TFSA Anion Research Articles

Operando Raman Spectroscopy for Evaluating Concentration Changes in Li- and Na-Based Solid Polymer Electrolytes

The solid polymer electrolytes (SPEs) can apply to all-solid-sate batteries using various cations as the carrier species. Herein, we performed operando Raman spectroscopy to evaluate the concentration changes with ionic transport in the Li- and Na-based SPEs composed of ether-based host polymer and MN(SO2CF3)2 (M = Li, Na; MTFSA), with the aim of extracting differences in behavior under electrochemical operation. In the [Li | Li-based SPE | Cu] cell, the integrated intensities of TFSA anions changed near the working electrode (WE) and counter electrode (CE) sides during cyclic voltammetry measurements, revealing changes in the anion concentration. Then, operando Raman spectroscopy was conducted for the [M | Li- or Na-based SPE | M (M = metallic Li or Na)] symmetric cells during chronoamperometry measurements. The cell using Li-based SPE exhibited increasing and decreasing integrated intensities for TFSA at the WE and CE sides, respectively, before reaching a steady state. In contrast, the integrated intensities of TFSA required a longer time to reach a steady state in the Na-based SPE system, suggesting that the solvation/desolvation of Na+ is more difficult during ionic transport. Overall, operando Raman spectroscopy contributed to elucidating the differences in ionic transport properties between Li- and Na-based SPEs during electrochemical processes.

Lithium salt-concentrated organogels prepared via one-step polymer network formation in acetonitrile-based solutions

We report lithium (Li) salt-concentrated organogel prepared via one-step gelation conducted in an acetonitrile (AN)-based solution containing tetra-arm poly (ethylene glycol) (TetraPEG) and the lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) salt at extremely high concentration. The polymer network formation via a Michael addition reaction between thiol-terminated and maleimide-terminated TetraPEG prepolymers successfully proceeded in concentrated LiTFSA/AN solutions under noncatalytic conditions at room temperature to yield a homogeneous polymer network with high reaction efficiency (>95%). We investigated the electrochemical properties of the concentrated organogel to apply it as a quasi-solid electrolyte material to the Li-ion capacitors comprising a graphite anode and activated carbon (AC) cathode. The concentrated organogel afforded the reversible Li-ion insertion/desertion into/from graphite; however, reactivity depended on the polymer concentration in the organogels. The reversible adsorption/desorption of TFSA anions into/from the AC was observed in the concentrated organogel, which was comparable to that of the corresponding electrolyte solution.

Structural aspect on “Salting-in” mechanism of PEG chains into a phosphonium-based ionic liquid using lithium salt

A study on the “salting-in” mechanism of poly(ethylene glycol) (PEG) into triethylpentylphosphonium bis(trifluoromethanesulfonyl)amide ([P2225][TFSA]) ionic liquid with LiTFSA salt is presented. There was no dissolution of PEG chains into [P2225][TFSA] in the absence of LiTFSA salt. However, salting-in dissolution occurred when LiTFSA salt was added irrespective of the molecular weight of PEG. Raman spectral analysis revealed that the TFSA anions trapped within the Li-ion coordination complex (i.e., [Li(TFSA)2]–) in [P2225][TFSA] solution are easily decoordinated when PEG-chains are added to form stable Li+⋯PEG complexes; in other words, Li ions preferentially interact with O atoms of PEG over those of TFSA anion. High-energy X-ray total scattering (HEXTS) and all-atom molecular dynamics (MD) simulations were combined to elucidate the detailed structures of the salting-in dissolved PEG chains at the molecular level. Some of the notable characteristics are as follows: (1) In PEG/[P2225][TFSA] solution (without LiTFSA salt), the PEG chains struggle to interact with the P2225 cations because of weaker PEG–P2225+ interactions, resulting in the incompatibility of PEG/[P2225][TFSA] with shrink polymer chains. (2) When LiTFSA is added to the solution, the Li ions bound to the PEG chains act as a solvation site for solvent IL (in this case, TFSA anions). (3) Interactions between PEG-coordinated Li+ and TFSA– trigger the polymer chains to expand, resulting in “salting-in” dissolution of PEG.

