Daniel M. Seo

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 ∼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.

Solvate Structures and Spectroscopic Characterization of LiTFSI Electrolytes

A Raman spectroscopic evaluation of numerous crystalline solvates with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI or LiN(SO2CF3)2) has been conducted over a wide temperature range. Four new crystalline solvate structures-(PHEN)3:LiTFSI, (2,9-DMPHEN)2:LiTFSI, (G3)1:LiTFSI and (2,6-DMPy)1/2:LiTFSI with phenanthroline, 2,9-dimethyl[1,10]phenanthroline, triglyme, and 2,6-dimethylpyridine, respectively-have been determined to aid in this study. The spectroscopic data have been correlated with varying modes of TFSI(-)···Li(+) cation coordination within the solvate structures to create an electrolyte characterization tool to facilitate the Raman band deconvolution assignments for the determination of ionic association interactions within electrolytes containing LiTFSI. It is found, however, that significant difficulties may be encountered when identifying the distributions of specific forms of TFSI(-) anion coordination present in liquid electrolyte mixtures due to the wide range of TFSI(-)···Li(+) cation interactions possible and the overlap of the corresponding spectroscopic data signatures.

Li+ cation coordination by acetonitrile—insights from crystallography

Solvation is a critical factor for determining the properties of electrolytes and lithium reagents, but only limited information is available about the coordination number for Li+ cations in solution with different solvents. The present manuscript examines the manner in which acetonitrile (AN) fully solvates Li+ cations. The results are also likely pertinent to other nitrile and dinitrile solvents. In particular, the crystal structure for a (AN)6:LiPF6 solvate is reported—this is the first 6/1 AN/Li solvate structure to be determined. The structure consists of Li+ cations fully solvated by four AN molecules (i.e., [(AN)4Li]+ species), uncoordinated PF6− anions and uncoordinated AN molecules (two per Li+ cation). This structure validates, in part, density functional theory (DFT) calculations which predict that there is little to no energetic benefit to coordinating Li+ cations with more than four AN solvent molecules.

Tetra-kis(acetonitrile-κN)lithium hexa-fluoridophosphate acetonitrile monosolvate.

In the title compound, [Li(CH3CN)4]PF6·CH3CN, the asymmetric unit consists of three independent tetrahedral [Li(CH3CN)4]+ cations, three uncoordinated PF6 − anions and three uncoordinated CH3CN solvent molecules. The three anions are disordered over two sites through a rotation along one of the F—P—F axes. The relative occupancies of the two sites for the F atoms are 0.643 (16):0.357 (16), 0.677 (10):0.323 (10) and 0.723 (13):0.277 (13). The crystal used was a racemic twin, with approximately equal twin components.

Poly[bis-(acetonitrile-κN)bis-[μ(3)-bis(tri-fluoro-methane-sulfonyl)-imido-κO,O':O'':O''']dilithium].

In the title compound, [Li2(CF3SO2NSO2CF3)2(CH3CN)2]n, two Li+ cations reside on crystallographic inversion centers, each coordinated by six O atoms from bis(trifluoromethanesulfonyl)imide (TFSI−) anions. The third Li+ cation on a general position is four-coordinated by two anion O atoms and two N atoms from acetonitrile molecules in a tetrahedral geometry.

Poly[[(acetonitrile)­lithium(I)]-μ3-tetra­fluoridoborato]

The structure of the title compound, [Li(BF4)(CH3CN)]n, consists of a layered arrangement parallel to (100) in which the Li+ cations are coordinated by three F atoms from three tetrafluoridoborate (BF4 −) anions and an N atom from an acetonitrile molecule. The BF4 − anion is coordinated to three different Li+ cations though three F atoms. The structure can be described as being built from vertex-shared BF4 and LiF3(NCCH3) tetrahedra. These tetrahedra reside around a crystallographic inversion center and form 8-membered rings.

Phase Behavior and Solvation of Lithium Triflate γ-Butyrlactone

Data describing the concentration and temperature dependent solvation and phase behavior of lithium trifluoromethanesulfonate (LiTf) in γ-butyrolactone (GBL) is presented. Differential scanning calorimetry (DSC), Raman spectroscopy and X-ray diffraction measurements are employed to elucidate the electrolyte interactions. DSC analysis show the presence of a crystallinity gap at concentrations between 2.56:1 and 5.00:1 GBL:LiTf (mole:mole) and a high melting solvate in more concentrated samples. Raman spectroscopic analysis indicates that the coordination of ions in the high melting solvate undergoes temperature dependent transitions. X-ray diffraction analysis showed that lithium triflate forms a 1:1 aggregated solvate having 6 ion pair coordinated by 6 solvent molecules. Taken together, data suggest ion transport in electrolytes is influenced by solvation.

A "Looking Glass" into Electrolyte Properties: Cyclic Carbonate and Ester-LiClO4 Mixtures

Many electrolyte mixtures have been introduced and tested for Li-ion batteries. Most of this work has been done by a trial-and-error approach. In part, this is due to the current limited understanding regarding electrolyte interactions at the molecular level—specifically ionic association and solvation. The properties of electrolytes are dictated by these molecular interactions, which are influenced by several factors, such as the structure of the anion and solvent, temperature and salt concentration. This study focuses on LiClO4 mixtures with different cyclic carbonate and ester solvents: ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), and γ-valerolactone (GVL). LiClO4, like LiBF4, is an intermediately associated salt and has been used for electrolytes in primary lithium batteries. Carbonate solvents, especially EC, are the most widely used solvents for electrolytes. To understand how the interactions are influenced by differences in solvent structure, the solventLiClO4 mixtures have been examined with thermal and vibrational spectroscopic analysis. From DSC measurements, phase diagrams have been prepared (i.e., Fig. 1). Crystalline solvate phases have been identified and the structures of these solvates obtained by single crystal XRD analysis. These solvates show the coordination between the ions and solvent in the crystalline phases. In addition, Raman spectroscopic analysis has been used to examine the solvent and anion interactions in both the solid-state and liquid phases. This information provides insight into the types of solvates and their relative amounts present in the liquid electrolytes. This then enables the direct correlation between the molecular-level interactions and electrolyte physical properties such as ionic conductivity, viscosity, wettability, volatility, etc. Acknowledgement: This research was fully supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award ER46655.