Amplifying the Differences in Aluminum‐Based Eutectic Electrolytes Through Electrodeposition on MXenes

Abstract Traditionally, rechargeable lithium metal battery systems relied on simple metal cationic species to enable metal nucleation and deposition. However, this mechanism is less applicable to room‐temperature rechargeable aluminum batteries (RABs), which utilize complex ionic species. This work takes advantage of two different MXenes, Mo 2 Ti 2 C 3 T x and Ti 3 C 2 T x , which have different metal‐termination group bond strengths, interlayer spacings, and surface termination compositions, to amplify and visualize the differences between two promising RAB electrolytes (i.e., urea/AlCl 3 and EMImCl/AlCl 3 ). This work allowed us to deduce the following: 1) despite lower coulombic efficiencies, smoother Al deposits are observed when the urea/AlCl 3 electrolyte is used; 2) unknown side reactions occur in all the different scenarios, however, they are likely less reversible in the EMImCl/AlCl 3 electrolyte; 3) metal‐halogen exchange reactions can occur in all scenarios, forming Al(OF) x and AlF x species; 4) the free urea in urea/AlCl 3 electrolyte behaves differently from the urea in aqueous solutions. Overall, due to the characteristics of aluminum, RABs involve vastly different internal mechanisms, resulting in the need for audacious attempts toward system understanding.