Improved Electrochemical Performance of Reversible Magnesium Electrolytes

Magnesium (Mg) is an attractive candidate for the anode material of a novel high-performance battery because of its high energy density, moderate electrochemical activity, chemical stability, as well as the natural abundance. There is a promising electrolyte, Mg(AlCl2EtBu)2/THF (Et:C2H5, Bu:C4H7), that enables Mg deposition and dissolution reversibly at room temperature, whose electrochemical characteristics, such as conductivity and electrochemical window, are superior to those of the conventional Grignard reagents in ether. Recently we have directly observed the complex structures in this electrolyte with the X-ray absorption fine structure (XAFS) measurements, revealing that (Mg2Cl2THF4), R2AlCl2, RAlCl3, R2AlClTHF, and R3AlTHF (R=Et or Bu) are formed in this electrolyte. In addition, we have mentioned the possibility to control the electrochemical properties by designing the complexes involved in the solutions. In this study, various components of the complex structures, such as alkyl groups, halogens and solvents, have been investigated as parameters to improve the electrochemical properties, which revealed their roles in the reversible Mg electrolytes. As a result, the enlarged electrochemical window was demonstrated in 0.25 mol/l Mg(AlCl2Me2)2/THF (Me:CH3). A solution of 0.25 mol/l Mg(AlCl2EtBu)2/THF (EL-EtBu) was synthesized with the method described elsewhere. A couple of 0.25mol/l solutions, Mg(AlCl2Et2)2/THF (EL-Et) and Mg(AlCl2Me2)2/THF (EL-Me) were also prepared. To investigate the local structures around Mg and Al in all electrolytes, Mg Kedge and Al K-edge XAFS measurements were carried out in the fluorescence mode at BL-11A in KEK-PF, Tsukuba, Japan, using the vacuum compatible homemade cells. In addition, NMR spectra were observed with a JEOL JNM-AL400 (400 MHz) at 24.4 MHz (Mg) and 104.1 MHz (Al). Electrochemical properties were studied by cyclic voltammetries (CV) using a potentiostat 1287 (Solartron). Specific conductivities were measured at 100 kHz with an impedance analyzer 1260 (Solartron). All measurements were performed at room temperature (25±2C). The results of XAFS measurements revealed that all the electrolytes showed the same aspects of the complex structures in each solutions, EL-EtBu, EL-Et, and EL-Me. 1. Valences of Mg are identical each other. 2. Mg atoms form tetrahedral dimmers Mg2Cl2THF4. 3. Valences of Al are identical each other. 4. Al atoms form tetrahedral monomers. These results indicate that all the alkyl groups are involved in the Al complexes in solutions, which was confirmed by the NMR measurements as follows. Figure 1 shows the Mg and Al NMR spectra for EL-EtBu, EL-Et, and EL-Me. In Fig.1(a), it is obvious that Mg take the same electronic structures among three, while Al have slightly different ones as in Fig.1(b), which is due to the different alkyl groups involved in the Al complexes. Figure 2 shows the results of CV measurements, showing that EL-Me have the larger electrochemical window than the others. In this presentation, it will be explained that the observed improvement is attributed to the designed complexes, Me2AlCl2, which have the lower HOMO energy than Et2AlCl2 and Bu2AlCl2. The roles of halogens as well as solvents in the electrolytes are also to be discussed.