The Relationship between the Relative Solvating Power of Electrolytes and Shuttling Effect of Lithium Polysulfides in Lithium–Sulfur Batteries

Abstract

Relative solvating power, that is, the ratio of the coordination ratios between a solvent and the reference solvent, was used to probe the quantitative structure-activity relationship of electrolyte solvents and the lithium polysulfide (LiPS) dissolution in lithium-sulfur batteries. Internally referenced diffusion-ordered nuclear magnetic resonance spectroscopy (IR-DOSY) was used to determine the diffusion coefficient and coordination ratio, from which the relative solvating power can be easily measured. The higher the relative solvating power of an ethereal solvent, the more severe will be the LiPS dissolution and the lower the coulombic efficiency of the lithium-sulfur battery. A linear relationship was established between the logarithm of relative solvating power of a solvent and the degree of LiPS dissolution, rendering relative solvating power an important parameter in choosing the electrolyte solvent for lithium-sulfur batteries.

A Selection Rule for Hydrofluoroether Electrolyte Cosolvent: Establishing a Linear Free‐Energy Relationship in Lithium–Sulfur Batteries

“…The recent development of IR‐DOSY and the D‐ α analysis techniques has overcome the limitations of vibrational spectroscopy (Fourier‐transform infrared and Raman spectroscopy) and realized the quantitative determination of the solvent solvation state . The concept of relative solvating power ( γ ), which is the ratio between the coordination percentage of a test solvent and a reference solvent, was established as a useful indicator for the LiPS shuttling effect …”

“…As a consequence of the non‐solvating nature of the group A HFEs, the γ values of TTE and TFETFE are very close to zero. Table also indicates that the solvating power of HFE is significantly smaller than that of various non‐fluorinated ethers . Even the group C HFE has a much lower γ than methyl‐ tert ‐butylether (MtBe), which is not able to solvate lithium effectively because of the steric hindrance of the t ‐butyl group.…”

“…[c] The experimental values of dimethoxyethane (DME), tetrahydrofuran (THF), methyltetrahydrofuran (MeTHF), and methyl‐ tert ‐butylether (MtBE) are taken from ref. . Key: Group A (TTE and TFETFE; α ‐ and β ′‐substituted fluoroalkyl groups), group B (TFEPE and TFEiBE; α ‐substituted fluoroalkyl group on one side of the molecule only), and group C (BTFE; β , β ′‐fluoroalkyl‐substituted HFE).…”

“…To fairly evaluate the LiPS shuttling effect, it is necessary to compare the Coulombic efficiencies of all the Li–S cells at the same specific charge/discharge capacity . The Coulombic efficiencies of the test cells at a specific discharge capacity of 850 mAh g −1 , and the corresponding shuttle factors, are shown in Table .…”

Intertwined Nitrogen‐Doped Carbon Nanotube Microsphere as Polysulfide Grappler for High‐Performance Lithium‐Sulfur Batteries

“…[1,2] LithiumÀ sulfur (LiÀ S) batteries are expected to be one of the most promising candidates for nextgeneration rechargeable batteries with a high theoretical specific capacity (1675 mAh g À 1 ), achievable specific energy (2500 Wh kg À 1 ), based on the electrochemical reactions between lithium metal and elemental sulfur as shown in Equation (1): [3] 2Li þ þ 2e À þ xS8 [1,2] LithiumÀ sulfur (LiÀ S) batteries are expected to be one of the most promising candidates for nextgeneration rechargeable batteries with a high theoretical specific capacity (1675 mAh g À 1 ), achievable specific energy (2500 Wh kg À 1 ), based on the electrochemical reactions between lithium metal and elemental sulfur as shown in Equation (1): [3] 2Li þ þ 2e À þ xS8…”