UCLA

Electrochemical energy storage devices with both high energy density and high power density are technologically necessary for the electrification of transportation and many other applications. This dissertation focuses on the development of fast-charging Li-ion batteries through a fundamental understanding of structural parameters that allow for fast and reversible redox reactions in electrode materials. Using solution-based routes, the atomic and nanoscale structure of electrode materials was controlled and connected to their electrochemical characteristics, including specific capacity, rate capability, and cycling longevity. Additionally, advanced in operando characterization techniques were employed to probe how materials' structure and properties evolve during cycling. Overall, these findings provide guiding principles for the design of fast-charging electrode materials going forward. The first two sections focus on size-dependent phase transition behavior in MoO2, a model tunnel structure anode (Chapters 2 and 3). Chapter 2 shows how the large first-order phase transition in bulk MoO2 becomes systematically suppressed in a series of size controlled nanoarchitectures. The phase transition remains first-order, but shrinks dramatically, in intermediate-sized nanoporous MoO2 and becomes entirely continuous solid-solution in smaller MoO2 nanocrystals. Accordingly, the signatures of slowed charge storage from the bulk phase transition disappear in the nanomaterials. In chapter 3, we employ this suite of materials to show that this change phase transition behavior is key to the development of pseudocapacitive properties in nanoscale MoO2 using electrochemical impedance spectroscopy. Chapters 4 and 5 characterize the charge storage properties of V9Mo6O40, which transforms into a disordered rock salt structure during cycling. In chapter 4, the crystal structure of V9Mo6O40 is shown to govern its transformation pathway and resulting morphology compared to a well-studied analog, V2O5, while chapter 5 highlights the role of optimized Li+ diffusion distances to realize fast-charging using nanoporous V9Mo6O40. Chapter 6 details the use of a newly developed technique to measure the insulating to conductive transition in two high rate anode materials, T-Nb2O5 and Nb18W16O93. The rate of the increase in conductivity is shown to explain the difference in performance. Finally, Chapter 7 focuses on synthesis and characterization of (W0.2V0.8)3O7, a unique V-based Wadsley-Roth shear structure that shows high rate capability.