The energy density of 2567 w h kg?1,

The growing concern about
global
warming and air pollution caused by the consumption of finite and nonrenewable
fossil fuels has triggered the establishment of an alternative sustainable society
by developing cleaner and renewable energy sources, such as wind and solar
power. These technologies require reliable, low-cost, environmentally friendly,
large-scale energy-storage systems for intermittent energy generation. There is
no doubt that the pursuit of advanced energy storage devices with higher energy
densities is critical for powering our future society1. Among the best
candidates for next generation high-energy-storage systems, metal?sulfur batteries, such as
li?s, na?s, and mg?s2,3, hold high theoretical
energy densities, making them especially attractive. Of these, the li?s battery has the highest
theoretical energy density of 2567 w h kg?1, calculated on
the basis of the li anode (?3860mah/g) and the s
cathode (?1675mah/g), making it a promising choice
for the next generation of high-energy rechargeable batteries.2,4

However, several complex challenges
need to be faced and overcome in order to achieve practical application:
(i) the insulating nature of sulfur and sulfides5,6, limits electron transport in the
cathode and leads to low active material utilization; (ii) polysulfide
dissolution7, gives rise to
shuttle phenomenon that sulfur species transport back and onward between
electrodes, contributing to low coulombic efficiency and active material loss;
(iii) volume change of sulfur during cycling leads to the electronic integrity
of the composite electrode being disrupted8, then continuous surface
side reactions are induced, causing a drastic capacity decay. To date,
tremendous efforts have been made to solve the above problems by constructing
advanced composite cathode materials, which have
involved embedding sulfur in n-doped9-11
carbons or in carbon of various morphologies including porous
carbon12,
porous hollow carbon13,14, disordered carbon nanotubes2,
double-shelled hollow carbon spheres15,16, spherical ordered mesoporous carbon
nanoparticles17, and so forth. It
is undoubted that these materials have made substantial progress in significant
improvements in the specific capacity, cycling stability, and cycle life of
li–s batteries. However, the fatal flaws of these processes are that they are
all commonly complex, which are not suitable for practical application.

The binder is an important ingredient in
battery functions to bond and keep the active materials in the electrode, which
helps to improve electrical contact between the active materials and conductive
carbon as well as to link the active materials with the current collector18. The choice of
suitable binder has turned out to affect battery performance significantly19-22.
For example, polyvinylidene fluoride (pvdf) is one kind of conventional binder used
in the process of electrode preparation. Many studies have pointed out that
pvdf binder is not suitable for electrode materials with serious volume expansion,
such as silicon and sulfur, because of the relatively weak bond strength. On
the other hand, the organic solvent n-methyl-2-pyrrolidone (nmp) with a high
boiling point is toxic and not conducive for industrial production and
environmental protection23,24. According to
previous research experience, a suitable binder for li–s batteries should have
the following characteristics: (i) good adhesive
strength21. An ideal
binder should have the ability to maintain the structural stability of the
electrode material with a large volume change during cycling. Novel binders,
such as la13225, sbr + cmc26 have been
developed to create a more robust network for the entire sulfur cathode. (ii) suitable swelling capacity22. For sulfur
cathodes, proper electrolyte absorption of the binders can improve the rate
performance of the batteries. Lacey et al22. Used peo in li–s
batteries as a binder to investigate the capacity improvement, and found that
peo locally modifies
the electrolyte system, improving reaction kinetics. Furthermore, they
demonstrated that binders reduce the porosity in carbon/sulfur composite
cathodes, which is harmful towards electrolyte immersion. A binder which is
more susceptible to swelling like peo will admit a large amount of electrolyte
into its volume and suppress cathode passivation during discharge. In other
words, swelling of the binder leads to an improved solvent system for the electrochemistry
of sulfur species27. (iii) effective adsorption of multi-lithium sulfide28.
The most serious problem that restricts the development of li–s batteries is
the dissolution of li2sn (4 < n > 8). Cui et al29. Demonstrated the
strong li–o interaction between poly (- vinylpyrrolidone) (pvp) and li2sn
(1 < n > 8) with theoretical calculations. Li2s cathodes with
pvp binder exhibited a stable cycling performance. Yang30 prepared a novel
multifunctional binder (b-cd p-n+) with a quaternary ammonium cation
originating from b-cyclodextrin. The introduced quaternary ammonium cations
play an important role in immobilizing polysulfides and suppressing the shuttling effect.
The b-cd p-n+ based cathode demonstrated an improved cycling performance and
rate capability.

From
the above discussion, the binder should be considered as an active component in
li–s batteries. However, it is difficult to meet the requirements for
application with a single binder. Rational use of different functional binders
is an effective strategy to improve the electrochemical performance of li–s
batteries.

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