Local Structure of Li+ in Superconcentrated Aqueous LiTFSA Solutions.

It has been reported that aqueous lithium ion batteries (ALIBs) can operate beyond the electrochemical window of water by using a superconcentrated electrolyte aqueous solution. The liquid structure, particularly the local structure of the Li+, which is rather different from conventional dilute solution, plays a crucial role in realizing the ALIB. To reveal the local structure around Li+, the superconcentrated LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) aqueous solutions were investigated by means of Raman spectroscopic experiments, high-energy X-ray total scattering measurements, and the neutron diffraction technique with different isotopic composition ratios of 6Li/7Li and H/D. The Li+ local structure changes with the increase of the LiTFSA concentration; the oligomer ([Lip(TFSA)q](p-q)+ (q > 2) forms at the molar fraction of LiTFSA (xLiTFSA) > 0.25. The average structure can be determined in which two water molecules and two oxygen atoms of TFSA anion(s) coordinate to the Li+ in the superconcentrated LiTFSA aqueous solution (LiTFSA)0.25(H2O)0.75. In addition, the intermolecular interaction between the neighboring water molecules was not found, and the hydrogen-bonded interaction in the solution should be significantly weak. According to the coordination number of the oxygen atom (TFSA or H2O), a variety of TFSA- and H2O coordination manners would exist in this solution; in particular, the oligomer is formed in which the monodentate TFSA cross-links Li+.

Record Enhancement of Curie Temperature in Host-Guest Inclusion Ferroelectrics.

Solid-state molecular rotor-type materials such as host-guest inclusion compounds are very desirable for the construction of molecular ferroelectrics. However, they usually have a low Curie temperature (Tc) and uniaxial nature, severely hindering their practical applications. Herein, by regulating the anion to control "momentum matching" in the crystal structure, we successfully designed a high-temperature multiaxial host-guest inclusion ferroelectric [(MeO-C6H4-NH3)(18-crown-6)][TFSA] (MeO-C6H4-NH3 = 4-methoxyanilinium, TFSA = bis(trifluoromethanesulfonyl)ammonium) with the Aizu notation of mmmFm. Compared to the parent uniaxial ferroelectric [(MeO-C6H4-NH3)(18-crown-6)][BF4] with a Tc of 127 K, the introduction of larger TFSA anions brings a lower crystal symmetry at room temperature and a higher energy barrier of molecular motions in phase transition, giving [(MeO-C6H4-NH3)(18-crown-6)][TFSA] multiaxial ferroelectricity and a high Tc up to 415 K (above that of BaTiO3). To our knowledge, such a record temperature enhancement of 288 K makes its Tc the highest among the reported crown-ether-based ferroelectrics, giving a wide working temperature range for applications in data storage, temperature sensing, actuation, and so on. This work will provide guidance and inspiration for designing high-Tc host-guest inclusion ferroelectrics.

Effects of Polysulfide Solubility and Li Ion Transport on Li–S Batteries Using Sparingly Solvating Electrolytes

