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Dec 18, 2018 - Compared with the CMC/SBR binder, the LA133 binder is found to possess not only higher charge densities (−49.6 versus −38.9 mV) but...
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Article Cite This: J. Phys. Chem. C 2019, 123, 250−257

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Comparative Study of Water-Based LA133 and CMC/SBR Binders for Sulfur Cathode in Advanced Lithium−Sulfur Batteries Weiwen Wang,† Xinyang Yue,‡ Jingke Meng,‡ Xinxin Wang,‡ Yongning Zhou,‡ Qinchao Wang,*,‡ and Zhengwen Fu*,† †

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Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, 220 Handan Road, Shanghai 200433, P. R. China ‡ Department of Materials Science, Fudan University, 220 Handan Road, Shanghai 200433, P. R.China S Supporting Information *

ABSTRACT: Two most widely used commercial water-based binders, polyacrylic latex (LA133) and sodium carboxymethyl cellulose/styrene butadiene rubber (CMC/SBR), are utilized for constructing sulfur cathodes to investigate their influence on the electrochemical properties of lithium−sulfur batteries. Compared with the CMC/SBR binder, the LA133 binder is found to possess not only higher charge densities (−49.6 versus −38.9 mV) but also better chain flexibility, which promises the homogeneous dispersion of the sulfur−carbon composite cathode materials and ensures an effective conducting framework, resulting in the high utilization of active sulfur. The electrode performance of the batteries further demonstrates that the LA133 cathode with higher dispersion degree delivers lower internal resistance, faster Li-ion diffusion rate, more efficient conversion of sulfur redox, higher reversible capacity (1176.2 versus 867.3 mAh g−1), and better rate capability and electrode stability.

1. INTRODUCTION Due to the rapid development of electric vehicles, unmanned aerial vehicles, and smart power grids, the ever-increasing demand for high-energy-density energy storage devices has driven the battery community to exploit advanced lithium− sulfur batteries, which has a theoretical energy density up to 5 times higher than the commerical Li-ion batteries (ca. 2500 versus ca. 420 Wh kg−1).1−4 Sulfur, an inexpensive, abundantly available, and environment-friendly material, is a promising cadidate for cathode materials with a high theoretical capacity (1675 mAh g−1), making Li−S batteries extremely attractive when combined with a metallic Li anode.5,6 Nevertheless, to realize industrial application, there are technical challenges native exclusively to the Li−S battery system, to wit, the polysulfide shuttle effect, as well as many challenges the same to those in traditional Li-ion battery systems, such as extreme volume expansion for the active materials (ca. 80%), wellrecognized electrode destruction over cycles, and the electronically insulating nature of sufur and discharge products (Li2S/ Li2S2).7−9 Extraordinary efforts have been made to deal with these extraordinary challenges. A highly promising approach is to use sulfur mixed with, impregnated in, and confined by conducting host materials, e.g., porous carbon,10,11 carbon nanotube,12,13 hollow carbon nanosphere,14,15 etc. With homogeneous distribution of sulfur and enhanced conductivity, these approaches considerably improve the electrochemical property of a Li−S battery. Nevertheless, considering the industrializa© 2018 American Chemical Society

tion, using the hydrophobic materials with high surface area that spontaneously overaggregate will inevitably plague the fabrication of the sulfur−carbon composite materials.16 The aggregation of the electrode materials would be serious and obvious when the solvent was removed from the slurry. This phenomenon would result in undesirable pinholes, crackings, or delamination on the electrode, causing the deterioration of the electrode performance and inducing unstable quality of the electrode from batch to batch.17,18 As an electrode component, binders are crucial in constructing a uniform coating on the current collector without any defects by using a traditional slurry coating process.19,20 The basic function of binders is to strongly bond active materials and conductive additives together on the current collector and keep electrode films integrated to ensure sufficient electrical contact and structural robustness during assembling and testing. More specifically, to avoid the spontaneous overaggregation, binder functional groups interact with active materials to change the liquid−solid interface and disperse active materials uniformly. However, the most commonly used binder, polyvinylidene fluoride (PVDF), cannot meet the requirement because of the problem of swelling or even dissolving in an organic electrolyte.21 Additionally, sulfur reduction intermediates such as soluble Received: November 4, 2018 Revised: December 11, 2018 Published: December 18, 2018 250

