Elsevier

Nano Energy

Volume 46, April 2018, Pages 436-446
Nano Energy

Full paper
25Mg NMR and computational modeling studies of the solvation structures and molecular dynamics in magnesium based liquid electrolytes

https://doi.org/10.1016/j.nanoen.2018.01.051Get rights and content

Highlights

  • Mixing 2 competing salts reduces the strong covalent interactions between Mg2+ and BH4- ions that hinder the application of Mg battery technology.

  • High field NMR, DFT, MD, and electrochemical evaluations were used to identify specific nanoscale solvation structures of mixed salts in diglyme.

  • Stable structures and an interconversion mechanism were verified with NMR and theory showing that solvent reordering prevails over Mg2+ hopping.

  • Solvent reordering permits increased uncoordinated anions, improved stability, and enhanced conductivity relative to a single anion solution.

Abstract

There is increasing evidence that the solvation structure of the active components in a liquid electrolyte solution strongly impacts the performance in electrochemical applications. In this work, the nanoscale solvation structures and dynamics of Mg(BH4)2 and Mg(TFSI)2 dissolved in diglyme (DGM) at various concentrations and ratios of Mg(BH4)2/Mg(TFSI)2 were investigated using a combination of natural abundance 25Mg NMR, quantum chemistry calculations of 25Mg NMR chemical shifts, classical molecular dynamics (MD) calculations, and electrochemical performance tests. By mixing two competing Mg salts, we were able to reduce the strong covalent interactions between Mg2+ and BH4 anions. A small increase is observed in the coordination number of Mg-TFSI and a significant increase in the interaction of Mg2+ ions with glymes. Through a combination of NMR, DFT and MD simulations, various stable species around 1 nm in size were detected in the mixed salt solution, which play key roles in the enhanced electrochemical performance of the mixed electrolyte. It is established that for the neat Mg(TFSI)2 in DGM electrolyte at dilute concentrations the TFSI- is fully dissociated from Mg2+. At higher concentrations, Mg2+ and TFSI- are only partially dissociated as contact ion pairs are formed. In contrast, at 0.01 M Mg(BH4)2 (saturated concentration) in DGM, the first solvation shell of a Mg2+ ion contains two BH4- anions and one DGM molecule, while the second solvation shell consists of five to six DGM molecules. An exchange mechanism between the solvation structures in the combined electrolyte containing both Mg(BH4)2 and Mg(TFSI)2 in DGM was found to result in the observation of a single 25Mg NMR peak. This exchange is responsible for an increase in uncoordinated anions, as well as improved stability and ionic conductivity as compared to single anion solution. Solvent molecule rearrangement and direct Mg-ion exchange between the basic solvation structures are hypothesized as likely reasons for the exchange. We elucidate that the solvent rearrangement is energetically much more favorable than direct Mg-ion hopping and is thus suggested as the dominant exchange mechanism.

