Lithium metal batteries (LMBs) have high energy density but face challenges from dendrite formation and unstable solid electrolyte interphase (SEI) layers, which limit their cycle life. Gel polymer electrolytes (GPEs) help address these issues by improving mechanical strength, forming stable SEI layers, and enhancing ion dissociation, though their structure can limit ion conductivity. To overcome this, we developed quasi-solid GPEs (qPEs) by incorporating a small fraction of liquid electrolyte (LE) into polyurethanes (PUs) with distinct hard segments, such as PUs with linear hexamethylene diisocyanate (PU-HD) and PUs with bulky isophorone diisocyanate (PU-ID). The PU-ID structure results in higher ionic conductivity, a greater Li-ion transference number, and lower interfacial resistance, as shown by Li||Li symmetric cell tests, which also revealed more stable and longer cycling performance (2500 h at 1 mA cm−2). XPS analysis found that PU-ID forms a LiF-rich SEI layer on the electrode surface, which helps suppress dendrite growth and enhances long-term battery stability with NCM 622 cathodes (82% capacity retention after 300 cycles at 0.5 C). These improvements highlight that modifying the polymer structure, especially with PU-ID, significantly boosts battery efficiency, cycle life, and electrode protection.