Li intercalation and anion/cation substitution of transition metal chalcogenides: Effects on crystal structure, microstructure, magnetic properties and Li+ ion mobility

Determination of the Critical Points for Intercalating Systems by the EMF Method Concerning Foreign Metal

The work is devoted to the study of the possibility of determining the critical points of phase diagrams by measuring the EMF of electrochemical cells relative to a metal that is not part of the test substance. The measurement results of the concentration dependences of the EMF for the Cu_xTiS₂ system in the electrochemical cells Na|Na⁺|Cu_xTiS₂ and Cu|Cu⁺|Cu_xTiS₂ in the concentration range 0 < x < 0.7 are compared. It was determined that the chemical potential of the ion is independent of x in a satisfactory approximation. The EMF difference in the given cells is close to the standard electrode potentials difference for respective metals. It is concluded that the dependences E(x) for both types of cells are similar and correlated. Therefore, one can choose any convenient metal for measurements of this kind. Using the example of measuring EMF in electrochemical cells for the Fe_xTiSe₂ system concerning sodium and lithium, we showed that the elastic contribution to free energy is constant over the entire concentration range 0 < x < 0.5.

Investigation of ionic and anomalous magnetic behavior in CrSe2 using 8Li β-NMR

We have studied a mosaic of 1T-CrSe₂ single crystals using β-detected nuclear magnetic resonance of ⁸Li from 4 to 300 K. We identify two broad resonances that show no evidence of quadrupolar splitting, indicating two magnetically distinct environments for the implanted ion. We observe stretched exponential spin lattice relaxation and a corresponding rate (1/T₁) that increases monotonically above 200 K, consistent with the onset of ionic diffusion. A pronounced maximum in 1/T₁ is observed at the low temperature magnetic transition near 20 K. Between these limits, 1/T₁ exhibits a broad minimum with an anomalous absence of strong features in the vicinity of structural and magnetic transitions between 150 and 200 K. Together, the results suggest ⁸Li⁺ site occupation within the van der Waals gap between CrSe₂ trilayers. Possible origins of the two environments are discussed.

We report the first nuclear magnetic resonance measurements of an atypical transition metal dichalcogenide, CrSe₂.

On the Mechanisms of Chemical Intercalation of Lithium in Electrode Materials

Reaction with n-butyl lithium (BuLi) is commonly used to study the kinetics of intercalation of lithium in electrode materials for batteries. We performed lithium isotope exchange experiments on TiS2 single crystals as model system to determine the irreversible step in the intercalation process. Single crystals of TiS2 were prepared by chemical vapor transport and intercalated by lithium with natural isotopic signature (7% 6Li 93% 7Li) using a 2.5 molar solution of BuLi in hexane. Crystals were homogenized at 313 K in a dry argon atmosphere for 1–2 months and then exposed for several days to ~1 molar BuLi solution enriched in the light lithium isotope (95% 6Li, 5% 7Li). After the isotopic exchange experiments concentration profiles of the isotopes were measured parallel to the a/b-plane of the crystal using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Profiles show that already intercalated lithium isotopes can be completely exchanged without significant increase in total lithium content. This observation implies that, after adsorption of BuLi at the crystal surface, the butyl radical can freely jump from one lithium atom to a neighbouring one. The jump sequence is either finished by desorption of a BuLi molecule or by combination of two adjacent butyl radicals forming an octane molecule or other kinds of deactivation of butyl radicals. The latter step is irreversible. Self diffusion coefficients determined by fitting of the lithium isotope profiles are in good agreement with chemical diffusivities determined by lithium intercalation experiments.

Li ion dynamics along the inner surfaces of layer-structured 2H-LixNbS2

Layer-structured materials, such as graphite (LiCy) or Lix(Co,Ni,Mn)O2, are important electrode materials in current battery research that still relies on insertion materials. This is due to their excellent ability to reversibly accommodate small alkali ions such as Li(+) and Na(+). Despite of these applications, microscopic information on Li ion self-diffusion in transition metal sulfides are relatively rare. Here, we used (7)Li nuclear magnetic resonance (NMR) spectroscopy to study translational Li ion diffusion in hexagonal (2H) LixNbS2 (x = 0.3, 0.7, and 1) by means of variable-temperature NMR relaxometry. (7)Li spin-lattice relaxation rates and (7)Li NMR spectra were used to determine Li jump rates and activation barriers as a function of Li content. Hereby, NMR spin-lattice relaxation rates recorded with the spin-lock technique offered the possibility to study Li ion dynamics on both the short-range and long-range length scale. Information was extracted from complete diffusion-induced rate peaks that are obtained when the relaxation rate is plotted vs inverse temperature. The peak maximum of the three samples studied shifts toward higher temperatures with increasing Li content x in 2H-LixNbS2. Information on the dimensionality of the diffusion process was experimentally obtained by frequency dependent Rρ measurements carried out at T = 444 K, that is in the high-temperature regime of the rate peaks. A slight, but measurable frequency-dependence within this limit is found for all samples; it is in good agreement with predictions from relaxation models developed to approximate low-dimensional (2D) jump diffusion.

Kinetics of Lithium Intercalation in Titanium Disulfide Single Crystals

Single crystals of titanium disulfide TiS2 were synthesized by chemical vapor synthesis and subsequently intercalated with n-butyl lithium (BuLi) in n-hexane. Experiments were carried out using a butyl lithium concentration between 0.8 and 10 mol L-1 and the temperature range was from 248 K to 328 K. The duration of the intercalation was varied from 2 h to 30 d. After the intercalation experiments concentration profiles of lithium, titanium and sulfur were measured parallel to the ab-plane of the crystal by LA-ICP-OES (LASER Ablation — Inductively Coupled Plasma - Optical Emission Spectroscopy). Chemical diffusion coefficients (D) were determined by fitting of the profiles to the specific solution of Fick's 2 nd law for the given boundary conditions.

The measured diffusivity in the ab-plane (D|| a/b ) varies between 10-13 and 10-15 m2 s-1 at room temperature. These variations of D cannot be correlated to the reaction time. On the other hand a systematic increase of D|| a/b with the concentration of butyl lithium (cBuLi ) was observed, with a corresponding decrease in activation energy from 59.6 ± 7.6 kJ mol-1 (cBuLi=1.6 mol L-1 ) to 42.6 ± 11.7 kJ mol-1 (cBuLi=10 mol L-1 ). Furthermore, profiles measured on the same crystals reveal D|| a/b values differing by up to a factor of 3. ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectromety) images gave evidence of inhomogenous insertion of lithium along crystal edges.

These findings indicate that stress induced by widening of the crystal layers plays a crucial role in the intercalation kinetics. SIMS profiling perpendicular to the ab-plane gives evidence that D⊥ a/b is at least four orders of magnitude lower than D|| a/b .