TY - JOUR

T1 - Li intercalation and anion/cation substitution of transition metal chalcogenides

T2 - Effects on crystal structure, microstructure, magnetic properties and Li+ ion mobility

AU - Bensch, Wolfgang

AU - Bredow, Thomas

AU - Ebert, Hubert

AU - Heitjans, Paul

AU - Indris, Sylvio

AU - Mankovsky, Sergiy

AU - Wilkening, Martin

N1 - Funding Information: We thank J. Wontcheu, O. Riemenschneider, M. Behrens and J. Ophey (workgroup of Prof. W. Bensch) for the sample preparation and contributions to the different experimental investigations. Furthermore, we thank J. Heine and B. Ruprecht (workgroup of Prof. P. Heitjans) for carrying out the NMR measurements in Hannover, Prof. D. Freude and M. Fernández (University of Leipzig) for the preliminary Ti NMR measurements as well as the workgroup of Prof. R. Bernd (University of Kiel) for the STM images. The corresponding single crystals were supplied by the workgroup of Prof. M. Binnewies (Leibniz University Hannover). We also thank Prof. C. P. Grey (Stony Brook University) for NMR cooperation. Financial support of the DFG (SPP 1136) is gratefully acknowledged.

PY - 2009/12

Y1 - 2009/12

N2 - 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 Cr5-yTiySe8. In addition, the effect of anion substitution in TiS2-zSez on the Li diffusion parameters was studied by 7Li nuclear magnetic resonance (NMR) spin-lattice relaxation measurements. For Cr5-yTiySe8 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 Cr5-yTiySe8. 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 LixTiS2-zSez (x ≈ 0.7) the Li diffusivity was investigated by various NMR techniques and compared with results obtained for the pure end members LixTiS2 and LixTiSe2. 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.

AB - 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 Cr5-yTiySe8. In addition, the effect of anion substitution in TiS2-zSez on the Li diffusion parameters was studied by 7Li nuclear magnetic resonance (NMR) spin-lattice relaxation measurements. For Cr5-yTiySe8 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 Cr5-yTiySe8. 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 LixTiS2-zSez (x ≈ 0.7) the Li diffusivity was investigated by various NMR techniques and compared with results obtained for the pure end members LixTiS2 and LixTiSe2. 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.

KW - Li NMR spectroscopy

KW - Band structure calculations

KW - In-situ X-ray diffraction

KW - Li diffusion

KW - Li intercalation

KW - Magnetic properties

KW - Transition metal chalcogenides

UR - http://www.scopus.com/inward/record.url?scp=71049147491&partnerID=8YFLogxK

U2 - 10.1016/j.progsolidstchem.2009.11.007

DO - 10.1016/j.progsolidstchem.2009.11.007

M3 - Review article

AN - SCOPUS:71049147491

VL - 37

SP - 206

EP - 225

JO - Progress in solid state chemistry

JF - Progress in solid state chemistry

SN - 0079-6786

IS - 2-3

ER -