Scalable Approach To Construct Free-Standing and Flexible Carbon

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A Scalable Approach to Construct Free-Standing and Flexible Carbon Networks for Lithium-Sulfur Battery Mengliu Li, Wandi Wahyudi, Pushpendra Kumar, FengYu Wu, Xiulin Yang, Henan Li, Lain-Jong Li, and Jun Ming ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12546 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 21, 2017

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ACS Applied Materials & Interfaces

A Scalable Approach to Construct Free-Standing and Flexible Carbon Networks for Lithium-Sulfur Battery Mengliu Li,† Wandi Wahyudi,† Pushpendra Kumar, Fengyu Wu, Xiulin Yang, Henan Li, LainJong Li, Jun Ming*

Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Kingdom of Saudi Arabia. E-mail: [email protected].

ACS Paragon Plus Environment


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ABSTRACT: Reconstructing carbon nanomaterials (e.g., fullerene, carbon nanotubes (CNTs) and graphene) to a multi-dimensional networks with hierarchical structure is a critical step in exploring their applications. Herein, a sacrificial template method by casting strategy is developed to prepare highly flexible and free-standing carbon film consisting of CNTs, graphene, or both. The scalable size, ultralight and binder-free characteristics, as well as the tunable process/property are promising for their large-scale applications, such as utilizing as interlayers in lithium-sulfur battery. The capability of holding polysulfides (i.e., suppressing the sulfur diffusion) for the networks made from CNTs, graphene or their mixture is pronounced, among which CNTs is the best. The diffusion process of polysulfides can be visualized in a specially designed glass tube battery. X-ray photoelectron spectroscopy analysis of discharged electrodes was performed to characterize the species in electrodes. A detailed analysis of lithium diffusion constant, electrochemical impedance, and elementary distribution of sulfur in electrodes have been performed to further illustrate the differences of different carbon interlayers for Li-S batteries. The proposed simple and enlargeable production of carbon-based networks may facilitate their applications in battery industry even as a flexible cathode directly. The versatile and reconstructive strategy is extendable to prepare other flexible films and/or membranes for wider applications. KEYWORDS: battery, carbon, sulfur, cathode, polysulfide


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INTRODUCTION Carbon nanomaterials, such as fullerene, carbon nanotubes (CNTs) and graphene, exhibit wide applications in materials science,1, 2 new energy,3 biological4 and environmental technologies5 due to their unique morphological and electronic structures. Except the researches on nanoscale devices (e.g., field emitter,6 molecular sensor7 and graphene barrister8), more and more efforts have been devoted to constructing a macro-sized two or three-dimensional films/membranes with different hierarchical structures satisfying for their industrial utilizations in the field of water treatment,9 gas separation10 and energy storage devices.11 Therefore, it would be very significant to develop one feasible strategy to reconstruct these single nano-units into hierarchical one with more intriguing performances. Although a series of superb multi-dimensional carbon-based films/membranes have been prepared,12-15 most synthetic strategies start from a uniform solution of carbon materials (e.g., CNTs, graphene (oxide), or their derivatives) and then form a layer on the filter paper by filtration. It is not easy to control the thickness and uniformity precisely during the filtration because carbon materials may precipitate from the solution and also it is challenging to pour the suspension uniformly onto the surface of filter particularly in initial step. Thus, it is still a challenge to fabricate a large area, thin and tunable films to date, especially in industrial production. In this investigation, multi-dimensional carbon networks are prepared simply by casting carbon suspension on a sacrificial template followed by carbonization. This approach is universally applicable for all carbon materials (e.g., CNTs, graphene, and their mixture), as well as any other solid materials in the form of suspension. It can be scalable and enables the mass production of carbon nanomaterial-based films, thereby largely facilitating their practical



