Electrolytic Conditioning of a Magnesium Aluminum Chloride Complex for Reversible Magnesium Deposition

Abstract

We describe in this report the electrochemistry of Mg deposition and dissolution from the magnesium aluminum chloride complex (MACC). The results define the requirements for reversible Mg deposition and definitively establish that voltammetric cycling of the electrolyte significantly alters its composition and performance. Elemental analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (SEM-EDS) results demonstrate that irreversible Mg and Al deposits form during early cycles. Electrospray ionization mass spectrometry (ESI-MS) data show that inhibitory oligomers develop in THF-based solutions. These oligomers form via the well-established mechanism of a cationic ring-opening polymerization of THF during the initial synthesis of the MACC and under resting conditions. In contrast, MACC solutions in 1,2-dimethoxyethane (DME), an acyclic solvent, do not evolve as dramatically at open circuit potential. From these results, we propose a mechanism describing how the conditioning process of the MACC in THF improves its performance by both tuning the Mg:Al stoichiometry and eliminating oligomers.

Understanding the Initial Stages of Reversible Mg Deposition and Stripping in Inorganic Nonaqueous Electrolytes

“…3,7,8,10,11 In all cases, Cl aligns toward the Mg surface, with adsorption on the HLW site preferred over the TOP site as shown in Figure 1 (also see Figure S2 in Supplementary Information), because in comparison to TOP, HLW and SIDE oriented adsorption (shown in Figure 5) maximizes the interaction of the Cl ion with the Mg surface sites as well as that of some dangling C and H atoms from the THF molecules with the Mg surface. Of all the charged magnesium-chloro complexes, (MgCl) + coordinated by 5 THF in the hollow site exhibits the strongest adsorption energy (-84.7 ± 10.5 kJ mol −1 ), and in general the strength of the adsorption reduces with sequential THF removal, as seen in the trend of adsorption energies of Figure 5.…”

“…10,32,33 We therefore also investigate the possible adsorption of THF products of polymerization such as poly-tetrahydrofuran (poly-THF) at the anode/electrolyte interface shown in the inset of Figure 2b. Here, two approximations are used to describe poly-THF: i ) by cleaving THF at the ethereal bonds and saturating by hydrogen atoms, forming the 1,4-dihydroxybutane (see Figure 2b), and ii ) by 1,2-dimethoxyethane also known as glyme (DME, see Figure 2c).…”

“…Recently, Barile et al 10 have observed that the addition of poly-THF in the MACC electrolyte increases the deposition overpotentials to 500 mV indicating that poly-THF may passivate more the Mg-surface. We conclude that hydroxyl-terminated polymer molecules (e.g.…”

“…5 The path to fully functioning and cost-competitive Mgion batteries begins with addressing a few but complex scientific questions, and perhaps the most pressing is to develop a robust understanding of the atomistic mechanism of reversible plating (or deposition) and stripping (or dissolution) of Mg at the anode/electrolyte interface during battery operation. To date, reversible Mg plating with low over-potential and reasonable anodic stability has been achieved in practice with only a specific class of electrolytes, namely organic or inorganic magnesium aluminum chloride salts (magnesium-chloro complexes) dissolved in ethereal solvents [6][7][8][9][10][11] . Aurbach and collaborators developed the first operational Mg ion full cells using electrolytes based on Grignard reagents (e.g.…”

“…First, extensive experimental and computational approaches have been used to characterize the equilibrium solvation structure of Mg 2+ in the bulk electrolyte, which defines the thermodynamic start (end) point of deposition (dissolution), and several possible coordinating complexes involving combinations of Mg 2+ , Cl − , and THF have been identified. 7,8,10,11,[15][16][17][18] These findings have subsequently been used to inform phenomenological models that describe the general sequence of ion desolvation, adsorption on the anode surface, charge-transfer, and metal incorporation that make up the overall deposition process (the sequence in reverse describes the dissolution process) see Ref. 11 and references therein.…”