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 reactions disturbances, hence explaining why it is frequently overlooked. Herein we show that this is no longer true by using advanced Electrochemical Quartz Microbalance (EQCM) based method in the form of ac-electrogravimetry¹. For proof of concept, we have studied the effect various solvent/salt mixtures, differing in their dipole moment and size/weight respectively, on the structure of the EDL forming at the surface of MoO₃. We showed that a significant amount of solvated lithium and anions contributed to charge compensation at the interface together with the primary contribution from bare lithium in the most polar solvent used (Propylene carbonate) combined with LiClO₄ salt. The amount of solvated lithium is significantly decreased when the solvent polarity is reduced by switching from an ethylene carbonate and dimethyl carbonate mixture (EC:DMC) to DMC; a solid experimental proof of the direct relationship between the solvation shell and solvent polarity. Moreover, we demonstrated a disappearance of the anionic response 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 in addition to classical EQCM are proven to be an elegant method to experimentally assess the chemical structure of the electrical double layer and we hope that the community will start to adopt it to better engineer interfaces of electrochemical energy storage devices.

1. Gabrielli, C., Garcia-Jareño, J. J., Keddam, M., Perrot, H. & Vicente, F. Ac-Electrogravimetry Study of Electroactive Thin Films. II. Application to Polypyrrole. J. Phys. Chem. B 106, 3192–3201 (2002).