Experimental Visualization of Interstitialcy Diffusion of Li Ion in β-Li₂TiO₃

Abstract

Monoclinic lithium metatitanate, β-Li₂TiO₃, is a member of the Li₂TiO₃ (M = Ti, Mn, Sn, Ru, and/or Ir) series and an important cation conductor for various energy applications such as Li-ion batteries and nuclear fusion reactors. Comprehensive knowledge of the crystal structure is vital to understand the Li-ion diffusion mechanism, and several possibilities were proposed previously. However, the exact crystal structure and Li-ion diffusion paths of β-Li₂TiO₃ are still unclear. Here, the results of a neutron diffraction study of high-purity ⁷Li-enriched β-Li₂TiO₃ are reported. The occupancy factor 0.033(3) and the atomic coordinates of the interstitial Li ion in the Li–O layer are successfully refined by Rietveld analysis of the time-of-flight neutron diffraction data. The three-dimensional network of Li-ion diffusion pathways is visualized by a combined technique of high-temperature neutron-diffraction and maximum-entropy methods. An interstitialcy diffusion mechanism, in which a lithium ion migrates through both the interstitial tetrahedral and lattice octahedral sites, is proposed for the Li₂TiO₃ series.