(644d) Aluminum-Ion Transport and Charge Transfer Mechanism in Chevrel Phase Electrodes for Rechargeable Aluminum Batteries

Rechargeable aluminum batteries are safe and low-cost alternatives to traditional lithium-ion batteries. However, the high Coulombic charge density of trivalent aluminum cations makes it difficult for them to intercalate and diffuse within crystalline transition metal compounds. The Chevrel phases Mo₆X₈, where X is a chalcogen atom (e.g., S or Se), are among the few materials known that can reversibly and electrochemically intercalate trivalent aluminum cations. Thus, understanding the ion intercalation and charge transfer mechanisms within these unique compounds may provide insights into the molecular-level design of new intercalation electrodes for rechargeable aluminum-ion batteries. Here, for the first time, we use multi-nuclear solid-state magic-angle-spinning (MAS) NMR spectroscopy combined with electrochemical and diffraction techniques to unravel the aluminum ion intercalation and charge transfer mechanisms in thio-chevrel Mo₆S₈ and seleno-chevrel Mo₆Se₈ electrodes.

In aluminum metal batteries with thio-chevrel Mo₆S₈ electrodes, ex situ single-pulse solid-state ²⁷Al MAS NMR measurements were performed at different states-of-charge to observe changes in the local environments of intercalated aluminum ions and quantify their relative populations within the host crystal structure. Additional aluminum species were observed associated with the formation of surface layers that reversibly form upon desolvation and intercalation of aluminum from the chloroaluminate anions in the ionic liquid electrolyte. Al-Mo₆Se₈ batteries were realized for the first time, enabling study of how changing the chalcogen anionic framework affects the electrochemical and ion transport properties. Solid-state⁷⁷Se and ⁹⁵Mo NMR measurements on the seleno-chevrel Mo₆Se₈ at charged and discharged states revealed the electron charge transfer mechanism upon aluminum ion intercalation, establishing that electrons are predominately stored on the chalcogen anionic framework, as opposed to the transition metal octahedra. Electrochemical measurements, particularly cyclic voltammetry (CV), variable-rate galvanostatic cycling, and GITT measurements were performed at different temperatures to study and compare the electrochemical performance and aluminum ion transport properties of the two different chevrel electrodes.

Overall, the multi-nuclear solid-state NMR measurements combined with the electrochemical measurements yield molecular-level insights into the coupled aluminum ion intercalation and electron charge transfer processes that occur in a model crystalline transition metal electrode. The results suggest materials deign principles aimed at designing aluminum-ion intercalation electrodes with improved electrochemical properties.