Molten Salt Driven Conversion Reaction Enabling Lithiophilic and Air‐Stable Garnet Surface for Solid‐State Lithium Batteries

Abstract Garnet‐type electrolytes show great potential in application of solid‐state lithium batteries due to their high ionic conductivity and wide electrochemical window. However, the formation of surface Li 2 CO 3 derived from air exposure triggers uneven contact with Li‐metal, leading to undesirable dendrite growth and performance deterioration. Herein, the Li 3 PO 4 layer replacing Li 2 CO 3 contaminant is built on garnet surface by taking molten NH 4 H 2 PO 4 salt driven conversion reaction. The high‐flowability molten salt contributes to conformal formation of Li 3 PO 4 , realizing the air‐stable garnet by preventing the re‐attack of H 2 O/CO 2 in air. Besides, the high work of adhesion for Li 3 PO 4 on Li‐metal along with the transformation from Li 3 PO 4 to Li 3 P/Li 2 O when contacting with molten Li‐metal enables a lithiophilic interlayer, leading to a seamless Li/garnet contact with ultralow interfacial resistance of 13 Ω cm 2 . Such ion‐conducting but electron‐insulating layer regulates the uniform distribution of Li‐flux, enabling a large critical current density of 1.2 mA cm −2 . Furthermore, the solid LiCoO 2 /Li cell with the modified garnet delivers a discharge capacity of 130 mAh g −1 at 30 °C, accompanied by a capacity retention of 81% after 150 cycles. This study proposes a promising solution for improvement of air stability and interfacial compatibility of garnet using facile molten salt treatment.