Study of Electrochemical Phenomena Observed at the Mg Metal/Electrolyte Interface

Tutusaus, Oscar; Mohtadi, Rana; Singh, Nikhilendra; Arthur, Timothy S.; Mizuno, Fuminori

doi:10.1021/acsenergylett.6b00549

Cited by 134 publications

(163 citation statements)

References 60 publications

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“…Mg electrodes aged in electrolytes with different iodine concentrations were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) ( Figure S2, Supporting Information). [22,26] According to a previous systematic XPS study of Mg metal, this surface layer could only be a few nanometer thick. To obtain a finer picture of the surface chemistry of the Mg electrodes in the electrolytes with low iodine concentrations, we performed X-ray photoelectron spectroscopy (XPS).…”

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confidence: 99%

Reducing Mg Anode Overpotential via Ion Conductive Surface Layer Formation by Iodine Additive

Gao

Han

et al. 2017

Advanced Energy Materials

125

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Another challenge is the development of electrolytes that can effectively utilize an Mg metal anode. Extensive effort has been put in to develop electrolytes capable of reversibly depositing/stripping Mg. Most work has focused on complex electrolytes that allow fast and reversible Mg deposition/stripping. Such electrolytes were first proposed by Aurbach's group in 2000, [8] and subsequently they developed the all phenyl complex showing high anodic stability. [9] Later, a non-nucleophilic electrolyte, Mg-HMDS, was developed by Muldoon and co-workers, and Fichtner and co-workers for Mg/S batteries. [10,11] To eliminate the organic Grignard species in the electrolyte, an all inorganic electrolyte, magnesium aluminum chloride complex, was reported. [12][13][14][15] These complex electrolytes have enabled the usage of Mg-cathode half-cells to evaluate various cathode materials due to their capability of facile Mg deposition/stripping; [8,10,[16][17][18] however, their complicated synthesis procedure, incompatibility with oxide cathodes, sensitivity to air and moisture, low ionic conductivity, and high cost have rendered them less attractive for practical applications than conventional organic electrolytes based on simple salts (salts containing anions of PF 6 − , BF 4 − , TFSI − , etc.). Unfortunately, most of these simple salt electrolytes are not compatible with Mg metal because poorly or nonconductive surface layers are normally formed on the Mg surface. In the electrolytes based on common aprotic organic solvents (such as AN, PC, etc.), the surface film is dominated by the decomposition of solvent due to their weak reduction stability, and the formed layer can block the deposition/stripping of Mg. [19] Even for solvents that are stable with Mg metal (such as glyme), the decomposition of salts or the reaction of trace moisture with Mg can still form a surface film covering the Mg surface, leading to a large overpotential for Mg deposition/stripping. [20][21][22][23] Therefore, the formation of a poorly or even nonconductive surface layer on Mg metal in common simple salt organic electrolytes is inevitable due to Mg's strong reducing ability, which remains a challenge for the application of a Mg metal anode in these simple salt electrolytes. Herein, we propose for the first time a facile approach to solve this problem by tuning the composition of the surface layer and forming an Mg ion conductive layer. By adding a small concentration of iodine (<50 × 10 −3 m) into the electrolyte, an insoluble magnesium iodide layer is formed on the surface of the Mg metal, which acts as a solid electrolyte Electrolytes that are able to reversibly deposit/strip Mg are crucial for rechargeable Mg batteries. The most studied complex electrolytes based on Lewis acid-base chemistry are expensive, difficult to be synthesized, and show limited anodic stability. Conventional electrolytes using simple salts such as Mg(TFSI) 2 can be readily synthesized, but Mg deposition/stripping in these simple salt electrolytes is accompanied by a large...

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Reducing Mg Anode Overpotential via Ion Conductive Surface Layer Formation by Iodine Additive

Gao

Han

et al. 2017

Advanced Energy Materials

125

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“…The hexacoordinate cluster of Mg 2+ (G1) 3 -6C has been observed as part of the crystal structure of Mg(BH 4 ) 2 in G1. 51 Interestingly, whenever the magnesium atom is coordinated by an odd number of oxygens, implying a dangling uncoordinated oxygen as G1 has two oxygens, the free energy associate this oxygen is always exothermic. This demonstrates that clusters in which all solvent oxygens are chelated to magnesium are more stable.…”

Section: Resultsmentioning

confidence: 99%

Exploring the Liquid Structure and Ion Formation in Magnesium Borohydride Electrolyte Using Density Functional Theory

