A High‐Performance Carbonate‐Free Lithium|Garnet Interface Enabled by a Trace Amount of Sodium

: Garnet-type solid-state electrolytes (SSEs) are promising for the realization of next-generation high-energy-density Li metal batteries. However, a critical issue associated with the garnet electrolytes is the poor physical contact between the Li anode and the garnet SSE and the resultant high interfacial resistance. Here, it is reported that the Li|garnet interface challenge can be addressed by using Li metal doped with 0.5 wt% Na (denoted as Li*) and melt-casting the Li* onto the garnet SSE surface. A mechanistic study, using Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) as a model SSE, reveals that Li2 CO3 resides within the grain boundaries of newly polished LLZTO pellet, which is difficult to remove and hinders the wetting process. The Li* melt can phase-transfer the Li2 CO3 from the LLZTO grain boundary to the Li*'s top surface, and therefore facilitates the wetting process. The obtained Li*|LLZTO demonstrates a low interfacial resistance, high rate capability, and long cycle life, and can find applications in future all-solid-state batteries (e.g., Li*|LLZTO|LiFePO4 ).