Control of H-Related Defects in & gamma;-MnO2 in a Hydrothermal Synthesis

Manganesedioxide is a good candidate for effective energy storageand conversion as it possesses rich electrochemistry. The compoundalso shows a wide polymorphism. The & gamma;-variety, an intergrowthof & beta;- and R-MnO2, has been extensively studied inseveral types of batteries (e.g., Zn/MnO2, Li-ion) andis a common electrode material for commercial batteries. It is wellknown that the insertion of protons thermodynamically stabilizes & gamma;-MnO2 with respect to & beta;-MnO2. Protons can enterthe structure either by forming groups of 4 hydroxyls around a Mn4+ vacancy, called a Ruetschi defect, or by forming a hydroxylgroup near a Mn3+ ion, called a Coleman defect. These defectsdifferently affect the electrochemistry of manganese oxide, and tailoringtheir amount in the structure can be used to tune the material properties.Previous studies have addressed the proton insertion process, butthe role of the synthesis pathway on the amount of defects createdis not well understood. We here investigate how the parameters ina hydrothermal synthesis of & gamma;-MnO2 nanoparticlesinfluence the amount and type of H-related defects. Structural investigationsare carried out using Pair Distribution Function analysis, X-ray absorptionspectroscopy, thermogravimetric analysis, and inelastic neutron scattering.We demonstrate the possibility to control the amount and type of defectsintroduced during the synthesis. While the amount of Ruetschi defectsincreases with synthesis temperature, it decreases with extended synthesistime, along with the amount of Coleman defects. Moreover, we discussthe arrangement of the defects in the & gamma;-MnO2 nanoparticles. Hydrogen may insert in two distinct waysin MnO2 tunnel structures, impacting their electrochemicalproperties. Here,we investigate the effect of hydrothermal synthesis parameters usinga combination of X-ray and neutron-based techniques. We show thattemperature and synthesis time affect the two defect types independentlyand that the defect density varies from particle core to surface.