All-solid-state batteries (ASSBs) are regarded as a promising route to enhanced safety and energy density through the use of lithium metal anodes. However, lithium dendrite growth and interfacial instability between Li and the solid electrolyte (SE) remain critical bottlenecks. To address these issues, a spontaneously preformed solid electrolyte interphase (PSEI) on the surface of the SE strategy was devised. By incorporating lithium difluoro(oxalato)borate (LiODFB) as a tailored SE additive into LiPSCl, an SEI is engineered that synergistically suppresses dendrite formation, stabilizes the Li/SE interface, and mitigates mechanical degradation. The resulting multifunctional PSEI, which consists of a hybrid organic-inorganic outer layer and an inorganic-rich inner layer, is found to both reduce the electronic conductivity of the SE (thus preventing Li nucleation within SEs) and accommodate mechanical degradation arising from Li plating/stripping to avoid contact loss. Consequently, Li-Li symmetric cells incorporating LPSC with PSEI achieved a critical current density (CCD) of 2.4 mA cm and ultrastable cycling for over 3500 h at 0.2 mA cm. The Li-LiNiCoMnO full cells retained 80% capacity after 310 cycles at 0.5 C and can also operate stably under 4.5 V. This work establishes a rational interface design for SEs that integrates electronic and mechanical modulation with electrochemical stability to unlock the potential of lithium metal anodes in ASSBs.