Jun Zhng ,Mingnn Li ,Hussein A.Younus ,Binshen Wng ,Qunhong Weng ,Yn Zhng ,＊＊,Shiguo Zhng ,＊

a College of Materials Science and Engineering,Hunan University,Changsha,410082,China

b Chemistry Department,Faculty of Science,Fayoum University,Fayoum,63514,Egypt

c Department of Chemistry and Biotechnology,Yokohama National University,79-5 Tokiwadai,Hodogaya-ku,Yokohama,240-8501,Japan

Keywords:Lithium–sulfur battery Advanced binder Polymer

ABSTRACT The lithium-sulfur battery(Li–S)is a promising energy storage system with many advantages over the commercialized lithium-ion battery.It has a high theoretical capacity of 1675 mAh g-1,a high theoretical energy density(2600 Wh kg-1),and is eco-environmentally friendly.Although only a small amount is used(＜10 wt%)in the electrode,binders may affect the discharge capacity and cycling stability of sulfur cathodes in the Li–S battery.In recent years,tremendous efforts have been made to develop functional binders with robust adhesive strength,fast ion/electron transportation,strong anchoring of lithium polysulfide(LiPS),and rapid redox kinetics,to improve capacity,coulombic efficiency,and energy density.This article reviews recent developments in binders for the Li–S battery.After brie fly introducing the fundamentals of the Li–S battery,the desireable characteristics of binders are discussed based on the correlation between the functions of the binder molecules and the performance of the battery.Future challenges in developing promising binders and potential solutions are provided in the conclusion.＊Corresponding author.

1.Introduction

With the soaring energy consumption and world-wide environmental problems,great efforts have been made to develop clean,renewable,and high-performance energy storage systems[1–4].Commercialized lithium-ion batteries(LIB),which are able to satisfy the needs of basic applications in daily life,have dominated the energy industry for decades[5].Despite the vast progress that has been made in improving the current LIB,the theoretically inherent capacity restrictions have impeded its further development[3].Also,the toxic and costly active materials(e.g.,LiFePO4and LiCoO2)put heavy pressures on the environment[6,7].Therefore,searching for environmentally-friendly energy systems with high theoretical capacity and low cost is extremely urgent.The lithium-sulfur(Li–S)battery is a promising energy storage system with many advantages.First,the Li–S battery which has a sulfur cathode(theoretical capacity of 1675 mAh g-1)and lithium metal anode(3860 mAh g-1)holds an ultra-high theoretical energy density of 2600 Wh kg-1,almost 10 times higher than the current LIB[8,9].Second,the cathodic active material S8,as a by-product of the oil refining process,is low in cost compared with the transition-metal oxide cathodes of the commercialized LIB[10–12].Sulfur is also non-toxic and eco-friendly,characteristics highly desired due to the increasingly worsening environmental issues[13].

The discharge mechanism of the Li–S battery is a multi-step/electron electrochemical reaction,as shown in Fig.1.Typically,there are two voltage plateaus during this process[14–16].Along with the initial processes of solid-liquid phase transition from elemental sulfur(S8)to the dissolved high-order lithium polysulfide intermediate LiPS(Li2S8),thefirst plateau,arising at about 2.4–2.1 V,indicates the formation of a series of soluble polysulfides(Li2Sx,4≤x≤8),corresponding to a theoretical capacity of 418 mAh g-1.The second lower plateau,appearing around 2.1–1.5 V,with a theoretical capacity of 1254 mAh g-1,corresponds to the liquid-solid transition from the dissolved low order LiPS(Li2S4)to insoluble Li2S2and Li2S,ending with the solid-solid transition from insoluble Li2S2to insoluble Li2S[17].

As the research progressed,some drawbacks of the Li–S battery were revealed(Fig.2a),including:(1)The high insulating nature of both the sulfur and thefinal discharge product(Li2S)lead to low utilization of the active materials.At room temperature,sulfur and Li2S are both electronic and ionic insulators with low conductivity of 5×10-30S cm-2and 10-14S cm-2,respectively[19].The electrochemical contact of sulfur with the conductive matrix is poor and the reduction of sulfur could only take place on the surface of the conductive matrix[20].With the continuous dissolution of LiPS(Li2Sx,4≤x≤8)into the electrolyte,the interior sulfur can participate in reactions and the low utilization of the active materials can be improved[21].Due to the insulating nature of sulfur/Li2S,the kinetics of the electrochemical reactions occurring in the sulfur cathodes largely depend on the electron transfer,eventually leading to sluggish kinetics[22–24].(2)The reduplicative volume expansion results in instability of the solid-electrolyte interface(SEI)film and the loss of electrical contact.The difference in the density between the Li2S(1.67 g cm-3)and the sulfur(2.03 g cm-3)induces a periodic volume change of up to～80%during cycling[18,25].The repetitive dissolution and re-deposition of the active material species tends to destroy the porous sulfur cathode,making the isolated sulfur unable to participate in the subsequent electrochemical reaction,resulting in the formation of“dead sulfur”and irreversible loss of capacity[26].(3)The“shuttle effect”induces the loss of active sulfur,corrosion of the lithium metal anode,rapid capacity fading,and low coulombic efficiency.Soluble LiPS(Li2Sx,4≤x≤8)inevitably migrates to the anode under the concentration gradient force during discharging,part of which would be reduced by the lithium metal into insulating Li2S and Li2S2,which are deposited on the surface of the anode[27].During charging,at the anode,plenty of the long-chain LiPS in the electrolyte would be reduced to shorter long-chain LiPS.Under the electricfield,this part of the LiPS migrates to the cathode,where it would be oxidized to a longer long-chain LiPS(Fig.2b).Eventually,the polysulfide keeps moving back and forth between the cathode and anode,which is the notorious“shuttle effect”of the Li–S battery[26–28].This effect leads to significant differences in energy consumption between charging and discharging[29,30].During charging,the concentration gradient of the ion diffusion is opposite to the direction of the electricfield,resulting in a continuous decrease in coulombic efficiency to compensate for the extra energy consumed by the migration of the LiPS.

