Multifunctional binder designs for lithium-sulfur batteries

Multifunctional binder designs for lithium-sulfur batteries

Accepted Manuscript Multifunctional binder designs for lithium-sulfur batteries Qi Qi , Xiaohui Lv , Wei Lv , Quan-Hong Yang PII: DOI: Reference: S2...

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Accepted Manuscript

Multifunctional binder designs for lithium-sulfur batteries Qi Qi , Xiaohui Lv , Wei Lv , Quan-Hong Yang PII: DOI: Reference:

S2095-4956(19)30005-1 https://doi.org/10.1016/j.jechem.2019.02.001 JECHEM 771

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

2 January 2019 30 January 2019 1 February 2019

Please cite this article as: Qi Qi , Xiaohui Lv , Wei Lv , Quan-Hong Yang , Multifunctional binder designs for lithium-sulfur batteries, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.02.001

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Review Multifunctional binder designs for lithium-sulfur batteries Qi Qia, Xiaohui Lva, Wei Lva,*, Quan-Hong Yangb a

Shenzhen Geim Graphene Center, Engineering Laboratory for Functionalized Carbon Materials,

b

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Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, Guangdong, China Nanoyang Group, State Key Laboratory of Chemical Engineering, School of Chemical Engineering

and Technology, Tianjin University, Tianjin 300072, China

*Corresponding author. E-mail address: [email protected] (W. Lv).

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Keywords: Polymer binders; Natural polymers; Artificial polymers; Combination; Modification

Abstract

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Lithium-sulfur (Li-S) batteries are promising next-generation high energy density batteries but their practical application is hinder by several key problems, such as the intermediate polysulfide shuttling

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and the electrode degradation caused by the sulfur volume changes. Binder acts as one of the most

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essential components to build the electrodes of Li-S batteries, playing vital roles in improving the performance and maintaining the integrity of the cathode structure during cycling, especially those

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with high sulfur loadings. To date, tremendous efforts have been devoted to improving the properties

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of binders, in terms of the viscosity, elasticity, stability, toughness and conductivity, by optimizing the composition and structure of polymer binders. Moreover, the binder modification endows them strong polysulfide trapping ability to suppress the shuttling and decreases the swelling to maintain the porous structure of cathode. In this review, we summarize the recent progress on the binders for Li-S batteries and discuss the various routes, including the binder combination use, functionalization, in-situ polymerization and ion cross-linking, etc, to enhance their performance in stabilizing the 1

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cathode, building the high sulfur loading electrode and improving the cyclic stability. At last, the design principles and the problems in the further applications are also highlighted.

Qi Qi received her B.S. degree from Beijing Forestry University in 2017.

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She is currently a Master Degree candidate in Nanoyang Group in the Graduate School at Shenzhen, Tsinghua University. Her research includes the design of novel binders and the catalysts for lithium-sulfur

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batteries.

Wei Lv received his PhD from Tianjin University in 2012 under the

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supervision of Prof. Quan-Hong Yang. He currently works as an Associate Professor in the Graduate School at Shenzhen, Tsinghua

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University. His research mainly focuses on novel carbon materials, such graphene

and

porous

carbons,

and

their

applications

in

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as

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electrochemical energy storage.

Quan-Hong Yang was born in 1972, joined Tianjin University as a full professor of nanomaterials in 2006 and became a chair professor in 2016. His research is related to novel carbon materials, from porous carbons, tubular carbons to sheet-like graphene and their applications in energy storage and environmental protection.

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1. Introduction Lithium sulfur (Li-S) batteries are promising next-generation energy storage devices because of their high theoretical specific capacity and the use of non-toxic and low-cost sulfur in cathodes [1–3]. However, several critical problems limit their practical applications. First, the highly insulating

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nature of sulfur (room temperature conductivity: 5×10-30 S cm-1) leads to its low utilization [4]. Second, the intermediate soluble lithium polysulfides (LiPSs) formed during cycling can pass through the separator to the lithium anode surface, known as “shuttle effect”, and then are reduced

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into insoluble Li2S or Li2S2 causing the loss of active materials [5–7] and the passivation of lithium anode [8–10]. Third, during the charging-discharging process, the conversion between sulfur and Li2S/Li2S2 generates large volume changes which destroy the electrode structure and lead to the fast

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capacity loss [11–13]. To date, the structure design of the sulfur-carbon/noncarbon hybrids [14–20], the cell configuration design (such as the interlayer design, sandwich electrode structure and

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non-encapsulation design) [21–26], the catalytic mechanism study (heterogeneous and homogeneous

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mediators) [27,28], polysulfides trapper design [29] and the lithium anode protection [30–32] have

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greatly improved the performance of Li-S batteries in the laboratory coin cells, in terms of energy density, cycling stability and rate capability. However, with the commercialized cell configuration

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which calls for the high sulfur loading (above 2–3 mAh cm-2) [33], the performance of Li-S battery falls badly with short cycling life which may be ascribed to the instability of thick cathode and the consumption of limited electrolyte [34,35]. It is well known that the electrode structure and its stability during cycling directly affect the performance of different batteries, such as the internal resistance, ion diffusion resistance [36] and cyclic stability [37]. As the linkers of active materials (sulfur) and conductive additives in the 3

