The global drive to create safer, higher capacity energy storage devices is increasingly focused on the relationship between the microstructures of electrochemically- active materials and overall battery performance. The advent of solid-state electrolytes with multi-layered, variable porosity microstructures opens new avenues to creating the next generation of rechargeable batteries, while creating new challenges for device integration and operation. In this dissertation, microstructures of solid-state Li-ion conducting electrolytes were characterized to identify the primary limiting factors on electrolyte performance and identify structural changes to improve porous electrolyte performance in dense-porous bilayer systems.

LLZO-based garnet electrolytes were fabricated with varied porosity and characterized using 3D Focused Ion Beam (FIB) Tomography, enabling digital reconstructions of the underlying 3D microstructures. Ion transport through the microstructures was analyzed using M-factors, which identified garnet volume fraction and bottlenecks as primary limiters on effective conductivity, followed by geometric tortuosity. Notably, a template-based porous microstructure displayed a low tortuosity plane and a high tortuosity direction, as opposed to the more homogenous tape-cast porous microstructures. To evaluate the performance of these microstructures in Li symmetric cells, dense-porous bilayers were digitally constructed using the FIB Tomography microstructures as porous layers with fully infiltrated Li-metal electrodes, and equilibrium electric potentials were simulated. The bilayers had area-specific resistance (ASR) values similar to the ASR value of the dense layer alone. The bilayer ASR also decreased as porous layer porosity increased, due to ion transport occurring primarily through the dense layer-electrode interface and higher porosity creating higher interfacial area. Artificial bilayers were created with porous layers composed of columns for a range of column diameters/particle sizes, porous layer porosities, and porous layer thicknesses. The bilayer ASR decreased with increasing porosity and decreasing column diameter, similar to the FIB Tomography bilayers. However, bilayer ASR dramatically increased when only partially infiltrated with electrodes, and instead increased with increasing porosity and decreasing column diameter. The simulation results showed that fabricating solid-state bilayer symmetric cells with low ASR required high porosity porous microstructures with small particle sizes, and electrodes completely infiltrated to the dense layer.