Local structure and lithium-proton ion exchange in Li1.33−x/3CoxMn1.67−2x/3O4 spinels

Lithium manganate spinels with the formula Li1+xMn2−xO4 undergo lithium↔proton ion exchange to form a protonated λ-MnO2-type phase. The Li1+xMn2−xO4 phases are metastable, and preparation at temperatures in excess of 400 °C results in phase separation. By partially substituting the manganese with cobalt, single-phase spinels of formula Li1.33−x/3CoxMn1.67−2x/3O4 (0<x<1) can be prepared with high crystallinity up to 750 °C. The local structure of these substituted lithium-rich spinels has been probed with X-ray absorption near-edge spectroscopy (XANES) and extended fine structure (EXAFS). Together with X-ray and neutron diffraction, it is shown that the cobalt is trivalent and that the cobalt and manganese are homogeneously distributed across the 16d octahedral site. Lithium extraction in aqueous acid occurs by lithium↔proton ion exchange. Incoherent inelastic neutron scattering (INS) shows that inserted protons are present predominately as structurally ordered hydroxyl, which is reexchangeable with lithium in aqueous solution.