Introduction

Recently, there has been an increased interest in the solvation structure of electrolytes as many of their important properties such as conductivity, viscosity, and even stability can be influenced by the local intermolecular interactions in the liquid [1], [2], [3], [4], [5], [6], [7]. The solvation structure of electrolytes refers to the detailed molecular interaction between molecular or ionic solutes and other species in solution, e.g. solvent molecules, such that the solute is surrounded by concentric shells of electrolyte molecules to form solvation complexes [8]. For energy applications, so-called ‘designer’ electrolytes, which are rationally developed to target specific solvation structures, have recently been shown to increase the electrolyte stability [9], [10], [11], [12], [13], which is particularly important for nascent energy storage technologies such as Li-S, Li-O, and multivalent intercalation [2], [14], [15], [16], [17]. In this work, we focus on elucidating the solvation structure in an organic liquid containing two different competing anions, which enables tuning of the solvation structure as a function of the separate salt concentrations. The common salt cation is chosen as Mg, which is motivated by the need for novel multivalent electrolytes with increased stability for electrochemical energy storage applications [1], [3], [4], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. In particular, a rechargeable Mg metal battery is an attractive future alternative to Li-ion as it could potentially triple the volumetric energy density (3833 mA h/cc) as compared to the Li counterpart graphite (~800 mA h/cc) [29]. On the other hand, despite the potential advantages, several obstacles need to be addressed, including the need for an electrolyte with a wide electrochemical window which also enables reversible plating/stripping of Mg [30]. The latter requirement is challenging as Mg metal forms an ionically blocking surface layer, i.e., the solid electrolyte interface (SEI), when exposed to oxygen, which inhibits Mg deposition [31]. Hence, the discovery of novel electrolytes in which no SEI or a weakened passivation layer is formed on the Mg metal surface is crucial for realizing highly reversible Mg metal deposition/dissolution [2], [32], [33], [34]. Electrolytes based on halo, organo, and organo-halo salts in ether solvents were found capable of reversible Mg deposition/dissolution by providing a passivation-free interface [35], [36]. However, Grignard solutions are limited by their insufficient anodic stability (< 2 V) and poor conductivity. Their nucleophilic and corrosive nature also makes them incompatible for use with high voltage electrophilic cathodes materials (such as sulfur and oxygen) and aluminum current collectors [35]. Non-nucleophilic electrolytes such as a HMDSMgCl/AlCl3 (THF) solution offer a wider electrochemical stability window (~ 3.3 V) and compatibility with sulfur cathodes, but suffer from capacity fading after the first cycle possibly due to the dissolution of polysulfide species and corrosion of current collectors due to the presence of chloride ions [37]. Simple and non-corrosive inorganic Mg salts such as Mg(TFSI)2 and Mg(BH4)2 have gained much popularity over the last few years. Mohtadi et al. successfully pioneered the use of magnesium borohydride, Mg(BH)4, taking advantage of its high thermodynamic and reductive stability [38]. However, the major drawbacks of Mg(BH4)2 are the low anodic stability (1.7 V vs. Mg on Pt) and low solubility. On the other hand, magnesium(II) bis(trifluoromethane sulfonyl) imide (Mg[N(SO2CF3)2]2), commonly known as Mg(TFSI)2), has also been reported as a highly competent electrolyte candidate due to its high resistivity towards oxidation, high conductivity, and compatibility with most cathode materials [36]. Unfortunately, it is limited by its high overpotential and low coulombic efficiency for deposition and dissolution [39], [40]. Previous reports have shown that the solvent or ligand dramatically influences the electrochemical properties of the electrolytes [10], [40], [41]. For example, Shao et. al. demonstrated that the coulombic efficiency of Mg(BH4)2 can be significantly enhanced by increasing the ligand strength, i. e. O donor denticity of the ethereal solvents [42]. However, little is known about the detailed changes of the solvation structures and dynamics in Mg electrolyte systems as a function of concentration – particularly when more than one salt is involved. In an effort to design halogen free (non-corrosive) and simple ionic electrolytes, we considered that combining two competing salts, Mg(BH4)2 and Mg(TFSI)2, would allow for exploration of the tunability of the solvation structure to access the different properties of the salts; BH4 for its excellent metal plating performance and TFSI for its exceptional redox stability and solubility. Recent studies have shown that the high overpotential for Mg plating/stripping and poor faradaic cycling efficiency of Mg(TFSI)2 electrolyte may be resolved by the addition of chloride ions [43], [44], [45]. However, chloride-containing magnesium electrolytes are corrosive towards non-noble metals [7]. A potential solution is to combine Mg(TFSI)2 with Mg(BH4)2 to access a combination of stability and solubility.

In this work, we adopted a multi-modal approach by combining theory and experiments at multiple lengths and time scales to elucidate the solvation structures as well as electrochemical performance of Mg(BH4)2 and Mg(TFSI)2 dissolved in diglyme (DGM) at various concentrations and ratios of Mg(BH4)2/Mg(TFSI)2 using a combination of natural abundance 25Mg NMR, quantum chemistry calculations of 25Mg NMR chemical shifts, classical molecular dynamics simulations, cyclic voltammetry, and coulombic efficiency measurements. We reveal the exchange mechanism between the basic nanometer sized solvation structures in the mixed electrolyte containing both Mg(BH4)2 and Mg(TFSI)2 in DGM as the primary factor contributing to the enhanced electrochemical performance achieved with this system. The objective of our research is thus to understand the fundamental solvation structure and exchange mechanisms in these electrolytes such that new design rules for multivalent electrolytes exploiting competing anion interactions lead to enhanced electrochemical performance via unique structural and dynamical properties.

Section snippets

Materials and sample preparations

Magnesium borohydride (Mg(BH4)2, 95%) was purchased from Sigma–Aldrich. Diglyme (DGM) was obtained from Novolyte Technologies, Inc. (Cleveland, US). Mg(TFSI)2 was obtained from Solvionic SA France. All sample preparations were performed in a MBraun Labmaster Ar-filled glove box (Stratham, NH) with water and O2 contents less than 1 ppm. A variety of samples containing a mix of Mg(BH4)2 and Mg(TFSI)2, were prepared by dissolving a constant concentration of 0.01 M Mg(BH4)2 and variable

Results and discussion

The fundamental understanding of the correlation between the solvation structure and the dynamical properties of ionic species in a multicomponent mixture and its effect on the electrochemical properties provides an important basis for designing optimized electrolytes. Here, we used MD simulations and NMR to understand the solvation structure and transport properties of neat Mg(BH4)2/DGM and neat Mg(TFSI)2/DGM solutions as well a more complex Mg(BH4)2 + Mg(TFSI)2 in DGM solution as a function

