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 ionicliquids in aluminum batteries. Elucidating DES composition is fundamental tounderstand aluminum electrodeposition in the battery anode. Despite numerousexperiemental efforts, the speciation of these DES remains elusive. This work showshow \textit{Ab initio} molecular dynamics (AIMD) simulations can shed light on themolecular composition of DES. For the particular example of AlCl$_{3}$:urea, one ofthe most popular DES, we carried out a systematic AIMD study, showing how anexcess of AlCl$_{3}$ in the AlCl$_{3}$:urea mixture promotes the stability of ionicspecies vs neutral ones and also favors the reactivity in the system. These two factsexplain the experimentally observed enhanced electrochemical activity in salt-richDES. We also observe the transfer of simple $[$AlCl$_{x}$(urea)$_{y}]$ clustersbetween different species in the liquid, giving rise to free $[$AlCl$_{4}]^{-}$ units. Thesmall size of these $[$AlCl$_{4}]^{-}$ units favors the transport of ionic species towardsthe anode, facilitating the electrodeposition of aluminum.