Controlled synthesis of MnO2 nanoparticles for aqueous battery cathodes: polymorphism-capacity correlation

Polymorphs of MnO2 are known to have different electrochemical activity, with gamma (gamma) and akhtenskite (epsilon) polymorphs often considered as the most active phases for aqueous battery cathodes. However, most synthetic samples contain a mixture of polymorph phases, which makes understanding of the structure-property correlations more complicated. In this paper, we report on a systematic study that correlates synthesis parameters with the morphology, phase composition and reversible storage capacity of the resulting nanoparticles. Rietveld analysis of X-ray powder diffraction patterns was used to accurately describe fractional composition of multi-phase nanoparticles. It was demonstrated that through control of the synthesis parameters desired phase compositions and nanoparticle morphologies can be achieved. The key synthesis parameters were found to be the concentration of Mn2+ precursor which strongly affects both the morphology and the crystalline structure of the products, and the time of reaction. The presence of surfactant only impacts the crystalline phase composition of the MnO2 nanoparticles and has insignificant effect on the morphology. It was also demonstrated that nanoparticles with higher fraction of the akhtenskite polymorph show higher reversible capacities in LiOH electrolyte (similar to 210 mAh/g) compared to other MnO2 phase compositions (similar to 120 mAh/g).