9 research outputs found
Multinuclear NMR Studies on Translational and Rotational Motion for Two Ionic Liquids Composed of BF<sub>4</sub> Anion
Two ionic liquids (ILs) based on the BF<sub>4</sub><sup>ā</sup> anion are studied by <sup>1</sup>H, <sup>11</sup>B,
and <sup>19</sup>F NMR spectroscopy by measuring self-diffusion coefficients
(<i>D</i>) and spinālattice relaxation times (<i>T</i><sub>1</sub>). The cations are 1-ethyl-3-methylimidazolium
(EMIm)
and 1-butyl-3-methylimidazolium (BMIm). Since two NMR nuclei (<sup>11</sup>B and <sup>19</sup>F) of BF<sub>4</sub><sup>ā</sup> exhibit narrow lines and high sensitivity, the <sup>11</sup>B and <sup>19</sup>F NMR measurements of <i>D</i><sub>BF4</sub> and <i>T</i><sub>1</sub>(BF<sub>4</sub>) were performed in a wide temperature
range. The temperature-dependent behaviors of <i>T</i><sub>1</sub>(<sup>19</sup>F) and <i>T</i><sub>1</sub>(<sup>11</sup>B) were remarkably different, although the values of <i>D</i><sub>BF4</sub>(<sup>19</sup>F) and <i>D</i><sub>BF4</sub>(<sup>11</sup>B) almost agreed. Since the Arrhenius plots of <i>T</i><sub>1</sub>ās for <sup>1</sup>H, <sup>19</sup>F,
and <sup>11</sup>B exhibited <i>T</i><sub>1</sub> minima,
the correlation times Ļ<sub>c</sub>(<sup>1</sup>H), Ļ<sub>c</sub>(<sup>19</sup>F), and Ļ<sub>c</sub>(<sup>11</sup>B)
were evaluated. The <i>D</i>(cation) and <i>D</i>(BF<sub>4</sub>) were plotted against 1/Ļ<sub>c</sub>(<sup>1</sup>H) and 1/Ļ<sub>c</sub>(<sup>19</sup>F), respectively,
and the relationships between translational and rotational motion
are discussed. The translational diffusion of the cations is related
to molecular librational motion and that of BF<sub>4</sub> is coupled
with reorientational motion. The Ļ<sub>c</sub>(<sup>11</sup>B) derived from <sup>11</sup>B <i>T</i><sub>1</sub> can
be attributed to a local jump. From the plots of the classical StokesāEinstein
(SE) equation, the empirical <i>c</i> values, which were
originally derived by theoretical boundary conditions, were estimated
for each ion. The empirical <i>c</i>(BF<sub>4</sub>) was
about 4.4<sub>5</sub>, while the <i>c</i> values of the
cations were smaller than 4
Dissociation and Diffusion of Glyme-Sodium Bis(trifluoromethanesulfonyl)amide Complexes in Hydrofluoroether-Based Electrolytes for Sodium Batteries
Physicochemical
properties and battery performance of [NaĀ(glyme)]Ā[TFSA]
complexes (TFSA: bisĀ(trifluoromethanesulfonyl)Āamide) dissolved in
a hydrofluoroether (HFE) were investigated. Glyme (tetraglyme (G4)
or pentaglyme (G5)) coordinates to Na<sup>+</sup> to form a 1:1 complex
[NaĀ(G4 or G5)]<sup>+</sup> cation. Raman spectroscopy revealed that
the complex structure of [NaĀ(glyme)]<sup>+</sup> is maintained in
the HFE solution, and free (uncoordinated) glymes are not liberated
on adding HFE. HFE molecules are scarcely involved in the first solvation
shell of Na<sup>+</sup> because of their low electron-pair-donating
ability. Raman spectra of the [TFSA]<sup>ā</sup> anion suggests
that the attractive interaction between the complex [NaĀ(glyme)]<sup>+</sup> cation and [TFSA]<sup>ā</sup> anion is enhanced on
adding HFE. The population of contact ion-pair (CIP) and/or aggregate
(AGG) is smaller for the G5 system than for the G4 one, and the [NaĀ(G5)]Ā[TFSA]/HFE
has higher ionic conductivity. The self-diffusion coefficients of
the [NaĀ(glyme)]<sup>+</sup> complex and [TFSA]<sup>ā</sup> were
measured by pulsed field gradient (PFG) NMR, and the dissociativity
of [NaĀ(glyme)]Ā[TFSA] was assessed. The dissociativity of the G5 system
is greater than that of the G4 one, and the dissociativity can be
correlated with the attractive interaction between [NaĀ(glyme)]<sup>+</sup> and [TFSA]<sup>ā</sup>, as evaluated by ab initio
calculations. The dissociativity of the complexes gave significant
effects on the battery performance
Static and Transport Properties of Alkyltrimethylammonium Cation-Based Room-Temperature Ionic Liquids
We have measured physicochemical
properties of five alkyltrimethylammonium cation-based room-temperature
ionic liquids and compared them with those obtained from computational
