Hierarchical and Highly Stable Conductive Network Cathode for

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Hierarchical and High-Stable Conductive Networks Cathode for Ultra-Flexible Li-S Batteries Xue-Song Wu, Zhao Wang, Chao Qin, Xinlong Wang, Haiming Xie, Zhenhui Kang, and Zhong-Min Su ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00377 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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ACS Applied Energy Materials

Hierarchical and High-Stable Conductive Networks Cathode for Ultra-Flexible Li-S Batteries Xuesong Wu, †,§ Zhao Wang, †,§ Chao Qin, † Xinlong Wang,*, † Haiming Xie, *,† Zhenhui Kang, *, ‡ and Zhongmin Su †

†

National & Local United Engineering Laboratory for Power Battery, Key Laboratory of

Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun 130024, Jilin, China ‡

Jiangsu Key Laboratory for Carbon-based Functional Materials and Devices, Institute of

Functional Nano and Soft Materials, Soochow University, Suzhou 215123, China

ABSTRACT: Flexible Li-S batteries have great potential for the next generation energy-storage which can meet the rising demand of rollable display and wearable electronic devices owing to the high theoretical energy density and competitive price. Here we design and fabricate an integrated electrode with hierarchical structure and interconnected 3D conductive networks as a cathode of flexible Li-S batteries. The composite cathode exhibits high electrochemical performance and cycling stability. The initial reversible discharge capacity is 1312 mA h g-1 at 0.2 C with sulfur load 2.0 mg cm-2 and the capacity decay rate is 0.09% per cycle within 500 cycles at current of 1 C. Notably, the composite electrode can sustain 15.2 MPa stress with 10% strain and remain structural integrity after 200,000 bending cycles–the highest number of bending found to date for any flexible S cathodes. The soft package batteries with different sizes and shapes are fabricated and they exhibit extraordinary flexibility and stability after bending

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and flattening over 2100 times. Moreover, their potential applications in rollable display, flexible lighting and wearable electronic devices are also investigated. KEYWORDS: Li-S batteries, flexible cathode, 3D conductive network, binder-free electrode, carbon cloth, hierarchical structure 1. INTRODUCTION The increasing need for flexible and wearable electronic devices urges us to develop flexible batteries with high performance.1-3 Among all the candidates, Li–S batteries exhibit great advantages with high theoretical capacity (1675 mA h g-1) and energy density (2600 W h kg-1), low toxicity, high natural abundance and competitive cost.4-8 Nevertheless, several crucial issues, such as poor electric conductivity of S8 and Li2S, dissolution and shuttle effect of polysulfide, need to be resolved for Li–S batteries before practical application.9-15 To overcome above issues, some 3D current collector and chemisorption by heteroatom doping have been fabricated.16-19 As for flexible Li-S battery, there are more severe problems need to address.20,21 Firstly, all materials, including the cathode, anode and current collector must be flexible enough to withstand deformations and afford stable electrochemical performance during the bending and folding. Secondly, the electrode materials should have tight connections with the current collectors to prevent delamination by repeatable deformations. Obviously the conventional current collectors, such as Al and Cu foil with limited flexibility, are not suitable to fabricate flexible Li-S batteries. At present, some flexible carbon based current collectors including CNT,22,23 carbon nanofibers,24 graphene,25,26 carbonized polymer27 and hybridized carbon28 have been explored for flexible Li-S batteries owing to their light weight and superior conductivities. However, the carbon frameworks in these flexible current collectors usually exhibit loose structures due to their interior weak physical interactions and cannot guarantee enough


