Abstract
High salt concentration has been shown to induce increased electrochemical stability in organic solvent-based electrolytes. Accompanying the change in bulk properties is a structural ordering on mesoscopic length scales and changes in the ion transport mechanism have also been suggested. Here we investigate the local structure and dynamics in highly concentrated acetonitrile electrolytes as a function of salt concentration. Already at low concentrations ordering on microscopic length scales in the electrolytes is revealed by small angle X-ray scattering, as a result of correlations of Li+ coordinating clusters. For higher salt concentrations a charge alternation-like ordering is found as anions start to take part in the solvation. Results from quasi-elastic neutron spectroscopy reveal a jump diffusion dynamical process with jump lengths virtually independent of both temperature and Li-salt concentration. The jump can be envisaged as dissociation of a solvent molecule or anion from a particular Li+ solvation structure. The residence time, 50-800 ps, between the jumps is found to be highly temperature and Li-salt concentration dependent, with shorter residence times for higher temperature and lower concentrations. The increased residence time at high Li-salt concentration can be attributed to changes in the interaction of the solvation shell as a larger fraction of TFSI anions take part in the solvation, forming more stable solvation shells.
Highlights
We will refer to this peak as the intermediate range order (IRO) peak
At low concentrations it is manifested as a shoulder/ broadening of the molecular peak but at higher concentrations (410 : 1) a separate peak can be observed around 2.4 ÅÀ1
The presence of Li+ will induce a structuring of the AN molecules beyond the first solvation shell. With this picture in mind, we propose that the origin of the intermediate range order in AN/Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solutions arises from correlations between the solvation cluster of Li+
Summary
Several electrolyte concepts have been proposed in order to improve both the capacity and the safety aspects.[4,5] An interesting approach is based on the finding that highly concentrated solutions (e.g. 3 to 420 m depending on the solvent/salt combination) of certain Li-salts in polar solvents behave completely different compared to standard electrolyte solutions where the Li-salt concentration is lower (B1 m).[6,7,8,9,10,11,12,13] One example is acetonitrile (AN), which is one of the most oxidation-tolerant organic solvents that could enable the use of high voltage electrodes (45 V). Ion transport in standard (dilute) electrolytes is governed by a vehicular-type mechanism where the Li-ion is moving together with its solvation shell.[19] The nanostructure and charge ordering present in highly concentrated electrolytes makes it plausible that the ion transport mechanism is more complex, but at present there is a limited understanding of the local dynamics and its relation to the nanostructure. The nature of solvation shells and their dynamics is of importance for fast ion transport, and for e.g., electrochemical processes and the formation of the solid electrolyte interphase.[33,34,35] In this work we focus on the local structure and dynamics in an archetypal highly concentrated electrolyte and their connection to the macroscopic transport properties, i.e., the conductivity. With QENS we probe the local dynamics directly on these length scales and show that the local dynamics can be described by a jump-diffusion type motion in the highly concentrated electrolytes as the first step of the conduction process
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