Atomic-scale direct observation of phase transformations in layered oxides by STEM

Abstract

The properties of materials applied in various industries are closely related to the phase formed by crystallization of each material. Therefore, it is very important to obtain a desired phase in the crystallization process in order to exhibit the properties of a suitable material. Even if a material having the properties necessary for a product is synthesized, a phase change may occur during the manufacturing process and use of the product due to changes in external/internal environment, external impact and chemical/electrochemical reactions. This leads to degradation of the product as it exhibits properties different from those of original materials. By using transmission electron microscope (TEM) analysis in this thesis, we studied the change of physical and chemical properties of cathode materials which used as lithium or zinc ion batteries. Understanding the atomic structure variation at the surface of electrode materials in contact with an electrolyte is an essential step toward achieving better electrochemical performance of rechargeable cells. Different types of water-based aqueous solutions have been suggested as alternative electrolytes to the currently used flammable organic solvents in Li-ion batteries. However, most research on aqueous rechargeable Li-ion cells has largely focused on the synthetic processing of materials and resulting electrochemical properties rather than in-depth atomic-level observation on the electrode surface where the initial charge transfer and the (de)intercalation reaction take place. By using LiFePO$_4$ and LiCoO$_2$ single crystals, we identify serious P and Co dissolution from LiFePO$_4$ and LiCoO$_2$ into aqueous solutions without any electrochemical cycling. Furthermore, both strong temperature-dependent behavior of P dissolution in LiFePO$_4$ and very unusual occupancy of Co in the tetrahedral interstices in LiCoO$_2$ are directly demonstrated via atomic-scale (scanning) transmission electron microscopy. Ab initio density functional theory calculations also reveal that this tetrahedral-site occupation is stabilized when cation vacancies are simultaneously present in both Li and Co sites. The findings in this work emphasize the significance of direct observation on the atomic structure variation and local stability of the cathode materials. An initial crystalline phase can transform into another phases as cations are electrochemically inserted into its lattice. Precise identification of phase evolution at an atomic level during transformation is thus the very first step to comprehensively understand the cation insertion behavior and subsequently achieve much higher storage capacity in rechargeable cells, although it is sometimes challenging. By intensively using atomic-column-resolved scanning transmission electron microscopy, we directly visualize the simultaneous intercalation of both H$_2$O and Zn during discharge of Zn ions into a V$_2$O$_5$ cathode with an aqueous electrolyte. In particular, when further Zn insertion proceeds, multiple intermediate phases, which are not identified by a macroscopic powder diffraction method, are clearly imaged at an atomic scale, showing structurally topotactic correlation between the phases. The findings in this work suggest that smooth multiphase evolution with a low transition barrier is significantly related to the high capacity of oxide cathodes for aqueous rechargeable cells, where the crystal structure of cathode materials after discharge differs from the initial crystalline state in general.