Fig.1.A typical discharge/charge profile of the Li–S battery.Reproduced with permission[18].Copyright 2017,Wiley-VCH.

2.Binders for Li-S batteries

Fig.2.(a)Main challenges in the sulfur cathode and(b)The“shuttle effect”in the electrolyte for the Li–S battery.Reproduced with permission[31].Copyright 2015,Royal Society of Chemistry.

Over the past few decades,researchers have attempted to mitigate the drawbacks of the Li–S battery using various approaches,mainly focusing on porous carbon materials[32],electrolyte additive[33],separator[34],and so forth,as shown in Fig.3.Although enormous efforts have been made to improve the capacity and cycle stability of Li–S batteries,most are impractical for industrial application[19].By contrast,less attention was turned to the electrically inactive components of the sulfur cathode,such as binders.Although the binder occupies a small proportion(usually＜10%)of the whole cathode[35–37],in addition to integrating the structure of the electrode,it plays an important role in the sulfur cathode.In the past,some binders,such as poly(ethylene oxide)(PEO)[38],polyacrylic acid(PAA)[39],and poly(vinylidene fluoride)(PVDF)were used in Li–S batteries[40,41].It was gradually realized that binders play significant roles in Li–S batteries.As demonstrated by Matthew et al.when PEO was used as a binder in the Li–S battery,the solvent system at the electrode/electrolyte interface would be significantly modified,leading to improved overall performance.It was pointed out that PEO could dissolve or swell in the liquid electrolyte,leading to the decay of the cathode's integrity[38].As a consequence,PVDF is the most frequently used Li–S battery binder.Widely used in various energy storage devices,PVDF is a fluorine-containing linear polymer with a wide electrochemical stability window(0–5 V Li/Li+),high thermal stability and considerable resistance to oxidation/chemical media[40,41].However,in recent years,disadvantages to its use as a binder in the Li–S battery have been revealed.For example,(1)poor adhesive ability,hampering its ability to maintain electrode integrity.PVDF interacts weakly with the electrode material only by van der Waals forces.The C–F bond in PVDF is not reactive,and thus no chemical bonds can be built with the active materials and the substrates[42].During the drying process,only part of the PVDF is used for adhesion,while the rest tends to crystallize,further decreasing the adhesive strength,since the crystalline part contributes little to adhesion.As a result,the active materials are easily detached from the substrates and pulverized,accompanied by the loss of capacity[30,43,44].(2)PVDF does not have any positive impact on the“shuttle effect”.The weak affinity of PVDF to polar active sulfur species leads to the loss of the sulfur-containing active phase[45,46].(3)PVDF is expensive and difficult to recycle,and electrode fabrication with PVDF involves toxic and volatile organic compounds(e.g.,N-methyl-2--pyrrolidone(NMP))[47].Recently,international communities have restricted the use of NMP,calling instead for environmentally friendly means of manufacturing that avoid the use of volatile organic compounds(VOCs)[13,48].In view of the existing difficulties of the Li–S battery and the disadvantages of PVDF,we brie fly summarize the characteristics of the ideal Li–S battery binders as follows(see Fig.4).

Fig.3.Research topics for the Li–S battery in recent years.Reproduced with permission[28].Copyright 2018,The Electrochemical Society.

Fig.4.Characteristics of the binders for Li–S batteries.

3.Characteristics required for Li-S battery binders

3.1.Strong adhesive strength

Traditionally,electrode slurries are composed of a binder solution and other desired active materials.In the manufacturing process,the solution is sufficient to wet the pores on the surface of the particles.Once the slurry is coated and dried,the adhesion of the composite framework is achieved.The adhesive strength can be adjusted by the functional groups of the polymer backbone and the chemical properties of the adherent surface(e.g.,Al foil,carbon paper,Ni foam)[49,50].With robust adhesive strength,structural integrity of the conductive network between active materials and conductive agents can be achieved and the resistance/polarization would be sharply decreased[51].First,the performance of the sulfur cathode is in fluenced by the weight ratio of the conductive additive to the binder(Table 1).Although a durable adhesive strength can be achieved with excessive binder,the resistance of the ion transport would be increased.For the conventional PVDF binder,a high percentage(～10 wt%)is generally required to fabricate sulfur cathodes,much higher than that of LIB(1.5–5.0 wt%)[52],thereby,neutralizing the advantage of the energy density and giving rise to interfacial resistance.Through optimizing the adhesive strength,a slight or much lower amount of binders can be achieved[10,53,54].Second,the external environment(e.g.,organic electrolyte solvent)has a complicated impact on the strength of the adhesion.The strength of the interfacial interaction between the binder and the substrate is decreased considerably by both the surface accumulated solvation molecular layers and the swelling stress induced by the solvent[55].As a result,the same binders usually exhibit different adhesive performance under different solvents.Establishing strong adhesion under an organic electrolyte for a long-timescale is still a great challenge for the Li–S battery binder.Third,the wettability of the slurries to the corresponding current collector should be considered,especially for water-based slurries.In fluenced by the relatively large surface tension(water:72.8 mNm-1,NMP:40.8 mNm-1,at 20°C),water-based slurries are difficult to spread over the current collector(e.g.,Al foil)or penetrate the porous structure(e.g.,Ni foam),especially for a hydrophobic substrate,to form a robust interface[54,56].It is much easier for the electrode to be pulverized or to detach,causing severe decay in the specific capacity.Accordingly,it is necessary to search for a strong adhesive binder to address these issues.

