Ligand competition within ternary deep eutectic solvents to extend cluster size distribution for moisture-immune superlubricity

Deep eutectic solvents (DES) based on hydrogen bonding are a key class of liquid superlubricious materials, with the typical binary DES composed of choline chloride and ethylene glycol. However, the limitations of conventional binary designs and their hydrogen bond acceptors/donors have hindered the development of advanced and sustainable superlubricious materials. This study introduces a novel ternary DES comprising the organic ligands 2-pyrrolidinone (PD) and succinonitrile (SN) combined with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Compared to the base lubricant (LP 1:3 ) with a single ligand (PD), the optimized system with two ligands (LPS 1:3:3 ) demonstrates superlubricity, achieving an ultralow coefficient of friction (∼0.007) and high load-bearing capacity (469 MPa). Competitive coordination between PD, SN, and Li + reduces viscosity and enhances the asymmetric contact probability of size-expanded liquid clusters, forming ultra-low shear sliding interfaces. Additionally, replacing water molecules with organic molecules in the Li + solvation shell improves resistance to various humidity levels. This study highlights the role of size-distribution-expanded clusters and adsorbed films in intensifying the ligand competition-induced asymmetric contact effect, providing fresh insights into the design of next-generation multivariate DES superlubricious materials.