Liquid-phase exfoliation of graphite is a promising method to produce graphene in large quantities. Selection of the solvent is the most important factor in determining the productivity of liquid-phase exfoliation and stability of the graphene dispersion. However, only low concentrations of graphene dispersion have been achieved in most of the solvents, and the solvents usually employed are toxic up to now. To address the performance and environmental safety issues of solvent, renewable solvents water and 1, 1, 3, 3-tetramethylurea (TMU) are explored as a mixture to exfoliate graphite into graphene in the present work. It is found that water-TMU mixed solvent is a powerful solvent to obtain a few-layer graphene, and the yield at the water volume faction of 0.3 was 4.5 times that in neat N-methylpyrrolidinone (NMP). The solvent-cosolvent interactions (water-TMU heteroassociation) are a critical factor in the liquid-phase exfoliation of graphene. The results reported in this work may provide a guide to rational design of a sustainable solvent system and to improve the yield and stability of the exfoliated graphene. (C) 2018 Elsevier Inc. All rights reserved.