Elucidating Consequences of Selenium Crystallinity on Its Electrochemical Reduction in Aluminum–Selenium Batteries

Selenium (Se) is an attractive positive electrode material for rechargeable aluminum (Al) batteries due to its high theoretical capacity of 2037 mA h g^–1 and its higher electronic conductivity compared to sulfur. Selenium can undergo a series of electrochemical reactions between Se(–II) and Se(IV), resulting in a six-electron capacity per Se atom. However, existing Al–Se battery literature is inconsistent regarding the different electrochemical reactions possible, while the conditions enabling the electrochemical reduction of Se to Al₂Se₃ are not well understood. Here, we demonstrate that this electrochemical reduction is achievable using amorphous selenium but is suppressed for crystalline selenium. We further show that the electrochemical oxidation of Se to SeCl₄, which occurs at higher potentials, reduces the long-range order of crystalline Se and enables its discharge to Al₂Se₃. Solid-state ⁷⁷Se nuclear magnetic resonance (NMR) measurements further establish that the local Se helical structures are maintained upon the loss of crystallinity.