Deetz¹,

Cao²,

Wang³

et al. 2018

J. Electrochem. Soc.

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Rechargeable magnesium batteries have attracted much interest due their high volumetric capacity, potential for safe operation, and the natural abundance of magnesium. However, the development of magnesium batteries for practical applications has been obstructed by the lack of understanding of the liquid structure of electrolytes. Herein, we use quantum density functional theory coupled with a continuum solvation model to investigate the structure of Mg(BH 4 ) 2 in two ethereal solvents: tetrahydrofuran (THF), and monoglyme (G1). The most energetically favorable clusters of Mg(BH 4 ) 2 , MgBH 4 + , and Mg 2+ , with associated solvent molecule ligands, are determined. The free energy required to generate monovalent ions in the electrolyte is positive and the formation of divalent complexes is prohibitive. Singly and doubly charged complexes are more stable in G1 than THF, which is consistent with experimental findings. From the standpoint of free energy, clusters containing multiple magnesium atoms are not favored. Theoretical 25 Mg-NMR, 11 B-NMR spectra, and infrared vibrational modes of borohydride were calculated for each cluster. The relationships between cluster charge and the signals of each spectrum are determined. These analytical descriptors could be useful to characterize the degree of ion dissociation in the electrolyte. For rechargeable batteries to become widespread in electric vehicle and grid energy storage applications, it is imperative that they are safe and low cost.1,2 These challenges have motivated researchers in recent years to consider alternatives to lithium-ion batteries, such as sodium 3,4 and multivalent 5,6 battery chemistries. Ever since the first prototypes were developed, 7,8 magnesium batteries have attracted much interest from the research community. Compared with lithiumion batteries, magnesium batteries have several advantages, such as their high volumetric capacity (3832 mAh/cm 3 ), which is a result of the divalency of magnesium. Additionally, the cycling of plating/stripping of Mg 2+ onto the magnesium metal anode does not produce dendrites, which alleviates one of safety concerns of lithium batteries. 9,10 However, magnesium batteries face crucial challenges before they can succeed in practical applications.11 In particular, the choice of the electrolyte is critical to their performance. 12,13 There are only a handful of electrolytes known to be stable 14 during battery cycling, due to the reactivity of the Mg anode. that an electrolyte consisting of magnesium borohydride in an ethereal solvent, such as tetrahydrofuran or monoglyme, could reversibly plate/strip Mg 2+ onto a magnesium metal anode. X-ray photoelectron spectroscopy 13 has shown that a Mg anode using this electrolyte did not yield signals for boron or carbon, implying that the electrolyte is electrochemically stable during charge and discharge cycles. This marked the discovery of the first stable, halide-free electrolyte for magnesium batteries. Since then, there have been a number of experimental 13,[20][21][22...

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“…The patchy deposition of Mg provides additional evidence that electrolyte and Cu electrode are not in contact over the full area, but that the contact area increases upon plating, as the plated Mg fills the gaps between electrolyte and electrode. Also for liquid electrolyte systems it has been observed that newly formed interfaces between Mg metal and electrolyte improve the performance24.…”

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confidence: 99%

Magnesium Ethylenediamine Borohydride as Solid-State Electrolyte for Magnesium Batteries

Roedern

Kühnel

Remhof

et al. 2017

Sci Rep

130

163

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Solid-state magnesium ion conductors with exceptionally high ionic conductivity at low temperatures, 5 × 10−8 Scm−1 at 30 °C and 6 × 10−5 Scm−1 at 70 °C, are prepared by mechanochemical reaction of magnesium borohydride and ethylenediamine. The coordination complexes are crystalline, support cycling in a potential window of 1.2 V, and allow magnesium plating/stripping. While the electrochemical stability, limited by the ethylenediamine ligand, must be improved to reach competitive energy densities, our results demonstrate that partially chelated Mg2+ complexes represent a promising platform for the development of an all-solid-state magnesium battery.

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Study of Electrochemical Phenomena Observed at the Mg Metal/Electrolyte Interface

Cited by 134 publications

References 60 publications

Reducing Mg Anode Overpotential via Ion Conductive Surface Layer Formation by Iodine Additive

Reducing Mg Anode Overpotential via Ion Conductive Surface Layer Formation by Iodine Additive

Exploring the Liquid Structure and Ion Formation in Magnesium Borohydride Electrolyte Using Density Functional Theory

Magnesium Ethylenediamine Borohydride as Solid-State Electrolyte for Magnesium Batteries

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