Lithium-tin (Li-Sn) alloys are perfect substrate materials for anodes in high-energy density lithium metal secondary batteries. A new approach is proposed to further prevent the Li deposit on Li-Sn alloy substrates from reaction with electrolytes using an artificial solid electrolyte interphase (ASEI) based on electrochemical oxidation and polymerization of 1,3-dioxolane precursor with LiTFSI additive. This ASEI layer is flexible, stable, ion conductive, and electrically insulating, which can provide very stable cycling of Li-Sn alloy substrate anodes for Li deposition/stripping with an average Coulombic efficiency of 98.4% at a current density of 1 mA cm-2. The Li-Sn alloy substrate is kept uniform and smooth without any dendrites and cracks after cycles. When the Li-Sn alloy substrate protected by ASEI is used as the anode of lithium-sulfur full cell, the cell shows much higher discharge capacity and better cycleability. This innovative and facile strategy of ASEI formation demonstrates a new and promising approach to the solution of the tough problems of Li dendrites in Li metal batteries.