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

Abstract

In this work we analyzed the phenomenon of quasi solid state (QSS) lithiation of sulfur-carbon (S/C) composite electrodes with sulfur confined in the micropores of carbon matrices based on our recent studies and data published in literature. We demonstrated that the existence of sulfur in the form of small molecules is not a necessary condition for the realization of QSS mechanism. QSS operation behavior was demonstrated both for carbons with small up to 1nm micropores and for carbons with larger pore size up to 2-3 nm. A key role in the operation of S/C electrodes via a QSS mechanism plays surface electrolyte interphase (SEI) which is formed on the surface of S/C composite during the initial discharge. The formation of SEI was supported by X-ray photoelectron spectroscopy and by scanning electron microscopy. Small pore size (up to 1 nm) of the carbon matrices has a positive effect on the cycling of S/C electrodes. A superior cycling performance for more than 3500 charge-discharge cycles was demonstrated for S/C composite electrodes based on carbons synthesized by carbonization of polyvinylidene dichloride (PVDC) resin. In recent years lithium-sulfur batteries have been the subject of very intensive research due to the high theoretical specific capacity of sulfur cathodes (1672 mA h g −1 ) which is an order of magnitude higher than that of lithiated transition-metal oxides and phosphates cathode materials used in commercial Li-ion batteries (140-200 mA h g −1 ). [1][2][3][4][5][6] This high capacity relates to the ability of sulfur atoms to accept two electrons resulting in the conversion of elemental sulfur to lithium sulfide (Li 2 S). Furthermore, sulfur is naturally abundant, environmentally friendly and relatively cheap.Due to the low electrical conductivity of elementary sulfur the addition of conductive additives or the use of conductive host materials it is necessary to ensure good performance of sulfur electrodes. Besides, during the discharge process elemental sulfur S 8 accepts electrons to give a chain of electroactive Li-polysulfides (Fig. 1a). The long-chain polysulfides Li 2 S n (4 ≤ n ≤ 8) are soluble in commonly used ethereal solvents and diffuse freely throughout the cell to the anode side where they are chemically reduced. This phenomenon known as the shuttle effect presents one of the main problems of Li-S cells. 7 The shuttle reactions prevent the possibility of extracting the full capacity of sulfur cathodes and lead to low Coulombic efficiency. Encapsulation of sulfur within activated carbons with high pores volume is one of the most effective approaches to mitigate the detrimental shuttle mechanism and stabilize composite sulfur cathodes during prolong cycling. 8-10The typical cyclic voltammetry and galvanostatic charge-discharge curves of the Li-S cell with C/S encapsulated cathode are shown in Fig. 1a. Two peaks observed in the cathodic voltammetric response of sulfur electrodes correspond to two plateaus observed in the voltage profiles of the discharge processes of these cathodes upon ...

Structural Design of Lithium–Sulfur Batteries: From Fundamental Research to Practical Application

“…Microporous structure plays an important role in the formation of small sulfur molecules. To confine the sulfur, a variety of microporous carbon hosts have been developed, such as nitrogen-doped microporous carbon, subnanometer 2D graphene, microporous structure carbon nanotubes, and pyrolyzed polymer derived carbon [140][141][142]. Lou et al [134] employed novel metal-organic-framework (MOF), ZIF-8, to develop a microporous carbon host for sulfur cathodes.…”

“…Further, there are many challenges and unrevealed electrochemical mechanisms related to the use of small sulfur molecule. One review paper published by Aurbach et al [142] raised questions on the mechanism of small sulfur molecules and stated that they are not the only way to make functionalized sulfur cathodes in carbonate electrolyte. The author proposed that the SEI layer formation on the cathode during the initial charging process played a key role in quasi-solid-state Li-S reactions.…”

Structure–Property Relationships of Organic Electrolytes and Their Effects on Li/S Battery Performance

“…[105] Among them, the latter approach is favored by most researchers. [105] Among them, the latter approach is favored by most researchers.…”

“…Very recently, Aurbach and co-workers [105] claimed that porous carbon, with a pore size of 2-3 nm, can not only accommodate short-chain sulfur but can also accommodate long-chain sulfur or octasulfur (S 5 to S 8 ) and also exhibits the same discharge plateau behavior, which invalidates the proposed effect of the confinement of small-chain sulfurs in the microporous carbon. Very recently, Aurbach and co-workers [105] claimed that porous carbon, with a pore size of 2-3 nm, can not only accommodate short-chain sulfur but can also accommodate long-chain sulfur or octasulfur (S 5 to S 8 ) and also exhibits the same discharge plateau behavior, which invalidates the proposed effect of the confinement of small-chain sulfurs in the microporous carbon.…”

Dual confinement of sulphur with rGO-wrapped microporous carbon from β-cyclodextrin nanosponges as a cathode material for Li–S batteries

“…A solid-solid mechanism of lithiation/delithiation reactions of the small S2-4 molecules was suggested [21], although it has been recently reported that small S molecules in the micropores of carbon matrices is not a necessary condition such that this mechanism is realized. [22] Microporous carbon were either obtained by means of templating [23] or by pyrolysis of biomass [24] but Ryu et al [25] had shown that it is possible to achieve high capacity sulphur cathode using solution impregnation only. Others reported ball mixing, melt infusion, and vapour phase diffusion to achieve homogenous impregnation of sulphur into the pores [5,8,10].…”