Electrochemical evaluation of the composite electrolytes

We find that the salt ratios of the electrolytes have pronounced effects on the Mg deposition and stripping properties. Fig. 9 compares the cyclic voltammetry (CV) curves acquired with each electrolyte at a scan rate of 20 mV/s. The results were analyzed and the activities are listed in Table 3, which shows that the pure Mg(BH4)2 electrolyte (saturated, ~ 0.01 M) exhibits very weak activities for Mg deposition, and both the Mg deposition current density and the Coulombic efficiency (15%) were

Conclusion

Our results indicate that even a small addition of Mg(TFSI)2 to Mg(BH4)2 dissolved in diglyme (DGM) can significantly disrupt the well-defined solvation structure and strong interaction between Mg and BH4, thus increasing the fraction of freely coordinated anions and resulting in significantly increased solubility of Mg(BH4)2 in DGM. This not only greatly enhances the dynamics but also improves the stability of the otherwise unstable TFSI anion. By taking advantage of the increased sensitivity

Acknowledgments

This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (BES). The NMR and first principle computational studies were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE's Office of Biological and Environmental Research (BER) and located at PNNL. The classical molecular

Jian Zhi Hu obtained his Ph.D. in Applied Physics in 1994 from a Joint-Training Pro-gram between Wuhan Institute of Physics, the Chinese Academy of Sciences and the Department of Chemistry, University of Utah, USA. He did his postdoctoral studies also from University of Utah. Currently, he is a senior staff scientist of Pacific Norwest National Laboratory, specialized in nuclear magnetic resonance (NMR) spectroscopy and imaging. He has published more than 190 peer reviewed papers related to

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    Jian Zhi Hu obtained his Ph.D. in Applied Physics in 1994 from a Joint-Training Pro-gram between Wuhan Institute of Physics, the Chinese Academy of Sciences and the Department of Chemistry, University of Utah, USA. He did his postdoctoral studies also from University of Utah. Currently, he is a senior staff scientist of Pacific Norwest National Laboratory, specialized in nuclear magnetic resonance (NMR) spectroscopy and imaging. He has published more than 190 peer reviewed papers related to NMR. He received 10 US patents and two R&D 100 awards.

    Nav Nidhi Rajput obtained her Ph.D. in Chemical Engineering at Louisiana State University in Louisiana, USA in 2013. She joined LBNL as a Postdoctoral Fellow at environmental energy technology division in 2013. Her research interests include molecular dynamics simulations to acceler-ate discovery of novel electrolytes for bat-teries and supercapacitors.

    Yuyan Shao received his Ph.D. from Harbin Institute of Technology. He is a Senior Scientist in the Energy Processes and Mate-rials Division of the Pacific Northwest National Laboratory. His research is focused on the fundamental study and high-performance functional materials for elec-trochemical energy conversion and storage, including fuel cells, batteries, supercapaci-tors, etc. He is named in Thomson Reuters' Highly Cited Researchers-2014. He has published over 80 papers.

    Nicholas Jaegers obtained his B.S. in Chemical Engineering from Iowa State University in 2014. He is a Ph.D. candidate in Chemical Engineering at Washington State University within Dr. Yong Wang's research group. He is currently located at Pacific Northwest National Laboratory as an intern focusing on applications of nuclear magnetic resonance in catalysis.

    Karl Mueller received his Ph.D. from the University of California, Berkeley and is the Chief Science and Technology Officer for Physical and Computational Sciences at Pacific Northwest national Laboratory. His research focuses on the use of NMR methods to address structural and dynamic questions in complex systems, including batteries and catalyst materials. He is a AAAS Fellow and has published over 140 papers.

    Jun Liu received his Ph.D. in materials science from University of Washington. He is a Laboratory Fellow and Energy Processes and Materials Division Director at the Pacific Northwest National Laboratory. Dr. Liu's main research interest includes synthesis of functional nanomaterials for energy sto-rage, catalysis, environmental separation and health care. He has received more than 40 U.S. patents, two R&D 100 Awards, two BES Awards for Significant Impact on DOE Missions, and was named 2007 Distinguished Inventor of Battelle. Dr. Liu is an AAAS Fellow and MRS Fellow. He is named in Thomson Reuters' Highly Cited Researchers-2014 in three categories (Materials science, Chemistry and Engineering). He has over 300 publications.

    Kristin Aslaug Persson obtained her Ph.D. in Theoretical Physics at the Royal Institute of Technology in Stockholm, Sweden in 2001. She is an Associate Professor in Materials Science and Engineering at UC Berkeley with a joint appointment as Faculty Staff Scientist at LBNL where she leads The Materials Project (www.materialsproject.org) and the Cross-Cutting thrust in the Joint Center for Energy Storage Research (JCESR) (www.jcesr.org). In 2009 she co-founded the clean-energy start-up Pellion Technologies Inc. (www.pelliontech.com), recipient of an ARPA-E award in 2010 for developing high-energy rechargeable magnesium batteries.

    1

    These authors have equal contribution to this work.

    2

    These authors have equal contribution to this work, but less than that of Jian Zhi Hu and Nav Nidhi Rajput.

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