methods. We have found that static properties (density and refractive
index) and transport properties (ionic conductivity, self-diffusion
coefficient, and viscosity) of these ionic liquids show close relations
with the length of the alkyl chain. In particular, static properties
obtained by experimental methods exhibit a trend complementary to
that by computational methods (refractive index ā [polarizability/molar
volume]). Moreover, the self-diffusion coefficient obtained by molecular
dynamics (MD) simulation was consistent with the data obtained by
the pulsed-gradient spināecho nuclear magnetic resonance technique,
which suggests that computational methods can be supplemental tools
to predict physicochemical properties of room-temperature ionic liquids
Surface Analysis of Ionic Liquids with and without Lithium Salt Using Xāray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) was applied to
a neat ionic
liquid 1-ethyl-3-methylimidazolium bisĀ(trifluoromethanesulfonyl)Āimide
[EMI<sup>+</sup>]Ā[Tf<sub>2</sub>N<sup>ā</sup>] and its lithium
salt solution at room temperature to clarify the composition and structure
of its near-surface region. Core level peaks were recorded for Li
1s, N 1s, C 1s, F 1s, O 1s, S 2s, and S 2p. Valence band XPS spectra
(0ā40 eV binding energy) were also studied. The XPS spectra
were analyzed using DV-XĪ± calculations. Results show that the
planar type isomer of the EMI<sup>+</sup> cation is dominant at the
near-surface region of EMI-Tf<sub>2</sub>N. Results of XPS measurements
show a spectrum of Li 1s in Li/EMI-Tf<sub>2</sub>N. The proposed models
for the preferred orientation of the ions exhibit good agreement with
results obtained from the DV-XĪ± calculations
Chelate Effects in Glyme/Lithium Bis(trifluoromethanesulfonyl)amide Solvate Ionic Liquids. I. Stability of Solvate Cations and Correlation with Electrolyte Properties
To develop a basic understanding
of a new class of ionic liquids
(ILs), āsolvateā ILs, the transport properties of binary
mixtures of lithium bisĀ(trifluoromethanesulfonyl)Āamide (LiĀ[TFSA])
and oligoethers (tetraglyme (G4), triglyme (G3), diglyme (G2), and
monoglyme (G1)) or tetrahydrofuran (THF) were studied. The self-diffusion
coefficient ratio of the solvents and Li<sup>+</sup> ions (<i>D</i><sub>sol</sub>/<i>D</i><sub>Li</sub>) was a good
metric for evaluating the stability of the complex cations consisting
of Li<sup>+</sup> and the solvent(s). When the molar ratio of Li<sup>+</sup> ions and solvent oxygen atoms ([O]/[Li<sup>+</sup>]) was
adjusted to 4 or 5, <i>D</i><sub>sol</sub>/<i>D</i><sub>Li</sub> always exceeded unity for THF and G1-based mixtures
even at the high concentrations, indicating the presence of uncoordinating
or highly exchangeable solvents. In contrast, long-lived complex cations
were evidenced by a <i>D</i><sub>sol</sub>/<i>D</i><sub>Li</sub> ā¼ 1 for the longer G3 and G4. The binary mixtures
studied were categorized into two different classes of liquids: concentrated
solutions and solvate ILs, based on <i>D</i><sub>sol</sub>/<i>D</i><sub>Li</sub>. Mixtures with G2 exhibited intermediate
behavior and are likely the borderline dividing the two categories.
The effect of chelation on the formation of solvate ILs also strongly
correlated with electrolyte properties; the solvate ILs showed improved
thermal and electrochemical stability. The ionicity (Ī<sub>imp</sub>/Ī<sub>NMR</sub>) of [LiĀ(glyme or THF)<sub><i>x</i></sub>]Ā[TFSA] exhibited a maximum at an [O]/[Li<sup>+</sup>] ratio
of 4 or 5
Li<sup>+</sup> Solvation and Ionic Transport in Lithium Solvate Ionic Liquids Diluted by Molecular Solvents
An
equimolar mixture of lithium bisĀ(trifluoromethanesulfonyl)Āamide
(LiĀ[TFSA]) and either triglyme (G3) or tetraglyme (G4) yielded stable
molten complexes: [LiĀ(G3)]Ā[TFSA] and [LiĀ(G4)]Ā[TFSA]. These are known
as solvate ionic liquids (SILs). Glyme-based SILs have thermal and
electrochemical properties favorable for use as lithium-conducting
electrolytes in lithium batteries. However, their intrinsically high
viscosities and low ionic conductivities prevent practical application.
