<p>Anode-free lithium batteries represent a promising avenue for high-energy-density storage, yet their practical application is hindered by lithium inventory loss from parasitic interfacial reactions, cathode degradation, and limited Li⁺ reversibility. Herein, we propose a polyolefin separator integrated with a Li₂S@C sacrificial layer, achieving multiscale interfacial stabilization in Ah-class anode-free pouch cells. This approach simultaneously replenishes the customized Li⁺ inventory during the formation cycle and establishes the lithium polysulfide-containing cathode interface with high-voltage tolerance (till 4.5 V). Real-time tracking via in-situ electrochemical impedance spectroscopy and transmission-mode operando X-ray diffraction reveals accelerated Li⁺ diffusion kinetics and stabilized phase evolution in LiNi_0.8Co_0.1Mn_0.1O₂ cathode interfaced with Li₂S@C|PE prelithiation separator. Consequently, a 1.22 Ah pouch cell with an Ag-modified Cu foil and LiNi_0.8Co_0.1Mn_0.1O₂ cathode is assembled with Li₂S@C|PE separator and exhibits gravimetric and volumetric energy densities of 450 Wh kg^-1 and 1355 Wh L^-1, respectively. This prelithiation protocol demonstrates upscaling potential and generic applicability to secure the interfacial chemistries for anode free/less lithium metal batteries.</p>