Anionic Effects on Solvate Ionic Liquid Electrolytes in Rechargeable Lithium–Sulfur Batteries

Abstract

A series of equimolar mixtures of Li salts (LiX) and glymes (triglyme (G3) and tetraglyme (G4)), [Li(glyme)]X with different anions (X: [N(SO2C2F5)2] = [BETI]; [N(SO2CF3)2] = [TFSA]; [CF3SO3] = [OTf]; BF4; NO3), were used as electrolytes to study the anionic effects of [Li(glyme)]X on the performance of lithium–sulfur (Li–S) batteries. The dissolution of lithium polysulfides (Li2S m ), which are discharge products of elemental sulfur, was significantly suppressed in the solvate ionic liquid (IL) electrolytes, as seen in [Li(G4)][BETI] and [Li(glyme)][TFSA], wherein all of the glymes participated in the formation of the complex cation [Li(glyme)]+. It was found that NO3 anions were irreversibly reduced at the composite cathode during discharge and BF4 anions formed unexpected byproducts through a chemical reaction with the polysulfide anions. Successful charge/discharge of Li–S cell could not be performed in [Li(glyme)]X in the presence of these anions because of the undesired side reactions. The solvate IL [Li(G4)][BETI] was found to be electrochemically stable in the Li–S cell and allowed a stable operation with a capacity of 600–700 mAh·g–1 and a Coulombic efficiency of 98.5% over 100 cycles, similar to that achieved by [Li(glyme)][TFSA]. In contrast, the Li–S cell with a concentrated electrolyte solution, [Li(G3)][OTf], showed a much lower capacity and Coulombic efficiency.

Preparation of Micropore-rich High Surface Area Activated Carbon from N-doped Carbon Precursor and its Application to Positive Electrode in Lithium-sulfur Battery

“…5(c) shows cycle performance of both cells. LiTFSI/ G4/HFE as "solvated ionic liquid electrolyte" was reported by Watanabe et al 6,7 They reported that in such electrolytes a glyme molecule is strongly coordinated to a Li ion and forms a stable [Li(glyme)] + complex cation, and HFE act as diluent to reduce the viscosity without taking part in the complex. This means that there would be no free glyme in the electrolytes.…”

Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

“…117,122 Consequently, solvate ILs are suitable for use as electrolytes in lithium ion batteries [111][112][113][114] and lithium-sulfur batteries. [115][116][117][118][119][120] The most striking and significant findings of our research on solvate ILs are the formation criteria of solvate ILs, 101 the enhanced oxidation stability of coordinating solvents, 95 the low coordinating properties of solvate cations and anions, and their electrode reactions with graphite. 121,122 Figure 8 shows how the anionic structures of lithium salts affect the formation of solvate ILs.…”

“…[115][116][117][118][119][120] The theoretical capacity of the S cathode is ten times greater than that of conventional cathode materials used in current Li-ion batteries. However, Li-S batteries suffer from the dissolution of lithium polysulfides, which are formed by the redox reactions at the S cathode.…”

“…Figure 10 shows the saturation concentrations (atomic concentration of S) of molecular sulfur (S 8 ) and Li 2 S x (x = 1-8) in different one-to-one mixtures of lithium salts and glymes. 116 A concentration based on the formula weight of Li 2 S 8 , for example, is one-eighth of the atomic concentration. It should be noted that the one-to-one mixtures (high concentration of lithium salt) do not always yield low saturated concentrations, which are typically seen in ca.…”

Electrochemical Deposition and Dissolution of Lithium on a Carbon Fiber Composite Electrode in a Solvate Ionic Liquid

“…Watanabe and coworkers have shown that lithium bis(trifluoromethylsulfonyl)amide (LiTFSA)-tetraglyme (G4) solvate ionic liquid is a suitable electrolyte for rechargeable Li-S battery because of low solubility of lithium polysulfides in the electrolyte. [2][3][4] The LiTFSA-G4 solvate ionic liquid, which is an equimolar mixture of LiTFSA and G4, is composed of [Li(G4)…”