Engineering High-Performance Sulfur Electrode from Industrial

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Engineering High-Performance Sulfur Electrode from Industrial Conductive Carbons Ning Ding, Jin Yang, Xiaodong Li, Zhaolin Liu, and Yun Zong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06837 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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ACS Sustainable Chemistry & Engineering

Engineering High-Performance Sulfur Electrode from Industrial Conductive Carbons Ning Ding,† Jin Yang,† Xiaodong Li,‡ Zhaolin Liu,*,† and Yun Zong*,† †

Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science,

Technology and Research), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Republic of Singapore ‡

Singapore Polytechnic, 500 Dover Road, Singapore 139651, Republic of Singapore

*(Z.L.) Email: [email protected], *(Y.Z.) E-mail: [email protected].

KEYWORDS: Industrial carbons; Surface area; Pore size; Electrical conductivity; Polysulfide adsorption; Single-walled carbon nanotubes; Lithium-sulfur battery

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ABSTRACT: As part of sustainable development, lithium-sulfur (Li-S) batteries exhibit great potential in grid energy storage and electrification applications, thanks to their high theoretical specific capacity, low cost and environmental benignity. Compared to scattered reports on anode and electrolyte development, huge effort in cathode research has led to the discovery of various new carbon materials with improved cell performance. Nevertheless, most of these carbons have cost issues with challenges in mass production, making their potential in practical Li-S batteries lean. On the other hand, a wide range of industrial conductive carbons are available with varied specifications and could be a good source for Li-S batteries if properly engineered. Herein, we systematically assessed 10 industrial conductive carbons, and found one particularly suitable to the fabrication of high-performance sulfur cathode. The carbon has a moderately high specific surface area and good electrical conductivity, and a moderate adsorption capacity to polysulfides. With a small portion of the carbon substituted by single-walled carbon nanotubes, an effective conductive network forms in the resultant sulfur electrode, facilitating sulfur reduction and polysulfide oxidation in cell operations. Consequently, it renders a reduced voltage hysteresis, increased cell capacity, uniform sulfur deposition in charge reaction on a mechanically enhanced electrode. At a sulfur content of 62.5 wt.%, the cathode delivers a specific discharge capacity of 452 mAh per gram of electrode at 1C, corresponding to a high energy density of 840 Wh kg-1 (1.7 times that of LiCoO2 cathode). A capacity retention of 75% was seen after 300 dischargecharge cycles, proving the feasibility of using industrial carbons to fabricate high-performance sulfur electrode for practical Li-S batteries.


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INTRODUCTION With the ever-growing need for portable electronic devices and electric vehicles, the demand on high-energy battery systems is surging. Lithium-ion batteries (LIBs), with their commercial launch by Sony in 1991, are dominating the energy storage device market.1 Further development opportunities in the technology are primarily to elevate the energy density by switching from low capacity cathode materials, e.g. LiCoO2, LiMn2O4, and LiFePO4, etc. with lithium-intercalation chemistry2 to high-valent metal oxides and elements, e.g. V2O5,3 Cr2O5,4 MnO2,5 chalcogens and iodine,6 hosting more lithium ions per formula in discharge. Sulfur, a by-product of oil refinery in abundance, shows particularly great promise. A sulfur electrode delivers a theoretical specific capacity of 1672 mAh g-1 (~ 12 times that of LiCoO2),7 making Li-S cells an attractive choice for the next-generation energy storage system. To date, practical adoption of rechargeable Li-S batteries is hampered by persistent technical hurdles. With the reversibility of lithium as a major issue at anode side, the primary concerns at the cathode side are the poor electrical conductivity of sulfur (5×10-30 S cm-1 at 25 °C) and the redox shuttles caused by the dissolution of lithium polysulfides (LiPSs) into the electrolyte in LiS cell reactions.8-10 Nanostructured carbon is often used in sulfur cathode to facilitate the electron transfer, accommodate Li2S or S deposition towards the end of discharge or charge reactions, and restrict LiPSs from diffusing into electrolyte via a “reservoir” mechanism. Various carbons have been tested in Li-S cells, including graphite, mesoporous carbons, carbon blacks, carbon nanofibers and nanotubes, graphene and biomass-derived carbons, etc., with a wide range of specific surface area (SSA, 1 to ~3000 m2 g-1), pore volume, and pore size distribution.11-15 For instance, a carbon aerogel with an SSA of 1208 m2 g-1 took up about 10% of LiPSs of its mass,16 more than that of industrial carbons (e.g. Super P and Vulcan carbons).17 Some heteroatoms


