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Mixed ionic and electronic conductor coatings for advanced electrode architectures in high performance Li-ion cells

Boek - Dissertatie

The climate crisis is necessitating an energy transition in which Li-ion batteries play a vital role by increasing the flexibility of the energy system. While Li-ion batteries have been commercialized for close to 30 years, increases in the energy and power density are still actively being pursued. A multitude of issues persist, such as intrinsic electrode instability, electrolyte decomposition, and transition metal dissolution. Studying these phenomena in the commonly used particle-based composite electrodes, containing active material along with carbon additives and binders, is no easy task, due to the expansive parameter space associated with these systems.In this thesis, a thin-film model system is shown to be a valuable tool to study both intrinsic electrode processes and interactions at the electrode-electrolyte interface. Thin-film electrodes allow good control over the electrode morphology, and remove the need for additives. Furthermore, it allows mature characterization techniques from the semiconductor industry to readily be transferred to Li-ion battery research, which can give valuable complementary information with respect to the electrochemical measurements.The thesis can be subdivided into two parts: first single electrode materials were studied, wherein the focus lies on the insertion process and interaction with the electrolyte solution. In the first research chapter the interaction between thin-film LiMn2O4 (LMO) and two electrolyte solutions (LiClO4 in PC and LiPF6 in EC:DMC) was studied. A clear picture of the interaction is obtained by combining electrochemical measurements with ERD and XPS. In the subsequent chapter the effect of conversion and alloying of indium tin oxide (ITO) on the film morphology, electronic properties, and structure was studied. AFM, C-AFM, and BSEM are demonstrated to give valuable information on the effect of the mechanisms associated with the conversion and alloying reactions. In the third research chapter, the effect of extensive chlorine modification on the insertion process in amorphous TiO2 was studied. The precise control of the electrode volume in combination with the use of ToF-SIMS enables determination of a reaction mechanism introduced by the extensive chlorine incorporation.In the second part, ITO, TiO2, and chlorine modified TiO2 were deposited on top of LMO as mixed ion and electron conductor coatings. The effect of highly conductive ITO coatings was studied using potentiostatic intermittent titration technique in the fourth chapter. Herein, the importance and effect of electronic junctions on the protective properties of coating material was demonstrated. In the final research chapter, the process of ALD of TiO2 and TiO2:Cl on LMO was studied using a combination of RBS and XPS. A mechanisms for the non-ideal deposition is proposed, and a comparative study between the two types of coatings was conducted.The ability to study different insertion mechanisms in Li-ion electrodes, interactions with the electrolyte, and the effect of coatings renders the thin-film electrode a valuable research tool. Transfer of some methodology to another material will be demonstrated in the conclusions. However, valuable insights obtained using the thin-film system need to be transferred to composite systems before these can be used in commercial batteries. Some barriers to transferring the insights obtained in thin-film electrode to particle-based system will be described in the concluding remarks.
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