Probing the local structure and the role of protons in lithium sorption processes of a new lithium-rich manganese oxide

Lithium exchange mechanisms, local structure, and its possible evolutions on lithium extraction and reinsertion are studied in Li1.6Mn1.6O4, a new lithium-rich manganese oxide showing improved lithium sorption. The parent Li1.6Mn1.6O4 and its lithium-extracted and lithium-reinserted forms are characterized by X-ray absorption (EXAFS and XANES) and incoherent inelastic neutron scattering (IINS) spectroscopies. Lattice hydroxyl groups are detected in the lithium-extracted sample; the chemical reinsertion of lithium removes most of the hydroxyl groups, but some protons remain in the structure mainly as structural water. X-ray absorption spectroscopy at the manganese K-edge indicates a local spinel structure of Li1.6Mn1.6O4 with manganese in oxidation state 4+. The structural arrangement and Mn oxidation state remain unchanged when lithium is extracted and reinserted. The lithium sorption occurs by Li+/H+ ion exchange process, as for Li1.33Mn1.67O4, the previously considered end member of the Li1+xMn2-xO4 spinel lithium-rich series. Since the cation-to-anion ratio of Li1.6Mn1.6O4 is not that which is typical for a spinel, the new lithium-rich manganese oxide has a spinel structure with the extra Li+ probably in interstitial sites, although lithium cannot be located unequivocally from these results.