Morphological control and multi-length-scale characterization of lithium iron phosphate
Keywords: Materials Science and Engineering.

As the world moves to decarbonize our energy ecosystem, scalable energy storage has emerged as a key technology in the future of the global economy. The two primary use cases are electric transportation and grid-leveling of intermittent renewable energy sources such as wind and solar. Many companies around the world are already producing electric vehicles at scale, with over 8.5\% of all cars sold in 2021 being electric vehicles. This was a tripling of electric vehicle production compared to the year 2020. Battery adoption for stationary storage is less developed. As of 2020, pumped hydro is still the dominant energy storage medium for power grids around the world, accounting for 95\% of the world's storage capacity. However, the strict geographic restrictions on dam construction necessitate the development of an alternative large-scale, carbon-neutral energy storage system. For both of these applications, lithium iron phosphate (LFP) batteries are emerging as a vital technology in the shift towards sustainable energy. Their high rate capability, extended cycling life and low materials cost make them a particularly scalable solution for the global energy storage demand. However, important fundamental questions remain regarding the fundamental materials properties that enable these impressive performance metrics. This dissertation aims to answer some remaining fundamental questions regarding structural, thermodynamic, and kinetic properties of LFP. My approach is centered around careful morphological control enabling advanced characterization methods. I will begin by discussing the techniques that will be used in this investigation. The unique requirements of these methods guide our model system choice, particularly regarding morphological parameters. Next, I will demonstrate the method by which these specific morphologies are synthesized and discuss the scientific implications of our chosen model systems. The first fundamental property I will investigate is the chemo-mechanical relationship between lithium content in LFP and the lattice parameters. This is difficult to measure at intermediate compositions due to the phase separation behavior of LFP. Next, I will provide a study of the tracer diffusion mechanism in LFP. I demonstrate that isotopic movement in LFP does not follow Fickian kinetics and provide the mathematical justification behind its non-standard behavior. Lastly, I will investigate the mechanism behind a recently discovered "pulse activation" effect observed in LFP, wherein applying a short burst of high current to an LFP electrode leads to increased kinetic performance for a significant time following the pulse. While electrode kinetics have been extensively studied under galvanostatic conditions, I perform these analyses for the first time on the transient effects induced by current pulses. This last aspect of my research is particularly relevant towards the practical application of lithium-ion batteries. In realistic operation, the cycling profile of batteries is not easily controlled, particularly in discharge. Therefore, understanding the transient response of the electrodes to changes in applied current is vital to optimal battery control. In summary, I aim to help to deepen our understanding of the kinetics and thermodynamics of LFP during (de)lithiation, fundamental properties that tie closely to the impressive rate capabilities and cycling lifetime of commercial LFP batteries