Nonflammable electrolytes for Li-ion batteries based on a fluorinated phosphate

In this work we demonstrate that the fluorination of alkyl substituents on phosphate can reduce the viscosity, enhance flameretarding efficiency, and increase electrochemical stability of the cosolvents. Therefore the new flame retardant can be used at a concentration greater than 20% in the electrolyte, as a cosolvent, in order to formulate a completely nonflammable electrolyte that works in lithium-ion cells. This new electrolyte formulation not only increases the safety feature of the lithium-ion battery, while maintaining the capacity utilization and rate capability, but also improves the capacity retention of the cell. The trade-off between electrolyte nonflammability and performance in the cell is thus alleviated by the introduction of fluorine in the structure of the flame retardants. The flammability of electrolyte solvents used in the state-of-theart lithium ion technology has caused serious safety concern, especially for high-energy battery packs designed for electric vehicle ~EV! or hybrid electric vehicle ~HEV! applications. 1-3 Recent efforts to address this problem have focused on developing flame retardants ~FRs! as electrolyte additives/cosolvents, which are exclusively based on organophosphorus~V! compounds. 4-7 So far, attempts to formulate an electrolyte that is nonflammable and also works well in lithium-ion cells has not been very successful. There always exists a trade-off between electrolyte flammability and its performance in cell either due to the electrochemical instability on graphitic anode ~e.g., triethylphosphate, TEP, and trimethylphosphate, TMP! or due to other adverse properties of FRs, such as high melting point and high viscosity ~e.g., hexamethylcyclophosphazene, HMPN!. Low efficiency of these FRs in flame retarding further worsens the above trade-off, 8 because as higher FR content is needed in the electrolyte to reduce the flammability to a certain level, the negative impact on performance increases proportionally with FR content. In order to identify an ideal FR that is ~i! more efficient in flame retarding and ~ii! electrochemically stable on both cathode and graphitic anode, we modified the structure of these FRs and synthesized a series of fluorinated organophosphorus compounds, with the known information that ~i! fluorinated substituents in carbonate molecules help forming stable solid electrolyte interface ~SEI! on the anode surface; 9 ~ii! fluorinated substituents also suppress the flammability of the molecule; 10 ~iii! introduction of fluorine into organic molecules usually reduces its boiling temperature and viscosity. 11 Among these new FRs synthesized and currently being investigated in our lab is the fluorinated version of TEP, tris ~2,2,2-trifluoroethyl ! phosphate ~TFP!