Evaluation of fluorinated alkyl phosphates as flame retardants in electrolytes for Li-ion batteries - I. Physical and electrochemical properties

The reduction of electrolyte flammability by the known phosphorus-based flame retardants is always realized at the expense of cell performance, i.e., either electrochemical instability causing severe capacity fading or high viscosity of these cosolvents affecting both capacity utilization and power. To alleviate this trade-off between cell safety and performance, we synthesized a series of fluorinated alkyl phosphates and studied their applicability as flame retarding cosolvents in electrolytes for Li-ion batteries. Summarized in this part of the work are the physical properties of these fluorinated phosphates and their effect on the flammability and ion conductivity as well as electrochemical stability of the electrolyte solutions containing them. We found that the addition of these phosphates to electrolyte solutions reduces the overall flammability at the expense of ion conduction, while electrochemical stability on carbonaceous anodes is improved as the result of the introduction of fluorine. By adjusting fluorine content in the phosphates, it is possible to find a cosolvent that makes the concept of nonflammable lithium ion electrolyte practical. (C) 2003 The Electrochemical Society.