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mechanical strength and integrity during the repeated deformations. Inspired by the structure of reinforced concrete, a common architecture but possessing an excellent mechanical property, we put forward a new strategy to construct a flexible integrated electrode with hierarchical structure and interconnected 3D conductive networks as cathode of Li-S batteries. In this work, the carbon cloth (CC) was selected intelligently as a current collector owing to the advantages of flexibility, stable quality and high mechanical strength, especially the unique merit of binder-free and freestanding.29-31 As sulfur host, the porous N-doped graphitic carbon containing cobalt (CoNC) derived from MOFs is coated tightly on the carbon fibers and then the interconnected multi-walled carbon nanotubes (MWCNTs) growing in-situ are further covered the coating layer forming a three-dimensional conductive network. The composite cathode exhibits high electrochemical performance and cycling stability. The initial reversible discharge capacity is 1312 mA h g-1 at 0.2 C with sulfur load 2.0 mg cm-2 and the capacity decay rate is 0.09% per cycle within 500 cycles at current of 1 C. It is rather remarkable that such a flexible electrode can withstand tremendous mechanical deformation and remain structural integrity even after 200,000 bending cycles–quadruple over reported maximum value for flexible S cathodes. Furthermore, soft package Li–S cells based on this electrode exhibit outstanding flexibility and stability after bending and flattening over 2100 times. We also explored their potential applications in rollable display, flexible lighting and wearable electronic devices. 2. Experimental Section 2.1. Fabrication of CC/S@CoNC/MWCNTs. Firstly, the carbon cloth with 10×10 cm2 was coating with ZIF-67 nanocrystals according to the process given in a previous report.32 Co(NO3)2·6H2O (1.25 g, 4.31 mmol), 2-methylimidazole (3.125 g, 38.13 mmol) and PEG (Mn = 4000, 0.625 g, 0.156 mmol) were mixed and grounded in


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planetary ball mill. Then the mixture was coated on a 10×10 cm2 carbon cloth, heated on thermocompressor at 200℃ for 10 min (Figure S1a). The as-prepared ZIF-67@carbon cloth were thermally converted to CC/CoNC composites through carbonization under a N2 flow at 900℃ for 4 h, at the same time, the short carbon nanotubes were formed under the catalysis by Co nanoparticles. Then the CC/CoNC was cut into disk of 1.3 cm diameter with approximate 2.5 mg CoNC coating on the carbon cloth (Figure S1b). Then CC/CoNC composites were put into a solution of 2 mL CS2 with 3.0, 5.0, 6.5 and 10 mg sulfur and desiccated by free volatilization. Then CC/S@CoNC was transferred to the electric oven at 155℃ for 20 h under Ar and a mass of carbon nanotubes were formed by the promotion effect of sulfur. The mass content of sulfur was determined by weighing and thermal gravimetric analysis. The CC/S@CoNC/MWCNTs composites with a sulfur loading density of 2.0, 3.0, 5.0 and 8.0 mg cm-2 were achieved respectively. 2.2. Fabrication of CC/S@NC. CC/S@NC was synthesized by a procedure similar to that was used for CC/S@CoNC/MWCNTs except Co was removed by using 2M HCl before encapsulating sulfur. 2.3. Fabrication of soft package Li–S cells. The soft package Li–S cells for bending tests were assembled into an aluminum pouch cell in drying room. All the components follows: Li foil anode, one-layer Celgard 2400 separator presoaked in an electrolyte (1 M LiTFSI and 0.1 M LiNO3 in 1:1 (v/v) 1, 3-dioxacyclopentane (DOL) and 1, 2-dimethoxyethane (DME)), and CC/S@CoNC/MWCNTs as cathode. 3. RESULTS AND DISCUSSION