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cathode of Li-S batteries [38,39], the polymer binders buffer the volume changes and protect the porous structure of electrode, and thus help maintain the integrity and stability of Li-S cathodes, particularly those with high sulfur loading and large thickness [40,41]. Poly(vinylidene difluoride) (PVDF) is the most widely used binder in Li-S batteries, which has a long polyethylene chain with

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two F atoms replacing two H atoms in every vinyl monomer [42,43]. However, due to its relatively poor adhesion strength, it cannot well maintain the structure integrity of the sulfur-based cathode due to the volume expansion during the sulfur lithiation [44–46]. Besides, as the electrolyte availability is

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critical to the activity and utilization of sulfur and the deposition of LiPSs dissolved in the electrolyte to Li2S/Li2S2 needs large surface area in the cathode, highly porous electrode structure is desired [47]. Unfortunately, Lacey et al. have demonstrated that PVDF restricted the access of electrolyte to the

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carbon surface and reduced pore volume available to the electrolyte, and thus, it is an unsuitable binder for Li-S battery [48].

(CMC),

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carboxymethylcellulose

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Until now, various natural and artificial binders, such as sodium alginate (SA), sodium

carboxymethylcellulose:styrene-butadiene

poly(ethylene rubber

(CMC:SBR),

oxide) poly(acrylic

(PEO), acid)

(PAA),

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poly(vinylpyrrolidone) (PVP) and gelatin have been investigated to improve the battery performance

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[49–53]. Because of their long carbon chains, abundant functional groups and water solubility, which could strengthen the structure of electrodes, adsorb LiPSs, tolerate the volume change during the discharge process and avoid the use of organic solvents, these binders can help construct the high sulfur loading electrode and greatly improve the sulfur utilization, the rate capability and the cycling stability [54,55]. Thus, the design of multifunctional binders shows a promising way to promote the practical use of Li-S batteries. 4

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This review summarizes the recent development on polymer binders in Li-S batteries and discussed their different functions on improving the battery performance (Fig. 1). Both natural and artificial binders possessing satisfactory adhesion properties are discussed. More importantly, how to introduce into the binders other functions beyond the adhesion function is discussed and highlighted,

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with the fundamental principles putting forward for designing novel and high-performance binders

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for Li-S batteries.

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Fig. 1. The binders with different functions in Li-S batteries.

2. Natural polymer binders A binder for the large scale use should have rich sources, low cost and strong adhesion strength, and thus, the bio-derived binders, such as sodium alginate (SA), chitosan and gelatin which have high molecular weight and rich functional groups have attracted great interests in these years [56,57]. Specifically, most of these binders are water-soluble which avoids the use of high cost and toxic 5

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N-methyl pyrrolidone (NMP) solvent and the following high temperature vacuum drying for the electrode fabrication, avoiding the loss of sulfur during the drying step [58]. The depressed swelling of these water-soluble polymers in the organic electrolyte helps maintain the established electrical connection between active particle and enhance the initial discharge capacity and stability of

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electrode. Besides, these natural binders always show outstanding toughness and can tolerate the volume expansion of active materials to maintain the electrode integrity [59]. At the same time, the oxygen-, nitrogen- and sulfur-containing functional groups which always exist in these bio-derived

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binders probably acting as chemical trappers to retard the LiPSs shuttling [60–62]. However, Godoi and co-workers also found that these functional groups promoted the LiNO3 decomposition, which decreased the reversibility of the sulfur cathode [63].

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Early in 2008, Huang et al. showed that the gelatin not only functioned as a highly adhesive agent, but also acted as an effective dispersion agent for the cathode materials enhancing the redox

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reactions of sulfur cathode [64]. A following research demonstrated that the water-soluble gelatin

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increased the wetting ability of electrode by the electrolyte because of the rich -COOH and -NH3 groups which also chemically bonded with LiPSs to suppress their shuttling, consequently improving

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the performance of Li-S batteries [65]. Another water-soluble natural binder, SA (Fig. 2a) which also

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contains rich carboxyl and hydroxyl groups was also found to enhance the reaction kinetics and lower the electrode resistance than PVDF as it resulted in much more pores in the electrode, favoring the adsorption of electrolyte and decreasing the ion resistance (Fig. 2b) [56]. The use of these polymers can also help build the high sulfur loading electrode with enhanced performance (Fig. 2c). Kim et al. built a high sulfur-loading electrode (10 mg cm-2) directly using elemental sulfur with the assistance of chitosan binder, which achieved the cycle retention of 91% at 50 cycles and a stable 6

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efficiency of 98% (Fig. 2f) [66]. As UV-vis spectra shows, chitosan (Fig. 2d and e), which contains abundant -OH and -NH2 functional groups, had the similar adsorption ability to the N-doped carbons towards LiPSs. For the electrode fabrication, electrodes using chitosan binder showed better dispersion morphology than those using PVDF binder, leading to the homogeneous distribution of

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sulfur with conducting agent in chitosan. Considering the strong adsorption ability of amide groups towards LiPSs, amino acid was also used as binder in Li-S batteries. Huang and co-workers demonstrated the γ-polyglutamic acid (PGA) as functional binder enhanced the cycling stability

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because of its abundant amide and carboxyl groups [67].

