​Guilhem PARADOL
Systèmes Moléculaires et nanoMatériaux pour l'Énergie et la Santé (SyMMES)/ Equipe STEP
​ UMR 5819
 CEA-CNRS-UGA-Grenoble-INP​

The question of energy storage is central to reach carbon neutrality. Li-ion batteries have enabled a significant rise in electric mobility since their introduction in the early 1990s. However, the current and future transformations of our lifestyles demand ever more efficient battery technologies. As a result, the materials currently used in the industry need to be replaced with new materials that are more energy-efficient, cleaner, and less expensive. In this context, silicon appears to be the ideal candidate to replace graphite as the anode material in Li-ion batteries. Upon lithiation, silicon sees its volume increase four times, which leads to particle cracking and pulverization, as well as the disconnection of some particles during contraction in the delithiation phase. Furthermore, the continuous formation of the SEI (Solid Electrolyte Interphase) on the surface of the particles leads to a gradual decline in the anode's performance. Thus, silicon shows poor cycling stability, and the path towards industrial production and commercialization of batteries that fully exploit silicon's potential is still an open research field. Nanostructuration of silicon can prevent particle cracking. As a result, many silicon nanoparticle-based anodes are being developed to improve the cyclability of this technology. This is the case for the anodes presented in this thesis, which were developed by teams at the CEA. I worked on nanoparticles produced at IRAMIS, as well as nanowires developed at IRIG. The performance of these anodes, as established during laboratory experiments, shows promising results. However, the reaction mechanisms of these particles, and the behavior of the anode in which they are assembled, remain poorly understood. To document the existence of intermediate phases of active materials, the stresses and strains on both the global and particle levels, the transformations and heterogeneities at the nanometric scale, and the microstructure and morphology at the component level, we chose to carry out operando characterization of these anodes using small-angle scattering techniques with neutrons and X-rays. Small-angle scattering is a powerful tool for obtaining information on the structure and composition of nanoparticles in a medium. Therefore, this technique is well-suited for studying silicon nanoparticles immersed in a liquid electrolyte, as is the case for the anodes studied in working conditions. Two operando small-angle scattering experiments are presented here. In this thesis, I develop the data processing methods that allowed us to determine the structural evolutions and reaction mechanisms of the studied silicon nanoparticles and nanowires. Operando small-angle neutron scattering enabled detailed study of the behavior during the first cycle of the anode, assembled with silicon nanoparticles as the sole active material. Depending on the electrode’s state of charge, the nanometric morphological evolution of the particles—swelling, SEI formation, activity level—was measured. This allows us to understand the key mechanisms involved in the electrode's performance. To study the performance of multi-component anodes, consisting of silicon nanowires, graphite, and nanoparticles of gold or tin, we conducted a small-angle X-ray scattering experiment simultaneously with a wide-angle scattering measurement. Here again, data processing highlighted the nanostructural evolutions of the particles and its interfaces and allowed us to interpret the various electrochemical measurements obtained in the lab on these materials.

Keywords :

li-ion battery, small angle scattering, silicon, nanostructure