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Highly concentrated, molten mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) 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)4TFSA dramatically decreased upon charge/discharge cycling, whereas Li(G3)1TFSA 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)4TFSA and Li(G1)2TFSA, which contain shorter ethers, but the corrosion was effectively suppressed in Li(G3)1TFSA. Furthermore, lithium polysulfides, which are formed as discharge intermediates at the sulfur cathode of the Li–S cell, were much less soluble in electrolytes with longer ethers. Therefore, a higher Coulombic efficiency and more stable cycle ability were achieved in Li–S cells with Li(G3)1TFSA. All the electrochemical properties in the batteries were dominated by the presence or absence of uncoordinating solvents in the concentrated electrolytes. This paper demonstrates that the structural stability of Li(glyme or THF)x+ cations in electrolytes plays an important role in the performance of Li batteries.
Zhang et al. (Thu,) studied this question.