Sparingly solvating electrolytes are an emerging class of electrolytes used in Li–S batteries. In this type of electrolytes, polysulfide dissolution and shuttling can be suppressed, leading to high Coulombic efficiency and cycle life. To optimize the electrolytes for high energy density Li–S cells, effects of polysulfide solubility and Li ion transport properties on Li–S battery performance were investigated for tetraglyme (G4)- and sulfolane (SL)-based electrolytes diluted with a hydrofluoroether (HFE). The Li2S8 solubility was found to be very low (1 mM in atomic S concentration) in [Li(G4)0.8][TFSA]-4.3HFE and [Li(SL)2][TFSA]-4.0HFE. Cells with [Li(SL)2][TFSA]-4.0HFE exhibited better rate capability compared with cells with [Li(G4)0.8][TFSA]-4.3HFE despite lower ionic conductivity of [Li(SL)2][TFSA]-4.0HFE. With regard to the Li ion coordination structure in [Li(SL)2][TFSA]-4.0HFE, the two oxygen atoms of the SL sulfonyl group coordinates to two different neighboring Li ions and TFSA anions form ionic clusters with Li ions. The unique SL- and anion-bridged, chain-like Li ion coordination structure was considered to be involved in the unusual Li ion self-diffusion behavior and Li ion tranport for [Li(SL)2][TFSA]-4.0HFE. The higher Li ion transference number measured under anion-blocking condition (t Li +) of [Li(SL)2][TFSA]-4.0HFE was suggested to contribute to the better rate performance, rather than the polysulfide solubility and ionic conductivity. Furthermore, [Li(SL)2][TFSA]-4.0HFE demonstrated an initial discharge capacity of 1130 mAh g−1 at a low electrolyte volume to sulfur weight ratio of 4, whereas a typical organic electrolyte failed to achieve such a high capacity owing to limitations of the redox mechanism mediated by dissolved polysulfides. In addition to the low solubility of polysulfides, the high t Li + is crucial for achieving high energy density Li–S batteries by reducing the electrolyte amount. The SL-based electrolytes were found to manifest a significant improvement in Li ion mass transfer as a sparingly solvating electrolyte, enabling the solid-state sulfur redox reactions in high-performance Li-S batteries. Acknowledgement This study was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). References 1) A. Nakanishi. et al, J. Phys. Chem. C, 2019, 123, 14229.2) M. Yanagi et al, J. Electrochem. Soc., 2020, 167, 070531. Figure 1

Preparation and Electrochemical Properties of Ambient Temperature Molten Chlorides Based on Quaternary Ammonium Chloride and MgCl2

The research and development of magnesium secondary batteries using magnesium anode have been attracted much attention as one of the prospective post-lithium ion batteries due to the high theoretical capacity (3830 Ah/dm3) and without no resource constraints. However, it has been quite difficult to construct the magnesium batteries because practical electrolytes for the battery has not yet been developed.We previously reported that TFSA-based ILs were not suitable for a Mg anode even in alkali metal binary molten salt such as MgTFSA2-KTFSA, which indicated that the existence of the TFSA anion disturbed a magnesium redox on Mg electrodes. [1] On the other hand, relatively smooth magnesium redox could be observed in 1-butyl-3-methylimidazolium chloride melts containing Mg[TFSA]2. [2] However, the electrochemical window of the imidazolium melt was limited by the reduction decomposition of the imidazolium.In this study, we reported that ambient temperature molten chlorides could be obtained by mixing a quaternary ammonium chloride and MgCl2, which did not contain the TFSA anion and also imidazolium cations. The thermal properties and also physicochemical properties were investigated. The electrochemical response of a magnesium metal in the melt were also shown to indicate the possibility as an electrolyte for a magnesium anode battery.[1] K. Kubota, H. Matsumoto, J. Electrochem. Soc., 167 (2020) 020541.[2] H. Matsumoto, K. Kubota, ECS Transactions, 86(14) (2018) 21.

Solvation Structure of Poly(benzyl methacrylate) in a Solvate Ionic Liquid: Preferential Solvation of Li-Glyme Complex Cation.