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applications in energy (e.g., solar, fuel fell, battery), environmental and materials science (e.g., effluent disposal, gas adsorption, multi-functional composite). Recently, considerable attention has been paid on the positive effects of carbon interlayers in the configuration of lithium-sulfur battery,16-18 because the intercalated layer between sulfur electrode and separator can effectively suppress the dissolution/migration of polysulfide.19 However, there are very few reports comparing the detailed effects of different carbon interlayers.20 Based on the proposed scalable approach of fabricating flexible carbon networks, herein we discover the morphologies and electrical characteristics of different carbonbased interlayer (e.g., CNTs, graphene). In-depth studies on the polysulfide diffusion process and the evolution of S cathodes with and without various carbon interlayers were performed to understand their effect for the properties of lithium-sulfur batteries, where we find that the superior battery characteristics can be achieved with the CNT layers even when it is utilized as current collector-free cathode directly. It can hold the polysulfide for hundred cycles with an extremely stable performance and robust rate capability, which is applicable for the development of wound-type battery. EXPERIMENT Materials Preparation Multi-walled carbon nanotubes and few-layer graphene were purchased from the CNano Company and EscortRam Company respectively. The CNTs and graphene were dispersed and stored in the N-Methyl-2-pyrrolidone (NMP) with a mass percent of 5 wt%. The suspensions of CNTs, graphene and mixture of CNTs-graphene (mass ratio of CNTs : graphene = 1:1) were prepared by diluting and mixing with the ethanol, and then casted on both sides of the flexible template (e.g., cellulose paper) by doctor blade with tunable thicknesses. After drying the casted


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samples at room temperature, they were calcined at 450 ºC (heating rate, 2 ºC min-1) for 3 hour under steady Ar flow, giving rise to final free-standing carbon networks. Electrochemical Measurement The sulfur (Sigma Aldrich) and Super P with the mass ratio of 7:3 were mixed and melted at 200 o

C to form a sulfur-containing composite. As-prepared S-Super P composite was mixed with

polyvinylidene difluoride (PVDF) with mass ratio of 9:1 in NMP to form a slurry and then casted on the Aluminum foil. After drying the electrode at 40 ºC in vacuum oven overnight, the electrode was punched into Ø13 mm circular discs for assembling the battery. The mass percent of sulfur in the S-C composites is 70 wt% (Figure S1) and the sulfur loading in electrode is about 974.4 µgs cm-2. The battery was assembled in Ar filled glovebox, in which the content of oxygen and moisture was strictly controlled at below 0.5 ppm. The lithium-sulfur battery with the configuration of electrode│interlayer│separator│lithium-metal was applied using the 2032-type coin cell, with the electrolyte of 1.0 M bis(trifluoromethane)sulfonamide lithium salt (LiTFSI) in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) (V/V, 1:1) (1 wt.% LiNO3 vs. LiTFSI, 0.04 M LiNO3). The polysulfide-based electrolyte of 1 M LiTFSI, 0.4 M LiNO3 and 0.05 M Li2S8 in DOL/ DME was prepared as below.21 Stoichiometric ratios of metallic lithium pieces and sulfur powders were fully reacted in the solvent of DOL/DME (1:1) first, subsequently LiTFSI and LiNO3 were then added. The battery was tested within the voltage window of 1.5-3.2 V by the Arbin battery testing system, USA. The high temperature electrochemical performance was tested in the oven at 60 ºC. For the battery using carbon networks as the current collector-free electrode directly, 130 µL polysulfide-based electrolyte was added and the cut-off voltage was 1.8-3.0 V. Cyclic voltammetry (CV) was carried out under the scan rate of 0.075-1.0 mV s-1 and



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electrochemical impedance spectroscopy (EIS) was conducted over a frequency range of 200 kHz-10 mHz using Multi-Channel Potentiostat BioLogic VMP3. Characterizations The structure and morphology of the carbon networks and the sulfur distribution after discharge were characterized by the field emission scanning electron microscope (FESEM, FEI Quanta 600) and energy-filtered transmission electron microscopy (EFTEM). The Raman spectrum was collected on a Witec alpha 300R Raman spectrometer at a 514 nm excitation wavelength. The observation of polysulfide diffusion in electrolytes of lithium-sulfur battery was performed using a specially designed glass tube glass battery (Figure S2). Thermogravimetry analysis (NETZSCH, STA 449) was carried from 40-450 ºC with a heating rate of 2 ºC min-1 under Ar atmosphere. XPS measurements were carried out using a Kratos Amicus spectrometer with monochromatic Al-Kα radiation. The pressure in the chamber was maintained less than 1.0×10-6 mbar and the sample dimensions were less than 10 mm. Before the elemental scans, a wide scan was acquired for all samples to calibrate the spectra. The binding energy was corrected based on C1s peak at 284.6 eV. The backgrounds were all Shirley type with the Gaussian/Lorentzian line shapes and the analyzing software is CasaXPS. RESULTS AND DISCUSSION Features of Approach and Carbon Networks The schematic in Figure 1 illustrates the preparation of the free-standing carbon networks. The suspension of carbon nanomaterials in ethanol was casted on the surface of cellulose paper uniformly by doctor blade. The thickness of layer can be precisely controlled by the height of blade within the micrometers. The carbon nanomaterials covered on the cellulose paper are bendable after drying (Figure 1b-c). Also, the light (i.e., 1.69 mg cm-2) and highly flexible CNTs