Fig. 2. (a) The molecular structure of SA; (b) Electrochemical impendence spectroscopic (EIS) of electrodes with PVDF and SA binders; (c) Discharge/charge curves of SA cathode and PVDF 7

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cathode at the 50th cycle [56]. Copyright (2013) Elsevier. (d) The structure of chitosan; (e) UV-vis spectra and photographs of lithium-polysulfide electrolyte with deionized water, PVDF and chitosan; (f) The cycling performance at 1/3 C [66]. Copyright (2016) American Chemical Society. Compared with above mentioned binders with linear carbon chains, the natural polymers

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containing grafted chains, such as Gum Arabic (GA) (Fig. 3a) and Guar Gum (GG), possess better flexibility and mechanical strength as well as more oxygen and nitrogen-containing functional groups, thus can more effectively buffer the volume change of sulfur and help trap LiPSs within the

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cathodes [68,69]. For instance, Li and co-workers reported that the electrode using GA as the binder had much better electrochemical performance than those using PVDF or gelation binder, showing superior electrochemical performance of 1090 mAh g-1 at 0.2 C after 50 cycles (Fig. 3b) [68]. As to

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GG with the similar molecular structure to GA that can transfer lithium ions, Lu et al. found that GG binder provided better mechanical properties than PVDF and confirmed its ability to immobilize

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sulfur by the shift of O–H stretching to lower wavenumbers in Fourier transform infrared

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spectroscopy (FT-IR) profiles (Fig. 3c) [70]. The triple-functions of GG including roles as a binder, an interface stabilizer and a lithium ion conductor were also revealed, which significantly improved

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the electrochemical performance of [email protected] ([email protected]) composite electrode (Fig. 3d)

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[69]. The 3D cross-linking structure of these polymer binders can also be used to functionalize the active materials, Zhou et al. designed the branched amylopectin wrapped graphene oxide-sulfur (GO-S) structure which successfully confined the sulfur particles and the LiPSs among the GO layers during cycling [71].

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Fig. 3. (a) The molecular structure of GA; (b) The cycling performance of electrode with GA, PVDF

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and gelatin binders at 0.2 C [68]. Copyright (2015) Wiley-VCH. (c) FT-IR spectra of GG, GG-S and GG-Li2S8 composite [70]. Copyright (2016) Elsevier. (d) The rate capacities of [email protected] composites

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with different binders [69]. Copyright (2016) The Royal Society of Chemistry.

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3. Artificial polymer binders

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Although the bio-derived natural binders discussed above can greatly improve the electrochemical performance of the sulfur cathode, their structural formula and molecular weight are difficult to identify and are easily affected by many uncertain factors as they are extracted from biomasses [64]. Considering the consistency and repeatability needed in the large scale uses, artificial binders with determined molecular weights have strong application prospects [72]. Moreover, these binders are easy to be functionalized to introduce additional functions. Various artificial binders, such as CMC, 9

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polyethylene oxide (PEO) [73], styrene butadiene rubber (SBR), LA132 [74,75], LA133 [76], polyacrylic acid (PAA) [77–80], polyethylene dioxythiophene (PEDOT) [81-82], polyaniline (PANI) [83,84], polypyrrole (PPY) [85,86], sulfonated polyether ether ketone (SPEEK) [87], poly(9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic chloride)

(AMAC)

(PFM) [55]

have

[50]

and

been

widely

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poly(acrylamide-co-diallyldimethylammonium

ester)

investigated until now. The most obvious advantage of these binders is their functions, i.e., conductivity, electrochemical activity and the adsorption ability towards LiPSs can be designed and

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tuned, thus helping reach better electrochemical performance [88,89].

CMC is a long chain of cellulose derivatives composed of glucosamine as shown in Fig. 4(a) [90]. In Li-S batteries, the abundant oxygen functional groups endow CMC with strong LiPSs capturing

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ability. Li et al. showed that the CMC binder enhanced the rate performance (Fig. 4b) of the sulfurized polyacrylonitrile (SPAN) cathode as it can preferentially form a continuous network and

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attach to the particles to inhibit the loss of active species during cycling [91]. The cellulose derivative

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called hydroxypropyl cellulose (HPC) was also used as binder in Li-S batteries, which can also provide better adhesion between electrode and Al foil than PVDF [92]. The combination of

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water-soluble CMC and SBR is generally used as the aqueous binder in lithium ion batteries in

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which the CMC is the thickening/setting agent and SBR is the primary binder [93]. It has been shown that the dispersion stability of graphite anode slurry was improved due to the mutual repulsion between CMC and SBR. In Li-S battery, such combination not only provides high adhesion to stabilize the electrode structure but also facilitates the uniform distribution of sulfur with conductive additives to improve the sulfur utilization. It can also suppress the agglomeration of Li2S [94]. Xiao and co-workers demonstrated the water-soluble polyamidoamine (PAMAM) dendrimers with 10

