Li+ cation coordination by acetonitrile—insights from crystallography

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116

Electrolytes with the salt lithium bis(fluorosulfonyl)imide (LiFSI) have been evaluated relative to comparable electrolytes with other lithium salts. Acetonitrile (AN) has been used as a model electrolyte solvent due to the simplicity of its solvation interactions (the AN molecule has only a single electron lone-pair for Li + cation coordination). The information obtained from the thermal phase behavior, solvation/ionic association interactions, quantum chemical (QC) calculations and molecular dynamics (MD) simulations (with an APPLE&P many-body polarizable force field for the LiFSI salt) of the (AN) n -LiFSI mixtures provides detailed insight into the coordination interactions of the FSI − anions and the wide variability noted in the electrolyte transport properties (i.e., viscosity and ionic conductivity). was first reported in a patent application in 1995 which indicated a method for its synthesis.1 Its use in research studies, however, has been restricted until quite recently due to the limited availability and high cost of this salt. A limited number of publications have been reported about the use of the FSI − anion for electrolytes in recent years, 2-55 but many of these have focused on FSI − -based ionic liquids instead of the LiFSI salt. Recent papers, however, indicate that battery electrolytes with LiFSI have very promising properties-i.e., a relatively high thermal and hydrolytic stability (relative to LiPF 6 ), a high conductivity (comparable to electrolytes with LiPF 6 ) and a wide liquidus range. 2-18One key concern originated from reports which indicated that the use of the LiFSI salt in electrolytes results in severe corrosion of the battery Al current collector at high potentials. 3,15 It has recently been demonstrated, however, that this was likely due to chloride impurities in the salt (from the synthesis procedures) rather than the LiFSI salt itself. 12To fully capitalize upon the use of LiFSI as an alternative lithium salt for advanced lithium batteries, it is necessary to understand the behavior of this salt in bulk electrolyte solutions. The preceding work in this series of publications has scrutinized the relationship between the solution structure and transport properties of electrolytes in detail. 56-59The present study extends this focus to acetonitrile-based (AN) n -LiFSI electrolytes to better understand the characteristics of this relatively * Electrochemical Society Student Member.* * Electrochemical Society Active Member. z E-mail: Wesley.Henderson@pnnl.gov; oleg.a.borodin.civ@mail.mil new electrolyte salt. Of particular interest are the recent reports which noted that highly concentrated (AN) n -LiTFSI and -FSI electrolytes have a high electrochemical stability to reduction at negative potentials (in sharp contrast to more dilute solutions with nitrile solvents) which allows for the rapid and reversible charging/discharging of graphitic electrodes. 5,60 ExperimentalMaterials.-Anhydrous AN (Sigma-Aldrich, 99.8%) and LiFSI (Suzhou Fluolyte Co., Ltd., >99.5%) were used as-received....

Section: Raman Characterization Of An-limentioning

confidence: 99%

Section: Raman Characterization Of Ionic Association-mentioning

confidence: 99%

Electrolyte Solvation and Ionic Association

Han

Borodin

Seo

et al. 2014

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116

“…A stable solvation structure of Li + in aprotic solvents is reported to be fourfold coordination. [77][78][79] Because this stable solvation number is smaller than the solvent/Li salt molar ratio, there are considerable amount of free solvent molecules in such dilute electrolytes. With increasing Li salt concentration, free solvent molecules become short for Li + coordination, and thus the Li salts tend to be more associated to form CIPs and AGGs rather than SSIPs.…”

