Effect of Chemical Extraction of Lithium on the Local Structure of Spinel Lithium Manganese Oxides Determined by X-ray Absorption Spectroscopy

The local structural modifications resulting from chemical extraction and reinsertion of lithium in spinels of composition LiMn 2 O 4 and Li 4/3 Mn 5/3 O 4 have been investigated by X-ray absorption spectroscopy (XAS) at the manganese K-edge. For LiMn 2 O 4, Mn−O and Mn−Mn distances determined from the extended X-ray absorption fine structure (EXAFS) decrease significantly when lithium is extracted in 0.1 M HCl, but are restored to their original values when ∼90% of the lithium content is reconstituted by sorption in LiOH solution. The Mn−O and Mn−Mn distances for Li 4/3 Mn 5/3 O 4 are shorter than for LiMn 2 O 4, consistent with a higher manganese oxidation state and lower unit cell parameter for this compound, and show much smaller variations on lithium extraction and reinsertion. In addition lithium extraction from LiMn 2 O 4 results in an increase in local structural order, followed by a return to the lower symmetry state of the parent material when lithium is reinserted. These changes are not observed in the spectra of Li 4/3 Mn 5/3 O 4 and its delithiated or relithiated products, which show rather the effects of local structural perturbations due to manganese vacancies in octahedral sites. Evolution of the X-ray absorption near-edge structure (XANES), in particular the position of the main edge discontinuity and modifications in the pre-edge spectral structure, supports chemical analyses in showing that lithium extraction from LiMn 2 O 4 produces a Mn IV -type spinel oxide from a mixed Mn III /Mn IV parent material and that reinsertion of lithium results in a return to a mixed oxidation state. By contrast, the XANES data for Li 4/3 Mn 5/3 O 4 show that here the dominant manganese state is Mn IV and that lithium extraction and reinsertion entail only trivial local electronic changes. The XAS results are therefore compatible with a mainly redox mechanism for lithium extraction and reinsertion for LiMn 2 O 4, but a predominantly ion-exchange process in the case of Li 4/3 Mn 5/3 O 4, with the insertion of charge-compensating protons into the lattice.