Understanding on Kinetics Change of Li-S Battery with Fluorinated Additives Using in-Situ/Operando Raman and X-Ray Absorption Spectroscopy

Abstract

Li-S battery concept is promising for the next generation energy storage due to its distinct advantage of high theoretical capacity (1675 mA/g) and energy density (2600 Wh/gm). Yet the formation of polysulfides as intermediate discharge products (Li2Sn, 4<n<8) and their dissolution result in the loss of active material and ultimate deposition at the anode. This shuttle phenomenon gives rise to irreversible loss of active material, rapid capacity fading, low coulombic efficiency and short cycle life with an ambiguity of its mechanism. To our best knowledge, we will be the first to employ the in-situ characterization techniques including Raman spectroscopy and X-ray absorption spectroscopy (XAS) to investigate the kinetics evolution with various electrolyte systems. This work presented the formation change of polysulfides intermediates at various fluorinated additives environment with different formula and composition based on in-situ/operando Raman spectroscopy. The Raman results clearly revealed the long-chain polysulfide conversion (400 cm-1) was pushed forward and an increasing concentration of short-chain polysulfide (453 cm-1) was appeared when fluorinated carboxylate ester-based electrolyte was used at its optimized formula compared to the baseline. Cyclic voltammetry results correspond to the chemistry evolution proposed by Raman. In-situ X-ray absorption spectroscopy was also conducted on sulfur K-edge to monitor the polysulfide intermediates formation. X-ray fluorescence mapping (XRF) on cathode and anode also confirmed that the sulfur shuttle phenomenon was significantly suppressed in fluorinated carboxylate ester-based electrolyte by changing the sluggish kinetics. Figure 1