Understanding structure property relationships in niobium–based oxides for high-rate anodes

Abstract

With the growing usage of portable electronic devices, electric vehicles, and grid level storage, a diverse set of energy storage devices is required for each application. Current commercial level lithium-ion batteries commonly utilize graphite as the anode material. While graphite possesses impressive energy storage, graphite struggles with high (dis)charge applications. One class of materials of interest to replace graphite are niobium-based oxides, some of which fall into a group of materials called Wadsley-Roth crystallographic shear compounds. Wadsley-Roth (W-R) compounds possess unit cells with nxm blocks of edge-shared octahedra, which boast high-rate capabilities, having higher volumetric capacities than graphite at various (dis)charge rates. While various (W-R) compositions of have been synthesized and their electrochemical properties explored, the origin of the excellent rate capabilities and capacities is unclear. Herein, niobium-based anodes for high-rate lithium-ion batteries are investigated to understand the structure-property relationships in W-R materials with different block sizes, levels of disorder, and composition. Additionally, a niobium oxide polymorph falls into a unique class of energy storage materials called pseudocapacitors, which possess high energy density while the charge storage mechanism mimics that of a capacitor. Sections of this work describe current and future investigations of pseudocapacitive niobium oxide to better understand the origin of this interesting material. Chapter 1 begins with a brief introduction, background, and motivation on the need for high-rate, high-capacity anode materials as an alternative for graphite, to address the growing need for high-power, high-energy density materials. Chapter II describes the synthesis of three structurally similar W-R compounds with different block sizes and investigates the electrochemical performance of each material. Chapters III and IV investigate methods to improve the electrochemical performances of W-R compositions through defects and dopants. Chapter V investigates the pseudocapacitive niobium oxide that also exhibits high-rate capabilities through a process called pseudocapacitance, in which the material possesses electrochemical characteristic similar to both batteries and capacitors. In the final chapter, Chapter VI, concludes the dissertation by describing further directions necessary to better understand the structure property relationships resulting in high-rate, high-capacity niobium-based oxide anodes.