Ultrastable Lithium Metal Batteries Enabled by the Fluorinated Li/Electrolyte Interphase with High Mechanical Strength.

A rationally designed solid-electrolyte interphase (SEI) is critical for enabling a high mechanical modulus, low mass transfer resistance, and uniform, rapid ion transport in lithium metal batteries. Herein, we report a promising artificial SEI layer fabricated by coating fluorinated hydroxyapatite on the commercially available polypropylene separator. Fluorination reaction for hydroxyapatite greatly improves its Young's modulus, physically suppressing Li dendrite growth. More importantly, the uniformly coated nanoscale fluorinated hydroxyapatite with a solid-solid nanoporous network enables efficient Li-ion transport during Li plating/stripping and F- release, which not only endows the formed SEI with local outer regions of ordered laminar Li2O as revealed by cryo-transmission electron microscopy but also effectively promotes the formation of a stable LiF-rich inner layer. The obtained double-deck SEI, with high mechanical strength, suppresses SEI fracture during repeated cycles, promotes uniform Li-ion flux, and stabilizes the Li/electrolyte interface. As a result, Li anodes based on this fluorinated hydroxyapatite exhibit an ultrastable cycling life over 1200 h with a capacity of 3 mAh cm-2 at a current density of 3 mA cm-2. When coupled with a LiFePO4 cathode, the obtained full cell shows a high capacity retention of 96% after 600 cycles.