Dual-functional Graphene Carbon as Polysulfide Trapper for High

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Dual-functional Graphene Carbon as Polysulfide Trapper for High Performance Lithium Sulfur Batteries Linlin Zhang, Fang Wan, Xinyu Wang, Hongmei Cao, Xi Dai, Zhiqiang Niu, Yijing Wang, and Jun Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18894 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Materials & Interfaces

Dual-functional Graphene Carbon as Polysulfide Trapper for High Performance Lithium Sulfur Batteries Linlin Zhang1, Fang Wan1, Xinyu Wang1, Hongmei Cao1, Xi Dai1, Zhiqiang Niu1*, Yijing Wang1* and Jun Chen1,2 1 Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) College of Chemistry, Nankai University, Tianjin 300071, P. R. China 2 Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, P. R. China *E-mail: [email protected]; [email protected] KEYWORDS: lithium sulfur battery; oxygen-doping carbon; separator; synergetic effect; superior cycling stability.

ABSTRACT. Lithium sulfur (Li-S) battery has attracted much attention due to its high theoretical capacity and energy density. But its cycling stability and rate performance are urgent to improve because of their shuttle effect. Herein, oxygen-doped carbon on the surface of reduced graphene oxide (labeled as ODC/rGO) was fabricated to modify the separators of Li-S batteries to limit the dissolution of the lithium polysulfides. The mesoporous structure in ODC/rGO can not only serve as the physical trapper, but also provide abundant channels for fast

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ion transfer, which is beneficial for effective confinement of the dissoluble intermediates and superior rate performance. Moreover, the oxygen-containing groups in ODC/rGO are able to act as chemical adsorption sites to immobilize the lithium polysulfides, suppressing their dissolution in electrolyte to enhance the utilization of sulfur cathode in Li-S batteries. As a result, owing to the synergetic effects of physical adsorption and chemical interaction to immobilize the soluble polysulfides, the Li-S batteries with ODC/rGO-coated separator exhibit excellent rate performance and good long-term cycling stability with 0.057% capacity decay per cycle at 1.0 C after 600 cycles.

1. Introduction Lithium sulfur (Li-S) battery has attracted great interests due to its high theoretical capacity (1675 mAh g-1) and high energy density (2600 Wh kg-1, more than five time of intercalation cathodes).1-8 In addition, the active sulfur in the cathode possesses conspicuous advantages such as low cost, nature abundance and environmental friendliness. However, some issues still hinder the practical application of Li-S batteries. For example, the poor conductivity of sulfur and lithium sulfide leads to the sluggish electrochemical kinetic process. Besides, long-chain lithium polysulfides are soluble in the organic electrolytes, leading to the low utilization of active sulfur materials. Furthermore, the migration of polysulfide intermediates between cathode and anode would also lead to side reaction (corrosion behavior with metal lithium anode), accompanying by severe capacity decay as well as low Coulombic efficiency. Recently, various strategies have been developed to solve these challenges for improving the performance of Li-S batteries, such as design of sulfur cathode hosts,9-17 preparation of unconventional electrolytes,18-20 and the modification of separators.21-30 Among these strategies, the modification of separators not only guarantees the unimpeded lithium ion transport but also


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blocks the dissolution of the polysulfides into electrolyte, further improving the utilization of active sulfur cathode in Li-S batteries.31-34 Two mechanisms including physical adsorption and chemical interaction are introduced into Li-S batteries to explain why the modified separator could mitigate the dissolution of polysulfide during electrochemical process. In the case of physical polysulfide trapper, various kinds of carbon materials with porous structure (such as reduced graphene oxide, carbon nanotubes and other carbon-based materials) are developed to modify the separator of Li-S batteries due to their large specific surface area, which provides much polysulfide trappers and abundant ion transfer channels.35-40 In addition to the common physical adsorption, chemical interaction between polysulfide intermediates and polar metal compounds41-46 is also applied in Li-S batteries to sufficiently restrict the shuttle effects. Therefore, the desired separator with synergetic effect combining chemical bonding and physical adsorption would further improve the effective restriction of soluble polysulfides, enhancing the utilization of active sulfur cathode in Li-S batteries. Currently, some strategies have been developed to achieve the synergetic effect by designing nanostructured metal oxides or conductive polymer and their composites with nanocarbon materials to suppress the shuttle phenomenon of Li-S batteries.47-54 But the low electron transport and the additional weight of these composites would decrease the energy density of the Li-S batteries. In contrast, heteroatom-doped carbon can overcome these issues.55-59 Recently, nitrogen-doped, sulfurdoped, or boron-doped carbon has been reported to mitigate the shuttle effect of Li-S batteries since the enhanced chemical interaction with polysulfides besides the physical adsorption.60-63 However, their complicated preparations (such as template method or extra chemical activation) still hinder its extensive use in Li-S batteries.64-65 Therefore, an in-situ oxygen-doped carbon with mesopores would be considerable for high performance of Li-S battery, which could


