A major feature of the electrolyte/electrode interface
(EEI) that
affects charge storage in lithium-ion batteries is the electrical
double layer (EDL), but most of the available experimental approaches
for probing its structuration have limitations due to electrical field
and redox reaction disturbances, hence explaining why it is frequently
overlooked. Herein we show that this is no longer true by using an
advanced electrochemical quartz crystal microbalance (EQCM)-based
method in the form of ac-electrogravimetry. For proof
of concept, we studied the effect of various solvent/salt combinations,
differing in their dipole moment and size/weight, respectively, on
the structure of the EDL forming at the EEI of Li
x
MoO3. We show that a significant amount of solvated
lithium ions and anions contribute to charge compensation at the interface,
and by varying the nature of the solvents (cyclic vs noncyclic), we
provide a solid experimental proof of the direct relationship between
the ions’ solvation and solvent polarity. Moreover, we demonstrated
a disappearance of the anionic motion in the less polar solvent (DMC)
most likely due to plausible formation of contact ion pairs and agglomerates
at the EDL level. Altogether, ac-electrogravimetry,
when combined with classical EQCM, stands as an elegant and powerful
method to experimentally assess the chemical structure and dynamics
of the electrical double layer. We hope that the community will start
to adopt it to better engineer interfaces of electrochemical energy
storage devices.
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