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

Abstract

The local structural modifications resulting from chemical extraction and reinsertion of lithium in spinels of composition LiMn2O4 and Li4/3Mn5/3O4 have been investigated by X-ray absorption spectroscopy (XAS) at the manganese K-edge. For LiMn2O4, 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 Li4/3Mn5/3O4 are shorter than for LiMn2O4, 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 LiMn2O4 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 Li4/3Mn5/3O4 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 LiMn2O4 produces a MnIV-type spinel oxide from a mixed MnIII/MnIV parent material and that reinsertion of lithium results in a return to a mixed oxidation state. By contrast, the XANES data for Li4/3Mn5/3O4 show that here the dominant manganese state is MnIV 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 LiMn2O4, but a predominantly ion-exchange process in the case of Li4/3Mn5/3O4, with the insertion of charge-compensating protons into the lattice.

X-Ray Absorption Fine Structure Spectroscopy as a Probe of Local Structure in Lithium Manganese Oxides

“…1 : 1 ratio, but the lithiumextracted sample contained 95% Mn>, with dissolution of ca. 25% of the oxide in the acid treatment, in agreement with reaction [1]. Thermogravimetry showed that no protons were present in the lithium-extracted phase, confirming that lithium-proton exchange had not occurred.…”

“…(1)(2)(3). These compounds are currently of immense technological importance for their applications as cathode materials in rechargeable lithiumion batteries (4,5).…”

“…The lithium insertion chemistry of these compounds is quite complex. Lithium extraction and insertion may take place with electron transfer and corresponding changes in the oxidation state of the transition metal ions, but also by ion exchange (1,(6)(7)(8)(9)(10)(11). Structural changes at a local level or involving a change in the symmetry of the entire crystalline lattice may also occur.…”

“…Such interpretations raise serious questions for present and future users of XAFS to study these and related compounds. We have earlier reported detailed Mn K-edge X-ray absorption studies of spinel lithium manganates and shown the structural and electronic effects of chemical extraction and insertion of lithium in compounds of different stoichiometries (1). We wish here to draw attention to certain errors in the chemical and structural interpretations of the XANES data presented in the aforementioned communications, by discussing the applicability of model compounds and ab initio methods to interpret X-ray absorption near-edge structure at the Mn K-edge.…”

Phase Diagram of Li[sub x](Mn[sub y]Fe[sub 1−y])PO[sub 4] (0≤x, y≤1)

“…The difference in amplitude is evident for k 3 of Mn 3ϩ , the significant damping suggests the enhancement of local disorder by the Jahn-Teller effect of Mn 3ϩ . [20][21][22][23][24] In addition, the spectrum shapes 3 , ͉F(r)͉, shows a radial atomic distribution-like function in real space about the emitting metal ions ͑Fig. 6͒.…”