Mn Cations Control Electronic Transport in Spinel CoxMn3-xO4 Nanoparticles

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 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 synchrotron X-ray emission spectroscopy 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.