Degradation of Ethylene Carbonate Electrolytes of Lithium Ion Batteries via Ring Opening Activated by LiCoO2 Cathode Surfaces and Electrolyte Species

High-performance lithium-ion batteries require electrolytes that are stable over wide operating voltages. We used density functional theory to investigate the degradation of ethylene carbonate (EC) electrolytes activated by interactions with LiCoO2 cathode surfaces and PF5 species in the electrolyte. We report detailed mechanisms for the activation of EC ring-opening reactions by Lewis acids to form CO2, organics, or organofluorines. We find that Lewis acid-base complexation between EC and either PF5 or LiCoO2 weakens the C-O bonds of the EC ring and consequently lowers the barrier to and energy of EC ring-opening reactions. Our results predict that ring opening activated by the LiCoO2 cathode surface forms a cathode-electrolyte interphase primarily composed of an organic and organofluorine film. Simultaneous degradation of an EC molecule and PF6- forms PF5 and a surface organofluorine with an activation barrier of 1.28 eV and reaction energy of -0.26 eV. Ring opening of EC activated by the cathode to form short organic oligomers results from sequential ring-opening reactions at the surface with an activation barrier of 1.04 eV and an overall reaction enthalpy of -1.15 eV for the case of EC dimer formation. Complexation of EC with PF5 lowers the barrier to EC ring opening to form CO2 from 1.96 to 1.68 eV and the reaction energy from 0.02 eV to -1.38 eV relative to unactivated CO2 formation. We expect that EC electrolyte degradation at the cathode surface will be dominated by EC dimer formation reactions activated by PF5 because of their low reaction barriers relative to CO2 formation.