First-principles XANES simulation for oxygen-related defects in Si-O amorphous materials

To understand the chemical reaction during the initial charging in an amorphous negative SiO (a-SiO) electrode for Li-ion batteries, we simulated the O K-edge X-ray absorption near-edge structure (XANES) spectra for oxygenexcess defects in silicon oxide crystals and glasses using the first-principles projector augmented-wave (PAW) method. Oxygen-excess defects, including the oxygen bridge-bonded (OBB) and peroxy linkage (POL) configurations, reproduce the pre-edge peak observed in the experimental spectrum of the as-prepared a-SiO material. It was found that the electronic states contributing to the pre-edge peaks are the sigma* state of the O-O bond for the OBB and POL configurations, and the pi* state of the O-O bond for an O2 molecule. The formation energy of these defects is distributed within the amorphous structure, and POL configurations with small formation energies were found in a-SiO. In addition, O2 molecules can exist in a-SiO2, whereas those in a-SiO are incorporated into the bond network. Through the insertion of Li+ ions, the pre-edge peak disappears, and weak peaks appear in the higher-energy region. These correspond to the experimental results in which the pre-edge peak in the O K-edge XANES of the as-prepared a-SiO disappeared during the initial charging. The chemical reaction between POL configurations in a-SiO and Li+ ions during the initial charging is thought to play an important role in improving cycle characteristics.