As the world's need for energy storage grows at an accelerated pace, researchers are looking for new strategies to take batteries to the next level of performance, both in terms of capacity and durability, while improving safety. The focus has been on the discovery of new materials but also on the development of a wide range of sophisticated diagnostic techniques. This allows on the one hand to anticipate their behavior in the presence of electrolytes in various electrochemical devices and on the other hand to study the processes leading to the evolution of interfaces. Mastering the interface processes is a real challenge in electrochemistry, and it is crucial for the development of electrochemical devices.

One strategy to overcome this challenge is the implementation of sensing technologies (optical¹, acoustic²...) providing a real-time monitoring of the interfacial processes occurring in a battery. In this regard, interest in piezoelectric sensors employed in junction with electrochemical analysis, has also been growing steadily in the energy storage community.^3,4 In our group, we aim at describing this complex interface by operando piezoelectric sensors, during electrochemical cycling of battery electrodes. This methodology, commonly known as Electrochemical Quartz Crystal Microbalance (EQCM) based interface analysis,^4-6 is able to provide vital understanding of the positional cohabitation of ions and solvent molecules within the electrical double layer (EDL), which itself conveniently falls in the penetration depth of the acoustic shear wave emanating from the QCM sensor.

In this contribution, we will describe our piezoelectric sensing strategy to investigate the impact of electrolyte composition on the electrochemical performance. The objective is to establish an interface model for different electrolyte compositions, via a study of the various solvent/salt mixtures, differing in their dipole moment and size/weight, respectively. Our experimental methodology lifts the challenge in the probing the EDL’s composition and structuration: specifically, accessing to the identity and the repartition profile of the charge carriers, as well as the role of solvation. Our results exemplified for both Li and Na ion cells,^5,6 will be complemented by the recent fiber optic based sensing methodology,^6,7 which provides the information on the chemical evolution of this interface.