Garnet-Based Solid-State Lithium Fluoride Conversion Batteries Benefiting from Eutectic Interlayer of Superior Wettability

Garnet-type Li–La–Zr–O electrolytes suffer from poor contact with the Li metal anode because of the facile formation of a passivation layer at the interface, thus leading to frustrating interfacial impedance or lithium dendrite penetration. Herein, we introduce a eutectic Na–Li intermediate layer between the Li anode and garnet electrolyte to enable sufficient wettability. The potential mechanism of solid–solid convection between Li and Na domains promotes the prompt extension of the eutectic zone, preservation of the highly soldered interface, and homogenization of Li plating during cycling. The Li–Na/garnet/Na–Li cell exhibits a small interfacial resistance of 18.98 Ω·cm2 at 60 °C and steady cycling for 3500 h (with a small overpotential of 10.8 mV). The first prototype of a solid-state battery combining a conversion-type FeF3 cathode with garnet electrolyte is successfully operated at 60 °C, with a high reversible capacity of 500 mAh·g–1 and ultrastable cycling for 300 cycles at 100 μA·cm–2, as well as high-rate endurance up to 500 μA·cm–2.