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network film with a large area over 50 cm2 (e.g., > 10 cm × 5 cm) was obtained after carbonization. The surficial and cross sectional morphologies of carbon networks were characterized by SEM (Figure 1d). Similarly, the networks based on graphene and mixture of CNT-graphene can be constructed using the same strategy (Figure 1e-g). To our knowledge, this is the first attempt to present a simple and scalable approach to construct the free-standing films based on carbon nanomaterials while finely controlling the areal density even below 1.0 mg cm-2 (Figure S3). This approach has other features: (i) It can be incorporated with metal (oxide) precursors (e.g., metal nitrates) to form a metal (oxide)/C composite; (ii) two sides of the cellulose template can be casted with different materials, thereby maximally enriching functionalities and serving for multiple purposes. The carbon networks not only have multi-dimensional structure rich in porosity, but also maintain their intrinsic sp2 carbon characteristics (Figure S4a). The typical 2D peak at around 2670-2695 cm-1 and G-mode (i.e., in-plane vibration of the C-C bond) at about 1594 cm-1 are Raman signature of graphitic sp2 materials with crystalline structure, while the carbonized cellulose paper itself only exhibits the D and G band (Figure S4b).22 The value of ID/IG (D band around 1350 cm-1, attributed to the disorder induced by the defects and curvature of carbon materials) of CNTs, CNTs-graphene, and graphene are 1.004, 1.005 and 0.981 respectively. The microstructures for these networks, consisting of knitted CNTs, randomly accumulated CNTsgraphene and self-assembled graphene are observed under the TEM images (Figure S5). The structural differences of various carbon networks also reflect on their tunable colors from deep black to gray black and gray (inset of Figure 1d-g).



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Applications for Lithium-Sulfur Battery The carbon networks were applied as the intercalated layer in lithium-sulfur battery (Figure 2a), and the electrochemical behaviors of batteries using different carbon layers were investigated. At the rate of 0.25C (i.e., 418.7 mA g-1, 1C = 1675 mA g-1), the battery with CNTs layer shows much better performance with a high capacity of 1658 mAh g-1, which is higher than 1285 mAh g-1, 1107 mAh g-1, 543 mAh g-1 of CNTs-graphene, graphene and pristine electrode, respectively (Figure 2b, c), demonstrating the highest sulfur utilization in the presence of CNTs network. The capacity can maintain at 556 mAh g-1 after 100 cycles with perfect charge-discharge profile and good coulombic efficiency around 98% (Figure 2b). It is much higher than 306 mAh g-1 of graphene and 92 mAh g-1 of pristine electrode. The advantage of CNTs-intercalated battery was further confirmed by the smoother and longer discharge voltage platform at around 2.41 V and 2.08 V. Particularly it has the lowest polarization of 191 mV, much lower than 251, 337 and 436 mV of CNTs-graphene, graphene and pristine electrode, respectively (Figure 2c). The difference among these interlayers is more pronounced in the C-rate test. For example, average capacities of 1301, 924, 756, 630, 466 mAh g-1 under the rate of 0.1, 0.25, 0.5, 1, 2.5C were obtained using CNTs interlayer, which are higher than that of 1112, 731, 586, 471 and 317 mAh g-1 using CNTs-graphene and those with graphene and without any interlayers (inset of Figure 2e). After the high rate test, the battery with the CNTs interlayer can recover to 738 mAh g-1 under the rate of 0.25C, confirming the fine rate capability. Note that the discharge-tail around 1.7-1.5 V in the CNTs and CNTs-graphene intercalated batteries may result from the reduction of polysulfide which are trapped and affected by the CNTs (Figure S6). Although there is a decay in initial cycles, the electrochemical performances are comparable and even better than many reported lithium-sulfur batteries with an intercalated layer (Table S1).


