Degradation of Ethylene Carbonate Electrolytes of Lithium Ion Batteries via Ring Opening Activated by LiCoO2Cathode 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 LiCoO2cathode surfaces and PF5species 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 PF5or LiCoO2weakens 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 LiCoO2cathode surface forms a cathode-electrolyte interphase primarily composed of an organic and organofluorine film. Simultaneous degradation of an EC molecule and PF6-forms PF5and 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 PF5lowers the barrier to EC ring opening to form CO2from 1.96 to 1.68 eV and the reaction energy from 0.02 eV to -1.38 eV relative to unactivated CO2formation. We expect that EC electrolyte degradation at the cathode surface will be dominated by EC dimer formation reactions activated by PF5because of their low reaction barriers relative to CO2formation.
Department of Chemical and Biological Engineering, University of Colorado , Boulder, Colorado 80309-0596, United States.
Jonathon L
JL
Tebbe
Department of Chemical and Biological Engineering, University of Colorado , Boulder, Colorado 80309-0596, United States.
Thomas F
TF
Fuerst
Department of Chemical and Biological Engineering, University of Colorado , Boulder, Colorado 80309-0596, United States.|Department of Chemistry and Biochemistry, University of Colorado , Boulder, Colorado 80309-0215, United States.
Charles B
CB
Musgrave
Lewis acid activated; cathode electrolyte interphase; electrolyte degradation; ethylene carbonate oxidation; ethylene carbonate ring opening