Effect of the Hydrofluoroether Cosolvent Structure in Acetonitrile-Based Solvate Electrolytes on the Li⁺ Solvation Structure and Li–S Battery Performance

Abstract

We evaluate hydrofluoroether (HFE) cosolvents with varying degrees of fluorination in the acetonitrile-based solvate electrolyte to determine the effect of the HFE structure on the electrochemical performance of the Li-S battery. Solvates or sparingly solvating electrolytes are an interesting electrolyte choice for the Li-S battery due to their low polysulfide solubility. The solvate electrolyte with a stoichiometric ratio of LiTFSI salt in acetonitrile, (MeCN)-LiTFSI, exhibits limited polysulfide solubility due to the high concentration of LiTFSI. We demonstrate that the addition of highly fluorinated HFEs to the solvate yields better capacity retention compared to that of less fluorinated HFE cosolvents. Raman and NMR spectroscopy coupled with ab initio molecular dynamics simulations show that HFEs exhibiting a higher degree of fluorination coordinate to Li at the expense of MeCN coordination, resulting in higher free MeCN content in solution. However, the polysulfide solubility remains low, and no crossover of polysulfides from the S cathode to the Li anode is observed.

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

“…[23][24][25][26][27] Moreover, several groups have indicated the importance of the solvent solvation state in the LiPS dissolution process. [28][29][30] As ar esult, an in-depth understanding of the relationship between LiPS dissolution and the solvation of electrolyte solvents is vital in developing efficient Li-S batteries.…”

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

“…[5][6][7] Fort hese new battery systems to be successful, it is critical to develop reliable electrolytes and to understand their working principles. [18,19] Because of their low solvating ability,H FEs are exceptionally versatile as electrolyte cosolvents.T hey offer several important advantages: 1) enhanced oxidative stability of electrolytes, [10][11][12] 2) they serve as excellent thinning reagents for reducing the viscosity of electrolytes, [20][21][22] and 3) they enable the construction of localized concentrated electrolytes,which are highly useful in lithium-metal batteries. [18,19] Because of their low solvating ability,H FEs are exceptionally versatile as electrolyte cosolvents.T hey offer several important advantages: 1) enhanced oxidative stability of electrolytes, [10][11][12] 2) they serve as excellent thinning reagents for reducing the viscosity of electrolytes, [20][21][22] and 3) they enable the construction of localized concentrated electrolytes,which are highly useful in lithium-metal batteries.…”

“…[13,[23][24][25] Significantly,they have gained popularity in Li-S batteries because of their effectiveness in suppressing the lithium polysulfides (LiPS) shuttle effect. [21,35] Yet, it is critical to also consider the position of the fluoroalkyl groups in the HFE structures when evaluating their solvation behavior because the lithium ion is coordinated to the oxygen atom of the ether molecule,a nd the inductive effect imposed by the fluoroalkyl groups on the oxygen is highly dependent on the proximity of the electron-withdrawing fluoroalkyl groups.I n other words,t he absolute number of electron-withdrawing fluorine atoms is not as dominant as the position of those fluorine atoms in controlling the short-chain (< 7c arbon atoms) HFE solvation property.Thel ithium-solvating ability of different HFE solvents, and the linear free-energy relationship (LFER) between the solvents and LiPS shuttle kinetics,w ere determined using internally referenced diffusion-ordered spectroscopy (IR-DOSY), calculations of the diffusion coefficient-coordination ratio (D-a), and knowledge of the relative solvating power (g). By comparing the electrochemical properties of four HFEs,Shin and co-workers concluded that the degree of fluorination (that is,the number of fluorine atoms) dictated the solvation behavior.…”

“…[8,9] First introduced as an electrolyte cosolvent ad ecade ago, [10] hydrofluoroethers (HFEs) have rapidly been employed by researchers all over the world to construct functional electrolytes for high-voltage lithium-ion, [10][11][12] lithium-metal, [13] lithium-sulfur (Li-S), [14] lithium-air, [15,16] lithium/selenium-sulfur, [17] and even sodium-ion batteries. [18,19] Because of their low solvating ability,H FEs are exceptionally versatile as electrolyte cosolvents.T hey offer several important advantages: 1) enhanced oxidative stability of electrolytes, [10][11][12] 2) they serve as excellent thinning reagents for reducing the viscosity of electrolytes, [20][21][22] and 3) they enable the construction of localized concentrated electrolytes,which are highly useful in lithium-metal batteries. [13,[23][24][25] Significantly,they have gained popularity in Li-S batteries because of their effectiveness in suppressing the lithium polysulfides (LiPS) shuttle effect.…”

“…By comparing the electrochemical properties of four HFEs,Shin and co-workers concluded that the degree of fluorination (that is,the number of fluorine atoms) dictated the solvation behavior. [21,35] Yet, it is critical to also consider the position of the fluoroalkyl groups in the HFE structures when evaluating their solvation behavior because the lithium ion is coordinated to the oxygen atom of the ether molecule,a nd the inductive effect imposed by the fluoroalkyl groups on the oxygen is highly dependent on the proximity of the electron-withdrawing fluoroalkyl groups.I n other words,t he absolute number of electron-withdrawing fluorine atoms is not as dominant as the position of those fluorine atoms in controlling the short-chain (< 7c arbon atoms) HFE solvation property.…”