Three‐Layer‐Structured Garnet‐Dominant Composite Solid Electrolyte for Solid‐State Batteries

Solid‐state batteries utilizing oxide, sulfide, and polymer electrolytes have garnered significant attention due to their robust safety and their applicability in high‐energy‐density advanced Li‐ion batteries. In this study, we present an innovative three‐layer‐structured solid electrolyte, including the garnet‐dominant composite solid electrolyte (GD‐CSE) for solid‐state batteries. This newly structured composite solid electrolyte (CSE) demonstrates a high ionic conductivity of around 1 mS/cm, substantial potential stability exceeding 5.0 V versus Li + /Li, and an exceptionally high Li‐ion transference number of about 0.8 at room temperature. Upon evaluating its application in batteries, symmetric Li cells employing the three‐layer CSE exhibit remarkable cycle stability lasting over 800 h at 0.1 mA/cm 2 . Furthermore, a solid‐state Li‐metal battery configuration (LiFePO 4 (LFP)|three‐layer‐CSE|Li‐metal) demonstrates favorable C‐rate performance, achieving 63% at 5.0 C compared to 0.1 C capacity, along with an impressive cycle retention of 99% during 200 cycles at 0.5 C. Additionally, we manufacture solid‐state Li‐metal pouch‐type cells measuring 3.4 × 5.0 cm 2 . These pouch‐type cells offer a capacity of 4.5 mAh and exhibit secure behavior even under bending and cutting conditions.