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Figure 1. Schematic illustration of fabrication process of the CC/S@CoNC/MWCNTs cathode. (a) fabrication of ZIF-67@CC by solvent-free hot-pressing method; (b) fabrication of CC/CoNC by direct thermolysis of ZIF-67@CC at the 900 °C under N2 atmosphere; (c) sulfur encapsulation by melt-diffusion method at 155 °C for 20 h under Ar atmosphere. Figure 1 shows the fabrication procedure of the CC/S@CoNC/MWCNTs cathode. Firstly, the nanocrystals of ZIF-67 were coated on the carbon cloth (denoted as ZIF-67@CC) by using a simple solvent-free hot-pressing method.32 Powder X-ray diffraction (PXRD) pattern confirmed the high purity of ZIF-67 (Figure S2). The scanning electron microscopy (SEM) images revealed the rhombic dodecahedral crystal of ZIF-67 with the uniform particle sizes of about 500 nm that attached tightly on the surfaces of carbon cloth (Figure S3a-b). Then the ZIF-67@carbon cloth was carbonized at the 900 °C under N2 atmosphere, cobalt, N-doped porous carbons coating the fiber of carbon cloth (abbreviated as CC/CoNC) is formed. In the meantime, some short MWCNTs were coated uniformly on the fiber of carbon cloth that were generated by Co nanoparticles catalysis under such temperature (Figure S3c).33 After annealing, the diffraction peaks of ZIF-67 disappeared entirely (Figure S4). Three apparent diffraction peaks at 2θ= 44.21°, 51.61°, and 75.91° corresponding to the (111), (200) and (220) of cobalt crystalline lattice, respectively, appeared in the PXRD pattern, suggesting that the cobalt ions of ZIF-67 were reduced to cobalt nanoparticles. To further probe the porocity of the CC/CoNC composite, N2 adsorption–desorption measurement was studied. Contrast to pure carbon cloth, the whole CC/CoNC composite has a larger surface area (Brunauer–Emmett–Teller surface area of 106.350 ± 4 m2 g-1). The composite has pore sizes of approximate 4.0 nm so that the sulfur could be


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confined into the pore well (Figure S5). The large surface area and mesoporous structure provide snug habitats for the intermediate polysulfides and enough expansion space for the sulfur particles during discharge. Finally, sulfur encapsulation is completed by a melt-diffusion method. The mass of encapsulated sulfur is determined by thermal analysis (Figure S6-7). To our surprise, plenty of long MWCNTs were formed in-situ during sulfur encapsulation. To verify this special phenomenon, we conducted this experiment several times and the same result was obtained. We speculated that the formation of MWCNTs might owe to the promotion effect of sulfur (Figure S3d).34 The morphology and structure of the resulting CC/S@CoNC/MWCNTs hierarchical cathode were investigated by PXRD, field-emission SEM and high-resolution TEM (HRTEM) measurements. No significant signal of sulfur is detected by the PXRD which proves that sulfur is encapsulated into the mesopores of CC/S@CoNC/MWCNTs (Figure S8). The MWCNTs with diameters of 50–100 nm are determined by HRTEM measurements (Figure 2). According to SEM images, the MWCNT network like a spider web spreading over the surface of the CC/CoNC which is conducive to anchoring the sulfur-cobalt, N-doped porous carbons particles and increasing the mechanical strength of the electrode. The MWCNTs-covered cathode provides pathways for electron transport rapidly on one hand, and alleviates polysulfide migration and increases capacity on the other hand (Figure S3d).


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Figure 2. (a) TEM image of the CNTs which are on the surface of CC/S@CoNC/MWCNTs cathode. (b) HRTEM image of the CNTs which are on the surface of CC/S@CoNC/MWCNTs cathode. (c-g) TEM image (c) and the corresponding Co (d), C (e), N (f) and S (g) elemental maps from the region. Energy

dispersive

X-ray

spectroscopy

(EDS)

and

elemental

mapping

of

the

CC/S@CoNC/MWCNTs detected significant amount of cobalt, carbon, nitrogen and sulfur which are uniformly distributed on the cathode surface (Figure S3e-h and Figure S9). The X-ray photoelectron spectroscopy (XPS) was used to study the composition in the CC/CoNC cloth composite (Figure S10). The binding energy of 284.8 eV, 285.8 eV and 287.6 eV at Gaussian fit peaks were indexed to sp2 carbon, C=N bonds and C–N species, respectively, which verified that C–N bonds are formed during the annealing procedure. The signal located at 398.9 eV is corresponding to a pyridinic nitrogen, and the signals located at 400.4 eV and 401.3 eV are


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ascribed to pyrrolic-N and graphitic nitrogen, respectively. In addition, the diffraction peak at 778.5 eV is the binding energy of metallic cobalt, confirming the existence cobalt nanoparticles, while the diffraction peak at 780.3 eV, assigned to Co2+, means that cobalt elements are incorporated into the framework comprising a carbon matrix and the Co–Nx species.35-37 The existence of cobalt elements not only effectively accelerate electrode kinetics process but also conduce to formation of MWCNTs on the surface of cathode. The complex interactions between elements and hierarchical structure of CC/S@CoNC/MWCNTs lead to a high-performance cathode and good cycling stability.