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highly branched, porous macromolecular architectures and abundant functional groups as the binder can help reach much better electrochemical performance than the conventional linear binders, such as CMC and SBR [95]. The high-density functional groups endow PAMAM with strong interfacial interaction between the active materials and good wetting ability by electrolyte, and it also acted as

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chemical trappers for LiPSs. Moreover, the interior pores with the diameter of 2 nm in PAMAM physically trap LiPSs, thus greatly enhancing the cyclic stability. By using such binder, it was easy to obtain electrodes with relatively high sulfur loading up to 4 mg cm-2, and at the same time, 98% of

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the original capacity at 0.2 C was achieved. Another water-soluble polymer with branched structure as Fig. 4(c) shows, polyethylenimine (PEI), which is characterized by the abundant amino groups was also used as the binder considering its strong affinity towards LiPSs. Wu and co-workers demonstrated that the amino groups in PEI formed strong electrostatic interaction with LiPSs using

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in situ UV–vis and operando XAS analysis. A high sulfur loading electrode (8.6 mg cm-2) was easily

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fabricated and the area capacity of 6.4 mAh cm-2 was maintained after 50 cycles (Fig. 4d) [96]. The

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nucleophilic substitution with CH3I to produce the cationic PEI (denoted as MPEII) could further

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enhance the interaction between PEI and LiPSs due to the positively charged N atoms [97].

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Fig. 4. (a) The molecular structure of CMC [90]. Copyright (2018) Wiley-VCH. (b) Rate performance of CMC binder for SPAN cathode [91]. Copyright (2017) The Royal Society of

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Chemistry. (c) The molecular structure of PEI binder; (d) Cycling performance of cathodes with PEI

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and PVDF binders [96]. Copyright (2017) Elsevier. The cationic polymers, such as

poly(diallyldimethylammonium triflate) (PDAT)

and

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poly[(N,N-diallyl-N,N-dimethylammonium)bis(trifluoromethanesulfonyl)imide] (PEB-1) can adsorb

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the negatively charged polysulfides through electrostatic attractions [36,40,98] to prevent polysulfides diffusion from the cathode through cycling. Moreover, Helms and co-workers

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demonstrated that PEB-1 could facilitate the transport of Li ions [36]. But the influences of the ion concentration, molecular structure, molecular weight and chemical properties of polycation binders on the cathode performance are needed to be further investigated in future. The utilization of conductive polymer binders can avoid the increase of the internal resistance of electrodes [99]. However, most of the conductive polymers are electroactive, which decreases its binding ability during electrochemical reactions. Manthiram and co-workers showed conductive and 12

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elastic binder design in Li-S batteries by combining the polypyrrole and polyurethane together (PPyPU) [100]. This nanocomposite was highly conjugated to form an electrical network which could also tolerate the volume change during cycling. As a result, such conductive binder reduced activation overpotential and improved electrode performance compared with PVDF [101]. The

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elasticity of polyurethane allowed the PPyPU binder to build flexible electrode and prevented premature electrode degradation. Frischmann et al. showed a new binder based on supramolecular redox mediators (perylene bisimide (PBI), Fig. 5a) which can offer both self-healing properties to

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maintain the electrode integrity and adaptive charge transport upon activation. Such binder reduced the impedance and enabled the high rate capability [102]. In the following study, they found the lithiated PBI binders in water yielded nanowire web morphologies (Fig. 5b) which increased the interfacial interaction between the active materials and binders [103]. 1.4-fold improvement in sulfur

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utilization at 3.0 C with the sulfur loading of 3 mg cm-2 and much higher discharge capacity after 400

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cycles under 0.5 C were reached.

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Fig. 5. (a) Self-assembly of PBI, structures before and after operando redox activation and structure of electrodes [102]. Copyright (2016) American Chemical Society. (b) Structure of PBI and

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deprotonation to yield water-soluble Li41, operando reduction of Li41 to Li61 and self-assembly of

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lithiated PBI [103]. Copyright (2018) American Chemical Society. Besides above electroactive binders, some other binders are found to have electrocatalytic activity.

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For example, the thiokol binder can transfer long-chain soluble LiPSs to insoluble short-chain ones

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confirmed by UV-vis adsorption spectra, which is benefit to the long cycling stability [104]. However, the mechanism has not been well understood. Polymeric ionic liquids (PILs) were also found to have the ability to improve the reduction of sulfur into Li2S4 and the redistribution of Li2S in the cathode, which enhanced the cyclic stability of the Li-S battery when they were used as the binder [73]. Dominko and co-workers compared the effect of five different PILs binders in Li-S batteries and found that they all performed better than PVDF [98]. The compatibility between the 14

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PIL and the produced sulfides inhibits swelling-induced degradation of the cathode because of the more even volume expansion, and the uptake of LiPSs by PILs also constrains the shuttling of LiPSs.

