Gas Evolution in Li-Ion Batteries: Modeling Ethylene Carbonate Decomposition on LiCoO₂ in the Presence of Surface Magnetism

Decomposition of liquid electrolytes used in Li-ion batteries can result in the formation of gaseous species and the release of flammable solvent vapor. These undesirable “gassing” reactions can be mediated by interactions between the electrode surfaces and the electrolyte. At present, the detailed mechanisms responsible for electrolyte gassing are not well understood. Here, first-principles calculations are used to characterize the thermodynamics and kinetics of potential gas-forming reactions that involve surface-mediated decomposition of ethylene carbonate (EC)—a commonly used electrolyte solvent—at the (101̅4) surface of a LiCoO₂ (LCO) cathode. The magnetic ordering of the (101̅4) surface was explicitly taken into account. Comparisons of EC adsorption on nonmagnetic, ferromagnetic, and antiferromagnetic surface orderings reveal differences in the adsorption geometry and reaction energetics. The antiferromagnetic surface exhibited the lowest surface energy overall; EC adsorption on this surface preferentially occurs on Li sites. In contrast, the nonmagnetic surface exhibits a stronger attraction for EC—adsorption is 0.5 eV more exothermic per molecule than for the antiferromagnetic surface—and adsorption occurs on Co sites. The thermodynamic driving force for EC decomposition on the antiferromagnetic surface was predicted for several potential reaction products resulting from more than 30 bond-breaking scenarios. The most exothermic of these reactions results in the formation of CO₂ and acetaldehyde (C₂H₄O), gaseous products that have been observed in prior experiments. Nevertheless, an evaluation of the minimum energy path for CO₂/C₂H₄O formation on fully lithiated LCO reveals a large reaction barrier for this process, implying a kinetically limited reaction. These data suggest that the rate of solvent decomposition at the cathode may be maximized in the charged state, where LCO is partially delithiated.