π-Electron-driven ionic liquids to enhance lithium recovery from spent batteries: Experimental and mechanistic insights

A novel strategy is proposed to enhance Li⁺ extraction from spent lithium-ion batteries (LIBs) by ionic liquid (IL)-based extractants with π-electron regulation. N-butyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide ([BMPy][Tf₂N]) + tributyl phosphate (TBP) was designed as an optimal extractant resulting in extraction efficiency of Li⁺ up to 98.4%, separation selectivities of Li⁺ over other ions β_Li⁺_/Co²⁺, β_Li⁺_/Ni²⁺, and β_Li⁺_/Mn²⁺ up to 705, 740, and 663, respectively, and Li₂CO₃ product with 93.2 wt.% purity. The molecular-level mechanism for enhancing Li⁺ extraction was unraveled by quantum chemical calculations and molecular dynamics simulations. It is found that the [BMPy][Tf₂N] + TBP system with moderate π-electron density is not able to disrupt the strong hydration of divalent metal ions Co²⁺, Ni²⁺ and Mn²⁺, but it can disrupt Li⁺ hydration by so-called the “π-electron density matching effect.” This work provides theoretical guidance for rational design of specific IL-based extractants to selective extraction of target metal ions from spent LIBs.