Lithium Diffusion Pathways in 3R-Li_xTiS₂: A Combined Neutron Diffraction and Computational Study

Abstract

Layered lithium transition-metal sulfides have long been discussed as early electrode materials for lithium-ion batteries. However, fundamental knowledge of lithium-ion migration in these solids is still lacking. In this study, we report on the diffusion dynamics in lithium-deficient high-temperature polymorphs of lithium titanium sulfides (3R-Li x TiS 2 ; x = 0.7, 0.9) as analyzed using powder neutron diffractometry and density functional theory (DFT) climbing-image nudged-elastic-band (cNEB) calculations. Two classes of probable migration pathways have been identified from the scattering-length density distributions (filtered using the maximum-entropy method [MEM]) and the probability density functions (PDFs, modeled from anharmonic Debye−Waller factors): direct diffusion in the (001) plane as the major mechanism and indirect diffusion through adjacent tetrahedral voids as a minor mechanism. Calculated activation barriers agree well with one-particle potentials (OPPs) derived from measurements for Li 0.7 TiS 2 (0.484 [14] and 0.88[4] eV) but deviate for Li 0.9 TiS 2 . The discrepancy at low defect concentration is attributed to the failure of the OPP derivation and the different nature of the methods (space-time averaged vs individual-ion perspective). This work elucidates the pathways of lithium-ion diffusion in 3R-Li x TiS 2 and points out pitfalls in established experimental/computational methods.

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

“…This finding, which is much more pronounced for Li 0.7 TiS 2 , indicates an additional thermally activated dynamic process superimposed on the normal temperature evolution of ADPs and comprising only the lithium ions. A future publication will substantiate, that it is indeed lithium-ion diffusion in plane, and elaborate on this phenomenon being well beyond the scope of this work [34].…”

“…Anharmonic effects in atomic displacement were discovered that will be adequately addressed in a publication to come [34].…”

LiBi3S5—A lithium bismuth sulfide with strong cation disorder

“…For an experimental mapping of lithium diffusion pathways in crystalline solids, neutron diffraction at-preferably multiple-high temperatures is the method of choice [30,31]. In the present case, however, effective evaluation of powder data would be severely hampered by cation disorder and low symmetry.…”

Diffusion Pathways and Activation Energies in Crystalline Lithium-Ion Conductors

“…In the final structure (right), the ion has migrated to the formerly vacant octahedral site. Adapted with permission from [41]. © 2015 American Chemical Society.…”

“…Because our powder neutron-diffraction study on Li 0.7 TiS 2 and Li 0.9 TiS 2 required high temperature to thermally activate lithium-ion diffusion, it dealt with the 3R phases exclusively [41]. We used PDF/OPP analysis to test three probable paths: two along different directions strictly in plane and one including connecting tetrahedral voids above/below the lithium-containing layer (see Figure 6).…”