We report the solvation structure of a lower critical solution temperature (LCST)-type thermoresponsive polymer in a solvate ionic liquid (SIL, i.e., an ionic liquid comprising solvate ions) to elucidate the predominant interaction for the dissolution of the thermoresponsive polymer in SIL at low temperatures. The solvation structure of poly(benzyl methacrylate) (PBnMA) and a model compound of its monomer in a typical glyme-based SIL, [Li(G4)][TFSA] (G4: tetraglyme; TFSA: bis(trifluoromethanesulfonyl)amide), have been investigated using high-energy X-ray total scattering and all-atom molecular dynamics simulations. In the model compound/SIL system, the intermolecular components extracted from the total G( r)s revealed that the ester moiety of BnMA is preferentially solvated by Li cations through a cation-dipole interaction, which induces slight desolvation of the G4 molecules, and the aromatic ring of BnMA is secondarily solvated by the [Li(G4)] cation complex through a cation-π interaction with maintaining the complex structure. In contrast, TFSA anions are attracted only by the [Li(G4)] cation. These interactions result in the formation of a solvation layer of SILs around the aromatic ring, which plays a key role in the negative entropy and enthalpy of mixing. Meanwhile, in the polymer solution, the coordination number of the Li cation around the ester moiety significantly decreased. This could be ascribed to the steric effect of the bulky side chains, preventing the approach of the [Li(G4)] cation complex to the ester moiety located near the main chain. These solvation structures lead to small absolute values of negative entropy and enthalpy of mixing, which together are key factors to understand the LCST-type phase behavior in the IL system.

Destabilized Passivation Layer on Magnesium-Based Intermetallics as Potential Anode Active Materials for Magnesium Ion Batteries

Passivation of magnesium metal anode is one of the critical challenges for the development of magnesium batteries. Here we investigated the passivation process of an intermetallic anode: Mg3Bi2 synthesized by solid-state and thin film process. The Mg3Bi2 composite electrode shows excellent reversibility in magnesium bis(trifluoromethansulfonylamide) dissolved in acetonitrile, while Mg3Sb2, which has same crystal structure and similar chemical properties, is electrochemically inactive. We also fabricated the Mg3Bi2 thin film electrodes, which show reversibility with low overpotential not only in the acetonitrile solution but also glyme-based solutions. Surface layer corresponding to the decomposed TFSA anion is slightly suppressed in the case of the Mg3Bi2 thin film electrode, compared with Mg metal. Comparative study of hydrolysis process of the Mg3Bi2 and the Mg3Sb2 suggests that the both intermetallic anodes are not completely passivated. The bond valence sum mapping of the Mg3Bi2 indicates that the fast Mg2+ diffusion pathway between 2d tetrahedral sites is formed. The electrochemical properties of the Mg3Bi2 anode is mainly due to the less passivation surface with the fast Mg2+ diffusion pathways.

Lithium Transference Numbers in PEO/LiTFSA Electrolytes Determined by Electrophoretic NMR

Recent literature shows a great controversy on the lithium transference number in salt-in-polymer electrolytes, namely Li-bis(trifluoromethanesulfonyl)amide (LiTFSA) in polyethylene oxide (PEO), as different experimental methods and theories yield strongly contradictory results [I. Villaluenga, D. M. Pesko, K. Timachova, Z. Feng, J. Newman, V. Srinivasan and N. P. Balsara, J. Electrochem. Soc., 165, A2766 (2018) and K. Pozyczka, M. Marzantowicz, J. R. Dygas and F. Krok, Electrochim. Acta, 227, 127 (2017)]. Here, we introduce the method of electrophoretic NMR (eNMR) as a model-free approach to shed light on the controversy. Electrophoretic mobilities of the ions in PEO/LiTFSA electrolytes of varying salt concentration are measured in order to directly obtain lithium transference numbers. At the field strengths involved, the mobilities depend on the electric field, due to nonlinear effects which can be cancelled out by an extrapolation to the low field regime. In all cases, the drift direction of lithium is opposite to the TFSA anion and a high effective charge of both ions (> 0.7) indicates mostly uncorrelated ion movement. The resulting lithium transference numbers are around 0.2 and are almost independent on salt concentration. They thus confirm the literature results from Bruce-Vincent methods, but not those from a Newman approach.

