All-solid-state batteries with metallic lithium (LiBCC) anode and solid electrolyte (SE) are under active development. However, an unstable SE/LiBCC interface due to electrochemical and mechanical instabilities hinders their operation. Herein, an ultra-thin nanoporous mixed ionic and electronic conductor (MIEC) interlayer (≈3.25 µm), which regulates LiBCC deposition and stripping, serving as a 3D scaffold for Li0 ad-atom formation, LiBCC nucleation, and long-range transport of ions and electrons at SE/LiBCC interface is demonstrated. Consisting of lithium silicide and carbon nanotubes, the MIEC interlayer is thermodynamically stable against LiBCC and highly lithiophilic. Moreover, its nanopores (<100 nm) confine the deposited LiBCC to the size regime where LiBCC exhibits “smaller is much softer” size-dependent plasticity governed by diffusive deformation mechanisms. The LiBCC thus remains soft enough not to mechanically penetrate SE in contact. Upon further plating, LiBCC grows in between the current collector and the MIEC interlayer, not directly contacting the SE. As a result, a full-cell having Li3.75Si-CNT/LiBCC foil as an anode and LiNi0.8Co0.1Mn0.1O2 as a cathode displays a high specific capacity of 207.8 mAh g−1, 92.0% initial Coulombic efficiency, 88.9% capacity retention after 200 cycles (Coulombic efficiency reaches 99.9% after tens of cycles), and excellent rate capability (76% at 5 C).