| Summary: | NASICON-type Li<sub>1.5</sub>Al<sub>0.5</sub>Ge<sub>1.5</sub>(PO<sub>4</sub>)<sub>3</sub> (LAGP) is a remarkable solid-state electrolyte due to its high ionic conductivity and excellent air stability. However, the weak LAGP|Li interfacial compatibility (e.g., chemical instability of LAGP with Li metal and lithium dendrite growth) limits its practical application. Herein, an ultrathin CuS layer was fabricated on the surface of the LAGP electrolyte by magnetron sputtering (MS). Then, an in situ Li<sub>2</sub>S/Cu nano-layer formed via the conversion reaction between CuS and molten Li was constructed at the LAGP|Li interface. The Li<sub>2</sub>S/Cu nano-layer enables effective hindering of the reduction reactions of LAGP with Li metals and the suppression of lithium dendrite growth. The assembled Li symmetric battery with the Li<sub>2</sub>S/Cu@LAGP electrolyte shows a promising critical current density (CCD) of 0.6 mA cm<sup>−2</sup> and a steady battery operation for over 700 h. Furthermore, the full LiFePO<sub>4</sub> battery comprising the Li<sub>2</sub>S/Cu@LAGP electrolyte shows excellent capacity retention of 94.5% after 100 cycles, providing an appropriate interface modification strategy for all-solid-state Li metal batteries.
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