Table 1 Different binders and their corresponding adhesive strength with various components and testing conditions.

3.2.Appropriate mechanical behaviors

In order to tolerate the reduplicative volume expansion,binders should have certain specific mechanical behaviors,e.g.,a relatively low glass transition temperature(Tg),a suitable degree of entanglement(chemical/physical cross-linking),and a self-healing property.The mechanical behaviors of binders have complex effects on battery performance and the search for an appropriate binder should be approached logically.First,a binder with a lowTg(i.e.,little tendency to crystallize,elastomeric)is a prerequisite to establish the stable cathode structure[55,62].Semi-crystalline polymers,e.g.,PVDF,carboxymethyl cellulose(CMC),and polyamides(PA),usually exhibit an elongation yield of only about 20%,which is much lower than the volume change(～80%)of the sulfur cathode during the charging/discharging[63].As a result,the cathode's integrity cannot be maintained.The repeated dissolution and re-deposition of the active materials in the destruction of the sulfur cathode's porous structure leads to the rearrangement of sulfur within the cathode,which tends to passivate the cathode and increase the impedance due to the uneven distribution of the insulating sulfur[14,23].The capacity of the second voltage plateau,which is associated with the formation of insulating solid species,is especially sensitive to the morphology and homogeneity of the cathode[64].In addition,as suggested by Xiong et al.the high crystallinity of binders adversely impacts the dispersion of the active materials(Fig.5)[65].In their research,sulfur particles were uniformly dispersed in the amorphous regions,and the cathode with the designed binder exhibited improved performance compared to that of PVDF.Second,a suitable degree of entanglement can provide additional energy dissipation networks to buffer the volume expansion of the electrode during the charging/discharging processes[53,66].Given this,three-dimensional networks are expected to displace traditional linear polymers.As suggested by Zhao[67]and Walu et al.[23],a binder with a high degree of chain entanglement,especially for water-dissolved binders,can easily form a three-dimensional porous structure after solvent evaporation.The obtained structure is able to tolerate larger volume changes of the electrode,favors electrolyte infiltration,and has electrochemically available interfaces and abundant ion pathways.In addition to the typical chemical cross-linking method[68–70],entanglements based on dynamic electrostatic interactions[70,71]and intermolecular H-bond interactions[72,73]have been employed in recent years.The resultant binders provided not only robust networks but also high ion-conductivity for the ordinary cathode,leading to excellent Li–S battery performance.Third,the self-healing property is also desireable for future binders.During charging/discharging processes in the Li–S battery,the volume expansion threatens the integrity and conductive network of the cathode,but introducing a self-healing feature to the binder system can reform these pathways and recover battery performance.Jen et al.developed a supramolecular polymer binder with self-healing characteristics and the cut-to-heal process could be clearly observed,as seen in the optical images in Fig.6[74].Together with good stretchability,the Li–S cathode could maintain its physical integrity.

In addition to the above characteristics,the swelling property and molecular weight of binders should be taken into consideration.A low degree of swelling is necessary for binders to prevent the porous structure of the electrode from collapsing during cycling.A higher degree of swelling tends to minimize the interactions among the binder,active material,and current collector[51,75,76].An obvious increase in charge transfer resistance is also expected because of the loss of electrical contact.Early research suggested that the swelling of binders in the electrolyte,such as PVDF,would block the pores of conductive carbons,negatively impacting the surface area available for the deposition of Li2S,followed by capacity fading[77].The molecular weight of the binder polymer also plays an important role in the performance of the battery,stemming from the relatively poor mechanical properties[78].

Fig.5.Illustration of the in f luence of binder crystallinity during the charge–discharge process in the Li–S battery.Reproduced with permission[65].Copyright 2017,Wiley-VCH.