Effect of Film-Forming Additive in Ionic Liquid Electrolyte on Electrochemical Performance of Si Negative-Electrode for LIBs

1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (EMI-TFSA) is one of the promising ionic liquids as electrolyte solvent to enhance the electrochemical performance of Si electrode for Li-ion batteries (LIBs) because of its low viscosity and high conductivity. However, it has low stability against reduction and its reductive decomposition product inhibits Li+ insertion to electrodes, leading to poor cycling stability. To exert a potential of EMI-TFSA, we employed vinylene carbonate (VC) as film-forming additive. Si electrode exhibited very high cycling stability and rate capability in 20 vol.% VC-added EMI-TFSA-based electrolyte. In addition, by replacing TFSA anion with bis(fluorosulfonyl)amide (FSA) for Li salt and ionic liquid solvent, an excellent cycling performance and outstanding rate capability was achieved. VC cannot only fabricate a good surface film but also lower the interaction between Li+ and FSA−, providing smooth desolvation of FSA− to obtain better high-rate performance. Non-flammability of the VC-added electrolytes was confirmed by fire resistance test in closed-system: no ignition was observed even at 300°C. Consequently, we found that mixture electrolyte consisted of EMI-based ionic liquid and VC, especially 1 M LiFSA/EMI-FSA with 20 vol.% VC, is a prospective candidate for simultaneously enhancing the electrochemical performance of Si electrode as well as safety of LIBs.

Anion Coordination Characteristics of Ion-pair Complexes in Highly Concentrated Aqueous Lithium Bis(trifluoromethanesulfonyl)amide Electrolytes.

We report on the structures of Li-ion complexes in salt-concentrated aqueous electrolytes based on lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), particularly focusing on the anion coordination behavior of the ion-pair complexes in the high concentration region cLi > 3.0 mol dm-3. Quantitative data analysis of the Raman spectra revealed the following. (1) Li ions do not coordinate with TFSA anions at lower cLi (<3.0 mol dm-3) to exist as ion pair-free ions. (2) In the concentrated region (cLi = 3.0 - 4.0 mol dm-3), the TFSA anions coordinate as monodentate ligands (mono-TFSA) with Li ions to form ion-pair complexes and coexist with free TFSA in the bulk. (3) Further increasing the cLi (4.0 - 5.2 mol dm-3) results in both monodentate and bidentate coordination (bi-TFSA) modes of TFSA anions to Li ions, yielding complicated ion-pair complexes in the first coordination sphere. The Walden plots, based on ionic conductivity and viscosity data, implied that the ion-conducting mechanism in the highly salt-concentrated region was considerably different from that in the dilute region (i.e., vehicle mechanism).

Solvation-Structure Modification by Concentrating Mg(TFSA)2-MgCl2-Triglyme Ternary Electrolyte.

Mg(TFSA)2/triglyme(G3)-based electrolytes (TFSA: bis (trifluoromethanesulfonyl) amide) are one of candidates for magnesium rechargeable batteries, but the passivation of Mg-metal anode due to the TFSA anion is fatal in practical use. In this work we show that at elevated temperatures around 150 °C a comparable amount of MgCl2 salt can be dissolved in concentrated Mg(TFSA)2/G3 solutions, and the passivation of Mg metal is markedly suppressed in such highly concentrated solutions (1 ≤ G3/Mg-salts ≤ 2) in comparison with in the dilute solutions (G3/Mg-salts ≫ 2). By decreasing the amount of G3 solvent, the solvation structure of Mg2+ ions is modified in that free TFSA anions are drastically lowered, which would consequently decrease the reactivity of TFSA anions. We also demonstrate that a full-cell using MgCo2O4 cathode with the electrolyte of Mg(TFSA)2/MgCl2/G3 1:1:2 at 150 °C delivers a cell voltage of ∼2 V versus Mg-metal anode.

