Three different cation-vacant spinels aiming at composition “Li2Mn4O9” were prepared by (1) low-temperature solid state reaction, (2) chemical delithiation, (3) electrochemical delithiation. Products of routes (1) and (2) were studied by neutron diffraction. Rietveld refinements confirmed the double-vacancy scheme proposed by de Kock et al. [Mater. Res. Bull. 25 (1990) 657] in the former. The exact composition of both samples departed significantly from the expected stoichiometry. They also differed in their electrochemical behavior. Both can be cycled at 3V with much better stability than stoichiometric LiMn2O4, probably due to the increased initial manganese valence with respect to LiMn2O4. Spinel with tetrahedral-vacancies only gives an additional reversible redox step ca. 150mV above the main plateau. This feature is observed on both discharge (lithium insertion) and charge (extraction).