MgCl₂: The Key Ingredient to Improve Chloride Containing Electrolytes for Rechargeable Magnesium-Ion Batteries

Abstract

The effect of MgCl 2 on a series of chloride containing magnesium electrolytes was investigated. In the presence of extra MgCl 2 , the electrochemical properties of Grignard reagents (RMgCl, R = Ph, Et, i Pr) were significantly improved, and the advantage of MgCl 2 was further demonstrated in Mg-Mo 6 S 8 rechargeable batteries with improved capacities and much smaller over-potentials. MgCl 2 was then further proven to be a powerful reagent to improve the performance of well-established strong Lewis acid derived magnesium electrolytes including the "all-phenyl" complex (APC) and alkoxide-based magnesium electrolytes. The results suggest that MgCl 2 salt is a very important species to benefit all chloride containing electrolytes for rechargeable magnesium-ion batteries.Currently, the lack of efficient energy storage devices is still the major limitation for the practical harvest of clean energies such as solar, wind or tide. Rechargeable batteries have been proposed as the promising solution for efficient energy storage and transmission. Although there is a great success of secondary lithium-ion batteries (LIBs) in portable electronic devices, the cost and safety barriers make traditional LIBs unable to meet the requirements for safe and economical power applications. As an alternative, magnesium-ion batteries are potential candidates for larger and safer energy storage due to the advantages in safety, cost and capacity. 1-3 For instance, in contrast to lithium, magnesium metal is non-toxic, more air resistant and free from dangerous dendrite formation. The greater natural abundance makes magnesium more than 20 times cheaper than lithium in cost. In addition, the divalent characteristic of magnesium ion (Mg/Mg 2+ ) enables magnesium to have nearly double the volumetric capacity versus lithium (3832 mA cm −3 vs 2062 mA cm −3 ). Despite these advantages, the lack of electrolytes which can plate/strip magnesium with wide electrochemical windows has retarded the progress of magnesium-ion batteries for quite a long time. 4 The discovery of efficient magnesium deposition can date back to early the 20 th century for Grignard reagents in ethereal solutions, but the relatively low oxidative stabilities (typically less than 2 V vs Mg/Mg 2+ ) prevented the use of Grignard reagents as effective electrolytes for high energy density secondary batteries. 5,6 The pioneering work by Gregory et al. in 1990 suggested that introducing strong Lewis acids into Grignard reagents can improve the oxidative stability, 7 and this strategy was later extended by Aurbach et al. in 2000 with the development of the famous dichloro-complex electrolyte (nBu 2 Mg + 2EtAlCl 2 ). 8 To date, strong Lewis acid neutralization has been proven one of the most successful methods to design magnesium electrolytes with more than 3 V (vs Mg/Mg 2+ ) stabilities, and it is believed that the addition of strong Lewis acids (aluminum or organoboron) helps to generate highly stable aluminate or organo-borate anions in solution, which in turn improves the oxidative st...

Anode-Electrolyte Interfaces in Secondary Magnesium Batteries

“…Chlorides, in the current context, pertain to both complex anions and cations, serving as ligands, and solvating or surface-adsorbed anions. 4,25,28,39 As noted before, the first solutions that supported fully reversible magnesium electroplating were Grignard reagents in ethers. 26 These RMgX reagents included compounds where X = Cl, Br, and probably also I.…”

Elucidating Solvation Structures for Rational Design of Multivalent Electrolytes—A Review

“…However, full operation of APC electrolytes with Chevrel phase-Mo 6 S 8 , TiS 2 , and V 2 O 5 cathodes has already been demonstrated by adding either LiCl or MgCl 2 as additives [ 80 , 81 ]. Pan et al demonstrated enhanced electrochemical performance for APC salts in Mg-Mo 6 S 8 with the addition of MgCl 2 salt and they observed a typical dimer structure Mg 2 (μ-Cl 3 ) from single-crystal X-ray diffraction [ 82 ]. Hence, it is likely that addition of MgCl 2 does not change the active species of APC salts but rather drive the Schlenk equilibrium to generate more electroactive species.…”

Elucidating Solvation Structures for Rational Design of Multivalent Electrolytes—A Review

“…However, full operation of APC electrolytes with Chevrel phase-Mo 6 S 8 , TiS 2 , and V 2 O 5 cathodes has already been demonstrated by adding either LiCl or MgCl 2 as additives [80,81]. Pan et al demonstrated enhanced electrochemical performance for APC salts in Mg-Mo 6 S 8 with the addition of MgCl 2 salt and they observed a typical dimer structure Mg 2 (μ-Cl 3 ) from single-crystal X-ray diffraction [82]. Hence, it is likely that addition of MgCl 2 does not change the active species of APC salts but rather drive the Schlenk equilibrium to generate more electroactive species.…”