Topotactic reactions, structural studies, and lithium intercalation in cation-deficient spinels with formula close to Li2Mn4O9

The composition Li2Mn4O9, reported as a spinel oxide containing vacancies o n both tetrahedral and octahedral sites [A. de Kock et al, Mater. Res. Bull . 25, 657 (1990)], was approached using three different preparation routes: low-temperature solid state reaction (A), chemical delithiation (B), and e lectrochemical delithiation (C). Rietveld refinements from neutron diffract ion data confirmed the double-vacancy scheme proposed previously for produc t A, but with more tetrahedral and fewer octahedral vacancies than in the i deal Li2Mn4O9 formula. Low-temperature solid state reactions systematically result in broad reflections. Sample B, which was obtained topotactically, exhibits much narrower reflections. But chemical analyses, thermogravimetry , and neutron diffraction show that the acid treatment introduces significa nt amounts of protons, resulting in a formula close to Li0.92HMn4O9. Sample s A and B were cycled electrochemically in lithium cells at 3 V with better stability than LiMn2O4, probably due to their higher initial manganese oxi dation state. No separate electrochemical step linked to the filling of vac ancies is observed in A, whereas B gives an additional redox step ca. 200 m V above the main plateau. This feature is not observed on compounds A or C; it is reversible, and seems to be a specific property of this spinel with a low initial cell parameter (8.09 Angstrom). Sample A2 with double cation vacancies is especially stable on cycling at 3 V, and shows a very small vo lume variation on lithium intercalation. (C) 2001 Academic Press.