Concentrated electrolytes: decrypting electrolyte properties and reassessing Al corrosion mechanisms

• Published in: Energy & environmental science Vol. 7; no. 1; pp. 416 - 426
• Main Authors: McOwen, Dennis W, Seo, Daniel M, Borodin, Oleg, Vatamanu, Jenel, Boyle, Paul D, Henderson, Wesley A
• Format: Journal Article
• Language: English
• Published: 01.12.2013
• Subjects: Aluminum; Carbonates; Cations; Crystal structure; Dilution; Electrolytes; Lithium batteries; Solvents
• ISSN: 1754-5692; 1754-5706
• DOI: 10.1039/c3ee42351d

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 super(+) cation) on electrolyte properties. The phase behavior of ethylene carbonate (EC)-LiTFSI mixtures indicates that two crystalline solvates form-(EC) sub(3):LiTFSI and (EC) sub(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 super(+) 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.