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A nickel-based hydroxide wrapped activated carbon cloth/sulfur composite with tree-bark-like structure for high performance freestanding sulfur cathode Zhen Meng, Shunlong Zhang, Jianli Wang, Xufeng Yan, Hangjun Ying, Xin Xu, Wenkui Zhang, Xianhua Hou, and Wei-Qiang Han ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00002 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Applied Energy Materials

A Nickel-Based Hydroxide Wrapped Activated Carbon Cloth/Sulfur Composite with Tree-Bark-Like Structure for High Performance Freestanding Sulfur Cathode Zhen Meng,†,‡,§ Shunlong Zhang,‡,§ Jianli Wang,† Xufeng Yan,‡,§ Hangjun Ying,‡,§ Xin Xu,‡,§ Wenkui Zhang,⊥ Xianhua Hou,II Wei-Qiang Han,†,‡* †

School of Materials Science and Engineering, Zhejiang University,

Hangzhou 310027, P. R. China ‡

Ningbo Institute of Material Technology and Engineering, Chinese Academy of

Sciences Ningbo 315201, P. R. China §

College of Materials Science and Opto-Electronic Technology, University of Chinese

Academy of Sciences Beijing 100049, P.R. China ⊥

College of Materials Science & Engineering, Zhejiang University of Technology

Hangzhou 310014, P. R. China II

School of Physics and Telecommunication Engineering, South China Normal

University Guangzhou 510006, P.R. China Corresponding authors:*Email: [email protected]

1

ACS Paragon Plus Environment

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

ABSTRACT:

Herein,

we

designed

a

simple

Page 2 of 28

and

energy-saving

“ethanol-transfer-adsorption” method to prepare the activated carbon cloth/sulfur composite (ACC/S). Then, the prepared composite was wrapped in a thin-layered nickel-based hydroxide (NNH) to form a tree-bark-like structure. After irreversibly reacting with lithium to form a barrier layer with both good Li+ permeability and abundant functional polar/hydrophilic groups, this layer could retard the diffusion of lithium polysulfides by both physical confinement and chemisorption. As a result, the freestanding NNH/ACC/S cathode with 4.3 mg cm-2 sulfur loading displayed a high areal discharge capacity of 4.3 mA h cm-2 over 100 cycles at 0.15 C. When cycled at 0.5 C, it still showed a discharge capacity of 650.0 mA h g-1 over 350 cycles. KEYWORDS:

lithium-sulfur

battery,

ethanol-transfer-adsorption

method,

freestanding cathode, tree-bark-like structure, high sulfur loading.

INTRODUCTION Lithium-sulfur (Li–S) battery is one of the most prospective high energy storage batteries due to the high theoretical capacity (1675 mA h g-1) of sulfur and high energy density (2576 W h kg-1) of the Li-S full cell.1-2 Compared with the traditional transition metal oxide cathode, sulfur has the characteristic of abundance, low cost and environmental benign.3 Despite these virtues, the commercialization of Li-S battery still faces many impediments. For instance, the insulating nature of sulfur (5× 10-30 S cm-1 at 25 oC) and its discharge product Li2S and Li2S2 inevitably results in low utilization of the active material and inferior cell kinetics.4-5 The high solubility of the intermediate polysulfides (Li2Sn, n ≥ 3) in the organic electrolyte accompanied by 2