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Polysulfide Diffusion Process and Interfacial Analysis To demonstrate the variable blocking effect of different intercalated layers, the visible electrolyte behaviors as cycling were studied by a special designed glass battery (Figure S2). Starting from the transparent electrolyte in Figure 3a-c, several characteristics can be summarized: (i) the electrolyte of CNTs-intercalated battery (Figure 3a) turns to light yellow even with a high utilization of sulfur after the 1st (dis-)charge cycle, demonstrating the efficient blocking effect of CNTs for the polysulfide. The side-leakage of polysulfide appeared until 5th cycle, further confirming the strong trapping ability of CNTs networks for polysulfides comparing to the graphene-intercalated battery. (ii) the electrolyte of graphene-intercalated battery (Figure 3b) immediately turns to yellow after the 1st (dis-)charge cycle, showing the poor capability of graphene networks for holding polysulfides. (iii) for the battery using pristine electrode without intercalated layer (Figure 3c), the color of electrolyte become light yellow slowly after many cycles, and it is ascribed to the low utilization of sulfur (low dissolution rate of S from cathodes to electrolytes), which cannot greatly change the color of electrolyte. The results confirm that the CNTs-based intercalated layer can largely enhance the sulfur utilization because of its higher conductivity and also it can trap the polysulfides close to the cathode side. These phenomena are consistent with the capacity measurement results in Figure 2. The XPS analysis of sulfur species in discharged electrodes further confirms the higher utilization of sulfur and other advantages after adding an intercalated layer. The S2p spectra for the cathode in the batteries with interlayers shows obvious peak at around 162.7 eV corresponding to Li-bonded S (Li2Sx),23, 24 while the S-S bond signals for elemental S8 at around 164.0 eV and 165.3 eV are very weak (e.g., the integrated peak area of elemental S8 for the cathodes in CNTs- and graphene-intercalated batteries are 5.8% and 10.6% respectively). It



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indicates that most elemental sulfur has been converted into short-chain polysulfides. By contrast, the cathode in the battery without the interlayer exhibits a relatively weaker peak from Li-bonded S but still presents a strong peak from elemental sulfur (i.e., 18.8%), demonstrating the low utilization of sulfur and the fast migration of dissolved polysulfide into the electrolyte. These results are consistent with the capacity in electrochemical tests and diffusion process of polysulfide in glass battery (Figure 2-3). Furthermore, the electrode of the batteries with interlayers shows the characteristic peak of LiTFSI at around 169.8 eV, while that without interlayers presents a prominent broad peak of decomposed LixSOy (i.e., 167.2 eV and 169 eV, which result from the decomposition of LiTFSI/LiNO3 on electrode surface).23, 24 The results suggest that the battery with interlayers helps to reduce the electrolyte decomposition likely due to their restriction for the polysulfides around the electrode. Meanwhile, the XPS F1s spectra in Figure 3e indicate the same conclusion. For example, the stronger CF3 and LiF peaks for the CNTs and graphene-intercalated battery means more LiTFSI salt were preserved on the electrode side and less LiTFSI/LiNO3 were decomposed to LixSOy.23, 24 Electrochemical Analysis The electrochemical behaviors of batteries were further analyzed by the cyclic voltammetry (CV) (Figure 4). In CV curves, two typical anodic peaks (i, reaction of S8 to Li2S6; ii, further reaction to Li2S2/Li2S) and one broad reversible cathodic peak (iii, several oxidation reactions from Li2S/Li2S2 to Li2Sx, 2 < x ≤ 8) are present (Figure 4a-c). These observations are consistent with the discharge plateaus at around 2.40 V and 2.08 V respectively and the charge plateau at about 2.22-2.42 V (Figure 4b).25-27 All batteries with different interlayers showed similar curves, but the CNTs-based one has three unique features: (a) a clear separation between discharge peaks (i) and (ii), demonstrating the stronger trapping-ability for polysulfide in networks; (b) a lower


