Development of Magnesium Borate Electrolytes: Explaining the Success of Mg[B(hfip)4]2 Salt

Magnesium batteries are one of the most promising post-lithium technologies. One of the main challenges preventing its commercialization is to find an efficient and safe electrolyte. The electrolyte, playing the role of the blood in a battery, interacts with all battery components and must be highly compatible with all of them. The development of Cl-free electrolyte systems is desired to avoid corrosion issues, and many studies suggest magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip)4]2) as one of the best candidates in terms of electrochemical properties and chemical stability. Here we present an in-depth analysis of the unique structure of this salt and the interactions generated in the electrolyte among the dissociated ions. The results show a delicate balance between electron-withdrawing effects and ligand stabilization in B(hfip)4−, crucial from the point of view of magnesium electrolytes. Moreover, the bulk nature of B(hfip)4− limits the anion-cation contacts to infrequent interactions through fluorine atoms. This has consequences not only for ion transport but also for hindering the anion decomposition towards the formation of MgF2. Taken together, we managed to demystify the exclusive nature of B(hfip)4− anion, thereby allowing for further rational development of new anion structures.