Concentrated electrolytes: decrypting electrolyte properties and reassessing Al corrosion mechanisms

Highly concentrated electrolytes containing carbonate solvents with lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) have been investigated to determine the influence of eliminating bulk solvent (i.e., uncoordinated to a Li+ cation) on electrolyte properties. The phase behavior of ethylene carbonate (EC)-LiTFSI mixtures indicates that two crystalline solvates form-(EC)(3):LiTFSI and (EC)(1):LiTFSI. Crystal structures for these were determined to obtain insight into the ion and solvent coordination. Between these compositions, however, a crystallinity gap exists. A Raman spectroscopic analysis of the EC solvent bands for the 3-1 and 2-1 EC-LiTFSI liquid electrolytes indicates that similar to 86 and 95%, respectively, of the solvent is coordinated to the Li+ cations. This extensive coordination results in significantly improved anodic oxidation and thermal stabilities as compared with more dilute (i.e., 1 M) electrolytes. Further, while dilute EC-LiTFSI electrolytes extensively corrode the Al current collector at high potential, the concentrated electrolytes do not. A new mechanism for electrolyte corrosion of Al in Li-ion batteries is proposed to explain this. Although the ionic conductivity of concentrated EC-LiTFSI electrolytes is somewhat low relative to the current state-of-the-art electrolyte formulations used in commercial Li-ion batteries, using an EC-diethyl carbonate (DEC) mixed solvent instead of pure EC markedly improves the conductivity.