The growing concern about global warming and airpollution, caused by the depletion of limited and nonrenewable fossil fuels,are receiving increasing attention, to the development of alternative sourcesof clean and renewable energy such as wind and solar energy has led to thecreation of another sustainable society. These technologies require a largeenergy storage system for reliable, low cost, environmentally friendly,intermittent energy generation.
Undoubtedly, the search of advanced energystorage devices with higher energy densities is crucial to the development ofour future society1. Among the best candidatesfor the next generation of high energy storage systems, metallic sulfurbatteries, such as Li-S, Na-S and Mg-S, have high theoretical energy densities,which makes them especially attractive. Of these, the Li-S battery has thehighest theoretical energy density of 2567 W h kg-1, calculated onthe basis of the Li anode (~ 3860mAh / g) and the S cathode (~ 1675mAh / g),making it a promising option for the next generation of high-energyrechargeable batteries2-4.
However, there are many complicated challenges in orderto achieve practical application for example (1)insulatingnature of sulfur and sulfides5,6, interrupt electrontransportation in the cathode, resulting low utilization of active materials;(2) dissolution of polysulfide7 grow up shuttle phenomenonwhich lead to columbic efficiency decay and loss of active material;(3) the volume change of sulfur during cyclingdestroys the electronic integrity of the composite electrode, induces acontinuous surface side reaction, causing a rapid capacity fading8.Up to now, enormous efforts have been made to solvethe above problems through the construction of advanced composite electrodematerials which have included sulfur embedding in N-doped9-11 carbon or differentmorphologies carbon, including porous carbon12, porous hollow carbon13,14, Disordered carbon nanotubes, double shelled hollow carbonspheres15,16, spherical orderedmesoporous carbon nanoparticles17 and so on.It is no doubt that these materials have madesubstantial progress in greatly improving the specific capacity, cyclestability and cycle life of Li – S cells.
However, the fatal defect of theseprocesses is that they are all generally complex and not suitable for practicaluse.The binder is an important component in the batteryfunction for binding and holding the active material to the electrode, improvesthe electrical contact between the active material and the conductive carbon,and binds the active material to the current collector18. The selection of anappropriate binder has been found to significantly affect battery performance19-22.
For example,polyvinylidene fluoride (PVDF) is a type of conventional binder used inelectrode preparation. Many studies indicate that PVDF binders are not suitablefor electrode materials that swell in volume, such as silicon and sulfur,because of their relatively weak bond strength. On the other hand,N-methyl-2-pyrrolidone (NMP), a high-boiling organic solvent, is toxic and notconducive to industrial production and environmental protection23,24. Based on past researchexperience, the appropriate lithium-sulfur battery binder should have thefollowing characteristics: (i) good adhesive strength21. The ideal binder should becapable of maintaining the structural stability of the electrode material witha large volume change during the cycle. New binders such as LA 13225, SBR + CMC26 were developed to build amore robust network for the entire sulfur cathode. (ii)Suitable swelling capacity22. For sulfur cathodes,proper electrolyte absorption of the binders can improve the rate performanceof the batteries.
Lacey et al22. Inaddition, they demonstrate that the binder reduces the porosity of the carbon /sulfur composite cathode, which is detrimental to electrolyte impregnation. Aswelling-sensitive binder such as PEO can contain a large amount of electrolytein its volume and inhibit cathode passivation upon discharge. In other words,the swelling of the binder leads to an improved solvent system for the sulfurspecies electrochemistry27. (iii) Effective adsorption of multi-lithium sulfide28. The most serious problemlimiting the development of Li – S batteries is the dissolution of Li2 Sn(4 n 8). Cui et al29.
demonstrated the strongLi–O interaction between poly (- vinylpyrrolidone) (PVP) and Li2Sn(1 < n > 8) with theoretical calculations. Yang30 prepared a novelmultifunctional binder. He introduced quaternary ammonium cations play animportant role in fixing polysulfides and inhibiting the shuttling effect.From the above discussion, the binder should beconsidered as the active component of the Li – S cell. However, it is difficultto satisfy the requirement for application with a single binder. The reasonableuse of different functional binders is an effective strategy for improving theelectrochemical performance of Li – S cells.In this work, we investigated the application ofGelatin and PEI composite as functional binders in Li–S batteries. Gelatin isgood adhesive material provide the elastic and mechanical properties which canbuffer the volume changes during repeatable charge and discharge process andPEI is polar in nature with abundant amine groups and hyperbranched networkstructures, which provide the strong affinity to absorb polysulfideintermediates.
When we used Gelatin/PEI composite as a binder, it remarkablyimproved cycling performance with good capacity retention and suppressing theshuttling effect of the polysulfides