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“shuttle effect” unavoidably leads to a continuous loss of the active material, severe capacity decay and low Coulombic efficiency.4, 6-7 Over the years, extensive efforts have been devoted to improve the electric conductivity of sulfur cathode and suppress the dissolution of the polysulfides. Since CMK-3 was employed as a sulfur matrix by Nazar et al., combining sulfur with structured

carbon

materials

including

micro/mesoporous

carbon,8-11

carbon

nanotube,12-13 carbon fiber,14-15 carbon cloth16-17 and graphene18-20 has become a world-wide approach. The porous structure of the carbon materials could physically confine sulfur and retard the diffusion of polysulfide into the electrolyte, accompanied with facilitating electron transfer in the cathode. Aurbach et al. employed a sulfur-impregnated activated carbon fiber cloth (ACF) as a binder-free cathode for rechargeable Li-S batteries.16 The ACF ensured good electronic conductivity and the porous structure of the ACF retarded the dissolution polysulfides. The electrochemical performance of Li-S battery with this composite as cathode had been significantly improved. However, most of the carbon materials are non-polar in nature with open porous structure, which showed limited ability in trapping polar polysulfide in the cathode effectively. Recently, Jiang et al. employed a thin-layered nickel-based hydroxide (Ni3(NO3)2(OH)4, NNH) to encapsulate sulfur effectively. During charging/discharging cycles, the NNH could irreversibly react with lithium to form a barrier

layer

with

both

good

Li+ permeability

and

abundant

functional

polar/hydrophilic groups. The S8@carbon black@NNH composite exhibited remarkable improvement in electrochemical performance compared with bare 3



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S8@carbon black. In the respect of preparing carbon/sulfur composite, the most common method is the “melt-diffusion strategy” reported by Nazar et al., in which carbon and sulfur are mixed uniformly and then heated at 150-160 oC. The melting sulfur is imbibed into the pores of the carbon by capillary forces. This method is convenient if the powdered carbon is employed as a sulfur matrix since the sulfur could be mixed with powdered carbon uniformly by grind. However, if the freestanding materials such as carbon cloth and graphene film are used as sulfur hosts, it is hard to mix them homogeneously with sulfur before heating, resulting in uneven sulfur distribution. Taking all these into account, herein, we designed a simple room-temperature “ethanol-transfer-adsorption” method to prepare the activated carbon cloth/sulfur (ACC/S) composite. This process was energy-saving, environment benign and easy to realize in large-scale production. More importantly, sulfur could be dispersed uniformly in the pores of the freestanding ACC easily with this process. ACC with abundant micropores served as both an effective sulfur matrix to suppress the dissolution of polysulfides and a fast electronic conduit. Then, we successfully wrapped the ACC/S in a thin reliable NNH layer. The NNH, just like tree bark, was coated intimately outside the ACC/S composite, severing as a guard to further trap the polysulfides. The double barriers consisting of micro/mesopores of the ACC and the NNH coating ensured most of the polysulfides were retained in the cathode. The prepared NNH-wrapped ACC/S (NNH/ACC/S) freestanding cathode could be used as electrode directly without any other binder, conductive additives and current collector, 4


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which would largely save the time and cost for assembling battery.

EXPERIMENTAL SECTION Materials preparation Preparation of ACC/S: The ACC/S was prepared by the “ethanol-transfer-adsorption” method. 200 mg of sulfur and a piece of ACC (8 cm×4 cm) were added into 100 ml of ethanol solution and stirred for 6 h to obtain the ACC/S. During this process, the amount of solid sulfur particles gradually decreased. The ACC/S was taken out from ethanol solution. Then, a piece of filter paper was used to suck the extra ethanol on the surface of the ACC/S. After that, the ACC/S was dried under vacuum. By increasing the quantity of the sulfur added in the ethanol solution to 300, 400 and 500 mg, ACC/S with different sulfur loading could be obtained. They were denoted as ACC/S-300, ACC/S-400 and ACC/S-500, respectively. Preparation of four NNH/ACC/S cathodes with different sulfur loading: The preparation

of

NNH/ACC/S

was

inspired

by

Jiang

et

al.4

0.5

g

of

hexamethylenetetramine/C6H12N4 (Energy chemical, assay ≥99.5 %) and 0.25 g of Ni(NO3)2·6H2O (Alfa Aesar, assay ≥98 %) were dissolved in 50 ml of distilled water. Afterwards, the prepared ACC/S (or ACC/S-300, ACC/S-400 and ACC/S-500) was put into the solution, which was then transferred into a Teflon-lined autoclave (80 ml) and held at 95 oC for 6 h. The obtained four NNH/ACC/S cathodes with different sulfur loading were washed several times with distilled water and dried under 60 oC. Removing NNH coating of the NNH/ACC/S for TGA: The prepared four NNH/ACC/S cathodes with different sulfur loading were put into 5



