Solvent‐Mediated Li₂S Electrodeposition: A Critical Manipulator in Lithium–Sulfur Batteries

Abstract

such as silicene [9a] and ionic compounds [10a] were shown to speed up the Li 2 S lateral passivation rate, leading to lower capacities.Solvent has a strong impact on Li 2 S deposition. The kinetics and morphology of Li 2 S deposition in glyme-based polysulfide solutions was studied and a progressive nucleation and a 2D island growth model was proposed. [6a] In addition, the use of discharge mediator was reported to slow down the impingement of insulating Li 2 S islands on carbon and transform the 2D growth to a 3D growth. [11] This is in line with Cuisinier et al. [12] reporting a distinct Li 2 S deposition mechanism in electron pair donor electrolytes compared to glymes due to the partial solvation of Li 2 S and the additional chemical pathways provided by the increased stabilization of polysulfide radicals. Recently, Pan et al. [4] reported that solvents with medium donor number (DN) yield flower-like Li 2 S morphology, low-DN solvents make Li 2 S films, and high-DN solvents give rise small particles. These pioneering studies suggest that the solution mediation process plays a critical role in Li 2 S deposition and highlight the urgent need for developing quantitative and comprehensive correlation between solvent property and Li 2 S nucleation and growth.In this work, we establish structure-property relationship of solvent in controlling solid Li 2 S deposition and develop quantitative solvent-mediated Li 2 S growth models as guides to solvent selection. We investigate three solvent's properties and their roles on Li 2 S deposition: (1) the donicity which governs the stability of the polysulfide anions (i.e., the precursor of Li 2 S deposition) through (Li + ) sol -polysulfide interactions; [12,13] (2) the polarity (dielectric constant) which governs the solvation ability of the final product Li 2 S; [14] (3) the viscosity which strongly affects the diffusivity of polysulfide and dissolved Li 2 S. [15] We show that these solvent-controlled properties are essential factors pertaining to the sulfur utilization, electrode kinetics, and reversibility of electrochemical reduction of elemental sulfur. Finally, we demonstrate the effectiveness of the solvent selection criteria developed in this study in identifying new and more effective electrolytes for Li-S batteries. Results and DiscussionWe study Li 2 S deposition in electrolyte model systems of two major groups: (i) ether-based solvents: 1,2-dimethoxyethane (G1), diethylene glycol dimethyl ether (G2), triethylene glycol Controlling electrochemical deposition of lithium sulfide (Li 2 S) is a major challenge in lithium-sulfur batteries as premature Li 2 S passivation leads to low sulfur utilization and low rate capability. In this work, the solvent's roles in controlling solid Li 2 S deposition are revealed, and quantitative solventmediated Li 2 S growth models as guides to solvent selection are developed. It is shown that Li 2 S electrodeposition is controlled by electrode kinetics, Li 2 S solubility, and the diffusion of polysulfide/Li 2 S, which is dictated...

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“…As seen in Table , DMC has a low dielectric constant ( ϵ =3.1), which is comparable to that of DEC ( ϵ =2.8); while the dielectric constant of EC is as high as 95.3, with a big donor number of 16 . This may be similar to observations in Li−S system by others, where the dissolution of polysulfides can be significantly suppressed with solvents with low dielectric constant and low donor number . Further work is needed to understand the effect of electrolyte on battery stability.…”

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