Lithium manganese nickel oxides Lix(MnyNi1-y)2-xO2 -: I.: Synthesis and characterization of thin films and bulk phases

The series Li-x(MnyNi1-y)(2-x)O-2 for x less than or equal to 1.33 and 0.38 less than or equal to y less than or equal to 0.50 shows a very close relationship to its parent series LixNi2-xO2. The refined lattice parameters for at least 0.93 less than or equal to x less than or equal to 1.26 are a linear function of the concentration ratio Li/(Mn + Ni) which in turn is proportional to the averaged valence state of the transition metals, Li-x(MnyNi1-y)(2-x)O-x is able to reversibly coprecipitate/reinsert Li2O and release/absorb O-2. This self-regulation mechanism seems to always adjust the number of cations to an undisturbed oxygen sublattice according to the rule "cations/anions = 1," which holds true at least for temperatures up to 800 degrees C and oxygen partial pressures above 10(-5) atm. Samples prepared in air and under O-2 did not show nucleation of Li2O, not even for x > 1.0. The series Li-x(MnyNi1-y)(2-x)O-2 where 0.38 less than or equal to y less than or equal to 0.50 crystallizes in a rhombohedral unit cell (space group R (3) over bar m) for x < 1.15 and transforms into a single monoclinic phase (space group C2/c) for x > 1.25. The similarity between LixNi2-xO2 and Li-x(MnyNi1-y)(2-x)O-2 strongly suggests a rhombohedral --> cubic transition at x approximate to 0.6 for the latter series. Derived from the linear dependence of the X-ray density on the stoichiometric parameter x, an equation was found with which the lithium concentration of Li-x(MnyNi1-y)(2-x)O-2 thin film phases over the entire range 0 less than or equal to x less than or equal to 1.33 can be determined accurately without extensive ion-beam analysis. XPS measurements on a film with the bulk stoichiometry Li1.10Mn0.39Ni0.51O2 gave evidence for Mn4+ and Mn3+, but no indication was found for nickel valence states other than Ni2+. In order to meet, the above-given stoichiometry, the averaged nickel valence state had to increase with film depth.