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

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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 βLi+/Co2+, βLi+/Ni2+, and βLi+/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.