Synergistic liquid–liquid extraction (LLE) of Li⁺ commonly relies on β-diketonates in combination with neutral phosphine oxides such as tri-n-octylphosphine oxide (TOPO), yet the co-ligand is typically treated as an empirical additive rather than a design element. Here we show that commercially available diphosphine dioxides, featuring a preorganised PO⋯PO donor set and a spacer-defined bite, act as powerful co-ligands for Li⁺ extraction with 3-benzoyl-1,1,1-trifluoroacetone (HBTA) under mild pH conditions. Compared to TOPO, the best-performing PO⋯PO co-ligand enhances Li⁺ transfer while suppressing Na⁺/K⁺ co-extraction, consistent with altered solution speciation and stoichiometry. These results establish a chelating co-ligand (PO⋯PO) as a simple, modular strategy to control synergy in Li⁺ extraction systems based on classical CO⋯CO extractants such as HBTA.