Ion Transport in (Localized) High Concentration Electrolytes for Li-Based Batteries

Abstract

High concentration electrolytes (HCEs) and localized high concentration electrolytes (LHCEs) have emerged as promising candidates to enable higher energy density Li-ion batteries due to their advantageous interfacial properties that result from their unique solvent structures. Using electrophoretic NMR and electrochemical techniques, we characterize and report full transport properties, including the lithium transference numbers (t + ) for electrolytes ranging from the conventional ∼1 M to HCE regimes as well as for LHCE systems. We find that compared to conventional electrolytes, t + increases for HCEs; however the addition of diluents to LHCEs significantly decreases t + . Viscosity effects alone cannot explain this behavior. Using Onsager transport coefficients calculated from our experiments, we demonstrate that there is more positively correlated cation−cation motion in HCEs as well as fast cation−anion ligand exchange consistent with a concerted ion-hopping mechanism. The addition of diluents to LHCEs results in more anticorrelated motion indicating a disruption of concerted cation-hopping leading to low t + in LHCEs.

Cluster analysis as a tool for quantifying structure–transport properties in simulations of superconcentrated electrolyte

“…First, the apparent Li + transference number is defined as , where D + / D − are the over self-diffusion coefficients of Li + and TFSI − , respectively. Although this definition relies on ideal solution assumptions and does not rigorously account for ion–ion and solvent–ion correlations, 47 it is widely used due to the routine measurement of self-diffusion coefficients using PFG-NMR. The second definition, the Li + transference number based on self-diffusion coefficients for each ionic species, is given as , 44,48 in which z + is the is charge of all cations in the i th ionic species ( e.g.…”

“…Note that the t CNE + should ideally be compared with experimental values that are based on directly measured electrophoretic mobility ( e.g. , through electrophoretic NMR 47 ), but such data are yet to be reported for a direct comparison. In similar systems composed of LiTFSI–DMC, the Li + transference number measured by eNMR did show an increase with rising salt concentration.…”

Restructuring Electrolyte Solvation by a Partially and Weakly Solvating Cosolvent toward High-Performance Potassium-Ion Batteries

“…HCEs, despite their potential benefits, are hindered from commercial deployment due to their prohibitive costs and elevated viscosity. LHCEs emerge as a solution to mitigate some of the challenges associated with HCEs . However, most fluorinated diluents cannot dissociate potassium salts, preventing an effective increase in the K + transference number.…”

Transitioning from Regular Electrolytes to Solvate Ionic Liquids to High-Concentration Electrolytes: Changes in Transport Properties and Ionic Speciation

“…Other studies involving electrolytes having a salt:solvent molar ratio of 1:1.9 or higher termed their electrolytes as HCEs. ,, Regardless of their definition, these studies demonstrated that SCEs or HCEs have a number of solvent molecules that do not fulfill the lithium-ion (Li + ) tetrahedral coordination structure of [Li­(solvent) 4 ] + , for a monodentate solvent. Under such conditions, the counterion participates in the Li + coordination, leading to the formation of ion pairs, aggregates, ,, and ionic networks. − The distinct Li + solvation structures in HCEs are responsible for the atypical electrolyte properties, such as the enhanced oxidative stability, , thermal stability, and improved Li + transport. ,, …”