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doped-carbons showed enhanced LiPS adsorption,18 while just like other reported carbons with sophisticated nanostructure, mass production are questionable with additional cost implications. Some industrial carbons possess nanostructures with desirable specifications and may readily be used for sulfur cathode fabrications. Herein we assessed 10 industrial carbons and found one particularly suitable for this purpose. The carbon possesses a moderately high surface area and good electrical conductivity, but a moderate adsorption capacity to LiPSs. With the assistance of a very small portion of single-walled carbon nanotubes (SWCNTs), a sulfur composite is made with notably improved behaviors in sulfur reduction and polysulfide oxidation reactions. At a sulfur content of 62.5 wt.%, a specific discharge capacity of 452 mAh per gram of the electrode was achieved at 1C, giving an energy density of 840 Wh kg-1 which is 1.7 times that of LiCoO2 electrode. The Li-S cell retained 75% of its initial capacity after 300 cycles, demonstrating the effectiveness of sulfur electrode engineering using industrial carbons for practical applications.

EXPERIMENTAL SECTION Chemicals and Materials. All carbons were from commercial sources, labelled with Carbon N (N = I to X) to avoid potential conflicts of interest. Precipitated sulfur powder (99.5%) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 98+%) were products of Alfa Aesar, with polyvinylidene fluoride (PVDF, Solef® 5130) from Solvay, anhydrous N-methyl-2-pyrrolidone (NMP, 99.5%), 1,2-dimethoxyethane (DME, 99.5%) and 1,3-dioxolane (DOL, 99.8%) from Sigma-Aldrich. Lithium nitrate (LiNO3, 99.99%) and lithium sulfide (Li2S, 99.98%) were from Aldrich. All the chemicals and materials were used as received unless stated otherwise. Characterization and quantitative polysulfide adsorption test. The specific surface area, pore size, and pore volume of the 10 carbons were determined by the Brunauer-Emmett-Teller (BET) method from nitrogen gas adsorption-desorption isotherms at 77 K (ASAP 2020,


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Micromeritics). The quantification tests of polysulfide adsorption on the carbons were conducted by soaking each carbon of fixed mass inside Li2S6 solution while stirring for sufficiently long time, filtering off the solid part while measuring the absorbance of remaining Li2S6 inside the filtrate on a UV-Vis-NIR spectrophotometer (Shimadzu UV-3600). Li2S6 solution was prepared from a stoichiometric reaction of sulfur and Li2S in anhydrous DME/DOL (v/v, 1:1) for 5 h at 90 °C to achieve a nominal sulfur concentration of 0.5 g L-1. All carbons were vacuum-dried for 15 h at 100 °C, and then stored in an Argon-filled glovebox for the adsorption tests. After mixing carbon and Li2S6 solution, quantitative dilution by DME/DOL (v/v, 1:1) solvent was used to improve the wetting of carbons by Li2S6 solution. The resultant suspension mixture was stirred for 24 h, and then filtered off a polytetrafluoroethylene (PTFE) membrane filter with 1 µm pores. After quantitative dilution, the filtrates were transferred into air-tight quartz cuvettes with their light absorbance recorded. The procedures were performed in the glovebox, except the final UVVis absorption spectra taken within 5 min after the quartz cuvette left the glovebox. Electrode preparation and electrochemical evaluation. In a typical preparation, 90 mg of precipitated sulfur was manually ground in an agate mortar with 90 mg of one of the 10 carbons, prior to the addition of 400 mg of 5 wt.% PVDF dispersion in NMP. The sulfur/carbon/PVDF mass ratio achieved was 45:45:10 in the final sulfur electrodes. Pure NMP may be added to facilitate the formation of a homogeneous slurry, where more NMP was needed for lower density carbons. The slurry was evenly cast onto an aluminum foil and dried for 2 h at 80 °C, from which the electrode was cut into discs of 15 mm in diameter. For good adhesion of sulfur/carbon composite onto the aluminum foil current collector, the mass loading of the sulfur composite was controlled to be ~ 2.2 mg. In the electrochemical studies, coin cells (CR2032 type) were assembled in an Argon-filled glovebox (MBraun, H2O and O2 < 0.1 ppm) using lithium foil as


