Abstract

From different precursors, including γ-, β-MnO₂, hollandite, and H⁺-form birnessite manganese oxides, well-crystallized Li_1.33Mn_1.67O₄ spinels were synthesized at 400 °C by using LiNO₃ as a flux. Alkali metal ion adsorptivities of the spinels after delithiation were investigated. SEM observation showed that each spinel obtained preserved the morphology of its precursor. The lack of morphology change, and the time-dependence of the lithium insertion into the precursors, illustrate that the formation of the spinel is a topotactic template reaction. None of the delithiated spinels preserved the original ion-exchange sites in their precursors but all showed nearly the same high affinity for Li⁺, while the amount of uniform acidic sites increased and the steric hindrance during lithiation decreased in the order of spinels obtained from birnessite, hollandite, and γ-, β-MnO₂. This is probably because birnessite has more (buffer) space for the accommodation of ions in its layered structure than other precursors with a tunnel structure and some part of the space remains after structural transformation to the spinel.