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

We investigated experimentally the effect of Li intercalation on the structural, microstructural and magnetic properties as well as on the Li ion diffusivity of the complex chalcogenides Cr_5?yTi_ySe₈. In addition, the effect of anion substitution in TiS_2?zSe_z on the Li diffusion parameters was studied by ⁷Li nuclear magnetic resonance (NMR) spin-lattice relaxation measurements.For Cr_5?yTi_ySe₈ the Li⁺ insertion is accompanied by an irreversible phase transition from monoclinic to trigonal symmetry which is electronically driven. The maximal Li content in the host material depends on the Ti content and decreases with increasing y in Cr_5?yTi_ySe₈. The intercalated materials can be deintercalated and the minimal Li content in the residual compound increases with Ti abundance. The intercalation process is accompanied by drastic changes of the microstructure. Electrochemical discharge curves depend significantly on the Ti. According to the results of XANES investigations performed on Cr₄TiSe₈, Ti is first reduced during Li uptake and Cr atoms accept electrons at later stages of the intercalation reaction. In-situ energy dispersive X-ray diffraction experiments show that the Li intercalation at room temperature proceeds via two different mechanisms while intercalation at 60 °C is faster and is dominated by one mechanism. ⁷Li MAS NMR measurements revealed a variety of transition metal environments around the Li sites corresponding to the Cr/Ti disorder. The NMR studies also indicate fast Li dynamics. The magnetism of the educts is dominated by strong antiferromagnetic exchange interactions in the high temperature region and by spin-glass behavior in the low temperature range. Intercalation of Li weakens the antiferromagnetic exchange and for fully intercalated materials ferromagnetic exchange is observed. The interpretation of the experimental results is supported by accompanying band structure calculations.In layer-structured Li_xTiS_2?zSe_z (x ≈ 0.7) the Li diffusivity was investigated by various NMR techniques and compared with results obtained for the pure end members Li_xTiS₂ and Li_xTiSe₂. In particular, anion substitution clearly influences the slopes of the low-T flanks of the diffusion induced NMR relaxation-rate peaks. The corresponding activation barriers characterizing local hopping processes are reduced in the mixed samples with 0 < z < 2 and can be explained by a domain model. DFT calculations yield very small hopping barriers along S-rich and Se-rich domain boundaries while the barriers for Li migration inside the domains are rather high. It is therefore assumed that Li migrates along the domain boundaries.