Optical sensors such as FBGs enable rapid sieving for the production of optimized electrolytes, and also provide access to the battery's thermodynamic parameters, enabling us to monitor its ageing and, therefore, its state of health. However, they cannot provide information on the nature of decomposition products associated with parasitic reactions that are detrimental to battery life. To overcome this limitation and gain access to the battery's chemistry, guided light must be allowed to interact with the electrolyte. This has led to the development of numerous optical sensors based on the general concept of evanescent waves. Long-pitch Bragg gratings (LPGs) and tilted Bragg gratings have been developed to track the variation in refractive index of the medium surrounding the fiber, and thus the concentration ratio of the components making up the electrolyte. One way of taking greater advantage of evanescent waves is to use unclad optical fibers (direct contact between the fiber core and electrolyte) and simply measure the attenuation of the optical signal by transmission (a technique known as FOEWS). Based on the same principle of evanescent waves, the IR technique makes it possible to identify the nature and composition of chemical species in electrolytes, with the sine qua non condition that the optical fiber must transmit in the mid-infrared range extending up to 20 mm, which is why we will be using chalcogenide fibers rather than silica. The longer and thinner the detection zone, the more sensitive such measurements will be.
Finally, new trends in probing the surrounding chemistry are based on the use of the plasmonic effect via the deposition of a nanometric layer of gold on inclined Bragg gratings or fibers without optical cladding. All these optical sensors will be described using examples mainly dedicated to batteries.
