Combination of first-principles molecular dynamics and XANES simulations for LiCoO2 -electrolyte interfacial reactions in a lithium-ion battery

We performed a first-principles molecular dynamics (FPMD) simulation of the interfacial reactions between a ${\mathrm{LiCoO}}_{2}$ electrode and a liquid ethylene carbonate (EC) electrolyte. For configurations during the FPMD simulation, we also performed first-principles Co K-edge x-ray absorption near-edge structure (XANES) simulations, which can properly reproduce the bulk and surface spectra of ${\mathrm{LiCoO}}_{2}$. We observed strong absorption of an EC molecule on the ${\mathrm{LiCoO}}_{2}$ {110} surface, involving ring opening of the molecule, bond formation between oxygen atoms in the molecule and surface Co ions, and emission of one surface Li ion, while all the surface Co ions remain ${\mathrm{Co}}^{3+}$. The surface Co ions having the bond with an oxygen atom in the molecule showed remarkable changes in simulated K-edge spectra which are similar to those of the in situ observation under electrolyte soaking [D. Takamatsu et al., Angew. Chem., Int. Ed. 51, 11597 (2012)]. Thus, the local environmental changes of surface Co ions due to the reactions with an EC molecule can explain the experimental spectrum changes.