Uncharted Waters: Super-Concentrated Electrolytes

Abstract

As a legacy left behind by classical analytical electrochemistry in pursuit of ideal electrodics, and classical physical electrochemistry in pursuit of the most conductive ionics, the study of non-aqueous electrolytes has been historically confined within a narrow concentration regime around 1 molarity (M). This confinement was breached in recent years when unusual properties were found to arise from the excessive salt presence, which often bring benefits to electrochemical, thermal, transport, interfacial, and interphasial properties that are of significant interest to the electrochemical energy storage community. This article provides an overview on this newly discovered and under-explored realm, with emphasis placed on their applications in rechargeable batteries.

Interactions and Transport in Highly Concentrated LiTFSI‐based Electrolytes

“…Hence, a key to more performant LIB cells is to improve the Li + conductivity. This can be made by either increasing the Li + mobility or the number of Li + charge carriers‐and employing highly concentrated electrolytes (HCEs) has the promise to do exactly this for LIBs by decoupling the mobility from the viscosity …”

“…no “free” solvent molecules exist. This is suggested to lead to changes in physical properties, interfacial chemistry and also alter the (Li + ) ion transport . While vehicular charge transport‐ions diffusing or migrating together with their respective solvation shells‐largely dominates in standard LIB electrolytes, Borodin and Smith showed using molecular dynamics (MD) simulation that in a 1 : 10 LiTFSI:EC electrolyte only ca .…”

“…For true HCEs, the situation is somewhat different; as there are too few solvent molecules per Li + to create a “normal” solvation shell, the Li + ions become incompletely solvated which results in both increased ion‐ion interactions and larger aggregates. In such systems, structural re‐organisation, hopping or cooperative ion transport modes are likely contributing to a larger extent . Additionally, while there is no clear picture, some studies report improved Li + transport numbers for HCEs .…”

“…In such systems, structural re‐organisation, hopping or cooperative ion transport modes are likely contributing to a larger extent . Additionally, while there is no clear picture, some studies report improved Li + transport numbers for HCEs . Altogether, this seems to render some HCEs adequate performance for LIB cells despite low total ionic conductivities .…”

Designing Advanced Lithium‐Based Batteries for Low‐Temperature Conditions

“…Nonetheless, recent works have shown how these electrolytes can be diluted with a non-coordinating fluorinated solvent to maintain the favorable aggregated ionic coordination state, all while improving viscosity and transport properties of the electrolyte and surface stability of the anode. [50,[52][53][54] Dilution also serves to reduce the bulk salt concentration down to an acceptably low value, minimizing mass and reducing overall cost while maintaining a highly aggregated coordination state. In fact, the appeal of fluorinated electrolytes has driven several investigations into their utility for low-temperature performance.…”