Toward Theoretically Cycling-Stable Lithium–Sulfur Battery


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Towards Theoretically Cycling-Stable Lithium-Sulfur Battery Using a Foldable and Compositionally Heterogeneous Cathode Lei Zhong, Kai Yang, Ruiteng Guan, Liangbin Wang, Shuanjin Wang, Dongmei Han, Min Xiao, and Yuezhong Meng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13247 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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Towards Theoretically Cycling-Stable Lithium-Sulfur Battery Using a Foldable and Compositionally Heterogeneous Cathode Lei Zhong a, Kai Yang a, Ruiteng Guan a, Liangbin Wang a, Shuanjin Wang a, Dongmei Han b, Min Xiao a * and Yuezhong Meng a* a

The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province/State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China;

b

Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, P. R. China.

ABSTRACT: Rechargeable lithium-sulfur (Li-S) batteries have been expected for new-generation electrical energy storages, which are attributed to their high theoretical energy density, cost effectiveness and eco-friendliness. But Li-S batteries still have some problems for practical application, such as low sulfur utilization and dissatisfactory capacity retention. Herein, we designed and fabricated a foldable and compositionally heterogeneous three-dimensional sulfur cathode with integrated sandwich structure. The electrical conductivity of the cathode is facilitated by three different dimension carbons, in which short-distance and long-distance pathways for electrons are provided by zero-dimensional ketjen black (KB), one-dimensional activated carbon fiber (ACF) and two-dimensional graphene (G). The resultant three-dimensional sulfur cathode (T-AKG/[email protected]) with an areal sulfur loading of 2 mg cm-2 exhibits a high initial specific capacity, superior rate performance and a reversible discharge capacity of up to 726 mAh g-1 at 3.6 mA cm-2 with an inappreciable capacity fading rate of 0.0044% per cycle after 500 cycles. Moreover,

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the cathode with a high areal sulfur loading of 8 mg cm-2 also delivers a reversible discharge capacity of 938 mAh g-1 at 0.71 mA cm-2 with a capacity fading rate of 0.15% per cycle and a coulombic efficiency of almost 100% after 50 cycles. KEYWORDS: lithium-sulfur battery, 3D cathode structure; cycling stability; fordable cathode; high sulfur loading

INTRODUCTION Numerous emerging applications (such as portable devices, electric vehicles and grid electrical storage) eagerly expect efficient and low-price electrical energy storage (EES) systems. Li-S battery is one of the best candidates as new-generation electrical energy storage owe to its marvellous specific capacity of 1675 mAh g-1 and breathtaking energy density of 2600 Wh kg-1. Moreover, elemental sulfur shows considerable advantages including resourceful, cost-effective, non-toxic, and eco-friendly.1-5 But Li-S battery is still hindered by some challenges, which mainly include the poor intrinsic electrical conductivity of both sulfur and final discharge products (Li2S and Li2S2), the large volumetric change during cycling (~80%), and the dissolution of lithium polysulfides (LiPSs, Li2Sn,

n>2)

intermediates in the organic

electrolyte solution,6-8 which results in the low sulfur utilization, irreversible loss of active sulfur, inferior capacity retention and unsatisfied coulombic efficiency during cycles.9-11 Considerable approaches have been devoted to solve all the above mentioned issues, which were explored to enhance the electrical conductivity of sulfur and

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prevent the dissolution and diffusion of LiPSs into the organic electrolyte. Various porous carbons have been used as the hosts for sulfur based cathode materials,12-16 which can effectively trap the LiPSs by physical/structural adsorption.

17

Carbon

materials of zero-dimensional (0D) meso-/micro porous or hollow carbons such as spheres with high surface areas,18-20 one-dimensional (1D) carbon nanotubes21 or nanofibers,22 and two-dimensional (2D) graphene sheet/paper23-24 have been developed. Recently, much progress has been made in further improving the specific capacity and stability of the sulfur cathode, and increasing the sulfur loading and exploiting new cathode structures. For example, many researchers continue to explore various new carbon materials. Guo et al.25 discussed a graphitic carbon nanocage serving as highly efficient mini-electrochemical nano-reactors as well as LiPSs reservoirs, and the electrode based on this material delivers an initial specific capacity of 900 mA h g-1 and remains a capacity of 706 mA h g-1 after 1000 cycles at 1 C. Lou et al.26 prepared a titanium [email protected] hollow sphere to enhance the conductivity of the sulfur cathode, meanwhile moderate the dissolution of LiPSs. Li et al.27 disclosed a carbon-cotton cathode with super-high sulfur loading and content (21.2 mg cm-2, 74 wt%) by simple carbonization process. Meanwhile, Manthiram et al.28 also demonstrated a similar work using carbon-cotton as current collector for ultrahigh sulfur-loaded Li-S batteries (61.4 mg cm-2 in sulfur loading, 80 wt% in sulfur content). On the other hand, many efforts have been paid to the structure design of the electrode. Both Li et al.29 and Guo et al.30 have presented the integrated sulfur