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1M of hydrochloric acid solution and stirred for 8h to remove the NNH coating. Then the cathodes were washed several times with distilled water and dried under 60 oC. Electrochemical Measurements The electrochemical measurements were carried out with 2032 coin cell with lithium foil as anode. The electrolyte was 0.5 M lithium bis(trifluoromethane) sulfonimide (LiTFSI) in a 1:1 (v/v) mixture of 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME) containing 5 wt% LiNO3. Galvanostatic charge/discharge tests were performed between 1.8 -2.8 V. Cyclic voltammetry (CV) was measured at a scan rate of 0.05 mV s-1. Material Characterizations The morphologies of the samples were captured using Hitachi S4800 field emission scanning electron microscope. The energy dispersive X-ray spectroscopy (EDS) was taken using FEI Quanta FEG 250 field emission microscopy. Thermogravimetric analysis (TGA) was performed by a Pyris Diamond analyzer under nitrogen flow with a heating rate of 10 oC min-1. X-ray diffraction (XRD) analysis was performed on a D8 advance XRD diffractometer. Nitrogen adsorption and desorption measurements were taken by a Micromeritics ASAP 2020M adsorption analyzer. X-ray photoelectron spectroscopy (XPS) spectra were measured by an Axis Ultra DLD imaging photoelectron spectrometer.

RESULTS AND DISCUSSION

6


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Figure 1. Schematic illustration of the preparation of the NNH/ACC/S.

The room-temperature “ethanol-transfer-adsorption” method was shown in Figure 1 (step I). A certain amount of sulfur and a shaped activated carbon cloth (ACC, 8 cm ×4 cm) were put into 100 ml of ethanol solution and stirred for about 6 h. Due to the low solubility of sulfur in ethanol (100 g of ethanol could only dissolve 0.065 g of sulfur at 25 oC),21 only a part of sulfur could be dissolved (Step I in Figure S1). The dissolved sulfur was easily adsorbed in the ACC by the strong adsorbability of ACC, leading to the subsequent dissolution of the solid sulfur particles (Step II and III in Figure S1). This process would proceed until the solid sulfur was consumed or the ACC was saturated with sulfur. During this process, sulfur experienced an evolution of “undissolved solid sulfur→ dissolved liquid sulfur→ adsorbed solid sulfur”. The ethanol was just like a hardworking stevedore to carry the sulfur from the solid particles to the pores in the ACC. After this adsorption process, the ACC/S was taken out from the ethanol solution and dried. Through varying the amount of sulfur added in the ethanol, ACC/S with different sulfur loading could be easily obtained (more details

were

in

the

experimental

section).

The

picture

to

prove

the

“ethanol-transfer-adsorption” method really worked was shown in Figure S2. Sulfur and ACC were added into the ethanol. After 0 h, 1 h, 2 h and 6 h, the ACC was taken 7



Page 8 of 28

out and the pictures of the ethanol and sulfur were taken. For comparison, the pictures of the ethanol and sulfur without ACC at different times were also taken. As the “ethanol-transfer-adsorption” treatment proceeded, the amount of solid sulfur gradually decreased. After 6 h, no obvious solid sulfur particles could be observed. However, in the mixture of ethanol and sulfur without the ACC treatment, obvious solid sulfur remained even after 6 h. The simple “ethanol-transfer-adsorption” method was energy saving and suitable for the freestanding sulfur matrix such as ACC. Even though some other methods have been reported to prepare freestanding sulfur cathode, special preparation conditions are usually employed.16,

22

The advantages of this

method compared with other preparation of sulfur cathode were shown in Table S1. The prepared ACC/S hybrids with different sulfur content were then uniformly packaged by NNH by a hydrothermal reaction (step II in Figure 1) to obtain four NNH/ACC/S electrodes with different sulfur loading.

