Thesis presented November 17, 2023

Abstract:
Transforming the world’s energy mix is essential if we are to limit greenhouse gas emissions. In this context, improvements in battery performances appear essential if society is to adapt to this technology and move away from the use of carbon-based resources for transport for example. Lithium-ion batteries in particular face two major challenges: they are required to store more energy, that is, have better gravimetric capacity, and to charge quicker, that is, perform better when kinetics of reactions are fast. Layered materials are good candidates of electrode active materials because they theoretically meet the criterion of fast charging performances. In addition, increasing the nickel content in LiNi_xMn_yCo_zO₂ (NMC_xyz), a family of layered materials, meets the criterion of improving the capacity of the limiting electrode, the positive electrode. Whether for this family of materials or for graphite, which is also layered, the evolution of the material structure during lithium insertion is still debated. Furthermore, making electrodes thicker is another way to increase their energy density. This strategy is not yet fully practical because of mechanisms that limit the full use of the electrode capacity when increasing the (dis)-charge rates. These limitations are the result of the reduction in reaction kinetics by impeded lithium transport in a mixed liquid/solid medium. These phenomena are exacerbated by the use of thicker electrodes.
In this context, the aim of our work is to resolve the mechanisms of the material structure evolution, and of the limitations of the electrode use for different layered materials. We have explored three approaches: fine characterisation of the lithiation mechanisms of NMCs, including a critical study of operando X-ray diffraction (XRD) measurement methods; characterisation of the limitations across the thickness of NMC, graphite and graphite/silicon electrodes; and the impact of ageing on these properties. Using operando X-ray diffraction studies, we have shown that the evolution of the structure and stresses in all NMCs, and more generally in layered transition metal oxides, can be simplified into a single mechanism, from which degradation follows. The observation of this mechanism may be biased by experimental conditions, that we have quantified and explained. Furthermore, we observed that the kinetic limitations already amplified by the increase of electrode thickness, are worsened by faster charge rates as expected. We also measured that these limitations are ten times more pronounced in a graphite electrode than in a NMC electrode. Finally, imperfect in-plane behaviour of aged electrodes was measured, corroborating the previous results.

Keywords:
Heterogeneities, Intercalation, X-ray diffraction, Batteries, NMC, Mechanism