Structural and Thermal Properties of Air‐Stable [Mg(1‐methylimidazole)6][N(SO2CF3)2]2

An air‐stable Mg salt, [Mg(MIm)6][TFSA]2 [MIm = 1‐methylimidazole; TFSA = bis(trifluoromethylsulfonyl)amide], was obtained by recrystallization from the MIm solution of Mg[TFSA]2. This compound melts at 481 K under sealed conditions, whereas it loses MIm at elevated temperatures (448 K, 5 % weight loss) under unsealed conditions. The crystal structure of [Mg(MIm)6][TFSA]2 was determined by X‐ray diffraction. The compound crystallizes in the monoclinic system (space group P21/c) with a = 8.2062(7) Å, b = 11.3185(10) Å, c = 24.454(2) Å, β = 90.229(2)°, V = 2271.3(3) Å3, and Z = 2. The Mg2+ cation is octahedrally surrounded by six non‐methylated N atoms of MIm ligands to form the [Mg(MIm)6]2+ complex cation, in which three imidazolium rings are crystallographically independent. The interplanar angles between two imidazolium rings, which are close to 90° in an ideal model, are 64.01(14), 72.86(18), and 81.91(17)° owing to the interaction of the [Mg(MIm)6]2+ cations with the TFSA– anions. The TFSA anion is completely isolated from the Mg2+ core and adopts the energetically favorable trans conformation. Raman spectroscopic measurements revealed the octahedral coordination of MIm to Mg2+ in crystalline [Mg(MIm)6][TFSA]2 and MIm solutions of Mg[TFSA]2.

Long-Range Ion-Ordering in Salt-Concentrated Lithium-Ion Battery Electrolytes: A Combined High-Energy X-ray Total Scattering and Molecular Dynamics Simulation Study

Herein, we report on a structural study for characterizing unique solution structures in the salt-concentrated electrolytes, which are promising new lithium (Li)-ion battery electrolytes. A combination of high-energy X-ray total scattering (HEXTS) experiments with all-atom molecular dynamics (MD) simulations was performed on the salt-concentrated electrolytes that were based on Li bis(trifluoromethanesulfonyl)amide (LiTFSA) and N,N-dimethylformamide (DMF). The radial distribution functions obtained from the HEXTS and MD approaches were in good agreement in the current LiTFSA/DMF solutions. We found that in the local structure: (1) the Li-ions were coordinated with both the DMF molecules and the TFSA anions in the concentrated solutions and (2) specific Li+···Li+ correlations were present in the radial distribution function over the r range of 3 A–15 A. The Li+···Li+ correlations originated from the extended multiple Li-ion complexes, that is, polymerized [Li+···TFSA–···Li+]n complexes so that they were hi...

Role of Solvent Bulkiness on Lithium-Ion Solvation in Fluorinated Alkyl Phosphate-Based Electrolytes: Structural Study for Designing Nonflammable Lithium-Ion Batteries

We report the characteristics of nonflammable tris(2,2,2-trifluoroethyl) phosphate (TFEP) as a nonaqueous solvent for lithium (Li)-ion battery electrolytes at a molecular level, particularly focusing on the solvent parameters, such as the electron pair-donating ability and molecular bulkiness of TFEP, and the solvation behavior around Li ions. The binding energy for the interactions between a Li ion and several solvents using density functional theory (DFT) calculations indicated that TFEP can be categorized as a nonaqueous solvent with weak electron pair-donating ability, resulting in the prediction of a small Gutmann’s donor number (the DN value is approximately 12.9). The Walden plots based on ionic conductivity and viscosity suggested that Li ions tend to exist as contact ion pairs (Li+···TFSA–) in the TFEP-based solutions, irrespective of the Li salt concentration cLi. Infrared and Raman spectroscopic studies demonstrated that Li ions are solvated by two TFEP molecules and one TFSA anion to form the ...

