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

The spinels Li2MM'O-3(8) (MM' = MgTi, CoTi, CoGe, NiGe and ZnGe) are cubic with space group P4(3)32. 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'O-3-X(8) form at both sides of the Li2MM'O-3(8) 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 < Delta HeV < 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+.