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anode (diameter: 15.8 mm) to couple with each of the as-prepared composite sulfur cathodes. The electrolyte was freshly prepared by dissolving LiTFSI into anhydrous mixture solvent of DME and DOL (v/v, 1:1) to achieve a lithium-ion concentration of 1 M, with 1 or 5 wt.% LiNO3 being added to suppress the polysulfide redox shuttles.19 To minimize the adverse impact of moisture, LiTFSI and LiNO3 salts were dried for 20 h at 180 °C prior to the transfer into glovebox. Each cell was filled with 20 µL of electrolyte and tested on a multi-channel battery tester (Neware). The electrochemical impedance spectroscopy (EIS) data was collected on a Metrohm Autolab in the frequency range of 100 kHz to 10 mHz at a bias of 0 versus the opencircuit potential. The electrical conductivity was measured using an Autolab TSC battery, with the temperature being controlled by Microcell HC system (± 0.1 °C). The morphology of sulfur electrode surfaces was characterized on a field-emission gun scanning electron microscope (FEG-SEM, JEOL JSM-7600) at an accelerating voltage of 5 kV.

RESULTS AND DISCUSSSION The ten carbons are commercial products available in large scale with a typical cost of a couple of US$ per kilogram, and some of them are currently used in lithium-ion battery manufacturing. They were characterized using BET to determine their specific surface area (SSA), pore size and pore volume, and the results are summarized in Table 1. Based on the SSA values, these carbons are divided into three categories: 1) high SSA carbons (SSA > 800 m2 g-1) for Carbons I-IV; 2) medium SSA carbons (260 m2 g-1 > SSA > 60 m2 g-1) for Carbons V-VIII; and 3) low SSA carbons (SSA < 20 m2 g-1) for Carbons IX and X. The high SSA carbons, Carbons I to IV, possess larger pore volume (1.17 to 3.80 cm3 g-1) and smaller average pore size (4.4 to 8.0 nm), which generally benefit Li-S cells for higher capacity (larger pore volume stores more sulfur) and improved cycle life (smaller pores better confine LiPSs). The medium SSA ones, Carbons V


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to VIII, have large pore size and low pore volume which are less suitable for Li-S cells. Carbons IX and X are essentially graphite with an average pore size comparable to that of Carbons V-VIII but an even lower pore volume, appearing to be the least attractive candidate for sulfur electrode. Their high electrical conductivity and dense-sheet morphology, however, could help enhance the electrode mechanical strength and the volumetric energy density.20

Table I. Summary of the BET data of the 10 studied industrial carbons. Carbon S/N

BET specific surface area (m2 g-1)

BJH Adsorption pore volume (cm3 g-1)

Adsorption average pore width (nm)

I

1580

2.96

8.0

II

1437

1.28

4.4

III

1126

3.80

7.8

IV

818

2.26

8.6

V

256

1.64

25.5

VI

220

0.68

12.9

VII

65.5

0.19

11.4

VIII

63.0

0.17

10.8

IX

17.8

0.07

15.0

X

0.82

0.005

25.1

Sulfur/carbon composites are often prepared from a mixture of porous carbon and sulfur at 155 °C, when the molten sulfur is in its least viscous state and hence most suitable for infiltration into the carbon pores.21 This preparation strategy is meant to have the discharged products of LiPSs confined inside the pores to suppress redox shuttles. Considering the 80% of volume expansion in sulfur lithiation, a specific pore volume of  1.8 cm3 g-1 is required to store -sulfur ( = 2.07 g cm-3) inside carbon pores in electrodes of practical batteries with a sulfur/carbon weight ratio of  2. From the data in Table 1 it seems that only Carbons I, III and IV would be able to satisfy such requirement. The validation to the necessity of a fairly large pore volume was conducted by evaluating all 10 carbons in sulfur electrode of Li-S cells, which will be discussed later.


