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

Deep eutectic solvents (DESs) have emerged as an alternative for conventional ionic liquids in aluminum batteries. Elucidating DESs composition is fundamental to understand aluminum electrodeposition in the battery anode. Despite numerous experimental efforts, the speciation of these DESs remains elusive. This work shows how ab initio molecular dynamics (AIMD) simulations can shed light on the molecular composition of DESs. For the particular example of AlCl3 :urea, one of the most popular DESs, we carried out a systematic AIMD study, showing how an excess of AlCl3 in the AlCl3 :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 DESs. We also observe the transfer of simple [AlClx (urea)y ] clusters between different species in the liquid, giving rise to free [AlCl4 ]- units. The small size of these [AlCl4 ]- units favors the transport of ionic species towards the anode, facilitating the electrodeposition of aluminum.