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cathodes with good electrochemical performance and cycling stabilities, in which cathode materials were combined with polypropylene separator together. Stable discharge capacities of 722 mAh g-1 over 200 cycles under 0.34 A g-1, and of 730 mAh g-1 for 500 cycles at 0.6 C were achieved, respectively. Guo et al.31 reported a three-dimensional (3D) nanocarbon-sulfur network cathode combining three different nano-carbons. The flexible energy storage systems are imperative for emerging flexible devices that are revolutionizing our life. In respect of Li-S batteries, foldable sulfur cathode materials are the first requisite for fabricating flexible soft-packaged Li-S batteries. In addition, poor flexibility of traditional Al-foil based sulfur containing cathodes limits their application in establishing foldable Li-S batteries with a high capacity.32 Many approaches recently have been done to address this problem. So, foldable and free-standing electrodes33-40 have been attracting increasingly more attentions. Previous studies gave us numerous inspirations (such as simplifying the manufacturing process, reducing the cost of materials, and controlling sulfur loadings) to further improve the performance of Li-S batteries. However, the high loading of insulating sulfur inevitably entail low specific capacities, poor C-rate properties, and inferior cycling stabilities. Consequently, rational designs and fabrications of the high conductive network structure cathode to meet the requirement of the different sulfur loading and sulfur contents are still in great needs for excellent high performance Li-S batteries. In one of our previous work on a self-standing and flexible cathode material consisting of a 3D activated carbon fiber host matrix and active sulfur,41 we

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demonstrated a high specific capacity with good cycling stability due to the continuous conductive framework of the activated carbon fiber matrix (ACFC). Whereas, this cathode structure is monotonous and difficult to be tailored, which leads to a low sulfur content in whole cathode. Moreover, we have reported a strategy of a chemically heterogeneous proton exchange membrane with sandwich structure for fuel cells application.42 Encouraged by these ideas, we propose here a foldable and compositionally heterogeneous sulfur cathode with sandwich structure, which is fabricated by a simple process. Owing to the incorporative introduction of the carbon fiber and graphene with a robust mechanical strength, the as-obtained composites can be easily tailored. To achieve the best performance, three different carbons are used to enhance the electrical conducting network and provide the adjustable areal sulfur loading. Consequently, this cathode can effectively satisfy the requirements of various sulfur loading by adjustment of reasonable design for outstanding performance Li-S batteries. The sandwich structure composite cathodes with various areal sulfur loadings of 2-8 mg cm-2 demonstrated high specific capacities, superior rate performances and cycling stabilities.

RESULTS AND DISCUSSION Physical and chemical characterization To obtain nano-scale sulfur particles containing inner conductive fillers, the [email protected] particle was prepared by a deposition method similar to the literatures43, 44 with some modifications. The simulated diagram of the synthesis processes of [email protected]

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is presented in Figure 1a. The active sulfur is favourably entrapped in the abundant mesoporous

structure

of

KB.

As

depicted

in

Figure

S1a,

the

BET

(Brunauer-Emmett-Teller) specific surface area of KB reaches 1278.6 m2 g-1 with a high pore volusme of 3.0 cm3 g-1 and a pore size distribution ranging from 2 to 5 nm, which enable the accommodation of large amount of sulfur. The sulfur content of the as-prepared [email protected] composite was confirmed by TGA (thermogravimetric analysis) and was found to be as high as 90% (Figure 1b). Compared with the pristine KB, [email protected] composite particles possess a low surface area of only 9.4 m2 g-1 (Figure S1b1) and a negligible pore volume of 0.06 cm3 g-1 (Figure S1b2), suggesting that some sulfur particles have deposited into the pores of KB during the preparation procedure. SEM (Figure S2) and TEM (Figure 1a) images show that the rough surface of KB becoming smoother and the particle size increases from about 20 nm to about 40-50 nm after sulfur loading. This can be attributed to the precipitation of extra sulfur on the surface of KB particles. The existence of both surface sulfur and pore-confined sulfur can be further confirmed by DTG curve (Figure 1b), which shows a fluctuation at near 350 oC, indicating two-step releasing of sulfur anchored on the outside surface or encapsulated in the porous structure of KB particles. The diffraction peaks of crystallized sulfur can be clearly observed from the XRD patterns of [email protected] as shown in Figure 1c.