4. The ways to enhance the binder performance

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4.1 Combination use of different binders As a single polymer binder may not well meet all the requirements to boost the performance of Li-S batteries and build the high sulfur loading electrode, many researchers tried to combine different binders together in the cathode [105–107]. For example, Huang and co-workers designed a gelatin

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and PEI composite (GPC) binder with a simple mixing process [108]. This binder could keep electrode stable as it is insoluble in the commonly used organic electrolyte solvents and the abundant amino groups effectively confine the LiPSs (Fig. 6a). Zhang and co-workers designed a mechanically

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robust network binder by weaving the dual biopolymers, wove guar gum (GG) and xanthan gum

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(XG) together, through intermolecular binding effect that evidenced by the shifted O–H stretching band for the composite N-GG-XG (Fig. 6b). Such network provided strong mechanical capability to

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support up to a super high sulfur loading electrode (19.8 mg cm-2) [109]. They also contain a large

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number of oxygen functional groups which can help adsorb the LiPSs. As a result, the electrode with a high sulfur loading of 11.9 mg cm-2 still showed stable cycling performance, with a discharge

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capacity of 733 mA h g-1 after 60 cycles at 1.6 mA cm-2 (Fig. 6c). In another example, Chen et al. used poly(ethylene glycol) diglycidyl ether (PEGDGE) to link polyethylenimine (e-PEI) forming a hyperbranched binder (denoted PPA) as shown in Fig. 6(d), endowing the excellent mechanical behavior to improve the electrode integrity. It also contained abundant polar functional groups, leading to chemical interaction of S–O bond and improving cycle life and rate capacity [110]. Wang et al. proposed a low binder content electrode (0.5 wt%) by using PVP with positive charge and the 15

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Nafion with negative charges as the binders together. Based on the layer-by-layer cross-linking effect between Nafion and PVP, the excellent binding strength was achieved and the assembled soft package battery showed obviously much lower charge transfer impedance and ohmic resistance

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compared with the battery with 10 wt% PVDF binders [111].

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Fig. 6. (a) The polysulfide adsorption ability of different binders [108]. Copyright (2018) Elsevier. (b)

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Polymer network of N-GG-XG binder; (c) The cycling performance of high sulfur loading [email protected] electrode [109]. Copyright (2017) The Royal Society of Chemistry. (d) The molecular

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structure of a hyperbranched PPA binder [110]. Copyright (2018) Wiley-VCH. 4.2 Functionalization of binders Introducing functional groups is a widely used method for the binder modification which is simple but effective. The hyperbranched network structure is always formed, maintaining the high mechanical strength to buffer the volume changes of sulfur during the electrochemical reactions, and

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at the same time, the introduced groups can endow much stronger affinity towards LiPSs and effectively suppress the shuttling of LiPSs in electrolyte. These two factors ensure the relatively high sulfur utilization and structure stability for the high sulfur loading electrode. Oxygen-containing functional groups, including hydroxyl, carbonyl and carboxyl groups,

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normally exist in most of the water-soluble binders. Increasing these functional groups in the binders cannot only increase the adsorption ability towards LiPSs, but also enhance their solubility in water, which can help construct a more uniform binder network. Huang and co-workers modified gelatin

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with phthalic anhydride to introduce more carboxyl groups, which decreased the charge transfer resistances and polarization of the cathode compared with the pure gelation binder [112]. Cui and co-workers compared the binding energy between Li2S and LiPSs with various functional groups

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such as carbonyl, ester, amide, imine, ether, nitrile, and halogenated groups and found that ester group had the strongest adsorption ability with Li2S and Li-S· species while halogenated groups had

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the weakest interaction with them [113]. Accordingly, PVP was selected as the promising

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bifunctional binder to help forming a uniform Li2S slurry and suppress the loss of LiPSs. The nitrogen functionalization is also widely used because these groups have much higher

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adsorption ability [114]. For example, Zhong et al. introduced the soy protein (SP) into PAA to get

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the novel binder with high mechanical strength and ion conductivity [115]. The polar groups (amine groups) in SP molecules showed strong affinity to polysulfides. Yan et al. synthesized a new binder (AFG) by polymerizing hexamethylene diisocyanate (HDI) with PEI (Fig. 7a) which was featured by the abundant amine groups and hyperbranched network structures [116]. The in-situ UV-vis absorption spectra in Fig. 7(e-f) clearly showed the much stronger binding strength between the AFG and LiPSs compared with PVDF, rendering high capacity retention (91.3%) after 600 cycles at 2C 17

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for the cathode with AFG binder. Besides, this binder had superior mechanical strength (Fig. 7b-c) and there was no swelling and structure changes in the electrolyte, and thus, the high sulfur loading electrode (8.0 mg cm-2) with the high specific capacity of 987 mAh g-1 was fabricated. Liang et al. showed

a

cationic

poly[bis(2-chloroethyl)

ether-alt-1,3-bis[3-(dimethylamino)

propyl]urea]

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quaternized (PQ) binder with high-density quaternary ammonium cations, which also have strong electrostatic absorption ability. They built a high sulfur loading electrode (7.5 mg cm-2) with a stable cycling capacity at around 7.0 mAh cm-2 with such binder [117]. Zeng et al. also introduced a

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quaternary ammonium cation into β-cyclodextrin which can immobilize LiPSs and accommodate the

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volume changes of the sulfur because of its hyperbranched network structure [59].