Figure 2. (a) Schematic illustration of ACC, ACC/S, NNH/ACC/S (the yellow spots represents sulfur; the light blue layer in the NNH/ACC/S represents the NNH layer). The corresponding 8


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scanning electron microscopy (SEM) images of (b, c) ACC, (d, e) ACC/S, (f, g) NNH/ACC/S.

The schematic illustration of ACC, ACC/S, NNH/ACC/S and the corresponding scanning electron microscopy (SEM) images were presented in Figure 2. The carbon cloth consisted of numerous carbon fibers (Figure 2b and c). No morphology change was observed on the surface of ACC after adsorbing sulfur (Figure 2d and e and Figure S3), indicating most of the sulfur existed in the pores of the ACC. N2 adsorption−desorption isotherms and pore distribution of the ACC, ACC/S, ACC/S-300, ACC/S-400, ACC/S-500 were displayed in Figure S4. The ACC with abundant micropores exhibited large surface area. After adsorbing sulfur, the specific surface area gradually reduced from 1900 m2 g-1 of the ACC to 3.6 m2 g-1 of the ACC/S-500. The pore volume decreased correspondingly from 0.97 cm3 g-1 to 0.003 cm3 g-1. These were consistent with the SEM images, proving the successful encapsulation of sulfur into the pores of ACC. The NNH/ACC/S exhibited a significant difference in the surface morphology compared with ACC and ACC/S (Figure 2f and g). Thin NNH layer was intimately coated on the ACC/S to form a rough surface, which was just like tree bark. No obvious redundant NNH aggregation was observed. The high resolution SEM image (inset in Figure 2g) further uncovered the detailed morphology information of the NNH/ACC/S. Many thin NNH sheets interlaced with each other to form the NNH coating layer, which was extremely thin and supple enough to conform the body shape of the ACC/S.4 Further characterization of the composites was shown in Figure 3. Both the line-scan analysis and the energy dispersive X-ray spectroscopy (EDX) elemental 9



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mapping revealed a homogeneous dispersion of C, S, Ni, O and N elements in the NNH/ACC/S (Figure 3a-f).23 Thermogravimetric analysis (TGA) was employed to confirm the sulfur content in the prepared NNH/ACC/S electrodes (Figure 3g).

Figure 3. (a) Line-scan analysis and (b-f) energy dispersive X-ray spectroscopy (EDX) elemental mapping of C, S, Ni, O and N elements in the NNH/ACC/S. (g) TGA curves of the NNH/ACC/S electrodes with different sulfur loading. (h) XRD patterns of the ACC and ACC/S with different sulfur loading.

Before the TGA measurement was carried out, the NNH/ACC/S cathodes were pretreated with acid to remove the NNH. The sulfur contents in the four NNH/ACC/S 10


