In Situ Construction of a Li ₂ S/Li x Sn Mixed Conductive Interlayer for Dendrite‐Free Garnet‐Based Solid‐State Lithium Metal Batteries

Solid‐state lithium metal batteries employing garnet‐type Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) electrolytes offer promising energy density and safety characteristics, yet their practical implementation suffers from high interfacial resistance and uncontrolled dendrite propagation. Herein, we construct an in situ Li 2 S/Li x Sn mixed conductive layer (MCL) on the LLZTO surface through a conversion reaction between a SnS coating layer and molten lithium at 250°C. This approach transforms the initial point contact between lithium metal and LLZTO into continuous planar contact, reducing the interfacial resistance to an ultralow value of 3.6 Ω cm 2 . Crucially, the MCL forms a mechanical modulus gradient that buffers stress at the rigid ceramic/soft metal interface and enables intimate interfacial contact and homogeneous Li + flux distribution. Consequently, the symmetric cell achieves a high critical current density of 1.4 mA cm −2 and demonstrates superior cycling stability for 9500 h at 0.1 mA cm −2 and 4000 h at 0.4 mA cm −2 . When integrated with commercial cathodes, the LiFePO 4 ‐based cells retain 91.2% of their initial capacity after 450 cycles at 1 C, while the LiNi 0.8 Co 0.1 Mn 0.1 O 2 cells maintain 82.8% capacity retention after 200 cycles at 0.2 C. This work provides fundamental insights into chemo‐mechanical interfacial engineering design principles for high‐performance solid‐state batteries.