Impact of the Mechanical Properties of a Functionalized Cross-Linked Binder on the Longevity of Li−S BatteriesC. Y. Kwok,† Q. Pang,† A. Worku,‡ X. Liang,† M. Gauthier,*,‡ and L. F. Nazar*,†

†Waterloo Institute of Nanotechnology and Department of Chemistry and ‡Department of Chemistry, University of Waterloo, 200University Ave. W., Waterloo, Ontario N2L 3G1, Canada

*S Supporting Information

ABSTRACT: One of the very challenging aspects of Li−Sbattery development is the fabrication of a sulfur electrodewith high areal loading using conventional Li-ion binders.Herein, we report a new multifunctional polymeric binder,synthesized by the free-radical cross-linking polymerization of[2-(acryloyloxy)ethyl]trimethylammonium chloride (AET-MAC) and ethylene glycol diacrylate (EGDA) to formpoly(AETMAC-co-EGDA), that not only helps to confine thesoluble polysulfide species but also has the desired mechanicalproperties to allow stable cycling of high-sulfur loadingcathodes. Through a combination of spectroscopic and electrochemical studies, we elucidate the chemical interactions thatinhibit polysulfide shuttling. We also show that extensive cross-linkage enables this polymeric binder to exhibit a low degree ofswelling as well as high tensile modulus and toughness. These attributes are essential to maintain the architectural integrity ofthe sulfur cathode during extended cycling. Using this material, Li−S cells with a high-sulfur loading (6.0 mg cm−2) and a low-intermediate electrolyte/sulfur ratio (7 μL:1 mg) achieve an areal capacity of 5.4 mA h cm−2 and can be (dis)charged for 300cycles with stable reversible redox behavior after the initial cycles.

KEYWORDS: Li−S batteries, high areal sulfur loading, functionalized cross-linked binder, polysulfide chemisorption,mechanical properties

1. INTRODUCTION

The rising demands in advanced energy storage devices forelectric vehicles, unmanned aerial vehicles, and robots call forforward-looking battery technologies that can exceed thespecific energy afforded by intercalation chemistry.1 Anapproach based on lithium−sulfur (Li−S) conversion chem-istry is promising for next-generation electrochemical batteries,due to sulfur’s high theoretical specific capacity of 1675 mA hg−1 and energy density of 2800 W h L−1.2 The high naturalabundance and innocuity of sulfur further contributes to thelow cost and appeal of such a system.3,4

The commercialization of Li−S batteries, however, has beenplagued by low areal sulfur loading, poor sulfur utilization, andcapacity degradation. These drawbacks originate from thedissolution and shuttling of lithium polysulfides, a large volumechange in the cathode during cycling, and a lengthenedelectronic/ionic pathway in thick electrodes.5−8 Considerableeffort has been devoted toward the optimization of cellcomponents, including the development of new hostmaterials,9−11 separators,12,13 and electrolytes.14,15 Despitethese recent advances,16,17 fabricating a high areal capacitystable-cycling Li−S battery still remains a major challenge.Low-sulfur loading (<2 mg cm−2) Li−S cells are oftenreported, whereas achieving high-sulfur loading cells with apractical electrolyte/sulfur ratio is critical to maximize cellenergy density.18 Although constructing a three-dimensional

(3D) carbon network on an electrode enables very high-sulfurloading,19−21 the large voids in these structures require excesselectrolyte to fully wet the electrode, thus compromising theoverall energy density.18 Casting active materials on thecurrent collector by a traditional slurry process is a more viableavenue, as it maximizes the packing efficiency and iscommensurate with current industrial protocols.5,22−24 How-ever, this process requires the use of functional binders toprovide an elastic interface for the essential cathodecomponents to prevent them from delaminating from thecathode over extensive cycling.25

Binders are polymers that hold the critical components inthe electrode together during battery operation. Their primaryfunctions are to (1) bridge individual particles together withthe current collector; (2) assist the carbon additive to maintainelectronic contact upon cycling; (3) facilitate Li+ ion transportat the electrode−electrolyte interface; and (4) providechemical interactions to improve electrochemical perform-ance.8 Although binders often contribute as much as 10% ofthe overall cathode weight, their vital function is usuallyoverlooked in Li−S cathodes.26 Poly(vinylidene fluoride)(PVDF) binder is conventionally used due to its excellent

Received: April 12, 2019Accepted: May 29, 2019Published: May 29, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

© 2019 American Chemical Society 22481 DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

Dow

nloa

ded

via

UN

IV O

F W

AT

ER

LO

O o

n O

ctob

er 4

, 201

9 at

20:

39:2

7 (U

TC

).Se

e ht

tps:

//pub

s.ac

s.or

g/sh

arin

ggui

delin

es f

or o

ptio

ns o

n ho

w to

legi

timat

ely

shar

e pu

blis

hed

artic

les.

thermomechanical properties and adhesion to the cathodematerials in lithium ion batteries.27,28 However, as Li−Schemistry is haunted by polysulfide dissolution, it is highlydesirable to instead arm the binder with active functionalgroups to bind these intermediates.3,8,29,30 This was demon-strated in early studies, which showed that such modifiedbinders exhibit improved cyclability in comparison withPVDF.31−38 The performance enhancement was attributed toamine, hydroxyl, carboxyl, and carboxylic groups interactingwith lithium polysulfides in these polymers.36,34,39−44 None-theless, the binding energy of these functional groups was stilltoo weak to suppress polysulfide shuttling,17,41 and the sulfurloading in these reports was far too low for any practicalapplications. Only recently have a few studies focussed ondesigning new binder systems for thick electrodes, exemplifiedby styrene−butadiene rubber,5 polyethylenimine,45 polyamido-amine dendrimers,43 gums,36 and cross-linked elastomericcarboxymethyl cellulose.37 Another report demonstrated that acationic polyelectrolyte binder is very effective at restrictingpolysulfide diffusion from the host sulfur/sulfide cathode.30 Inparallel, researchers are starting to recognize the importance ofthe mechanical properties of binders to maintain the structuralintegrity of multicomponent Li−S cathodes,8 although there isstill a lack of understanding on the impact of the binder’scharacteristics in addressing the continuous volume expansion/contraction experienced during Li−S cell cycling.Herein, we report a new ammonium chloride-based cross-

linked binder. We show that not only does its stronginteraction with lithium polysulfides contribute to greatlysuppressed polysulfide shuttling but that its optimal interfacialand mechanical properties allow stable cycling of high-sulfurloading cathodes. The strong charge-transfer interactionsbetween the ammonium cation and polysulfide anion andthe consequent molecular reorganization of these species aredemonstrated by spectroscopic and electrochemical studies.More importantly, we correlate the electrochemical perform-ance of sulfur cathodes with the mechanical properties of threedifferent polymeric binders, including one similar in chemical

functionality but with lower tensile strength. Our results showthat cross-linked polymers with a low swelling ratio and a hightensile strength, Young’s modulus, and toughness are criticalfor delamination tolerance. Furthermore, excellent compati-bility of the polymer binder with the selected casting solventallows the utilization of a conventional slurry process toconstruct mechanically stable electrodes.5,36,46 Using thisapproach, a high-sulfur loading (6.0 mg cm−2) Li−S cellutilizing a low electrolyte/sulfur ratio of 7:1 (μL:mg) canoperate for 300 cycles with a low capacity fade rate.

