Exploiting Pulping Waste as an Ecofriendly Multifunctional Binder for

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Exploiting pulping waste as an ecofriendly multifunctional binder for lithium sulfur batteries Xiufen Wu, Chao Luo, Leilei Du, Yinglin Xiao, Shuai Li, Jun Wang, Chaoyang Wang, and Yonghong Deng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00054 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 13, 2019

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Exploiting pulping waste as an ecofriendly multifunctional binder for lithium sulfur batteries Xiufen Wu† #, Chao Luo† #, Leilei Du†, Yinglin Xiao†, Shuai Li‡, Jun Wang†, ‡, Chaoyang Wang§, Yonghong Deng†∗

†Department

of Materials Science & Engineering, Southern University of Science and

Technology (SUSTech), Shenzhen 518055, P. R. China

‡Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology (SUSTech), Shenzhen 518055, P. R. China

§Research

Institute of Materials Science, South China University of Technology, Guangzhou

510640, P. R. China

#X.

Wu and C. Luo contributed equally to this work

*Corresponding author

Tel.: +86-0755-88015462; Fax: +86-0755-88015462;

E-mail: [email protected] (Yonghong Deng)

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Abstract: Lithium sulfur (Li-S) batteries have drawn tremendous interest owing to high energy density, low cost and environmental friendliness. However, the practical application of Li-S batteries is severely restricted by limited cycle life and high self-discharge rate. Here, for the first time, one papermaking waste, calcium lignosulfonate (LSCa) is employed as a novel ecofriendly multifunctional binder for Li-S batteries. The LSCa electrode retains a capacity of 453 mAh g-1 after 500 cycles at 1C (1C = 1675 mA g-1), and it delivers a high capacity of 571 mAh g-1 even at 5C, the performance of which is much better than that of conventional PVDF electrode. Furthermore, a preferable areal capacity of 4.16 mAh cm-2 after 100 cycles at 0.05C is obtained with a high sulfur loading of 7.64 mg cm-2. These achievements are ascribed to the better adsorption ability to polysulfides, more favorable Li+ transportation and superior adhesion property of the LSCa, compared to PVDF.

Keywords: Li-S batteries, binder, lignosulfonate, papermaking waste, areal capacity

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Introduction Lithium ion batteries (LIBs), due to limited energy density, are unable to fully meet the ever-increasing demands for energy storage and electric vehicles. Accelerated development of advanced battery systems is critical to satisfy the requirements of high energy density for long endurance mileage.1,2 Recently, lithium sulfur (Li-S) batteries have attracted much attention owing to their ultrahigh theoretical specific capacity (1675 mAh g-1), low cost and environmentally friendly property. They are considered as a promising candidate for the next generation high-energy batteries.3,4 However, there are still several challenges in Li-S batteries, including the insulating nature of sulfur, the notable shuttle effect caused by soluble polysulfides, and the large volume expansion (~78%) going from sulfur to Li2S upon lithiation, which impede their commercial application.5-8 To solve these problems, considerable efforts have been done, designing various novel sulfur composites9-12 and optimizing electrolyte components.13,14 Besides the above efforts, novel binders are also introduced to improve the performance of Li-S batteries. Due to scarce adsorption ability and weak adhesion strength from Van Der Waals Force, conventional polyvinylidene difluoride (PVDF) binder cannot accommodate the uncontrolled shuttle effect and the large volume expansion of Li-S batteries upon lithiation.15 Ideal binders for Li-S batteries should benefit good electrochemical performance by facilitating electron/ion transportation, mitigating shuttle effect, and buffering volume change. Various functional binders, such as perylene bisimide (PBI),16 polypyrrole and polyurethane (PPyPU),17 poly (ethylene glycol) diglycidyl ether with polyethylenimine (PPA)18 have been exploited to enhance the electrochemical performance of Li-S batteries. These binders require undesired complex synthesis, employing toxic and expensive