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cathodes with different sulfur loading after removing NNH were 32, 40, 54 and 59 wt%, corresponding to 4.3, 6.1, 10.8, 13.2 mg cm-2 of sulfur, respectively. It should be noted that by weighting the NNH/ACC/S cathodes before and after acid treatment, the content of NNH on each cathode could be easily calculated. For the cathodes with sulfur loading of 4.3, 6.1, 10.8, 13.2 mg cm-2, the NNH contents were 7.1, 5.9, 4.8, 4.2 wt% in the cathodes, corresponding to 0.96, 0.91, 0.96, 0.94 mg cm-2. The TGA curves of the four NNH/ACC/S cathodes exhibited that most of the sulfur evaporated between 300-430 oC. That was significantly different from the TGA curve of pure sulfur, which showed main weight loss under 300 oC.10 The high temperature of sulfur evaporation in the NNH/ACC/S cathodes indicated most of the sulfur was highly confined in the pores of ACC.13, 24-25 The X-ray diffraction (XRD) spectrum of the ACC (Figure 3h) exhibited a broad peak between 20-30°, which was characteristic of non-graphitic carbons.26 The absence of typical sulfur diffraction peaks in the ACC/S, ACC/S-300, ACC/S-400 and ACC-S-500 revealed that sulfur was well confined within ACC. The XRD spectra accorded well with the TGA curves and the SEM images. All these above implied the successful impregnation of ACC with sulfur by the “ethanol-transfer-adsorption” strategy. CR2032-type coin cells were assembled to evaluate the electrochemical performance of NNH/ACC/S cathode with lithium foil as anode. The galvanostatic discharge-charge cycling stability of the NNH/ACC/S with sulfur loading of 4.3 mg cm-2 was displayed in Figure 4a. To verify the positive effect of the NNH coating on the `cycling stability, the electrochemical performance of the cell with ACC/S `as 11



Page 12 of 28

cathode with similar sulfur loading was also tested. After an initial activated cycling at 0.05 C, the ACC/S cathode exhibited a discharge capacity of 1006.3 mA h g-1 at the second cycle with a current density of 0.15 C (251 mA g-1, 1 C=1675 mA h g-1), which decayed to 565.3 mA h g-1 after 100 cycles. In sharp contrast to the ACC/S cathode, the NNH/ACC/S cathode displayed a discharge capacity of 1095.7 mA h g-1 at the second cycle, which could be maintained at 1002.4 mAh g-1 after 100 cycles. The areal capacity of the NNH/ACC/S after 100 cycles was 4.3 mA h cm-2, which was comparable with that of the traditional lithium-ion batteries (LIBs) (4 mA h cm-2). More importantly, after suffering a slightly capacity fading over the initial 18

Figure 4. Electrochemical performance of the NNH/ACC/S and ACC/S. (a) Cycle performance at 0.15 C. (b) Sulfur utilization at the second cycle and the 100th cycle. (c) Electrochemical impedance spectra. (d) Cycle performance at 0.5 C. (e) Rate performance.

cycles (from 1095.7 to 1002.8 mA h g-1), the NNH/ACC/S cathode exhibited little capacity loss. The charge-discharge profiles of the NNH/ACC/S and ACC/S at 0.15 C were shown in Figure S5. The NNH/ACC/S cathode exhibited longer plateaus and 12


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lower polarization than the ACC/S cathode in the first cycle at 0.15 C. This phenomenon became more obvious in the 100th cycle, indicating the presence of hydroxide layer could ensure a more kinetically efficient redox reaction.27-28 In addition, the Coulombic efficiency of the NNH/ACC/S after 100 cycles was 95.3 %, higher than that of the ACC/S (94.5 %). The polysulfides generated during charge and discharge could spontaneously dissolve in the liquid organic electrolyte easily, which would lead to fast capacity decay, low Coulombic efficiency and largely reduce the utilization of sulfur.29-30 The sulfur utilizations of the NNH/ACC/S and ACC/S in the second cycle were 65.4 % and 60.1 %, respectively (Figure 4b). After 100 cycles, the sulfur utilization of the NNH/ACC/S cathode remained 59.8 %, much higher than that of the ACC/S cathode (33.7 %). The high sulfur utilization, good cycle stability and enhanced Coulombic efficiency of the NNH/ACC/S cathode over the 100 cycles proved that the polysulfides could be suppressed in the cathode successfully by the NNH coating. The electrochemical impedance spectra of the cells with NNH/ACC/S and ACC/S as cathodes at open-circuit voltage were shown in Figure 4c. The depressed semicircle in high frequency region was related to the charge-transfer resistance (Rct) and the inclined line in low frequency region was involved with the resistance originating from the ion diffusion in the cathode (Warburg impedance). The NNH/ACC/S and ACC/S exhibited similar semicircle diameters, implying that the thin NNH coating had little effect on charge transfer impedance. Cyclic voltammetry (CV) curve of the NNH/ACC/S was shown in Figure S6. Two reduction peaks at 13