2. RESULTS AND DISCUSSION

Two highly cross-linked polymers based on ammoniumchloride functional groups were prepared via radical polymer-ization (see Experimental Methods for details). Ethylene glycoldiacrylate (EGDA) was used as the cross-linking agent to co-po lymer i ze w i th e i the r [2 - (ac ry loy loxy)e thy l ] -trimethylammonium chloride (AETMAC) or diallyldimethy-lammonium chloride (DADMAC) monomers (Figure 1a,b) tofabricate the three-dimensional polymeric network; these arereferred to as poly(AETMAC-co-EGDA) and poly(DADMAC-co-EGDA), respectively. The two cross-linked polymers havesimilar functional groups in their subunits that regulate lithiumpolysulfide transport in the electrolyte. However, they exhibitvery different mechanical properties due to differences in thereactivity of the monomers with EGDA, which controls thedensity of the cross-linked network. The density was optimizedwith respect to their electrochemical performance as binders inLi−S cells. Successful cross-linking co-polymerization wasconfirmed by the insolubility of the products in water, unlikethe monomers which are highly soluble. Their Fouriertransform infrared (FTIR) spectra, shown in Figure 1c,drespectively, show bands typical for the functional groups, asdescribed in the literature (see Figure S1 for details).47,48

The interactions between the functional groups in thepolymer binders and lithium polysulfides (as represented byLi2S4) were examined by FTIR spectroscopy to elucidate themechanism of the charge-transfer binding effect. The N−C

Figure 1. Synthesis of cross-linked polymers via radical polymerization. Schematic representation of the cross-linker (EGDA), (a) AETMAC and(b) DADMAC to form a three-dimensional polymeric network. FTIR spectra for the highly cross-linked polymeric binders, namely, (c)poly(AETMAC-co-EGDA) and (d) poly(DADMAC-co-EGDA).

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22482

moieties (3400 and 1640 cm−1) and the ester CO moieties(1700 cm−1) are potential indicators of this process.Homopolymers were mixed with Li2S4 dissolved in 1,2-dimethoxyethane (DME), and the solids were extracted forthe FTIR analysis (see Experimental Methods for details). TheFTIR spectra obtained for the pristine homopolymer andLi2S4, as well as the homopolymer−Li2S4 mixtures withpoly(AETMAC), poly(DADMAC), poly(EGDA), and PVDFare compared in Figure 2. Homopolymers were used in thisanalysis to ensure that the interactions observed in the FTIRspectra are only attributed to specific functional groups. Thepristine poly(AETMAC) in Figure 2a exhibits a sharp peak at1638 cm−1 and a broad band at 3425 cm−1, corresponding tothe unsaturated primary amine and C−N stretching vibrationsin the secondary amine group.47 Upon contact with Li2S4,these peaks shifted to much lower wavenumbers of 1622 and3400 cm−1, and their intensity significantly decreased.Similarly, for poly(DADMAC), these bands also shifted byabout 10 cm−1 (from 1636 to 1628 cm−1 and from 3447 to3412 cm−1) when blended with Li2S4 (Figure 2b).48 Inaddition, the rearrangement of the sulfur chain in thepolysulfide is observed, signified by a shift in the symmetricalS−S band from 484 to 453 and 449 cm−1 in thepoly(AETMAC)-Li2S4 and poly(DADMAC)-Li2S4 materials,respectively.49,50 All these molecular rearrangements, de-lineated by the shifts in their respective FTIR spectra, indicatethat charge transfer takes place between the ammoniumcations and the polysulfide anions.51 The acidic character ofthe ammonium cations enables their direct electrostaticbinding to the polysulfide anions.52 This interaction causesthe original C−N and S−S bond lengths to increase as a resultof bond weakening. Thus, a red-shift in their FTIR spectra

(Figure 2a,b, purple curve) is observed.40 However, we observeno shifts in vibrational modes associated with carbonyl groups,likely because the carbonyl groups instead bind to lithium ionsvia the oxygen empty 2p orbital. Such indirect interactions donot disturb the S−S and CO bonds,39,41 as seen in theinvariant sharp peak ascribed to the ester CO bond inpoly(AETMAC) (1736 cm−1, Figure 2a) or poly(EGDA)(1726 cm−1, Figure 2c). Of course, the fluoride group in PVDF(Figure 2d) cannot engage in any interactions with lithiumpolysulfide. Accordingly, the PVDF−Li2S4 materials show onlyfeatures characteristic for polysulfide and a C−F asymmetricstretching band (1402 cm−1).53

In light of the interactions described above, we furtherexamined the polysulfide adsorptivity of these binders via afacile titration technique: electro-oxidization of polysulfides.54

Although lithium polysulfides are known to disproportionate inglyme, Li2S4 and Li2S6 are the most prevalent soluble speciesduring sulfur redox and were hence selected as representativeprobe molecules.30,55 Each binder material was accuratelyweighed into a stock solution of Li2S4 or Li2S6 in DME. Thesupernatant was then collected and subjected to electro-chemical titration of the remaining Li2Sn in the solution (seeExperimental Methods for details). Knowledge of theconcentration in the stock solution of Li2S4 (Figure S2a) andLi2S6 (Figure S2b) enables the determination of the amount ofLi2Sn removed from the supernatant via adsorption onto thebinder materials.The results of adsorptivity measurements of Li2S4 and Li2S6

by PVDF, poly(DADMAC-co-EGDA), and poly(AETMAC-co-EGDA) are summarized in Figure 2e. Unsurprisingly, PVDFadsorbs much less Li2Sn compared with the two cross-linkedpolymers. These results are in strong agreement with the FTIR

Figure 2. Chemical interactions between functional groups present in the binders and lithium polysulfide. FTIR spectra to probe the interactionsbetween the functional group in each homopolymer and Li2S4 for (a) poly(AETMAC); (b) poly(DADMAC); (c) poly(EGDA); (d) PVDF. (e)Summary of calculated polysulfide adsorptivity per 10 mg of PVDF (green), poly(DADMAC-co-EGDA) (blue), and poly(AETMAC-co-EGDA)(red). The lighter color bar represents Li2S4 and the darker color represents Li2S6.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22483

analysis, showing there are no chemical interactions betweenthe fluoride moieties and lithium polysulfides.29 On the otherhand, the ammonium subunits in both poly(AETMAC-co-EGDA) and poly(DADMAC-co-EGDA) network engage inacid−base interactions with the polysulfide anions. Wespeculate that the polysulfide adsorptivity of these polyelec-trolytes could be further enhanced via ion exchange of Cl− foran even more weakly associated counter anion to reduceinteractions with the cationic subunit in the network.56,57

Nonetheless, both of these cross-linked polymer bindersexhibit comparable polysulfide adsorptivity to some cathodehost materials, which have demonstrated a superior ability toreadily bind polysulfides.17,11

Further correlation can be drawn between Li2Sn adsorptivityon the cross-linked polymers and the chain-length of thepolysulfides. Both poly(AETMAC-co-EGDA) and poly-(DADMAC-co-EGDA) have a higher affinity toward thehigher order hexasulfide than the tetrasulfide. Poly-(AETMAC-co-EGDA) features the highest adsorptivity (4.25mg Li2S6 per 10 mg of polymer). Recent work by Li et al.30

also showed that lithium ions are less tightly bound to theterminal sulfur anions in high-order polysulfides. Theircomputational and our experimental results hence complementeach other well.We examined the electrochemistry of these binders in Li−S

cells using porous hollow carbon spheres (PHCSs) as sulfurhosts (see Experimental Methods and Figure S3a,b fordetails).10 These high surface area carbon spheres (FigureS3c,d) enable a large fraction of sulfur to be loaded in thecathode, while maintaining good electronic conductivity. Sulfurwas loaded into these spheres via melt-diffusion at 160 °C,yielding 75 wt % sulfur in the composite material (S-PHCS,Figure S4), which is the maximum loading achievable fornanostructured carbon spheres.10,58

Constructing a compact cathode via the traditional slurryprocess demands that the binders be evenly distributed on thecurrent collector. The conductive carbons (Super P and carbonnanotubes) and active composite material (S-PHCSs) in thecasting solvent need to be at a concentration to achieve aviscosity that ensures effective mixing, dispersion, andpenetration of the binder material into the electrode.59 The

hydrophilic character of these cross-linked polyelectrolytebinders provides excellent compatibility with water. A binarysolvent system comprised of water and dimethylformamide(DMF) was used for the two cross-linked polymers, whereasonly DMF was used to prepare the PVDF casting solution.DMF homogenizes the nonpolar S-PHCSs and carbon,whereas water ensures that the binder is well dispersed inthe solvent. The S-PHCS composite, carbon additives, and thebinder solutions were mixed and cast on both sides of P50carbon paper via a layer-by-layer deposition technique tomaximize the packing efficiency. Our use of P50 as a currentcollector is dictated by several factors. In comparison with anAl foil current collector, the micron-sized carbon fibers (FigureS5a) provide a 3D electronically conductive network thatenables excellent electron transfer to the adherent cathodematerials, while precluding any aluminum corrosion that couldresult from the reaction of the chloride counter-ions of thecross-linked polyelectrolytes with an Al current collector.56

