Article pubs.acs.org/JPCC
Structural Interactions within Lithium Salt Solvates: Cyclic Carbonates and Esters Daniel M. Seo,† Taliman Afroz,† Joshua L. Allen,† Paul D. Boyle,‡ Paul C. Trulove,§ Hugh C. De Long,∥ and Wesley A. Henderson*,†,⊥ †
Ionic Liquids & Electrolytes for Energy Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States ‡ X-ray Structural Facility, Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States § Department of Chemistry, U.S. Naval Academy, Annapolis, Maryland 21402, United States ∥ Air Force Office of Scientific Research, Arlington, Virginia 22203, United States ⊥ Electrochemical Materials & Systems Group, Energy & Environment Directorate, Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: Only limited information is available regarding the manner in which cyclic carbonate and ester solvents coordinate Li+ cations in electrolyte solutions for lithium batteries. One approach to gleaning significant insight into these interactions is to examine crystalline solvate structures. To this end, eight new solvate structures are reported with ethylene carbonate, γ-butyrolactone, and γ-valerolactone: (EC)3:LiClO4, (EC) 2 :LiClO 4 , (EC) 2 :LiBF 4 , (GBL) 4 :LiPF 6 , (GBL) 1 :LiClO 4 , (GVL) 1 :LiClO 4 , (GBL)1:LiBF4, and (GBL)1:LiCF3SO3. The crystal structure of (EC)1:LiCF3SO3 is also re-reported for comparison. These structures enable the factors that govern the manner in which the ions are coordinated and the ion/solvent packingin the solid-stateto be scrutinized in detail.
■
INTRODUCTION Understanding the structural interactions within solutions remains a key challenge. This is particularly true for the solvent−salt mixtures used as electrolytes for lithium batteries. When a lithium salt is dissolved, the anions and/or solvent molecules form coordinate bonds to the Li+ cations through donor atom electron lone-pairs. Anion solvation in aqueous (or protic) solutions occurs principally through hydrogen-bonding with solvent molecules, but the aprotic solvents used in battery electrolytes do not have acidic protons and thus do not form hydrogen bonds with the anions. In lithium battery electrolytes, therefore, a direct competition exists between the unsolvated (or naked) anions and solvent molecules for coordination to the cations in the Li+ cation coordination shells. Solid-state solvate structures serve as informative (but necessarily limited) models for the interactions within liquid electrolytes by providing critical structural insight into the manner in which Li+ cations are coordinated within energetically favorable crystalline phases.1−8 Ethylene carbonate (EC) is one of the prevalent solvents used for commercial Li-ion battery electrolytes. Only a few lithium solvate structures with EC, however, have been reported to datenamely, (EC)4:LiPF6, (EC)4:LiAsF6, (EC)4:LiBOB (where BOB− is bis(oxalato)borate), (EC)2:LiBenzIm(BF3)2, and (EC)1:LiCF3SO3.6−8 Structurally similar ester solvents such as γ-butyrolactone (GBL) and γ-valerolactone (GVL):have also garnered strong interest for electrolyte applications,9−15 but no © XXXX American Chemical Society
lithium solvate structures have been reported for these solvents. A study of the solvates formed with the cyclic carbonate and ester solvents with varying lithium salts enables the discernment of how variability of the anion and solvent structures leads to differences in the Li+ cation coordination. This information may greatly aid in the development of a molecularscale understanding of bulk electrolyte interactions which, in turn, will provide strong guidance for a transformation from empirical to directed electrolyte formulations tailored for specific requirements for advanced batteries.
■
EXPERIMENTAL METHODS EC (electrolyte-grade, Novolyte) was used as-received. GBL (≥99%, Sigma-Aldrich) and GVL (99%, Sigma-Aldrich) were dried over molecular sieves prior to use. The water content of the solvents was checked using a Mettler Toledo DL39 Karl Fischer coulometer and verified to be