Figure 3. (a) The cyclic voltammetry curves of the CC/S@CoNC/MWCNTs composite electrode (sulfur loading: 2.0 mg cm−2 ) at a rate of 0.1mV/s in the potential range 1.7-2.7 V. (b) The rate capability of the CC/S@CoNC/MWCNTs and CC/S@NC composite electrode at increasing current rates from 0.1 C to 2 C. (c) The corresponding voltage capacity profiles of CC/S@CoNC/MWCNTs at various rates. (d) Cycling performance and coulombic efficiency of the CC/S@CoNC/MWCNTs and CC/S@NC composite electrode at 0.2 C. (e) Its corresponding voltage capacity profiles of CC/S@CoNC/MWCNTs at 0.2 C. (f) Cycling performance and


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coulombic efficiency comparisons of CC/S@CoNC/MWCNTs and CC/S@NC composite electrodes at 1 C. To assess the electrochemical performance of the composite electrode, we actualize a series of electrochemical tests. Figure 3a gives the cyclic voltammetry (CV) plots of the CC/S@CoNC/MWCNTs cathode (S loading: 2.0 mg cm-2) in the initial five cycles. In the first cathodic scan, two remarkable peaks at ≈ 2.35 and 2.03 V are agreement with the typical plateaus of S cathode. The former peak was associated with the transition from S to polysulphides (long-chain Li2Sx, 4≤x≤8), and the other one corresponded to the further reduction of polysulphides (short-chain) to Li2S2 and Li2S that provides a main proportion of the cell capacity. In the anodic scan, two overlapping anodic peaks at about 2.38 and 2.42 V were found, corresponding to the reversible reaction in the charging stage, namely the conversion from Li2S to polysulphides and eventually, to elemental S. It is worth noting that no obvious changes are observed for the oxidation and reduction peaks in subsequent second to fifth cycles, meaning a high electrochemical stability of the cathode. For purposes of comparison, the CV plots of the CC/CoNC cathode were also investigated. There is no obvious redox peak indicating CC/CoNC composite itself does not have capacity contribution (Figure S11). Table 1. Summary of the best electrochemical performance of representative flexible sulfur cathodes. Sample description

CC/S@CoNC/MWCNTs (our work)

Capacity at low

Capacity retention

Capacity at high

Capacity

rate [mAh g-1]

(%)

rate [mAh g-1]

retention (%)

1312 (0.2C)

63% (100 cycles)

764 (1C)

55%(500 cycles)

Category of flexible cathodes

Hierarchical carbon cloth based flexible cathode

buckypaper/sulfur/buckypaper (B/S/B) cathodes

1010 (0.2C)

51% (400 cycles)

891 (0.5C)

57% (400 cycles)

31

Commercial

carbon

cloth/paper-based

flexible

cathodes S–rGO paper 37

1317 (0.06C)

67.5% (200 cycles)

657 (0.9C)

90% (168 cycles)

Graphene-based

flexible


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cathodes LRC/S@EFG electrode 39

1215 (0.2C)

78% (200 cycles)

688 (1C)

Carbonized

polymer-based

flexible cathodes C@WS2/S composite electrode

40

1318 (0.2C)

954 (1C)

88% (500 cycles)

Inorganic-based

flexible

composite cathodes NPGO-S electrode

41

1330 (0.2C)

1120 (1C)

70% (200 cycles)

Polymer-glued

flexible

composite cathodes G–NDHCS–S electrode 42

1360 (0.2C)

69% (100 cycles)

850 (0.5c)

65% (200 cycles)

Hybridized

carbon-based

flexible cathodes S/DPAN/MWNCT cathode

43

1450 (0.2C)

86% (260 cycles)

930 (2C)