The fibrous structure of the P50 carbon paper furthermoreallows sufficient electrolyte penetration, without compromisingthe overall sulfur content, to overcome mass-transport barriersin thick electrodes. In particular, the nature of the carbon paperenables the measurement of the intrinsic electrode’s mechan-ical properties, which is the focus of the study reported here.Furthermore, Al and P50 current collectors have a similaraverage areal mass of 4.5 and 5.0 mg cm−2, respectively.Therefore, from a gravimetric point of view, the overall sulfurcontent is not sacrificed. The high-magnification scanningelectron microscopy (SEM) image (Figure S5b) and energy-dispersive X-ray (EDX) analysis (Figure S5c,e) of therepresentative poly(AETMAC-co-EGDA)-based sulfur elec-trode show that not only are the materials evenly distributedonto the P50 carbon paper but also that the overall structure ofthe cathode exhibits sufficient porosity for the electrolytepenetration.37 These properties do not exist for Al foil and arevital to showcase the properties of the binder itself, namely, itsmechanical properties that enable long-term cycling even athigh-sulfur loading, as shown below.The electrochemical performance of the resulting Li−S cells

was examined by galvanostatic cycling (Figure 3). Electrodeswith low loadings of ∼3.5 mg cm−2 (based on sulfur) were first

Figure 3. (a) Electrochemical profiles (C/20) and (b) discharge capacities at different C-rates for sulfur cathodes at a low-sulfur loading of ∼3.5mg cm−2; (c) long-term cycling stability of the same electrodes conducted at a C/5 rate.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22484

fabricated with the different polymeric binders, to rank theimpact of the functional group and the mechanical propertieson sulfur utilization and cycling stability before examining highloading Li−S cells. During the first activation cycle at C/20(1C = 1675 mA h g−1), the poly(AETMAC-co-EGDA),poly(DADMAC-co-EGDA), and PVDF-based electrodes ex-hibited similar discharge capacities of 1115, 1160, and 1175mA h g−1, respectively (Figure 3a; the sloping region below∼1.8 V is due to negative electrode passivation from the slightreduction of the LiNO3 additive on the first cycle.60). Cyclicvoltammetry (CV) studies on these cathodes (Figure S6a−c)further confirm the galvanostatic cycling results and show thatthe electrochemical profiles of these cathodes exhibit a typicaltwo-peak discharge step behavior corresponding to theformation of high-order polysulfides at ∼2.3 V andnucleation/precipitation of insoluble Li2S2/Li2S at ∼2.1 V vsLi/Li+.21,36

The discharge capacity of these cathodes at different C-ratesis compared in Figure 3b. The Li−S cells fabricated with eitherpoly(AETMAC-co-EGDA) or poly(DADMAC-co-EGDA) ex-hibited a reversible capacity of ∼600 mA h g−1 at 2C. Thecapacity recovered to ∼850 mA h g−1 when the C-rate revertedto C/10. In sharp contrast, the capacity of the PVDF-basedLi−S cell dropped to ∼250 mA h g−1 at 2C and showed

capacity fluctuation during subsequent cycling at C/10. Wenote that the carbon spheres only provide physical confine-ment for the polysulfides by limiting diffusion, but these sulfurspecies can be ultimately be solubilized in the electro-lyte.16,17,39 For this reason, the electrode with the PVDFbinder only retains 52% capacity over 100 cycles (Figure 3c),whereas electrodes prepared with the poly(AETMAC-co-EGDA) and poly(DADMAC-co-EGDA) binders retained 85and 65% of their initial capacities, respectively. Their highconcentration of ammonium cations forms a 3D network ofinteracting sites to more effectively bind with the polysulfideanions.39,48 Nonetheless, the behavior of these two cross-linkedbinders diverges after 30 cycles. Although the capacity of thecathode fabricated with poly(DADMAC-co-EGDA) continuedto decline past this point, that of poly(AETMAC-co-EGDA)remained relatively constant. This is reflected in the moreinvariant CV profile exhibited for the sulfur cathode fabricatedwith poly(AETMAC-co-EGDA) (Figure S6a; after the first“activation” cycle) by comparison with the evolving nature ofthe CV observed for poly(DADMAC-co-EGDA) (Figure S6b).These binders do not display a drastic difference in polysulfideadsorptivity and share the same cross-linker with an optimizedcross-linking density. The underlying difference lies in theirmechanical properties, as we show below.

Figure 4. Mechanical characterization of the polymeric binders to simulate the sulfur electrode environment in Li−S cells. (a) Time-dependent

swelling profiles for the polymers in the electrolyte (DOL/DME, 1:1 v/v). The swelling ratio was calculated according to the equation−W W

Wwet dry

dry,

where Wdry is the weight of the dry polymer and Wwet is the weight after soaking. (b) Tensile stress−strain curves and (c) toughness of sulfurcathodes using these polymeric binders on P50 carbon paper, until fracture. SEM images of the surface of the sulfur cathodes (d−f) before and (g−i) after 100 cycles fabricated with (d, g) PVDF; (e, h) poly(DADMAC-co-EGDA); and (f, i) poly(AETMAC-co-EGDA).

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22485

The swellability of the polymers was studied by soakingthem in a dioxolane/dimethyloxyethane solution (DOL/DME,1:1 v/v). The change in the weight corresponding to thesolvent uptake was monitored after extracting the solid atdifferent contact times. The weight gain shown in Figure 4agives a rough estimate of the polymer−solvent interactions.Poly(AETMAC-co-EGDA) was fabricated with a lower cross-linker ratio (4:1 molar ratio of AETMAC/EGDA) and swelledfar less (∼60 wt %) than PVDF (∼140 wt %) but also less thanpoly(DADMAC-co-EGDA) (∼180 wt %), which was preparedwith a higher cross-linker ratio of 2:1 mole ratio of DADMAC/EGDA. This is attributed to the large chain transfer constant ofthe allylic DADMAC monomers, which interferes with thegrowth of an extended network.61 It results in a much loosernetwork and higher swellability compared to the AETMACnetwork. The lower degree of swelling and higher chemicalstability observed in poly(AETMAC-co-EGDA) are importantbinder characteristics to maximize the acid−base interactionsbetween ammonium chloride and lithium polysulfide.39 This isparticularly important for high-sulfur loading cathodes, wherecell performance is strongly dependent on the amount of liquidelectrolyte available.62 Although a moderate degree ofpolymeric binder swelling can result in superior electro-chemical performance,39,63 the high swelling observed for bothPVDF and poly(DADMAC-co-EGDA) leads to the uptake ofsignificant amounts of liquid electrolyte. Consequently, thepolymer no longer binds the electrode components.Furthermore, in the case of poly(DADMAC-co-EGDA),extensive swelling can weaken the interactions between theammonium chloride and lithium polysulfides, which hindersthe entrapment of polysulfides. Thus, poly(DADMAC-co-EGDA) adsorbs less Li2Sn than poly(AETMAC-co-EGDA), asdemonstrated in Figure 2e.The sulfur electrode fabricated with poly(AETMAC-co-

EGDA) also better supports the continuous stress duringcycling. The tensile strength, as well as the Young’s modulusand toughness of the cathodes (cut into rectangles 3″ × 1″),were evaluated on a tensometer (see Experimental Methods).The force and the displacement recorded in real time wereused to calculate the stress (σ), given by eq 1

