Structures, Electronic States, and Reactions at Interfaces between LiNi₀.₅Mn₁.₅O₄ Cathode and Ethylene Carbonate Electrolyte: A First-Principles Study

Electrolyte decomposition on cathode surfaces of lithium-ion batteries has attracted considerable attention because it leads to battery degradation and formation of a cathode solid–electrolyte interphase. In this study, we used density functional theory (DFT) calculations to investigate the distribution of the adsorption modes of ethylene carbonate (EC) electrolyte molecules and EC decomposition reactions on the (100) surfaces of lithiated (pristine) and delithiated forms of spinel-type LiNi₀.₅Mn₁.₅O₄ (LNMO) as model cathode surfaces. DFT molecular dynamics (MD) simulations indicated that EC molecules have two characteristic adsorption modes. These two modes can satisfactorily explain the experimental observations and suggest a new feature of electrolyte–cathode interfaces in terms of the control of interfacial dipoles. On the basis of the DFT-MD results, we examined several possible pathways of EC decomposition on the LNMO (100) surfaces and estimated their activation barriers. We then found that the pristine LNMO (100) surface was inert with respect to the EC decomposition, whereas on delithiated surfaces, twofold-coordinated surface oxygen atoms generated by the delithiation process served as active sites for nucleophilic attack on the carbonyl carbon and the methylene group of adsorbed EC molecules. The induction of ring opening of the EC molecule by the former attack, and hydrogen abstraction from the methylene group and subsequent CO₂ generation by the latter were consistent with experimental observations.