Page 14 of 28

around 2.3 V and 2.0 V represented the conversion from sulfur to Li2Sn (4≤n≤8) and further to Li2S2 or Li2S, respectively. Two oxidation peaks overlaid with each other at 2.3-2.6 V involved the reverse process, namely the oxidation of Li2S2 or Li2S to Li2Sn (4≤n≤8) and further to sulfur.31 The overlap of the redox peaks of the second and third cycles suggested a stable electrochemical process.32 Long-term cycling stability of the NNH/ACC/S cathode at 0.5 C was also investigated (Figure 4d). After activation at 0.05 C, the cathode exhibited a discharge capacity of 838.2 mA h g-1 at the second cycle, which increased gradually to 974.9 mA h g-1 at the eighth cycle. The increasing of the discharge capacity in the first several cycles resulted from the further activation of the cathode.33-34 After 350 cycles, the cell retained a discharge capacity of 650.0 mA h g-1, corresponding to capacity retention of 76.4 %. The NNH/ACC/S cathode nearly exhibited a constant discharge capacity of around 650 mA h g-1 after 270 cycles and was subject to little capacity loss in the following cycles. The capacity decay was as low as 0.064 % per cycle. The ACC/S cathode, however, exhibited a low discharge capacity of 450 mA h g-1 after just 198 cycles. The excellent cycle stability of the NNH/ACC/S cathode was superior to many recent reports of sulfur cathodes (Table S2). The rate performance of the NNH/ACC/S was shown in Figure 4e. Average reversible discharge capacities of 1041.2, 884.5, 728.6 and 522.4 mA h g-1 were obtained at 0.15 C, 0.25 C, 0.5 C and 0.75 C, respectively. In comparison, the ACC/S delivered lower discharge capacities of 906.6, 713.8, 532.3 and 345.0 mA h g-1. When the C-rate was switched to 0.15 C, a high discharge capacity of 948.3 mA h g-1 was 14


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recovered for the NNH/ACC/S cathode, much higher than that of the ACC/S (618.5 mA h g-1). The good cycle stability of the NNH/ACC/S cathode was attributed to the NNH coating, which largely retarded the polysulfides dissolution. To confirm the enhanced ability of the NNH coating to trap polysulfides, in-situ visual-electrochemical experiments were performed since the strong color of the polysulfides in the liquid organic electrolyte could be used to monitor the dissolution of the polysulfides.35 As shown in Figure 5a, during discharging, the colorless electrolyte with ACC/S as cathode turned to dark yellow, indicating most of the polysulfides diffused into the electrolyte. Even after discharging for 7 h, no obvious change in color could be observed, suggesting lots of polysulfides remained in solution. In contrast, the electrolyte with NNH/ACC/S as cathode just exhibited light yellow color during discharge, which faded with the discharge process proceeding after discharging for 2.5 h. The color of the separators obtained from coin cells after cycling were in consistent with the in-situ visual-electrochemical experiments (Figure 5b). The separator of the NNH/ACC/S electrode appeared less yellow than that of the ACC/S, implying less polysulfides on the separator. These provided the visual evidence of that the dissolution of the polysulfides could be effectively suppressed by NNH coating.

15



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Figure 5. (a) The in-situ visual-electrochemical experiments. (b) The separators obtained from coin cells after cycling. (c) Voltage–capacity profile of the 3rd cycle (continuous discharge) and the 4th cycle (12h rest period during discharge). (d) The 4th cycle percent capacity loss from the 3rd cycle.