σ =−F F

Az z0

(1)

where Fz and Fz0 are the forces experienced by the electrodeduring the experiment and at the beginning of the experiment,respectively, and A is the cross-sectional area of the electrode.For a 1″ width P50 carbon paper with a sulfur loading of 3.5mg cm−2, the cross-sectional area of the specimen was 7 mm2.The strain (ε) was calculated according to eq 2

ε =−

×x x

x100%

z z

z

0

0 (2)

where xz and xz0 are the distances between the two grip headswhile the electrode was being stretched.The stress−strain curves for these electrodes in Figure 4b

show a profile typical of fiber nanocomposites: a linear curvethat obeys Hooke’s law, followed by a sudden drop in stress.64

This sharp decrease in stress corresponds to the tearing of theelectrode composite when the maximum stress is exceeded,and provides the value of the tensile strength. The poly-(AETMAC-co-EGDA)-based electrode exhibits the highesttensile strength (7.0 GPa), compared to the electrodes

prepared from poly(DADMAC-co-EGDA) (5.7 GPa) andPVDF (3.8 GPa). Furthermore, the Young’s modulus of theelectrode fabricated from the two cross-linked binders is verysimilar (∼4.5 GPa), and greatly improved over the PVDFbinder (2.5 GPa). The cross-linked binders form a 3D networkthat also improves the connectivity between the cathodecomponents (S-PHCSs and carbon additives) and the carbonfibers in the P50 carbon paper. This material entanglementdecreases the likelihood of delamination during cycling andstiffens the electrode structure, thus improving the tensilestrength and Young’s modulus.36,65

Toughness (U) is an intrinsic property that describes theenergy of mechanical deformation per unit volume prior tofracture. It provides a metric to gauge the durability of theelectrode and is calculated by integrating the area under thestress−strain curve, as described in eq 366

∫ σ= ·−

Ux x

xxd

x

x z z

zz

rz

z0 0

0 (3)

where−x x

xz z0

z0

is the strain that the electrode experiences and xzr

is the distance between the two grip heads when the electroderuptures.The toughness measured for sulfur electrodes with different

binders is compared in Figure 4c. The electrode prepared withpoly(AETMAC-co-EGDA) exhibited the highest value of 55MJ m−3. Both poly(DADMAC-co-EGDA) (36 MJ m−3) andPVDF (34 MJ m−3) exhibit similar but much lower toughness.This suggests that at the same sulfur and carbon additiveloading, poly(AETMAC-co-EGDA) forms a more resistant 3Dnetwork with the S/C composite providing superiormechanical bonding compared to the other two polymers.SEM studies were carried out before (Figure 4d−f) and after

cycling (Figure 4g−i) to validate the impact of the mechanicalproperties in maintaining cathode architecture. Cross-sectionalareas of these electrodes (Figure S7) were also examined toinvestigate electrode material delamination from the currentcollector. The low-magnification SEM images in Figure 4d−fshow that all three electrodes were relatively flat and free ofcracks. Furthermore, the cross-sectional SEM images in FigureS7a−c show that most of the cathode materials penetrate a fewtens of microns into the carbon fiber matrix. Upon cycling, thePVDF-based electrode (Figure 4g) shows large cracks. This isaccompanied by severe delamination of the cathode materialfrom the P50 current collector, as shown in the cross-sectionalarea SEM image upon cycling at C/5 for 10 cycles (FigureS7d). We attribute this to the fact that PVDF exhibits thelowest tensile strength, Young’s modulus, and toughnessamong all of the binders studied and thus cannot supportdrastic volume changes upon cycling. On the other hand,electrodes cast with the poly(DADMAC-co-EGDA) binderexhibited fewer cracks and less material delamination underidentical cycling conditions (Figures 4h and S7e). This isattributed to the higher tensile strength and Young’s modulusresulting from the incorporation of the cross-linker. Fur-thermore, the improved capacity retention benefited from thelarge number of active sites in the polymer network, providingadditional polysulfide chemisorption. However, the highlyswollen poly(DADMAC-co-EGDA) network is unable tomaintain uniform interaction with lithium polysulfides, causingthe capacity of the electrode to diminish rapidly (Figure 3c).The architecture of the poly(AETMAC-co-EGDA)-basedelectrode remained intact upon deep cycling (Figures 4i and

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22486

S7f). This further confirms that the individual S/C compositeparticles were strongly bound to the current collector. Insummary, the low swelling capability of poly(AETMAC-co-EGDA) ensures sufficient interactions between its ammoniumcations and the polysulfide anions. This cross-linked polymeralso conveys a higher tensile strength, stiffness, and toughnessto the electrode, for increased tolerance to delamination, andfavors electrode durability.To further correlate the electrochemical performance with

the mechanical properties of the sulfur electrodes fabricatedwith these binders, electrochemical impedance spectroscopy(EIS) studies were performed during cycling (Figure S8). Theinterfacial resistance of both the PVDF- and poly(DADMAC-co-EGDA)-based electrodes increased during the first 50cycles. This is in strong agreement with the SEM analysis(see surface topology in Figure 4 and cross-section in FigureS7), which suggests that the architecture of these electrodesslowly deteriorates as a result of their inferior physicalproperties (Figure 4b,c). In contrast, the EIS of thepoly(AETMAC-co-EGDA)-based electrode (Figure S8c) re-mained largely unchanged. This indicates that although bothpoly(DADMAC-co-EGDA) and poly(AETMAC-co-EGDA)contain ammonium cation subunits that engage in charge-transfer interactions with the polysulfide anions,8,51 the latterpolymer provides superior mechanical binding strengthbetween the cathode components as well as with the currentcollector, preserving the electrode integrity during cycling.High-sulfur loading (∼6.0 mg cm−2) electrodes were

fabricated using these binders as a further proof-of-concept.All of the cathodes were maintained at open circuit voltage for4 h prior to cycling, to allow equilibrium swelling of the binder.Due to the electrode thickness, these cathodes wereconditioned at a low current density of C/50 for one cycle(where all cathodes exhibit a similar initial discharge capacitybetween 800 and 1000 mA h g−1 or areal capacity between 4.5

and 5.6 mA cm−2) before switching to cycling at C/10. On thefirst cycle, specific capacities of 905, 710, and 640 mA h g−1 orareal capacities of 5.4, 4.0, and 3.7 mA h cm−2 were obtainedfor electrodes fabricated with poly(AETMAC-co-EGDA),poly(DADMAC-co-EGDA), and PVDF respectively (Figure5a,b). The initial areal capacities of the cathodes in this studyare not as high as for some recently reported 3D-architecturedcathodes, but we note that those were studied over only 100cycles using a catholyte-type cell (i.e., a high electrolyte: sulfurratio).19 Considering the relatively low electrolyte/sulfur (7:1)ratio used in our study, the approach is promising for higherenergy density Li−S cell applications.Long-term cycling of the electrodes over 300 cycles (Figure

5c) provides further insight into the ability of the binder tostabilize the cycling of high loading Li−S cells. The PVDF-based electrode showed fluctuation in capacity with rapidfading. The capacity drop after 60 cycles indicates cumulativesurface passivation from randomly precipitated Li2S/S8particles on the electrode surface, which becomes pronouncedat the 79th cycle and causes cell death. However, the sulfurcathodes fabricated with poly(DADMAC-co-EGDA) andpoly(AETMAC-co-EGDA) exhibited good reversible dischargeareal capacities over the first 150 cycles. The gradual capacitydecay experienced on continued cycling (>150 cycles) of thepoly(DADMAC-co-EGDA)-based sulfur cathode is ascribed toits high degree of swelling as well as to the low tensile strengthand toughness of the polymer. At such a high degree ofswelling, the ammonium cations may interact with the solventmolecules as much as with the polysulfides. The poly-(AETMAC-co-EGDA)-based electrode reached a highercapacity of 5 mA h cm−2 (after the first few conditioningcycles needed to fully wet the electrode)37,67−69 and ultimatelystabilized at ∼3 mA h cm−2 on the 50th cycle. Mostimportantly, capacity retention was excellent over the lattercycling period.