Section: Liquidus Concentration Rangementioning

confidence: 99%

Review—Superconcentrated Electrolytes for Lithium Batteries

Yamada

2015

702

667

Ever-increasing demand for better batteries has set extraordinarily high standards for electrolyte materials, which are far beyond the realm of a conventional nonaqueous electrolyte design. Superconcentrated (or highly concentrated) solutions are emerging as a new class of liquid electrolytes with various unusual functionalities beneficial for advanced lithium (Li) battery applications. This article reviews unique features, as well as basic physicochemical properties, of highly concentrated electrolytes from the viewpoint of their peculiar solution structure, and discusses their future contributions to advanced battery technologies. © The Author(s) An electrolyte is an indispensable component in rechargeable batteries. In general, it serves as a carrier ion-conductive and electroninsulating medium facilitating ion transport between a pair of electrodes, simultaneously withstanding the strong reducing and oxidizing forces of negative and positive electrodes, respectively. From this perspective, an ion-transport property and reductive/oxidative stability are the major two metrics in a battery electrolyte design. For lithium (Li)-ion batteries, the reductive/oxidative stability is of particular importance, because they employ extremely strong reducer and oxidizer as negative and positive electrodes, respectively, to deliver the highest voltage (currently ca. 3.8 V) among various battery chemistries. To address this severe situation, an electrochemically stable nonaqueous electrolyte, rather than a highly conductive aqueous electrolyte, has been used with some compromise on ionic transport properties.The composition of nonaqueous electrolytes makes critical impacts on the performance of Li-ion batteries.1-3 This fact is seemingly paradoxical to the "rocking chair" concept of Li-ion batteries, in which electrolyte components principally do not participate in the battery faradaic reactions. This paradox arises, at least partly, from the inherent thermodynamic instability of the electrode/electrolyte interfaces; in most cases, the interfaces are stabilized in a kinetic way (by electrode passivation) based on electrolyte decomposition products, 4 and thus the nature of the interfaces strongly depends on electrolyte composition. In other words, the design of electrolytes often corresponds to the control of the interfaces where faradaic charge-transfer reactions occur, thus making significant effects on the battery performance.A nonaqueous electrolyte design seems to have infinite variations of aprotic solvents (and additives), Li salts, and their mixing ratios (i.e., electrolyte concentrations). Indeed, however, there are many restrictions in the choice of aprotic solvents and Li salts in terms of the kinetic stability of the interfaces they form. First, only specific organic carbonates (e.g., ethylene carbonate (EC)) are compatible with graphite negative electrodes. 5,6 This peculiarity results from solvent's passivation ability. Because almost all aprotic solvents are unstable at such a low potential as ∼0 V vs. Li +...

“…Such high values are consistent with earlier calculations for solvent-Li + and anion-Li + complexes. [1][2][3][4][5] The M06-L/6-311+G * * level was therefore chosen as the most reliable for the investigation of the larger ion clusters.…”

Section: A502mentioning

confidence: 99%

“…Previous manuscripts have examined in detail acetonitrile electrolytes with numerous lithium salts: (AN) n -LiX. [1][2][3][4][5][6] Electrolytes with the most strongly associated anions studied (i.e., CF 3 SO 3 − and CF 3 CO 2 − ), in which anion. .…”

mentioning

confidence: 99%

Electrolyte Solvation and Ionic Association

Borodin

Han

Daubert

et al. 2015

Self Cite

Molecular dynamics (MD) simulations of acetonitrile (AN) mixtures with LiBF 4 , LiCF 3 SO 3 and LiCF 3 CO 2 provide extensive details about the molecular-and mesoscale-level solution interactions and thus explanations as to why these electrolytes have very different thermal phase behavior and electrochemical/physicochemical properties. The simulation results are in full accord with a previous experimental study of these (AN) n -LiX electrolytes. This computational study reveals how the structure of the anions strongly influences the ionic association tendency of the ions, the manner in which the aggregate solvates assemble in solution and the length of time in which the anions remain coordinated to the Li + cations in the solvates which result in dramatic variations in the transport properties of the electrolytes. Understanding the origin of the widely varying transport properties (e.g., viscosity, ionic conductivity and diffusion coefficients) of battery electrolytes remains a key challenge. The present work continues a detailed study into the solution structure of electrolytes-ion solvation and ionic association interactions-to provide mechanistic explanations for electrolyte properties. Previous manuscripts have examined in detail acetonitrile electrolytes with numerous lithium salts: (AN) n -LiX. [1][2][3][4][5][6] Electrolytes with the most strongly associated anions studied (i.e., CF 3 SO 3 − and CF 3 CO 2 − ), in which anion. . . Li + cation coordination (i.e., ionic association interactions) is a prominent feature of solvate formation, were found to have a much lower ionic conductivity than electrolytes with more weakly coordinating anions (e.g., PF 6 − and ClO 4 − ). 3 This is as one might expect once the ionic association tendency of the salts is well understood, but the details of why the conductivity values are low are missing.Two particularly notable points from the previous studies are that:(1) The average solvation numbers, obtained from a Raman spectroscopic analysis, for the (AN) n -LiCF 3 CO 2 solutions were found to be almost constant near a value of 1 (i.e., one AN molecule per Li + cation) over a wide concentration range, even for very dilute solutions. It remains unclear, however, as to why such electrolytes have such a low degree of solvation and why highly concentrated (>5 M) liquid electrolytes-perhaps contrary to expectations-can be prepared with LiCF 3 CO 2 with this being the case.(2) Experimental measurements of the solution structure and transport properties of (AN) n -LiCF 3 SO 3 mixtures were unobtainable due to the rapid crystallization of a solid solvate phase (i.e., (AN) 1 :LiCF 3 SO 3 ) from the electrolytes. 1 Liquid electrolytes with this salt can, however, be readily studied using molecular dynamics (MD) simulations to better understand how and why solution interactions differ for differing anions.The present work therefore delves deeper into the molecular-and mesoscale-level interactions using MD simulations applied to the characterization of (AN) n -LiCF 3 SO 3 and -LiCF ...