CNT-based flexible cathodes

The CC/S@CoNC/MWCNTs cathode displays good rate performance which has stepwise capacities of 1323.2, 982, 847.4, 764.4 and 653.7 mA h g-1 at 0.1, 0.2, 0.5, 1, and 2 C, respectively. Moreover, the battery recovered the capacity of 904.3 mA h g-1 at 0.2 C, demonstrating a good reversibility (Figure 3b and 3c). Li-S batteries usually suffer from insufficient long-life performance owing to the shuttle effect of polysulfides and dissolution in organic electrolytes. Fortunately, the CC/S@CoNC/MWCNTs cathode exhibits a long-life cycle performance. Figure 3d and 3e show the long-life performance of cathode at 0.2 C. The battery provided a discharge capacity of 1312 mA h g-1 at the first cycle, which accounts for 78.33% of the theoretical value. The gradual increase for the first few rounds took place during electrochemical activation step on account of the electrolyte diffusion through separator film and the CC/S@CoNC/MWCNTs.32 At the end of recharging for 100 cycles at 0.2 C, the battery still kept a capacity of 827 mA h g-1, accounting for a retention of 63%. It is worth mentioning that even after 500 cycles at 1 C, the battery still remained a capacity of 464 mA h g−1 (Figure 3f), corresponding to 55% retention of initial capacity of 846 mA h g−1. Compared with the best electrochemical performance of commercial carbon cloth/paper-based flexible cathodes reported,31 the initial discharge capacity of CC/S@CoNC/MWCNTs cathode exhibited a


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significant enhancement from 1010 mA h g-1 to 1312 mA h g-1. The relative high initial capacity and stable cycle performance of composite cathode are also comparable to the best results of other type flexible sulfur cathodes reported, such as graphene-based,38 carbonized polymerbased39 and inorganic-based flexible cathodes40 (Table 1). The improved electrochemical performance and cycling stability of the electrode owes to the nitrogen doping on the CC/S@CoNC/MWCNTs, especially the pyridine-N, whose lone electrons have strong interactions with Li2Sx and trap polysulfide species.35 For comparison, CC/NC composite without Co was also fabricated as a sulfur host. After Co was removed, the MWCNTs were disappeared (Figure S12). PXRD patterns for CC/NC and CC/S@NC composite show that Co was removed and S@NC homogeneously distributed on carbon fibers of CC substrate (Figure S13). A series of characterizations of CC/NC and CC/S@NC such as N2 adsorption–desorption measurement (Figure S14), TG (Figure S15), CV (Figure S16), and EIS (Figure S17) have been carried out. The cycling performance of CC/S@NC electrode was studied under the same conditions as CC/S@CoNC/MWCNTs electrode. The CC/S@NC cathode only deliver capacity of 814 mA h g-1 at 0.2 C, and the capacity was faded to 348.8 mA h g-1 after 100 cycles. The CC/S@NC cathode displays a relatively poor rate performance which has stepwise capacities of 828, 574, 512.4, 438.2 and 374.6 mA h g-1 at 0.1, 0.2, 0.5, 1, and 2 C, respectively. As for the high rate, the coin cells assembled by CC/S@NC electrode mostly break down after 100 cycles. These results demonstrate that cobalt is very important for the improvement of battery performance (Figure 3b, d, f and Figure S18).


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Figure 4. XPS Characterizations of at CC/S@CoNC/MWCNTs composite electrode. (a) Typical five voltage points for XPS characterization in a discharge/charge voltage profile. (b-f) XPS spectra at five different voltage stages. In order to further verify the role of cobalt metal, the XPS spectra were performed at five states (Figure 4a) at 2.4 V (state A, Figure 4b), 2.1 V (state B, Figure 4c), 1.5 V (state C, Figure4d), 2.3 V (state D, Figure 4e) and 2.6 V (state E, Figure 4f) respectively in one discharge/charge cycle. The energy peak located at 169.3 eV and 170.5 eV can be observed at five states, which should be ascribed to the chemical bonding formation between cobalt and sulfur. The higher oxidation states of Co and the detected chemical bonding related to S both indicate possible chemical interactions between the inlaid Co nanoparticles and S species, which would help trapping and immobilizing S species during the whole electrochemical processes.44, 45 At the state A, the weak signal at 162.3 eV and 163.5 eV of Li-S bond and S-S bond of S8 at 163.6 eV and 164.8 eV can be observed, simultaneously, high valence sulphate was generated as side reaction. From state A to B, it can be clearly observed that the ratio of Li-S bond increase accompanied by the decrease


