Interface Stabilization via In Situ Lithiated Sn Interlayer in All‐Solid‐State Li‐Metal Batteries: Toward Pellet‐Type Cell to Pouch‐Type Cell

ABSTRACT All‐solid‐state batteries (ASSBs) are among the most promising candidates for next‐generation energy storage due to their high energy density and intrinsic safety. However, the practical use of lithium metal anodes in ASSBs remains limited by interfacial instability and dendritic Li growth, particularly under moderate stack pressures. Here, we report a nano‐sized tin (nSn) interlayer that forms a highly reversible Li–Sn alloy interface via spontaneous in situ lithiation, offering a robust, lithiophilic, and electronically conductive interface between the Li metal and sulfide‐based solid electrolyte. The resulting Li _x Sn layer promotes uniform Li‐ion transport, suppresses dendrite formation, and enhances interfacial charge transfer. Symmetric cell measurements demonstrate markedly improved critical current densities and stable cycling, while full‐cell tests reveal high Coulombic efficiency, improved rate performance, and excellent capacity retention. The nSn interlayer was further scaled into a sheet‐type architecture using a warm isostatic press (WIP) process for integration into pouch‐type cells. The pouch cell exhibited high areal capacity, retaining 81% after 500 cycles, and delivered energy densities of 351 Wh kg ⁻¹ and 918 Wh L ⁻¹ under a low external pressure of 2 MPa. This work introduces a simple and scalable alloying‐based interfacial engineering strategy that enables high‐performance, fast‐charging ASSBs with practical viability.