Figure 5. Electrochemical profiles for the sulfur cathodes fabricated with the polymeric binders at high-sulfur loading (∼6.0 mg cm−2). Firstdischarge/charge voltage profile at a current rate of C/10 as a function of (a) mass-specific capacity and (b) areal capacity. (c) Long-term cycling atC/10 for the same sulfur cathodes (the 1st conditioning cycle is at C/50). The CE for the polyelectrolyte binders over the cycling duration was∼90%.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22487

3. CONCLUSIONS

Our work demonstrates the importance of cross-linkedpolymeric binders that exhibit both strong chemisorptionand robust mechanical properties, utilizing poly(AETMAC-co-EGDA) as a model system, for the fabrication of stable andhigh loading sulfur cathodes. The combination of a 3Dnetwork in the cross-linked polymer and strong charge-transferinteractions between the quaternary ammonium group andpolysulfide lead to a very significant improvement in theelectrochemical performance compared with a simple linearpolymer binder such as PVDF. The tighter polymer network,compared to poly(DADMAC-co-EGDA), which containssimilar chemical functionality, reduces electrolyte swellingand increases the tensile strength, Young’s modulus, andtoughness. These factors are all critical for delaminationtolerance. Excellent capacity retention over 300 cycles forcompact thick electrodes (6.0 mg cm−2) was exhibited in Li−Scells with a moderate electrolyte−sulfur ratio. Moreover,traditional slurry processes can be used to cast the cathodematerial onto the current collector, which fits industrialprotocols. We believe that these findings will inspire thebattery community to place more focus on utilizing highlyfunctional binders to achieve high areal capacity Li−Scathodes, including solid-state Li−S batteries that equallysuffer from volume expansion during cycling.

4. EXPERIMENTAL METHODS4.1. Preparation of Poly(AETMAC-co-EGDA) Cross-Linked

Polymer. Free-radical polymerization was employed to cross-linkAETMAC and EGDA at 80 °C in dry ethanol. The molar ratiobetween the monomer and the cross-linker was maintained at 4:1.AETMAC (80 wt % in H2O, Sigma-Aldrich) containing 600 ppmmonomethyl ether hydroquinone inhibitor was first passed through aninhibitor-remover column (Sigma-Aldrich) twice prior to use. Next,804 mg of the purified AETMAC aqueous solution, 157 mg of EGDA(90% technical, Sigma-Aldrich), 39 mg of AIBN (99%, recrystallized,Sigma-Aldrich) and 4 g of dry ethanol (Fisher Scientific) wereintroduced into a 50 mL round-bottom flask. The flask was sealedwith a rubber septum, and the mixture was stirred rapidly for 15 min.After a clear solution was obtained, the mixture was purged gentlywith dry nitrogen for an additional 10 min before placing the flask inan oil bath (80 °C) on a stir plate. The stirred reaction wasterminated after 12 h, and the cross-linked polymer was recovered byprecipitation in acetone (ACS, Sigma-Aldrich). Finally, the polymerwas filtered and rinsed with cold isopropanol (anhydrous, Sigma-Aldrich) to remove any unreacted precursors.4.2. Preparation of Poly(DADMAC-co-EGDA) Cross-Linked

Polymer. The free-radical cross-linking polymerization betweenDADMAC and EGDA was achieved in a similar manner to theexperimental procedure described above. However, the co-polymer-ization temperature for poly(DADMAC-co-EGDA) was set to 60 °C,and the molar ratio of monomer to cross-linker was 2:1. In a typicalprocedure, a mixture of 604 mg of DADMAC (97% AT, Sigma-Aldrich), 353 mg of EGDA, 44 mg of AIBN, and 4 g of anhydrousethanol in a 50 mL round-bottom flask was used for the reaction. Theflask was sealed with a rubber septum, and the mixture was stirred forat least 15 min to allow dissolution of the reactants; the solution wasthen purged gently with dry nitrogen for 10 min. The flask was placedin an oil bath (60 °C), and the mixture was stirred for 12 h, althoughthe contents turned cloudy 10 min after the start of polymerization.Finally, the cross-linked polymer was filtered and rinsed with coldisopropanol to remove any unreacted precursors.4.3. Preparation of the Porous Hollow Carbon Sphere

(PHCS) Sulfur Host. The host material was synthesized, aspreviously reported.10 Elemental sulfur was melt-diffused into thePHCSs at 160 °C for 12 h to afford the S-PHCS composite, with a

sulfur content of 75 wt %, as determined by thermogravimetricanalysis (TGA).

4.4. Preparation of Li2Sn. In an Ar-filled glovebox, Li2S4 andLi2S6 were synthesized by allowing elemental sulfur (99.5%, Sigma-Aldrich) to react with lithium triethylborohydride (1 M, Sigma-Aldrich) in anhydrous tetrahydrofuran in the appropriate stoichio-metric ratio. The resultant solution was vacuum-dried, followed by afinal wash with toluene to isolate the powders. The lithium polysulfidepowder was collected by vacuum drying in a Buchi vacuum oven at 60°C for 6 h.

4.5. Preparation of the Li2S4−Homopolymer Mixture. All ofthe solid materials were vacuum-dried in a Buchi vacuum oven at 60°C for 24 h prior to use. In an Ar-filled glovebox, 20 mg ofhomopolymer and 20 mg of Li2S4 were stirred in 2 mL of DME for 6h. The material for FTIR analysis was collected by centrifugation (10krpm, 10 min) followed by vacuum drying overnight at roomtemperature for 12 h.

4.6. Lithium Polysulfide Adsorptivity Measurement. Theadsorptivity of lithium polysulfides onto the polymeric binders wasdetermined by an electrochemical sulfur titration technique.54 In anAr-filled glovebox, the binders and lithium polysulfide (Li2S4 or Li2S6)powders were stirred in DME at an concentration of 10 mg mL−1 for18 h to allow saturation of the materials with Sn

2−. The suspensionwas then centrifuged (10k rpm, 10 min) to remove the solid powders.The supernatant was collected and further diluted with 2 M LiTFSI inDME, utilizing a stainless steel current collector (type 316, 100 mesh,Alfa Aesar). The counter electrode compartment was filled with 1 MLiTFSI in DME and 2 wt % LiNO3. A porous glass frit was used toseparate the two solutions in a two-electrode Swagelok cell. Theoxidation potential of the sample solution was held at 3.0 V vs Li/Li+

to oxidize the excess lithium polysulfide to elemental sulfur, and thecapacity was determined by integrating the time needed for thecurrent to reach essentially zero. Lithium polysulfide solutions werealso prepared in the absence of binders to establish a standard curve.

4.7. Tensile Tests. The P50 carbon paper samples with thecomposite material coated in the same manner as for electrodepreparation were cut into 3″ × 1″ rectangular shapes and mounted viagrip heads. Mechanical tests were carried out on a universal macro-tribometer (UNMT-2MT, Centre for Tribology, Inc.) with a 10 kgload cell. The force and displacement were controlled by a step-motorat a pull rate of 0.01 mm s−1.

4.8. Physical Characterization. FTIR spectra were obtained on aBruker Tensor 37 spectrometer. For polysulfide-contact experiments,the samples were mixed with KBr in an Ar-filled glovebox beforetransfer to a N2-filled chamber where the analysis was conducted.Lithium polysulfide adsorptivity was measured on a VMP3 multi-channel potentiostat (Bio-logic) by chronoamperometry. SEM andEDAX elemental analysis studies were carried out on a Zeiss Ultrafield emission SEM instrument equipped with an EDX attachment(Zeiss). TEM was performed on a ZEISS Libra 200 MC TEMinstrument operating at 200 kV. The specific surface area and porevolume were measured using a Quantachrome Autosorb-1 system at77 K. The specific surface area and pore size distribution werecalculated by the Brunauer−Emmett−Teller method and thequenched solid density functional theory method, respectively.TGA, used to determine the sulfur content of the materials, wascarried out on a TA Instruments SDT Q600 at a heating rate of 10 °Cmin−1 from room temperature to 800 °C under air flow.

