Combining the Pair Distribution Function and Computational Methods To Understand Lithium Insertion in Brookite (TiO2)

X-ray pair distribution function (PDF) methods and first-principles calculations have been combined to probe the structure of electrochemically lithiated TiO2 Brookite. Traditional powder diffraction studies suggest that Brookite amorphizes upon lithium insertion, with the Bragg reflections disappearing. However, PDF analysis indicates that the TiO2 framework connectivity is maintained throughout lithium intercalation, with expansions along the a and b axes. The Li+ ions within the framework are poorly observed in the X-ray PDF, which is dominated by contributions from the more strongly scattering Ti and O atoms. First-principles calculations were used to identify energetically favorable Li+ sites within the Brookite lattice and to develop a complete structural model of the lithiated material. This model replicates the local structure and decreased intermediate range order observed in the PDF data. The analysis suggests that local structural distortions of the TiO2 lattice accommodate lithium in five-coordinate sites. This structural model is consistent with the observed electrochemical behavior.