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Figure 1 (a) Schematic illustration of synthesis process of [email protected] and high-resolution TEM images of the corresponding KB and [email protected] nanoparticles and (b) TGA and DTG curves of the [email protected] and (c) XRD patterns of sulfur, KB, and [email protected]

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Figure 2 Simulated diagram of the structure of T-AKB/[email protected] cathode, and schematic illustration of the LiPSs trapping process by ACF and KB in T-AKG/[email protected] cathode during charge/discharge cycling. Figure 2 illustrates the sandwich structure of the 3D network cathodes of T-AKG/[email protected] obtained by successively vacuum suction filtration of the dispersions of AKG, AKG/[email protected] and again AKG. All the samples of foldable 3D cathodes can be easily taken off from the filter paper. As shown in Video S1, the 3D cathode exhibits good flexibility and favourable mechanical durability subjecting to coiling with glass rod (diameter ~4 mm). The soft packing Li-S battery with as-fabricated 3D foldable cathodes can readily light up the LED lamps (contains 32 LEDs) even after bending nearly 180° deformation. (Figure S3). Its excellent flexibility is due to the high length-diameter ratio (>70) of ACF. As seen in Figure S1c, ACF has high specific surface area (320.5 m2 g-1) and plenty of micropores (0.25 cm3 g-1). Elemental

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analysis proves that ACF contains 4.78% of nitrogen as listed in Table S1. The presence of nitrogen can improve the electronic distribution of carbon conductive network and increase the chemical adsorption of the soluble LiPSs (Li2S4-8).45,

46

Furthermore, porous carbon materials are understood to have the ability of catching Li2S4-8. The shuttle effect of Li2S4-8 is expected to be suppressed by a synergistic effect of chemical adsorption of nitrogen element, physical adsorption of porous carbon materials and rational design of 3D conductive network with three layers of AKG material (labels as T-AKG). Based on the above analysis, T-AKG network can equalize the electron distribution and keep the Li2S4-8 within the cathode to enhance the utilization of sulfur. A schematic illustration of sulfur reduction and Li2S6-8 trapping process in T-AKG is showed in Figure 2. In order to reveal the Li2S6-8 adsorption ability of T-AKG, T-AKG of 20 mg was added to the Li2S6-8 (40 µL, 0.1mol L-1) solution (DOL/DME, 1:1 (vol./vol.), 5 mL) for 3-6 h as shown in Figure 3. The color changed from golden to colorless with adsorption time increasing, demonstrating the obvious adsorption ability of T-AKG

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Figure 3 Static adsorption of Li2S6-8 and UV-vis absorption spectra of the Li2S6-8 solution before and after being adsorbed by T-AKG.

for Li2S6-8. The UV-vis absorption spectra of the Li2S6-8 solutions (Figure 3) clearly present the concentration changes of Li2S6-8 upon T-AKG adsorption. The characteristic absorption peaks of S82-/S62- between 260 and 310 nm47,

48

almost

disappear after 6 h adsorption. In this sense, The UV-vis results further suggest that the T-AKG can intercept the migration of Li2S6-8 to some extent. Meanwhile, the cell with areal sulfur loading of 2 mg cm-2 after 450 cycles at 3.6 mAh cm-2 (1 C) was disassembled and no obvious yellow-colored separator and only slight anodic corrosive behavior was observed (Figure S4), reconfirming the merit of T-AKG in confining the polysulfide. A series of 3D network electrodes (T-AKG/[email protected]) with different sulfur loadings

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(2-8 mg cm-2) were prepared by changing the amount of [email protected] (28-112 mg). T-AKG/[email protected] cathodes have heterogeneous structure due to successive suction filtration of AKG suspension, AKG/[email protected] dispersion and again AKG dispersion. Consequently, in as-prepared T-AKG/[email protected] cathodes, the [email protected] is mainly distributed in the middle layer of the cathodes (Figure S5a). However, after several charge-discharge cycles, the sulfur in T-AKG/[email protected] partly spreads out and becomes well uniform (Figure S5b).

