De Jesus Baez, Luis Rafael (2018-05). The Secret Lives of Battery Materials: Core-Level Spectroscopy as a Probe of Compositional and Electronic Structure Inhomogeneities. Doctoral Dissertation.
Thesis
The invention of rechargeable batteries has dramatically changed our landscapes and lives, underpinning the explosive worldwide growth of consumer electronics, ushering in an unprecedented era of electric vehicles, and potentially paving the way for a much greener energy future. Unfortunately, current battery technologies suffer from a number of challenges, e.g., capacity loss and failure upon prolonged cycling, limited ion diffusion kinetics, and a rather sparse palette of high-performing electrode materials. This dissertation will focus on elucidation of the influence of electronic structure on intercalation phenomena. Mechanistic understanding of compositional and electronic structure heterogeneities spanning from atomistic to mesoscale dimensions is imperative to facilitate the rational design of novel electrode chemistries and architectures. First, this dissertation provides an introduction to the fundamental science challenges involved in electrode design utilizing Vv2Ov5 as a model system to review means of defining ionic and electronic conduction pathways. Subsequently, the oxidative chemistry of graphite, a canonical anode material, is evaluated with the purpose of understanding the spatial localization and connectivity of functional groups in graphene oxide, which is of utmost relevance to the design of high-performing electrode composites. Furthermore, scanning transmission X-ray microscopy (STXM) observations indicate the formation of lithiation gradients in individual nanowires of layered orthorhombic Vv2Ov5 that arise from electron localization and local structural distortions. Electrons localized in the Vv2Ov5 framework couple to a local structural distortion, giving rise to small polarons, which are observed to be trap Li-ions and are found to represent a major impediment to Li-ion diffusion. In addition, this dissertation presents the first direct visualization of patterns of compositional inhomogeneities within cathode materials during electrochemical discharge. Two distinct patterns are evidenced: core--shell separation and striping modulations of Li-rich and Li-poor domains within individual particles. 3D compositional maps have been developed and translated to stress and strain maps, providing a hitherto unprecedented direct visualization of stress and strain inhomogeneities. Finally, a cluster of interlaced LivxVv2Ov5 nanoparticles is evaluated by scanning transmission X-ray microscopy. Increased heterogeneity at the interface between particles suggests the exchange of Li-ions, implying a "winner-takes-all" behavior (corresponding to particle-by-particle lithiation of an ensemble of particles). Such behavior portends the creation of localized hot-spots and provides insight into a possible origin of failure of Li-ion batteries.