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Porous carbons are known to adsorb LiPSs, as often shown by qualitative images. To acquire a quantitative understanding, we studied the LiPS adsorption capacity of the carbons on a UV-Vis absorption spectrophotometer, using Li2S6 as the representative of LiPS for its good stability and high solubility.22 The UV-Vis spectra in Figure 1a display an absorption peak centered at λ = 263 nm,23 with the peak absorbance growing linearly to the increased Li2S6 concentration. The trend of absorbance change of the “filtrates” off different carbons (cf. Experimental Section) suggests a linear correlation between the Li2S6 adsorption capacity and the carbon specific surface area, increasing from 0.24 mg gcarbon-1 for Carbon X (SSA: 0.8 m2 g-1) to 8.1 mg gcarbon-1 for Carbon IV (SSA: 818 m2 g-1), and further to 19.3 mg gcarbon-1 for Carbon I (SSA: 1580 m2 g-1, Figure 1b). This trend implies the physical adsorption nature of LiPS onto carbon surfaces. It is worth noting that for practical applications the sulfur/carbon weight ratio in electrode is  2, at least 100 times that of the quantity of Li2S6 adsorbed by these nanostructured industrial carbons (< 2.0 wt.%). Clearly, the contribution to the LiPS adsorption by the porous carbons in actual cell operations is insignificant which may be simply neglected. As the industrial carbons studied herein possess the specs (surface area, pore size, and pore volume) covering a fairly wide range, the obtained results should represent the typical behaviors of nanostructured carbons used as conductive matrix in LiS batteries. Hence, extra caution must be taken in future to attribute improved Li-S performance (particularly the cycling stability) to carbon-based polysulfide adsorption. Quantitative analysis would be needed as the gauge of validation to avoid misinterpretations.


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Figure 1. (a) UV-visible absorption spectra of Li2S6 solutions of regularly varied concentrations. Inset: The correlation between the Li2S6 concentration and the absorbance @ λ = 263 nm. (b) Li2S6 adsorption capacities on carbons of varied specific surface areas. The data points of Carbons VI and VII overlapped coincidently.

To evaluate the electrochemical performance in Li-S cells, all 10 carbons were prepared into sulfur/carbon composite by manually grinding each carbon with precipitated sulfur and PVDF binder (cf. Experimental), fabricated into electrode and evaluated in respective cells. The voltage profiles of the Li-S cells are shown in Figure 2, with two voltage plateaus each in the discharge. The upper at ~ 2.3 V originates from sulfur to Li2S4 reduction, and the lower at 2.1 V is from the subsequent conversion of Li2S4 to Li2S as precipitates on the carbon substrate in sulfur cathode.24 In addition, the cells using Carbons I to VIII show another voltage plateau at ~ 1.8 V in the first


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several cycles due to the irreversible reduction of LiNO3 on carbon substrate, which becomes more prominent for carbons of higher specific surface area.25 It artificially drives the capacity of the cells (e.g. using Carbons I, IV and V in the electrode) beyond the theoretical capacity of Li-S cells. To facilitate proper discussion on the cell discharge capacity, we only consider the capacity from sulfur reduction taking place at voltages of 1.85 V and above. Another observation in Figure 2 is the high cell capacity from high surface area carbons. With Carbons I - IV the cells delivered a high initial specific discharge capacity of ~ 1250 mAh g-1 (at 0.06C), corresponding to a sulfur utilization of 75% (1250 vs. 1672 mAh g-1) (Figure 2a-d). The use of Carbons V - VIII gave a slightly lower capacity of 1150 mAh g-1 (Figure 2e-h); while the cells using Carbons IX and X suffered a drastic capacity drop to about 750 mAh g-1 (Figure 2i,j). This is anticipated, as a larger surface generally accommodates more Li2S. High SSA porous carbon often has low electrical conductivity, yielding big charge-discharge voltage hysteresis. This was also observed in the cells using Carbons I-V (Figure 2a-e), with big drops in the discharge voltages while cycling at 0.25C (close-packed red lines). With Carbon II of poor electrical conductivity, the cell even ceased operations after few cycles (Figure 2b). In contrast, the voltage hysteresis was less serious for the cells using high conductivity carbons, e.g. Carbons VI, VII or VIII, where higher discharge voltage plateaus were seen with much smaller voltage drops at higher rate (0.25C, Figure 2f-h). The low SSA graphite Carbons IX and X with good electrical conductivity, however, only showed marginal improvement in their cell voltage hysteresis. This may be due to their large particle size (typically a few µm) and ultra-low pore volume, hampering the formation of an effective conductive network which is needed for smooth cell operations.


