The volume of the metallic lithium anode in all-solid-state Li metal batteries increases significantly due to the lithium dendrite formation during the battery cycling,and the rough surface of lithium metal also reduces Li-ion transport in Li/electrolyte interface.In this work,we developed a solid polymer composite by adding the low-cost Si3N4 particles to protect the lithium anode in all-solid-state batteries.The Fourier transform infrared spectroscopy (FTIR) data show that the surface of 10 wt ％ Si3N4 particles interacts with the polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt;the interaction restricts the anion mobility and improves the ionic conductivity (1 × 10-4 S·cm-1) and lithium-ion transference number (0.28) of the composite electrolyte.The lithium metal anode is well protected by the composite electrolyte in all-solid-state cells,including symmetric and Li/LiFePO4 cells.The lithium dendrite growth suppression by this composite electrolyte indicates the possible application of these low-cost composite electrolytes for lithium metal protection.