Passivation Layer of Magnesium-Based Negative Electrodes

Rechargeable magnesium batteries are getting more interests as a potential beyond Li-ion chemistry in the battery field. Of course a lot of the challenges of the rechargeable magnesium batteries are remaining for cathodes, anodes and electrolytes. In the case of the anode active materials, since formation of the passivation layer at the surface of the magnesium metal is the most critical problem for the reversible deposition and dissolution process, halide based electroltye solutions are preferred. On the other hand, the halide-based electrolyte sotluion always has voltage limitation due to the corrosion of the current collector. As a consequence, it is very challenging to develop a high voltage magnesium batteries. Previously we reported that the magnesium metal can be easily passivated in the Mg(TFSA)2 BuMe-triglyme solution using XPS and in situ FTIR. It is mainly due to the electrochemical reduction of the TFSA anion to form the passivation layer resulting in the high overpotential. However, a magnesium-based intermetallic compound: Mg3Bi2 showed good reversibility in the Mg(TFSA)2 BuMe-tryglyme solution with very low overpotential. Even though the decomposition of the TFSA anion itself was still observed by XPS, the surface layer formed on the Mg3Bi2 could not be enough stable to work as the passivation layer. In the present study, we synthesized various magnesium-based intermetallic compounds such as Mg3Bi2, Ma3Sb2 and Mg2Sn via solid-state process and investigated the stability of the passivation layer formed at the surface of the intermetallic compounds. In the ambient atmosphere, Mg3Bi2 and Mg2Sn were easily decomposed to form Bi and Sn respectively. On the other hand, the Mg3Sb2 was extremely stable even with water. These results well matches with the electrochemical performances of the each intermetallic compounds. The obtaind results suggest that the formation of the alloy or intermetallic compounds de-stabilizes the surface of the magnesium. We think the destabilization of the magnesium improves the performance of the anode active materials.

Passivation Layer Formation of Magnesium Metal Negative Electrodes for Rechargeable Magnesium Batteries

A comparative study for electrodeposited magnesium metal was conducted using an organohaloaluminate-based electrolyte solution and a conventional ionic electrolyte solution. The deposition/dissolution process of the magnesium metal in the ionic electrolyte solution shows high overpotential with low coulombic efficiency, compared with the organohaloaluminate-based electrolyte solution. The magnesium metal deposited in the ionic electrolyte solution shows porous morphology and poor crystallinity. An initial electrolytes decomposition process on Pt substrate was analyzed with in situ FTIR. The in situ FTIR spectra show passivation process of the electrode during the 1st cathodic scan due to the decomposition of the conventional ionic electrolyte solution. The surface layer on magnesium metal was characterized by using XPS. The XPS spectra for the magnesium metal immersed in the ionic electrolyte solution proved that a MgF2-based passivation layer corresponding to the decomposed TFSA anion is formed.

The Solvation Structure of Lithium Ions in an Ether Based Electrolyte Solution from First-Principles Molecular Dynamics.

The solvation and desolvation of the Li ion play a crucial role in the electrolytes of Li based secondary batteries, and their understanding at the microscopic level is of great importance. Oligoether (glyme) based electrolytes have attracted much attention as electrolytes used in Li based secondary batteries, such as Li-ion, Li-S, and Li-O2 batteries. However, the solvation structure of the Li ion in glyme based electrolytes has not been fully clarified yet. We present a computational study on the solvation structure of lithium ions in the mixture of triglyme and lithium bis(trifluoromethylsulfonyl)-amide (LiTFSA) by means of molecular orbital and molecular dynamics calculations based on density functional theory. We found that, in the electrolyte solution composed of the equimolar mixture of triglyme and LiTFSA, lithium ions are solvated mainly by crown-ether-like curled triglyme molecules and in direct contact with an TFSA anion. We also found the aggregate formed with Li ion and TFSA anions and/or triglyme molecule(s) is equally stable, which has not been reported in the previous classical molecular dynamics simulations, suggesting that in reality a small fraction of Li ions form aggregates and they might have a significant impact on the Li ion transport. Our results demonstrate the importance of performing electronic structure based molecular dynamics of electrolyte solution to clarify the detailed solvation structure of the Li ion.

