Phase transformations in layered electrode materials for sodium ion batteries

Abstract

In this thesis, I investigate sodium ion intercalation in layered electrode materials for sodium ion batteries. Layered metal oxides have been at the forefront of rechargeable lithium ion battery technology for decades, and are currently the state of the art materials for sodium ion battery cathodes in line for commercialization. Sodium ion intercalated layered oxides exist in several different host phases depending on sodium content and temperature at synthesis. Unlike their lithium ion counterparts, seven first row layered TM oxides can intercalate Na ions reversibly. Their voltage curves indicate significant and numerous reversible phase transformations during electrochemical cycling. These transformations arise from Na-ion vacancy ordering and metal oxide slab glide but are not well understood and difficult to characterize experimentally. In this thesis, I explain the nature of these lattice differences and phase transformations for O and P-type single-transition-metal layered systems with regards to the active ion and transition metal at hand. This thesis first investigates the nature of vacancy ordering within the O3 host lattice framework, which is currently the most widely synthesized framework for sodium ion intercalating oxides. I generate predicted electrochemical voltage curves for each of the Na-ion intercalating layered TM oxides using a high-throughput framework of density functional theory (DFT) calculations and determine a set of vacancy ordered phases appearing as ground states in all NaxMO₂ systems, and investigate the energy effect of stacking of adjacent layers. I also examine the influence of transition metal mixing and transition metal migration on the materials' thermodynamic properties. Recent work has established the P2 framework as a better electrode candidate structure type than O3, because its slightly larger interlayer spacing allows for faster sodium ion diffusion due to lower diffusion barriers. However, little has been resolved in explaining what stabilizing mechanisms allow for the formation of P-type materials and their synthesis. This work therefore also investigates what stabilizes P2, P3 and O3 materials and what makes them synthesizable at given synthesis conditions, both for the optimization of synthesis techniques and for better-guided material design. It is of further interest to understand why some transition metal oxide systems readily form P2 or P3 compounds while others do not. I investigate several possible stabilizing mechanisms that allow P-type layered sodium metal oxides to by synthesized, and relate these to the choice of transition metal in the metal oxide structure. Finally, this work examines the difficulty of sodium ion intercalation into graphite, which is a commonly used anode material for lithium ion batteries, proposing possible reasons for why graphite does not reversibly intercalate sodium ions and why cointercalation with other compounds is unlikely. This thesis concludes that complex stabilizing mechanisms that go beyond simple electrostatics govern the intercalation of sodium ions into layered systems, giving it advantages and disadvantages over lithium ion batteries and outlining design principles to improve full-cell sodium ion battery materials.