Liu, JunZhu, Boyu2026-04-202026-04-202026-04-202026Zhu_washington_0250O_29280.pdfhttps://hdl.handle.net/1773/55524Thesis (Master's)--University of Washington, 2026Polyolefin separators play a central role in regulating ion transport and interfacial stability inlithium metal batteries, yet their performance can vary markedly across testing environments. In this study, Celgard 2400 and Celgard 2500 polypropylene separators, the Celgard 2325 trilayer separator, and a Nanoshel polyethylene separator were systematically compared in CR2032 cells under controlled assembly conditions with fixed electrolyte volume and consistent cell hardware through both Li|Cu plating/stripping tests and high-voltage NMC811|Li full-cell cycling. In Li|Cu cells, all separators exhibited high coulombic efficiency, but a reproducible separator dependence emerged, with cells using Celgard 2400 and Celgard 2500 polypropylene separators showing higher efficiency than those using the polyethylene separator and Celgard 2400 delivering the highest average value. Replicate statistics further indicated small yet measurable scatter, highlighting that instrument precision can contribute to apparent fluctuations when efficiencies approach unity. In contrast, separator ranking changed in full cells cycled between 2.8 and 4.4 V, where the polyethylene separator provided the highest capacity retention, coulombic efficiency closest to unity, and the most stable long-term cycling behavior, while Celgard 2325 trilayer separator exhibited the poorest durability with pronounced late-cycle instability. These findings indicate that separator performance is not universal but depends strongly on the battery chemistry and operating conditions.application/pdfen-USnoneBattery materialsLithium batteriesEngineeringMaterials ScienceChemical engineeringMaterials science and engineeringThesis