Self-stratified photonic radiative cooling composites with asymmetric thermal conductivity

High-power outdoor electronics, such as 5G base stations, need energy-efficient thermal management. Passive daytime radiative cooling (PDRC) represents a promising solution, but faces practical limitations due to low thermal conductivity and performance degradation associated with coloration. Herein, we demonstrate a hierarchically structured asymmetric bilayer composite, fabricated through a scalable and feasible self-stratification process, which integrates a cholesteric photonic lattice of cellulose nanocrystals (CNCs) with a highly thermally conductive framework of boron nitride (BN) nanosheets. The top photonic CNC layer provides vivid structural color and high mid-infrared emissivity (εMIR = 91.5%), while the bottom BN-rich layer delivers high solar reflectance (96.9%) and enhanced through-plane thermal conductivity (8.9 W m^-1 K^-1). The material achieves a temperature drop of up to 17.8 °C under realistic solar and thermal loads, while its asymmetric heat transfer property suppresses parasitic heat gain from the environment. Furthermore, the composite enables scalable structural color patterning <i>via</i> screen printing without compromising the cooling performance, offering both aesthetic customization and environmental durability. This work presents a scalable self-assembly strategy for high-performance, aesthetically versatile radiative cooling materials that address key challenges in next-generation electronic thermal management.