Characterising Lithium-Ion Electrolytes via Operando Raman Microspectroscopy

Abstract

<div><div><div><p>Knowledge of electrolyte transport and thermodynamic properties in Li-ion and ”beyond Li-ion” technologies is vital for their continued development and success. Here, we present a method for fully characterising electrolyte systems. By measuring the electrolyte concentration gradient over time via operando Raman microspectroscopy, in tandem with potentiostatic electrochemical impedance spectroscopy, the Fickian ”apparent” diffusion coefficient, transference number, thermodynamic factor, ionic conductivity and resistance of charge-transfer were quantified within a single experimental setup. Using lithium bis(fluorosulfonyl)imide (LiFSI) in tetraglyme (G4) as a model system, our study provides a visualisation of the electrolyte concentration gradient; a method for determining key electrolyte properties, and a necessary technique for correlating intermolecular electrolyte structure with the described transport and thermodynamic properties.</p></div></div></div>

Three-dimensional operando optical imaging of single particle and electrolyte heterogeneities inside Li-ion batteries

“…The concentration gradient we observe extends as far as 1.5 μm. This in agreement with previous theoretical and experimental studies which have shown electrolyte concentration gradients around electrode interfaces extending between 500 nm 84 and 10s μm 74,79,85 depending on the electrode, electrolyte composition (mass to volume ratio) and cycling conditions 86,87 .…”

“…A gradient in the 2PEF intensity also becomes present above potentials of 4.0 V around LCO particles. This 2PEF/electrolyte concentration gradient, which decays quasi homogeneously away from particles, originates from the difference between the rate of delithiation at the LCO surface and the rate at which PF6anions diffuse towards the LCO particles [78][79][80][81] to balance Li + ions released and maintain electroneutraility 74,82,83 . Furthermore, at all potentials the 2PEF is relatively uniform in regions not containing LCO (bottom row) and upon discharge, the concentration gradient around particles disappears.…”

Complete characterization of a lithium battery electrolyte using a combination of electrophoretic NMR and electrochemical methods

“…S2.1 (ESI †) differ substantially from the measurements of Fawdon et al, who determined D from time resolved Raman microscopy. 36 Such discrepancies are often found in the literature. 6,37,38 Further work is needed to resolve such discrepancies.…”

Characterising the Ionic Transport and Thermodynamic Properties of Potassium-ion Electrolytes

“…[17][18][19][20][21][22][23] Spectroscopic techniques, capable of determining these properties for Li-ion electrolytes through direct visualisation of concentration gradients, have also recently been developed. These include X-ray spectroscopy, 24 Raman spectroscopy, 25 or magnetic resonance imaging (MRI). 26,27 However, there is currently no study which has fully characterised the ionic transport and thermodynamic properties for nonaqueous K-ion electrolytes.…”

“…The trends match those found for other nonaqueous electrolytes. 16,17,25,38 With increasing concentration, first coulombic ion-ion interactions decrease the salt free energy relative to the DME, reducing the salt activity coefficient and hence causing a drop in χ M . As the concentration increases further, ion-solvent interactions increase, resulting in DME being increasingly bound, decreasing solvent vapour pressure, hence increasing the salt activity coefficient and χ M .…”