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Fig. 7. (a) Copolymerization of PEI with HDI; (b) 13C-NMR spectrum of AFG. (c-d) Tensile strength test of AFG; (e-f) In situ UV-vis spectra of electrodes with AFG and PVDF binders during cycling [116]. Copyright (2017) Wiley-VCH. Besides the oxygen functionalization and nitrogen functionalization, the sulfur-based functional groups can also play the same role in the binders [118]. Yan and co-workers cross linked 4,4’-biphenyldisulfonic acid with polypyrrole and connected them onto CMC to form a double-chain 18

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polymer network [119]. The dissolved LiPSs can be effectively confined in the cathode due to the repulsive effect of sulfonate anion groups bound on the polymer backbone, in addition to the good conductivity, enhancing the interfacial polysulfide redox and Li2S deposition. The electrode showed much smaller electrochemical resistance and lower overpotential during discharge process, a high

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area capacity of 9.2 mAh g-1 was delivered even with the high sulfur loading of 9.8 mg cm-2. Moreover, the cross-linked networks formed after the functionalization have superior mechanical strength that can maintain the integrity of electrodes because of their strong interaction between

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molecules. 4.3 Integrated structure designs for binders and active materials

The main function of the binder is to combine the conductive additives and active materials together

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to form an integrated conductive network. However, due to the volume changes of the electrode or the swelling of the binders, such network is easy to be destroyed. In order to solve this problem, Cui

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and co-workers used the polydopamine (PD) as “nano-binder” to glue carbon and sulfur at nanoscale,

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forming a stable double-shelled sulfur cathode (Fig. 8a-c). PD with high viscosity, isotropy and high

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elastic constant (E = 6.748 kNm kg-1) not only prevented the conductive carbons detaching from cathode but also played a key role in accommodating the volume expansion of sulfur during cycling

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except for its hollow structure. Such structure showed excellent cycling performance with 0.014% capacity decay per cycle during 2500 cycles at 0.5 C (Fig. 8d) [120]. Besides the above mentioned approach, the in-situ polymerization of binder with the active

materials and conductive additives together is another efficient way to achieve enhanced interfacial actions within the cathode framework, which is beneficial to improve the electrode stability and build the compact and high sulfur loading cathode [121]. Nazar group prepared the stable and high sulfur 19

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loading cathodes by coupling multifunctional sulfur composites with an in-situ cross-linked polymer binder (Fig. 8e) [122]. The cathode slurry was prepared by mixing N-doped graphene and graphitic C3N4/sulfur composite with CMC, citric acid, CNTs and Super P together, and after the casting and drying, CMC was polymerized by small citric acid (CA) molecules forming a reticular structure by

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the dehydration condensation reaction. Thus, there was elastomeric contact between these components, which assists the interparticle physical binding and electrical connection to build a compact thick electrode with the sulfur loading up to14.9 mg cm-2 (Fig. 8f) and low electrolyte/sulfur

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ratio (3.5:1, μL:mg). It is noted that only with the low binder content (5 wt%), the sulfur content in

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the electrode can reach 65.5 wt%.

Fig. 8. (a) The view of the PD double-shelled cathode before and after discharge process; (b) and (c) The SEM and TEM images of PD electrode; (d) Cycling performance of the PD cathode and other configurations tested under the same conditions [120]. Copyright (2016) Wiley-VCH. (e) The cross-linking of CMC binder with CA as the linker; (f) The cycling stability of the cathodes with sulfur loadings of 10.2 and 14.9 mg cm−2 at 1.0 mA cm−2 [122]. Copyright (2016) Wiley-VCH. 20

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4.4 Binding ability enhancements by ion cross-linking Because of the large volume change of sulfur during cycling, the mechanical properties to maintain a stable electrode structure and keep an integrated conductive network greatly affect the cathode performance in Li-S batteries. Besides the design of hyperbranched network structure and the

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enhancement of the interfacial contact between binders and other materials, to strength the binding ability of the polymer chains is another way to improve the performance of binders. Metal ions enhanced cross-linking of the hydrogel have been widely investigated, and ionic bonding between

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the metal ions and the oxygen functional groups (e.g., carboxylic groups) was found to play dominate role in the hydrogel framework formation and strengthen the mechanical properties [123]. Thus, the functions of metal ions can also be used to enhance the performance of the water-soluble binders with rich functional groups. For example, Lin and co-workers introduced Cu2+ to SA binder

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as ionic cross-linking additive (Fig. 9a) [124]. Every Cu2+ ion was adsorbed by four hydroxyl and

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carboxyl groups through electrostatic adhesion, endowing the robust mechanical properties of the

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binder to build the high sulfur loading electrode. At the same time, such positively charged ions can immobilize the negatively charged polysulfides through chemical bonds to restrain the shuttle effect,

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in addition to the absorption ability contributed by oxygen containing functional groups, which

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greatly enhanced the cycling stability. As a result, the electrodes had enhanced electrochemical performance of 758 mAh g-1 after 250 cycles at 1 C as shown in Fig. 9(b). In addition to Cu2+, Li et al. used Mg2+ ions as the cross linkers to enhance the PSS:PEDOT network by the coordination between Mg2+ and SO3-, which helped form a 3D polymer framework (Fig. 9c) to better tolerate the volume changes during cycling [125]. Due to the metal ions crosslinking, the binders could be highly branched, very elastic and chemical stable because of the formation of 3D hydrogel-like network. 21

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This is benefit to decrease the binder content in the electrode and thus, improving the sulfur loading

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and decreasing the internal resistance (Fig. 9d).

