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

Abstract

Hydrofluoroethers (HFEs) have been adopted widely as electrolyte cosolvents for battery systems because of their unique low solvating behavior.The electrolyte is currently utilized in lithium-ion, lithium-sulfur,lithium-air,and sodiumion batteries.B ye valuating the relative solvating power of different HFEs with distinct structural features,a nd considering the shuttle factor displayed by electrolytes that employ HFE cosolvents,w eh ave established the quantitative structureactivity relationship between the organic structure and the electrochemical performance of the HFEs.Moreover,wehave established the linear free-energy relationship between the structural properties of the electrolyte cosolvents and the polysulfide shuttle effect in lithium-sulfur batteries.T hese findings providev aluable mechanistic insight into the polysulfide shuttle effect in lithium-sulfur batteries,a nd are instructive when it comes to selecting the most suitable HFE electrolyte cosolvent for different battery systems.Over the past few decades,lithium-ion batteries have been applied extensively in consumer electronic devices owing to their relatively high energy density and operating voltage. [1][2][3] To meet the demands of electric vehicle applications, however, the energy density of state-of-the-art lithium batteries must be increased substantially. [4][5][6] Va rious new lithium-battery systems with high theoretical energy density have been proposed. [5][6][7] Fort hese new battery systems to be successful, it is critical to develop reliable electrolytes and to understand their working principles. [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. [26][27][28][29][30][31][32][33][34] Despite the extensive application of HFEs,t he effect of their intricate molecular structure on electrochemical behavior has hardly been studied. 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 e...

Theoretically predicting the feasibility of highly-fluorinated ethers as promising diluents for non-flammable concentrated electrolytes

“…The dilution effect has been examined by considering five different fluorinated ether molecules. Among them, three common HFEs (viz, ETE, BTFE, TTE), which have been actually studied 10,12,17 . In addition, we introduced, for the first time, two other fluorinated diethyl ether molecules, bis(1,1,2,2-tetrafluoro ethyl) ether (B2E) and bis(pentafluoro ethyl) ether (BPE) that consist of eight and ten fluorine atoms, respectively (Table 1).…”

“…| (2020) 10:21966 | https://doi.org/10.1038/s41598-020-79038-y www.nature.com/scientificreports/ a high ability to enhance the oxidative stability of the electrolytes 13 , excellent thinning reagents for reducing the viscosity of electrolytes 14 and to construct a localized-concentrated electrolyte, which is strongly requested for the formation of a stable salt-derived SEI film 9,12,16 . Regarding the general structure of HFEs, the common HFEs were classified according to the vicinity of the fluoroalkyl groups to the oxygen atom 17 . The HFEs, with the closest position of a fluoroalkyl group to the oxygen atom, showed the lowest lithium-solvating power, while the distant fluoroalkyl-substituted HFEs, as bis(2,2,2-trifluoroethyl) ether (BTFE), did the opposite effect in addition to the highest conductivity 17 .…”

“…Regarding the general structure of HFEs, the common HFEs were classified according to the vicinity of the fluoroalkyl groups to the oxygen atom 17 . The HFEs, with the closest position of a fluoroalkyl group to the oxygen atom, showed the lowest lithium-solvating power, while the distant fluoroalkyl-substituted HFEs, as bis(2,2,2-trifluoroethyl) ether (BTFE), did the opposite effect in addition to the highest conductivity 17 . Also, ethyl 1,1,2,2-tetrafluoroethyl ether (ETE) showed a high ionic conductivity in addition to BTFE for acetonitrile-based electrolytes 10 .…”

“…However, the practical application of nonflammable HC electrolyte still suffers from some limitations as the low ionic conductivity and the high viscosity of the electrolyte 4,5 . In challenges to overcome these drawbacks, some researchers are focusing on the promising approach of dilution [9][10][11][12][13][14][15][16][17] . In fact, the selection and designing of diluents are very important to improve transports properties of electrolyte anions without losing their nonflammability characteristic and the formation of stable salt-derived SEI film 11,12 .…”

Non-fluorinated non-solvating cosolvent enabling superior performance of lithium metal anode battery

“…Thus far, design strategies for ideal non-solvating cosolvents are largely unexplored and thus limited to only fluorinated non-solvating cosolvents (FNSCs) [14][15][16] . The negative inductive effect of fluorine withdraws electrons near the adjacent oxygen within the solvent molecules, which lowers lithium solubility 17 . Further, this decreases cathodic stability, and it results in a facile decomposition to LiF-rich SEI [18][19][20] .…”