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Figure 2. Galvanostatic voltage profile of Li-S cells using different carbons: a) Carbon I; b) Carbon II; c) Carbon III; d) Carbon IV; e) Carbon V; f) Carbon VI; g) Carbon VII; h) Carbon VIII; i) Carbon IX and j) Carbon X. The specs of all 10 carbons are summarized in Table 1. Each sulfur electrode consists of 45 wt.% sulfur, 45 wt.% carbon and 10 wt.% PVDF binder. LiNO3 concentration in the electrolyte was 5 wt.%. The cells were stabilized at 0.1 mA (or 0.06C) for 6 cycles (colour curves), then cycled at 0.25C for 10 cycles (close-packed red lines at reduced capacity) in the voltage window of 1.5-3.2 V vs. Li/Li+.


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The addition of 1 to 5 wt.% of LiNO3 into the electrolyte is a typical approach to suppressing LiPS redox shuttles in Li-S cells. The gradual reduction of LiNO3 on the electrodes, however, leads to continuing decay in the cell capacity.26 It is noteworthy that the dissolution of LiPS into the electrolyte also causes the cell capacity decay, which continuously deteriorates the cells using LiNO3-free electrolyte but reaches a stabilized state in their counterparts with LiNO3 added in the electrolyte. In Li-S cells with sulfur cathodes fabricated from the 10 carbons, the capacity decay was generally found faster in the initial 6 cycles for those using Carbons I-VIII than their counterparts using Carbons IX and X. Such difference is likely due to the formation of LiNO x layer on carbon surfaces in discharge, with more LiNO3 consumed on Carbons I-VIII of notably larger specific surface area. With the LiNOx layer growing thicker, the dissolution/precipitation of Li2S/sulfur in the following charge cycles is getting more difficult, leading to the cell capacity loss and a shortened cycle life.27 The undesirable LiNO3 reduction on carbon surfaces can be mitigated by setting the cut-off discharge voltage above 1.85 V. Among the 10 cells shown in Figure 2, balanced performance in capacity and rate capability is seen in the four cells using electrodes fabricated with Carbons IV, VI, VII and VIII. When new cells using these four carbons were tested with a cut-off discharge voltage of 1.85 V, the voltage plateau arising from LiNO3 reduction almost vanished. All four cells showed a capacity retention of > 80% after 100 cycles at 0.25C (Figure 3a-d), which further validates that the difference in the specific surface area of the carbons has almost no influence on the cycling stability of Li-S cells. Nonetheless, high surface area carbons provide more active sites for Li2S/S deposition. For instance, the cell with Carbon IV in sulfur cathode was able to deliver a high specific discharge capacity of ~ 1000 mAh g-1 at 0.06C and ~ 800 mAh g-1 at 0.25C (Figure 3a), respectively. The associated lower conductivity from its high surface area, as anticipated, resulted in an abrupt


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drop of the discharge voltage for cycling at 0.25C which in average is 0.13 V lower than that of other cells (Figure 3b-d). Collectively, the Li-S cell with Carbon IV was found to deliver a 15% higher specific energy.

Figure 3. (a-d) Galvanostatic voltage profiles of Li-S cells with sulfur/carbon (w/w: 1:1) electrode. a) Carbon IV; b) Carbon VI; c) Carbon VII and d) Carbon VIII. The cell was stabilized at 0.1 mA (0.06 C) for 10 cycles (blue lines), followed by cycling at 0.25C for 100 cycles (red lines). The concentration of LiNO3 in the electrolyte was 5 wt.%. (e-g) Galvanostatic voltage profiles of Li-S cells with sulfur/carbon (w/w: 2.3:1) electrode at 0.1C. (e) sulfur/Carbon IV, (f) sulfur/Carbon IV+SWCNTs (1 wt% SWCNTs in


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all carbons) and (g) sulfur/SWCNTs. The sulfur content was 62.5 wt.%, with a loading of 1.0 mg sulfur per electrode. The concentration of LiNO3 in the electrolyte was 1 wt.%. (h) Conductivity of Carbon IV, SWCNTs, and the sulfur/Carbon IV with and without SWCNTs.

