Highlights A robust and high-wettability cellulose separator with unique nano-cracked structures is constructed through hydrogen bond-driven reassembly of cellulose molecules using a green binary solvent. The nano-cracked structures endow the separator with high porosity (70.2%) and excellent electrolyte retention (329%). The supercapacitors with nano-cracked separators exhibit high specific capacitance (93.6 F g−1 at 1.0 A g−1) and a long cycling life (99.5% retention after 10,000 cycles).