Ion-Pair Dissociation on α-MoO3 Surfaces: Focus on the Electrolyte-Cathode Compatibility Issue in Mg Batteries

Nanosizing has been explored as one option to enhance typically sluggish Mg ion mobility in cathode materials. Here, we explore the effects of the surfaces of alpha-MoO3 in facilitating or hindering Mg ion desolvation at the electrolyte cathode interface. It is well-known in the Mg battery community that Mg anion contact ion pairs may form in the electrolyte and thus the insertion of Mg into the cathode must involve desolvation and dissociation, where Mg is detached from the counterion at the cathode surface. Here we compare two representative Mg anion pairs, i.e., (Mg-Cl)(+) and (Mg-TEST)(+) (TEST-, bis(trifluoromethanesulfonyl)imide), differentiated according to the size and polarizability of the anion, and examine their dissociations on different alpha-MoO3 surfaces. We find that (Mg Cl)+ requires a higher dissociation energy than (Mg-TFSI)(+), which implies that the Mg-TFSI-type electrolyte is more suitable to be integrated with the MoO3 cathode when creating an electrochemical cell. The dissociation of (Mg-TEST)(+) occurs, however, not on the lowest energy (010) surface where the van der Waals (vdW) gap opens, but on the hydroxyl-terminated (001) surface. Therefore, to optimize the performance of alpha-MoO3 toward (Mg-TEST)(+) dissociation, preferential growth of the hydroxyl-terminated (001) surface is required.