3.3.LiPS anchoring

To increase the utilization of active materials and enhance the performance of the battery,multiple approaches have been proposed to inhibit the“shuttle effect”,e.g.,functional sulfur host[79–83],microporous carbon matrix[84],concentrated electrolyte[85],and separators[86].However,most of the above solutions sacrifice the energy density of the battery and usually have an ultra-high inherent cost,which is impractical for large-scale applications.In the early stage,numerous carbon materials were utilized to physically confine LiPS in the pores.However,as suggested by Nazar et al.it is effective only for short-and medium-term cycling(e.g.,a few hundred cycles)[27].Thus,seeking suitable alternatives that confine the LiPS within the sulfur cathode is highly desireable.In 2013,Cui et al.based onab initiosimulations,first demonstrated a feasible approach to suppressing the shuttle effect of LiPS through the use of a bi-functional binder,poly(vinylpyrrolidone)(PVP),which possesses strong Li–O interactions with both Li2S and Li–S[87].The corresponding results and the electrochemical performance of the Li–S batteries are illustrated in Fig.7,revealing that organic binder polymers with heteroatom-containing groups(e.g.,esters,ketones,and amides)have much stronger affinities with the Li-ions of LiPS through polar-polar interactions than PVDF.The battery using the Li2S cathode achieved a high initial discharge capacity of 760 mAh g-1at 0.2C(1090 mAh g-1of S),with a capacity retention of 94%even after 100 cycles.Even after prolonged cycling over 500 cycles at 0.2C,the electrode with PVP retained 69%of their initial capacity.Based on thesefindings,Lacey et al.reported a water-soluble binder polymer composite of PVP and PEO(Table 2)[88].In addition to the intensive interactions between PVP and soluble sulfur species,the combination of PEO synergistically reduced the transfer resistance of the Li ions due to the improved environment of the electrode/electrolyte interface[38].Similarly,Li et al.reported a binder polymer composite of PVP,Li-Nafion,and nano-silica,which further provides fast Li transport paths and significantly increases the performance rate[89].In addition to polar-polar interaction between Li+and LiPS,other interactions(e.g.,electrostatic interactions)are also effective(Fig.8).

Table 2(continued)

Table 2 Comparison of various binders for Li–S batteries in recent years.

Fig.6.The self-healing process of the PP-Py binder.Reproduced with permission[74].Copyright 2019,Royal Society of Chemistry.

Fig.7.(a)Various binding energies between Li2S/Li–S species and various functional groups.Specific capacity of Li2S cathodes using PVP binder:(b)cycled over 200 cycles at 0.2C;(c)cycled at various C-rates from 0.2C to 2C;(d)cycled 500 cycles at 0.2C.Reproduced with permission[87].Copyright 2013,Royal Society of Chemistry.

In recent years,researchers have explored various kinds of polymers with LiPS anchors,e.g.,PAA[58,90],polyamide-6(PA6)[65],PEO[50],PVA[91],Poly(ethylenimine)(PEI)[22,92],CMC[93],Alginate derivatives[94],Gum Arabic[16,95],Cellulose derivatives[46],PES[8],and L-Cysteine-Modified Acacia Gum(L-AG)[96].It should be noted that most of the abovementioned binders are linear electroneutral polymers.Considering the reduplicative volume expansion of the sulfur cathode,a three-dimensional(3D)network or multi-side-chain structure binders with LiPS affinity are highly desireable.In 2017,Yan et al.developed a polymer binder for the Li–S battery with a 3D network and flexible structure through direct copolymerization of PEI with hexamethylene diisocyanate(HDI)[97].As confirmed by theoretical simulation and in situ ultraviolet–visible(UV–Vis)characterization,such a hyperbranched polymer with abundant amino functional groups(AFG)exhibited strong adsorption for LiPS,thus mitigating the serious shuttle effect of soluble LiPS and improving the reversibility of the redox reactions.In addition,the robust mechanical property with high elastic modulus,originating from the cross-linking and hyper-branched network structure,alleviated the huge volume changes during cycling.Subsequently,analogous copolymers with 3D cross-linked structure were successively reported,e.g.,PPA[98],AHP[99],N-GG-XG[72],CMC-CA[100],PAM[101],PEI-ER[102],LBSIP[15],and PTH[103].The cathodes with all these binders demonstrated stable cycle performance without exception(Fig.9a–c),which can be ascribed to the relatively strong binding energy with polar LiPS and appropriate mechanical properties.Compared to electroneutral heteroatom groups-containing polymers,cationic/anionic alternatives anchor LiPS more effectively.In 2014,Zeng et al.first reported a water-soluble,hyper-branched and cationic binder polymer(named β-CDp-N+),through introducing a large number of quaternary ammonium cations into the electroneutralβ-cyclodextrin[104].The unique multidimensional hyper-branched cationic network not only provided strong electrostatic adsorption to the LiPS,but it also provided a strong adhesive strength to active materials and conductive additives to current collectors.In 2017,Ling et al.presented a kind of quaternized cationic binder polymer,which was denoted as poly[bis(2-chloroethyl)ether--alt-1,3-bis[3(dimethylamino)propyl]urea](PQ),to trap the LiPS through electrostatic interactions[105].Its binding strength was evaluated by theoretical calculation and experimental characterizations including time-lapse UV–Vis spectroscopy.This cationic binder could confine the LiPS within the cathode through electrostatic interactions.Similarly,Li et al.proposed a polyelectrolyte binder(PEB-1),named poly[(N,N-diallyl-N,N-dimethylammonium) bis(tri fluoromethanesulfonyl)imide],which enabled the stable performance of the Li–S battery[106].Due to the cationic charge characteristics,the anionic polysulfides would be confined by anion exchange reactions between counter anions(TFSI)and the LiPS.In order to understand the effect of cationic binder polymer properties(e.g.,ion concentration,molecular structure)on the performance of the battery,Su et al.investigated two representative cationic binders(D11 and PDAT)for Li–S batteries[107].The two binders-based Li–S batteries,especially the PDAT,delivered a highly improved cycling performance compared with the PVP.Further analyses including LiPS adsorption tests combined with UV–Vis spectra,XPS,and electrochemical quartz crystal microbalance experiments revealed that the PDAT had a much stronger LiPS affinity and superior sulfur utilization compared to D11 and PVP,possibly due to its higher concentration of ions,yet,both D11 and PDAT binders demonstrate a clear advantage over PVP in terms of capacity and cycling stability.In addition to cationic structural features,counter anions play important roles in LiPS anchoring.In a series of quaternary ammonium cation-based binders with various anions,the binder containing TFSI anion was found to hold the best polysulfide adsorption compared to other anions including BF4-,PF6-,andCl-[108].However,the binding strength between the cationic backbones and the anionic polysulfide species would be sharply minimized under the in fluence of the swelling tendency.In order to solve this problem,two highly cross-linked polymer binders based on ammonium chloride functional groups were developed by Kwok et al.[75]possessing a low degree of swelling and higher chemical stability,to maximize the acid-base interactions between the ammonium groups and polysulfides,ensuring stable cycling for high-sulfur loading cathodes.It is well known that quaternary ammonium cations,holding a positive charge,could capture soluble LiPS intermediates and inhibit the“shuttle”effect.The corresponding cathodes usually deliver the lowest capacity decay and lowest polarization while maintaining the best cycling stability,as illustrated in Fig.9 d-e.