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polarization and narrower anodic/cathodic peaks, revealing the uniform distribution of polysulfide within electrode/electrolyte in specific area rather than a wide range of reactions in the mixture, which actually occurred in the battery without using blocked layer (Figure 4a); (c) a small peak around 2.43 V appears under the slow scan rate (e.g., 0.075 mV s-1) but not in the batteries with and without a graphene interlayer (Figure 4a-c), confirming the reduction of polysulfide (i.e., Li2S6/Li2S8) into elemental sulfur step by step with help of CNTs.28, 29 To make an intuitive demonstration for the effect of blocking layers, the diffusion coefficient of lithium ion was calculated by the Randles-Sevcik equation,30, 31 which describes the relationship between peak current and the scan rate as below: ip = 2.69 × 105n3/2AD1/2Cυ1/2 where ip is the peak current (Amps), n is the number of electrons transferred in the reaction (here is 2, corresponding to the reaction of S8 to Li2S6),32 A is the electrode area (Ø13, which is 1.33 cm2), C is the change in concentration of Li+ (e.g., 0.54 mmol cm-3 calculated by the variation of lithium ions based on the amount of sulfur, 974.4 µgs cm-2 on electrode), υ is the scan rate (V s-1) and D is the diffusion coefficient (cm2 s-1). A plot of normalized peak current (ip) with the square root of the scan rate (υ1/2) was shown in Figure 4d. The results clearly reveal that the lithium ion diffusion coefficient with the CNTs interlayer is the highest, 1.93 × 10-7 cm2 s-1, over four times than 4.15 × 10-8 cm2 s-1 of the pristine one (Table S2). It is also higher than 1.39 × 10-7 cm2 s-1 of CNTs-graphene and 7.69 × 10-8 cm2 s-1 of graphene, which is in accordance with the battery cycling and rate test results. To get a better understanding of the effects, electrochemical impedance was investigated. The Nyquist plots were fitted to an equivalent circuit (inset of Figure 5) perfectly.33-35 Re represents the electrolyte ohmic resistance, corresponding to the intercept of Z’-axis at high



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frequency regions. The similar Re value of different batteries at around 1.4-2.1 Ω confirms the uniformity of battery. R1, R2 and R3 are the relative charge-transfer resistance at the interfaces of anode/electrolyte, cathode/electrolyte and blocked layer/electrolyte, and the sum of three resistances is 41.1 Ω (CNTs), 56.1 Ω (CNTs-graphene), 61.3 Ω (graphene) and 52.9 Ω (pristine electrode) respectively (Table S2). The electrical resistance using CNTs layer is much lower than that of graphene. Note that the charge-transfer resistance of CNTs-based battery decreases significantly to 2.6 Ω after the cycling, which is much lower than 14.8 Ω of CNTs-graphene, 6.9 Ω of graphene and 10.3 Ω of pristine one. The reason should be ascribed to the better conductive pathways along CNTs for lithium ions and its naturally higher conductivity, thereby giving rise to the superior performance. Further, the sharper slope of CNTs-based battery at the low frequencies (i.e., Warburg Impedance Zw) directly confirms the fastest lithium diffusion rate in the battery (Figure 5c). Structural Analysis and Further Extensions The blocking effect of interlayer was characterized by the energy dispersive X-Ray spectroscopy (EDX) of discharged electrodes (Figure 6a-f). The cross-sectional area of carbon layer are rich of sulfur, demonstrating the holding ability of intercalated layers for the polysulfide. The capability of CNT unit to trap polysulfides was further revealed by the energy filtered transmission electron microscopy (EFTEM) (Figure 6g-l), where the intensive sulfur signals were found at the locations with CNTs. The tubular structure of CNTs in knitted frame may be favor for holding and trapping the polysulfide in comparison to the accumulated graphene, which are more obvious at temperature tests. As shown in Figure 7a-b, the discharge capacity of battery with the CNTs interlayer can achieve 1205 mAh g-1, higher than 912 mAh g-1 of CNTs-graphene, 858 mAh g-1 of graphene, and 672 mAh g-1 of pristine electrode. A significant improvement was


