Synthesis And Electrochemical Properties Of Rechargeable Magnesium Battery Electrolytes

Rechargeable magnesium batteries may be a potential candidate for green storage batteries due to higher energy density, cheap raw material and easy to handle of magnesium. Magnesium is in the diagonal position of lithium and has similarity with lithium in many respects, such as ionic radius and chemical properties. Magnesium has high negative potential (-2.37 V/acidity and -2.69 V/basicity) and large theoretical specific capacity (2205 mAh·g^-1). In addition, magnesium is relatively inexpensive (about 1/24 of lithium), environmentally friendly and safe to handle. So, rechargeable magnesium batterry with magnesium as anode has become a new research focus. However, the electrolyte restricts the development of magnesium battery. In many nonaqueous solutions, a reversible process of electrochemical deposition and dissolution of magnesium is hard achieved because of the formation of compact passive film. Therefore, it is necessary to find new electrolyte for the development of magnesium battery.In this paper, we systematically studied the electrochemical performance of Mg(CF3SO3)2/imidazolium cation-based ionic liquids (0.3 mol·L^-1) as the rechargeable magnesium battery electrolytes, and then the electrochemical behavior of pyrrolidinyl magnesiuhalide (1 mol·L^-1 C4H8NMgX/THF, X = Br and Cl) and pyrazolate magnesiuhalide (1 mol·L^-1 C3H4N2MgBr/THF) dissolved in tetrahydrofuran (THF). The main research contents are as follows:The performance of imidazolium cation-based ionic liquids (ILs) containing 0.3 mol·L^-1 Mg(CF3SO3)2 as the electrolytes for magnesium deposition-dissolution was examined. Firstly, we studied the effects of the imidazolium cation-based ILs with different anions and cations on the electrochemical performance of the electrolyte. The cathodic satiability and the viscosity of the electrolyte can be affected by the different branched chains of the imidazolium cations. And the different anions determine the magnesium deposition-dissolution properties in the electrolyte. The reversible deposition-dissolution behavior can be found in the electrolytes which contain the BF4- anion. And the 0.3 mol·L^-1 Mg(CF3SO3)2/BMImBF4 electrolyte exhibits good behavior, but with larger viscosity which has negative impact on the electrochemical performance. After adding of a certain amount of THF, the viscosity can be reduced and the conductivity of the solution, stability of deposition-dissolution process improved. Then, we further studied the effects of Mg power counter electrode on the deposition-dissolution process.A new kind of electrolyte for rechargeable magnesium batteries, which included pyrrolidinyl magnesiuhalide (1 mol·L^-1 C4H8NMgX/THF, X = Br and Cl) and pyrazolate magnesiuhalide (1 mol·L^-1 C3H4N2MgBr/THF) dissolved in tetrahydrofuran (THF), was prepared. These solutions were characterized in term of conductivity, anodic stability, and reversibility of magnesium deposition and dissolution. Furthermore, the effect of metal substrates on magnesium deposition process was studied. Compared with Grignard reagent of EtMgBr (1 mol·L^-1 EtMgBr/THF), the electrolytes show higher conductivity, wider electrochemical window, higher initial cycling efficiency and so on. For different working electrodes, the magnesium deposition-dissolution behaviors and the deposition morphology in the electrolytes were different.