DOI: 10.1021/acs.jpcc.8b10736 J. Phys. Chem. C 2019, 123, 250−257

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Figure 1. Molecular structures of CMC/SBR and LA133 binders.

polysulfide anions (Sn2− with n = 4−8) are highly reactive and will react with PVDF binder.22 These would result in the gradual degradation of the conductive network and the desquamation of electrode materials. Furthermore, N-methylpyrrolidinone (NMP), the solvent for PVDF, is a hazardous and volatile organic solvent, which induces serious problems of safety and pollution. The hydrophobic sulfur−carbon composite materials with high surface area will adsorb a huge amount of NMP solvent during the slurry coating process, causing the low content of solids in the coating slurry. Consequently, the slurry coating tends to peel off from the current collector due to the large volume shrinkage when removing solvent from the slurry.23,24 By comparison, water-based binders are more attractive options for preparing the sulfur cathode, as they show negligible solvent absorption in an organic electrolyte thanks to their hydrophilic nature, and the environmental-friendly solvent water is used. Various water-based binders, such as poly(ethylene oxide) (PEO),25 poly(acrylic acid) (PAA),26 sodium carboxymethyl cellulose/styrene butadiene rubber (CMC/SBR),27 carbonyl-β-cyclodextrin,28 and polyacrylic latex (LA133),29 have been applied as binders in Li−S batteries to replace PVDF and improve electrochemical performance. Among them, CMC/SBR and LA133 are the most extensively used to disperse and bind various kinds of sulfur−carbon composites,14,19,26,29−35 mainly due to their excellent adhesion, flexibility, and dispersion properties.26,36 However, as far as we know, there is no systematic research to explore the electrode performance of the sulfur cathode prepared by these two binders. In this study, we evaluate the electrochemical properties of the sulfur cathode by using LA133 or CMC/SBR as binders. The differences in morphology and electrode performance of the sulfur cathode have been investigated. The present study shows the LA133 binder is more suitable for making the sulfur cathode and delivering better electrochemical performance.

(1:1 wt %; Alfa Aesar) was used as the binder. After the components were intensely mixed in a aqueous solution for about 12 h with magnetic stirring, the slurry was cast onto an Al foil substrate, solvent-evaporated, and then dried at 60 °C under a vacuum for 10 h. The obtained electrodes were cut into discs 12 mm in diameter with the sulfur loading of ∼1 mg cm−2. 2.2. Electrochemical Measurement. To investigate the electrochemical performance of the sulfur cathode by using LA133 or CMC/SBR as binders, coin batteries of CR2032type were assembled in an argon-filled glovebox with oxygen and water contents less than 1.0 ppm. Li metal foil (China Energy Lithium Co., Ltd.) was used as a negative electrode. The electrolyte was 1 M lithium bis(trifluoromethane)sulfonimide (LiTFSI) in 1:1 v/v 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) containing 1 wt % lithium nitrate (LiNO3). The batterites contained a polypropylene microporous membrane Celgard 2400 as the separator. The galvanostatic discharge−charge behavior of the coin batteries was performed on a Land CT2001 multichannel battery tester in the voltage range 1.8−2.6 V at different rates (1 C = 1675 mA g−1) in ambient temperature. Electrochemical impedance spectroscopy (EIS) and cyclic voltagram (CV) measurements were performed on a CHI660e electrochemical workstation (Chenhua). EIS were conducted from 100 kHz to 0.1 Hz with an ac amplitude of 5 mV. The CV data were collected at different scan rates from 1.8 to 2.6 V versus Li/Li+. All experiments were conducted at 25 °C. 2.3. Material Characterization. The S-KB composite was dispersed in a water solution of LA133 and CMC/SBR (1:1 wt %) binder by ultrasonic treatment to give homogeneous colloids. The concentrations of the S-KB composite in both the binders were 2 mg mL−1. A Malvern Zetasizer Nano-ZS90 was used to measure the ζ potential and particle size distribution of the S-KB composite. The surface morphology and component of the sulfur cathode were characterized by a scanning electron microscope (SEM) (Hitachi S-4800).

