Lithium diffusion pathways in metastable ramsdellite-like Li2Ti3O7 from high-temperature neutron diffraction

• In ramsdellite-like Li2Ti3O7, interstitial lithium ions are complexly distributed.
• Framework cation positions are not significantly occupied by lithium ions.
• Heating successively leads to relaxation, increased mobility, and decomposition.
• Spacious interstitial channels contain the major pathway of lithium diffusion.
• Framework vacancies are the most probable minor pathway of lithium diffusion.

Ramsdellite-like Li2Ti3O7 is an excellent, strongly anisotropic lithium-ion conductor with various proposed applications in energy storage and lithium processing. Diffusion pathways have not yet been thoroughly studied, in spite of their importance for these purposes; even the precise crystal structure of this material is a subject of ongoing discussion. Herein, we use variable-temperature neutron diffraction in the metastability range of ramsdellite-like Li2Ti3O7 to probe the nontrivial lithium-ion distribution. Refinement with anisotropic displacement parameters including anharmonic terms identifies the room-temperature structure as statically disordered snapshot of the dynamic behavior at synthesis temperature. Mapping of scattering-length density, reconstructed using maximum-entropy methods (MEM), shows the signature of partial relaxation and activation of lithium movement with increasing temperature. Scrutinous topological analyses of procrystal voids and Voronoi–Dirichlet partitioning (VDP) reveal lithium diffusion along ribbons in the interstitial channels as the major and diffusion through framework vacancies as the most probable minor mechanism. By rationalizing former empirical results, we provide deeper insight into this paradigmatic example of strongly anisotropic lithium-ion conductors.

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