4 research outputs found
Complex Nature of Ionic Coordination in Magnesium Ionic Liquid-Based Electrolytes: Solvates with Mobile Mg<sup>2+</sup> Cations
The
Raman shifts of the TFSI<sup>ā</sup> expansion-contraction
mode in <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium
bisĀ(trifluoromethanesulfonyl)Āimide ionic liquid (IL) electrolytes
were analyzed to compare the ionic coordination of magnesium with
lithium and sodium. In the Mg<sup>2+</sup>-IL electrolytes, the TFSI<sup>ā</sup> anions are found in three different potential energy
environments, while only two populations of TFSI<sup>ā</sup> are evident in the Na<sup>+</sup>- and Li<sup>+</sup>-IL electrolytes.
For Mg<sup>2+</sup>, the high frequency peak component is associated
with a TFSI<sup>ā</sup> that is in a bidentate coordination
with a single metal cation and can therefore be considered a contact
ion pair (CIP) solvate. The mid frequency component is attributed
primarily to bridging aggregate (AGG) TFSI<sup>ā</sup> solvate
or a weakly bound monodentate CIP TFSI<sup>ā</sup>. The low
frequency peak is well-known to be associated with āfreeā
TFSI<sup>ā</sup> anions. The average number of TFSI<sup>ā</sup> per Mg<sup>2+</sup> cation (<i>n</i>) is 3 to 4. In comparison,
the value of <i>n</i> is 4 at very low concentrations and
decreases with increasing salt mole fraction to 2 for Li<sup>+</sup> and Na<sup>+</sup>, where <i>n</i> of Na<sup>+</sup> is
larger than that of Li<sup>+</sup> at any given concentration. The
results imply the existence of anionic magnesium solvates of varying
sizes. The identity of the Mg<sup>2+</sup> charge-carrying species
is complex due to the presence of bridging AGG solvates in solution.
It is likely that there is a combination of single Mg<sup>2+</sup> solvate species and larger complexes containing two or more cations.
In comparison, the primary Li<sup>+</sup> and Na<sup>+</sup> charge-carrying
species are likely [LiĀ(TFSI)<sub>2</sub>]<sup>ā</sup> and [NaĀ(TFSI)<sub>3</sub>]<sup>2ā</sup> in the concentration range successfully
implemented in IL-based electrolyte batteries. These solvates result
in Mg<sup>2+</sup> cations that are mobile in the IL-based electrolytes
as demonstrated by the reversible magnesiation/demagnesiation in V<sub>2</sub>O<sub>5</sub> aerogel electrodes
Mechanisms of Magnesium Ion Transport in Pyrrolidinium Bis(trifluoromethanesulfonyl)imide-Based Ionic Liquid Electrolytes
Inert
polar aprotic electrolytes based on pyrrolidinium bisĀ(trifluoromethanesulfonyl)Āimide
ionic liquids were investigated for Mg battery applications. On a
molecular scale, there are two TFSI<sup>ā</sup> populations
coordinating Mg<sup>2+</sup> ions: one in a bidentate coordination
to a single Mg<sup>2+</sup> and one in a bridging geometry between
two Mg<sup>2+</sup> ions. On average, each Mg<sup>2+</sup> cation
is surrounded by three to four TFSI<sup>ā</sup> anions. The
electrolytes, in general, remain amorphous far below ambient conditions,
which results in a wide useable temperature range in practical devices.
