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.