Ab Initio Molecular Dynamics Investigations of the Speciation and Reactivity of Deep Eutectic Electrolytes in Aluminum Batteries

Abstract

Deep eutectic solvents (DES) have emerged as an alternative for conventional ionic<br>liquids in aluminum batteries. Elucidating DES composition is fundamental to<br>understand aluminum electrodeposition in the battery anode. Despite numerous<br>experiemental efforts, the speciation of these DES remains elusive. This work shows<br>how \textit{Ab initio} molecular dynamics (AIMD) simulations can shed light on the<br>molecular composition of DES. For the particular example of AlCl$_{3}$:urea, one of<br>the most popular DES, we carried out a systematic AIMD study, showing how an<br>excess of AlCl$_{3}$ in the AlCl$_{3}$:urea mixture promotes the stability of ionic<br>species vs neutral ones and also favors the reactivity in the system. These two facts<br>explain the experimentally observed enhanced electrochemical activity in salt-rich<br>DES. We also observe the transfer of simple $[$AlCl$_{x}$(urea)$_{y}]$ clusters<br>between different species in the liquid, giving rise to free $[$AlCl$_{4}]^{-}$ units. The<br>small size of these $[$AlCl$_{4}]^{-}$ units favors the transport of ionic species towards<br>the anode, facilitating the electrodeposition of aluminum.