Track-etched polyimide separator decorated with polyvinylpyrrolidone for self-assembling a robust protective layer on lithium-metal anode

• Track-etched polyimide separator with vertically-aligned nanochannels is designed. • Vertically-aligned nanochannels of the separator allow a fast Li + migration. • Hydrophilic polyvinylpyrrolidone can regulate Li + flux uniformly during cycling. The practical application of Li metal is hampered by critical obstacles, such as the indiscriminate dendritic growth of metallic Li and severe dimensional changes during cycling. Effective surface engineering is necessary to stabilize the interfacial reactions of Li metal to resolve these technical issues. Many strategies have been proposed to mitigate this inherent limitation of Li metal. The interfacial stabilization using an electrolyte additive or polymer coating is the most common approach to enable the uniform distribution of Li + on the surface of Li metal. In parallel, the development of functional separators is also considered a promising alternative strategy for effective suppression of the fatal growth of Li dendrites because it promotes Li + transfer at the interfaces. The synergistic effect of regulating Li + flux and promoting Li + transfer is expected to effectively prohibit the dendritic growth of metallic Li. Herein, we report a track-etched polyimide (TEPI) separator with vertically aligned nanochannels decorated with polyvinylpyrrolidone (PVP) that enables a self-assembled robust protective layer on the surface. From a structural viewpoint, the vertically aligned nanochannels of the TEPI separator allow for fast Li + migration. Additionally, hydrophilic PVP can regulate Li + flux uniformly through the nanochannels by forming a stable Li 3 N-dominant solid electrolyte interphase (SEI) layer, thereby enhancing interfacial kinetics and suppressing the Li dendrite growth during cycling. We demonstrate that the TEPI separator decorated with PVP is effective in securing high reversibility and the improved cycle performance of Li-metal batteries (LMBs).