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

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 Li2CO3 derived from air exposure triggers uneven contact with Li‐metal, leading to undesirable dendrite growth and performance deterioration. Herein, the Li3PO4 layer replacing Li2CO3 contaminant is built on garnet surface by taking molten NH4H2PO4 salt driven conversion reaction. The high‐flowability molten salt contributes to conformal formation of Li3PO4, realizing the air‐stable garnet by preventing the re‐attack of H2O/CO2 in air. Besides, the high work of adhesion for Li3PO4 on Li‐metal along with the transformation from Li3PO4 to Li3P/Li2O when contacting with molten Li‐metal enables a lithiophilic interlayer, leading to a seamless Li/garnet contact with ultralow interfacial resistance of 13 Ω cm2. 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 LiCoO2/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. The uniform Li3PO4 layer in place of Li2CO3 contaminant is in situ built on garnet surface by making use of the molten NH4H2PO4 salt driven conversion reaction. The Li3PO4‐modified layer enables the highly air‐stable garnet electrolyte without Li2CO3 formation for even 20 days as well as the lithiophilic interface with dendrite‐free Li deposition.