Liquid Structure with Nano-Heterogeneity Promotes Cationic Transport in Concentrated Electrolytes

Abstract

Using molecular dynamics simulations, small-angle neutron scattering, and a variety of spectroscopic techniques, we evaluated the ion solvation and transport behaviors in aqueous electrolytes containing bis(trifluoromethanesulfonyl)imide. We discovered that, at high salt concentrations (from 10 to 21 mol/kg), a disproportion of cation solvation occurs, leading to a liquid structure of heterogeneous domains with a characteristic length scale of 1 to 2 nm. This unusual nano-heterogeneity effectively decouples cations from the Coulombic traps of anions and provides a 3D percolating lithium-water network, via which 40% of the lithium cations are liberated for fast ion transport even in concentration ranges traditionally considered too viscous. Due to such percolation networks, superconcentrated aqueous electrolytes are characterized by a high lithium-transference number (0.73), which is key to supporting an assortment of battery chemistries at high rate. The in-depth understanding of this transport mechanism establishes guiding principles to the tailored design of future superconcentrated electrolyte systems.

Understanding Electrochemical Stability and Lithium Ion‐Dominant Transport in Concentrated Poly(ethylene carbonate) Electrolyte

“…The lower ability of the polymer chain to form a stable complex‐like structure should provide a cation‐dominated conduction in SPEs. In addition, Suo, Borodin, and coworkers, have reported that highly concentrated 1,3‐dioxolane (DOL)‐dimethoxyethane (DME)/LiTFSI electrolytes and H 2 O/LiTFSI aqueous electrolytes have high t + values. A situation similar to the PEC electrolyte may have occurred in these highly concentrated liquid electrolytes.…”

Aqueous Symmetric Supercapacitors with Carbon Nanorod Electrodes and Water‐in‐Salt Electrolyte

“…At the same time, the water molecules and lithium ions simultaneously increased on the surface of the negative electrode interface layer. In this case, almost all of water molecules were strongly coordinated to lithium ions, resulting in the remarkably reduced water activities near negative electrode ,. Therefore, the EDLC using 21 m WIS electrolyte could completely reach a high operation voltage of 2.2 V.…”

“…Recently, Wang et.al showed that the water‐in‐salt (WIS) electrolyte based on lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and water system with concentration above 20 m (mol kg −1 ) exhibits a wide stability window of 3.0 V on stainless steel gird electrodes . This wide stability window is attributed to the forming special shell structure in the electrolyte, in which the water molecules are strongly coordinated to the lithium ions, resulting the passivation of water activities . The special structure also brings up the problem about ionic conductivity.…”

“…[13] This wide stability window is attributed to the forming special shell structure in the electrolyte, in which the water molecules are strongly coordinated to the lithium ions, resulting the passivation of water activities. [14] The special structure also brings up the problem about ionic conductivity.…”

Combined Theoretical and Experimental Studies of Sodium Battery Materials

“…The dynamic behavior in liquid electrolytes can be treated with molecular dynamics (MD) simulations, which provide insight into solution structures, transport mechanisms, and electrolyte decomposition pathways . With the help of supercomputers, a system consisting of up to 400–500 atoms can be simulated with first principles MD simulations with a plane wave basis set and pseudopotential method.…”