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 3 V 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. 150 mV above the main plateau. This feature is observed on both discharge (lithium insertion) and charge (extraction).