Manganese oxide stability and morphology in sulfuric acid electrolyte

Digestion of Mn₂O₃ and Mn₃O₄ in a range of H₂SO₄ solutions (0.01-10.0 M), at a variety of temperatures (20-140 °C) has led to the formation of a series of kinetically stable manganese dioxide samples via a dissolution-precipitation mechanism involving disproportionation of a soluble Mn(III) intermediate. The resultant manganese dioxide samples were characterised in terms of their domain of phase stability, chemical composition, structure, morphology and electrochemical performance. γ-MnO₂ predominated at all but high H₂SO₄ concentrations (>5 M), where α-MnO₂ was formed, and high temperatures (>80 °C) where β-MnO₂ was formed. The structural variety of γ-MnO₂ in this domain of stability was interpreted in terms of the fraction of De Wolff defects (Pr), which was found to increase as the H₂SO₄ concentration was decreased and the temperature was increased, microtwinning (T_w), which despite being less statistically significant, was found to follow a similar trend, and cation vacancy fraction and Mn(III) fraction. Both the latter structural properties decreased as the temperature was increased; however, decreasing the H₂SO₄ concentration led to a decrease in cation vacancy fraction but an increase in Mn(III) fraction. These structural characteristics, in particular De Wolff defects, were interpreted on a molecular level in terms of soluble Mn(III) intermediate condensation in which the electrolyte conditions determine the relative proportions of equatorial-axial edge sharing (ramsdellite domains only) and equatorial-axial comer sharing (both ramsdellite and pyrolusite domains) that occurs. Morphological differentiation was easily established due to the different characteristics of each phase. γ-MnO₂ existed as fine needles (250 nm x 50 nm), β-MnO₂ was formed as much larger columns (1 μm x 100 nm), while α-MnO₂ was present as small spheres up to 400 nm in diameter. Electrochemical characterisation by voltammetry in an aqueous 9.0 M KOH electrolyte demonstrated that the performance of the γ-MnO₂ samples was comparable to that of commercial EMD, whereas α- and β-MnO₂ suffered from diffusional limitations which lowered their operating voltage. For γ-MnO₂, superior performance resulted when lower temperatures and H₂SO₄ concentrations were used, corresponding to intermediate levels of De Wolff defects and micro twinning, but a cation vacancy fraction minimum. Examination of the kinetics of transformation of Mn₂O₃ into γ-MnO₂ involved two reactions; i.e., Mn₂O₃ disappearance and MnO₂ formation, which were both characterised using an autocatalytic first-order mechanism. In general, the kinetics of Mn₂O₃ disappearance and MnO₂ formation increased as acid concentration and temperature were raised. The kinetic rate of MnO₂ formation is somewhat smaller compared with the Mn₂O₃ disappearance rate, which probably associates with the time needed for nucleation during the induction period. Digestion of LiMn₂O₄ in 0.2 - 7.0 M H₂SO₄ at temperatures ranging from ambient to 120 °C resulted in a number manganese oxides including λ-MnO₂, R-MnO₂, β-MnO₂ and γ-MnOOH. λ-MnO₂ can be regarded as an intermediate species, formed after Li-extraction by acid, leading to other types of manganese oxides upon acid digestion. Morphological examination showed that the particle sizes of the digested products were decreased while maintaining their shape. A kinetic study on this system resulted in a contrary phenomenon compared to the one found in Mn₂O₃ system, in which the disappearance of the starting LiMn₂O₄ materials and R-MnO₂ formation were decreased as acid concentration and temperatures were increased. Digestion of partially reduced EMD and λ-Mn0₂, a general formula HMn₂O₄, in 0.2-7.0 M H₂SO₄ at various temperatures exhibited different mechanisms. Upon acid digestion, the reduced EMD was reverted back to EMD (γ-MnO₂) structure, whilst the reduced λ-MnO₂ was converted into R-MnO₂. The surface morphology of reduced EMD and the product after acid digestion were to some extent similar to EMD; however, the digested product was less compact compared with the reduced EMD. Morphological examination of reduced λ-MnO₂ showed it was different when compared to the λ-MnO₂. The sample has an interior of compact column/tube-shaped agglomerates covered loosely by column-shaped particles. After acid digestion both resultant samples were found to be needle-like particles of various lengths.