Evaluation of fluorinated alkyl phosphates as flame retardants in electrolytes for Li-ion batteries - II. Performance in cell

A series of fluoroalkyl phosphates are synthesized and evaluated as flame retardants for electrolytes of Li-ion batteries. In this work, nonflammable electrolytes based on these phosphates are formulated with the knowledge obtained in Part I of these papers through ion conduction and electrochemical studies. The performance of these nonflammable electrolytes in lithium-ion cells is preliminarily evaluated by investigating the effect of the degree of their concentration and fluorination on the cycling behavior, rate capability, and low temperature performances of the cells. It was found that the electrolytes based on phosphates, with at least two alkyls fluorinated, demonstrate stable cell performance at room temperature, and the presence of these phosphates not only delivers higher cell safety, but also improves the cell capacity retention over a long period of testing. However, the rate capability and low temperature performance of these nonflammable electrolytes decline with increasing concentration of these phosphates, as a result of higher cell impedance. Nevertheless, compared with their nonfluorinated counterparts, fluorination does introduce higher flame-retarding efficiency and lower performance impact. This alleviated trade-off between electrolyte flammability and cell performance made it possible to formulate a series of nonflammable electrolytes, which contain 15-20% of fluoroalkyl phosphates and can work in a lithium-ion cell with minimum sacrifice in performance. (C) 2003 The Electrochemical Society.