Electronic Effects of Substituents on Redox Shuttles for Overcharge Protection of Li-ion Batteries

Abstract

The redox behavior and kinetic parameters of five ferrocene derivatives were investigated in 1M LiPF6 in 50:50 volume percent EC:EMC, a typical electrolyte used in lithium-ion batteries. Using cyclic voltammetry (CV) and rotating disk electrode voltammetry (RDE) techniques, the effect of electron donating and withdrawing substituents on each derivative was evaluated from the view point of the Hammett substituent constant. We found that electrochemical rate constants of the ferrocene derivatives can be related to the Hammett equation which gives an accurate approximation for predicting the oxidation potential of redox shuttles when changes are desired in their electron donating and electron withdrawing properties by means of functional group substitution. Our results show that the exchange current density and reaction rate for oxidation decrease as the electron withdrawing property of the substituent increases. It is also shown that electron donating and electron withdrawing property of a substituent affect the exchange current density and electrochemical oxidation reaction rate obeying a trend opposite to that of the Hammett substituent constants (σ). The correlations found here are expected to improve the ability to systematically design chemical overcharge protection reagents through judicious substitution of functional groups on redox shuttles.