(Georgia Institute of Technology, 2023-06-07)
Bistri, Donald
Solid-State-Batteries (SSBs) present a promising technology for next-generation batteries due to their superior properties including increased energy density and safer electrolyte design. Traditional SSB-architecture features a Lithium-metal anode, a solid-state composite cathode, and a stiff ceramic electrolyte. While an attractive alternative, commercialization of SSBs faces a series of chemo-mechanical issues across its constituents, namely growth induced fracture of solid-state electrolyte (SSE) due to metal deposition, interphase formation at the anode/SSE interface and damage of the various phases in composite electrodes. Computational modeling for SSBs is at its infancy and constitutes the focus of this thesis, with an emphasis on mechanical integrity of composite electrodes and deposition-induced fracture of SSE. Both cathode and anode may consist of a composite of active particles surrounded by a ceramic SSE matrix. During cycling, active particles undergo volumetric changes against the stiff SSE, which can lead to fracture. Developing models for these systems requires an understanding of three critical components, namely active particles, SSE, and the combined behavior of the composite. Towards modeling the role of mechanical damage on performance, we propose a novel coupled chemo-mechanical interface element, analogous to cohesive elements used in fracture mechanics. The framework enables for modeling galvanostatic charging of composite electrodes and captures the continuous evolution of mechanical stresses, interfacial damage, and non-uniform current distribution across particles in a microstructure. We specialize on an LCO-LGPS composite cathode and study the evolution of interfacial damage under varying material and microstructural properties. Specifically, we model electrolyte compositions with varying SSE stiffness and active particle volumetric change to assess their effect on interfacial stresses, mechanical damage, and overall electrochemical response. Subsequently, we discuss how variations in microstructural composition alter the state of interfacial damage. Both packing effects and particle size distribution are studied to understand how these factors impact integrity of the interface and performance.
Towards modeling the phenomenon of Li-metal filament growth, we formulate a continuum electro-chemo-mechanical gradient theory which couples phase-field damage and electrochemical reactions. The proposed framework is fully-coupled with electrodeposition impacting mechanical deformation, stress generation and subsequent SSE fracture. Conversely, electrodeposition kinetics are affected by mechanical stresses through a thermodynamically-consistent driving force that distinguishes chemical, electrical, and mechanical contributions. Importantly, the theory captures the interplay between crack propagation and electrodeposition by tracking damage and electrodeposition via distinct phase-field variables such that filament growth is preceded by and confined to damaged regions within the SSE. An attractive feature of such approach is its ability to simulate the nucleation, propagation and branching of cracks and Li-filaments in arbitrary orientations. While the framework is general in nature, we specialize it towards modeling the growth of Li-metal filaments in an inorganic LLZO electrolyte. We demonstrate the capacity to capture intergranular and transgranular crack and Li-filament growth, both of which have been experimentally observed. In doing so, we elucidate the manner in which mechanics and fracture of the SSE impact electrodeposition kinetics and Li-filament growth mechanism. From a manufacturing standpoint, we additionally elucidate the role of mechanical boundary conditions (i.e. mechanical confinement) on the rate of crack propagation across the electrolyte versus electrodeposition of Li-metal within cracks. Under specific mechanical boundary conditions, we show the capacity of the framework to capture the experimentally observed phenomenon of crack fronts propagating ahead of Li-metal filaments, as cracks traverse the entire electrolyte in advance of Li-deposits.