Paul Rudnicki PhD Thesis Defense

Paul Rudnicki
PhD Candidate
Chemical Engineering
Academic advisor: Professor Jian Qin

Abstract: Morphology, stability, and kinetics at the lithium-electrolyte interface

Batteries incorporating a lithium metal anode can reach much higher energy densities than today’s dominant lithium-ion battery chemistry, enabling the electrification of high-value transport applications. However, safety and cycle life issues persist due to heterogeneous, high surface area lithium deposits which form at the lithium-electrolyte interface through a complex set of electrochemical, mechanical, and transport-based processes. This dissertation describes work using both theoretical and simulation-based approaches toward better understanding and control of the evolution of this critical interface.

First, we developed a coarse-grained Brownian dynamics simulation to model the evolution of lithium morphology during charging in the presence of a thin polymer coating. We created a threedimensional simulation of lithium deposition incorporating a morphology-dependent electric potential field, a physical effect known to encourage heterogeneous, dendritic morphologies. In accord with experimental findings, we showed that a qualitatively viscoelastic polymer coating, which displays both mechanical strength and flowability, outperformed both purely liquid-like and maximally stiff, solid-like polymers.

Next, we extended a field theoretical approach to calculate the free energy landscape for the lithium metal electrodeposition reaction at the electrode-electrolyte interface, accounting for the orientational and electronic polarization of a mixed-solvent electrolyte. Free energy curves for the initial and final states along a reaction coordinate allowed direct evaluation of the reorganization energy and activation free energy which govern the reaction kinetics.

Finally, we developed the first linear stability analysis of lithium deposition through a polymer electrolyte to include the coupled effects of ion transport, elastic mechanical response, and Marcus kinetics. This technique determines whether small-scale, randomized disturbances at the lithiumelectrolyte interface will grow into large-scale heterogeneous morphologies. Our model predicted a novel stability regime for polymer electrolytes with moderate shear modulus and high partial molar volume of lithium ions. This emphasized the underappreciated importance of ionic partial molar volume and can lead to the design of novel electrolyte materials for more stable lithium metal batteries. Control of the lithium-electrolyte interface is crucial for development of reliable, safe lithium metal batteries. In this dissertation, we present several studies which generated novel understanding of interfacial evolution and behavior during charging, which can serve as a guide for design of high-performance electrolytes and coatings for lithium metal batteries.