Ternary spinel oxides are actively used and researched for applications ranging from electronics, sensors, catalysis, data storage, to energy storage due to their useful magnetic, catalytic, and electronic properties. It is known that the most prominent mechanism of charge transport in oxides is through hopping that follows the polaron models. But the polaron hopping models are too crude to account for the configurational differences that occur in ternary transition metal spinels, where the two different cation types can exhibit large degrees of disorder between the two cation lattice sites and possess a variety of oxidation states. The current model, small polaron nearest neighbor hopping, was proposed in 1961 and has multiple limitations. Understanding the mechanisms of charge transport in ternary and higher order spinels would enable design principles for enhancing their electronic and electrochemical properties for energy applications. Here, we investigate colloidal nanoparticles of the ternary CoxMn3-xO4 spinel system and determine the relationship between electronic properties and cation site occupation, based on the octahedral site, donor-acceptor formalization introduced in the polaron hopping model. We employ advanced synchrotron x-ray emission spectroscopy (XES) to characterize the donor-acceptor cation pairs in the lattice. We find that the stochiometric volcano trend in electronic conductivity correlates well with the concentration of Mn4+/Mn3+ hopping pairs, while the concentration of Mn3+/Mn2+ pairs is unchanged with stoichiometry. We also find that Co does not directly contribute to conductivity, however, the Co does create configurational disorder that leads to the generation of hopping pairs for Mn at octahedral sites. From these results we conclude that Mn4+/Mn3+ pairs are the dominant active species and the origin of the stoichiometry-dependent behavior for polaron charge transport. This work provides a starting point for understanding and optimizing charge transport for higher order spinels to improve energy electrodes.

Controlling charge transport in spinel oxides through manipulation of cation site occupation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.