Investigation of the Dipole Moment Effects of Fluorofunctionalized Electrolyte Additives in a Lithium Ion Battery

Abstract

Bismaleimides modified through fluorosubstitution constitute two new electrolyte additives that have been successfully synthesized and used in lithium ion batteries. Fluorosubstitution significantly eliminates the interface impedances of the two electrodes and improves the rate performance of batteries because of the reduced LiF and Li2CO3 content in a solid electrolyte interphase. This study indicates that between the symmetric (two fluorosubstitutions) and asymmetric (one fluorosubstitution) designs of fluorosubstitution in bismaleimide structures, the symmetric design effectively inhibits the direct electrochemical reduction of —C═C— in bismaleimide on a graphite surface and reacts with ethylene carbonate in accordance with the high strength of the dipole moment. Cyclic voltammetry and battery measurements indicate considerable improvements due to the addition of bismaleimide modified through symmetric fluorosubstitution. The morphology of the anode electrode and chemical composition of the solid electrolyte interphase after 20 cycles were examined through scanning electron microscopy, X-ray photoelectron spectroscopy, and nuclear magnetic resonance. A kinetic analysis of solid electrolyte interphase formation was performed on all electrolyte systems through an electrochemical quartz crystal microbalance. Battery performance demonstrated that the addition of bismaleimide modified through symmetric fluorosubstitution was able to increase the capacity for anode half-cells and c-rate performance for full cells at 2 C by 7% and 164%, respectively. These novel electrolyte additives not only prevented unnecessary compound formations in the solid electrolyte interphase but also enhanced the ionic diffusivity and reversibility of the electrochemical reaction in the solid electrolyte interphase, cathode electrolyte interphase, and two-electrode surfaces.