Lithium Insertion into Li2MoO4: Reversible Formation of (Li3Mo)O4 with a Disordered Rock-Salt Structure

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.