Figure 4 Performances of the T-AKG/[email protected] cathode with a sulfur loading of 4 mg cm-2: (a) Discharge-charge profiles at 0.71 mA cm-2, (b) C-rate performance at 0.36-7.1 mA cm-2, and (c) cycling performance at 0.71, 2.13 and 3.55 mA cm-2.

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The electrochemical performances of the T-AKG/[email protected] cathodes with various sulfur content were subsequently performed. Figure 4 reveals the performance of the T-AKG/[email protected] with a controlled sulfur loading of 4 mg cm-2 in the voltage range of 1.7-2.8 V. The discharge curve (Figure 4a) clearly shows two plateaus (located at 2.3, 2.1 V), which corresponds to the two step reduction reaction of sulfur including S8 to Li2S4-8, and short-chain LiPSs (Li2S2 and Li2S) during the discharge process.15 One long oxidation plateau of 2.25-2.45 V only appears in the charge curve, which is corresponding to the formation of Li2Sn (n > 2) and the ultimate oxidation to S8. In addition, an increase of reversible capacities (from 3 to 50 cycles) is observed, which might be due to the decrease of polarization after activate cycles. Figure4b presents the C-rate performances of the cathode. When cycled at the current density from 0.36 to 7.2 mA cm-2, the discharge capacities of T-AKG/[email protected] decreases from 1261 mAh g-1 to 478.9 mAh g-1. However, it recovers to around 1000 mAh g-1 when the current density of charge-discharge switched back to 0.71 mA cm-2 and then keeps at about 995.8 mAh g-1 for another continuous 60 cycles, indicating the good rate performance of T-AKG/[email protected] cathodes. Figure 4c displays the cycling performance of T-AKG/[email protected] cathode at various current densities. It delivers a discharge capacity of 893 mAh g-1 at 0.71 mA cm-2 followed by activation and its capacity remains 884 mAh g-1 after 100 cycles with a very low capacity decay rate of 0.01% per cycle. When cycling at 2.13 mA cm-2/3.55 mA cm-2, T-AKG/[email protected] cathode delivers a capacity up to 767 mAh g-1/730 mAh g-1 with a capacity retention of 89.4%/87.9%

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upon 100 cycles. Obviously, T-AKG/[email protected] cathode with sulfur loading of 4 mg cm-2 exhibits prominent rate performance and cycling performance at various current rates.

Figure 5 (a) Cycling performance at 1.33 mA cm-2 of the electrode with a sulfur loading of 6 mg cm-2, (b) cycling performance at 0.71 mA cm-2 of the electrode with a sulfur loading of 8 mg cm-2 and (c) rate performances. Moreover, the cathodes with higher areal sulfur loading of 6 and 8 mg cm-2 also reveal excellent performances. The cathode with areal sulfur loading of 6 mg cm-2 demonstrates a reversible capacity of 1086 mAh g-1 and still holds a capacity of 939 mAh g-1 after 100 cycles at 1.33 mA cm-2 (Figure 5a). Meanwhile, the cathode with high areal sulfur loading of 8 mg cm-2 shows the discharge capacity up to 865 mAh g-1 over 50 cycles at a current density of 0.71 mA cm-2 (Figure 5b). Additionally, it also reveals good C-rate (0.36-3.54 mA cm-2) performances, as depicted in Figure 5c,

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the battery capacity is ~668 mAh g-1 at 3.55 mA cm-2, and the capacity returns to ~948 mAh g-1 as the current density of charge-discharge comes return to 0.71 mA cm-2.

Figure 6 Long-term cycling performance of the T-AKG/[email protected] cathode with an areal sulfur loading of 2 mg cm-2 at 3.6 mAh cm-2 (1 C). In addition, multi-rate and long-term cycling of T-AKG/[email protected] cathode with a sulfur loading of 2 mg cm-2 was performed to investigate its rate capability and cyclability (Figure S6 and Figure 6). The electrode affords a highly reversible capacity of 639 mAh g-1 at a high rate up to 7.2 mA cm-2 (2 C) and recovers to around 1095 mAh g-1 when switched back to 0.1 C, and then upon cycling at 0.5 C, a low capacity fading rate of 0.03% per cycle over 500 cycles was revealed. Moreover, the electrode exhibits outstanding cycling stability with a negligible capacity fading rate of 0.0044% per cycle after 500 cycles at 3.6 mA cm-2 (1 C), demonstrating its fast reaction kinetics and excellent electrode integrity.

