Article pubs.acs.org/JPCC
Structural Interactions within Lithium Salt Solvates: Acyclic Carbonates and Esters Taliman Afroz,† Daniel M. Seo,† Sang-Don Han,† Paul D. Boyle,‡ and Wesley A. Henderson*,†,§ †
Ionic Liquids & Electrolytes for Energy Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, North Carolina 27695, United States ‡ X-ray Structural Facility, Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695, United States § Electrochemical Materials & Systems Group, Energy & Environment Directorate, Pacific Northwest National Laboratory (PNNL), 902 Battelle Boulevard, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: Solvate crystal structures serve as useful models for the molecular-level interactions within the diverse solvates present in liquid electrolytes. Although acyclic carbonate solvents are widely used for Li-ion battery electrolytes, only three solvate crystal structures with lithium salts are known for these and related solvents. The present work, therefore, reports six lithium salt solvate structures with dimethyl and diethyl carbonate, (DMC)2:LiPF6, (DMC)1:LiCF3SO3, (DMC)1/4:LiBF4, (DEC)2:LiClO4, (DEC)1:LiClO4, and (DEC)1:LiCF3SO3 and four with the structurally related methyl and ethyl acetate, (MA)2:LiClO4, (MA)1:LiBF4, (EA)1:LiClO4, and (EA)1:LiBF4.
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(with 2-trifluoromethyl-4,5-dicyanoimidazole (TDI−), difluoro(oxalato)borate (DFOB−) and tris(oxalato)phosphate (TOP−) anions, respectively)−have been reported.2,3,6 Interestingly, two recent publications have shown promising Li-ion battery performance for electrolytes based upon EA without the inclusion of any cyclic carbonate solvents.7,8 These are examples of new electrolyte formulations which are increasingly being demonstrated for battery chemistries/applications beyond those of/for current commercial Li-ion batteries. To provide more insight into the solvates formed with these solvents, the present paper therefore reports ten new solvate structures with different lithium salts and the acyclic solvents DMC, DEC, MA and EA.
INTRODUCTION Solvate crystal structures with lithium salts provide direct information about solvation (solvent···Li+) and ionic association (anion···Li+) interactions. In particular, these structures indicate the energetically favorable coordination present in the solid state. As such, they are useful, but necessarily limited, models for the myriad solvate species which exist in liquid electrolytes. A scrutiny of the trends and differences in the coordination that exist with varying solvent and anion structures, however, aids in building a foundation of knowledge about the molecular-level interactions present for disparate electrolyte compositions. Very few solvate crystal structures have been reported for carbonate and ester solvents with lithium salts until quite recently when three publications reported the structures of ten new solvates with the cyclic solvents ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), and γvalerolactone (GVL).1−3 Given that the electrolytes used in commercial batteries use mixtures of ethylene carbonate with acyclic carbonates,4,5 it is informative to compare these solvates with those formed with the structurally similar acyclic solvents dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl acetate (MA), and ethyl acetate (EA).
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EXPERIMENTAL AND COMPUTATIONAL METHODS DMC (≥99%, anhydrous, Sigma-Aldrich), DEC (≥99%, anhydrous, Sigma-Aldrich), MA (99.5%, anhydrous, SigmaAldrich), and EA (99.8%, anhydrous, Sigma-Aldrich) were used as-received. The water content of the solvents was checked using a Mettler Toledo DL39 Karl Fischer coulometer and verified to be