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effectively suppress the shuttle effect by synergetic effects combining physical adsorption and chemical interactions with polysulfides. Herein, we have fabricated an oxygen-doped carbon layer with hierarchical architecture on the surface of reduced graphene oxide (defined as ODC/rGO) by a facile and template-free approach. The hierarchical structure in ODC/rGO not only provides short ion transfer paths but also acts as the physical adsorption trapper for soluble polysulfides. In addition, the simultaneous introduce of oxygen atom could also anchor polysulfide intermediates through strong chemical affinity, effectively mitigating the severe shuttle phenomenon. As a consequence, in view of the synergetic effect of the physical and chemical interactions with lithium polysulfides, the battery with ODC/rGO-coated separator exhibits a high initial capacity and superior long-term cycling stability. More importantly, even with large sulfur loading of 4.0 mg cm-2, it also displays stable electrochemical performance. 2. Experimental section 2.1 Preparation of ODC/rGO The aqueous graphene oxide (GO) dispersion was prepared by a modified Hummer’s method.66 ODC/rGO was prepared as followed: 1.5 g tartaric acid and 0.75 g sodium borate were added into 20 mL deionized water. After that 16.6 mL GO (2.0 mg mL-1) was dropwise into the above solution. The mixture solution was continuously stirred, dried and then heated at 600 °C for 2 h under Ar atmosphere. Then the obtained powder was washed with 1 M HCl by deionized water. After that, the black powder was dried under ambient conditions to achieve the ODC/rGO. The contrast experiments are prepared by the similar process (TB is obtained from the calcination mixture of sodium borate and tartaric, T from the calcination of tartaric acid, B/rGO


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from calcinating the mixture of graphene oxide and sodium borate, rGO from the calcination of graphene oxide). SEM images of these composites are shown in Figure S1. 2.2 Preparation of ODC/rGO-coated separator and sulfur cathode The ODC/rGO-coated separator was prepared by mixing the ODC/rGO, super P and polyvinylidene fluoride (PVDF) (mass ratio as 8:1:1) on glass fiber and then dried at 80 °C under vacuum. The sulfur content in the electrode is measured by TGA, as shown in Figure S2. The loading of ODC/rGO is about 0.50 mg cm-2. The sulfur electrode is prepared by mixing sulfur power, super P, and PVDF (7:2:1) on aluminum foil and dried at 60 °C in a vacuum oven. The electrolyte/sulfur ratio is 25 µl mg-1. 2.3 Preparation of Li2S6 solution: Li2S6 solution Li2S6 solution was prepared by dissolving stoichiometric amounts of sulfur and Li2S in Li-S electrolyte. After that, the mixed solution was then stirred and heated at 60 ºC overnight in an Ar-filled glove box, yielding the brown Li2S6 solution. 2.4 Electrochemical measurements and characterization Li-S batteries were assembled in an Ar-filled glove box (Mikrouna Universal 2440/750) with Li metal foil, ODC/rGO-coated separator, and sulfur as anode, separator, and cathode, respectively. The electrolyte is 1 M lithium bis-(trifluoromethanesulfomyl) imide in 1,3dioxolane and 1,2-dimethoxyethane (1:1,v/v) with 1wt% LiNO3 addition. The morphologies were characterized by field-emission SEM (JEOL JSM7500F) and TEM (JEOL 2100F). Raman spectra were recorded using a confocal Raman microscope (DXR, Thermo Fisher Scientific) with a 532 nm excitation from an argon-ion laser. The surface chemical composition was performed by X-ray photoelectron spectrometry (XPS) (Perkin Elmer PHI 1600 ESCA) with the bonding energy based on C 1s at 285 eV. The mass fraction of sulfur in electrode was tested