There is a change in the ratio of bidentate:bridging TFSI<sup>ā</sup> and in the conductivity, viscosity, and diffusion behavior at a
salt mole fraction of 0.12ā0.16. At concentrations above this
threshold, there is a more dramatic decrease of the diffusion coefficients
and the conductivity with increasing salt concentration due to slower
exchange of the more strongly coordinated bidentate TFSI<sup>ā</sup>. The mechanism of ion transport likely proceeds via structural diffusion
through exchange of the bridging and āfreeā TFSI<sup>ā</sup> anions within adjacent [Mg<sub><i>n</i></sub>(TFSI)<sub><i>m</i></sub>]<sup>(<i>m</i>ā2<i>n</i>)ā</sup> clusters and exchange of bidentate anions
via a bidentate to bridging mechanism. The vehicular mechanism likely
makes only a small contribution. At concentrations above approximately
0.16 mole fraction, the structural diffusion is more closely related
to the tightly bound bidentate anions
Conformations and Vibrational Assignments of the (Fluorosulfonyl)(trifluoromethanesulfonyl)imide Anion in Ionic Liquids
Investigations of the (fluorosulfonyl)Ā(trifluoromethanesulfonyl)Āimide
(FTFSI) anion, incorporated in various ionic liquids, by means of
density functional theory (DFT) methods and differential scanning
calorimetry (DSC), X-ray diffraction (XRD), and Raman techniques are
reported in this work. Theoretical studies using DFT methods (B3LYP/6-31G**)
show that there are three likely anion geometries (syn, gauche, and
anti) separated by less than 3 kJĀ·mol<sup>ā1</sup>. The
energy barrier to conversion between the anti and syn/gauche conformers
is between 10 and 14 kJĀ·mol<sup>ā1</sup> and lower than
10 kJĀ·mol<sup>ā1</sup> for rotations around the SNSF and
SNSC dihedral angles, respectively. The FTFSI anion has a characteristic
vibration at 730 cm<sup>ā1</sup> assigned to the expansion
and contraction of the entire anion that is sensitive to ionic interactions
with metal cations. DSC, XRD, and Raman studies indicate that an alkali
metal salt containing the FTFSI anion, KFTFSI, exists in two crystalline
forms. Form II converts to form I via a solidāsolid phase transition
at 96.9 Ā°C. The FTFSI expansionācontraction mode at 745
cm<sup>ā1</sup> in KFTFSI form I shifts to 741 cm<sup>ā1</sup> in form II. It can be hypothesized that this shift is due to the
presence of different anion geometries or varying ionic interactions
in the two crystalline forms
Crystalline Complexes of Pyr<sub>12O1</sub>TFSI-Based Ionic Liquid Electrolytes
This
study examines the formation of previously unreported crystalline
phases of <i>N</i>-methoxyethyl-<i>N</i>-methylpyrrolidinium
bisĀ(trifluoromethanesulfonyl)Āimide (Pyr<sub>12O1</sub>TFSI). The melting
point of pristine Pyr<sub>12O1</sub>TFSI, determined by conductivity
measurements, is between ā20 and ā17.5 Ā°C. Formation
of this crystalline phase is difficult and only occurs under specific
conditions. Pyr<sub>12O1</sub>TFSI readily forms 1:1 phases with both
NaTFSI and MgĀ(TFSI)<sub>2.</sub> The results of single crystal structure
determinations are presented. The Na<sup>+</sup> crystalline phase
provides clear evidence that the Pyr<sub>12O1</sub><sup>+</sup> cation
can coordinate some metal ions, but this coordinative interaction
does not occur with all metal cations, e.g., Mg<sup>2+</sup>, and
in all states of matter, e.g., Na<sup>+</sup>-IL solutions. The TFSI<sup>ā</sup> ions are found in two different aggregate solvates
in the Pyr<sub>12O1</sub>TFSI:NaTFSI 1:1 phase and in contact ion
pair and aggregate solvates in the Pyr<sub>12O1</sub>TFSI:MgĀ(TFSI)<sub>2</sub> 1:1 phase. The Pyr<sub>12O1</sub>TFSI:MgĀ(TFSI)<sub>2</sub> crystalline phase gives insight into the local structure of the
liquid electrolyte, where it is likely that a maximum of approximately
30% of the total TFSI<sup>ā</sup> can likely be coordinated
in a bridging geometry, and the rest are in a bidentate coordination
geometry. This ratio is determined from both the crystal structure
and the Raman spectroscopy results