2. EXPERIMENTAL SECTION 2.1. Sulfur Cathode Preparation. The Sulfur-Ketjen Black (S-KB) composite material was made by a conventionally used melting diffusion strategy with a mixture of sulfur powder and KB (weight ratio S: KB = 7:3) heated under an Ar atmosphere at 155 °C for 12 h. To fabricate the sulfur cathode, a slurry, containing 80 wt % S-KB composites, 10 wt % acetylene black (AB), and 10 wt % binders was mixed in water solvent. LA133 (water-based binder, Indigo) or CMC/SBR

3. RESULTS AND DISCUSSION The chemical structure of CMC/SBR and LA133 binders is illustrated in Figure 1. CMC is a linear polymeric derivative of natural hydrophobic cellulose with varying degrees of carboxymethyl substitution. The carboxymethyl (−COO−) and hydroxyl (−OH) groups cause the chains to be soluble in the aqueous phase. At the same time, the main chains of 251

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Figure 2. (a) Particle size and (b) ζ potential distribution profiles of the S-KB composite dispersed in CMC/SBR and LA133 binders. (c) Suspensions of the S-KB composite dispersed in CMC/SBR and LA133 before or after the 24 h settlement. SEM images of the S-KB composite dispersed in (d) CMC/SBR and (e) LA133 binders.

Figure 3. CV profiles of Li−S batteries employing S-KB electrodes fabricated with water-based (a) LA133 binder and (b) CMC/SBR (1:1 by weight) binder at scan rates of 0.1 mV s−1.

cellulose are still hydrophobic,37 which adsorb onto the similar sulfur−carbon particles by a hydrophobic interaction. Additionally, nonionic SBR as the elastomer demonstrates high flexibility, a strong binding force, and good heat resistance. With the combination of SBR and CMC as binders, which can provide a strong dispersion medium, a high adhesion agent, and mechanical stability, the sulfur electrode would endure extreme volume expansion during the charge−discharge process.26 Compared to merely high-polarity functional groups for CMC/SBR binders, the copolymer LA133 binder consists of acrylamide (AM), lithium methacrylate (LiMAA), and acrylonitrile (AN), involving not only high-polarity hydrophilic cyano/amide groups that provide sufficient adhesion strength among the conductive agent, active material, and current collector but also a relatively low-polarity oleophilic ester group that supplies appropriate flexibility for the electrode. Moreover, the LA133 binder with backbones of vinyl−vinyl structure is more flexible than the CMC binder with ring−ring structure. As a result, LA133 has higher flexibility to change the conformations and make the particles charged. Furthermore, the micelle structure of LA133 formed in the wet state indicated that it is also capable of buffering the extreme volume expansion of active sulfur and constrain soluble polysulfides.36 Additionally, the phenomenon that CMC prefers bacterial growth (common for cellulose) would shorten the storage life. A three-dimensional network will be formed in the cathode slurries, because of bridging of the particles with polymer chains where segments of polymer chain (−COO−) adsorb on the particles to lower the surface energy, sequentially making

the particles negatively charged to form electrostatic repulsion among the particles.7,38 After solvent evaporation, the cathode keeps the initial morphology in the wet state. Therefore, the final morphology of the dried cathode mainly relies on the dispersity of the slurry. As for Li−S batteries, a uniform dispersion of the cathode slurry is even more important because evenly dispersed sulfur and conducting agent particles make for better utilization of active material and minimize isolated bulks of sulfur dead-sites with higher surface area exposure.26 Additionally, during charge/discharge cycles, the cathode structure may be destroyed as a result of voids created by the sulfur dissolution. This problem can be greatly relieved by the utilization of evenly dispersed nanosized sulfur, which would lead to smaller voids remaining after sulfur dissolution. Thus, getting a uniform distribution of the S-KB composite in the solvent is the underlying goal for the slurry preparation. Parts a and b of Figure 2 show the particle size and ζ potential distribution profiles of the S-KB composite in binders. The most probable particle size of S-KB particles in binder slurries is similar to the original size of KB (∼48 nm),23 which demonstrates the good dispersion of the S-KB particles in these two kinds of binder slurries. However, the LA133-binder slurry exhibited a higher ζ potential compared to the CMC/ SBR-binder slurry (−49.6 versus −38.9 mV). A high ζ potential value indicates a strong electrostatic repulsive force among the particles, which can efficiently prevent the aggregation and stabilize the homogeneous distribution of the particles in the slurries. Moreover, the optical images of the two slurries after the 24 h settlement (Figure 2c) also 252

