As draw solute is the core element of forward osmosis (FO) technology, here Li-Bet-Tf2N synthesized from a customized ionic liquid betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]) and Li2CO3 recovered from lithium-ion battery (LIB) wastes is proposed as a novel draw solute to treat Li+-containing wastewater from LIB manufacturing through FO filtration. Having high dissociation ability and an extended structure, Li-Bet-Tf2N generates a sufficiently high osmotic pressure to drive the FO filtration efficiently along with insignificant reverse solute diffusion. Li-Bet-Tf2N produces a water flux of 21.3 L·(m2·h)−1 at 1.0 mol∙L–1 against deionized water, surpassing conventional NaCl and MgCl2 draw solutes with a higher water recovery efficiency and a smaller solute loss. Li-Bet-Tf2N induces a more stable and higher water permeation flux with a 10.0% water flux decline than NaCl and MgCl2 for which the water fluxes decline 16.7% and 16.4%, respectively, during the treatment of 2000 mg∙L–1 Li+-containing wastewater for 12 h. More remarkably, unlike other draw solutes which require intensive energy input and complicated processes in recycling, Li-Bet-Tf2N is easily separated from water via solvent extraction. Reproducible results are achieved with the recycled Li-Bet-Tf2N. Li-Bet-Tf2N thus demonstrates a novel class of draw solute with great potentials to treat wastewater economically.