THE SURFACE-STRUCTURE OF THE PROTON-EXCHANGED LITHIUM MANGANESE OXIDESPINELS 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 ins erted manganese oxide spinels. The Li+ extraction from the orthorhombi c LiMn2O4 spinel results in the formation of the cubic spinel, the cry stal structure of which is the same as lambda-MnO2. The LiMn2O4 spinel prepared from an electrolytically prepared manganese dioxide is conve rted to HMn2O4 on dilute acid treatment. X-ray photoelectron spectrosc opic analysis revealed that the valence state of the surface Mn ions r emains unchanged during the Li+ extraction and insertion, which indica tes an occurrence of the Li+-H+ ion exchange reaction at the surface i rrespective of the solution pH. The Fourier transform infrared photoac oustic spectra of the Li+ extracted spinel demonstrated the existence of the surface hydroxyl groups, which is considered to be associated w ith the vacant 8a tetrahedral sites in the spinel structure. Based on the thermal stability of the surface hydroxyl groups and the bulk hydr oxyl groups, the Li+ extracted spinel structure is discussed in relati on to its lithium-ion sieve property. (C) 1997 Academic Press.