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Figure 4. (a) First discharge−charge profiles and (b) cycling performance of Li−S batteries employing S-KB electrodes fabricated with water-based LA133 binder and CMC/SBR (1:1 by weight) binder. Discharge−charge profiles of Li−S batteries employing S-KB electrodes fabricated with water-based (c) LA133 binder and (d) CMC/SBR (1:1 by weight) binder at various C-rates. (e) Rate capabilities of Li−S batteries employing SKB electrodes fabricated with water-based LA133 binder and CMC/SBR (1:1 by weight) binder.

solid S8 and the soluble polysulfide L2S4, while the cathodic peak at around 2.0 V and anodic peak at about 2.30 V can be ascribed to the transformation between soluble Li2S4 and insoluble discharge products Li2S2 or Li2S. In the first scaning, the cathodic peaks for the LA133-binder-based cathode are at 2.32 and 2.05 V, both higher than those of the CMC/SBRbinder-based cathode (2.21 and 2.02 V), which can be attributed to the faster charge transfer kinetics of the LA133 cathode with highly dispersed sulfur. More importantly, the highly consistent overlap of the reduction and oxidation peaks during the cycle indicates the good stability and reversibility of the LA133 cathode. The galvanostatic charge−discharge profiles of the cathode with the two different binders at the first cycle are shown in Figure 4a. Both sulfur cathodes gave the two-plateau discharge profile typical of conventional Li−S batteries, i.e., the formation of soluble long-chain polysulfides (Li2Sx, 4 ≤ x ≤ 8) at about 2.3 V and insoluble short-chain Li2S2/Li2S at around 2.1 V,12,41 which is consistent with the CV profiles of the sulfur cathode (Figure 3). Compared to that of the CMC/ SBR, Li−S batteries with LA133 demonstrate modified electrochemical kinetics and reduced electrochemical polarization, which is confirmed by an increased reversible capacity

demonstrate that the LA133 slurry still remains a uniform mixture while the particles dispersed in CMC/SBR solution suffer from serious aggregation and get noticeably coagulated. Notably, the stability of the slurry would dramatically influence the morphology of the electrode. During the drying proccess, the binder chains are prone to migrate with the evaporation of the solvent water, which promotes the coagulation of the particles. Thus, with greater stability, the slurry is more likely to retain the homogeneous distribution morphology in the wet state. As SEM images showed in Figure 2d,e, the LA133-binder slurry possessed a better water-evaporated distribution morphology than the CMC/SBR-binder slurry. These phenomena proved that the LA133-binder slurry can achieve a homogeneous cathode framework by preventing the aggregation of the particles in the slurry. Electrochemical measurements were performed to estimate the application of LA133 or CMC/SBR as binders in Li−S batteries. Figure 3 displays CV profiles of the sulfur cathode with the different binders by keeping the same amount of sulfur loading. Both profiles show two two pairs of distinct redox peaks, which both exhibit the typical CV characteristics of sulfur cathodes.39,40 The cathodic peak at about 2.30 V and anodic peak at about 2.35 V relate to the reaction between 253

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Figure 5. Nyquist plots of the sulfur cathodes with the different binders (a) before and (b) after 50 cycles. The inset is the equivalent circuit model for fitting the impedance.

Figure 6. CV profiles of Li−S cells employing S-KB electrodes fabricated with water-based (a) LA133 and (b) CMC/SBR (1:1 by weight) binder at various scan rates from 0.2 to 0.5 mV s−1. Plots of CV peak current for (c) the first cathodic reduction process (IC1: S8 → Li2S4), (d) the second cathodic reduction process (IC2: Li2S4 → Li2S2/Li2S), (e) the first anodic oxidation process (IA1: Li2S2/Li2S → S4), and (f) the second anodic oxidation process (IA2: Li2S4 → S8) versus the square root of the scan rates.

