First-Principles Study on the Initial Oxidative Decompositions of Ethylene Carbonate on Layered Cathode Surfaces of Lithium-Ion Batteries

Understanding solvent decomposition mechanisms at the electrode-electrolyte interface is of great importance to mitigate capacity fading and improve the cycling performance of batteries. First-principles calculations were conducted to study the oxidative decomposition reactions of ethylene carbonate (EC) on the (110) surfaces of LiCoO2 (LCO) and LiNi1/3Co1/3Mn1/3O2 (NCM) in lithium-ion batteries. All of the possible oxidative decomposition reaction steps of EC on cathode surfaces, including the H-abstraction reaction and ring-opening reactions caused by C-c-O-e and/or C-e-O-e bond cleavage, were analyzed from both thermodynamic and kinetic aspects. Our calculation results indicated that EC decompositions were initiated by the ring-opening reaction as the first-step reaction on both LCO and NCM surfaces, which was caused by C-c-O-e bond cleavage with the activation energies of 0.96 and 0.57 eV, respectively. In the second step, the H-abstraction reaction was prone to occur on LCO surfaces with a reaction barrier of 0.90 eV and a reaction energy of -1.51 eV. However, the proton was much easier to be transferred from EC to NCM cathode surfaces with one small reaction barrier of 0.26 eV, and the reaction energy (-1.64 eV) was similar to that on LCO. We concluded that the main oxidative decomposition reactions of EC on layered cathode surfaces were initiated by the ring-opening reaction, followed by proton transfer reaction contributing to the hydroxyl group (-OH) formation. Additionally, the Li-releasing behavior was observed on both cathode surfaces, which might contribute to the capacity fading of LCO and NCM.