Composite solid electrolytes (CSEs) possess significant advantages over individual polymer or inorganic solid electrolytes. However, conventional CSEs suffer from multiple scale issues, including interruptions in ionic transport pathways, incompatibility at heterogeneous interface, and the excessive thickness of electrolytes. Herein, a novel CSE with ultrathin structure (22 µm) based on the strategy of multiscale collaborative optimization (MC-CSE) is designed. This strategy involves continuous, rapid, and homogeneous Li