(1176.2 versus 867.3 mAh g−1) and the voltage difference between charge and discharge plateaus (133 versus 172 mV), as given in Figure 4a. Considering the electrochemical stability, the LA133 cathode keeps a reversible capacity of 709.2 mAh g−1 even after the 150th cycling, whereas the CMC/SBR cathode just delivers a reversible capacity of 612.1 mAh g−1 (Figure 4b). Notably, although the LA133 cathode performed at a higher capacity, it seems to decay faster than the CMC/ SBR cathode. On the basis of previous researches,6,30 the polysulfide shuttle effect is the main reason for the fast capacity

decay during cycles, as just using commercial KB as sulfur hosts. Usually, complex porous structure materials will be used as the sulfur host in order to restrain the diffusion of polysulfides, so the corrosive reaction between the lithium anode and lithium polysulfides during the discharge process can be suppressed effectively.4,42 Moreover, because of the much lower sulfur utilization for the CMC/SBR than for the LA133 cathode during the first cycle (867.3 versus 1176.2 mAh g−1), some of the unused sulfur may be actived during the following cycles, which could compensate for the active 254

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Figure 7. SEM images of the sulfur cathodes: (a) LA133 cathode before cycling, (b) LA133 cathode after 50 cycles, (c) CMC/SBR cathode before cycling, and (d) CMC/SBR cathode after 50 cycles.

impedance spectroscopy (EIS) analysis. Nyquist plots of the sulfur cathodes before and after 50 cycles are given in Figure 5. The EIS spectrum is composed of one depressed semicircle at the high-frequency region, which indicates the charge transfer resistance (Rct) controlled by the electrode−electrolyte interface resistance and the electronic conductivity of the cathode, and one inclined line at the low-frequency region.43 Using the circuit model to explain the EIS, as presented in Table S1, the fitted data reveal that the LA133 cathode before cycling has a much lower charge transfer resistance (Rct = 9.83 Ω) than that of the CMC/SBR cathode (Rct = 26.19 Ω). Morever, after 50 cycles, the Rct of the LA133 was kept almost unchanged (12.06 Ω) while the CMC/SBR drastically increased to 88.28 Ω, implying the less effective electric contact among the cathode materials with CMC/SBR as binders. To further investigate the reaction kinetic behavior of the sulfur cathodes, CV measurements with different scan rates were performed to calculate the Li-ion diffusion coefficient. As shown in Figure 6, all the cathodic and anodic current peaks (IC1, IC2, IA1, and IA2) have a linear relationship with the square root of scanning rates, indicative of the diffusion-limited process. Therefore, the Li-ion diffusion process can be estimated using the classical Randles−Sevcik equation:44

materials reacted with the lithium anode. As a result, the capacity decay of the CMC/SBR cathode is lower than that of the LA133 cathode. This deduction could be verified by the slightly increased capacity of the CMC/SBR cathode at about the 10th cycle (Figure 4b). The sulfur cathodes with different binders were further tested by galvanostatic charge/discharge at different current rates from 0.1 to 3 C (1 C = 1675 mA g−1). Corresponding charge/discharge voltage profiles and rate performance at different current rates are shown in Figure 4c−e. When the current rates are increased to 0.1, 0.2, 0.5, and 1 C, the LA133 cathode can present high discharge capacities of 1238, 1018, 933, and 856 mAh g−1, respectively. More importantly, even at a high current rate of 3 C, the LA133 cathode can still achieve a reversible capacity of 669 mAh g−1. And the potential hysteresis between discharge and charge profiles is not evident, reflecting the rapid electrochemical kinetics of the cathode. However, for the CMC/SBR-based cathode, the rate capacity fades quickly (from 887 to 417 mAh g−1) as the current rate increases from 0.1 to 3 C. Furthermore, due to the high potential hysteresis under the high current rate, the two-step discharge behaviors of the sulfur cathode cannot be fully exploited in the CMC/SBR cathode, implying its slow electrochemical kinetics. The phenomenon that the restored capacity is only 591 mAh g−1 (versus 969 mAh g−1 for the LA133) after resetting the current back to 0.2 C, further confirmed the speculation. The favorable electrochemical kinetics property of the sulfur cathode with LA133 is further backed by the electrochemical

