Li intercalation and anion/cation substitution of transition metal chalcogenides

Effects on crystal structure, microstructure, magnetic properties and Li⁺ ion mobility

verfasst von

Wolfgang Bensch, Thomas Bredow, Hubert Ebert, Paul Heitjans, Sylvio Indris, Sergiy Mankovsky, Martin Wilkening

Abstract

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, i.e., Li intercalation reduces the crystallite size and induces strain in the material. In the electrochemical discharge curves a significant dependence of the lengths and voltage of the plateaus on the Ti content is observed. 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 magnetic properties of the host materials are significantly influenced by Li uptake. 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.

Organisationseinheit(en)

Institut für Physikalische Chemie und Elektrochemie

Externe Organisation(en)

Christian-Albrechts-Universität zu Kiel (CAU)
Rheinische Friedrich-Wilhelms-Universität Bonn
Ludwig-Maximilians-Universität München (LMU)

Typ

Übersichtsarbeit

Journal

Progress in solid state chemistry

Band

37

Seiten

206-225

Anzahl der Seiten

20

ISSN

0079-6786

Publikationsdatum

12.2009

Publikationsstatus

Veröffentlicht

Peer-reviewed

Ja

ASJC Scopus Sachgebiete

Allgemeine Materialwissenschaften, Physik der kondensierten Materie, Physikalische und Theoretische Chemie

Elektronische Version(en)

https://doi.org/10.1016/j.progsolidstchem.2009.11.007 (Zugang: Unbekannt)