4.9. Electrode Preparation. The electrodes were prepared froma water/dimethylformamide (6:4) slurry containing 80 wt % S-PHCS,5 wt % Super P, 5 wt % 8 nm multiwall carbon nanotubes, and 10 wt% polymeric binder onto P50 carbon paper. Only dimethylformamidewas used when PVDF is served as a binder. The sulfur loading onthese electrodes was either 3.5 or 6.0 mg cm−2. The electrodes weredried in a 60 °C oven for 24 h before they were transferred to an Ar-filled glovebox. Li−S coin cells (2325) were assembled with a lithiumfoi l anode and an e lectrolyte of 1 M li th ium bis-(trifluoromethanesulfonyl)imide (LiTFSI) and 2 wt % LiNO3 in a1:1 v/v mixture of 1,2-dimethoxyethane (DME) and 1,3-dioxolane(DOL) (BASF). The electrolyte/sulfur ratio was 7:1 (μL:mg) for the

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22488

electrochemical studies. Cycling performance and rate capabilitystudies were performed on a multi-channel Arbin instrument. Cyclicvoltammetry and electrochemical impedance spectroscopy studieswere recorded on a Bio-logic VMP3 electrochemical station with ascan rate of 0.1 mV s−1 and amplitude of 10 mV in the frequencyrange of 200 mHz to 500 kHz, respectively.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b06456.

FTIR transmission spectra for the cross-linked polymersand the mechanical blends of their homopolymer;calibration plots for the polysulfide adsorptivity measure-ments; SEM and TEM images, TGA curves, N2 isothermanalysis results for the PHCSs and S-PHCSs; SEMimages of the P50 current collector; elemental mappingof a cast sulfur cathode, cyclic voltammetry and Nyquistplots and cross-sectional SEM images for the three sulfurcathodes with the different binders (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: gauthier@uwaterloo.ca (M.G.).*E-mail: lfnazar@uwaterloo.ca (L.F.N.).ORCIDC. Y. Kwok: 0000-0003-1007-8699M. Gauthier: 0000-0001-9486-1810Author ContributionsC.Y.K., L.F.N., and M.G. conceived the concept and designedexperimental work. C.Y.K. and A.W. prepared and charac-terized the cross-linked polymers, and optimized the cross-linking density of the polyelectrolytes. C.Y.K. prepared thecathode materials, performed the physical characterization ofthe materials and electrodes as well all of the electrochemicalstudies, with assistance from Q.P., A.W., and X.L. All authorthoroughly discussed the analysis of the data. C.Y.K., A.W., andL.F.N. wrote the manuscript with contributions from all of theauthors.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported by the Joint Center for EnergyStorage Research (JCESR), an Energy Innovation Hub fundedby the US Department of Energy (DOE), Office of Science,Basic Energy Science. The authors thank Kelvin Liew and Prof.Boxin Zhao at the University of Waterloo for their generoushelp in performing the mechanical measurements on theelectrodes. We thank Dr Shahrzad Hosseini Vajargah for thecollection of the TEM images. This research was also partiallysupported by the Natural Sciences and Engineering Council ofCanada (NSERC) through their Discovery and CanadaResearch Chair programs to L.F.N. and a doctoral scholarship(PGS-D) to C.Y.K.

■ REFERENCES(1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M.Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012,11, 19−29.(2) Adelhelm, P.; Hartmann, P.; Bender, C. L.; Busche, M.; Eufinger,C.; Janek, J. From Lithium to Sodium: Cell Chemistry of Room

Temperature Sodium-Air and Sodium-Sulfur Batteries. Beilstein J.Nanotechnol. 2015, 6, 1016−1055.(3) Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S.Rechargeable Lithium-Sulfur Batteries. Chem. Rev. 2014, 114,11751−11797.(4) Evers, S.; Nazar, L. F. New Approaches for High Energy DensityLithium-Sulfur Battery Cathodes. Acc. Chem. Res. 2013, 46, 1135−1143.(5) Lv, D.; Zheng, J.; Li, Q.; Xie, X.; Ferrara, S.; Nie, Z.; Mehdi, L.B.; Browning, N. D.; Zhang, J.-G.; Graff, G. L.; Liu, J.; Xiao, J. HighEnergy Density Lithium-Sulfur Batteries: Challenges of Thick SulfurCathodes. Adv. Energy Mater. 2015, 5, No. 1402290.(6) Pope, M. A.; Aksay, I. A. Structural Design of Cathodes for Li-SBatteries. Adv. Energy Mater. 2015, 5, No. 1500124.(7) Zhou, W.; Guo, B.; Gao, H.; Goodenough, J. B. Low-CostHigher Loading of a Sulfur Cathode. Adv. Energy Mater. 2016, 6,No. 1502059.(8) Chen, H.; Ling, M.; Hencz, L.; Ling, H. Y.; Li, G.; Lin, Z.; Liu,G.; Zhang, S. Exploring Chemical, Mechanical, and ElectricalFunctionalities of Binders for Advanced Energy-Storage Devices.Chem. Rev. 2018, 118, 8936−8982.(9) Lee, J. T.; Zhao, Y.; Thieme, S.; Kim, H.; Oschatz, M.;Borchardt, L.; Magasinski, A.; Cho, W. I.; Kaskel, S.; Yushin, G.Sulfur-Infiltrated Micro- and Mesoporous Silicon Carbide-DerivedCarbon Cathode for High-Performance Lithium Sulfur Batteries. Adv.Mater. 2013, 25, 4573−4579.(10) He, G.; Evers, S.; Liang, X.; Cuisinier, M.; Garsuch, A.; Nazar,L. F. Tailoring Porous in Carbon Nanospheres for Lithium-SulfurBattery Cathode. ACS Nano 2013, 7, 10920−10930.(11) Zheng, J.; Tian, J.; Wu, D.; Gu, M.; Xu, W.; Wang, C.; Gao, F.;Engethard, M. H.; Zhang, J.-G.; Liu, J.; Xiao, J. Lewis Acid-BaseInteractions between Polysulfides and Metal Organic Framework inLithium Sulfur Batteries. Nano Lett. 2014, 14, 2345−2352.(12) Ahn, W.; Lim, S. N.; Lee, D. U.; Kim, K.-B.; Chen, Z.; Yeon, S.-H. Interaction Mechanism between a Functionalized Protector Layerand Dissolved Polysulfide for Extended Cycle Life of Lithium SulfurBatteries. J. Mater. Chem. A 2015, 3, 9461−9467.(13) Do, V.; Deepika; Kim, M. S.; Kim, M. S.; Lee, K. R.; Cho, W. I.Carbon Nitride Phosphorus as an Effective Lithium PolysulfideAdsorbent for Lithium-Sulfur Batteries. ACS Appl. Mater. Interfaces2019, 11, 11431−11441.(14) Shyamsunder, A.; Beichel, W.; Klose, P.; Pang, Q.; Scherer, H.;Hoffmann, A.; Murphy, G. K.; Krossing, I.; Nazar, L. F. InhibitingPolysulfide Shuttle in Lithium-Sulfur Batteries trough Low-Ion-Pairing Salts and a Triflamide Solvent. Angew. Chem., Int. Ed. 2017,56, 6192−6197.(15) Gordin, M. L.; Dai, F.; Chen, S.; Xu, T.; Song, J.; Tang, D.;Azimi, N.; Zang, Z.; Wang, D. Bis(2,2,2-trifluoroethyl) Ether as anElectrolyte Co-solvent for Mitigating Self-Discharge in Lithium-SulfurBatteries. ACS Appl. Mater. Interfaces 2014, 6, 8006−8010.(16) Li, S.-Y.; Wang, W.-P.; Duan, H.; Guo, Y.-G. Recent Progresson Confinement of Polysulfides through Physical and ChemicalMethods. J. Energy Chem. 2018, 27, 1555−1565.(17) Pang, Q.; Liang, X.; Kwok, C. Y.; Nazar, L. F. Advances inLithium-Sulfur Batteries Based on Multifunctional Cathodes andElectrolytes. Nat. Energy 2016, 1, No. 16132.(18) Eroglu, D.; Zavadil, K. R.; Gallagher, K. G. Critical Linkbetween Materials Chemistry and Cell-Level Design for High EnergyDensity and Low Cost Lithium-Sulfur Transportation Battery. J.Electrochem. Soc. 2015, 162, A982−A990.(19) Chung, S.-H.; Chang, C.-H.; Manthiram, A. A Carbon-CottonCathode with Ultrahigh-Loading Capability for Statically andDynamically Stable Lithium-Sulfur Batteries. ACS Nano 2016, 10,10462−10470.(20) Du, W.-C.; Yin, Y.-X.; Zeng, X.-X.; Shi, J.-L.; Zang, S.-F.; Wan,L.-J.; Guo, Y.-G. Wet Chemistry Synthesis of MultidimensionalNanocarbon-Sulfur Hybrid Materials with Ultrahigh Sulfur Loadingfor Lithium-Sulfur Batteries. ACS Appl. Mater. Interfaces 2016, 8,3584−3590.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22489