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It should be pointed out that the cyclability of T-AKG/[email protected] cathodes are much better than most recently reported sulfur electrodes based on carbon fiber as conductive framework in lithium sulfur batteries (Supporting Information, Table S2). The stable redox behavior of T-AKG/[email protected] cathode was further discussed by CV measurements. Two emblematic reduction peaks at around 1.89 V and 2.26 V can be clearly observed from Figure 7a, which are assigned to the formation of the soluble long-chain (Li2S4-8), and insoluble short-chain (Li2S2 or Li2S), respectively. The overlapped oxidization peaks at 2.43 V and 2.52 V are attributed to the oxidization of Li2S1-8 to S8. It is in good agreement with discharge-charge plateau from Figure 4a during the lithiation and delithiation process. It should be noted that the broad peak between 1.5 and 1.8 V in first cycle is presumably due to the irreversible reduction of LiNO3 electrolyte additive.46 Then sulfur is redistributed after activation in first cycle due to the gradually infiltration of electrolyte resulting in a smaller polarization of T-AKG/[email protected] The oxidation peaks gradually shift to low voltage and the reduction peaks slightly shift towards high voltage, which leads to the reduced peak potential difference. Moreover, there is no obvious change in peak current. These results demonstrate a slight polarization and eminent reversibility of T-AKG/[email protected] EIS results were carried out to further explore the reaction kinetics of T-AKG/[email protected] cathode with sulfur loading of 4 mg cm-2 for the fresh cell and the one after 10 cycles at 0.1 C (0.71 mA cm-2). Figure 7b shows the Nyquist plots of the cathodes. There exist a semicircle and short inclined line at the high frequency and low frequency region, respectively. The higher frequency represents (intercept at axis)

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Figure 7 (a) Cyclic voltammetry curves of the electrode with a sulfur loading of 4 mg cm-2 and (b) Nyquist plots before cycle and after 10 cycles. is ohmic resistance (Ro), generally attributed to the intrinsic electrolyte and electrode impedance, the semicircle appeared on high-to medium frequency region is mainly on account of the charge-transfer impedance (Rct).49 The straight line appeared is Warburg impedance (Wo), which currently contributed to the diffusion process of Lithium ion.50 An appropriate equivalent circuit model is used to fit these the Nyquist plots. As depicted in Figure 7b and Table S3, the Ro of T-AKG/[email protected] cathode has no distinct change before and after cycling. Nevertheless, the Rct significantly decreases after cycling, indicating that Rct is the predominant parameter in the Rtotal of Li-S batteries.51 This suggests that the redistribution of sulfur within T-AKG/[email protected] cathodes after cycling results in a lower resistance of charge transfer, and thus observably boosts the electrons/ions transport. Consequently, the reaction kinetics of T-AKG/[email protected] electrode can be well enhanced achieved. This also interprets the reason for the increase in reversible capacities and the decrease in polarization during the first dozens of cycles as shown in Figure 4a,-4c.

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CONCLUSIONS We have demonstrated a rational, simple method for design and controllable fabrication of three-dimensional, free-standing and foldable cathodes (T-AKG/[email protected]) using three different carbons (KB, ACF and G) and nanocarbon-sulfur ([email protected], 90 wt% S) but without any additional binder. The T-AKG/[email protected] cathodes show heterogeneous sandwich structure by a simply step by step vacuum filtration method, the areal sulfur loading can be readily tailored by changing the amount of [email protected] in the middle layer. The 3D structural cathode not only has the continuity of “point-line-plane” conductive network constructed by three kinds of carbon materials, but also encapsulates LiPSs within the cathode due to high specific surface area and plenty of micropores of ACF, KB, and N-doping. Consequently, this 3D cathode with an areal sulfur loading of 4 mg cm-2 exhibits a high initial specific capacity, superior rate performance, and a reversible discharge capacity of up to 900 mAh g-1 at a high current density of 0.71 mA cm-2 with good capacity retention of 99% over 100 cycles.

METHODS Preparation of [email protected] composite On the basis of a modified deposition method,43, 44 0.1 g of ketjen black (KB, Lion Corporation, Japan), 1.8 g of sulfur (Aladdin) and 50 mL of ethanediamine (EDA) (Aladdin) were firstly mixed in a beaker with stirring for 20 min. Subsequently, an 8% HNO3 (Aladdin) aqueous solution was slowly added into the mixture until the pH approaching 6-7 to precipitate S from its complex with EDA. After continuously stirring for 2 h, [email protected] composite was obtained by filtration and repeatedly washing

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with water followed by drying at 50 oC for 24 h.

