The popularization of Li ion technology, the need to give batteries a second life, and the meteoric rise of connected objects mean that the battery is becoming a key element in our society, the equivalent of the heart in the human body. By analogy with medicine, we need to develop technical solutions to know the state of health (SOH) of the battery at all times, so that we can intervene in the event of failure. One solution, which will be presented, consists of injecting miniaturized sensors, notably optical ones, capable of transmitting/receiving information within the battery, so that it is no longer a black box and we can know its SOH at all times. This lecture will review the configuration of single-mode and multimode optical fibers and how they function as waveguides. The operating principle of optical Bragg grating sensors (FBGs) will then be introduced, and their vast range of applications (aeronautics, rail transport, composite materials, civil engineering, medicine) will be described. We'll also look at how these sensors can be integrated non-intrusively into a battery.
It will be shown that these optical sensors can be used to image cell temperature and detect pressure variations within the battery, as well as the stresses associated with Li intercalation within electrode materials. In addition, the nucleation and growth of the solid electrolyte-electrode interface (SEI) can be monitored during its formation, as well as its dynamic evolution during cycling. In addition, we will detail how, by combining several Bragg sensors, we can access the heat generated by the battery without resorting to conventional isothermal calorimetry, which ignores the battery's specific heat. Having access to this heat allows us to fully parameterize the thermal model. These results are of great practical importance, as they offer a scalable solution for screening electrolyte additives, rapidly identifying better cell formation processes and designing battery thermal management systems with enhanced safety.
