length of ZnCo-MOF-5 microcubes is between 5 and 10 µm. Figures 1b and c show that the surface of each microcube is encapsulated by numerous protrusion-like nanosheets. TEM images shown in Figure 1d illustrate that the ZnCo-MOF-5 microcubes are not solid, but consist of a number of nanosheets, which are different from the natural micropores in a typical MOF-5. Such an open structure is desirable to prepare for the sulfur host which can potentially tolerate the volume changes of sulfur species during cycling. More discussion will follow. These ZnCo-MOF-5 microcubes are well crystalized as shown in XRD analysis (Figure S1).43


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Figure 1. (a), (b) and (c) SEM images of ZnCo-MOF-5 microcubes; (insert: Model of ZnCo-MOF-5 microcubes); (d) TEM images of ZnCo-MOF-5 microcubes (insert: TEM images of ZnCo-MOF-5 microcubes). The as-synthesized hierarchical ZnCo-MOF-5 microcubes were then carbonized at 1000 °C under an Ar atmosphere at a heating rate of 3 °C min-1. After the carbonization, the precursors were converted to a composite consisting of Co and porous graphitic carbon (defined as Co@GC-PC). As shown in Figures 2a and b, the cubic structure of the ZnCo-MOF-5 microcubes remained after the carbonization treatment, but the nanosheets are transformed into packed nanoparticles with abundant macro/meso pores. TEM image (Figure 2c) also shows the existence of macro/meso pores as the white spots dispersed in the carbon frameworks. The black spots in Figure 2c are uniformly dispersed in the carbon frameworks of the Co@GC-PC and further characterized by HRTEM. Figure 2d shows the interval between the two lattice fringes is



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~ 0.20 nm, corresponding to the (111) interplanar distance of metallic Co and consistent with the SAED result (the inserted Figure in Figure 2d).20 A few layers of film can be seen on the Co nanoparticles. The lattice spacing of these layers is ~ 0.34 nm, corresponding to the (002) plane of graphitized carbon.20 The formation of graphitic carbon was attributed to the catalysis of Co during the carbonization process.35 The existence of metallic state of cobalt is also shown in XRD analysis. XRD pattern (Figure 2e) shows that all reflections of the powders after the heat treatment match with standard cobalt (JCPDS No. 15-0506). Besides, though Co nanoparticles are encapsulated by just few-layer carbon shells, an indistinct diffraction peak at 26.0° could be observed in the XRD pattern (Figure 2e), indicating the (002) reflection of graphitized carbon. Co@GC-PC with the conductive Co and graphene shells makes it a good electrical conductor, resulting

in

an

improved

polysulfide

redox

of

the

S/Co@GC-PC

cathode.44

Brunauer-Emmett-Teller (BET) analysis (Figure 2f) shows the specific surface area of Co@GC-PC was 724 m2 g-1, and the pore size was between 1 and 40 nm (centered at ~20 nm) as shown in Figure S2. The presence of a large surface area and adequate pores in Co@GC-PC is beneficial for Li-S cathode in terms of tolerating the volume expansion of sulfur species during electrochemical cycling, offering possible entrapment for intermediate polysulfides, and improving the electrolyte penetration in the electrode.15


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Figure 2. (a) SEM and (b), (c)TEM images of Co@GC-PC composite; (d) HRTEM images of Co@GC-PC composite (inset: corresponding SAED pattern); (e) XRD patterns of Co@GC-PC composite; (f) N2 adsorption/desorption isotherm and BET data of Co@GC-PC composite. To investigate the interactions between polysulfides and Co@GC-PC, we mixed the same mass of Co@GC-PC and AB with Li2S6 solution to test the adsorption of polysulfides. As shown in Figure S3a, initially, the colour for the mixture of Li2S6 and electrolyte is yellow. It, however, changed to colourless after the addition of Co@GC-PC due to the strong adsorption of porous carbon towards Li2S6, leading to the disappearances of yellow colour. On the contrary, after the addition of AB to the mixture of Li2S6 and electrolyte, the solution still exhibits a yellow colour, suggesting a weak adsorption of AB towards polysulfides. X-ray photoelectron spectroscopy (XPS) analysis also provided evidence on the interaction between polysulfides and Co@GC-PC by examining the specimens taken out from the Li2S6 solution after the polysulfide adsorption test. As presented in Figure S3b, the XPS spectrum of Co@GC-PC shows the spin orbit doublet



