(Invited) Molecular-Level Understanding of Ion Intercalation Mechanisms in Aluminum and Zinc Battery Electrodes Revealed By Solid-State NMR Spectroscopy

Solid-state nuclear magnetic resonance (NMR) spectroscopy is a powerful and quantitative characterization tool that can elucidate the local chemical, structural, and electronic changes that battery materials undergo upon electrochemical cycling. However, to date no solid-state NMR experiments have been reported that directly probe multivalent aluminum or zinc cations within intercalation electrodes, in part due to their challenging quadrupolar nature and in part due to the small number of structures that reversibly intercalate multivalent ions. Here, multi-nuclear solid-state magic-angle-spinning (MAS) experiments will be presented on rechargeable aluminum and zinc intercalation electrodes for the first time, revealing insights into their ion intercalation and charge transfer mechanisms. For aluminum or zinc batteries using the thio-Chevrel Mo6S8 and seleno-Chevrel Mo6Se8 as cathode materials, solid-state 27Al and 67Zn NMR experiments establish quantitatively the relative populations of intercalated aluminum or zinc cations in different local environments as a function of state-of-charge. For the seleno-chevrel electrodes, solid-state 77Se NMR experiments reveal the effects of aluminum- and zinc-ion intercalation on the local electronic structures of the Mo6Se8 frameworks. For comparison, aluminum-graphite batteries will also be analyzed, where solid-state 27Al NMR measurements yield insights into the local environments of monovalent chloroaluminate anions after intercalation into natural graphite. Opportunities and challenges will be discussed regarding the application of NMR spectroscopy to aluminum and zinc battery materials. Overall, the solid-state NMR and electrochemical results pave the way towards a better understanding of material design principles aimed at realizing multivalent intercalation electrodes with enhanced electrochemical properties.