Grensvlakreacties aan nikkelrijke kathodes in Li-ionbatterijen: inzichten vanuit dunnefilmelektroden als model

The lithium-ion battery has revolutionized the modern world and has become an essential part of our everyday lives. From mobile phones and laptops to electric vehicles, the lithium-ion battery has enabled massive technological advancements in the past 30 years. These batteries can also be combined with renewable energy sources, such as wind and solar, to limit our dependence on fossil fuels. However, the constant societal demand for increasing energy necessitates the development of batteries with higher capacities. One method to increase the capacity of Li-ion batteries is to increase the nickel content in the positive electrode. This led to the development of the LiNiO2 electrode. While this material has a high capacity and energy density, it suffers from multiple stability issues that ultimately result in the degradation of the material. To avoid this, some of the nickel is substituted with cobalt and manganese to form Li(NixMnyCoz)O2 (more commonly called NMCxyz) where x + y + z = 1. However, as the goal remains to achieve the maximum capacity out of the material, the nickel content is being incrementally increased to meet societal demands. However, as the nickel content increases, the stability of the material decreases, resulting in capacity loss over multiple cycles. It is also incompatible with aqueous processing due to the dissolution of active materials. To overcome the stability limitations of nickel-rich NMC materials, current studies focus on investigating their interfacial behavior. However, the complexity of the electrode configurations can make studying these processes difficult. In this thesis, we utilized a thin-film model system to simplify the electrode geometry and composition. This allowed for the precise investigation of the interfacial reactions occurring on the NMC electrode. In addition, it enables characterization techniques from the semiconductor industry to be used to gain valuable insights into the interfacial reactions occurring on lithium-ion battery materials. In the first part of this thesis, we developed the NMC thin-film system. During this process, we have found multiple side reactions between the NMC and substrate during the annealing procedure. These reactions were investigated using SEM, ERD, and electrochemical measurements, and strategies were implemented to compensate for and mitigate the unwanted reactions. Once the thin-film was developed we used this film in the next part of this thesis, to study the interfacial reactions occurring on NMC when it is exposed to air. A combination of ERD, XPS, and SEM was employed to achieve this goal. These results were combined with thermodynamic data to propose a mechanism of the reactions occurring on NMC during storage in a humid environment. Finally, the thin-film was used to study the reaction between NMC and water to determine whether there is an electrochemical component or if the reaction is purely chemical. This setup mimics the reactions that occur on the surface of an NMC material during aqueous processing. The thin-film allowed for the investigation of the chemical and electrochemical reactions occurring on electrode materials without the influence of binders and carbon additives. As these reactions primarily happen on the uppermost surface of the electrode, they are often not immediately detected on particle-based systems. Therefore, the thin-film system can be an invaluable tool for studying surface reactions in battery materials and testing potential solution strategies before implementing them into a particle-based electrode system. The concluding chapter discusses potential pathways that future research can take.