Although polysulfide adsorption could improve the utilization of the active materials to a certain degree,the conversion process from Li2S2to Li2S(418 mAh g-1theoretical capacity)with sluggish reaction kinetics,which is regarded as a key point to improve the specific capacity,is still rarely mentioned[61,101].In this regard,several catalytic binders have been explored to prompt LiPS conversion.In 2016,Helms et al.reported redox-active supramolecular polymer binders for the Li–S battery(Fig.10a)[111].The redox-active core of these binders aided in charge transfer and reduced the impedance upon activation,enabling the conversion from LiPS to Li2S and improving the utilization of the active materials(Fig.10b)[112].Similarly,Chen et al.reported a stable free radical polymer,named poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate)(PTMA),as an active binder in the Li–S battery(Fig.10c)[113].The activated binder showed strong LiPS absorption ability,and provided additional active site-PTMA+to improve the kinetics of the cathode reaction.In addition to electroactive binders,other functional binders are showing the potential to facilitate LiPS conversion processes[114].Recently,Zuo et al.designed a novel self-healing poly(dimethylsiloxane)polymer with a dynamic disulfide bond,which could reversibly break and form,facilitating the conversion of polysulfides[115].Electrochemical tests,XPS,and DFT calculations confirmed that the reversible breakage/formation of the S–S bonds of binders could capture LiPS intermediates and accelerate their conversion.

With the in-depth study of polysulfide adsorption,some reports indicated that binders are beneficial for inhibiting the agglomeration of Li2S.Dominko et al.used poly(ionic liquid)s(PILs)binder as an active agent to prompt uniform Li2S precipitation[116].A smooth and flat morphology of the cathode surface,which established a continuous conductive structure for the electrodeposition of solid sulfur and Li2S in the solid-liquid-solid conversion,could be observed for the PILs-based Li–S battery(Fig.11).

3.4.High Li-ion conductivity or electron conductivity

Owing to the low ionic/electronic conductivity of S and insoluble Li2S2/Li2S,it is difficult for Li+and electrons to be transferred and participate in the oxidation/reduction reactions during charging/discharging,leading to strong polarization,low utilization efficiencies,and irreversible energy losses[62].Meanwhile,Li+conductivity rather than the electronic conductivity becomes the rate-limiting step of the electrochemical performance in terms of the charge/discharge processes,especially at high rates[89,117,118].To achieve a higher theoretical energy density of the commercialized lithium-ion battery,the sulfur loading in the cathode should not be less than 6 mg cm-2.However,such a high sulfur loading increases the thickness of the electrode because of the low density of sulfur,resulting in deteriorated capacity[119].Binders with intrinsic ionic conductivity,which could facilitate thekinetics of battery reactions and improve the performance rate at high rates are proposed to decrease the internal resistance.Zhang et al.synthesized a binder with abundant negative charged sulfonate coordination sites,which can provide the cathode with high lithium-ion conductivity and fast lithium-ion diffusion[53].Zhong et al.used soy protein to design a good ion-conductive binder(Fig.12)[58]and fabricated an Li–S battery with high sulfur loading which exhibited excellent rate performance. Deprotonated carboxyl and polyphosphates were also demonstrated to be effective lithium-ion conductive functional groups,in comparison with PVDF.Such anionic binders have potential applications in the Li–S battery(Fig.10a).

Proper swelling properties and better electrolyte wetting abilities are also desireable to promote the transportation of Li+,especially for Li+non-conductive binders(PVDF,CMC,Alginate,etc.)[120,121].With an enhanced electrolyte uptake at the electrode,faster electrochemical reactions and ion diffusions can be enabled,which can reduce the voltage polarization generated by the mass transfer impedance.Deng et al.compared the electrolyte wetting ability of different binderfilms and found that poor electrolyte wetting ability was associated with an inferior performance rate[47].Some factors,such as the property of the binder and the solvent in which the binder is dissolved,interfere with electrolyte wetting ability[63,122].The high crystallinity binder PVDF would induce large cracks after oven-drying,and these large cracks that lead to a rough surface are beneficial for electrolyte wetting[78].For water-soluble binders,a high evaporation rate would lead to the formation of voids and pores upon drying,which also has a beneficial impact on electrolyte wetting ability[67].

