Kinetics of Lithium Intercalation in TiX2 Single Crystals (X = S, Se, Te) under Hydrostatic Pressure

Abstract

Abstract Chemical intercalation of lithium in single crystals of TiX2 (X = S, Se, Te) was studied at pressures of 10–500 MPa and temperatures of 293–363 K. Crystals were synthesized by chemical vapor synthesis and cut with a LASER into squares (2.2 × 2.2 mm). Samples and 1.6 mol · L –1 n-butyl lithium solutions in n-hexane (BuLi) were sealed in gold capsules. Experiments were carried out in cold seal pressure vessels for 5–20 days using Ar as pressure medium. A large fraction of samples showed degradation after the experiments, i.e., at low pressures and at high temperatures. On samples with minor degradation, lithium profiles were measured parallel to the crystallographic ab-plane, using femtosecond-LASER Ablation-Inductively Coupled Plasma-Optical Emission Spectroscopy and -Mass Spectrometry (fs-LA-ICP-OES and -MS). Most of the profiles could be well fitted by the specific solution of Fick's second law for the given boundary conditions. Lithium diffusivities (D Li) and lithium contents at the crystal edges (x Li,S) were determined from the fits. At 500 MPa and room temperature lithium diffusivities are similar in TiS2 ((1.07 to 2.41) × 10 –15 m2 · s –1) and TiSe2 ((1.48 to 2.10) × 10 –15 m2 · s –1) but significantly lower in TiTe2 ((0.07 to 0.55) × 10 –15 m2 · s –1). Moreover, diffusivity of Li in TiTe2 decreases significantly with increasing pressure whereas for the other chalcogenides a pressure dependence cannot be evidenced. Lithium surface contents are lower for TiS2 (x = 0.15 to 0.36) than for TiSe2 (0.66 to 0.71) and TiTe2 (0.59 to 0.99), slightly elevated compared to ambient pressure experiments. These findings indicate, that the ionicity of the Ti–X bonding as well as the size of the interlayer space (van der Waals gap) play a crucial role for the degree of lithium insertion and for the pressure dependence of lithium diffusivity.