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Abstract All‐solid‐state batteries with metallic lithium (Li_BCC) anode and solid electrolyte (SE) are under active development. However, an unstable SE/Li_BCCinterface 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 Li_BCCdeposition and stripping, serving as a 3D scaffold for Li⁰ad‐atom formation, Li_BCCnucleation, and long‐range transport of ions and electrons at SE/Li_BCCinterface is demonstrated. Consisting of lithium silicide and carbon nanotubes, the MIEC interlayer is thermodynamically stable against Li_BCCand highly lithiophilic. Moreover, its nanopores (<100 nm) confine the deposited Li_BCCto the size regime where Li_BCCexhibits “smaller is much softer” size‐dependent plasticity governed by diffusive deformation mechanisms. The Li_BCCthus remains soft enough not to mechanically penetrate SE in contact. Upon further plating, Li_BCCgrows in between the current collector and the MIEC interlayer, not directly contacting the SE. As a result, a full‐cell having Li_3.75Si‐CNT/Li_BCCfoil as an anode and LiNi_0.8Co_0.1Mn_0.1O₂as 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). 

Abstract Proton conduction underlies many important electrochemical technologies. A family of new proton electrolytes is reported: acid‐in‐clay electrolyte (AiCE) prepared by integrating fast proton carriers in a natural phyllosilicate clay network, which can be made into thin‐film (tens of micrometers) fluid‐impervious membranes. The chosen example systems (sepiolite–phosphoric acid) rank top among the solid proton conductors in terms of proton conductivities (15 mS cm⁻¹at 25 °C, 0.023 mS cm⁻¹at −82 °C), electrochemical stability window (3.35 V), and reduced chemical reactivity. A proton battery is assembled using AiCE as the solid electrolyte membrane. Benefitting from the wider electrochemical stability window, reduced corrosivity, and excellent ionic selectivity of AiCE, the two main problems (gassing and cyclability) of proton batteries are successfully solved. This work draws attention to the element cross‐over problem in proton batteries and the generic “acid‐in‐clay” solid electrolyte approach with superfast proton transport, outstanding selectivity, and improved stability for room‐ to cryogenic‐temperature protonic applications.