Electrolytes For Rechargeable Magnesium Batteries

Lithium and magnesium possess similar chemical properties due to the diagonal position in the periodic table of the elements. Magnesium has high negative potential(-2.37 V/acidity and-2.69 V/basicity) and larger theoretical volumetric capacity than lithium(3832 and 2062 mAh·cm-3,respectively), and cheaper price(about 1/25 of lithium) and simple process for handle. Rechargeable magnesium batteries with magnesium as the anode have become a hot spot of the study. Mg surface is easily covered by a compact passivation layer which prevents magnesium ion penetration in commonly used aprotic solvents. The study of electrolyte is one of the two focuses of rechargeable magnesium batteries. The preparation process of most conventional electrolytes is complicated. On the other hand, the electrolytes are generally composed of organic magnesium salts with halogen elements, which may corrode non-inert current collectors. In view of the problems which the electrolytes of rechargeable magnesium batteries are facing, we studied two kinds of electrolytes, which are pyrazolyl-based halides/THF solutions prepared by a simple process and magnesium borohydride/tetraglyme solutions, which shows high anodic stability on stainless-steel(SS).Firstly, the performance of pyrazolyl-based halides/THF solutions as the electrolytes for magnesium deposition-dissolution was researched. We studied the effects of the pyrazolyl-based halides/THF solutions with different reaction ratio、different groups of pyrazole system、different kinds of Grignard reagent and different collector on the electrochemical performance of the electrolyte. The pyrazolyl-based halides/Tetrahydrofuran solutions were characterized in term of reversibilityof magnesium deposition-dissolution and anodic stability on different metal substrates using galvanostatic charge/discharge and cyclic voltammetry techniques. Furthermore, the composition and morphology of the deposition were analyzed using X-ray diffraction and scanning electron microscopy. 1mol?L-1 PhMgCl/THF-1-methylpyrazole(at 1:1 molar ratio) electrolyte is the optimum system according to the experimental analysis. This electrolyte shows a low voltage for magnesium deposition-dissolution, a high cycling reversibility and easy preparation.Secondly, we studied the effects of the borohydride/tetraglyme solutions with different solvents on the electrochemical performance.Mg(BH4)2/ether solutions were prepared by dissolving inorganic magnesium salt Mg(BH4)2 in different ether solvents such as diglyme(DGM), tetrahydrofuran(THF), dimethoxyethane(DME) and tetraglyme(TG). The Mg(BH4)2/ether solutions as electrolytes for rechargeable magnesium batteries were characterized in term of anodic stability on different current collector(Pt、Ni、Cu、SS) by using cyclic voltammetry technique. The results indicate that 0.5 mol?L-1 Mg(BH4)2/TG electrolyte shows the best electrochemical performance with 2.4 V(vs. Mg RE)anodic stability on SS. 90 oC heating treatment was further performed in the preparation process and Li BH4 as a chelating agent was added to improve the electrochemical performance. Constant-current discharge/charge tests of coin cells show that the stable cycle efficiency of magnesium deposition/dissolution can reach to 100% on SS. Furthermore, the compatibility of the 0.5 mol?L-1 heated Mg(BH4)2/LiBH4/TG([LiBH4]=1.5mol?L-1) solution with Mo6S8 cathode material was verified using coin cells with Mg anode, Mo6S8 cathode coating on SS and Mg(BH4)2/LiBH4/TG([LiBH4]=1.5 mol?L-1) electrolyte. The results indicated that the discharge capacity can reach to 76.8 mAh?g-1 with 92.4% capacity retention for the remaining 107 cycles. The good compatibility of the electrolyte with Mo6S8 cathode indicates the feasible application in rechargeable magnesium batteries.