Effect of electrolyte on electrochemical performance of magnesium rechargeable batteries

Abstract

The rechargeable magnesium battery is considered as a next generation energy storage and conversion system for last few decades. Magnesium metal is an attractive candidate for the anode material owing to their characters of high theoretical specific capacity (3,832 mAh cm-3), great material abundance and good safety. In comparison with conventional Li-ion battery, Magnesium is non-dendrite system, which is a significantly improving battery safety. Magnesium batteries can be used for magnesium (Mg) metal anode. In spite of these advantages, Mg rechargeable batteries have some problems. Divalent Mg ion is the kinetically slow Mg insertion/deinsertion and diffusion reaction in the cathode materials. Mg rechargeable batteries have also been hindered by the surface chemistries of Mg, which very limits the choice of available electrolytes and cathodes materials. To improved performance can be obtained by search for suitable cathode materials and less passivated Mg anode. In this paper, main work is concerned with development of novel electrolytes to improve cell performance during charging and discharging. The prototype electrolytes suffer from the use of very volatile solvents, such as THF, and organohaloaluminate electrolytes with a highly corrosive nature toward current collectors, high air sensitivity, and low anodic stability (below 3.0 V versus Mg reference electrode), which limits the choice of cathodes. In order to shortcomings of the existing electrolytes and obtain the better performance, new electrolyte system should be developed. As a first work, this paper presents the effects of various existing electrolyte systems on Mg deposition and dissolution. The anodic stability of the electrolytes were determined by means of linear sweep voltammetry(LSV) and cycling experiments for Mg/Cu cells were galvanostatically conducted. On the basis of galvanostatic cycling of Mg/Cu cells, highly reversible Mg deposition and dissolution processes were obtained in 0.2 M MgBu2-(AlCl2Et)2/THF, 0.4 M (PhMgCl)2-AlCl3/THF electrolytes with and without TPFPB. Addition of the TPFPB Lewis acid could significantly improve the anodic stability of 0.4 M (PhMgCl)2-AlCl3/THF electrolytes By Linear sweep voltammetry experiments. Also, the surface morphology and chemical component of the Mg metal and Cu electrode after Mg deposition and dissolution were observed using a field emission scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). It revealed that the shape of the Mg deposit strongly depends on the electrolyte composition. As a second work, to improve stability of electrolyte against electrochemical oxidation and reduction, we present new electrolytes based on magnesium (II) bis(trifluoromethane sulfonyl)imide (Mg(C2F6NO4S2)2, Mg(TFSI)2) dissolved in glyme-based solvents. To check the solvation of Mg2+ ions, we measured ion-dipole interaction with Fourier-transform infrared (FT-IR) analyses. Also we presents typical linear sweep voltammmetry (LSV) curves for various electrolytes and solvents. 0.3M Mg(TFSI)2 in glyme/diglyme (1/1, v/v) showed significant higher anodic stability than AlCl3-(PhMgCl)2/THF electrolytes. And highly reversible Mg deposition and dissolution processes were obtained in Mg(TFSI)2 dissolved in glyme-based solvents on the galvanostatic cycling. Moreover, the properties of glyme-based electrolytes in Mg/poly(2,2,6,6-tetramethylpiperidinyl-1-oxy-4-yl methacry late) (PTMA), Mg/CMK3-S, and Mg/Mo6S8 cells are discussed.