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of Co 2p3/2 and Co 2p1/2 at binding energy of 777.8 and 792.8 eV, respectively, corresponding to metallic Co.24 After the polysulfide adsorption test, the attenuated signal of Co 2p shows a low signal-to-noise ratio indicating the interaction between Li2S6 and Co nanoparticles. Moreover, the Co 2p peak shifted to a higher binding energy, indicating the formation of chemical bonds between Li2S6 and Co in the Co@GC-PC-Li2S6 composite. The Li 1s peak of pristine Li2S6 at ~ 54.4 eV became wider after contacting with Co@GC-PC, suggesting the interaction between Li2S6 and Co@GC-PC (Figure S3c). However, the signal of Li 1s of the AB after the polysulfide adsorption test can hardly be detected, indicating the negligible capability towards polysulfide adsorption (Figure S3d). S/Co@GC-PC was then obtained by a simple melt diffusion technique at 155 °C. XRD pattern of S/Co@GC-PC (Figure S4) indicates the coexistence of S and the metallic state of Co. To determine the actual S content in the S/Co@GC-PC, thermogravimetric analysis (TGA) was conducted (Figure S5). The S content in S/Co@GC-PC was about 70 wt%, which was consistent with the predetermined proportion. Though some cubes agglomeration could be seen in Figure 3a, the inset of Figure 3a and Figure 3b show the single cube of the S/Co@GC-PC which is similar to the Co@GC-PC composite, suggesting the morphology is well-preserved after S loading. Figure 3c shows that the Co metal nanoparticles are uniformly dispersed in the carbon frameworks of the S/Co@GC-PC. The lattice spacing of Co metal nanoparticles and the graphene shells could also be observed in S/Co@GC-PC (shown in Figure 3d). Moreover, the elemental analysis characterization revealed in Figures 3e-h shows uniform distributions of C, S and Co.


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Figure 3. (a) SEM image of S/Co@GC-PC composite (inset: SEM image of S/Co@GC-PC composite); (b) and (c) TEM images of S/Co@GC-PC composite; (d) HRTEM image of S/Co@GC-PC composite; (e) The HAADF-STEM image of the S/Co@GC-PC composite; Elemental mapping of C (f), Co (g), and S (h).

The electrochemical behaviour of S/Co@GC-PC for Li-S batteries was evaluated using the 2025 coin cells with S/Co@GC-PC cathode and lithium foil as anode. For comparison, AB was employed as the S host since it is a common conductive materials in batteries.44 Our initial research focused on the cycling capacity of an electrode, a fundamental property for the commercialized Li-S batteries. Figure 4a shows that the S/Co@GC-PC cathode exhibits a better capacity retention over long cycles than that of S/AB cathode. At the current rate of 0.2 C, the discharge capacity of S/Co@GC-PC cathode is stabilized at 790 mAh g-1 after 220 cycles, however, the S/AB cathode delivers a discharge capacity of 188 mAh g-1. Note that, the size of Co@GC-PC cubes is few micrometers, so the fully reaction of sulfur and lithium ion is a



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gradually dynamic process due to diffusion.45 However, during the subsequent cycles, more and more sulfur can react with lithium ion, thus exhibiting an obvious activation process, corresponding to the gradually increase of the reversible capacity.46 Similar phenomena can be found in previous literatures.24, 47As shown in Figure 4b, when a 0.5 C current rate is used (the ﬁrst two cycles were activated at 0.05 C), a similar trend can be seen. The S/Co@GC-PC cathode maintains a reversible capacity of 744 mAh g-1 after prolonged 100 cycles, as for S/AB cathode, the capacity drops to 558 mAh g-1. Figure 4c shows galvanostatic charge/discharge curves of S/Co@GC-PC cathode at different cycles within a voltage window of 1.8-2.6 V at 0.5 C. A typical discharge voltage plateau at ~2.3 V relates to the transformation of S8 to soluble long-chain polysulfides (Li2Sn, n≥4), and the one at ~2.1 V corresponds to the further reduction of soluble long-chain polysulfides to insoluble short-chain polysulfides (Li2Sn, n

A Novel Sulphur Host Comprised of Cobalt and ... - ACS Publications

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