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organic chemicals as solvent. Therefore, it is of great significance to develop more effective, ecofriendly and lower-cost binders for Li-S batteries. Lignin, as the main waste byproduct of pulping industry, has a production of about 70 million tons every year. However, little of it is reasonably used. Consequently, much work so far has focused on the high-value applications of lignin, such as flame retardants, sorbents and medical chemicals.19-21 There are also promising applications in energy field due to its inexpensiveness, abundance and environmentally friendly features. For example, carbon materials have been developed from lignin for supercapacitors, LIBs and sodium ion batteries, etc.21,22 In our previous work, lignin derivative was prepared and introduced to silicon anode as a highly effective binder, and the batteries showed noticeably improved electrochemical performance.23 While, to the best of our knowledge, lignin derivative has not been used for the cathode binder for Li-S batteries yet. Lignosulfonate, the waste from sulfite pulping process, which is water soluble and rich in polar sulfonate groups, may have high ionic conductivity and good polysulfides adsorption.24,25 Inspired by the above-mentioned aspects, we directly introduce the lignosulfonate as a binder for Li-S batteries. Compared to conventional PVDF binder, we demonstrate the superiority of lignosulfonate as a water-soluble binder for sulfur cathode by exhaustive electrochemical, optical and mechanical studies.

Experiment section Preparation of Binders and Sulfur Cathode PVDF(HSV900) was purchased from Sigma-Aldrich. Sodium lignosulphonate (LSNa) and calcium lignosulfonate (LSCa) (typical chemical unit structure is shown in Figure S1) were purchased from Macklin. LSCa and LSNa were dissolved in deionized water while PVDF was

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dissolved in N-methyl pyrrolidone. Sulfur-carbon composite was prepared by mixing sulfur and Ketjenblack carbon (KJC) at a mass ratio of 2:1. Then, the mixture was heated at 155 °C for 16 h in a vacuum oven to melt sulfur into the pores of the carbon matrix. The obtained sulfur-carbon composite was labeled as S/KJC. Cathode electrodes were prepared by mixing S/KJC (90 wt%) and binder (10 wt%) without the addition of extra conductive agent. The obtained slurry was cast on carbon-coated aluminum foil, followed by drying under vacuum at 60 °C for 12 h. The areal loading of pure sulfur is typically 1.3-1.5 mg cm-2.

Materials Characterization The S content in S/KJC composite was 64.8%, confirmed by thermogravimetric analysis (TGA, Mettler) at a heating rate of 10 °C min-1 from 30 to 600 °C under nitrogen atmosphere (Figure S2). The surface morphology of sulfur electrodes was recorded by field-emission scanning electron microscopy (SEM, Tescan mira3, Oxford) at an accelerating voltage of 10 kV. Contact angle measurements were carried out with contact angle tester (AST VCA OPTIMA) by dropping the electrolyte solution on the surface of electrodes. Peel force studies were conducted with universal tensile machine (AG-Xplus HS). The adhesive tape was removed by peeling at an angle of 180° and a constant displacement rate of 5 mm min-1. Nano indentation investigations (Keysight UTM150) were performed on Berkovich indenter. The maximum load was 200 mN with a surface approach velocity of 10 nm s-1 and a peak hold time of 5 s. For polysulfides adsorption measurement, 0.5 m2 binder powder was added into 2 mM Li2S6 solution in 2 mL DOL/DME mixture (1:1, v/v) for 48 h.26