To achieve higher energy density, the sulfur/carbon weight ratio in the electrode was increased to 2.3:1 (i.e. sulfur, Carbon IV and PVDF in 62.5, 27.5, and 10 wt.%). As anticipated, the higher sulfur content compromised the electrode conductivity and thus widened the voltage gap, giving a faster capacity decay in the cycling tests (Figure 3e). In addition, the initial discharge capacity (1104 mAh g-1) and the average voltage (2.01 V) at 0.1C were also lower than that of the cells shown in Figure 3a-d, suggesting lower sulfur utilization rate and increased polarization. Despite the reversible deposition of Li2S/sulfur in the first discharge/charge with a Coulombic efficiency of ~ 100%, the voltage plateau from Li2S4 to Li2S reduction was found vanished over the next few cycles, accounting for 80.5% of capacity loss (1104 to 215 mAh g-1). This is likely due to the increased polarization from the redistribution of sulfur on the initially ineffective conductive network over cycling, shifting the Li2S4 to Li2S reduction plateau to a voltage below 1.80 V.28 The construction of an effective conductive network was realized by substituting a very low portion of Carbon IV with the same amount of single-walled carbon nanotubes (TUBALLTM, 0.2 wt.% of SWCNTs dispersion in NMP). Impressively, with merely 1 wt.% of carbon being the SWCNTs, the sulfur electrode delivered an energy density (Ed = specific capacity × average operating voltage × sulfur content) of 1282 Wh kg-1 at 0.1C (Figure 3f, 5th cycle data, average voltage: 2.13 V), 26% higher compared to the cell using Carbon IV with a lower sulfur content (Figure 3a) or 4 times higher than the one with Carbon IV and the same sulfur content but in the absence of SWCNT (Figure 3e). Interestingly, SWCNTs as the sole carbon source (60 wt.% sulfur, 30 wt.% SWCNTs and 10 wt.% PVDF) gave a very low cell capacity of ~ 200 mAh g-1 even if the discharge voltage was extended to 1.5 V (Figure 3g). As the SWCNT film prepared


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via chemical vapor deposition at 1100 °C has been reported to enable a high-performance sulfur electrode,29 the much lower capacity in this case may be due to the aggregation of SWCNTs into bundles upon solvent evaporation in electrode fabrication (starting from a SWCNTs suspension) and/or the surfactant residue on SWCNTs, leaving notably smaller exposed surfaces for Li2S/S deposition. Further deterioration appears when SWCNTs bundles stuck onto the aluminum foil current collector to form a dense film that impedes the dissolution and diffusion of sulfur/LiPSs, lowering sulfur utilization rate and cell capacity. Hence, a binary-carbon system of Carbon IV (for high SSA) and SWCNTs (facilitating conductive network formation) in the sulfur electrode would integrate their advantages to enable much improved cell performance. To better understand such a binary-carbon based sulfur electrode, the electrical conductivity of Carbon IV, SWCNTs, sulfur/Carbon IV, and sulfur/Carbon IV+SWCNTs were measured and shown in Figure 3h. In the experiment, 0.2 g of each sample was loaded into a TSC battery under an applied force of 300 N (Φ = 12 mm) to ensure good electrical contact, with the conductivity data recorded at a few temperature points between -30 and 100 °C. Interestingly, Carbon IV was found to possess higher electrical conductivity than SWCNT, with the consistent results obtained from the sample of Carbon IV partially substituted by SWCNTs in sulfur electrode (59% lower, 3.42 vs. 8.44 S cm-1 at 25 °C). Obviously, “conductivity enhancement” is not an immediate role of the SWCNTs in the Carbon IV/SWCNT/sulfur composite electrode (as perceived previously). The actual functions need to be further investigated.