Therefore, we diluted SILs with molecular solvents in order to enhance
their ionic conductivities. To determine the stabilities of the complex
cations in diluted SILs, their conductivity and viscosity, the self-diffusion
coefficients, and Raman spectra were measured. [LiĀ(G3)]<sup>+</sup> and [LiĀ(G4)]<sup>+</sup> were stable in nonpolar solvents, that
is, toluene, diethyl carbonate, and a hydrofluoroether (HFE); however,
ligand exchange took place between glyme and solvent when polar solvents,
that is, water and propylene carbonate, were used. In acetonitrile
(AN) mixed solvent complex cations [LiĀ(G3)Ā(AN)]<sup>+</sup> and [LiĀ(G4)Ā(AN)]<sup>+</sup> were formed. [LiĀ(G4)]Ā[TFSA] was more conductive than [LiĀ(G3)]Ā[TFSA]
when diluted with nonpolar solvents due to the greater ionic dissociativity
in [LiĀ(G4)]Ā[TFSA] mixtures. In view of the stability of the Liāglyme
complex cations, the enhanced ionic conductivities, and the intrinsic
electrochemical stabilities of the diluting solvents, [LiĀ(G4)]Ā[TFSA]
diluted by toluene or HFE, can be a candidate for an alternative battery
electrolyte
Local Structure of Li<sup>+</sup> in Concentrated Ethylene Carbonate Solutions Studied by Low-Frequency Raman Scattering and Neutron Diffraction with <sup>6</sup>Li/<sup>7</sup>Li Isotopic Substitution Methods
Isotropic
Raman scattering and time-of-flight neutron diffraction
measurements were carried out for concentrated LiTFSA-EC solutions
to obtain structural insight on solvated Li<sup>+</sup> as well as
the structure of contact ion pair, Li<sup>+</sup>Ā·Ā·Ā·TFSA<sup>ā</sup>, formed in highly concentrated EC solutions. Symmetrical
stretching vibrational mode of solvated Li<sup>+</sup> and solvated
Li<sup>+</sup>Ā·Ā·Ā·TFSA<sup>ā</sup> ion pair were
observed at Ī½ = 168ā177 and 202ā224 cm<sup>ā1</sup>, respectively. Detailed structural properties of solvated Li<sup>+</sup> and Li<sup>+</sup>Ā·Ā·Ā·TFSA<sup>ā</sup> contact ion pair were derived from the least-squares fitting analysis
of first-order difference function, Ī<sub>Li</sub>(<i>Q</i>), between neutron scattering cross sections observed for <sup>6</sup>Li/<sup>7</sup>Li isotopically substituted 10 and 25 mol % *LiTFSA-EC<i>d</i><sub>4</sub> solutions. It has been revealed that Li<sup>+</sup> in the 10 mol % LiTFSA solution is fully solvated by ca.
