Atomic-Scale In-Situ Tem Study on the Phase Transitions during Rapid Solidification and Li-Ion Battery Operation

Phase transitions in nanomaterials are the basis for their broad applications. However, due to the short length and fast kinetics at the nanoscale, gaining a mechanistic understanding of such transient processes is extremely challenging. In this dissertation, in-situ transmission electron microscopy (TEM) studies have been performed to reveal the atomic-scale processes during vitrification of metallic liquids and to uncover the reaction and degradation mechanisms in lithium-ion battery electrodes. It has been a long-standing goal for scientists to vitrify single-element metallic liquids. Here, we report an experimental approach that successfully vitrifies melts of pure refractory body-centered cubic metals by achieving an unprecedented high liquid quenching rate of 10^14 Ks^−1. The availability of monatomic metallic glasses being the simplest glass formers offers unique possibilities to study the structure-property relationships of glasses. Distinctive tendencies towards shear localization have been observed in sub-100-nm metallic glasses, which may shed light on the relationship between atomic structure and mechanical property of metallic glasses. Phase transitions in anode materials during battery operation often induce large volume change and pulverization. By building a nanobattery inside the TEM, the plasticity and strain accommodation in one-dimensional anode materials during lithiation/delithiation were, for the first time, visualized under atomic-scale resolution. Lithiation of SnO2 nanowires was initiated by preferred lithium insertion along the (020) plane, which developed into multiple reaction fronts where stress-driven dislocation plasticity was found to be a precursor towards solid-state amorphization. Revealing the thermodynamics and kinetics in intercalation compounds has been technically challenging, due to the subtle structural changes associated with lithium intercalation/extraction. By tracking the evolution in the electron diffraction pattern of a multi-particle system consisting of 200-300 nanoparticles, we found that the lithiation of anatase TiO2, previously believed to be biphasic, switches to a single-phase reaction with a high lithiation rate up to 60 C when the crystal size decreases to 20 nm. This dissertation provides a novel non-equilibrium processing methodology for investigating the fast kinetics and structures of supercooled liquids under deep quench, and advances the fundamental understanding of the mechanical degradation and the size-dependent kinetics and thermodynamics in nanostructured electrode materials