The surface structure of the proton-exchanged lithium manganese oxide spinels and their lithium-ion sieve properties

The spinel structure and the valence states of the Mn ions both at the surface and the bulk were characterized for the Li+ extracted and inserted manganese oxide spinels. The Li+ extraction from the orthorhombic LiMn2O4 spinel results in the formation of the cubic spinel, the crystal structure of which is the same as lambda-MnO2. The LiMn2O4 spinel prepared from an electrolytically prepared manganese dioxide is converted to HMn2O4 on dilute acid treatment. X-ray photoelectron spectroscopic analysis revealed that the valence state of the surface Mn ions remains unchanged during the Li+ extraction and insertion, which indicates an occurrence of the Li+-H+ ion exchange reaction at the surface irrespective of the solution pH. The Fourier transform infrared photoacoustic spectra of the Li+ extracted spinel demonstrated the existence of the surface hydroxyl groups, which is considered to be associated with the vacant 8a tetrahedral sites in the spinel structure. Based on the thermal stability of the surface hydroxyl groups and the bulk hydroxyl groups, the Li+ extracted spinel structure is discussed in relation to its lithium-ion sieve property. (C) 1997 Academic Press.