Fig.8.Comparison of LiPS anchoring mechanism between various binders and LiPS(Li2S x,2≤x≤8).Reproduced with permission[109],Copyright 2018,American Chemical Society.Reproduced with permission[110],Copyright 2019,American Chemical Society.Reproduced with permission[15],Copyright 2019,American Chemical Society.Reproduced with permission[106],Copyright 2017,Macmillan Publishers Limited.Reproduced with permission[108],Copyright 2018,Elsevier.Reproduced with permission[75],Copyright 2019,American Chemical Society.Reproduced with permission[107],Copyright 2017,American Chemical Society.Reproduced with permission[53],Copyright 2019,Wiley-VCH.Reproduced with permission[92],Copyright 2018,American Chemical Society.Reproduced with permission[98],Copyright 2017,Wiley-VCH.

The electronic conductivity of the binder strongly affects the overall performance of the battery.A large amount of highly conductive carbon additive(e.g.carbon black,super-P,MWCNTs)is essential in traditional cathode systems,which results in lower energy density[43].However,multifunctional binders based on conductive polymers are currently being studied because they can integrate the advantages of organic conductors and conventional adhesive components,thus avoiding or decreasing the use of conductive additives and obtaining a higher energy density.Multifunctional electronic conductive binders have been designed in which different groups are introduced into the polymer backbone to enable functionalities(e.g.electronic structure,mechanical behaviors,and electrolyte uptake)[123].Traditional conductive polymers(e.g.polyaniline(PANI),polypyrrole(PPY),polyacetylene,and their derivatives)commonly hold a polymer backbone with extended conjugatedπbonds[124].They can be chemically doped to convert them from insulators to highly conducting materials using appropriate dopants while still maintaining flexibility.In addition,the heteroatoms(O,N,and S)are doped into conductive polymers(N+for PANI and PPY,S+for PEDOT)as their lone electron pairs can bind with Li ions in LiPS.Li et al.[73]systemically studied the binding energies between Li2S and Li2Snspecies(4≤x≤8)in a series of heteroatom-doped conductive polymers involving PEDOT,PPY,and PANI,as shown in Fig.13[125].The weak interactions of PPYor PANI with the Li2S and Li–S·species are ascribed to the fact that only separatedπ–σforce exists between the heteroatoms and Li ions of Li2S and Li2Sn.Recently,it has been acknowledged that conductive polymers can entrap LiPS through electrostatic interactions between the positively charged cations on the backbones and polysulfide anions[126,127].Gao et al.prepared S-wrapped PANI nanofibers(PANI NFs)without any thermal treatment for the Li–S battery[128].In this composite,the inner conductive PANI NFs with a positive backbone(z=+45.17 mV)not only offer an electron transport pathway but also bind with negatively charged(2≤x≤8)through electrostatic interactions,other than the limited coordination interactions.A stable architecture was formed and retained,leading to the excellent cycling stability of the Li–S battery.

3.5.Dispersion of active materials

Fig.9.Compared cathodes with different(a)anionic,(b)cationic,(c)electroneutral polymer binders,(d)organic-dissolved binder,and(e)water-soluble binder in recent researches.

Fig.10.Schematic illustration of the redox processes of cathodes with activated binders.(a)Overview of the self-assembly process of PBI into supramolecular polymers.Reproduced with permission[111].Copyright 2016,American Chemical Society.(b)Energy levels from sulfur and different polyimides.Reproduced with permission[111].Copyright 2017,Elsevier.(c)[-assisted polysul f ide redox through the conductive matrix.Reproduced with permission[113].Copyright 2017,Elsevier.