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Fig. 9. (a) Schematic diagram of the Cu2+ cross-linking for the binders; (b) The cycling performance

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of electrode with Cu2+ coordination binder at 1C [124]. Copyright (2018) The Royal Society of Chemistry. (c) Schematic of PEDOT:PSS-Mg2+ binder and the formed electrode structures with

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uncross-linked and cross-linked binders; (d) Electrochemical impendence spectroscopic (EIS) data of

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Chemistry.

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cathodes with PVDF and PEDOT:PSS-Mg2+ binders [125]. Copyright (2018) The Royal Society of

5. Conclusions To date, various polymer binders with superior performance to traditional binders like PVDF have been utilized in Li-S batteries (Table 1). It has been shown that the binder content and the binder types are greatly related to the electrochemical performance. For different cathode materials, the requirement on the binder should be different. For example, in the cathode fabricated by directly 22

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mixing sulfur, conductive additives and binders together, the binders with high electric and ionic conductivities are preferred to enhance the sulfur utilization as the binders are directly coated on the insulative sulfur particles surface. At the same time, the high mechanical strength as well as low swelling ratio is also needed to tolerate the volume change of sulfur and maintain the porous

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structure of electrode for the deposition of insoluble products from electrolyte. Moreover, the binder should also have low viscosity and abundant functional groups to help realize the uniform dispersion of sulfur in the cathode. In contrast, for the cathode with sulfur stored in the porous carbon host, the

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needs on the binder should be similar to the currently-used batteries. But the functional groups are also desired as they can help suppress the shuttling of LiPSs. Considering the low sulfur content in these cathodes, the much better binding ability is preferred to lower the content of binders in the

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cathode, which can help improve the sulfur loading on the electrode level. Thus, the binder modification in different cathodes should be further optimized. However, the clear description on the

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relationship between the binder type and cathode performance cannot be achieved yet, which needs

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more efforts to be done in the future studies.

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Table 1. Typical polymer binders and the resulted electrochemical performance of Li-S batteries.

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Binders

23

Cathode materials

Binder content (%)

Sodium alginate

Carbon black/S

10

Chitosan

Multi-walled carbon nanotube/S

10

Gelatin

Acetylene black/S

8

PGA

Acetylene black/S

7

Electrochemical performance 508 mAh g−1 at 0.2 C, capacity retention of 74.7% after 50 cycles 1050 mAh g−1 at 0.3 C, capacity retention of 91% after 50 cycles 463 mAh g−1 at 0.2 C, capacity retention of 61.0% after 100 cycles 727 mAh g−1 at 0.2 C, capacity retention of 70.2%

Ref.

[56]

[66]

[65]

[67]

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after 100 cycles

20

Guar Gum

Acetylene black/S

15

Amylopectin

GO/S

14

Carbonyl-β-cyclodextrin

SPAN

10

Starch

Super P/S

10

CMC : SBR

Carbon black/S

PEO

Ketjen black/S

PVP

Carbon black/Li2S

5

Active carbon/S

5

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10

10

PEDOT

Acetylene black/S

10

PANI

Super P/S

2

SPEEK

Ketjen black/S

10

PFM

Acetylene black/S

10

AC 24

10

Carbon black/S

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PAA

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LA132

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Carbon black/S

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Gum Arabic

1090 mAh g−1 at 0.2 C, capacity retention of 78.6% after 50 cycles 777 mAh g−1 at 0.2 C, capacity retention of 86.3% after 150 cycles 441 mAh g−1 at 0.3 C, capacity retention of 68% after 175 cycles 1456 mAh g−1 at 0.2 C, capacity retention of 94.4% after 50 cycles 594.3 mAh g−1 at 0.2 C, capacity retention of 94% after 100 cycles 580 mAh g−1 at 0.06 C, capacity retention of 66.7% after 60 cycles 800 mAh g−1 at 0.08 C, capacity retention of 74.7% after 30 cycles 714 mAh g−1 at 0.2 C, capacity retention of 94% after 100 cycles 470 mAh g−1 at 0.5 C, capacity retention of 52.1% after 100 cycles 325 mAh g−1 at 0.2 C, capacity retention of 42.9% after 50 cycles 625 mAh g−1 at 0.1 C, capacity retention of 50% after 100 cycles 375 mAh g−1 at 0.36 C, capacity retention of 51.7% after 100 cycles 270 mAh g−1 at 0.6 C, capacity retention of 54% after 300 cycles 840 mAh g−1 at 0.1 C, capacity retention of 67.2% after 100 cycles

[68]

[70]

[71]

[62]

[58]

[94]

[73]

[113]

[75]

[77]

[50]

[126]

[87]

[50]

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Super C65/S

10

PEI

Super C45/S

10

PBI

CTAB-modified S-GO

10

PDAT

[email protected]

