Fiber optics for electric batteries

Every two weeks, a current scientific topic is explored by a researcher from the Collège de France.

In a world where electric mobility is developing rapidly, improving electric batteries is becoming crucial. Despite some promising avenues, the technology remains similar to that which prevailed several decades ago. A finer understanding of the chemical mechanisms within batteries opens the way to greater durability and efficiency.
Interview with Charlotte ^{Gervillié-Mouravieff*}, materials chemistry researcher at the Collège de France.

Chemistry lies at the heart of lithium-ion batteries, the model most widely used and mastered worldwide. These batteries consist of two electrodes that behave like sponges. They are capable of " absorbing " and " releasing " lithium ions, enabling them to store electricity. To do this, they are bathed in a liquid called electrolyte, which allows lithium ions to circulate between the two electrodes. Although this basic principle is well known, there are many other chemical phenomena between the electrodes and the electrolyte that still elude us. In fact, once closed and in operation, the chemical reactions inside batteries are difficult to observe directly. Yet these reactions influence battery performance and longevity." It remains crucial to improve our understanding of the chemical reactions taking place inside batteries to maximize their efficiency ", says chemist Charlotte Gervillié-Mouravieff. In fact, a better understanding of these chemical phenomena will enable us to pursue new avenues of research, such as the rapid development of new cobalt-free electrode materials, or the replacement of lithium ions by sodium ions, which are far more abundant and accessible. The aim is not only to increase battery storage capacity, but above all to solve problems linked to safety, stability and environmental impact. The researcher points out that these " invisible and uncontrolled chemical reactions between the electrolyte and the electrodes are at the origin of certain incidents, which can even lead to fires ". A crucial reminder that safety remains a major issue in this quest for improvement.

Fiber optics at the heart of new solutions

One of the most innovative solutions for better understanding what goes on inside batteries lies in the use of fiber optics. Usually used to transport light over long distances, this technology lends itself to an entirely different application in the battery field, that of capturing chemical changes inside cells in real time." We use sensors on optical fibers to observe phenomena invisible in batteries ", explains Charlotte Gervillié-Mouravieff. Her inspiration comes from methods used in medicine, where sensors are used to monitor vital information in the human body. Applied to batteries, fiber optics could detect the formation of passivation layers during the first charging cycles, a critical stage in a battery's life cycle. This layer, which forms on the negative electrode, can both protect the battery and extend its life, and lead to rapid degradation if it forms unstably.

On the surface, the principle seems simple. Infrared light sent through an optical fiber can interact with the chemical compounds present in the battery. When the light encounters certain molecules, it is partially absorbed. By analyzing the missing wavelengths at the output of the fiber, researchers can identify which compounds are present and how they evolve over time." Many molecules absorb only part of the spectrum. When we recover what's left of our infrared light, the bands that correspond to all the molecules we've come across on our fiber optic path are missing ", summarizes the researcher.

However, this approach is not without its technical challenges. The chemical environments inside batteries are extremely corrosive, and the optical fiber itself can be degraded. The choice of materials for manufacturing the fibers is the subject of close collaboration with the Institut des Sciences Chimiques de Rennes, a laboratory specializing in the synthesis of transparent glass in the infrared range.

The challenges of industrialization

Although battery research is progressing rapidly, the transition to an industrial scale remains complex." In Europe, the battery industry remains underdeveloped. We buy the majority of our batteries abroad ", regrets Charlotte Gervillié-Mouravieff, which slows down the commercial diffusion of new technologies emerging from laboratories.

Yet current research offers encouraging prospects in terms of performance, repairability and recycling. The integration of optical sensors makes it possible to monitor the evolution of battery cells in real time, and even diagnose problems before they become critical. This would not only extend battery life, but also reduce waste by accurately identifying defective cells.

However, this transition to intelligent, repairable batteries will require major investment and increased cooperation between scientific and industrial players. The first applications of these technologies are likely to be in laboratory testing, before they can be deployed commercially. Charlotte Gervillié-Moutavieff remains optimistic :" There is a real trend towards more diagnostics in batteries ". The future of batteries, illuminated by infrared light, may already be taking shape, provided the challenges of industrialization are met.

*Charlotte Gervillié-Mouravieff is a researcher at the Chemistry of Materials and Energy Laboratory at Collège de France, directed by Prof. Jean-Marie Tarascon.