The Effect of Monomers of Conducting Polymer in the Electrolytes on the Electrochemical Behavior of Lithium−Sulfur Batteries
Authors: Lee Seungmin , Eom KwangSup 
Published: December 2020

The recent demand for the energy storage system with a higher energy density has turned the attention to the lithium−sulfur (Li−S) batteries due to its higher gravimetric/volumetric capacities (1672 mAh g-1 and 3467 mAh cm-3, respectively) than the metal oxide-based cathodes (e.g., 148 mAh g-1 and 1363 mAh cm-3 for LiCoO2) [1] However, in the Li−S batteries the intermediate lithium-polysulfides (LiPS) dissolve into the electrolyte followed by their shuttle between Li metal anode and S cathode, which is called the shuttle effect, has led to the performance degradation of Li−S batteries. [2-3] Thus, the strategies that reduce the dissolution of the LiPS from the cathode side, for example forming a chemical/physical protective layer on S cathode, have been mainly studied. [2-3] Among them, adding an additive into the electrolytes that can form a protective layer on S cathode during charge/discharge or pre-treatment has been considered as an effective way because it has achieved the improvement of electrochemical performance by more facile and cost-effective method compared to the ex-situ methods [4]. However, the effect of electrolyte additives and their chemical/physical change during electrochemical treatment (or cycling) on the electrochemical behavior of Li−S batteries have been rarely studied. In particular, the effect of electrochemical variables (current or voltage), which determines the electrochemical behavior of Li−S batteries and consequent electrochemical performance, has been not fully understood, therefore is needed to be delineated in detail.
In this work, we employed three monomers (aniline, thiophene, pyrrole) of conducting polymers as an electrolyte additive and evaluated their effect on the electrochemical behavior of Li−S batteries. In particular, the monomers are expected to be electrochemically polymerized into the conducting polymers during battery cycling [4]. Moreover, since the three monomers contain heteroatomic bonds (C−S or C−N) where LiPS can be adsorbed, it might suppress the dissolution of LiPS [4].
The galvanostatic charge/discharge test employing four different electrolytes was conducted first to investigate the stability of electrolytes containing monomers and the interaction among the electrolyte composites. (Fig. 1) Both batteries with base electrolyte and base+thiophene electrolyte (Fig. 1a-b), respectively, manifested the typical charge/discharge behavior of Li−S batteries. Meanwhile, the battery with aniline showed the impeded lower plateau reaction (from Li2SX to Li2S2/Li2S, 4≤X≤8) and that with pyrrole showed the absence of higher plateau reaction (from Li2S2/Li2S to Li2SX, 4≤X≤8) at the initial cycles. (Fig. 1c-d) The corresponding overpotential (Ƞ) at the 10th discharge and charge states was smallest in the case of base+thiophene followed by base electrolyte and base+aniline as shown in Fig. 1f. Given that the overpotential at the discharge and charge states respectively represents the resistance of liquid-to-solid and solid-to-liquid reaction, this disparity of overpotential suggests that the different molecular interaction between the electrolyte components/electrodes and monomers leads to different electrochemical behavior of Li−S batteries. [5] The effect of monomers on the solvents’ properties and the electrochemical polymerization mechanism using the monomers will be further studied, to design the highly stable Li−S battery.

References

[1] H. Peng et al. Chem. Soc. Rev. 46 (2017) 5237–5288
[2] A. Eftekhari et al. J. Mater.Chem. A, 5 (2017) 17734–17776
[3] S. Lee et al. J. Power Sources. 408 (2018) 111–119
[4] S. Lee et al. Adv. Funct, Mater. 30 (2020) 2000028
[5] H. A. Gasteiger et al., J. Electrochem Soc, 165 (2018) 16, A4027–A4033

Figure 1