Synthesis of NaxMnO2+δ by a Reduction of Aqueous Sodium Permanganate with Sodium Iodide

Abstract

Reduction of sodium permanganate with sodium iodide in aqueous solutions has been investigated systematically. The products formed have been characterized by X-ray diffraction, wet-chemical analysis, and surface area and magnetic susceptibility measurements after firing at various temperatures. The results reveal that the sodium content x in the reduction products NaxMnO2+δ depends strongly on the reaction pH and mildly on the relative concentrations of the reactants. Na0.7MnO2+δ obtained at pH>11 followed by firing at T>500°C adopts the P2 layer structure (hexagonal) with cation vacancies arising from a δ≈0.3. Na0.7MnO2+δ crystallizing in a distorted P2 structure (orthorhombic) without cation vacancies (δ≈0) could be obtained by annealing the hexagonal Na0.7MnO2+δ (δ≈0.3) in N2 atmosphere around 600°C. While the orthorhombic Na0.7MnO2+δ (δ<0.05) is stable during ion-exchange reactions with lithium salts at 25≤T≤180°C, the hexagonal Na0.7MnO2+δ (δ≈0.3) transforms to spinel-like phases due to the presence of cation vacancies. Na0.5MnO2+δ obtained at a controlled pH of 9.3 adopts a metastable layer structure on firing at 500°C and a tunnel structure isostructural with Na4Mn4Ti5O18 on firing at T≥600°C. The tunnel structure is stable to ion-exchange reactions without transforming to spinel-like phases. In addition, washing the reduction products with various organic solvents before firing at higher temperatures is found to influence the reaction kinetics, composition, and crystal chemistry.