A well-dispersed slurry with a uniform distribution of insulating sulfur particles and conductive additives is indispensable to fabricate the Li–S cathode with an efficient conductive network containing minimized isolated bulks of sulfur dead-sites[43,129].However,because of the particle aggregation in the traditional electrode fabrication processes,physical mixing is often insufficient to create uniform dispersions of the active materials[49].To address this issue,it is necessary to establish a fundamental understanding of the drying process.A three-dimensional network is built in the electrode slurries since segments of polymer chains and particles entangle each other.After the solvent evaporates(Fig.14a),the dried composite electrode maintains the initial morphology in the wet state and the particles are completely shrunk altogether by the polymer chains[67,93,130–132].Based on this,the slurry with higher stability is able to maintain a well-dispersed morphology inherited from the wet state,while for water-based slurries,particle aggregation is more easily found between conductive carbon additives because of their hydrophobic surface.To achieve the homogeneous dispersion of all components in water-based slurries,surfactants or dispersing agents are introduced to enable the dispersion of particles and prevent them from agglomerating in two ways:(1)Surfactants could significantly reduce the contact angle between the particle surface and the slurry solvent and the resistance between the internal force of the agglomerate and the particle-solvent force,thus facilitating the dispersion of the agglomerate[133].(2)Surface adsorption changes the attractive force between the clusters and thus the complex particle reorganization process.This reorganization process in the slurry has great in fluences on thefinal agglomerate size,distribution of active substances and conductive agents,the uniformity of the slurry,the morphology of the clusters,and so forth[122,134].Several binders(e.g.,SBR-CMC,LA133)are able to act as dispersing agents to interact with the surface of the active materials/conductive agents and prevent the merging and agglomeration of the adjacent particles through steric hindrance or electrostatic action[93,133].They could reduce the contact angle and facilitate the wetting process on the particle surface.Due to the capillary effect of the particle/adhesive paste,the binder solution with a low contact angle helps to form a conductive network during the drying process.These pathways increase the conductivity and improve the performance of the electrode.For example,when using water-soluble SBR-CMC as a Li–S battery binder,the surface of the conductive carbon additives tends to be anchored by the carboxylate groups of CMC,giving rise to a negative surface charge to stabilize the dispersion through an electrostatic double-layer repulsion mechanism.Thefinal electrode shows a more even and smooth surface,accompanied by an improved cycling performance[93].The SBR-CMC binder is not only a high adhesion agent but also a dispersion medium,which favors the uniform distribution between insulating sulfur particles and conductive carbon black,and ensures a good electrical contact,leading to higher sulfur utilization.As demonstrated by Wang et al.a uniformly dispersed slurry is able to prevent the cathode structure from being destroyed[133].Otherwise,the solid Li2S2and Li2S layers on the surface of the cathode would sharply increase the charge-transfer resistance and hamper the diffusion of Li+within the cathode,resulting in severe polarization.Even worse,most of the middle active substance Li2Sn(2＜n≤8)would be rapidly converted into Li2S,which is not soluble in the electrolyte,especially at high current densities.It is known that the accumulation of the insulating Li2S2and Li2S would prevent the diffusion of the Li+and restrict the electrochemical reaction inside the sulfur particles[133,135–137].However,this phenomenon could be greatly relieved by using a binder which is beneficial for the homogeneous dispersion of active material.The resultant cathode tends to cause much smaller voids following the dissolution of the sulfur.Eventually,much lower internal resistance,a faster Li+diffusion rate,and higher reversible capacity are acquired.It is crucial to control the morphology of the sulfur cathode to attain a stable structure together with a uniform carbon host.

Fig.11.PILs binder facilitates Li2S precipitation during the charging process.Reproduced with permission[116].Copyright 2018,American Chemical Society.

Fig.12.An illustration of the binder with a robust network,strong LiPS anchoring and ion conductivity by 3D cross-linked.Reproduced with permission[58].Copyright 2019,Royal Society of Chemistry.

Fig.13.The most stable configurations and corresponding binding energies of(b)Li2S and(c)Li–S·species with the heteroatoms(O,S,or N atoms)in different kinds of conductive polymers(PEDOT,PPY,and PANI).Reproduced with permission[125].Copyright 2013,American Chemical Society.

In many cases,the application of a surfactant/dispersant improves the dispersion and uniformity of the active material/conductive agent slurry.However,excess surfactant may cause many problems.First,the surfactant still exists on the surface of the active material/conductive agent after the drying process,so it may damage the conductivity of the electrode.Using a volatile surfactant(e.g.,ethanol,isopropanol)that will disappear during the electrode drying process is a possible solution[75,138].The other problem is that several anionic/cationic surfactants/-dispersants used in water-based slurries have the potential to cause current collectors to corrode(e.g.,Al foil),which is neglected in most previous work[14].

The rheological properties of the slurry are also critical for the conventional casting process,especially for industrial requirements.However,only a few reports have focused on this point.Increased slurry viscosity at low shear rates can slow down sedimentation during the drying process,which avoids spontaneous aggregation and results in a homogenous cathode[13,139].Normally,the presence of binder polymers is able to increase the viscosity of the electrode slurry,which ensures high-quality(i.e.,uniform and smooth)coating onto the current collector.In the past few years,some methods have been demonstrated to obtain a satisfactory slurry coating[140–143].For example,the deprotonation or pre-lithiation of PAA binder(PAA-X,X=NH4+,Li+,Na+,etc.)is beneficial for the rheological properties and the lamination processes of the slurry.At low shear rates,the viscosities of PAA-X slurries sharply increase with the increasing depronation degree of the PAA.As indicated by Hu et al.this can be attributed to the reduced mobility of the particles[138].The deprotonation or pre-lithiation process of the PAA binder usually sacrifices the adhesive strength,due to the decreased content of the carboxyl groups.

In addition,the molecular weight of the binder polymer plays an important role in the rheological properties of slurries.If the molecular weight of the binder polymer is too low,the obtained slurry cannot be used for the normal coating process,due to its low viscosity,as suggested by Jeon et al.[78].Even worse,the resultant electrode is not able to completely endure large changes of the active materials,which is associated with its low mechanical strength,because of its low molecular weight.To circumvent this problem,other additives such as thickening agents and surfactants were deliberately added into the composite solution,especially for aqueous slurries.

3.6.Electrochemical stability and fl ame retardant

Qualified binders must be chemically and electrochemically stable while the battery is working.One of the reasons PVDF has been intensively used as a binder for the LIB battery is its electrochemical and chemical stability[2,29].Typically,the electrochemical window of the binder is determined by cyclic voltammetry experiments or calculated by the gap between the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO)[63].However,different from the real composite electrodes,most tests are performed between two stainless-steel electrodes,usually leading to an overestimation.As for the second characterization analysis method,the energy levels of these orbitals do not exactly equate to the redox potentials of the material,and therefore,it should be regarded as a rough guideline.

