“In-N-out” design enabling high-content triethyl phosphate-based non-flammable and high-conductivity electrolytes for lithium-ion batteries

Safety issues related to flammable electrolytes in lithium-ion batteries (LIBs) remain a major challenge for their extended applications. The use of non-flammable phosphate-based electrolytes has proved the validity in inhibiting the combustion of LIBs. However, the strong interaction between Li⁺ and phosphate leads to a dominant solid electrolyte interphase (SEI) with limited electronic shielding, resulting in the poor Li⁺ intercalation at the graphite (Gr) anode when using high-phosphate-content electrolytes. To mitigate this issue and improve Li⁺ insertion, we propose an “In-N-Out” strategy to render phosphates “non-coordinative”. By employing a combination of strongly polar solvents for a “block effect” and weakly polar solvents for a “drag effect”, we reduce the Li⁺-phosphate interaction. As a result, phosphates remain in the electrolyte phase (“In”), minimizing their impact on the incompatibility with the Gr electrode (“Out”). We have developed a non-flammable electrolyte with high triethyl phosphate (TEP) content (>60 wt.%), demonstrating the excellent ion conductivity (5.94 mS cm ⁻¹ at 30 °C) and reversible Li⁺ intercalation at a standard concentration (~1 mol L ⁻¹). This approach enables the manipulation of multiple electrolyte functions and holds the promise for the development of safe electrochemical energy storage systems using non flammable electrolytes.