Self-assembled functional layers onto separator toward practical lithium metal batteries

• The development of advanced separator for practical Li-metal batteries. • Scalable and facile fabrication by electrostatic layer-by-layer techniques. • Self-assembled functional layers on separator consisted of advanced polymers. • Improved physical/electrochemical characteristic by self-assembled functional layer. • Stable Li-LiNi 0.8 Co 0.15 Al 0.05 battery cycling under the lean electrolyte condition. Li-metal batteries are among the most promising candidates for the development of small- to large-scale energy storage devices owing to their high theoretical energy density and processability. However, the practical application of Li-metal is limited by safety issues arising from uncontrollable Li-dendrite growth and various side reactions that occur during long-term cycling. Herein, an advanced separator composed by self-assembled functional layers is introduced for highly stable Li-metal batteries. The nanoscale self-assembled layers are formed via an ionic complexation of mechanically robust chitosan and ionic conductive polystyrene sulfonate onto the separator. This layered structure improves the physical properties of the separator and electrochemical properties. When using this separator with Li-metal anode, these improvements provide a robust mechanical barrier and regulation of Li-ion transport which could suppress Li-dendrite growth and stable cycling. Therefore, even under lean electrolyte conditions, the assembled cells (Li-Li, Li-Cu, and Li-LiNi 0.8 Co 0.15 Al 0.05 ) exhibit enhanced battery cycling behaviors.