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Figure 4. (a-g) SEM images of sulfur particles (a), the pristine electrodes of sulfur/Carbon IV (b) or sulfur/Carbon IV+SWCNTs (c), the cycled electrodes in full-charge state with sulfur/Carbon IV (d,e) and sulfur/Carbon IV+SWCNTs (f,g). The images of EDX mapping (e,g) show the precipitated sulfur on the carbon substrates. (h,i) Nyquist plots of a Li-S cells with sulfur/Carbon IV or sulfur/Carbon IV+SWCNTs electrode in (h) fully-charged state and (i) fully-discharged state. The cells were first stabilized at 0.1 mA for 2 cycles and relaxed for 10 hours before the EIS test. The electrode composition was 62.5 wt.% sulfur, 27.5 wt.% carbon and 10 wt.% PVDF. For sulfur/Carbon IV+SWCNTs electrode, the carbon was made of 99 wt.% of Carbon IV and 1 wt.% of SWCNTs.

Under an optical microscope notable difference was observed between sulfur electrodes with and without SWCNTs (Figure 4). With sulfur particles in the size of tens of micrometers (Figure 4a), SWCNTs-free sulfur electrode has micrometers-wide cracks (Figure 4b) which compromise


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electron conduction at the electrode level. In sharp contrast, with 1 wt.% of carbon as SWCNTs, a crack-free sulfur electrode was obtained (Figure 4c). This improvement may be attributed to a better “gelling” of sulfur particles with the carbon matrix, such that the dispersed SWCNTs with a flexible 1D structure effectively bridge the carbon granules to enhance mechanical integrity of the electrode (Figure 4b,c, insets). Upon discharge, the lithiated sulfur dissolves into electrolyte to leave voids among carbons which in the following charge are refilled by sulfur deposition to give smooth surfaces (Figure 4d,f). In sharp contrast to the peel-off of sulfur/carbon from Al foil current collector (Figure 4d, inset) in the SWCNTs-free electrode, uniform sulfur was seen on its SWCNTs-contained counterpart at full-charge state (Figure 4f, inset). This is also evidenced by the sulfur energy-dispersive X-ray (EDX) mapping (Figure 4e,g), showing completely different sulfur distribution patterns on the electrodes. The full coverage of sulfur in Figure 4g indicates that the incorporated SWCNTs favor uniform sulfur deposition. Moreover, the notably improved mechanical integrity of the sulfur electrode by SWCNTs-incorporation effectively facilitates the charge transfer in the electrode, giving a much lower resistance of 73 Ω for SWCNTs-contained cell as compared to its SWCNTs-free counterpart (146 Ω, twice as much as the former) at the full-charge state (Figure 4h). Similarly, notably lower cell resistance was also found at the fullydischarged state for the SWCNTs-contained cell (Figure 4i), which should have benefitting from the improved electrode integrity. Using the sulfur/Carbon IV+SWCNTs composite cathode, a capacity of 650 and 547 mAh g-1 was achieved after 50 and 300 cycles at 1C, respectively (Figure 5a). The increase of SWCNTs to 2 wt.% shows no effect on cell capacity and cycling performance (~ 75% capacity retention after 300 cycles) but gives a slightly higher discharge voltage plateau (1.84 V on average, cf. Figure 5a, inset). With the content of SWCNTs in the carbons increased to 4 wt.% the average


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discharge voltage was further elevated to 1.92 V, but at the cost of a 12% loss in the cell capacity (570 vs. 650 mAh g-1). The integration of discharge voltage profiles gives the electrode with 2 wt.% SWCNTs (relative to the overall carbons) the highest energy density of 840 Wh kg-1 at 1C (Figure 5b), 1.7 times that of LiCoO2 cathode in LIBs (for LiCoO2 calculations: the capacity as 140 mAh g-1 at 1C; the average voltage as 3.7 V; with 95 wt.% LiCoO2 in the electrode). This advantage is readily achieved at a low areal sulfur loading of ~0.7 mg cm-2 (based on ~ 2.0 mg of sulfur/carbon composite, a sulfur content of 62.5%, a circle electrode with  = 15 mm), and may be further enlarged by the use of a much higher areal sulfur loading of 7 mg cm-2 in free-standing sulfur electrode film strengthened by SWCNTs which will be part of our follow-up studies to be reported elsewhere.