Ip = (2.69 × 105)n1.5AD Li+ 0.5C Liν 0.5

in which IP is the peak current, n is the number of charge transfer (n = 2 for Li−S batteries), A is the active electrode area, DLi+ is the Li-ion diffusion coefficient, CLi is the Li-ion 255

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concentration in the cathode, and ν represents the scan rate. Because n, S, and CLi are constant in the battery system, the slope of the profile (Ip/ν0.5) represents the Li-ion diffusion rate. Then the calculated Li-ion diffusion coefficients are summarized in Table S2. The result shows that the Li-ion diffusion coefficients are almost of the same order of magnitude, and the LA133 yields the highest value, especially at the peak C2 for the second cathodic reduction process (near 10 times higher than the CMC/SBR). In fact, the second cathodic reduction process where the long-chain lithium polysulfides convert to solid lithium sulfides, which contributes the 75% of the theoretical capacity (1255 mAh g−1), is severely plagued by the energy required for nucleation of the solid state phase and the sluggishness of solid state diffusion in the bulk.2 However, with a faster Li-ion diffusion rate, the sulfur cathode using LA133 as binder enables more efficient conversion of sulfur redox, lower electrochemical polarization, and larger reversible capacities, which is consistent with the electrode performance, as demonstrated in Figure 4. Surface morphology analysis was used to further explore the remarkable performance of the LA133 cathode. Optical images of the sulfur cathodes using CMC/SBR and LA133 as binders have little much difference before/after 50 cycles (Figure S1), indicating the excellent adhesion ability for both of the binders.45 Furthermore, Figure 7 exhibits SEM images of the sulfur cathode with different binders before and after 50 cycles. For the cathode with LA133 binder, as shown in Figure 7a,b, the components are reasonably dispersed before cycling and almost unchanged after 50 cycles, demonstrating both the good dispersion and the structural stability of the LA133. However, compared with the LA133 cathode, the surface of the CMC/SBR cathode before and after cycling is less uniform and of higher granularity (Figure 7c,d). The agglomeration of active sulfur is less effectively in contact with the conducting framework and less accessible for the Li-ion in the electrolyte, causing the lower utilization of sulfur and lower reversible capacities. Moreover, owing to the poor dispersibility and extreme volume expansion for active sulfur, there are visible cracks on the CMC/SBR cathode, even the better elastic robustness is imparted with the blending of elastic SBR.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b10736.

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Details of the fitted EIS data, the calculated Li-ion diffusion coefficients, and optical images of the sulfur cathodes (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.W.). *E-mail: [email protected] (Z.F.). ORCID

Yongning Zhou: 0000-0002-9791-3468 Zhengwen Fu: 0000-0002-4649-0194 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21773037), National Key Scientific Research Project (Grant No. 2016YFB0901504).

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REFERENCES

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4. CONCLUSIONS In conclusion, this study has systematically investigated the electrochemical performance of the sulfur cathode with watersoluble LA133 and CMC/SBR as binders and the dispersion properties of binders in the slurries. With higher charge densities and better chain flexibility, LA133 binder favors a homogeneous distribution of the sulfur−carbon composite as well as excellent dispersing stability. The electrochemical performance of the Li−S batteries would dramatically depend on the dispersion degree of the cathode, which ensures an effective conducting framework and uniform dispersion of active sulfur and alleviates the effects of volume expansion for active materials. So compared with the CMC/SBR, the LA133 cathode with higher dispersion degree delivers lower internal resistance, faster Li-ion diffusion rate, and a much higher reversible capacity (1176.2 versus 867.3 mAh g−1). The SEM images of the cathode morphologies before and after cycling further confirm the well dispersion and the structural stability of LA133. All these results clearly reveal that LA133 is a highly promising binder for the sulfur cathode to improve the electrochemical performance with an efficient conversion of sulfur redox and a high sulfur utilization. 256

DOI: 10.1021/acs.jpcc.8b10736 J. Phys. Chem. C 2019, 123, 250−257

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DOI: 10.1021/acs.jpcc.8b10736 J. Phys. Chem. C 2019, 123, 250−257