(21) Zhou, G.; Li, L.; Ma, C.; Wang, S.; Shi, S.; Koratkar, N.; Ren,W.; Li, F.; Cheng, H.-M. A Graphene Foam Electrode with HighSulfur Loading for Flexible and High Energy Li-S Batteries. NanoEnergy 2015, 11, 356−365.(22) Ye, X.; Ma, J.; Hu, Y.-S.; Wei, H.; Ye, F. MWCNT PorousMicrospheres with an Efficient 3D conductive Network for HighPerformance Lithium-Sulfur Batteries. J. Mater. Chem. A 2016, 4,775−780.(23) Song, J.; Gordin, M. L.; Xu, T.; Chen, S.; Yu, Z.; Sohn, H.; Lu,J.; Ren, Y.; Duan, Y.; Wang, D. Strong Lithium PolysulfideChemisorption on Electroactive Sites of Nitrogen-Doped CarbonComposites for High-Performance Lithium-Sulfur Battery Cathodes.Angew. Chem., Int. Ed. 2015, 54, 4325−4359.(24) Zeng, F.; Wang, W.; Wang, A.; Yuan, K.; Jin, Z.; Yang, Y.-S.Multidimensional Polycation β-Cyclodextrin Polymer as an EffectiveAqueous Binder for High Sulfur Loading Cathode in Lithium-SulfurBatteries. ACS Appl. Mater. Interfaces 2015, 7, 26257−26265.(25) Yu, X.; Manthiram, A. Electrode-Electrolyte Interfaces inLithium-based Batteries. Energy Environ. Sci. 2018, 11, 527−543.(26) Lacey, M. J.; Jeschull, F.; Edstrom, K.; Brandell, D. PorosityBlocking in Highly Porous Carbon Black by PVdF Binder and ItsImplications for the Li-S System. J. Phys. Chem. C 2014, 118, 25890−25898.(27) Zhang, S. S.; Jow, T. R. Study of Poly(acrylonitrile-methylmethacrylate) as Binder for Graphite Anode and LiMn2O4 Cathodefor Li-ion Batteries. J. Power Sources 2002, 109, 422−426.(28) Zhang, Z.; Zeng, T.; Lai, Y.; Jia, M.; Li, J. A ComprehensiveStudy of Different Bunders and Their Effects on ElectrochemicalProperties of LiMn2O4 Cathode in Lithium Ion Batteries. J. PowerSources 2014, 247, 1−8.(29) Ai, G.; Dai, Y.; Ye, Y.; Mao, W.; Wang, Z.; Zhao, H.; Chen, Y.;Zhu, J.; Fu, Y.; Battaglia, V.; Guo, J.; Srinivasan, V.; Liu, G.Investigation of Surface Effects through the Application of theFunctional Binders in Lithium Sulfur Batteries. Nano Energy 2015, 16,28−37.(30) Li, L.; Pascal, T. A.; Connell, J. G.; Fan, F. Y.; Meckler, S. M.;Ma, L.; Chiang, Y.-M.; Prendergast, D.; Helms, B. A. MolecularUnderstanding of Polyelectrolyte Bunders that Actively Regulate IonTransport in Sulfur Cathodes. Nat. Commun. 2017, 8, No. 2277.(31) Cheon, S.-E.; Cho, J.-H.; Ko, K.-S.; Kwon, C.-W.; Chang, D.-R.;Kim, H.-T.; Kim, S.-W. Structural Factors of Sulfur Cathodes withPoly(ethylene oxide) Binder for Performance of RechargeableLithium Sulfur Batteries. J. Electrochem. Soc. 2002, 149, A1437−A1441.(32) Wang, Y.; Huang, Y.; Wang, W.; Huang, C.; Yu, Z.; Zhang, H.;Sun, J.; Wang, A.; Yuan, K. Structural Change of the Porous SulfurCathode using Gelatin as a Binder during Discharge and Charge.Electrochim. Acta 2009, 54, 4062−4066.(33) Wang, L.; Wang, D.; Zhang, F.; Jin, J. Interface ChemistryGuided Long-Cycle-Life Li-S Battery. Nano Lett. 2013, 13, 4206−4211.(34) Lacey, M. J.; Jeschull, F.; Edstrom, K.; Brandell, D. Functional,Water-Soluble Binders for Improved Capacity and Stability ofLithium-Sulfur Batteries. J. Power Sources 2014, 264, 8−14.(35) Wang, Z.; Chen, Y.; Battaglia, V.; Liu, G. Improving thePerformance of Lithium-Sulfur Batteries using Conductive Polymerand Micrometric Sulfur Powder. J. Mater. Res. 2014, 29, 1027−1033.(36) Li, G.; Ling, M.; Ye, Y.; Li, Z.; Guo, J.; Yao, Y.; Zhu, J.; Lin, Z.;Zhang, S. Acacia Senegal-Inspired Bifunctional Binder for Longevityof Lithium-Sulfur Batteries. Adv. Energy Mater 2015, 5, No. 1500878.(37) Pang, Q.; Liang, X.; Kwok, C. Y.; Kulisch, J.; Nazar, L. F. AComprehensive Approach toward Stable Lithium-Sulfur Batterieswith High Volumetric Energy Density. Adv. Energy Mater. 2017, 7,No. 1601630.(38) Liu, J.; Galpaya, D. D.; Yan, L.; Sun, M.; Lin, Z.; Yan, C.; Liang,C.; Zhang, S. Exploiting a Robust Biopolymer Network Binder for anUltrahigh-Areal-Capacity Li-S Battery. Energy Environ. Sci. 2017, 10,750−755.

