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Schlagwörter:
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Zusammenfassung:
During Li-insertion in some complex transition metal molybdates with a
NASICON structure, which serve as cathodes in Li-ion rechargeable cells,
a formation of a cubic rock-salt-type phase was often detected between 1
and 2 V vs Li+/Li. Detailed information about elemental composition and
stability of this compound was missing, and suggestions were made toward
a solid solution composed of lithium oxide and two-valence transition
metal oxide MO with M a 3d element. In the present work, we showed that
Li2MoO4 with a phenacite-type structure without any additional
transition metal can reversibly accommodate Li-ions at room temperature
with the formation of the NaCl-type compound. Reversible
Li-incorporation into the Li2MoO4 structure is accompanied by a
reduction of Mo ions and changes in their oxygen coordination. Li-ions
are shifted from a tetrahedral to an octahedral site, resulting in the
formation of a cubic (Li3Mo)O-4 framework with a random distribution of
Li and Mo on one site. This mixed occupancy is remarkable because of
significant charge and size differences between Li+ and Mo5+. The novel
compound shows Li-deficiency at least up to x(Li) = 0.2, which can be
deduced from charge flow in the galvanostatic cycling of the
electrochemical cells with a (Li3Mo)O-4 cathode between 1.5 and 2.75 V
vs Li+/Li. An increase in the cell potential above 3 V leads to the
oxidation of (Li3Mo)O-4 back to Li2MoO4 with phenacite-type structure.
The reaction of (Li3Mo)O-4 to Li2MoO4 also occurs upon a short exposure
to air.
