Deciphering the intrinsic kinetics of liquid lithium polysulfides redox process in ether-based flowing electrolyte for Li–S batteries

Abstract Owing to their high theoretical capacity and low material cost, lithium-sulfur batteries (LSBs) are considered the most promising next-generation energy storage devices. In response to existing LSBs problems, facilitating the conversion of lithium polysulfides (LiPSs) is considered an effective method, and the LiPSs conversion in the liquid phase provides the majority of the capacity of LSBs. Understanding the kinetics of redox processes of LiPSs in liquid-electrolyte LSBs is crucial. In this study, we have designed a new flowing electrolyte three-electrode system, equipped with a tiny H-type electrolytic cell isolated using the Nafion membrane, to quantitatively measure the intrinsic kinetics of LiPSs conversion. As a result, the onset redox potentials, exchange current densities, reaction orders, and apparent activation energy of the LiPSs conversion were obtained on the basis of the steady-state polarization curves. The onset reduction potentials of main LiPSs were 2.427, 2.348, 2.281, 2.212, and 2.139 V (vs Li+/Li) for S8, Li2S8, Li2S6, Li2S4, and Li2S2, respectively. The calculated exchange current density of Li2S6 redox was the highest (188.55 µA/cm2), while the S8 reduction was the minimal (0.97 µA/cm2). In addition, the reaction orders and apparent activation energies of Li2S8, Li2S6, Li2S4 were studied. The kinetic parameters obtained were related to the formation of the potential plateaus and the sloping regions, especially for the exchange current densities associated with LSBs charge/discharge process. These parameters were used to decipher the charge/discharge curves and CV profile of S8 in detail. This study could assist in understanding of the LSBs reaction mechanism.