In Situ Construction of a Li₂S/Li_xSn Mixed Conductive Interlayer for Dendrite-Free Garnet-Based Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries employing garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (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 Li₂S/Li_xSn 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 cm². 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 LiFePO₄-based cells retain 91.2% of their initial capacity after 450 cycles at 1 C, while the LiNi_0.8Co_0.1Mn_0.1O₂ 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.