Design of fluorinated polymer interphases for anodeless lithium metal batteries

Increasing the energy density of batteries is critical for accelerating transportation electrification and expanding the use of renewable energy sources. Anodeless lithium metal batteries (ALLMBs), which eliminate the need for excess lithium inventory, are a promising route to higher energy density. However, their practical implementation is hindered by the reversibility of Li plating and stripping on copper current collectors, and both the attainable reversibility and dominant failure modes depend strongly on electrolyte chemistry. Here, we report a norbornene-derived polymer bearing pentafluorophenyl side chains as an interface layer for ALLMBs. In this design, the rigid polynorbornene backbone enables a mechanically robust, continuous coating, while the pentafluorophenyl groups are intended to act as a fluorine source, promoting LiF-rich passivation during initial Li deposition. Through spin coating, we investigated how controlled coating thickness impacts the cycling performance of ALLMBs in carbonate-and ether-based electrolytes. We identify an optimal thickness window that improves Coulombic efficiency and capacity retention in both ether and carbonate-based cells, whereas thinner coatings fail to provide durable protection and thicker coatings increase polarization. These results establish a thickness-governed balance between ion transport and interfacial passivation that is modulated by electrolyte chemistry, offering practical design guidelines for fluorinated polymer interphase layers for high-energy ALLMBs.