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obtained in initial cycles particularly for the CNTs layer, and the voltage platform of chargedischarge profiles are fine as those performed at room temperature (Figure 2c). Stimulated by the feasibility of producing carbon networks in mass production (Figure S7), we further applied them as current collector-free electrode simulating the applications in the wound-type battery, in which the polysulfide-based electrolyte was utilized as sulfur source (Figure 7c). The electrochemical behaviors are quite different from the batteries using the sulfur electrode with intercalated layer. The battery has a very stable cycle performance, high rate capacities (e.g., 873, 736, 581 and 351 mAh g-1 at the rate of 0.5, 1, 2 and 5C respectively), and typical voltage vs. capacity profiles (Figure 7d-f). Clearly, the carbon networks can serve as diverse roles in the battery applications. To our best knowledge, this is the first time to use rolls of carbon networks for the wound-type lithium-sulfur battery, and we believe that many new things can be further discovered. CONCLUSION In summary, we present a new and versatile sacrificial template method for constructing flexible carbon networks in industrial-scalable production. The different effect of as-prepared carbon networks in lithium-sulfur battery have been investigated in details. The observation of polysulfide diffusion process from cathodes to electrolytes and the analysis of X-ray photoelectron spectra of discharged electrode were carried out to interpret how the intercalated layers work in the Li-S battery. In-depth analysis of lithium diffusion constant, electrochemical impedance, elemental sulfur distribution also confirmed that CNTs network is better than graphene even at high temperatures. The strong capability of CNTs networks as a current collector-free electrode for holding sulfur over hundred cycles has been further confirmed by rate capability tests. Importantly, the proposed fabrication method herein opens a convenient door for



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reconstructing the nanomaterials into free-standing networks which may facilitate their wider and practical applications.


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Figure 1 Schematic illustration of networks preparation and their features. (a) Double-side casting the CNTs slurry on sacrificial template of cellulose paper by doctor blade. (b, c) Flexible CNTs/template composite, and their corresponding SEM images of surficial and cross-sectional interface. (d) As-prepared CNTs networks after the carbonization. Surficial morphology, sample photo, schematic draw and cross-sectional images of (e) CNTs, (f) graphene and (g) CNTsgraphene networks.



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0.0

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Figure 2 (a) Schematic diagram of lithium-sulfur battery with intercalated layer and (b-e) the different effect of different carbon networks for performances. (b) Comparative voltage vs. capacity profiles and (c) cycle performances of lithium-sulfur battery with and without using intercalated layer. (d) Typical cycling and (e) rate voltage vs. capacity profiles of battery using CNTs layer. Inset of (e) is the comparative rate capabilities of batteries without and with using different carbon networks (black, CNTs; red, CNTs-graphene; green, graphene; blue, pristine electrode).


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CNTs

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Figure 3 Operando photos of cycled lithium-sulfur batteries with the intercalated layer of (a) CNTs, (b) graphene and (c) pristine battery. Comparative analysis of (d) S2p and (e) F1s spectra for the discharged electrode in the Li-S battery with and without an intercalated layer.



(a) 10

0.075 mV s-1 0.25 mV s-1 -1 1 mV S

8

0.1 mV s-1 0.5 mV s-1

(b)

(iii)

10 8 6

Current / mA

Current / mA

6 4 2 0 -2

4 2 0 -2

(i) (ii) Connected area

-4 -6 1.50

1.75

2.00

2.25

2.50

-4 2.75

3.00

-6

3.25

1.50

1.75

2.00

Voltage / V

(c)

2.25

2.50

2.75

3.00

3.25

Voltage / V

25

(d)0.007

20

0.006

15 10

0.005

2.43 V ip / A

Current / mA

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

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5 0

CNTs Mixture of CNTs-Graphene Graphene Without intercalated layer

0.004 0.003

-5

0.002

Seperated area

-10

0.001

-15 1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

0.005

0.010

0.015

0.020

0.025

0.030

0.035

υ1/2 / (V/s)1/2

Voltage / V

Figure 4 Typical cyclic voltammetry (CV) profile of (a) pristine electrode, (b) graphene, (c) CNTs under different scan rate from 0.075 mV s-1 to 1 mV s-1 and (d) graph of normalized peak current vs. square root of the scan rate.


