AbstractsEngineering

Many efforts have been focused on reducing the rate at which the batteries degrade, i.e., the loss of capacity and power over time. Among the many causes of reduced capacity and power, the instability of the electrode/electrolyte interface has emerged as one of the most prominent issues, but, at the same time, it is likely the least understood issue. This instability is mostly associated with chemical and mechanical degradation processes of electrode/electrolyte interfaces. The aim of this dissertation is to elucidate the mechanisms by which degradation occurs at the electrode/electrolyte interface by evaluating the changes in the properties of the interface and correlating these changes with the capacity and power fade of Li-ion batteries. Various types of cells and experimental methodologies have been developed, and the combination of several analytical techniques has been utilized to obtain a conclusive understanding of the changes that occur in the electrode/electrolyte interface. As a complementary tool, atomistic-scale simulations including molecular dynamics (MD) and density functional theory (DFT) have been used for better understanding and explanation what we have observed. This dissertation addresses the chemical and mechanical degradation occurring at the graphite/electrolyte and LiMn2O4/electrolyte interfaces. First, degradation mechanisms of the solid electrolyte interphase (SEI) induced by elevated temperatures and dissolved Mn-ions were identified (Chapter 2 and 3). Second, the mechanical aspect of the SEI layer was investigated by evaluating its elastic modulus computationally and experimentally (Chapter 4). Third, the effects of fluoroethylene carbonate (FEC) were investigated and the associated mechanisms were identified (Chapter 5). Fourth, the influence of dissolved manganese ions on the structural degradation of graphite was investigated (Chapter 6). The findings from this research can provide a fundamental understanding of chemical and physical processes that underlie the degradation of the SEI layer and concomitant other degradation phenomena. Such an understanding is essential to gain insights into strategies for controlling or optimizing the properties of the SEI to provide the mechanical and chemical stability of the electrode/electrolyte interface.