Microscopic insights into the ring-opening reaction of ethylene carbonate on LiCoO2 by on-the-fly machine learning molecular dynamics†

Abstract

The cathode–electrolyte interface plays a crucial role in determining the structural stability, electrochemical behavior and cycling performance of Li-ion batteries (LIBs). However, the dynamic structural evolution and microscopic reaction mechanisms at the interface remain poorly understood. Here, we provide a microscopic picture of the dynamic structure and the initial dynamics of the ring-opening reaction of ethylene carbonate (EC) at the EC–LiCoO₂ interface, one of the most commonly used battery systems, identifying two distinct mechanisms for the initial EC decomposition reactions, based on on-the-fly machine learning accelerated molecular dynamics simulations. Explicit solvent modeling reveals various binding configurations of EC, with multiple binding sites and orientations, which tend to influence its reactivity at the interface. Notably, the interaction between the carbonyl carbon of EC and the oxygen sites on the LiCoO₂ (104) facet is strongly correlated with ring-opening of the bound EC. At 300 K, the C–O (ether O) bond of the EC molecules can be cleaved by a nucleophilic attack from the surface oxygen of LiCoO₂, leading to ring opening. In the absence of additional chemical species, spontaneous ring closure is usually observed on the stoichiometric LiCoO₂ surface, presenting a dynamic equilibrium between transient ring-opening and self-healing for the adsorbed EC molecules at the interface. Furthermore, a prolonged ring-opened state of EC was observed, facilitated by Li⁺ ion extraction from the stoichiometric LiCoO₂, with an intermediate process of Co³⁺ oxidation to Co⁴⁺. A greater Li⁺ deficiency in the substrate was found to further promote EC ring opening. These findings not only provide fundamental insights into electrode–electrolyte interfacial reactions but also offer guidance for the efficient design of LIBs with enhanced stability and electrochemical/cycling performance.