A sulfur-enriched solid electrolyte interface (SEI) is known to enhance the cycling stability of a Li metal electrode, however, using the conventional additive approach, the positive effect is hard to maintain during prolonged cycling. Here, we present a method of forming a sulfur-enriched SEI in sustainable manner during battery cycling. A polymer electrolyte layer containing a sulfur chain is inserted between the lithium metal electrode and liquid electrolyte phase. The interfacial layers of the poly(alpha-lipoic acid-co-sulfur) readily form Li2S and Li2S2 at the Li metal surface, inducing dendrite-free, planar Li deposition beneath the layer. As a result of the regeneration of the sulfur-enriched SEI during repeated cycling, a Li metal electrode with the interfacial layer in a Li symmetric cell operated for more than 400 cycles at a high current density of 3 mA cm(-2) and a high areal capacity of 3 mAh cm(-2). The SEI- forming interfacial layer approach provides sustainable protection of the Li metal electrode during prolonged cycling in Li metal batteries.