10

Thiokol

Acetylene black/S

16.7

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HPC

520 mAh g−1 at 0.2 C, capacity retention of 49.5% after 100 cycles 500 mAh g−1 at 0.05 C, capacity retention of 48.2% after 70 cycles 430 mAh g−1 at 1 C, capacity retention of 71.7% after 150 cycles 941 mAh g−1 at 0.1 C, capacity retention of 77.8% after 50 cycles 501 mAh g−1 at 0.1 C, capacity retention of 61.1% after 200 cycles

[92]

[97] [102] [127] [40]

[104]

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The several key problems that hinder the practical application of Li-S batteries have not been well solved, including the low sulfur loading, poor cyclic stability induced by the LiPSs shuttling, low Coulombic efficiency and the instability of sulfur cathode and Li anode. According to the above

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discussion, we can find that binder plays the key role to stabilize the cathode structure, improve the

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sulfur loading and sulfur utilization and enhance the cyclic stability although it only occupies less than 10 wt% in the electrode. It has been shown the low-cost water-soluble binders can help build the

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compact high sulfur loading electrode while their abundant functional groups help trap the LiPSs to

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suppress the shuttling. In addition, their elasticity can well buffer the volume changes of electrode directly using sulfur as the active materials and maintain the electrode integrity, thus showing great

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promise in practical uses. However, how to further optimize their performance in the battery is still a critical problem.

An ideal binder for Li-S battery should possess these following characters: (i) the strong adhesion strength which ensures the intimate contacts between active materials and conductive additives and anchor them on the current collector; (ii) the high flexibility to tolerate the volume expansion of the electrode; (iii) the high chemical stability that cannot react with the highly reactive polysulfides but 25

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can provide the anchoring ability towards them; (iv) the high conductivity to promote the transportation of Li ion and electron, decreasing the internal resistance and facilitating the kinetics of polysulfides redox reactions; (v) the stability in the organic electrolyte which can maintain the porous structure of the electrode, and at the same time, it should have good wetting ability by the

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electrolyte. Adhesion is the most important function of binders, which plays the dominate role to combine the active materials, the conductive additives and the current collector together, so the molecular weight

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of binder must be large enough to ensure efficient interfacial contact between above components. Nevertheless, binders with high viscosity like SA and CMC are brittle which lead to electrodes collapse during cutting and encountering the large volume changes of sulfur during cycling. The

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satisfactory elasticity, stability and toughness are all needed for polymer binders considering the large sulfur volume expansion during the discharge process. But a single polymer can hardly well

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balance the viscosity, solubility and conductivity which are generally needed for achieving

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high-performance cathodes. The combination use of different binders is a good choice, but the synergistic effect between the different binders has not been well investigated.

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Many reports have shown the functional groups like oxygen, nitrogen and sulfur-containing

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groups in the binder contribute to the immobilization of LiPSs. Accordingly, the functionalization of binders is an effective way to endow them with much stronger affinity towards LiPSs and suppress the shuttling of LiPSs in electrolyte. However, the viscosity of binder should be reevaluated after the functionalization and the LiPSs adsorption, and the electroactive functional groups possibly negatively impact the stability of binders. Design the binder with highly conjugated 3D networks using nanotechnology or in-situ polymerization is also promising to build the high sulfur loading 26

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electrode with enhanced ion and electron transportation. Considering the gelation properties of most water-soluble polymers, introducing the cross-linkers, such as the metal ions, is another simple way to strengthen the electrode mechanical properties. It is noteworthy that although the above optimization strategies of binders are all effective, the

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performance of polymer binders cannot be maximally enhanced by using any of them alone, the investigation on the combined use of these strategies is urgently needed, and the synergistic mechanisms should be thoroughly studied by not only experimental studies but also theoretical

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calculation. Moreover, to achieve deep understanding of the binder functions to optimize the electrode structures, the real-time investigations using the advanced in-situ testing technologies should be developed and used. In addition to the utilization of organic polymer binders, the novel

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aqueous inorganic polymers should also be considered, for instance, Zhou et al. developed an ammonium polyphosphate (APP) binder with moderate binding strength [128]. APP acts as a strong

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trapping agent towards LiPSs which retains the LiPSs in the electrode and facilitates Li ion transfer

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promoting the reaction kinetics. More importantly, it was flame-retardant, which opens a new direction for the advanced multifunctional binder design.

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To sum up, the optimization of binders in Li-S batteries seems the simple but effective way to

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promote their commercialization, but many challenges still need to be overcome and the understanding of their complex roles in the batteries should also need to be enhanced in future.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51772164 and U1601206), the Guangdong Natural Science Funds for Distinguished Young Scholars (2017B030306006), the Local Innovative and Research Teams Project of Guangdong Pearl River 27

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Talents Program (2017BT01N111), the Guangdong Special Support Program (2017TQ04C664), and the Shenzhen Basic Research Project (Grant Nos. JCYJ20170412171359175).

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This review discussed the recent progress of binders for lithium-sulfur batteries and the ways to enhance their physicochemical properties, such as modification, combination, in-situ polymerization

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and ion cross-linking.

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