Inter- and Intramolecular Interactions in Ether-Functionalized Ionic Liquids.

The intra- and intermolecular interactions in ether-functionalized ionic liquids (ILs) are studied by means of infrared (IR) spectroscopy measurements of N-ethoxyethyl-N-methylpiperidiniumbis(fluorosulfonyl)imide (P1,2O2-FSI) and N-ethoxyethyl-N-methylmorpholiniumbis(fluorosulfonyl)imide (M1,2O2-FSI). The temperature dependence of the spectra in the medium IR range allows the study of the anion conformer distribution and its variation during phase transitions. In particular, it is found that for both ILs the trans conformer of FSI is more stable than the cis conformer, and the enthalpy differences between them are calculated and are found to decrease upon the addition of a Li salt. The results obtained in the far IR range, combined with ab initio calculation of the ionic couple performed using the B3LYP-D functional and considering both empirical dispersion corrections and the presence of a polar solvent, provide evidence for the occurrence of a hydrogen bonding between the O atom of the anion and its closest H atoms directly linked to a C atom of the cation. The comparison with samples having the same cations but with bis(trifluoromethanesulfonyl)imide (TFSI) as an anion, that is, M1,2O2-TFSI and P1,2O2-TFSI, as well as with samples having cations without the ether-functionalization neither in the ring nor in the side chain, such as N-propyl-N-methylpyrrolidinium-FSI (PYR13-FSI) and 1-butyl-1-methylpyrrolidinium-TFSI (PYR14-TFSI), indicates that the occurrence of such highly directional interaction between anion and cation is better observable in the ether-functionalized samples, in particular in those containing FSI as an anion.

The effect of ether-functionalisation in ionic liquids analysed by DFT calculation, infrared spectra, and Kamlet-Taft parameters.

The effect of ether-functionalisation on ionic liquids (ILs) is discussed based on Kamlet-Taft parameters and the infrared (IR) spectra of N-ethoxyethyl-N-methylpiperidinium bis(trifluoromethanesulfonyl)imide ([P1,2O2][TFSI]) and N-ethoxyethyl-N-methylmorpholinium bis(trifluoromethanesulfonyl)imide ([M1,2O2][TFSI]). The results are analysed taking into consideration their ion conformers, electronegativity and hardness, and the IR active vibrations obtained by means of DFT calculations. From the evaluation of Kamlet-Taft parameters, the ether-functionalisation in the cationic ring is found to improve the polarity and hydrogen bond acidity of the ILs. This correlates with the computational result which designates that the oxygen atom in the cationic ring increases the electronegativity of the cation. The comparison with the IR spectra, which were obtained experimentally and computationally, revealed that trans-[TFSI] was preferably formed in [M1,2O2][TFSI] compared to [P1,2O2][TFSI]. Although the Kamlet-Taft parameters indicate that [M1,2O2][TFSI] has a higher polarity, this IL preferably adopts trans-[TFSI], which is normally stabilised with the cations having a lower polarity. This may be due to the presence of the oxygen in the cationic ring which delocalises the electron density of the lowest unoccupied molecular orbitals (LUMO) and increases the conformational freedom of the hydrogen bonds between cations and anions. Moreover, the mixtures of pure ILs with a suitable Li-salt were also investigated to analyze the effect of the Li salt on the polarity and the ion conformers.

A high-power and fast charging Li-ion battery with outstanding cycle-life.

Electrochemical energy storage devices based on Li-ion cells currently power almost all electronic devices and power tools. The development of new Li-ion cell configurations by incorporating innovative functional components (electrode materials and electrolyte formulations) will allow to bring this technology beyond mobile electronics and to boost performance largely beyond the state-of-the-art. Here we demonstrate a new full Li-ion cell constituted by a high-potential cathode material, i.e. LiNi0.5Mn1.5O4, a safe nanostructured anode material, i.e. TiO2, and a composite electrolyte made by a mixture of an ionic liquid suitable for high potential applications, i.e. Pyr1,4PF6, a lithium salt, i.e. LiPF6, and standard organic carbonates. The final cell configuration is able to reversibly cycle lithium for thousands of cycles at 1000 mAg-1 and a capacity retention of 65% at cycle 2000.

Tailoring the physical properties of the mixtures of ionic liquids: a microscopic point of view.

A detailed investigation of the phase diagram of the mixtures of the two ionic liquids N-trimethyl-N-propylammonium bis(trifluoromethylsulfonyl)imide (TMPA-TFSI) and N-trimethyl-N-hexylammonium bis(trifluoromethylsulfonyl)imide (TMHA-TFSI) has been performed in the temperature range between 140 and 330 K by means of DSC and infrared spectroscopy measurements. In the low temperature crystalline states, a large concentration of the trans-TFSI conformer is present in TMPA-TFSI, while almost only cis-TFSI is retained in TMHA-TFSI. For the mixtures (TMPA-TFSI)100-x (TMHA-TFSI)x, at concentrations close to the extremes, solid crystalline phases are still present and they show a strong predominance of the trans conformer of the TFSI ion for x < 15 or a large concentration of the cis conformer of TFSI for x > 85. At intermediate concentrations (33 < x < 67) no crystalline phase develops at low temperatures and both conformers of TFSI survive in the whole temperature range investigated here. We suggest that the competition between the two TFSI conformers at low temperatures can be the origin of the lack of crystalline phases for intermediate concentrations and can be exploited as a valid tool to tailor the physical properties of the mixtures of ionic liquids.

Stabilization of different conformers of bis(trifluoromethanesulfonyl)imide anion in ammonium-based ionic liquids at low temperatures.

The infrared absorption spectra of two ionic liquids with bis(trifluoromethanesulfonyl)imide (TFSI) as an anion and ammonium with different alkyl chains as cations are reported as a function of temperature. Using the comparison with ab initio calculations of the infrared-active intramolecular vibrations, the experimental lines were ascribed to the various ions composing the ionic liquids. In the liquid state of the samples, both conformers of the TFSI ion are present. In the solid state, however, the two conformers survive in N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI), while only cis-TFSI is retained in N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA-TFSI). We suggest that the longer alkyl chains of the former compound stabilize the less stable conformer of TFSI by means of stronger interactions between anions and cations.

Ionic liquid-based membranes as electrolytes for advanced lithium polymer batteries.

Gel-type polymer electrolytes are formed by immobilizing a solution of lithium N,N-bis(trifluoromethanesulfonyl)imide (LiTFSI) in N-n-butyl-N-ethylpyrrolidinium N,N-bis(trifluoromethanesulfonyl)imide (Py24TFSI) ionic liquid (IL) with added mixtures of organic solvents, such as ethylene, propylene and dimethyl carbonates (EC, PC, and DMC, respectively), into a poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) matrix, and their properties investigated. The addition of the organic solvent mixtures results in an improvement of the ionic conductivity and in the stabilization of the interface with the lithium electrode. Conductivity values in the range of 10⁻³-10⁻²  S cm⁻¹ are obtained in a wide temperature range. These unique properties allow the effective use of these membranes as electrolytes for the development of advanced polymer batteries based on a lithium metal anode and an olivine-type lithium iron phosphate cathode.