Cell Assembly and Electrochemical Investigations

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CR2016 coin cells were assembled in an argon-filled glove box (Vigor). The sulfur electrode was cut into a 12 mm disc as cathode while lithium metal foil was used as anode and porous polypropylene (2400, Celgard) was used as separator. The electrolyte was 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a mixture of 1, 3-dioxolane (DOL) and 1, 2-dimethoxyethane (DME) (1:1, v/v) with 1 wt.% LiNO3 as additive. The amount of the electrolyte used in each cell was 40 μl. For cells with a high sulfur loading, the amount of the electrolyte was controlled by approximately 16 μl of electrolyte per mg of S. Galvanostatic cycling measurements were performed on Neware battery tester in the voltage range of 1.7 - 2.8 V at 30 °C. The cells were discharged/charged at various C rates from 0.1C to 5C and then back to 1C (1C = 1675 mA g-1). To estimate the self-discharge behavior, fresh cells were discharged/charged at 1C for one cycle, and then rested at fully charged state for 60 h, followed by running at 1C for two cycles. This procedure was repeated for five times, but the fifth rest time was 240 h. Cyclic voltammetry (CV) measurements were conducted on an electrochemical workstation (Solartron, U.K.) in the voltage range of 1.7 - 2.8 V. Electrochemical impedance spectroscopy (EIS) measurements were conducted in a frequency range of 10-2-106 Hz with an AC voltage amplitude of 5 mV at the open-circuit voltage. Voltages mentioned in this study were all versus Li/Li+.

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Results and Discussion The cycling behaviors of Li-S batteries with different binders were measured at 1C for 500 cycles, with pre-cycling at 0.1C (3 cycles) and 0.5C (2 cycles), which are shown in Figure 1. As can be seen, the LSCa electrode exhibits a much higher initial specific reversible capacity of 1360 mAh g-1 with a sulfur utilization of 81%, compared to those of the PVDF electrode (706 mAh g-1) and the LSNa electrode (1231 mAh g-1). After 500 cycles, the LSCa electrode remains a capacity of 453 mAh g-1. In comparison, the capacity of the PVDF electrode drastically decreases to 173 mAh g-1 and the LSNa electrode fails in less than 500 cycles. The LSCa electrode shows a lower capacity decay rate (~0.10% per cycle) than that of the PVDF electrode (~0.15% per cycle). Furthermore, the LSCa electrode exhibits higher Coulombic efficiency than the PVDF and LSNa electrodes for all cycles, presenting its improved reversibility.

Figure 1. Cycling performance of Li-S batteries with different binders at 1C.

Figure 2a shows the rate capability of Li-S batteries with different binders. The LSNa and the LSCa electrodes display high specific discharge capacities of 1223 mAh g-1 and 1307 mAh g-1 at 0.1C, respectively, which are much higher than that of the PVDF electrode. When the current 7/19

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density increases from 0.1C to 5C, the capacities of the LSNa and the PVDF electrodes rapidly drop to around 250 mAh g-1. In comparison, the LSCa electrode retains a much higher specific capacity of 571 mAh g-1 at 5C. To further estimate rate capability, we investigated the capacity retention at 5C, compared to the capacity at 1C. The LSCa electrode shows much higher capacity retention of 71.9% than those of the PVDF (34.6%) and the LSNa (34.1%), proving the better rate capability of the LSCa.

Figure 2. (a) Specific capacities of Li-S batteries with different binders at various current densities; (b) Discharge/charge curves of Li-S batteries with different binders at 0.1C; Electrochemical impedance spectroscopy results of different electrodes (c) before cycling and (d) after 50 cycles.

The discharge/charge curves of the different electrodes at 0.1C are shown in Figure 2b. All the electrodes display typical two-plateau discharge curves. The first discharge plateau at around 2.3 V corresponds to the reduction of cyclic sulfur (S8) to soluble long chain polysulfides (Li2Sn, 4 ≤ n ≤ 8) and the second discharge plateau at about 2.05 V represents the transformation of long 8/19

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chain polysulfides to insoluble Li2S2/Li2S.15,27 Obviously, the voltage difference (ΔU) between charge and discharge plateaus of the LSCa electrode is the smallest, exhibiting its lowest polarization, compared to the other two electrodes.28 Table 1. Derived Rct values of different electrodes before cycling and after 50 cycles. Before cycling

After 50 cycles

Rct(Ω)

Rct (Ω)

