High-Performing Electrolytes for Secondary Magnesium Batteries Based on EMImCl, AlCl3, and δ-MgCl2

After several studies demonstrating the reversibility of Mg deposition and stripping processes from Grignard solutions early Mg secondary batteries, which comprised a metallic magnesium anode, a composite cathode and a polymer electrolyte based on polyethylene glycol and an innovative δ‐MgCl2 salt were first proposed in 1998. Subsequently, electrolytes based on Grignard and other organo‐Mg compounds were explored. Nevertheless, ethereal solutions of Grignard compounds are hazardous, due to the volatility and flammability of the solvents. Here, we report about a class of high-performing electrolytes based on 1-ethyl-3-methylimidazolium chloride (EMImCl) doped with AlCl3 and a highly amorphous form of MgCl2 [4]. The measurements of the phase diagrams of these materials allowed us to identify four thermal transitions, which are strongly dependent on salt concentration. Vibrational studies indicated the presence of two different types of concatenated Mg-chloroaluminate complexes. Electrochemical measurements at 25°C revealed a promising redox reversibility in non-blocking conditions with: 1) an exchange current of 0.54-1.68 mA/cm2; 2) a Coulombic efficiency as high as 98.4%; 3) a deposition overpotential <100 mV; and 4) an anodic stability of ca. 2.2 V. Broadband electrical spectroscopy studies provided information about the conduction mechanism of the electrolyte materials in terms of dielectric and polarization phenomena. A relatively uniform Mg chemical environment is revealed by 25Mg NMR spectroscopy. Finally, a structural 3D-model, which explains all the revealed experimental evidences is proposed. Mg-anode cells assembled with the electrolytes and a vanadium oxide cathode were cycled at least 50 times at a high rate (35 mA/g) revealing an initial capacity of 80 mAh/g and an open-circuit voltage of 2.7 V.