Lyocell fibers are derived from renewable plant resources and serve as promising sustainable materials, but the conventional fabrication of Lyocell micro/nano fibrils (LMNFs) requires excessively high mechanical energy input; incomplete fibrillation also leaves unrefined raw fibers, leading to large pore defects and low filtration performance of the resulting water filter papers. Herein, a hydrated polycarboxylic deep eutectic solvent (DES) consisting only of choline chloride, citric acid and deionized water was adopted to address these limitations. Carboxylation pretreatment with the developed DES enabled low-energy mechanical fibrillation to prepare carboxylated Lyocell micro/nano fibrils (C-LMNFs), which featured a finer diameter (~100 nm), more uniform diameter distribution, superior dispersibility, a carboxyl content of 0.6 mmol g⁻¹, a zeta potential of −77 mV, and a mass yield above 98%. Furthermore...