Magnesium Borohydride-Based Electrolytes Containing 1-butyl-1-methylpiperidinium bis(trifluoromethyl sulfonyl)imide Ionic Liquid for Rechargeable Magnesium Batteries

Abstract

The electrolytes containing halide-free inorganic magnesium salt Mg(BH 4 ) 2 dissolved in ether solvents have shown reversible Mg deposition-dissolution performance. Herein, we improved the anodic stability of the electrolytes on non-inert stainless-steel electrode by mixing PP 14 TFSI ionic liquid with tetraglyme (TG) and dimethoxyethane (DME) ether solvents. The effect of mixing ratios and salt concentrations on the electrochemical behavior of the electrolyte were investigated. High anodic stability, good ionic conductivity, excellent cycling efficiency, feasibility of the preparation and good compatibility toward Mo 6 S 8 and TiO 2 insertion cathode make the electrolytes promising for the potential application in rechargeable magnesium batteries. 3-5 The development of magnesium electrolytes is considered as the most important challenge for the commercial application of rechargeable magnesium batteries because electrolyte properties govern battery performance and determine the class of cathodes to be utilized. 6 The significant progress is the reports of 0.25 mol L −1 Mg(AlCl 2 BuEt) 2 /THF (Bu=butyl, Et=ethyl) electrolyte 7,8 and 0.4 mol L −1 (PhMgCl) 2 -AlCl 3 /THF electrolyte, 9,10 which have high anodic stability (2.5 V and 3.3 V vs. Mg RE on inert Pt electrode, respectively) and reversibility of Mg deposition-dissolution. Recently, a family of novel boron based electrolytes with high ionic conductivity, excellent Mg deposition reversibility as well as high anodic potential were proposed.11,12 On the other hand, phenolate-based 13 and alkoxide-based 14 electrolytes exhibit air insensitive character and excellent magnesium depositiondissolution performance. Meanwhile, inorganic magnesium salt solutions synthesized by the acid-base reaction of MgCl 2 and Lewis acidic compounds such as AlCl 3 show high coulombic efficiency, low overpotential for magnesium deposition-dissolution and good anodic stability. [15][16][17][18] However, these electrolytes may corrode non-inert current collectors at lower anodic potentials duo to the presence of halides in the cation and anion components of the electrolytes, although some of these electrolytes have shown impressive stability against electrochemical oxidation.19 Hence, it is still necessary to find electrolytes with high stabilities on non-inert current collectors for a practical rechargeable Mg battery system. Nelson et al. showed that decreasing the chloride content in Mg electrolytes by switching the Lewis acid from AlCl 3 to Al(OPh) 3 greatly improves the anodic stability up to 5 V on both Pt and stainless steel electrodes.20 Ha et al. proposed a new class of electrolytes based on magnesium (II) bis(trifluoromethane sulfonyl)imide (Mg[N-(SO 2 CF 3 ) 2 ] 2 , Mg(TFSI) 2 ) dissolved in glymebased solvents with unique characteristics such as highly reduced corrosive nature toward the current collector, low volatility, high solvating power, and the ability to form an appropriate solvation sheath structure for Mg deposition-dissolution. 21 Recently, a whole new promisi...

Mg Cathode Materials and Electrolytes for Rechargeable Mg Batteries: A Review

“…However, as the percentage of IL increases the ionic interactions between the ions in the electrolytes typically become an obstacle to ion transfer . As a result, the viscosity of the electrolyte increases and the ionic conductivity decreases, leading to increased over‐potentials and decreased current densities in reduction and oxidation cycling …”

“…Eventually, the distribution of the reduced Mg 0 particles covers all of the Pt substrate, preventing dendrite formation and producing a cauliflower type morphology. [32,55,83,84] Mg salts and other electrolyte species can influence the overpotential for Mg reduction and oxidation, especially in the presence of trace H 2 O in the electrolytes. For instance, while Grignard salts, organohaloaluminate Mg salts and Mg[BH 4 ] 2 are highly reactive with water tending to inhibit its role in the Mg electrochemistry, less reactive electrolytes, such as those containing MgCl 2 or Mg[NTf 2 ] 2 are easily hydrated and difficult to dry thoroughly by pre-heating.…”

Lightweight complex metal hydrides for Li-, Na-, and Mg-based batteries

“…Also, the LiBH 4 -containg cell revealed stable operation over 107 cycles, with an average discharge of 71 mA h/g and 92% of capacity retention. Lower values were obtained for the same cell with Mg(BH 4 ) 2 -LiBH 4 in DGM/TG/PP 14 TFSI as SSE [73]. The initial discharge-charge capacity was 74 mA h/g and 57 mA h/g, respectively.…”

Rechargeable Magnesium Batteries using Conversion‐Type Cathodes: A Perspective and Minireview

“…Another simple Mg salt, Mg(BH 4 ) 2 , can also react with electrophilic sulfur cathodes. It should be noted that by using ether‐based solvents with more donating ligands and the addition of LiBH 4 , as demonstrated by Mohtadi et al, Shao et al, Buttry et al, and Nuli et al, electrolytes based on simple Mg(BH 4 ) 2 show optimized electrochemical properties . Non‐nucleophilic boron clusters including (C 2 B 10 H 11 )MgCl, Mg(CB 11 H 12 ) 2 , and Mg(HCB 9 H 9 ) 2 , discovered by Mohtadi and co‐workers and Guo and co‐workers have also been demonstrated to show reversible Mg stripping/plating properties with much higher oxidative stability (3.6 V vs Mg) compared with Mg(BH 4 ) 2 electrolytes (Table ) .…”