In Situ Construction of an Ultra‐Stable Conductive Composite Interface for High‐Voltage All‐Solid‐State Lithium Metal Batteries

The garnet electrolyte presents poor wettability with Li metal, resulting in an extremely large interfacial impedance and drastic growth of Li dendrites. Herein, a novel ultra‐stable conductive composite interface (CCI) consisting of Li y Sn alloy and Li 3 N is constructed in situ between Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) pellet and Li metal by a conversion reaction of SnN x with Li metal at 300 °C. The Li y Sn alloy as a continuous and robust bridge between LLZTO and Li metal can effectively reduce the LLZTO/Li interfacial resistance from 4468.0 Ω to 164.8 Ω. Meanwhile, the Li 3 N as a fast Li‐ion channel can efficiently transfer Li ions and give their uniform distribution at the LLZTO/Li interface. Therefore, the Li/LLZTO@CCI/Li symmetric battery stably cycles for 1200 h without short circuit, and the all‐solid‐state high‐voltage Li/LLZTO@CCI/LiNi 0.5 Co 0.2 Mn 0.3 O 2 battery achieves a specific capacity of 161.4 mAh g −1 at 0.25 C with a capacity retention rate of 92.6 % and coulombic efficiency of 100.0 % after 200 cycles at 25 °C.