Quantitative Molecular-Level Understanding of Electrochemical Aluminum-Ion Intercalation into a Crystalline Battery Electrode

Few materials are known to electrochemically intercalate trivalent aluminum cations, a charge storage mechanism central to rechargeable aluminum-ion battery electrodes. Here, using the chevrel phase Mo6S8 as a model crystalline electrode material, we couple electrochemical and solid-state 27Al NMR methods to understand quantitatively the aluminum-ion intercalation mechanism up from the molecular level. Unlike divalent Mg2+ cations, trivalent Al3+ cations intercalate simultaneously, as opposed to sequentially, into two cavities within the chevrel framework during galvanostatic discharge. Minimal Al3+ cation trapping occurs upon deintercalation (<7%). The simultaneous ion intercalation mechanism, as well as slow solid-state ion diffusion, can both be understood in terms of the high charge density of Al3+ cations. We also reveal that an amorphous surface layer forms upon aluminum-ion desolvation from molecular chloroaluminate anions in the ionic liquid electrolyte. The results yield quantitative molecular-level understanding of aluminum-ion intercalation in a model crystalline electrode material and establish solid-state 27Al NMR as a powerful characterization tool for rechargeable aluminum-ion batteries.