Simulated X-Ray Spectroscopy and Dynamical Stability of Lithiated Graphite Anode Material

Lithium graphite intercalation compounds (Li-GICs) are model anode materials for modern Lithium-ion batteries, and the properties of these systems has been extensively studied over the past years. Graphite itself presents well oriented, hexagonal carbon layers and anisotropic physio-chemical properties which is further enhanced by intercalation process. The so-called “staging” mechanism of lithium intercalation into graphite, whereby the lithium ions progressively fill/empty entire layers as a function of electrode voltage, has been established by a variety of electrochemical and characterization measurements, however a deep fundamental understanding of the lithium dynamics and the associated local charge fluctuations, and the role of these effects in modifying the electrode thermodynamic stability during staging has not been established. Here, we performed ab-initio molecular dynamics (AIMD) on a model Li-GIC at various staging. We explicitly calculate the entropy of the Lithium and Carbon atoms by considering the anharmonic and quasi-diffusional motions from our MD trajectories and uncover a major stabilizing force in these systems, namely long-range, corporative Lithium librations which manifests as enhanced low energy peaks in the vibrational spectrum at far IR frequencies. Moreover, we associated these low energy librations to specific changes in the electronic structure, ad probed by simulated X-ray adsorption spectra (XAS), calculated within a many-body, matrix transfer element framework. By comparison with available experimental data, we show that specific aspects of the Lithium ion librations is in fact encoded in the XAS spectrum, leading to the observed changes in the relative intensity of the C K-edge π^* and σ^* during staging. Thus, we show that the XAS response is a quantitative measure of the lithium-ion mobility and charge dynamics.