PVDF

154.1

13.8

LSNa

97.2

8.4

LSCa

71.1

5.3

Sample

EIS measurements were conducted for the electrodes before cycling and after 50 cycles to investigate lithium diffusion kinetics. As shown in Figure 2c, the Nyquist plots of the electrodes before cycling consist of one semicircle at the high frequency region and a straight line at the low frequency region. The diameter of the semicircle represents the value of charge transfer resistance (Rct) and the slope of the straight is relevant to the mass transfer process.28,29 The corresponding fitting values of Rct are listed in Table 1. Before cycling, the value of Rct for the LSCa electrode is significantly smaller than those of the PVDF and the LSNa electrodes. After 50 cycles, although all the Rct values of the electrodes decrease, the LSCa electrode still exhibits a lower Rct value, indicating faster charge transfer than the other two electrodes. It is noteworthy that the Nyquist plot of the PVDF sample exhibits one quarter circle in the medium frequency region, which may correspond to the formation of Li2S2 and Li2S films during discharge/charge process (Rf). This phenomenon indicates that the surface of the PVDF electrode has been seriously deposited and covered with mass solid films.29 The coverage makes the active materials wetting by the electrolyte difficult, finally leading to low electrochemical reaction rate and serious 9/19

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electrochemical polarization.

Figure 3. (a) Cyclic voltammetry of the LSCa electrode between 1.7 and 2.8V at different scan rates; Linear fits for the cathodic (b) and anodic (c) peak currents versus scan rates of different electrodes; (d) Contact angles of electrolyte on different binder films.

To further show the excellent rate capability of the LSCa electrode, we conducted CV tests for the PVDF, the LSNa and the LSCa electrodes at different scan rates between 1.7 V and 2.8 V. Figure 3a shows the detailed CV curves of the LSCa electrode, in which the redox peaks agree well with the discharge/charge curves. The PVDF and the LSNa electrodes show similar CV curves (Figure S3). The linear fittings on peak current Ip versus ν1/2 are shown in Figures 3b and 3c. According to the Randles-Sevick Equation,30,31 it can be seen the larger slope K, the larger diffusion coefficient D. The corresponding diffusion coefficient of the LSCa electrode is larger than those of the LSNa and the PVDF electrodes both in cathodic reaction (Figure 3b) and anodic reaction (Figure 3c), indicating the less concentration polarization and the more favorable 10/19

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electrode kinetics of the LSCa electrode.32 As for the obvious electrochemical difference between the LSCa and the LSNa electrodes, contact angle study was carried out and the corresponding results are shown in Figure 3d. Compared to the LSCa film, the LSNa film has a relatively worse electrolyte wetting ability, which partly contributes to its inferior rate performance. 33

Figure 4. (a) Open circuit voltage versus rest time during the fifth self-discharge rest for 240 h of the different electrodes; (b) Self-discharge rates of the different electrodes.

It is well known for Li-S batteries that severe self-discharge behavior, caused by the shuttle effect, leads to voltage drop and capacity decay.34 In this work, the self-discharge behaviors of the different electrodes were investigated and the results are shown in Figure 4. It is observed that the open circuit voltage of the LSCa electrode shows negligible voltage drop, while obvious voltage drops are seen for the PVDF and the LSNa electrodes (Figure 4a). The self-discharge rate (η) is calculated according to the equation: η=(C0-C1)/C0 (C0 is the discharge capacity before rest; C1 is the following discharge capacity after rest). The smallest η of the LSCa electrode among them shows its best ability to prevent capacity loss, due to the most sulfur retention and the least long chain polysulfides dissolution, as shown in Figure 4b.35 The results about the absorptivity of the different binders toward polysulfides also demonstrate that the LSCa binder has stronger adsorption ability than the PVDF electrode (Figure S4). The better ability on the adsorption of 11/19

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polysulfides owes to the abundant sulfonate groups in lignosulfonate, which have the electron-rich oxygen with lone pairs to confine polysulfides by forming lithium-oxygen (Li-O) bond.24,36

Figure 5. SEM images of the PVDF, the LSNa and the LSCa electrodes (a-c) before cycling and (d-f) after 50 cycles.

