Restricting the Solubility of Polysulfides in Li‐S Batteries Via Electrolyte Salt Selection

Abstract

1 M lithium bis(trifl uoromethanesulfonyl)imide (LiTFSI) in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME). [ 21 ] Switching to other solvents with high dielectric constant such as dimethyl sulfoxide (DMSO) or dimethylacetamide (DMA) facilitates the production of sulfur radicals and the full utilization of the sulfur, but the cycling stability of the cell is also sacrifi ced and the unstable interface between the reactive DMSO/DMA solvent and Li metal raises other concerns. [ 22,23 ] Alternative cell designs incorporating solid-state electrolytes completely prevent the dissolution of polysulfi des, but the utilization of the sulfur (i.e., the reversible capacity) is also largely reduced. [ 24 ] Therefore, the solubility of the polysulfi des in cells with liquid electrolytes needs to be carefully tuned and well understood. In most studies, a low sulfur loading (less than 1 mg cm -2 ) is used, but a high S loading is desired for high-capacity full cells. [ 25,26 ] In this paper, lithium trifl uoromethyl-4,5-dicyanoimidazole (LiTDI) is studied for Li-S cells. We study the effect of LiTDI on the solubility of polysulfi des. The performance of this electrolyte was investigated with a high sulfur (i.e., practically usable) loading at 3 mg cm -2 under a high current density.A comparison of the physical properties of electrolytes with either 1 M LiTFSI or LiTDI in DOL/DME (1/1, v/v) is shown in Table 1 . Both electrolytes have a comparable conductivity and viscosity, suitable for the electrochemical needs, but an 83% decrease in the Li 2 S 8 solubility is observed in the LiTDI electrolyte. Restricting the polysulfi de solubility to a low level is expected to improve the Li-S cell cycle life because of the suppression of the polysulfi de shuttle mechanism, reduction of anode contamination, and the minimization of the loss of active material from the cathode. [ 22 ] To understand the relationship between the polysulfi de solubility and electrolyte supporting salt, 7 Li and 17 O nuclear magnetic resonance (NMR) spectroscopic measurements were used to aid in understanding the solvation interactions. The δ ( 7 Li) for the LiTDI and LiTFSI-based electrolytes without polysulfi des was -0.8 and -1.2 ppm (relative to Li + cations fully solvated by H 2 O molecules) ( Figure 1 a-c), respectively. The δ ( 17 O) for the DME molecules shifts to ≈ -27.7 and -26.9 ppm in the LiTDI and LiTFSI-based electrolytes, respectively, from -24.3 ppm in a neat DOL/DME solvent mixture. The δ ( 17 O) for the DOL molecules near 35 ppm shifts by 1-2 ppm. The smaller shift of δ ( 17 O) for the DOL molecules relative to the DME molecules and signifi cant broadening of the peaks for the latter suggest that the Li + cations may be preferentially solvated by the DME. [ 27 ] It has been reported that the solvation of Li + cations in the electrolyte affects the short-chain polysulfi de solubility by solvating the Li + ion in the polysulfi des, but it did not affect the

Towards practical sulfolane based electrolytes: Choice of Li salt for graphite electrode operation

“…Such structures are found in various LiTDI/solvent crystalline structures [44] and it was found that, in ether mixtures including LiTDI, larger polysulfide clusters are formed as compared with equivalent LiTFSI electrolytes [45]. DFOB, on the other hand, mostly interacts with Li + via its carbonyl groups, and only to a lesser extent, via its fluorine atom in the liquid phase and is coordinated similarly to LiTFSI [46].…”

Modelling transport-limited discharge capacity of lithium-sulfur cells

“…It is generally believed that the dissolution of these polysulfide species into the electrolyte, and their subsequent diffusion away from electrochemically active surfaces, contribute to a major degradation mechanism as well as sulfur-utilization limitation [4,5]. In light of this loss mechanism, numerous strategies have been proposed to encapsulate polysulfides in the cathode, including optimization of the cathode nanomorphology (as reviewed in [6,7]), doping of polysulfide-adsorption sites [8,9], and coating of polysulfide blocking or adsorption layers [10,11].…”

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

“…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. 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. [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.…”