Ultrafast sintering (UFS) is a compelling approach for fabricating Li₇La₃Zr₂O₁₂ (LLZO) solid-state electrolytes (SSEs), paving the way for advancing and commercializing Li-garnet solid-state batteries. Although this method is commonly applied to the sintering of LLZO ceramics, its use for producing dense, phase-pure LLZO SSEs has thus far been primarily limited to millimeter-thick pellets, which are unsuitable for commercial solid-state batteries. This study presents ultrafast sintering as a highly effective approach for fabricating self-standing, dense, 45 µm-thick LLZO membranes. The chemical and structural evolution of LLZO membranes during the UFS process is characterized through in situ synchrotron X-ray diffraction and thermogravimetric analysis-mass spectrometry, complemented by an in-depth investigation of surface chemistry using X-ray photoelectron spectroscopy. The membranes in Li/LLZO/Li symmetrical cell configuration exhibit a high critical current density of up to 12.5 mA cm⁻² and maintain superior cycling stability for 250 cycles at a current density of 1 mA cm⁻², with an areal capacity limit of 1 mAh cm⁻². The electrochemical performance of LLZO membranes is also assessed in full cell configuration using a pyrochlore-type iron (III) hydroxy fluoride cathode.