Operating mechanisms of electrolytes in magnesium ion batteries: chemical equilibrium, magnesium deposition, and electrolyte oxidation

Abstract

Since the early nineties there have been a number of reports on the experimental development of Mg electrolytes based on organo/amide-magnesium chlorides and their transmetalations. However, there are no theoretical papers describing the underlying operating mechanisms of Mg electrolytes, and there is no clear understanding of these mechanisms. We have therefore attempted to clarify the operating mechanisms of Mg electrolytes by studying the characteristics of Mg complexes, solvation, chemical equilibrium, Mg-deposition processes, electrolyte-oxidation processes, and oxidative degradation mechanism of RMgCl-based electrolytes, using ab initio calculations. The formation and solvation energies of Mg complexes highly depend on the characteristics of R groups. Thus, changes in R groups of RMgCl lead to changes in the equilibrium position and the electrochemical reduction and oxidation pathways and energies. We first provide a methodological scheme for calculating Mg reduction potential values in non-aqueous electrolytes and electrochemical windows. We also describe a strategy for designing Mg electrolytes to maximize the electrochemical windows and oxidative stabilities. These results will be useful not only for designing improved Mg electrolytes, but also for developing new electrolytes in the future.

Exploration of the Detailed Conditions for Reductive Stability of Mg(TFSI)2 in Diglyme: Implications for Multivalent Electrolytes

“…7,32 Since the ion configurations and their solvation structures at the interface are predetermined by those in the bulk solution, in most of the studies the electrochemical stability windows are derived from the properties of electrolytes computed for the bulk. 6,31,33,34 However, in the conventional picture of multivalent ion deposition, 35 the details of the transition from a fully charged and highly coordinated form of a metal cation to its partially discharged and lower coordinated form remain largely unclear. The vicinity of the electrified anode surface may significantly impact the ion coordination and the solvent dynamics 36 and thus the relative time scales of the solvent reorganization and intrinsic electron transfer.…”

“…The concentration-dependent instability of the Mg(TFSI) 2 salt at Mg metal potentials was assigned to a drastic decrease of dissociation energy of the C−S bond in the cation-reduced (MgTFSI) 0 complex leading to its decomposition into neutral [Mg + −CF 3 SO 2 NSO 2 − ] 0 and CF 3 fragments. 31 However, the relationship between the solute concentration and the electrochemical stability of anions is far from being fully understood and is still debated 4,6,28,30,53 since the proposed mechanism does not exclude other processes involving solvent impurities or predicted cleavage of the (TFSI) − ion on the Mg surface. 6,54 Given the complexity and richness of the electrochemistry of the Mg(TFSI) 2 /G2 electrolyte, we present an in-depth ab initio study to elucidate the connection between relevant solvation structures of complexes formed in the bulk solution and their electrochemical stability in the vicinity of the Mg metal anode and formulate a set of general guidelines for selection or design of stable electrolytes.…”

“…28,50,51 In pursuit of understanding the Mg(TFSI) 2 electrolyte behavior and its potential improvement, extensive computational studies were carried out. 5,6,27,31 Using a combination of classical MD simulations and quantum chemistry calculations hybridized with implicit solvent models, 52 it was shown that the solvent donor strength and its chelating ability largely determine the solubility of Mg(TFSI) 2 and the ion-pair solvation structure. The concentration-dependent instability of the Mg(TFSI) 2 salt at Mg metal potentials was assigned to a drastic decrease of dissociation energy of the C−S bond in the cation-reduced (MgTFSI) 0 complex leading to its decomposition into neutral [Mg + −CF 3 SO 2 NSO 2 − ] 0 and CF 3 fragments.…”

Magnesium-Sodium Hybrid Battery With High Voltage, Capacity and Cyclability

“…It therefore shifts the equilibrium toward the formation of [MgCl] + , arguably the electro-active carrier for Mg 2+ deposition (Kim et al, 2014; Liu et al, 2014) providing an effective way to tune the performance of the Mg-NaCrO 2 battery by controlling the ratio between Mg-APC and Na(CB 11 H 12 ) in dual-salt electrolyte. Using non-carbon-coated NaCrO 2 cathode, we found that the highest discharge capacity was achieved when using a 3:7 volumetric mixture of 0.4 M Mg-APC and 0.5 M Na(CB 11 H 12 ), which was the dual salt electrolyte used in the following tests.…”

Ionic liquid hybrids: Progress toward non-corrosive electrolytes with high-voltage oxidation stability for magnesium-ion based batteries

“…Mg 0.07 V 2 O 5 is a promising cathode in Mg batteries. [49,50] We sought to determine the properties of this material in the presence of the best performing ionic liquids as described above. Figure 6A shows cyclic voltammetry data within the voltage stability range for 1M1PPi/acetonitrile.…”