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of the ratio of S-S bond. When discharged to 1.5 V, the ratio of Li-S bond increased further and the peak of S-S bond was assigned to Li2S2. When recharging to 2.4 V, the ratio of Li-S bond and the amount of high valence sulphate decreased while the ratio of the bond of S-S increased which means Li2S2 was translated to polysulfide. Finally, when recharging to 2.6 V, the peaks of Li-S bonds and high valence sulphate were disappeared which further means that all the Li2Sx species were converted to sulfur (S8) completely. As shown in Figure S19, the binding energy of around 786.0 eV at Gaussian fit peak was indexed Co-S and there is always the Co-S bond at the corresponding different voltage state which also further confirmed that cobalt particle can bind polysulfides via Co-S bond. XPS analysis showed that the cobalt particle played an important role in improving the capacity and cycling stability of Li-S battery, which can not only bind polysulfides via metal-sulfur bonding, but also can catalyse the polysulfides from long-chain to short-chain effectively.


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Figure 5. (a) Cycling performance and coulombic efficiency of CC/S@CoNC/MWCNTs electrode with sulfur loading 3.0 and 5.0 mg cm−2 at 0.2 C. (b) EIS plots for CC/CoNC@S/MWCNTs composite electrode. (c) The cyclic voltammetry curves of composite electrode at a rate of 0.1mV/s in the potential range 1.7-2.7 V with sulfur loading: 3.0 mg cm−2 and (d) 5.0 mg cm−2. The problems of low sulfur content and low areal loading in cathode must be solved before the practical application of Li–S battery system.46, 47 So the electrodes with a sulfur loading density of 2–8 mg cm-2 are prepared to test the ability of the CC/CoNC for encapsulating sulfur. The electrochemical performances of CC/S@CoNC/MWCNTs electrodes with different sulfur loading are investigated. As shown in Figure 5a and Figure S20, compared to the CC/S@CoNC/MWCNTs cathode with the sulfur loading of 2.0 mg cm-2, the electrode with a


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higher sulfur loading density of 3.0 mg cm-2, 5.0 mg cm-2 or 8.0 mg cm-2 exhibits slightly decreased specific capacities which is probably due to the slow electrode kinetics process on the interface of electrode and the reduce of electric conductivity owing to high sulfur loading. In spite of the slight decrease of the specific capacity, a capacity of nearly 600 mA h g-1 can still be obtained for 8.0 mg cm-2. Furthermore, there is no obvious capacity decay after ten cycles for sulfur loading density of 3.0 mg cm-2, 5.0 mg cm-2 or 8.0 mg cm-2. This relatively high capacity mainly attributed to the large surface area and abundant mesopores. In order to deeper understanding of the electrochemical performance of the CC/S@CoNC/MWCNTs cathode, the internal resistances of fresh Li–S cells with a sulfur loading density of 2.0, 3.0, 5.0 and 8.0 mg cm-2 are evaluated by electrochemical impedance spectroscopy (EIS) measurements (Figure 5b and Figure S21). A typical semicircle in the high-medium frequency range related to the charge transfer resistance (Rct) and an inclined line at low-frequency associated to the mass transfer processes are observed in the Nyquist plots for the cells with the CC/S@CoNC/MWCNTs cathode display. The Rct values of the batteries with sulfur loading of 2.0, 3.0, 5.0, 8.0 mg cm-2 are 46.46 Ω, 54.52 Ω, 56.62 Ω and 86.47 Ω, respectively. From the above data, we can draw a conclusion that the CC/S@CoNC/MWCNTs cathode has a good electric conductivity. The CV plots of the Li–S cells with sulfur loading densities of 3.0, 5.0 and 8.0 mg cm-2 are also measured (Figure 5c-d and Figure S22). No obvious changes are observed for the oxidation and reduction peaks from the second to the fifth cycles when sulfur loading density up to 8.0 mg cm-2, indicating a high electrochemical stability of the cathode.


