Concentration Dependent Solution Structure and Transport Mechanism in High Voltage LiTFSI–Adiponitrile Electrolytes
Christopher J. Franko, Chae-Ho Yim, Fabian Årén, Gustav Åvall, Pamela S. Whitfield, Patrik Johansson, Yaser Abu‐Lebdeh, Gillian R. Goward
Abstract
The physiochemical properties of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in adiponitrile (ADN) electrolytes were explored as a function of concentration. The phase diagram and ionic conductivity plots show a distinct relationship between the eutectic composition of the electrolyte and the concentration of maximum ionic conductivity in the 25 °C isotherm. We propose a structure-based explanation for the variation of electrolyte ionic conductivity with LiTFSI concentration, where the eutectic concentration is a transitionary region at which the structure changes from solvated contact ion pairs to extended units of [Li z (ADN) x TFSI y ] z−y aggregates. It is found through diffusion coefficient measurements using pulsed-field gradient (PFG) NMR that both <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>L</mml:mi> <mml:mi>i</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo stretchy="true">/</mml:mo> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>T</mml:mi> <mml:mi>F</mml:mi> <mml:mi>S</mml:mi> <mml:mi>I</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>L</mml:mi> <mml:mi>i</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo stretchy="true">/</mml:mo> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>A</mml:mi> <mml:mi>D</mml:mi> <mml:mi>N</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> increase with concentration until 2.9 M, where after Li + becomes the fastest diffusing species, suggesting that ion hopping becomes the dominant transport mechanism for Li + . Variable diffusion-time (Δ) PFG NMR is used to track this evolution of the ion transport mechanism. A differentiation in Li + transport between the micro and bulk levels that increases with concentration was observed. It is proposed that ion hopping within [Li z (ADN) x TFSI y ] z−y aggregates dominates the micro-scale, while the bulk-scale is governed by vehicular transport. Lastly, we demonstrate that LiTFSI in ADN is a suitable electrolyte system for use in Li-O 2 cells.