4 EC molecules. The nearest neighbor Li<sup>+</sup>Ā·Ā·Ā·OĀ(EC)
distance and Li<sup>+</sup>Ā·Ā·Ā·OĀ(EC)ī»CĀ(EC) bond
angle are determined to be 1.90 Ā± 0.01 Ć
and 141 Ā±
1Ā°, respectively. In highly concentrated 25 mol % LiTFSA-EC solution,
the average solvation number of Li<sup>+</sup> decreases to ca. 3
and ca. 1.5. TFSA<sup>ā</sup> are directly contacted to Li<sup>+</sup>. These results agree well with the results of band decomposition
analyses of isotropic Raman spectra for intramolecular vibrational
modes of both EC and TFSA<sup>ā</sup>
Unusual Li<sup>+</sup> Ion Solvation Structure in Bis(fluorosulfonyl)amide Based Ionic Liquid
Raman spectra of 1-ethyl-3-methylimidazolium
bisĀ(fluorosulfonyl)Āamide
[C<sub>2</sub>mIm<sup>+</sup>]Ā[FSA<sup>ā</sup>] ionic liquid
solutions dissolving LiFSA salt of various concentrations were measured
at 298 K. FSA<sup>ā</sup> ((FSO<sub>2</sub>)<sub>2</sub>N<sup>ā</sup>) is an analogue anion of bisĀ(trifluoromethanesulfonyl)Āamide
((CF<sub>3</sub>SO<sub>2</sub>)<sub>2</sub>N<sup>ā</sup>; TFSA<sup>ā</sup>). We found that a solvation number of the Li<sup>+</sup> ion in [C<sub>2</sub>mIm<sup>+</sup>]Ā[FSA<sup>ā</sup>] is
3, though it has been well established that Li<sup>+</sup> ion is
solvated by two TFSA<sup>ā</sup> anions in the corresponding
ionic liquids below the Li<sup>+</sup> ion mole fraction of <i>x</i><sub>Li<sup>+</sup></sub> < 0.2. To yield further insight
into larger solvation numbers, Raman spectra were measured at higher
temperatures up to 364 K. The Li<sup>+</sup> ion solvation number
in [C<sub>2</sub>mIm<sup>+</sup>]Ā[FSA<sup>ā</sup>] evidently
decreased when the temperature was elevated. Temperature dependence
of the Li<sup>+</sup> ion solvation number was analyzed assuming an
equilibrium between [LiĀ(FSA)<sub>2</sub>]<sup>ā</sup> and [LiĀ(FSA)<sub>3</sub>]<sup>2ā</sup>, and the enthalpy Ī<i>H</i>Ā° and the temperature multiplied entropy <i>T</i>Ī<i>S</i>Ā° for one FSA<sup>ā</sup> liberation toward
a bulk ionic liquid were successfully evaluated to be 35(2) kJ mol<sup>ā1</sup> and 29(2) kJ mol<sup>ā1</sup>, respectively.
The Ī<i>H</i>Ā° and Ī<i>S</i>Ā°
suggest that the Li<sup>+</sup> ion is coordinated by one of bidentate
and two of monodentate FSA<sup>ā</sup> at 298 K, and that the
more weakly solvated monodentate FSA<sup>ā</sup> is liberated
at higher temperatures. The high-energy X-ray diffraction (HEXRD)
experiments of these systems were carried out and were analyzed with
the aid of molecular dynamics (MD) simulations. In radial distribution
functions evaluated with HEXRD, a peak at about 1.94 Ć
appeared
and was attributable to the Li<sup>+</sup>āOĀ(FSA<sup>ā</sup>) correlations. The longer Li<sup>+</sup>āOĀ(FSA<sup>ā</sup>) distance than that for the Li<sup>+</sup>āOĀ(TFSA<sup>ā</sup>) of 1.86 Ć
strongly supports the larger solvation number of
the Li<sup>+</sup> ions in the FSA<sup>ā</sup> based ionic
liquids. MD simulations at least qualitatively reproduced the Raman
and HEXRD experiments
Li<sup>+</sup> Local Structure in LiāTetraglyme Solvate Ionic Liquid Revealed by Neutron Total Scattering Experiments with the <sup>6/7</sup>Li Isotopic Substitution Technique
Equimolar mixtures
of lithium bisĀ(trifluoromethaneĀsulfonyl)Āamide
(LiTFSA) and tetraglyme (G4: CH<sub>3</sub>Oā(CH<sub>2</sub>CH<sub>2</sub>O)<sub>4</sub>āCH<sub>3</sub>) yield the solvate
(or chelate) ionic liquid [LiĀ(G4)]Ā[TFSA], which is a homogeneous transparent
solution at room temperature. Solvate ionic liquids (SILs) are currently
attracting increasing research interest, especially as new electrolytes
for Liāsulfur batteries. Here, we performed neutron total scattering
experiments with <sup>6/7</sup>Li isotopic substitution to reveal
the Li<sup>+</sup> solvation/local structure in [LiĀ(G4)]Ā[TFSA] SILs.
The experimental interference function and radial distribution function
around Li<sup>+</sup> agree well with predictions from ab initio calculations
and MD simulations. The model solvation/local structure was optimized
with nonlinear least-squares analysis to yield structural parameters.
The refined Li<sup>+</sup> solvation/local structure in the [LiĀ(G4)]Ā[TFSA]
SIL shows that lithium cations are not coordinated to all five oxygen
atoms of the G4 molecule (deficient five-coordination) but only to
four of them (actual four-coordination). The solvate cation is thus
considerably distorted, which can be ascribed to the limited phase
space of the ethylene oxide chain and competition for coordination
sites from the TFSA anion