We have seen that optical sensors, based on the use of surface plasmons which are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal-dielectric interface, are sufficiently sensitive to probe chemical phenomena influencing refractive index. This technique also makes it possible to determine the kinetic and thermodynamic data linked to the grafting and deposition of molecules, and thus to monitor the ageing of the electrolytes studied as a function of time.
In terms of the mechanical properties of electrodes, we will demonstrate the benefits of acoustic emission. These properties manifest themselves in the release of energy in the form of transient elastic waves within the material under load. To give an element of comparison, we recall that seismic waves are acoustic waves. The acoustic emission of a material depends not only on its intrinsic characteristics (crystallographic structure, grain size, homogeneity and phase transformations), but also on the stresses to which it is subjected. Acoustic emission is therefore ideally suited to monitoring electrode materials that constantly expand and contract during cycling, leading to cracking or crazing. The application of acoustic emission to batteries will be presented through a number of examples. A distinction will be made between the passive approach, which uses a simple sensor attached to the battery surface, and the active approach, which studies the propagation of an acoustic wave through the battery. In addition, we'll show how acoustic and/or ultrasonic measurements can be used to determine electrolyte dewatering during cycling, enabling monitoring of the battery's state of health. Finally, we will introduce the concept of " electrochemical-acoustic time-of-flight ", one of the latest developments in the field.
