Bj. Neudecker et al., LITHIUM MANGANESE NICKEL OXIDES LI-X(MNYNI1-Y)(2-X)O-2 - I - SYNTHESIS AND CHARACTERIZATION OF THIN-FILMS AND BULK PHASES, Journal of the Electrochemical Society, 145(12), 1998, pp. 4148-4159
Citations number
34
Categorie Soggetti
Electrochemistry,"Materials Science, Coatings & Films
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 lattic
e parameters for at least 0.93 less than or equal to x less than or eq
ual 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 copre
cipitate/reinsert Li2O and release/absorb O-2. This self-regulation me
chanism 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 p
artial 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 L
i-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 p
hase (space group C2/c) for x > 1.25. The similarity between LixNi2-xO
2 and Li-x(MnyNi1-y)(2-x)O-2 strongly suggests a rhombohedral --> cubi
c transition at x approximate to 0.6 for the latter series. Derived fr
om the linear dependence of the X-ray density on the stoichiometric pa
rameter 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 les
s than or equal to x less than or equal to 1.33 can be determined accu
rately without extensive ion-beam analysis. XPS measurements on a film
with the bulk stoichiometry Li1.10Mn0.39Ni0.51O2 gave evidence for Mn
4+ and Mn3+, but no indication was found for nickel valence states oth
er than Ni2+. In order to meet, the above-given stoichiometry, the ave
raged nickel valence state had to increase with film depth.