Figure 5 displays SEM images of the three electrodes before cycling and after 50 cycles. It is clear to see that the active materials are homogeneously distributed at fresh state for all the electrodes (Figures 5a-c). After 50 cycles, distinct cracks and compact solid films are observed for the PVDF electrode (Figure 5d), coinciding with EIS results that the surface of the PVDF electrode is seriously deposited and covered with mass solid films. In the LSNa electrode, less and smaller cracks are observed, compared to the PVDF electrode (Figure 5e). As for the LSCa electrode, the surface is well maintained without conspicuous cracks after cycling (Figure 5f). The lignosulfonate electrodes also show smaller increment ( < 10%) in thickness than that of PVDF electrode (~25%) after cycling (Figure S5). These results indicate the superior mechanical strength of the LSCa electrode that can accommodate large volume expansion during discharge/charge than the PVDF and the LSNa electrodes.

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Figure 6. (a) Force-distance curves of the different electrodes; (b) Nano-indentations of the different binders.

180° peeling measurement was performed to confirm the excellent adhesion ability of the LSCa sample. The corresponding results are shown in Figure 6a. The average peeling forces of the LSCa and the LSNa samples are ~ 1.0 N, which are obviously higher than that of the PVDF sample (~ 0.5 N). In lignosulfonate electrodes, the higher average peeling force probably comes from the π-π interactions between lignosulfonate molecule and the surface of host carbon (KJC), which is stronger than Van der Waals force interaction in the PVDF electrode.37 The improved adhesion of the LSCa sample also contributes to the well-maintained morphology of the corresponding electrode after cycling and the superior electrochemical performance. Nano-indentation measurements on electrodes with different binders were also performed to assess mechanical properties, the results of which are shown in Figure 6b. The steps, increasing load to maximum with a certain velocity, holding a certain time and then unloading at the same rate, are used to imitate the expansion and contraction behaviors of the electrodes upon discharging/charging.38 The PVDF sample shows the largest displacements, indicating that its mechanical strength is the weakest to accommodate the expansion among the three binders. Therefore, there are obvious cracks on the PVDF electrode after cycling, as shown in Figure 5d. The order of the mechanical strength from the displacements is also in line with the behavior of

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cracks and the average adhesion strength of the different electrodes. More importantly, areal capacity is vital to the total capacity of an electrode, i.e. the energy density of a battery.39 We also prepared electrodes with high sulfur loading on Ni foam current collector and investigated their electrochemical performance. As shown in Figure 7, when the mass loading is fixed at 6 - 8 mg cm-2, the LSCa electrode (7.64 mg cm-2) maintains a superior areal capacity of 4.16 mAh cm-2 after 100 cycles at 0.05C than the other two electrodes, further proving the promising performance of the LSCa as an ecofriendly multifunctional binder for Li-S batteries.

Figure 7. Cycling performance of the different electrodes with high mass loading of 6 - 8 mg cm-2 at 0.05C.

Conclusions In summary, we have developed an aqueous ecofriendly binder LSCa with high performance for Li-S batteries. The binder is capable of facilitating ion transportation and inhibiting shuttle effect. What’s more, its good adhesion is beneficial for buffering volume expansion. These advantages contribute to its long cycling performance. The demonstrated high areal capacity further supports its potential practicability. Our work not only shows that LSCa is a green, inexpensive and promising binder for L-S batteries, but also suggests a facile and sustainable way 14/19

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for the high-value utilization of waste lignin.

Acknowledge The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 51732005), Natural Science Foundation of Guangdong Province, China (2016A030311031),

and

Natural

Science

Foundation

of

Shenzhen

(No.

JCYJ20170410160631170).

Supporting information The chemical structure of PVDF and lignosulfonate, TGA curve of S/KJC composite material, CV curves of the PVDF and the LSNa electrodes, optical images of polysulfide adsorption tests, the cross-sectional SEM images of electrodes before and after cycling.

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Abstract graphic (For Table of Contents Use Only)

Synopsis The feasibility for directly introducing the pulping waste as ecofriendly binder is developed for lithium-sulfur batteries.

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