Cédric Leau, PhD student in materials chemistry

Your thesis concerns the development of a system for observing chemical reactions inside batteries. Didn't we know what was going on?

We know how batteries work in general. A battery consists of two electrodes. They are like sponges for lithium, passing from one to the other under the effect of an electric current during charging and discharging. While this basic principle is well known, many other chemical phenomena still elude us. We have an idea of what they are; some measurements are carried out a posteriori, by opening the batteries. The novelty of my work is to develop a procedure we call operando, under operational conditions. The idea is to get as close as possible to reality with a minimally invasive system. This will enable us to know in real time what's going on inside the batteries. We hope this will improve their lifespan.

Your method uses an optical fiber. How does it work?

Optical fibers can be found just about everywhere in everyday life, mainly in telecommunications. It's all about communicating with light. An optical fiber is a long glass wire designed to guide a beam of light. As part of my thesis project, we're trying to work with a special optical fiber that is transparent only in the infrared range, which is light invisible to the naked eye. This infrared light has the particularity of being easily absorbed by the organic molecules present in batteries. The advantage of this light is that it can be used for spectroscopic purposes: we obtain a profile of the variation in light intensity as a function of the interaction of this light with its surrounding environment and the matter through which it passes. This profile gives us an indication of the chemical processes underway. The difficulty lies in finding the right optical fiber that does not absorb these infrared rays, hence the particularity of the one we are using. It has been developed by the Glass and Ceramics team at the Institute of Chemical Sciences in Rennes, with whom we work.

What measurements can you perform?

One of the first measurements made possible by this technique concerns the electrolyte, the liquid present in batteries that enables lithium to be transported from one electrode to the other. For example, we can see the evolution of lithium concentration in this electrolyte. While it's relatively simple to measure a liquid that surrounds the fiber, it becomes more complicated for solid materials present within the battery. This is what I'm trying to achieve in my research.

How can this method help improve commercial batteries?

The aim of research is to understand the world better, which can then lead to doing things better. While in the short term we're a long way from being able to deploy this method in batteries available to the general public, it can guide the design of future commercial batteries. The team's publication in Nature Energy at the end of 2022 concerned the electrolyte, which I mentioned earlier. The work described in this paper showed the disappearance of a certain chemical species, even though it was originally present in the electrolyte. We knew that it was consumed over time, but this is the first time we've been able to measure this evolution live.

One of the keys to battery performance lies in the so-called "SEI", the solid electrolyte interface. This is a solid layer that forms inside the lithium battery when we start using it. The proper formation of this solid layer acts as a protective barrier, separating the electrolyte from the electrodes. This prevents numerous parasitic reactions during future use, guaranteeing better battery performance. The chemical species I was talking about contributes to the quality of this layer. A precise understanding of its consumption enables us to better understand the formation of this layer and to build it up in the best possible conditions, thus improving battery life.

What's a typical day like for you?

Like many researchers, there's no such thing as a typical day - that's what makes research so interesting! However, you could divide my work into different stages. In all cases, I assemble the batteries from the two electrodes and the electrolyte. All the products we use are, for the most part, sensitive to water and sometimes to air. We work in a glovebox, to ensure a controlled environment. My project is to bring together the world of chemistry and materials with that of optics. So I have to implement an optical fiber in this battery cell assembly. This is at the heart of my work: determining the best way of mounting a battery around an optical fiber in an experimental set-up that can then be applied to a real battery. This is a major challenge for my project. Finally, I "cycle" the assembly I've created. These are walls with numerous slots to which I connect these cells in various forms, depending on what we're working on. This involves charging and discharging them at varying speeds to take measurements. These provide us with electrochemical charging and discharging data from which we can derive information. For my part, I "cycle" them alongside my spectrometer to take spectroscopic measurements, which involve evaluating the variation in the light signal entering and leaving the fiber. All these measurements, whether optical or electrochemical, then have to be put into dialogue.

What kind of studies are needed to work in a field that combines disciplines as diverse as chemistry and optics?

I didn't come from a purely chemical background. I studied at the École supérieure de physique et de chimie industrielles de la ville de Paris (ESPCI Paris - PSL), an engineering school specializing in industrial chemistry and physics with a strong experimental component. This allows me to bring these two fields together in my thesis. Although in the laboratory we mainly use chemistry, I need to have some knowledge of physical optics. What's more, although at the Collège de France we do fundamental research, my field can have direct industrial applications. So this engineering course is very useful for me. The ESPCI's vocation is to train engineers for industrial research and development. What sets it apart is that the majority of its students go on to do doctoral theses. This is an exception among French engineering schools. The scientific training is very solid and the lectures are close to the laboratories with which we interact.

What do you intend to do after your doctorate?

I'm working in a field which, although it's fundamental research, is still applied. I'm not yet sure which would interest me most. There's so much to be done in the two worlds of academia and industry! With a subject like mine, I have a foot in both worlds. We know that the needs for batteries are going to be huge in the years to come. Fundamental research as well as industrial research and development will play a major role in this energy transition.

Cédric Leau works in the Chemistry of Materials and Energy Laboratory, under the supervision of Prof. Jean-Marie Tarascon. His thesis is entitled "Détection infrarouge operando via fibre optique de l'évolution de la chimie de la batterie".

Photos © Patrick Imbert
Interview by Aurèle Méthivier