The dissolved polysulfides in the electrolyte spontaneously diffused to the anode and reacted with lithium metal, leading to undesirable loss of the active material and inevitable self-discharge of the Li-S battery.29-30, 36. Self-discharge experiments of the NNH/ACC/S and ACC/S cathodes at an intermediate state of discharge (SOD) were employed to evaluate the self-discharge and adsorptivity of the cathodes.36 As shown 16


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in Figure 5c, after 3 formation cycles at 0.05 C, the 4th discharge process was interrupted at the end of the first discharge plateaus, where the soluble polysulfides is maximal. After resting for 12 h, the discharge was restarted. The discharge and charge capacity losses between the 3rd and 4th cycles were denoted asΔD and ΔC, respectively. ΔD is related to the self-discharge of the battery andΔC is associated with irreversible capacity loss.36 TheΔD of the NNH/ACC/S cathode was 49.5 mA h g-1, corresponding to 4.0 % of the discharge capacity in the 3rd cycle (Figure 5d), which was much lower than that of the ACC/S cathode (95.2 mA h g-1, 8.1 %). Similarly, NNH/ACC/S exhibited lowerΔC (94 mA h g-1, 7.58 %) than ACC/S (179 mA h g-1, 15.2 %). The low ΔD and ΔC of the cell with NNH/ACC/S as cathode suggested that the NNH layer could effectively suppress the dissolution of the polysulfides into the electrolyte, leading to low self-discharge, low irreversible capacity loss and an enhancement of the cycling stability. The NNH layer was intimately coated on the ACC/S, serving as a shield to physically confine the polysulfides in the cathode. During the charging/discharging cycles, Ni in the NNH coating transform from Ni(II) to Ni(III), forming (Li, Ni)-mixed hydroxide compounds with good Li+ permeability.4 The high-resolution XPS spectra of Ni 2p were shown in Figure 6a. Before charging/discharging cycles, two main peaks located at 855.5 and 872.9 eV, respectively, with satellite peaks at high binding energies of 861.7 and 879.3 eV, corresponding to Ni 2p3/2 and Ni 2p1/2 spectra from Ni(II).23, 37 After charging/discharging cycles, these two peaks shifted to 856.1 and 873.5 eV, in consistent with the spectra of Ni(III).23, 37 The transformation 17



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of the oxidation states from Ni(II) to Ni(III) accorded well with previous report,4 implying

the

formation

of

(Li,

Ni)-mixed

hydroxide

compounds

during

charging/discharging cycles. The (Li, Ni)-mixed hydroxide compounds shell could physically retard the diffusion of the polysulfides into the electrolyte. 4

Figure 6. (a) The high-resolution Ni 2p XPS spectra of the NNH/ACC/S cathode before and after cycling. (b) Schematic illustration of the function of the NNH coating layer. (During discharging, the diffusion of polysulfides could be largely suppressed by the NNH layer with both physical confinement and chemisorption.)

In addition to the physical confinement of the NNH layer, the polar surface of the NNH with functional hydrophilic groups (for example, hydroxyl groups) also benefited the trapping of hydrophilic polysulfides by chemical interaction.2, 33, 38-41 The Li 1s XPS spectra of the NNH/ACC/S and the ACC/S cathodes after 5 full charging/discharging cycles and then discharged to 2.1 V (the concentration of polysulfides in the electrolyte was thought to be the highest at this voltage) were also probed (Figure S7). For the ACC/S cathode, the peak of Li 1s XPS spectrum centered at 56.0 eV, which interestingly shifted downward to 55.7 eV in the NNH/ACC/S 18


