Sulfide-based all-solid-state battery (ASSB) with a lithium metal anode (LMA) is a promising candidate to surpass conventional Li-ion batteries owing to their inherent safety against fire hazards and potential to achieve a higher energy density. However, the narrow electrochemical stability window and chemical reactivity of the sulfide solid electrolyte towards the LMA results in interfacial degradation and poor electrochemical performance. In this direction, we introduce an organic additive approach, that is the mixing of prelithiated trithiocyanuric acid, Li₃TCA, with Li₆PS₅Cl, to establish a stable interface while preserving high ionic conductivity. Including 2.5 wt % Li₃TCA alleviates the decomposition of the electrolyte on the lithium metal interface, decreasing the Li₂S content in the solid-electrolyte interface (SEI) thus forming a more stable interface. In Li|Li symmetric cells, this strategy enables a rise in the critical current density from 1.0 to 1.9 mA cm⁻² and stable cycling for over 750 hours at a high current density of 1.0 mA cm⁻². This approach also enables Li|NbO-NCM811 full cell to operate more than 500 cycles at 0.3 C.