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Ref :

https://www.electropages.com/blog/2020/01/why-lithium-sulphur-batteries-are-taking-so-long-be-used-commercially

Because they continually shuttle between the electrodes, they never change back into a neutral state (which the surface polysulphide ions do)

I don't think this explains the 'shuttle effect' clearly. I still don't understand what it is.

During discharging, lithium ions and sulfur form Li2S8 at the cathode, then it becomes Li2S4, Li2S2, Li2S with continued discharging. During charging, the Li2S move towards the cathode, now, why can't it be reversely reduced to Li2S4, Li2S8. Why can't they ever change back into a neutral state?

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OP's title indicated OP is looking for the real mechanism of the Shuttle Effect in $\ce{Li-S}$ batteries. Yet, OP is asking different questions in the text body. I'd try to address as much as possible in this answer.

A most recent review article (Ref.1) states that:

Lithium-sulfur ($\ce{Li-S}$) batteries are one of the most promising batteries in the future due to its high theoretical specific capacity ($\pu{1675 mAh g−1}$) and energy density ($\pu{2600 Wh kg−1}$). However, the severe capacity fading caused by shuttle effect of polysulfide needs to be addressed before the practical application of $\ce{Li-S}$ batteries.

To understand what is the Shuttle Effect, OP need to first understand the working chemical mechanism of $\ce{Li-S}$ batteries, and can take a head stat in that direction with this article about Lithium–Sulfur Battery, which gives a fair description of the chemistry involve in the battery. The same Wikipedia article simply described the Shuttle Effect as follows:

The lithium polysulfide $\ce{Li2S}_x$ ($6 < x < 8$) is highly soluble in the common electrolytes used for $\ce{Li-S}$ batteries. They are formed and leaked from the cathode and they diffuse to the anode, where they are reduced to short-chain polysulfide and diffuse back to the cathode where long-chain polysulfide are formed again. This process results in the continuous leakage of active material from the cathode, lithium corrosion, low coulombic efficiency, and low battery life.

The formation of polysulfide shuttle effect is classified into five steps in Ref.1:

  1. Formation of long-chain polysulfide,
  2. Detaching of polysulfide from sulfur host,
  3. Dissolution of polysulfide into electrolyte,
  4. Migration of polysulfide toward lithium anode side, and
  5. Reaction between polysulfide and lithium anode.

If interested, OP may read Ref.1 and Ref.2, which thoroughly discussed the mechanism and possibilities of how to prevent the Shuttle Effect in depth. For example, one such techniques is modifying the separator, which has been done successfully recently (Ref.3):

UiO-66-Modified Polypropylene Separator

References:

  1. Wenchen Ren, Wei Ma, Shufen Zhang, Bingtao Tang, "Recent advances in shuttle effect inhibition for lithium sulfur batteries," Energy Storage Materials 2019, 23, 707-732 (https://doi.org/10.1016/j.ensm.2019.02.022).
  2. Chao Deng, Zhuowen Wang, Shengping Wang, Jingxian Yu, "Inhibition of polysulfide diffusion in lithium–sulfur batteries: mechanism and improvement strategies," J. Mater. Chem. A 2019, 7(20), 12381-12413 (https://doi.org/10.1039/C9TA00535H).
  3. Yanpeng Fan, Zhihui Niu, Fei Zhang, Rui Zhang, Yu Zhao, Guang Lu, "Suppressing the Shuttle Effect in Lithium–Sulfur Batteries by a UiO-66-Modified Polypropylene Separator," ACS Omega 2019, 4(6), 10328-10335 (https://doi.org/10.1021/acsomega.9b00884).
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    $\begingroup$ I've read this paper before asking my question. But it doesn't say, why does leakage of cathode hurts the performance, why the sulfur deposited on the anode destroys the battery, or why is corrosion of anode a problem. Specifically, of the 12345 items in the quote, why does each pose a problem? $\endgroup$
    – seilgu
    Feb 24 '20 at 1:37
  • $\begingroup$ This battery is based on lithium and sulfur. Any time you loose one of them hurts the performance (similar to basic dry battery, which normally loose zinc electrode). Here, when lithium polysulfide ($\ce{Li2S}_x$, $x \gt 6$) is formed it is dissoved in organic electrolyte, making it mobile. $\endgroup$ Feb 24 '20 at 19:17
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    $\begingroup$ I think the cell works BECAUSE the polysulfides dissolve in the electrolyte and become mobile. If it never dissolves, only the lithium atoms will attach to the the sulfur electrode and the battery will stop working. When you say you "lose" electrode materials, how come you can't regain it by charging? In Zn/Cu battery, when discharging, Zn dissolves and Cu deposits on the Cu electrode, when charging, Zn deposits on the Zn electrode and Cu dissolves. It's reversible. Why isn't Li-S battery similarly reversible? $\endgroup$
    – seilgu
    Feb 25 '20 at 12:01
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Apparently, the main drawback of the Lithium-Sulfur battery is that it should work so : At the anode made of lithium, $Li^+$ ions are emitted and go to the cathode made of sulfur. This sulfur should accept the electrons, to make $S^{2-}$ ions which neutralize the arriving $Li^+$ ions. And it should produce a deposit of insoluble $Li_2S$ (or $Li_2S_2$, or $Li_2S_4$) on the cathode. The trouble is that sometimes, the sulfur does accept electrons, but the $S$ atom is part of a big molecule $S_6$ or $S_8$, and that this molecule fixes an electron producing $S_6^-$ or $S_8^-$ ions, which do not neutralize $Li^+$ ions. On the contrary, they cross the electrolyte in the "wrong" sense, and get deposited on the anode. This destroys the cell. Nobody has yet found the way of getting rid of this stupid problem.

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    $\begingroup$ But why does the sulfur deposited on the anode destroy the battery? Why can't the deposited sulfur continue to react during a subsequent charge/discharge cycle? I think this is my major confusion. $\endgroup$
    – seilgu
    Feb 24 '20 at 1:22

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