Lithium metal is a highly desirable anode for lithium rechargeable batteries, having the highest theoretical specific capacity and lowest electrochemical potential of all material candidates. Its most notable problem is dendritic growth upon Li plating, which is a major safety concern and exacerbates reactivity with the electrolyte. Here we report that Li-rich composite alloy films synthesized in situ on lithium by a simple and low-cost methodology effectively prevent dendrite growth. This is attributed to the synergy of fast lithium ion migration through Li-rich ion conductive alloys coupled with an electronically insulating surface component. The protected lithium is stabilized to sustain electrodeposition over 700 cycles (1,400 h) of repeated plating/stripping at a practical current density of 2 mA cm−2 and a 1,500 cycle-life is realized for a cell paired with a Li4Ti5O12 positive electrode. These findings open up a promising avenue to stabilize lithium metal with surface layers having targeted properties.