Advances in Organic and Inorganic Chemistry

Making the Most of Neutron-Diffraction Data: Lithium Diffusion Pathways in Ramsdellite-Like $\ce{Li2Ti3O7}$

Dennis Wiedemann1 and Alexandra Franz2

1Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, 10623, Berlin, GER

2Abteilung Struktur und Dynamik von Energiematerialien, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109, Berlin, GER

Ramsdellite-like $\ce{Li2Ti3O7}$ exhibits fast and strongly anisotropic lithium-ion conduction. Although proposed applications range from energy storage to lithium processing, the crucial diffusion pathways in this material have not yet been studied in depth; even aspects of its crystal structure are still under discussion.1 In our studies, we have examined $\ce{Li2Ti3O7}$ using variable-temperature neutron diffraction to probe its nontrivial lithium-ion distribution. At 24\,°C, a -refined structural model with anharmonic-anisotropic displacement parameters shows a statically disordered snapshot of the dynamic behavior during synthesis. We found no significant occupation of the framework cati-on positions by lithium ions and agree with the recently favored formulation as $\ce{[Li2\square_5]_{i}[(Ti3\square_{0.5})O7]_{f}}$ (\square: vacancy, i: interstitial, f: framework). Reconstruction of the scattering-length density via maximum-entropy methods (MEM) indicates successive partial relaxation and activation of lithium movement with increasing temperature in the metastability range. Using topological analyses of procrystal voids and – partitioning (VDP), we have identified two pathways of lithium diffusion: interstitial migration along ribbons as the major, framework migration through vacancies as the most probable minor mechanism. Thusly, we explain former empirical results and shed light on this paradigmatic lithium-ion conductor.2

  1. D. Tang, C. M. Teng, J. Zou, F. H. Li, Acta Crystallogr., Sect. B: Struct. Sci. 1986, 42, 340–342; A. Orera, M. T. Azcondo, F. García-Alvarado, J. Sanz, I. Sobrados, J. Rodríguez-Carvajal, U. Amador, Inorg. Chem. 2009, 48, 7659–7666.

  2. D. Wiedemann, S. Nakhal, A. Franz, M. Lerch, Solid State Ionics 2016, 293, 37–43.

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Void structure of the $\ce{Ti3O7^{2–}}$ framework at 422\,°C (red: oxide, gray: titanium ions; blue: preferred, green: probabilistic voids/channels).