Synergistic Solvent Design for Fluorine-Free Electrolytes in High-Performance Lithium-Ion Batteries

The development of fluorine-free electrolytes offers a promising route for more environmentally sustainable lithium-ion batteries (LIBs). However, the low solubility of fluorine-free lithium salts in conventional carbonate solvents limits this progress. Additionally, while strongly coordinating solvents enhance salt dissolution, their use can lead to co-intercalation in graphite, complicating electrolyte design. Herein, lithium bis(oxalate)borate (LiBOB) is chosen as lithium salt, and a synergistic electrolyte design strategy is employed, combining a strongly coordinating solvent (tris(2-methylpropyl) phosphine oxide, TMP) and weakly coordinating solvent (dimethyl carbonate, DMC). This approach enables the use of fluorine-free lithium salts while ensuring compatibility with graphite by modulating the primary coordination shell. Furthermore, this solvation structure enables the formation of an inorganic-dominated cathode electrolyte interphase (CEI) on the LiMn₂O₄ (LMO) and a solid electrolyte interphase (SEI) on the graphite, improving lithium-ion conductivity and overall electrochemical performance. Additionally, compared to lithium hexafluorophosphate (LiPF₆), fluorine-free lithium salt minimized the formation of HF by-products, suppressing transition metals (TMs) dissolution. As a result, 1 Ah LMO||graphite pouch cell with fluorine-free electrolyte retains 92.8% of its capacity after 200 cycles, demonstrating excellent cycling stability and delivering high-rate capacity of 0.8486 Ah at 2 C, compared to 84.8% retention and 0.652 Ah with the conventional electrolyte.