Effects of functional groups and anion size on the charging mechanisms in layered electrode materials

We report on an extensive molecular simulations study about the influence of the nature of functional groups and anion size on the charging mechanisms and volume expansion/contraction in layered materials used as electrodes for energy storage applications. The study of the electrochemical behavior of graphene and Ti3C2 MXene (with three different functional groups) in five ionic liquids shows that the electrodes expand when charged negatively and no major dependence on the anion type is observed. When the electrode is positively charged, the volume changes are more complex and no specific trend could be observed depending on the anion size. The volume changes are in most cases, very well explained by the quantities of adsorbed ions. In specific cases, e.g., for FSI− and TFSI− anions, the reorientation of the ions can also affect the interlayer volume. We demonstrate that the charging mechanisms are changing consistently with the anion sizes and are largely correlated with the surface charge of the electrode material in the neutral state.