(39) Park, K.; Cho, J. H.; Jang, J.-H.; Yu, B.-C.; de la Hoz, A. T.;Miller, K. M.; Ellison, C. J.; Goodenough, J. B. Trapping LithiumPolysulfides of a Li-S Battery by Forming Lithium Bonds in a PolymerMatrix. Energy Environ. Sci. 2015, 8, 2389−2395.(40) Ma, L.; Zhuang, H.; Lu, Y.; Moganty, S. S.; Hennig, R. G.;Archer, L. A. Tathered Molecular Sorbents: Enabling Metal-SulfurBattery Cathodes. Adv. Energy Mater. 2014, 4, No. 1400390.(41) Seh, Z. W.; Zhang, Q.; Li, W.; Zheng, G.; Yao, H.; Cui, Y.Stable Cycling of Lithium Sulfide Cathodes through Strong Affinitywith a Bifunctional Binder. Chem. Sci. 2013, 4, 3673−3677.(42) Zheng, G.; Zhang, Q.; Cha, J. J.; Yang, Y.; Li, W.; She, Z. W.;Cui, Y. Amphiphilic Surface Modification of Hollow CarbonNanofibers for Improved Cycle Life of Lithium Sulfur Batteries.Nano Lett. 2013, 13, 1265−1270.(43) Bhattacharya, P.; Nandasiri, M. I.; Lv, D.; Schwarz, A. N.;Darsell, J. T.; Henderson, W. A.; Tomalia, D. A.; Liu, J.; Zhang, J.-G.;Xiao, J. Polyamidoamine Dendrimer-based Binders for High-LoadingLithium-Sulfur Battery Cathodes. Nano Energy 2016, 19, 176−186.(44) Zhang, L.; Ling, M.; Feng, J.; Liu, G.; Guo, J. EffectiveElectrostatic Confinement of Polysulfides in Lithium/Sulfur Batteriesby a Functional Binder. Nano Energy 2017, 40, 559−565.(45) Wang, H.; Ling, M.; Bai, Y.; Chen, S.; Yuan, Y.; Liu, G.; Wu, C.;Wu, F. Cationic Polymer Binder Inhibit Shuttle Effects throughElectrostatic Confinement in Lithium Sulfur Batteries. J. Mater. Chem.A 2018, 6, 6959−6966.(46) He, M.; Yuan, L.-X.; Zhang, W.-X.; Hu, X.-L.; Huang, Y.-H.Enhanced Cyclability for Sulfur Cathode Achieved by a Water-SolubleBinder. J. Phys. Chem. C 2011, 115, 15703−15709.(47) Shen, S.; Zhu, Z.; Liu, F. Introduction of Poly[(2-acryloyoxyethyl trimetal ammonium chloride)-co-(acrylic acid)]Branches onto Starch for Cotton Warp Sizing. Carbohydr. Polym.2016, 138, 280−289.(48) Aly, M. A. S.; Gauthier, M.; Yeow, J. On-Chip Cell Lysis byAntibacterial Non-Leaching Reusable Quaternary Ammonium Mono-lithic Column. Biomed. Microdevices 2016, 18, 2.(49) Evers, S.; Yim, T.; Nazar, L. F. Understanding the Nature ofAbsorption/Adsorption in Nanoporous Polysulfide Sorbents for theLi-S Battery. J. Phys. Chem. C 2012, 116, 19653−196558.(50) Das, P.; Ray, S.; Bhaumik, A.; Banerjee, B.; Mukhopadhyay, C.Cubic Ag2O Nanoparticle Incorporated Mesoporous Silica with LargeBottle-Neck like Mesoporous for the Aerobic Oxidative Synthesis ofDisulfide. RSC Adv. 2015, 5, 6323−6331.(51) Ling, M.; Yan, W.; Kawase, A.; Zhao, H.; Fu, Y.; Battaglia, V. S.;Liu, G. Electrostatic Polysulfides Confinement to Inhibit RedoxShuttle Process in the Lithium-Sulfur Batteries. ACS Appl. Mater.Interfaces 2017, 9, 31741−31745.(52) Zhang, S. S. New Insight into Liquid Electrolyte ofRechargeable Lithium/Sulfur Battery. Electrochim. Acta 2013, 97,226−230.(53) He, F.; Luo, B.; Yuan, S.; Liang, B.; Choong, C.; Pehkonen, S.O. PVDF Film Tethered with RGD-click-Poly(glycidyl methacrylate)Brushes by Combination of Direct Surface-Initiated ATRP and ClickChemistry for Improved Cytocompatibility. RSC Adv. 2014, 4, 105−107.(54) Hart, C. J.; Cuisinier, M.; Liang, X.; Kundu, D.; Garsuch, A.;Nazar, L. F. Rational Design of Sulphur Host Materials for Li-SBatteries: Correlating Lithium Polysulphide Adsorptivity and Self-Discharge Capacity Loss. Chem. Commun. 2015, 51, 2308−2311.(55) Pascal, T. A.; Wujcik, K. H.; Wang, D. R.; Balsara, P. N.;Prendergast, D. Thermodynamic Origins of the Solvent-DependentStability of Lithium Polysulfides from First Principles. Phys. Chem.Chem. Phys. 2017, 19, 1441−1448.(56) Liao, J.; Ye, Z. Qu Quaternary Ammonium Cationic Polymer asa Superior Bifunctional Binder for Lithium-Sulfur Batteries and Effectsof a Counter Anion. Electrochim. Acta 2018, 259, 626−636.(57) Liao, J.; Liu, Z.; Liu, X.; Ye, Z. Water-Soluble LinearPoly(ethyleneimine) as a Superior Bifunctional Binder for Lithium-Sulfur Batteries of Improved Cell Performance. J. Phys. Chem. C 2018,122, 25917−25959.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22490

(58) Sun, Q.; He, B.; Zhang, X.-Q.; Li, A.-H. Engineering of HollowCore-Shell Interlinked Carbon Spheres for Highly Stable Lithium-Sulfur Batteries. ACS Nano 2015, 9, 8504−8513.(59) Nunes, R. W.; Martin, J. R.; Johnson, J. F. Influence ofMolecular Weight and Molecular Weight Distribution on MechanicalProperties of Polymers. Polym. Eng. Sci. 1982, 22, 205−228.(60) Zhang, L.; Ling, M.; Feng, J.; Mai, L.; Liu, G.; Guo, J. TheSynergetic Interaction between LiNO3 and Lithium Polysulfides forSuppressing Shuttle Effect of Lithium-Sulfur Batteries. Energy StorageMater. 2018, 11, 24−29.(61) Mark, J. E. Physical Properties of Polymer Handbook, 2nd ed.;Springer: New York, 2007.(62) Zheng, J.; Lv, D.; Gu, M.; Wang, C.; Zhang, J.-G.; Liu, J.; Xiao,J. How to Obtain Reproducible Results for Lithium Sulfur Batteries? J.Electrochem. Soc. 2013, 160, A2288.(63) Xu, K. Electrolytes and Interfaces in Li-Ion Batteries andBeyond. Chem. Rev. 2014, 114, 11503.(64) Gu, H.; Guo, J.; Wei, H.; Guo, S.; Liu, J.; Hang, Y.; Khan, M.A.; Wang, X.; Young, D. P.; Wei, S.; Guo, Z. StrengthenedMegnetoresistive Epoxy Nanocomposite Paper Derived fromSynergistic Nanomagnetite-Carbon Nanofiber Nanohybrids. Adv.Mater. 2015, 27, 6277−6282.(65) Gu, H.; Tadakamalla, S.; Zhang, X.; Huang, Y.; Jiang, Y.;Colorado, H. A.; Lou, Z.; Wei, S.; Guo, Z. Epoxy ResinNanosuspensions and Reinforced Nanocomposite from PolyanilineStabilized Multi-Walled Carbon Nanotubes. J. Mater. Chem. C 2013,1, 729−743.(66) Tomastik, J.; Ctvrlik, R. Nanoscratch Test − A Tool forEvaluation of Cohesive and Adhesive Properties of Thin Films andCoatings. EPJ Web Conf. 2013, 48, No. 00027.(67) Xiao, L.; Cao, Y.; Xiao, J.; Schwenzer, B.; Engelhard, M. H.;Saraf, L. V.; Zie, Z.; Exarhos, G. J.; Liu, J. A Soft Approach toEncapsulate Sulfur: Polyaniline Nanotubes for Lithium-SulfurBatteries with Long Cycle Life. Adv. Mater. 2012, 24, 1176−1181.(68) Wang, C.; Wan, W.; Chen, J.-T.; Zhou, H.-H.; Zhang, X.-X.;Yuan, L.-X.; Huang, Y.-H. Dual Core-Shell Structured Sulfur CathodeComposite Synthesized by a One-Pot Route for Lithium SulfurBatteries. J. Mater. Chem. A 2013, 1, 1716−1723.(69) Zhang, S. A Concept for Making Poly(ethylene oxide) BasedComposite Gel Polymer Electrolyte Lithium/Sulfur Battery. J.Electrochem. Soc. 2013, 160, A1421−A1424.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b06456ACS Appl. Mater. Interfaces 2019, 11, 22481−22491

22491