The High-Temperature Transformation from 1T- to 3R-Li x TiS2 (x = 0.7, 0.9) as Observed in situ with Neutron Powder Diffraction

Abstract

Abstract Layered titanium disulfide is used as lithium-ion intercalating electrode material in batteries. The room-temperature stable trigonal 1T polymorphs of the intercalates Li x TiS2(x ≤ 1) are widely-investigated. However, the rombohedral 3R polymorphs, being stable at higher temperatures for large x, are less well known. In this study, we report on the synthesis of phase-pure 1T-Li x TiS2(x = 0.7,   0.9) and its transformation to the 3R phase between 673 and 873 K as monitored using high-temperature neutron powder diffractometry. For the 3R polymorph, full Rietveld refinements show lithium ions to be statistically distributed over octahedral voids at the fractional coordinates 0, 0, 1/2 , exclusively. The comparison of Madelung energies with results of periodic quantum-chemical calculations reveals that the evolution of lattice parameters and the room-temperature stability of the 1T phase are not governed by electrostatics, but by correlation and polarization. The insights gained do not only elucidate the structure of 3R-Li x TiS2, but also help to understand and control polymorphism in layered transition-metal sulfides.