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using a TG-DSC analyzer (NETZSCH, STA 449 F3) from room temperature to 800 °C with a heating rate of 5 ºC min−1 in nitrogen atmosphere. Electrochemical impedance spectroscopy measurement was carried out in a frequency range from 100 kHz to 10 mHz with voltage amplitude of 5 mV (Zahner IM6ex). Nitrogen adsorption isotherms were carried out at 77K using a micromeritics ASAP 2020 analyzer. The adsorption experiment was performed by a Cary 60 UV-vis spectrophotometer (Agilent Technologies, USA). 3. Results and discussion Figure 1a schematically illustrates the experimental process of preparing the ODC/rGO. Typically, aqueous GO solution was first added into the aqueous mixture of tartaric acid and sodium borate dropwise. Owing to the electrostatic interaction between positive and negative charges, the formed boric acid and sodium tartrate would homogeneously adsorb on the surface of GO sheets. After that, the mixed solution was dried and then calcinated at 600 °C for 2 h under an argon atmosphere, obtaining the flake-like ODC/rGO with porous structure. During the calcination, carbon dioxide/monoxide gas and water molecule were released from the decomposition of sodium tartrate and boric acid. As a result, abundant mesopores were obtained in carbon layer on the surface of rGO.67-68 At the same time, resultant carbon with mesopores was doped by the oxygen atoms, which is derived from the decomposition of sodium tartrate. The oxygen-doping carbon with mesopores was supported by rGO layer, which was converted from GO sheets. The as-prepared ODC/rGO shows a flake-like and porous structure (Figure 1b), where micropores with 5-10 µm sizes are uniformly distributed in ODC/rGO. The wall of the micropores consists of the rGO sheets, which is covered with amorphous carbon layer, as displayed in Figure 1c. Furthermore, it is noted that a large amount of mesopores are distributed


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on amorphous carbon (Figure 1d). The mesoporous structure of ODC/rGO is also confirmed by its N2 adsorption-desorption isotherm with a type-IV isotherm and a H3 hysteresis loop in the middle relative pressure region (Figure 1e). The ODC/rGO possesses a large pore volume of 1.12 cm3 g-1 and a specific surface area of 432.5 m2 g-1, which is larger than other composites (Table S1). The abundant mesopores in flake-like structure provide fast ion transfer channels and abundant physical trappers for polysulfides during Li-S batteries operation, effectively improving the utilization of active sulfur in cathode even at rapid electrochemical process. The homogeneous distributions of carbon and oxygen were further verified by elemental mappings, as shown in Figure S3. The oxygen functional groups will effectively bond with long-chain polysulfide, as a result, ODC/rGO coating separator would be beneficial for restricting the dissolution of polysulfides and good cycling stability of the Li-S batteries.69 The distinct D band at 1326 cm-1 and G band at 1585 cm-1 of ODC/rGO in Raman spectrum (Figure 2a) are generally ascribed to local defects and ordered graphitic carbon, respectively. The polar oxygen containing groups is negative in comparison with the adjacent carbon atom, which provide chemical bonding sites for polysulfides.70 Therefore, the ODC/rGO will anchor the lithium ion and polysulfide anions simultaneously through electrostatic interaction, suppressing the dissolution of the lithium polysulfides into electrolyte and further diffusion to lithium metal anode. X-ray photoelectron spectroscopy (XPS) was also measured to illustrate the surface chemical component of ODC/rGO (Figure 2b). The C 1s spectrum of ODC/rGO is divided into two peaks centered at 284.8 and 285.0 eV (Figure 2c), ascribing to C-C and C-O functional groups, respectively. The high resolution XPS spectrum of O 1s (Figure 2d) is fitted as a sum of three peaks, assigning to -OH, -O-C=O and -C=O groups located at 536.5, 533.6, and 531.9 eV, respectively. Accompanying with the adsorption of negative polysulfide ions, the ODC/rGO


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would be electronegative and further hinder the diffusion of polysulfides to metal Li anode through electrostatic interaction, resulting in the confined shuttling effect in Li-S batteries. The ODC/rGO-coated separator instead of conventional separator (glass fiber) was assembled in the coin-type Li-S batteries to explore its electrochemical performance. Cyclic voltammogram (CV) was performed in a voltage window of 1.7-2.8 V at a scan rate of 0.1 mV s-1. Two distinct discharge plateaus at around 2.4 and 2.1 V (Figure 3a) correspond to the reduction reaction of element sulfur to soluble long-chain lithium polysulfide (Li2Sn, 4

Dual-functional Graphene Carbon as Polysulfide Trapper for High

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