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 LiCoO 2 cathode surfaces and PF 5 species in the electrolyte. We report detailed mechanisms for the activation of EC ring-opening reactions by Lewis acids to form CO 2, organics, or organofluorines. We find that Lewis acid–base complexation between EC and either PF 5 or LiCoO 2 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 LiCoO 2 cathode surface forms a cathode–electrolyte interphase primarily composed of an organic and organofluorine film. Simultaneous degradation of an EC molecule and PF 6 – forms PF 5 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 PF 5 lowers the barrier to EC ring opening to form CO 2 from 1.96 to 1.68 eV and the reaction energy from 0.02 eV to −1.38 eV relative to unactivated CO 2 formation. We expect that EC electrolyte degradation at the cathode surface will be dominated by EC dimer formation reactions activated by PF 5 because of their low reaction barriers relative to CO 2 formation.