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Issue 5, 1998
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Crystal chemistry and physical properties of complex lithium spinels Li2MM′3O8 (M=Mg, Co, Ni, Zn; M′=Ti, Ge)

Abstract

The spinels Li2MM′3O8 (MM′=MgTi, CoTi, CoGe, NiGe and ZnGe) are cubic with space group P4332 . Simple crystal field theory qualitatively explains the distribution of M over tetrahedral and octahedral sites: Ni occupies only octahedral sites, whereas Zn, Mg and Co show strong preference for tetrahedral sites. 1:3 cation ordering of Li/M and M′ occurs on the octahedral sites. The titanates undergo an order–disorder phase transition involving the octahedral cations at high temperatures, whereas the ordered phase is maintained until melting for the germanates. Solid solutions Li2–2XM1+3XM′3–XO8 form at both sides of the Li2MM′3O8 stoichiometry for the titanates; but there is no substantial range of solid solution for Li2ZnGe3O8 and Li2NiGe3O8 . The occurrence of order–disorder phenomena and solid solutions in the titanates is attributed to the similarity in size of Li, M and Ti, whereas the smaller Ge is less able to disorder with Li/M. M is shown to be divalent from magnetic susceptibility measurements (for Co and Ni) with the support of conductivity data. The samples containing Co and Ni are paramagnetic down to 5 K. From impedance measurements on pellets with blocking electrodes, the main conductive species is deduced to be Li+ : the activation energies for conduction are high, 0.55<ΔH/eV<2.14. Cyclic voltammograms show a set of reversible peaks at ca. 1.5 V vs. Li/Li+ for the titanates, attributed to the Ti3+/4+ couple, but no Li could be electrochemically extracted from either titanates or germanates up to 5 V vs. Li/Li+ .

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Article type: Paper
DOI: 10.1039/A800234G
Citation: J. Mater. Chem., 1998,8, 1273-1280
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    Crystal chemistry and physical properties of complex lithium spinels Li2MM′3O8 (M=Mg, Co, Ni, Zn; M′=Ti, Ge)

    H. Kawai, M. Tabuchi, M. Nagata, H. Tukamoto and A. R. West, J. Mater. Chem., 1998, 8, 1273
    DOI: 10.1039/A800234G

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