Elucidating Reversible Electrochemical Anionic Redox in Intercalation Electrodes for Rechargeable Multivalent-Ion Batteries
Authors: Jadhav Ankur , Xu Jeffrey , Messinger Robert J. 
Published: December 2020

Battery electrodes that enable electrochemical intercalation of multivalent ions result in multiple electron transfers per ion, respectively, yielding significant enhancements in electric charge storage capacity. When multivalent cathodes are paired with their corresponding metal anodes, potentially transformative gains in energy density are possible. However, few positive electrode materials exist that intercalate multivalent ions, in part due to the slow solid-state diffusion of highly charged multivalent ions. The chevrel phases Mo6X8, where X is a chalcogen atom (e.g., S or Se) are among the few materials known to reversibly and electrochemically intercalate both divalent (e.g. Mg2+, Zn 2+, etc.) and trivalent (Al3+) ions. Thus, understanding the ion intercalation and electron charge transfer mechanisms within these unique compounds may provide insights into the molecular-level design of new intercalation electrodes for rechargeable multivalent ion batteries.
Here, we couple electrochemical and solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) measurements to reveal the concurrent aluminum-ion intercalation and electron charge transfer mechanisms in chevrel electrodes Mo6S8 and Mo6Se8 up from the molecular level. Solid-state 27Al single-pulse MAS NMR measurements were performed on the thio-chevrel Mo6S8 electrodes at different states-of-charge to observe changes in the environments and to quantify the local populations of aluminum ions within the host crystal structure, revealing both intercalated ions and additional aluminum species associated with surface layers and/or decomposition products. Al-Mo6Se8 batteries were made for the first time and their electrochemical properties were compared to the thio-chevrel Mo6S8. Solid-state 77Se and 95Mo MAS NMR measurements on the seleno-chevrel Mo6Se8 at charged and discharged states revealed that electrons are transferred preferentially to the anionic chalcogen framework upon aluminum ion intercalation, as opposed to the transition metals (Mo). Anionic redox was further confirmed by density functional theory (DFT) calculations and X-ray absorption near-edge structure (XANES) measurements. Lastly, we generalize the results to other ion valencies, demonstrating reversible electrochemical anionic redox in the Chevrel phase upon divalent Zn2+ cations in aqueous Zn batteries.
Overall, for the first time, we demonstrate reversible electrochemical anionic redox as a charge storage mechanism for rechargeable batteries, which preserves the crystalline framework structure and does not involve the breaking and forming of chemical bonds. This charge storage mechanism is fundamentally different than that observed for lithium-ion intercalation into transition metal oxides, where the electrons are stored within the d-orbitals of the transition metal. The results suggest materials design principles aimed at designing multivalent-ion intercalation electrodes with improved electrochemical properties.

Figure 1