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Figure 6. (a) Stress–strain curve of electrode. (b) The electrical conductivity of the electrode over 200000 cycles. (c) The microscope photograph of curved portion after 200,000 bending cycles. (d) Voltage variation of soft package Li-S cells with fold, (e) cylinder, (f) wearable electronic device. The insert figures correspond to the demonstrations of flexibilities. Thanks to the above-mentioned integrity and versatility of CC/S@CoNC/MWCNTs cathode, we investigate some practical applications of soft package batteries in the field of foldable and flexible wearable electronics. In the production process, the composite electrode can be easily tailored and be made into different sizes and shapes to meet practical applications. We first fabricated soft package Li–S cells with different shapes, such as circular, hexagonal, rhomboid, triangle and V shape (Figure S23), which delivered similar area specific capacity around 1.8 mA h cm-2 at 0.5 C (Figure S24). Meanwhile, we also fabricated a series of soft package cells with different sizes, including 4×4 cm2, 6×6 cm2, 8×8 cm2 and 10×10 cm2, owing to the favorable handle ability of the cathode (Figure S23). A soft package Li–S cell of 4×4 cm2 can deliver an initial capacity 905 mA h g-1 at 0.5 C, which is comparable with that of a coin cells. After 25


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cycles, the discharge capacity of the soft package remained stable with a small capacity decay of 0.64% per cycle (Figure S25). In contrast to the regular electrochemical batteries, flexibility must be considered for the electrodes and soft package batteries from the practical use perspective. However, in most studies of flexible Li-S batteries, the flexibility is only presented by photographs of bent or curved, which are lack of quantitative evaluations. According to the review by Zhang et al.,48 only 25 previous literatures give the quantitative assessment on the flexibility of electrodes. For this purpose, various quantitative methods are applied in our study for evaluation of the flexibility. In order to evaluate the mechanical strength of our cathode, the tensile stress/strain curves of electrodes at different calcination temperatures of 600°C, 650°C, 700°C, 750°C, 800°C, 850°C and 900°C were tested. Meanwhile, to highlight the merit of S@CoNC/MWCNTs composite coating on carbon cloth, as a comparison, we also tested the mechanical strength of the original carbon cloth. The results of the tests indicated that the calcination temperature has no obvious effect on the tensile strength of electrode, and the tensile strength of the composite electrode was ≈15.2 MPa with 10% strain and Young’s modulus was 150 MPa, as twice as that of the original carbon cloth (stress ≈7.1 MPa with 13% strain) (Figure 6a, Figure S26). These unambiguous values are comparable to or even higher than those of the most reported flexible electrodes.48 The good mechanical strength of the cathode inspired us to conduct bending tests and further probe its persistence for mechanical deformation. The results showed the composite cathode has extraordinary persistence for tremendous mechanical deformation. The initial electrical conductivity of the electrode (400 S m-1) still remained stable even after 200,000 bending cycles (Figure 6b). The microscope photograph for the curved portion of the electrode showed that the


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fibers of carbon cloth kept integrity without any fractures (Figure 6c). Compared with the best results known to date (Table 2),49 the persistence ability of this composite cathode for mechanical bending is increased four times. To be closer to the deformation conditions for practical use of the flexible batteries, the flexibility test of the soft package Li-S battery based on this cathode was performed in a twostage process. In the first stage, the battery was folded into V shape with the bending angle φ≈ 165° and the bending radius ρ≈0.8 cm for 100 times (Figure 6d), which presents some special folding situations. In the second stage, the battery was further bent into a cylinder for 2000 times with the bending angle φ≈140° and the bending radius ρ≈2.5 cm (Figure 6e), which presents the most common service condition for flexible batteries. The test results showed that the soft package Li-S battery could withstand repeated folding more than 2100 times and maintain stable initial voltage plateau around 2.4 V. To our knowledge, there are only three comparable examples in the literature that evaluate the long-term stability of soft package batteries by repeated bending tests, the bending cycles of whose are 50,2 10046 and 5003 (Table 2), respectively. Therefore, there is no doubt that both the persistence and the flexibility of the cathode have significant advance over reported state-of-the-art Li-S batteries. These decent performance of the cathode make it eligible for simulating some practical applications in life. Firstly, a wearable electronic device imbedded with soft package cell was simulated. The elbow pad including a soft package Li–S cell is tied to the arm. The battery can keep the initial voltage around 2.3975 V after bending 100 times (Figure 6f, the bending angle φ ≈60° and bending radius ρ≈4 cm). Secondly, we took a soft package Li–S cell of 15×8 cm2 as a power source of a red LED light. The battery was rolled up into a cylinder with a diameter of about 1.8 cm and then put into a


