The Coupling between Stability and Ion Pair Formation in Magnesium Electrolytes from First-Principles Quantum Mechanics and Classical Molecular Dynamics

Multi-valent (MV) battery architectures based on pairing a Mg metal anode with a high-voltage (∼ 3 V) intercalation cathode offer a realistic design pathway toward significantly surpassing the energy storage performance of traditional Li-ion based batteries, but there are currently only few electrolyte systems that support reversible Mg deposition. Using both static first-principles calculations and ab initio molecular dynamics, we perform a comprehensive adsorption study of several salt and solvent species at the interface of Mg metal with an electrolyte of Mg 2+ and Cl − dissolved in liquid tetrahydrofuran (THF). Our findings not only provide a picture of the stable species at the interface, but also explain how this system can support reversible Mg deposition and as such we provide insights in how to design other electrolytes for Mg plating and stripping. The active depositing species are identified to be (MgCl) + monomers coordinated by THF, which exhibit preferential adsorption on Mg compared to possible passivating species (such as THF solvent or neutral MgCl2 complexes). Upon deposition, the energy to desolvate these adsorbed complexes and facilitate charge-transfer is shown to be small (∼ 61 -46.2 kJ mol −1 to remove 3 THF from the strongest adsorbing complex), and the stable orientations of the adsorbed but desolvated (MgCl) + complexes appear favorable for charge-transfer. Finally, observations of Mg-Cl dissociation at the Mg surface at very low THF coordinations (0 and 1) suggest that deleterious Cl incorporation in the anode may occur upon plating. In the stripping process, this is beneficial by further facilitating the Mg removal reaction.

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“…glyme, diglyme, tetraglyme, and poly-THF) is expected to inhibit Mg 2+ ion delivery at the surface. 34,35 C. THF on MgO…”

Section: B Poly-thf On Mgmentioning

confidence: 99%

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

et al. 2015

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“…The self-diffusivities decrease as its concentration increases because the ion-ion interaction becomes stronger at higher concentrations. One might think that the self-diffusivity measured by NMR might not be exactly the same as the values measured with traditional methods such as mutual diffusion measurement 18,19 or MD simulations; [25][26][27][28][29][30][31][32][33] however, the difference between NMR measurement and MD simulation was reported to be minimal. 34 The aim of this article is to provide a theoretical method to evaluate the transport properties from self-diffusivities and activity coefficients; therefore, more detailed discussion regarding the differences between the measured self-diffusivities in different methods is out of scope of the paper and will be the subject of subsequent studies.…”

Section: Experimental Values and Evaluation Of Transport Properties Omentioning

confidence: 99%

A Method for Estimating Transport Properties of Concentrated Electrolytes from Self-Diffusion Data

Kim

Srinivasan

2016

J. Electrochem. Soc.

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A theoretical method is introduced that calculates the electrolyte-transport properties using Onsager-Stefan-Maxwell (OSM) theory (concentrated-solution theory) and the generalized-Darken relation. OSM theory is used to relate binary diffusivities to the transport properties of the electrolyte, including transference number, ionic conductivity, and Fickian diffusivity. The generalized-Darken relation is proposed to calculate the binary diffusivities of multicomponent systems from self-diffusivities. Finally, an example is demonstrated to show the details of how this method can be used to estimate transport properties. The calculated properties were reasonably close to the previously reported experimental values. © The Author To develop a physics-based lithium-ion battery model or battery management systems, accurate estimation of electrolyte transport properties are essential. This is especially true as new high-energy cells are designed with thick electrodes wherein electrolyte transport dominates the end of discharge at high current densities.However, the experimental measurements of the electrolyte properties are not trivial because it requires well-controlled experimental apparatus to obtain accurate values. Depending on the technique used, the estimation of the transport properties can be very sensitive to both natural and induced convection when estimating the diffusion coefficient. In addition, solvent reduction; inevitable when using lithium metal electrodes in electrochemical techniques, results in side reactions, and gas formation, leading to convolution of the obtained data. To properly describe mass transport in a solution, n(n − 1)/2 transport properties are necessary for n chemically distinct species because the system has n(n − 1)/2 binary diffusivities (D ij ).2 In a general lithium-ion battery, the electrolyte includes a cation, an anion, and a solvent; therefore, three independent transport properties are required. The cation transference number relative to the solvent t 0 + , ionic conductivity κ, and Fickian diffusivity D serve to describe the mass transport of the electrolyte. 3,4 In this work, we introduced a novel method of evaluating the electrolyte-transport properties using the Onsager-Stefan-Maxwell (OSM) theory (concentrated-solution theory) 2-10 combined with the generalized-Darken relation. 11,12 This method provides a way to estimate transport properties with minimized experimental measurements. The key to this method is converting self-diffusivities to binary diffusivities of multicomponent systems using the generalized-Darken relation. After the conversion, OSM theory can be used to calculate the electrolyte transport properties from the binary diffusivities. This paper is intended to provide the details of this conversion and evaluate its accuracy. As an example, we demonstrate the applicability by calculating the three transport properties of LiPF 6 in PC, and show that the estimated properties compare favorably with published literature data. Onsager-Stefan-Maxwell (OSM) Theor...

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“…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.…”

mentioning

confidence: 99%

“…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][23][24][25] and theoretical 12,23,26 studies aimed at improving battery performance. Presently, there are some problems with this electrolyte, including low ionic conductivity and solubility.…”

mentioning

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 Coupling between Stability and Ion Pair Formation in Magnesium Electrolytes from First-Principles Quantum Mechanics and Classical Molecular Dynamics

Cited by 286 publications

References 74 publications

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

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

A Method for Estimating Transport Properties of Concentrated Electrolytes from Self-Diffusion Data

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

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