Single step transformation of sulphur to Li2S2/Li2S in Li-S batteries

Abstract

Lithium-sulphur batteries have generated tremendous research interest due to their high theoretical energy density and potential cost-effectiveness. The commercial realization of Li-S batteries is still hampered by reduced cycle life associated with the formation of electrolyte soluble higher-order polysulphide (Li2Sx, x = 4–8) intermediates, leading to capacity fading, self-discharge, and a multistep voltage profile. Herein, we have realized a practical approach towards a direct transformation of sulphur to Li2S2/Li2S in lithium-sulphur batteries by alteration of the reaction pathway. A coconut shell derived ultramicroporous carbon-sulphur composite cathode has been used as reaction directing template for the sulphur. The lithiation/delithiation and capacity fading mechanism of microporous carbon confined sulphur composite was revealed by analyzing the subsurface using X-ray photoelectron spectroscopy. No higher-order polysulphides were detected in the electrolyte, on the surface, and in the subsurface of the cathode composite. The altered reaction pathway is reflected by a single-step profile in the discharge/charge of a lithium-sulphur cell.

Unique aqueous Li-ion/sulfur chemistry with high energy density and reversibility

“…S4, only the spectra in the range of 158−166 eV were selected to fit the S2p of S 8 , S 0 , S 1− , and S 2− (Fig. 2B) (28,29). At SOC = 0%, only S 8 (164.4 eV) was observed, whereas polysulfide species S x 2− (8 ≥ x ≥ 2) appeared during lithiation, which were fitted with S 0 at 164.0 eV and S −1 at 162.2 eV, respectively.…”

Review—On the Mechanism of Quasi-Solid-State Lithiation of Sulfur Encapsulated in Microporous Carbons: Is the Existence of Small Sulfur Molecules Necessary?

“…Effective surface films make easier desolvation of Li ions, which plays a key role for QQS reaction. 15,20,21,45 Interestingly, in the case of S/C cathodes prepared with AC1 an additional higher voltage plateau was observed during two initial charges with the IL electrolyte solution (Fig. 4b).…”

“…This type of behavior schematically presented in Fig. 1b is generally observed for organic carbonatebased electrolyte solutions, [13][14][15][16][17][18][19][20][21][22]24,25,27,31,33,35,36,39,40 as well as for bis(fluorosulfonyl)imide (FSI) anion-based ionic liquid electrolyte solutions. 28,37,38 For this type of the operation of Li-S cells voltage profiles differ from those commonly measured for conventioanal Li-S composite electrodes and are characterized by a single galvanostatic charge/discharge voltage plateau observed around 2.0 V vs. Li/Li + .…”

“…18,[42][43][44] In the case of liquid electrolytes Wang et al proposed that this type of unique behavior (namely, the single plateau observation) relates to the reaction of Li ions after desolvation with sulfur encapsulated in the micropores under solvent deficient conditions and called this mechanism "quasi-solid-state reaction". 15,45 In most cases this mechanism is observed for composite S/C electrodes with very narrow micropores which width is lower than 1 nm, [14][15][16]19,20,22,23,28,40 but in some cases this type of operation was described also for larger pore size. 13,29,[37][38][39] Several explanations for this single-plateau behavior can be found in the literature.…”

“…Desolvation of the solvated Li ions ions by the micropores which pore size is below 1 nm may play a critical role, since solvated ions tend to be desolvated in micropores with a size close to the ion size. 15,20,21,45 The reduction of sulfur still produces polysulfides as intermediates, but the polysulfides remain in the pores instead of being dissolved. Li desolvation prevents or slows down the dissolution of polysulfides as the solvent concentration is very low or close to zero in the micropores.…”