The and so forth. It is undoubted

The growing concern aboutglobalwarming and air pollution caused by the consumption of finite and nonrenewablefossil fuels has triggered the establishment of an alternative sustainable societyby developing cleaner and renewable energy sources, such as wind and solarpower. These technologies require reliable, low-cost, environmentally friendly,large-scale energy-storage systems for intermittent energy generation. There isno doubt that the pursuit of advanced energy storage devices with higher energydensities is critical for powering our future society1.

Among the bestcandidates for next generation high-energy-storage systems, metal?sulfur batteries, such asLi?S, Na?S, and Mg?S2,3, hold high theoreticalenergy densities, making them especially attractive. Of these, the Li?S battery has the highesttheoretical energy density of 2567 W h kg?1, calculated onthe basis of the Li anode (?3860mAh/g) and the Scathode (?1675mAh/g), making it a promising choicefor the next generation of high-energy rechargeable batteries.2,4 However, several complex challengesneed to be faced and overcome in order to achieve practical application:(i) the insulating nature of sulfur and sulfides5,6, limits electron transport in thecathode and leads to low active material utilization; (ii) polysulfidedissolution7, gives rise toshuttle phenomenon that sulfur species transport back and onward betweenelectrodes, contributing to low Coulombic efficiency and active material loss;(iii) volume change of sulfur during cycling leads to the electronic integrityof the composite electrode being disrupted8, then continuous surfaceside reactions are induced, causing a drastic capacity decay.

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To date,tremendous efforts have been made to solve the above problems by constructingadvanced composite cathode materials, which haveinvolved embedding sulfur in N-doped9-11carbons or in carbon of various morphologies including porouscarbon12,porous hollow carbon13,14, disordered carbon nanotubes2,double-shelled hollow carbon spheres15,16, spherical ordered mesoporous carbonnanoparticles17, and so forth. Itis undoubted that these materials have made substantial progress in significantimprovements in the specific capacity, cycling stability, and cycle life ofLi–S batteries. However, the fatal flaws of these processes are that they areall commonly complex, which are not suitable for practical application.The binder is an important ingredient inbattery functions to bond and keep the active materials in the electrode, whichhelps to improve electrical contact between the active materials and conductivecarbon as well as to link the active materials with the current collector18. The choice ofsuitable binder has turned out to affect battery performance significantly19-22.For example, Polyvinylidene fluoride (PVDF) is one kind of conventional binder usedin the process of electrode preparation.

Many studies have pointed out thatPVDF binder is not suitable for electrode materials with serious volumeexpansion, such as silicon and sulfur, because of the relatively weak bondstrength. On the other hand, the organic solvent N-methyl-2-pyrrolidone (NMP)with a high boiling point is toxic and not conducive for industrial productionand environmental protection23,24. According toprevious research experience, a suitable binder for Li–S batteries should havethe following characteristics: (i) good adhesivestrength21. An idealbinder should have the ability to maintain the structural stability of theelectrode material with a large volume change during cycling. Novel binders,such as LA13225, SBR + CMC26 have beendeveloped to create a more robust network for the entire sulfur cathode. (ii) Suitable swelling capacity22. For sulfurcathodes, proper electrolyte absorption of the binders can improve the rateperformance of the batteries.

Lacey et al22. used PEO in Li–Sbatteries as a binder to investigate the capacity improvement, and found thatPEO locally modifiesthe electrolyte system, improving reaction kinetics. Furthermore, theydemonstrated that binders reduce the porosity in carbon/sulfur compositecathodes, which is harmful towards electrolyte immersion. A binder which ismore susceptible to swelling like PEO will admit a large amount of electrolyteinto its volume and suppress cathode passivation during discharge. In otherwords, swelling of the binder leads to an improved solvent system for theelectrochemistry of sulfur species27. (iii) Effective adsorption of multi-lithium sulfide28.The most serious problem that restricts the development of Li–S batteries isthe dissolution of Li2Sn (4 < n > 8). Cui et al29.

demonstrated thestrong Li–O interaction between poly (- vinylpyrrolidone) (PVP) and Li2Sn(1 < n > 8) with theoretical calculations. Li2S cathodes withPVP binder exhibited a stable cycling performance. Yang30 prepared a novelmultifunctional binder (b-CD p-N+) with a quaternary ammonium cationoriginating from b-cyclodextrin. The introduced quaternary ammonium cationsplay an important role in immobilizing polysulfides and suppressing the shuttling effect.

The b-CD p-N+ based cathode demonstrated an improved cycling performance andrate capability. From the above discussion, the binder shouldbe considered as an active component in Li–S batteries. However, it isdifficult to meet the requirements for application with a single binder.Rational use of different functional binders is an effective strategy toimprove the electrochemical performance of Li–S batteries.



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