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

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