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 Li_2–2XM_1+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 Ti^3+/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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