Spinel Lithium Manganates: Characterisation by Probe Spectroscopic and Structural Methods, and a Synthetic Route to a Nanocrystalline form

Abstract

Lithium manganate phases having the spinel crystal structure have been investigated in terms of the relationships between their overall stoichiometry, local structural order, and chemical properties in regard to topotactic extraction and insertion of lithium. A series of lithium manganates have been prepared with compositions Li(1+x)MnIII(1-3x-y)MnIV(1+2x+y)O(4+0.5y), where 0≤x≤0.33 and 0≤y≤(1-3x), from different starting materials in classical solid state reactions at 400 °C and 800 °C. The lithium manganates, and their λ-MnO2 products after lithium extraction in 0.2M HCl and lithium reinsertion in 0.1 M LiOH, have been characterised by X-ray diffraction, elemental analysis, thermogravimetry and differential thermal analysis. The characterisation supports previously described models where chemical extraction and insertion of lithium occurs by oxidation and reduction of manganese to the measure that the parent lithium manganates contain trivalent manganese, but also by ion exchange with protons as the MnIV/MnIII ratio in the parent material is increased. X-ray absorption spectroscopy at the manganese K-edge has been applied as a probe method to characterise the oxidation state of manganese and the local environment around the manganese centres in lithium manganates of approximate composition LiMn2O4, LiMn2O9/2 and Li4/3Mn5/3O4, and to investigate the chemical and structural changes in LiMn2O4 and Li4/3Mn5/3O4 when lithium is chemically extracted from the spinel lattices, and then reinserted. Analysis of the extended X-ray absorption fine structure (EXAFS) in the spectrum of a model λ-MnO2 compound has been performed in detail, and structural parameters refined using both phase and amplitude functions for Mn-O and Mn-Mn atom pairs drawn from the tables of McKale and ab initio calculated functions for λ-MnO2 calculated using the FEFF code. Refinements in both cases give very similar results, and the parameters have been successfully transferred to the data treatment of the EXAFS spectra of the other lithium manganate compounds and their products. The local structure determined for the LiMn2O9/2 and Li4/3Mn5/3O4 compounds is compatible with the crystallographic model of approximately 5/6 occupancy of 16d octahedral sites by manganese in these phases and consequent local disorder in the lattice. The LiMn2O4 compound showed full 16d site occupancy by manganese, but local disorder due to the presence of both MnIII and MnIV in the material. Energy positions of absorption edge structures provide supporting evidence for the predominance of MnIV in the LiMn2O9/2 and Li4/3Mn5/3O4 compounds. Extraction of lithium results in modifications of the pre-edge structure and position of the edge which confirm that oxidation of manganese plays a more important role in the reaction with LiMn2O4 than in the case of Li4/3Mn5/3O4, and energy shifts of the multiple scattering resonances in the near edge structure (XANES) correlate with changes in Mn-Mn distance determined from the EXAFS. Reinsertion of lithium results in almost complete restoration of the EXAFS and XANES spectra of the parent lithium manganates. The proton sites in λ-MnO2 materials have been characterised using a combination of inelastic neutron scattering (INS) and infrared spectroscopies. The INS spectra of λ-MnO2 compounds derived by extraction of lithium from lithium manganates of compositions close to Li4/3Mn5/3O4 are dominated by a mode at 908 cm-1, also observed in the infrared spectra, which is assigned to lattice hydroxyl groups associated with vacant 8a tetrahedral sites. The presence of an INS mode at 1080 cm-1 was observed for one of the λ-MnO2 samples and is attributed to protons in interstitial sites, associated with a small amount of MnIII in the material. Variable temperature infrared spectroscopy, INS, X-ray diffraction and EXAFS have shown that dehydration between 80 and 120 °C results in destruction of the protonated λ-MnO2 structure and suggests that water plays an important role in stabilising the λ-MnO2 lattice in the protonated materials. A model is proposed in which protons associated with oxygen atoms at 16d octahedral vacancies form lattice water species. Chemical insertion of protons into λ-MnO2 by chemical reduction in nonaqueous solution has given a distorted spinel phase also containing lattice water. A sol-gel route to the preparation of nanocrystalline spinel lithium manganates formed in situ within a silica support matrix has been developed, and the materials characterised by X-ray diffraction and electron microscopy. The method consists of the preparation of pre-doped silica gel from polyoxyethylene surfactant solutions containing lithium and manganese nitrates, followed by pyrolysis of the surfactants to give crystallisation of lithium manganate. Modification of the gel by the surfactants prevents the segregation and crystallisation of manganese(III) oxide as a separate phase observed in non-modified silica, so that the bimetallic lithium manganese oxide is the only crystalline phase observed after calcination at temperatures from 270 °C. The crystallinity and crystal morphology of the lithium manganate, and the microstructures of the composite materials, appear to be strongly determined by the surfactant. Crystallites of dimensions 20-40 nm are formed in the calcined surfactant gels for metal oxide contents up to 40 wt % in the silica support. X-ray absorption spectroscopy at the manganese K-edge has been applied to characterise the manganese states and structures at all stages of the synthesis, and after acid treatment of the materials. The data suggests that manganese(II) is at least partly coordinated by polyoxyethylene oxygen atoms in the surfactant-modified gels, comparable to the formation of glycolate species when ethylene glycol is used in an analogous process. The manganese environment in the surfactant-modified gels after calcination is characterised by a lithium manganate phase coexisting with manganese ions in a more disordered environment. An increase in the amount of crystalline phase is achieved by increasing the Li/Mn ratio at the pre-doping stage and raising the calcination temperature to 400-500 °C. Comparison of the X-ray absorption data with those for the lithium manganates prepared by classical methods has shown that the resulting phase contains manganese predominantly in a tetravalent state, and undergoes lithium extraction by an analogous mechanism.