Chelate Effects in Glyme/Lithium Bis(trifluoromethanesulfonyl)amide Solvate Ionic Liquids, Part 2: Importance of Solvate-Structure Stability for Electrolytes of Lithium Batteries

Highly concentrated, molten mixtures of lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) and ether solvents (tetrahydrofuran (THF), monoglyme (G1), diglyme (G2), and triglyme (G3)) were investigated as electrolytes for Li batteries. To compare the electrochemical reactions in the electrolytes with different solvents, the ratio of ether–oxygen atoms and Li+ ([O]/[Li]) in the electrolytes was fixed at four. The capacity of a Li–LiCoO2 cell with [Li(THF)4][TFSA] dramatically decreased upon charge/discharge cycling, whereas [Li(G3)1][TFSA] allowed the cell to have a stable charge–discharge cycles and a Coulombic efficiency of greater than 99% over 100 cycles. Corrosion of the Al current collector of the cathode was also affected by the composition of the electrolytes. Persistent Al corrosion took place in [Li(THF)4][TFSA] and [Li(G1)2][TFSA], which contain shorter ethers, but the corrosion was effectively suppressed in [Li(G3)1][TFSA]. Furthermore, lithium polysulfides, which are formed as discharge inte...