Operando soft X-Ray Absorption and Raman Spectroscopic Study on Magnesium Deposition By Using Ether-Based Electrolyte

Magnesium rechargeable battery, which uses magnesium metal anode, is one of the candidates for the next generation batteries. The magnesium metal anode has a high theoretical volumetric capacity (3832 mAh/cm3), and relative low reduction potential (-2.356 V vs. standard hydrogen potential). However, conventional electrolytes consisting of inorganic salts and ester solvents, as widely used in lithium ion batteries, do not occur reversible deposition/dissolution of magnesium metal. This is because a passivation layer is formed at the magnesium metal anode surface that blocks both magnesium ions and electron transport. The limitation of available electrolytes prevents the magnesium battery from being used in a practical application.Recently, it has been reported that reversible magnesium deposition/dissolution reactions can occur in inorganic magnesium salts dissolved in glyme-based solvents electrolytes 2, 3. However, the reaction mechanism of magnesium metal anode has not been clearly understood. To clarify the reaction mechanism, it is necessary to investigate the interfacial structure of magnesium ion at anode/electrolyte interface. In this study, we investigated the bulk structure of the glyme-based electrolyte using Raman spectroscopy, X-ray absorption spectroscopy (XAS) and the theoretical calculation. Furthermore, we observed the electronic and local structure of the electrolyte at the interface during magnesium deposition using operando soft x-ray absorption spectroscopy.0.5 M Mg(TFSA)2/triglyme and 0.5 M Mg(TFSA)2/2-MeTHF electrolytes were prepared by stirring Mg(TFSA)2 with triglyme or 2-MeTHF in an Ar-filled glove box. Raman spectroscopic measurements were carried out using LabRAM HR-800 equipped with He-Ne laser (633 nm) at room temperature. operando XAS measurements at Mg K-edge were conducted at the beam line BL27SU at SPring-8 (Japan). XAS spectra were obtained at different potential during magnesium deposition. Magnesium deposition/dissolution occurred in Mg(TFSA)2/triglyme as observed in previous report3. In contrast to Mg(TFSA)2/triglyme, magnesium metal deposition did not observe in Mg(TFSA)2/2-MeTHF. Raman spectroscopy showed that TFSA anion exists as uncoordinated state with magnesium ion in the Mg(TFSA)2/triglyme, while TFSA anion strongly coordinates with magnesium ion in the 2-MeTHF-based electrolyte. operando Mg K-edge XAS observed that the local structure attributed to first coordination shell changed more smaller with decreasing potential in the Mg(TFSA)2/triglyme than in the Mg(TFSA)2/2-MeTHF at anode/electrolyte interface. In the Mg(TFSA)2/triglyme, the solvation structure of magnesium ion without coordination of TFSA anion, may suppress its local structure change during approaching the anode/electrolyte interface, resulting in the reversible magnesium deposition/dissolution. ACKNOWLEDGMENTS This study was partially supported by Synchrotron radiation experiment was performed at BL27SU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). REFERENCES [1] Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich M.; Levi, E., Nature, 2000, 407, 724-727.[2] Ha, S.-Y.; Lee, Y.-W.; Woo, S. W.; Koo, B.; Kim, J.-S.; Cho, J.; Lee, K. T.; Choi, N.-S., ACS Appl. Mater. Interfaces, 2014, 6, 4063-4073.[3] Orikasa, Y.; Masese, T.; Koyama, Y.; Mori, T.; Hattori, M.; Yamamoto, K.; Okado, T.; Huang, Z. -D.; Minato, T.; Tassel, C.; Kim, J.; Kobayashi, Y.; Abe, T.; Kageyama, H.; Uchimoto, Y., Sci. Rep., 2014, 4, 5622.