RESUMO
Surface coating is one of the techniques used to improve the electrochemical performance and enhance the resistance against decomposition of cathode materials in lithium-ion batteries. Despite several experimental studies addressing the surface coating of secondary Li-ion batteries using α-Al2O3, the reactivity of the material toward the electrolyte components is not yet fully understood. Here, we have employed calculations based on the density functional theory to investigate the adsorption of the organic solvent ethylene carbonate (EC) on the major α-Al2O3(0001) surface. During adsorption of a single EC molecule, it was found that it prefers to bind parallel to the surface through its carboxyl oxygen. As the surface coverage (θ) was increased up to a monolayer, we observed larger adsorption energies per EC molecule (E ads/N EC) for parallel interactions and a reduction for perpendicular interactions. We also noted that increasing the surface coverage with both parallel and perpendicularly interacting EC molecules led to a decrease of the surface free energies and hence increased stability of the α-Al2O3(0001) surface. Despite the larger E ads/N EC observed when the molecule was placed parallel to the surface, minimal charge transfer was calculated for single EC interactions and at higher surface coverages. The simulated scanning tunneling microscopy images are also presented for a clean corundum α-Al2O3 surface and after adsorption with different coverages of parallel and perpendicularly placed EC molecules.
RESUMO
Understanding the surface reactivity of the commercial cathode material LiMn2O4 towards the electrolyte is important to improve the cycling performance of secondary lithium-ion batteries and to prevent manganese dissolution. In this work, we have employed spin-polarized density functional theory calculations with on-site Coulomb interactions and long-range dispersion corrections [DFT+U-D3-(BJ)] to investigate the adsorption of the electrolyte component ethylene carbonate (EC) onto the (001), (011) and (111) surfaces of the fully lithiated and partially delithiated Li1-xMn2O4 spinel (0.000 < x < 0.375). The surface interactions were investigated by evaluating the adsorption energies of the EC molecule and the surface free energies. Furthermore, we analyzed the impact of EC adsorption on the Wulff crystal morphologies, the molecular vibrational frequencies and the adsorbate/surface charge transfers. The adsorption energies indicate that the EC molecule strongly adsorbs on the (111) facet, which is attributed to a bidentate binding configuration. We found that EC adsorption enhances the stability of the (111) facet, as shown by the Wulff crystal morphologies. Although a negligible charge transfer was calculated between the spinel surfaces and the EC molecule, a large charge rearrangement takes place within the surfactant upon adsorption. The wavenumbers of the C[double bond, length as m-dash]O stretching mode for the interacting EC molecule are red-shifted with respect to the isolated adsorbate, suggesting that this bond becomes weaker. The surface free energies show that both the fully lithiated and partially delithiated forms of the LiMn2O4 surfaces are stabilized by the EC molecule.