Page 19 of 27

a 40

b 35 30 Resistance / Ω

-Im (Z) / Ω

30

20

10

25 20 15 10 5

0

0 0

10

20

30

40

50

60

70

80

Re

R1

R2

R3

Re

R1

R2

R3

Re (Z) / Ω c 40

d 35

20 CNTs CNTs-Graphene Graphene Wihout blocked layer

10

Resistance / Ω

30 30

-Im (Z) / Ω

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


25 20 15 10 5 0

0 0

10

20

30

40

50

60

70

80

Re (Z) / Ω

Figure 5 Niquist plot and resistance of lithium-sulfur batteries with and without using the intercalated layer (a, b) before and (c, d) after the cycle. Inset of (a) is the equivalent circuit, in which three double layer capacitances of the anode (CPE1), cathode (CPE2) and conductive intercalated layer (CPE3) were applied to fit.



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

a

b

c

d

e

f

g

h

i

j

k

l

20 nm

Figure 6 Cross-sectional SEM images of cycled (a, d) CNTs, (b, e) graphene and (c, f) CNTsgraphene blocked layer and the EDX of sulfur distribution in green color. The scale bar of (a-c) is 30 µm. EFTEM of elemental sulfur distribution on (g, h, i) CNTs and (j, k, l) graphene intercalated layer after the discharge. Blue and red color represent elemental carbon and sulfur respectively.


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Page 21 of 27

b 3.5

CNTs-Graphene CNTs Graphene Pristine electrode

750

3.0 Voltage / V

1000

500

2.5 2.0 1.5

250 0

0

5

10

15

20

25

Lithium Li2S8

Separator

Model

0

30

Cycle Number

c

d

f

1st 200

25th 50th 75th 400 600 800 Capacity / mAh g-1

100th 1000

400

600

800

1000

1200

1000

Voltage / V

Voltage / V

3.0 0.5C 2.5 2.0 1.5 1.0 0

200

CNTs-Graphene Prisitne electrode

Capacity / mAh g-1 800 600 400 200 0

Roll of CNTs-Networks

e

CNTs Graphene

1.0

Capacity / mAh g-1

Capacity / mAh g-1

a 1250

Wound-type battery

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


0.5C 0

20

3.5 3.0 2.5 2.0 1.5 1.0

5C 0

1C

2C

40 60 Cycle Number

200

2C

400 600 Capacity / mAh g-1

80

1C

5C 100

0.5C 800

1000

Figure 7 (a) High temperature performance of lithium-sulfur battery using different intercalated layers and (b) the typical voltage vs. capacity profiles. (c) Roll of CNTs network applied in the current collector-free battery simulating the applications in the wound-type battery. (d) Cycle performance of batteries using polysulfide-based electrolyte at variable rate of 0.5-5C, and its typical (e) cycle and (f) rate voltage vs. capacity profiles.



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

ASSOCIATED CONTENT Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. Thermogravimetric analysis, schematic of designed glass battery, SEM, TEM images, Raman spectra, electrochemical performances and comparative results, lithium diffusion and impedance analysis, and photo of roll of CNTs networks. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions †M. Li and W. Wahyudi contributed equally. ACKNOWLEDGMENT The research was supported by KAUST.


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Page 27 of 27

TOC Different Effect of Intercalated Layer

0.007 0.006 0.005

ip / A

Lithium Metal

Separator

Li2Sx

Intercalated Layer

Carbon-sulfur

Current Collector

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


CNTs Mixture of CNTs-Graphene Graphene Without intercalated layer

0.004 0.003 0.002

ip = 2.69×105n3/2AD1/2Cυ1/2

0.001 0.005

0.010

0.015

0.020

0.025

υ1/2 / (V/s)1/2


0.030

0.035

Scalable Approach To Construct Free-Standing and Flexible Carbon

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