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Abstract Solid state lithium metal batteries using garnet solid electrolytes such as LLZTO (Li_6.4La₃Zr_1.5Ta_0.5O₁₂) promise substantial improvements in energy density and safety. However, practical implementation is hindered by lithium dendrite penetration at high current densities. Recent work shows that internal electrochemically induced mechanical stresses are large enough to propagate lithium dendrites and subsequently fracture solid electrolytes. This study builds on this understanding and demonstrates that stress‐driven dendrite propagation can be controlled via deflection at weakly bonded internal interfaces. This approach, based on a fracture‐mechanics analysis of multilayered composites, is investigated with a variety of interlayer materials that are embedded into LLZTO. The viability and effectiveness of dendrite deflection are most clearly evident with reduced graphene oxide where the critical current density increased from 0.6 to 3.8 mA cm⁻². In this material, both the weak interface with LLZTO and the mixed ionic–electronic conducting nature of the interlayer appear to contribute to the improved performance. Additional insight into the mechanics of multilayered electrolytes is also obtained with finite element modeling. The overall results present a promising proof‐of‐concept demonstration along with important generalized design guidelines for creating multilayered solid electrolyte architectures that can enable high‐performance solid‐state batteries. 

We have investigated the surface of lithium metal using x-ray photoemission spectroscopy and optical spectroscopic ellipsometry. Even if we prepare the surface of lithium metal rigorously by chemical cleaning and mechanical polishing inside a glovebox, both spectroscopic investigations show the existence of a few tens of nanometer-thick surface layers, consisting of lithium oxides and lithium carbonates. When lithium metal is exposed to room air (∼50% moisture), in situ real-time monitoring of optical spectra indicates that the surface layer grows at a rate of approximately 24 nm/min, presumably driven by an interface-controlled process. Our results hint that surface-layer-free lithium metals are formidable to achieve by a simple cleaning/polishing method, suggesting that the initial interface between lithium metal electrodes and solid-state electrolytes in fabricated lithium metal batteries can differ from an ideal lithium/electrolyte contact.