Determination of Lithium Insertion Sites in Li_xTiP₄ (x = 2−11) by Electron Energy-Loss Spectroscopy

A combined computational and experimental study of the electron energy-loss near edge structures for Li_xTiP₄ at the lithium K edge is presented. Li_xTiP₄ is considered to be a possible negative electrode material for lithium batteries. The complex behavior of this compound during lithium insertion/extraction was studied using first-principles simulations of the fine structures in electron energy-loss spectra (EELS) and band structure calculations. These calculations were performed with the WIEN2k code, within the density functional theory and were compared to experimental spectra obtained for the following lithium compositions (x = 2, 4.3, 5.5, 9, 11). This study brings to light the existence of spectroscopic features characteristic of lithium atoms inserted in tetrahedral sites of the TiP₄ matrix. These sites are shown to be preferred with respect to octahedral ones for low lithium compositions. The spectroscopic differences between the electrodes formed through oxidation or reduction processes allowed us to discriminate between two different electrochemical pathways in charge and discharge:  a biphasic process is involved in charge (oxidation), whereas a solid solution occurs in discharge (reduction). This study demonstrates the sensitivity of the Li K edge to these two different behaviors.