The flame retardant property is also a vital safety parameter for binders,especially in industrial production.By introducing the inorganic ammonium polyphosphate(APP)group,Cui et al.developed a novel flame-retardant binder[109],which not only improves the polysulfide adsorption capability,but which also endows the sulfur electrode with flame-retardant properties to improve the safety of the Li–S batteries.The S-APP electrode showed a highly reversible average capacity of 1035 mAh g-1at 0.2C,and 520 mAh g-1at 4C,indicating fast reaction kinetics.These capacities are much higher than those for the PVDF-S tested under the same conditions.The cycling stability of different sulfur electrodes was tested at 0.5C.The S-APP electrode exhibited an initial reversible capacity of 753 mAh g-1,and remained at 640 mAh g-1after 400 cycles with stable coulombic efficiency,and a capacity retention of 85.0%(Fig.15).

Fig.14.(a)SEM and TEM images of the electrodes:(I,II)NVPF-CMC;(III,Ⅳ)NVPF-PVDF.(Ⅴ,Ⅵ)illustration of the electrode during drying.Reproduced with permission[67].Copyright 2018,Wiley-VCH.(b)Comparison of the distribution of the sulfur/conductive additives(CB)when using SBR-CMC(top)and PVDF(bottom)as binders of the Li–S battery.Reproduced with permission[93].Copyright 2011,American Chemical Society.(c)Zeta potential and dispersion morphology of optical microscope images of different binders solutions.Reproduced with permission[134].Copyright 2016,American Chemical Society.

3.7.Eco-friendly and low cost

In order to reduce the energy industry's impact on the environment,the binder materials should be abundant,eco-friendly,and easily available,thus making the Li–S battery“greener”.In recent years,many organic solvents,such as NMP,DMF,and DMAc,have been regulated by EU's(European Union's)REACH and are rapidly being substituted in many industries[13].Even so,taking organic solvents as the dispersion medium is still a trend,as shown in Figs.10d and 9e,since the particle aggregation is suppressed differing from water-based slurries,as mentioned above.Suitable binder alternatives that can be dissolved in water or less environmentally hazardous solvents are highly desireable in recent years.Water-soluble polymers,especially the low-cost and eco-friendly biopolymers,generally have abundant polar functional groups,which can enhance the adhesive strength and inhibit the diffusion and migration of the LiPS from the sulfur cathode[78,121].Zhang et al.utilized water-soluble Gum Arabic(GA)as a high-performance binder for the Li–S battery,which eliminated the use of organic solvent[95].In fluenced by the functional groups,high binding strength and inhibited shuttle effect were achieved.Eventually,a cycling performance of 841 mAh g-1(0.2C,75 wt%S)was achieved even after 500 cycles.

4.Conclusions and perspective

Although a small amount of binder is added in the sulfur cathode,it can in fluence the performance of the battery in many aspects.In this review,we have discussed the properties required of the ideal binder for the advanced Li–S battery,including strong adhesion to overcome volume expansion,LiPS anchoring to increase the utilization of active materials and coulombic efficiency,and good Li+conductivity to enhance the performance rate.Binders should also have appropriate mechanical behavior,good dispersion,and electrochemical/thermal stability.

Even though great achievements have been made in binders towards high areal loading,stable cycling performance,and high performance rate in recent years,there are still several challenges that need to be overcome for the rational design of high-performance binders in the future,including the following points.(a)To overcome fast decay of the battery performance,highly adhesive and more efficient 3D-crosslinked binders are desirable to benefit the connection between active material,conductive agent,and current collector,and enhance the integrity of the electrode.(b)Further in-depth investigation to establish the binder structure-electrode correlation is beneficial to design the task-specific binder for the special cathodic host.For example,binders with high elastic modulus are required for cathodes possessing fewer pores to buffer large volume expansion.(c)Green binders whose synthesis and fabricating process involves no or less volatile organic solvents are promising to alleviate environmental pollution.Therefore,water-soluble binders or natural polymer-based binders as well as their derivatives may attract more attention in the future.(d)Introducing multiple functions into a single binder is expected to produce an“all-in-one”binder for the Li–S battery.However,such multi-functional binders usually require complicated synthesis and high cost.There may also be a tradeoff between different functions when cross-linked architecture,LiPS anchoring sites,and conductive segments meet.

Fig.15.(a)Self-discharge behavior of Li–S batteries using APP and PVDF binders.(b–c)Charge/discharge voltage profiles and(d)potential polarization of S-APP and S-PVDF at different current densities.(e)Long-term cycling performance and coulombic efficiency at 0.5C for 400 cycles.(f)Schematic of the flame-retardant mechanism.(g)The specific burning time test of sulfur electrodes with different binders.Reproduced with permission[109].Copyright 2018,American Chemical Society.

Declaration of competing interest

None.Acknowledgments

This work was supported by the National Natural Science Foundation of China(Grant No.52072118,51772089 and 21872046),the Youth 1000 Talent Program of China,the Outstanding Youth Scientist Foundation of Hunan Province(Grant No.2018JJ1009),the Natural Science Foundation of Hunan Province(Grant No.2020JJ4174),the Provincial Science and Technology Innovation Platform and Talent Plan-Changsha,Zhuzhou and Xiangtan High-level Talents Accumulation Project(Grant No.2017XK2023),the Research and Development Plan of Key Areas in Hunan Province(Grant No.2019GK2235),and the Key Research and Development Program of Ningxia(2020BDE03007).