Long-Range Ion-Ordering in Salt-Concentrated Lithium-Ion Battery Electrolytes: A Combined High-Energy X-ray Total Scattering and Molecular Dynamics Simulation Study

Abstract

Herein, we report on a structural study for characterizing unique solution structures in the salt-concentrated electrolytes, which are promising new lithium (Li)-ion battery electrolytes. A combination of high-energy X-ray total scattering (HEXTS) experiments with all-atom molecular dynamics (MD) simulations was performed on the salt-concentrated electrolytes that were based on Li bis­(trifluoromethanesulfonyl)­amide (LiTFSA) and N,N-dimethylformamide (DMF). The radial distribution functions obtained from the HEXTS and MD approaches were in good agreement in the current LiTFSA/DMF solutions. We found that in the local structure: (1) the Li-ions were coordinated with both the DMF molecules and the TFSA anions in the concentrated solutions and (2) specific Li+···Li+ correlations were present in the radial distribution function over the r range of 3 Å–15 Å. The Li+···Li+ correlations originated from the extended multiple Li-ion complexes, that is, polymerized [Li+···TFSA–···Li+] n complexes so that they were highly structurally ordered. We concluded that this type of an ion-ordered structure plays a crucial role in the electrochemical stability and the ion-conducting mechanism, resulting in a unique LIB performance employing these salt-concentrated electrolytes.

Anion Coordination Characteristics of Ion-pair Complexes in Highly Concentrated Aqueous Lithium Bis(trifluoromethane- sulfonyl)amide Electrolytes

“…This corresponded well with plots from our previous work on typical organic electrolyte systems containing LiTFSA salt. 28,36,45 The distance between the experimental plot and the ideal line did not change significantly, even while increasing the cLi up to 3.0 mol dm -3 . In the highly concentrated solutions (cLi > 3.0 mol dm -3 ) according to the Raman and DFT results in this work, Li ions directly interacted with TFSA anions to form Li + •••TFSAcontact ion-pair complexes; i.e., the dissociativity of the LiTFSA salt was significantly low.…”

“…Indeed, recent simulation studies suggested that an extended ion-ordering structure based on a multiple Li-ion complexes formed in a concentrated electrolyte system contribute to the unusual Li-ion transport with the Grotthuss mechanism. 13,28,46…”

“…14 In a previous study, we reported on the structural properties of Li-ion complexes formed in highly LiTFSA salt-concentrated N,N-dimethylformamide (DMF) as a model solvent having a large electron-pair donating ability (Gutmann's donor number, DN = 26.6). 27 On the basis of our systematic combined experimental (Raman spectroscopy and high-energy X-ray total scattering) and theoretical [density functional theory (DFT) calculations and MD simulations] study, 18,28 we proposed that (1) the Li ions are coordinated by both DMF and TFSA in the concentrated region (cLi > 2 mol dm -3 ) to form long-range ionordering structures based on extended multiple Li-ion complexes,…”

Structural Variation in Carbonate Electrolytes by the Addition of Li Salts Studied by X‐Ray Total Scattering

“…In fact, X‐ray total scattering is becoming a useful tool for the structural characterization of electrolytes in the field of rechargeable batteries in combination with the molecular dynamics simulation. [ 32,33 ] In addition, the measurement time is much shorter (≈1 h) than the neutron total scattering (≈10 h), which makes it easier to systematically investigate various samples. Furthermore, using a 2D detector, the measurement time can become even shorter (≈ a few minutes), which provides an opportunity to carry out an operando measurement.…”

Re‐weighted random series path integral simulations of molecular clusters: Applications to lithium solvated by a mixed Stockmayer cluster

“…Even though the body of knowledge gathered to date on clusters is vast, we find surprisingly little work dedicated to the study of Stockmayer and ion‐Stockmayer clusters in the literature. The present goals are of fundamental importance for the research and development of lithium ion batteries, as a practical application. Lithium ion batteries are complex bulk systems, and their direct simulations will likely require more realistic models to extend the scope of the present investigation.…”