MECHANISM OF PROTON INSERTION AND CHARACTERIZATION OF THE PROTON SITES IN LITHIUM MANGANATE SPINELS

Protonated forms of the spinel manganese dioxide phase lambda-MnO2 have been prepared by the ion-exchange of three different lithium manganate precursors. The mechanisms of lithium extraction and proton insertion were examined in each case by X-ray diffraction and chemical and thermal analyses. The amount of protons inserted by ion exchange differed according to the composition of the precursor lithium manganate, the distribution of cations between 8a tetrahedral and 16d octahedral sites, and the oxidation state of the manganese, all of which depend on the method of preparation. The proton sites in the lambda-MnO2 materials were characterized using a combination of inelastic neutron scattering (INS) and infrared spectroscopies. The strongest features in the INS spectra, at 910 and 1080 cm(-1), are assigned to OH deformation modes. These results are discussed in relation to thermal analysis and infrared data for the manganese dioxides pyrolusite (beta-MnO2) and synthetic ramsdellite, and the proton sites explained using a conventional model for the manganese oxide lattice. The 910 cm(-1) mode dominated the INS spectra of all three lambda-MnO2 materials and is assigned to lattice hydroxyl groups associated with the vacant 8a tetrahedral sites in the structure. The presence of an INS mode at 1080 cm(-1) was observed for only one of the lambda-MnO2 samples and is attributed to protons in interstitial sites, associated with a small amount of Mn-III in the material. Variable temperature infrared spectroscopy and X-ray diffraction showed that the loss of water above 100 degrees C results in destruction of the lambda-MnO2 spinel lattice and suggests that water plays an important role in stabilizing the protonated lambda-MnO2 structure. A model is proposed in which protons associated with oxygen atoms at 16d octahedral vacancies form lattice water species.