Figure 5. (a) Cycling data and voltage profiles (the inset) at 1C for cells with SWCNTs-contained electrode. The SWCNT content was 1, 2 and 4 wt.% of the total carbons. (b) Summary of sulfur electrode energy density in this study. The data of sulfur/Carbons IV, VI-VIII were derived from 1st discharge plateau at 0.25C, and those with SWCNTs were calculated based on the 5th discharge plateau at 1C.


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It is worth noting that self-discharge could be a concern for Li-S cells due to the parasitic reactions occurring on the both electrodes,19 e.g. the loss of sulfur from cathode into electrolyte via the formation of soluble high-order polysulfides (Li2Sn, n= 6-8) (chemical reduction by loworder polysulfides, Li2Sn, n = 2-4), and the chemical oxidation of lithium anode by the soluble polysulfides (Li2Sn, n= 4-8).30 Self-discharge reactions are proven to be mitigatable by a few approaches which are used alone or in combinations, e.g. addition of LiNO3 into electrolyte,31 use of fluorinated solvent32 or ionic liquid33 in electrolyte, a gel-ceramic multilayer electrolyte,34 an ion-selective separator,35,36 a functionalized interlayer,37 and/or an well-protected lithium anode,38 etc. Moreover, the use of lithium metal anode also has safety implications due to the formation of lithium dendrites which may pierce through the membrane to cause internal shortcircuit of the cell. This may be addressed by using a Li2S cathode to provide both the sulfur and lithium sources, coupling with a lithium metal-free anode to form a new type of Li-S cells.39,40 Hence, it would be inappropriate to overly stress on the performance of any single component in the development of Li-S cells for practical applications due to the underlying interlinks among its key components which may compromise the performance of one another. A holistic engineering approach would help identify the best compromise (or synergy) to ensure the competitiveness of final Li-S cells in terms of energy density, power density, cycle life and cost factor, paving the path to their commercialization. More and greater success is anticipated along this line in near future.

CONCLUSIONS We investigated 10 industrial nanostructured carbons covering a wide range of specifications on their suitability for the preparation of sulfur electrode in practical Li-S batteries. A carbon with a moderately high surface area and a good electrical conductivity (Carbon IV) stands out by


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its balanced cell performance in capacity and rate capability, if a moderate sulfur content is used. At higher sulfur content the electrode using such carbon alone cracks, leading to much poorer cell performance. Crack-free electrode readily forms, if an ultra-small portion of such carbon is substituted with SWCNTs. SWCNTs alone are not a good substrate for sulfur electrode; while its incorporation at low content into the electrode effectively lowers the cell resistance and enables uniform sulfur precipitation. With a SWCNTs-incorporated sulfur electrode at a sulfur content of 62.5 wt.%, the specific energy achieved at 1C was 840 Wh kg-1, 70% higher than that of LiCoO2 electrode used in LIBs. This work demonstrates the potential of sulfur electrode engineering with industrial carbons for practical Li-S battery applications. It deepens understanding in the role of the nanostructured conductive carbons in Li-S cell operations, inspiring the development of new sulfur substrates to accelerate the commercialization of Li-S batteries.

AUTHOR INFORMATION Corresponding Author *E-mail addresses: [email protected] (Z.L. Liu) *E-mail addresses: [email protected] (Y Zong)

ACKNOWLEDGMENTS This work was conducted under a core grant for collaborative research (IMRE/14-1C0243), supported by Institute of Materials Research and Engineering, with partial support from grants under the Advanced Energy Storage Research Programme of Science and Engineering Research Council (SERC, award numbers: 1229904044 & 1229904045), Agency for Science, Technology and Research (A*STAR), Singapore.


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SYNOPSIS

Industrial carbon is engineered into high-quality sulfur electrode with the assistance of very low content of SWCNTs as additives, giving greatly enhanced mechanical integrity, uniform sulfur deposition and good cell performance.

TOC Graphic: For Table of Contents Use Only


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Engineering High-Performance Sulfur Electrode from Industrial

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