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cathode. This might be caused by the chemical interaction between NNH and lithium polysulfides, most likely to be the electron transfer from O on the surface of NNH to Li+. The O has electron-donating ability,42-43 which could donate electronic charge to Li+ in the lithium polylsulfides, forming Li-O interaction, similar with the Li-N interaction39, 44-48 and Li-O interaction in previous reports.49 SEM images of the ACC/S and NNH/ACC/S after 100 cycles were captured to further verify the function of NNH coating (Figure S8). Obvious sulfur agglomerates varied from several microns to dozens of micron could be observed outside the ACC in the ACC/S cathode. Even though the pores in the ACC could physically adsorb part of the polysulfides, the carbon material was non-polar in nature, which was not favorable for the polar polysulfides adsorption. In the cell with ACC/S cathode, during charging/discharging, polysulfides escaped out of the pores and dissolved in the electrolyte (Figure 6b). When the polysulfides were oxidized to sulfur, it formed agglomerates outside the ACC. Sulfur agglomerations with such large size retarded/impeded charge transfer process and reduced the utilization of sulfur.19 On the contrast, in the NNH/ACC/S cathode, due to the physical confinement and chemisorption of the NNH coating, most of the polysulfides could be restricted in the pores of the ACC and adsorbed on the surface of the hydrophilic NNH surface. Therefore, few sulfur agglomerations were observed in the NNH/ACC/S cathode (Figure S8b). Achieving high areal sulfur loading in cathode is critical for high-areal capacity. The electrochemical performances of the NNH/ACC/S with different sulfur loading 19



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were also investigated (Figure 7). With the sulfur loading increasing, higher voltage polarization was observed due to slower dynamics and increased electrode resistance in the electrodes (Figure 7a).50 After an activated process, the cells with sulfur loading of 6.1, 10.8, 13.2 mg cm-2 exhibited discharge capacities of 1151.1, 1103.2, 1062.2 mA h g-1, respectively (current density: 300 µA cm-2) (Figure 7a and b), corresponding to areal capacities of 7.0, 11.9, 14.0 mA h cm-2 (Figure 7c). After 25 cycles, the retained areal capacities of these three electrodes were 6.1, 8.7, 9.6 mA h cm-2, respectively, which were higher than that of the LIBs.51 Even though the sulfur loadings and the cycling performance of this work were not the highest and the best in literatures,51-52 they were comparable with some recent reports (Figure S9).

Figure 7. (a) The voltage profiles, (b) cycle performance at 300 µA cm-2 and (c) the areal capacity of the NNH/ACC/S cathodes with higher sulfur loading.

CONCLUSION In conclusion, we successfully infused sulfur uniformly into the pores of the ACC to prepare the freestanding ACC/S cathode by a “ethanol-transfer-adsorption” method, which was a simple, energy-saving and environment benign process and easy to realize for large-scale production. This method showed prospect in preparation of 20


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other carbon/sulfur composite. A NNH thin layer was further coated intimately on the ACC/S to prepare the NNH wrapped ACC/S cathode (NNH/ACC/S), which irreversibly reacted with lithium to form a barrier layer with both good Li+ permeability and abundant functional polar/hydrophilic groups. With both physical confinement and chemisorption, this tree-bark-like layer could restrict most of the polysulfides in the cathode. As a result, the freestanding NNH/ACC/S cathode with 4.3 mg cm-2 sulfur loading exhibited excellent cycle stability.

Supporting Information: N2 sorption data, SEM images, CV curve, XPS spectra, comparison of our “ethanol-transfer-adsorption” method with other methods and comparison of the electrochemical performance

ACKNOWLEDGEMENT The authors are grateful for the financial support by the National Natural Science Foundation of China (Grant No. 51504234 and No. 51371186), Zhejiang Provincial Natural Science Foundation of China (Grant No. LY16E040001) the “Strategic Priority Research Program” of the Chinese Project Academy of Science (Grant No. XDA09010201), the Ningbo 3315 International Team of Advanced Energy Storage Materials, Zhejiang Province Key Science and Technology Innovation Team (Grant No. 2013TD16), the Natural Science Foundation of Ningbo (Grant No. 2015A610252). REFERENCES (1) Ji, X.; Lee, K. T.; Nazar, L. F., A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Batteries. Nat. Mater. 2009, 8, 500-506. 21



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