Mechanism of Proton Insertion and Characterization of the Proton Sites in Lithium Manganate Spinels

Abstract

Protonated forms of the spinel manganese dioxide phase A-MnCb 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 A-MnOa 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 /3-2) 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 A-MnOa 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 A-MnOa samples and is attributed to protons in interstitial sites, associated with a small amount of Mnm in the material. Variable temperature infrared spectroscopy and X-ray diffraction showed that the loss of water above 100 °C results in destruction of the A-MnOa spinel lattice and suggests that water plays an important role in stabilizing the protonated A-MnOa structure. A model is proposed in which protons associated with oxygen atoms at 16d octahedral vacancies form lattice water species.

The Surface Structure of the Proton-Exchanged Lithium Manganese Oxide Spinels and Their Lithium-Ion Sieve Properties

“…10a, in the range between 1500 and 1700 cm\ and above 3200 cm\. The band at 910 cm\ observed for sample A is ascribed to a lattice coupling vibration of hydroxyl groups (17,19). This band was not observed in the starting LiMn O spinel irrespective of heating.…”

“…The Li>-H> ion-exchange reaction is also likely to occur in spinels with increased number of tetravalent Mn ions (17), i.e., Li > Mn \ O with increasing value of up to 0.33. Proton sites in the manganese oxide spinel, the stoichiometric composition of which is based on Li Mn O ( "0.33), have therefore been characterized recently with the inelastic neutron scattering (INS) technique (19). This study showed that in the spinel lattice the inserted protons can exist in the form of hydroxyl groups and structural water molecules, which are associated with the vacant 8a tetrahedral sites and oxygen atoms at 16d octahedral vacancies, respectively.…”

Topotactic Reactions, Structural Studies, and Lithium Intercalation in Cation-Deficient Spinels with Formula Close to Li2Mn4O9

“…In a "rst step, the structure was re"ned with lithium, manganese, and oxygen atoms only. Several proton sites located along O}O lines in the spinel structure were proposed previously for the formation of strong OH bonds on the basis of infrared spectroscopy data (17). Tests for all these yielded negative or negligible hydrogen occupations.…”

Preparation and Characterization of a Cylinder-Type Adsorbent for the Recovery of Lithium from Seawater

“…4. This peak shift is commonly observed after lithium extraction 10) and indicates that the lithium extraction proceeded topotactically while the original spinel structure was maintained.…”