Grafting of Lithiophilic and Electron‐Blocking Interlayer for Garnet‐Based Solid‐State Li Metal Batteries via One‐Step Anhydrous Poly‐Phosphoric Acid Post‐Treatment

Abstract Garnet‐based solid‐state Li‐metal batteries (GSSBs) have the merits of high energy density and high safety. However, the realization of a stable and well‐matched Li|garnet interface for GSSBs remains challenging due to electron leakage and lithiophobic Li 2 CO 3 impurity. To address these issues, herein, new surface chemistry is reported that converts the undesired Li 2 CO 3 contaminant into an ultra‐thin lithium polyphosphate (Li‐PPA) layer through anhydrous polyphosphoric acid ‐induced in situ substitution reaction without damaging the water‐sensitive garnet electrolyte. In particular, the Li‐PPA interlayer not only facilitates the homogenous spreading of molten Li but also creates a robust electron‐blocking shield to suppress Li dendrite formation. As a result, the assembled Li symmetric cell exhibits a low interfacial impedance (4 Ω cm 2 ) and high critical current density (1.8 mA cm −2 ) at 25 °C, which enables the cell to continuously cycle over 2500 h at 0.2 mA cm −2 . Furthermore, the GSSBs paired with LiFePO 4 deliver a high capacity of 149.3 mAh g −1 at 1 C and maintain 92.3% of the initial capacity after 500 cycles and can be used for solar energy storage, suggesting the feasibility of this interfacial engineering strategy for GSSBs.