Deciphering the Double-Layer Structure and Dynamics on a Model LixMoO3 Interface by Advanced Electrogravimetric Analysis

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 LixMoO3. 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.

Automated summary

In this study, an advanced ac-electrogravimetry method combined with classical electrochemical quartz crystal microbalance (EQCM) was used to investigate the effect of solvent/salt combinations on the structure of the electrical double layer (EDL) formed at the electrolyte/electrode interface (EEI) of LixMoO3. The results provided solid experimental evidence of the direct relationship between the solvation of ions and solvent polarity. This elegant and powerful method can be adopted to improve the engineering of interfaces in electrochemical energy storage devices.