Preparation of T-AKG/[email protected] composite cathodes The activated carbon fibers (ACF, with 0.5-1 mm in length and 7 µm diameter purchased from LISO, Shanghai) were further treated and modified to obtained nitrogen-doped ACF on the basis of the previously reported procedure.41 20 mg of graphene (G, Aladdin) were evenly dispersed in 10 mL NMP (Aladdin) by ultrasonication to get G/NMP dispersion (2 mg mL-1). For three 20 mL bottles, the mixture of 24 mg ACF, 14 mg KB and 1 mL G/NMP were added. For each of the three bottles, different amount of [email protected] (28, 56, 84 and 112 mg) was introduced in order to obtain different cathodes with varying sulfur loadings (2 to 8 mg cm-2). Thereafter, 5 mL of 0.5% sodium alginate aqueous solution (as a dispersant) was charged into the above bottles, respectively, and stirring for 30 min. The 3D network cathodes of T-AKG/[email protected] were fabricated by successively vacuum suction filtration of the different dispersions of ACF+KB+G (AKG), [email protected] (AKG/[email protected]) and again ACF+KB+G (AKG). In addition, 3D conductive network material of T-AKG without sulfur was also prepared by the same method, and all of samples were dried at 50 oC for 24 h before testing. Characterization of materials X-ray diffraction (XRD) was conducted by a Bruker DX-1000 diffractometer with Cu Kα radiation. Thermal gravimetric analysis (TGA) was carried out with a PerkinElmer Pyris Diamond TG/DTA thermal analyzer under nitrogen protection at a

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heating rate of 10 oC min-1 from 30 to 600 oC. Element analyses (EA) was performed on the instrument of element analyser (Elementar Analysensysteme GmbH). The microstructure and morphology of the cathode were observed by scanning electron microscopy (FE-SEM, HITACH S4800) equipped with an energy dispersive spectrometer

(EDS)

and

transmission

electron

microscopy

(TEM,

JEOL

JEM-2010CHR, at 200 kV). The specific surface area (SSA) was analyzed based on Brunauer-Emmett-Teller (BET) and the pore size distribution was derived from the adsorption branches of isotherms using the Barrett-Joyner-Halenda (BJH) model.

Cell assembling and electrochemical measurements Cathodes were first cut into circular disks with Ф12 mm. CR2032 coin cells were assembled in an Ar-filled glove box with lithium foil as anodes, Cellgard 2500 film as separator, and 1 M LiTFSI and 5 wt% LiNO3 solution in DME (dimethoxymethane) and DOL (1, 3-dioxolane) (1:1, by volume) as electrolyte. The amount of electrolyte was about 80-100 µL for per T-AKG/[email protected] cathode, Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were recorded by Solartron 1255B frequency response analyzer coupled with a Solartron 1287 electrochemical interface at a scan rate of 0.01 mV s-1 from 1.5 to 3.0 V versus Li+/Li and in the frequency range between 0.1 Hz to 100 MHz with an applied voltage amplitude of 5 mV, respectively. Measurements of charge/discharge were carried out in a voltage window of 1.7-2.6 V at various current densities and at room temperature by a Land battery tester. A soft packing Li-S battery was also assembled in a glove box using

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T-AKG/[email protected] cathode with a controlled sulfur loading of 4 mg cm-2. ASSOCIATED CONTENT Supporting Information Nitrogen adsorption-desorption isotherms and pore size distribution of the KB, [email protected] and ACF; SEM images of KB and [email protected];the as-fabricated soft-packaged Li-S battery; photographs of cathode, separator and anode of the cell after cycles; SEM images of T-AKG/[email protected] cathode and corresponding elemental mapping of carbon and sulfur; C-rate performance of T-AKG/[email protected] cathode with a sulfur loading of 2 mg cm-2; elemental composition of ACF; areal loading and specific capacities in high-loading Li-S battery cathodes in previously reported works; flexibility of T-AKG/[email protected] cathode shown in Video S1. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (Y. Z. Meng) or [email protected] (M. Xiao)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors would like to thank the Link Project of the National Natural Science Foundation of China and Guangdong Province (Grant No. U1301244); the National Natural Science Foundation of China (Grant No. 51573215, 21506260); Guangdong Province Sci & Tech Bureau Key Strategic Project (Grant No. 2016B010114004); Guangdong Natural Science Foundation (2014A030313159, 2016A030313354); and

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Guangzhou

Scientific

and

Technological

Planning

Project

(2014J4500002,

201607010042, 2016A050503001) for financial support of this work.

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257x176mm (150 x 150 DPI)

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