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glass bottle. The light of LED was stable during the whole roll-up process (Figure S27a-b,). Similar result was obtained when it was used to simulate energy supply of Google Glass by wrapping around the leg of the glasses (Figure S27c). Table 2. Summary of flexibility for flexible sulfur cathodes and soft package batteries. Material

Electrode

Soft package battery -1

Stress (MPa)

Strain (%)

Conductivity (S m )

Repetitions (Times)

Repetitions (Times)

15.2

10

400

200,000

2100

20

~2.9

100

GO/CMK-3@S/CNT 52

~31–34

~12.5–15

1000

AAO-templated S–CNT 22

10

9

800

12000

0.13

~50

125

22000

6

3.3

800

50000

CC/S@CoNC/MWCNTs 51

rGO/CNT/S fiber

S–PDMS/GF

53

C/S composites

49

Carbon felt 2

50

Foldable DWCNT/S CNT/CMK-3@S

50

100

3

500

Finally, we test the application of this Li-S battery on flexible display. As shown in Figure S27d, Supporting Information, a strip-shaped soft package Li–S cell of 42×4 cm2 was fixed on the bottom of a traditional Chinese painting printed on a transparent polyethylene glassine paper (60×34 cm2) and then eight LED lights were arranged uniformly on the edge of the painting. With the lights turning on, the painting was rolled up gently and then was slowly unfolded. After repetitive operation for several times, the light of LED still hold stable, which demonstrated the battery was stable during the bending and flattening (Video S1, Supporting Information). All these results suggest that the composite electrode exhibit an excellent flexibility, high electrical conductivity and mechanical strength, a long-term cycle performance and bending durability, which is a promising candidate for high performance flexible Li-S batteries. 4. CONCLUSIONS


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In summary, we successfully designed and fabricated a CC/S@CoNC/MWCNTs integrated composite cathode with a hierarchical structure for flexible Li-S battery. Using carbon cloth as current collector, this flexible cathode is freestanding, economical and scalable. This flexible cathode can undertake 15.2 MPa stress with only 10% strain and exhibit high electrical conductivity of 400 S m-1, which remain stable after 200,000 bending cycles. The initial reversible discharge capacity is about 1312 mA h g-1 at 0.2 C with sulfur loading 2.0 mg cm−2. The capacity fading rate was 0.09% per cycle within 500 cycles at current of 1 C. The soft package Li–S cell based on the composite electrode exhibits an excellent flexibility which can withstand repeated folding more than 2100 times and maintain stable initial voltage plateau around 2.4 V. On the whole, both the persistence and the flexibility of the cathode have significant advance over reported state-of-the-art Li-S batteries. Furthermore, we simulate some applications of soft package Li-S battery for wearable electronic device and rollable display for the first time. Although, the mass energy density of such cathode is not high due to the relatively heavy quality of carbon cloth, it can be overcome by replacing carbon cloth with other light substrates such as carbon nanotube or graphene network. This strategy may guide us to develop flexible and high performance Li-S batteries for flexible lighting, rollable display and wearable electronic device in the future.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM, TG curves, and PXRD data of CC/S@NC and CC/S@CoNC/MWCNTs; Movie S1; additional electrochemical and characterization data (PDF).


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected]. Author Contributions §

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (No. 21771035, 21671034, 51725204, 21771132, 51572179, 21471106, 51422207, 21501126), the Collaborative Innovation Center of Suzhou Nano Science and Technology, and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES (1)

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TOC

An ultra-flexible composite cathode with hierarchical structure and high-stable conductive networks is proposed for flexible Li-S batteries. The electrode exhibits high initial discharge capacity of 1312 mA h g-1 at 0.2 C and remains structural integrity after 200,000 bending cycles. The soft package battery presents extraordinary flexibility and stability after bending and flattening over 2100 times.


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Hierarchical and Highly Stable Conductive Network Cathode for

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