Characterization of Engineered Complex Cathode Materials for Li-ion Batteries

Abstract

Lithium-ion batteries have become a vital part of our modern life and play an essential role in electric vehicle development. One of the most feasible strategies to enhance the energy density of Li-ion batteries is to use layered, Ni-rich cathode materials. However, higher nickel content causes several problems and therefore, several methods, including doping and coating, have been utilized to stabilize their structure and boost their performance. This thesis aims to understand the microstructure of such engineered complex cathodes and provide valuable contributions by comprehensively understanding and establishing a link between the composition, structure, performance, and properties of these complex materials. In this regard, the most advanced electron- and photon-based techniques have been used to uncover the fundamental underlying reasons for the enhanced performance or degradation in these complex cathode structures. This study shows that introducing W cation inside the LiNiO2 results in new W-variants with a heterogeneous concentration on the top surface and through grain boundaries of the host secondary particles. These W-rich regions play a reinforcing role in grain boundaries and protect the outer surface of LiNiO2 particles. However, synthesis defects, such as porosities, could reduce these benefits by increasing the electrolyte infiltration inside the cathode particles. It is also demonstrated that the degradation process can be studied through the changes in electron energy loss near-edge structure spectra. The investigation of a coating approach on LiNi0.8Co0.15Al0.05O2 materials through the mechanofusion process illustrates more microscopic-scale details regarding the thickness unevenness of the coating and some degree of physical intermixing between the core (LiNi0.8Co0.15Al0.05O2) and coating (LiFePO4 and alumina) precursors. In addition to good physical contact between the core and coating materials, further analysis at higher resolution reveals some nanoscale grains and defective areas near the top surface of the secondary particles following the mechanofusion coating process.