All-solid-state lithium metal batteries (ASSLMBs) that incorporate solid electrolyte (SE) and lithium metal anode suggest considerable potential in addressing the security concerns and energy density limitation of conventional lithium-ion batteries (LIBs). However, the practical application of ASSLMBs is always restricted by the interfacial instability of lithium metal anode/electrolyte and inevitable lithium dendrites propagation in SE. Herein, a solvate ionic liquid is adopted to modify the interface stability of lithium metal anode/electrolyte and inhibit the growth of lithium dendrites via an in-situ formation of a robust solid electrolyte interphase (SEI) on the surface of lithium metal anode. Consequently, the ASSLMBs assembled with Li₆PS₅Cl (LPSCl) electrolyte, lithium metal anode that protected by robust SEI layer, and LiNbO₃@NCM622 cathode exhibit high initial capacity of 126.5 mAh·g⁻¹ and improved cycling stability with a capacity retention of 80.3% over 60 cycles at 0.1 C. This work helps to provide a facile route for the design of robust SEI in the application of ASSLMBs.