Solid state electrolytes are known for non-flammability, dendrite blocking, and stability over large potential windows. Garnet-based solid state electrolytes have attracted much attention for their high ionic conductivities and stability with lithium metal anodes. However, high interface resistance with lithium anodes hinders their application to lithium metal batteries. Herein, we develop several efficent and effective techniques that exhibit a significantly improved contact and lead to an interface resistance as low as ~ 7 Ω·cm². We also use neutron depth profiling (NDP), a nondestructive and uniquely Li-sensitive technique, to reveal the interfacial behaviors of garnet SSE in contact of metallic Li through monitoring Li plating-stripping processes in situ. The NDP measurement has demonstrated the predictive capabilities for diagnosing short-circuits in garnet-based solid state batteries. Two types of cells, symmetric lithium/garnet/lithium (LGL) cells and asymmetric lithium/garnet/carbon-nanotubes (LGC) ones have been fabricated to emulate the behavior of Li metal and Li-free Li metal anodes, targeting full-cell applications with Li-free cathode (e.g., sulfur, and oxygen) and Li-containing cathode (e.g., LiCoO₂, LiFePO₄, etc.), respectively. The NDP data implies the limitation of Li-free Li metal anode in forming reliable interfacial contacts, and strategies of excessive Li and better interfacial engineering need to be investigated.