Sophie Le Caër

Ultrasensitive spectroscopy of ionic reactive intermediates in the gas phase performed with the first coupling of an IR FEL with an FTICR-MS

First example of coupling a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR-MS) with an infrared Free Electron Laser (FEL) is presented. This experimental setup is ideally suited for the direct structural characterization of reactive polyatomic ions. Ultrasensitive measurements of the infrared vibrational spectrum of ionic reactive intermediate selectively prepared is allowed by the association of the high peak power of the FEL, its wide tunability, and the flexibility of FTICR-MS, where several mass selections and ion-molecule reactions can be combined. These possibilities are demonstrated in the case of Fe + complexes where two photofragmentation pathways compete. The resulting infrared spectrum is in excellent agreement, both with respect to the position and to the relative intensities of the infrared transitions, with predicted by ab initio electronic structure calculations. r 2003 Elsevier Science B.V. All rights reserved.

Radiolysis as a solution for accelerated ageing studies of electrolytes in Lithium-ion batteries

Diethyl carbonate and dimethyl carbonate are prototype examples of eco-friendly solvents used in lithium-ion batteries. Nevertheless, their degradation products affect both the battery performance and its safety. Therefore, it is of paramount importance to understand the reaction mechanisms involved in the ageing processes. Among those, redox processes are likely to play a critical role. Here we show that radiolysis is an ideal tool to generate the electrolytes degradation products. The major gases detected after irradiation (H2, CH4, C2H6, CO and CO2) are identified and quantified. Moreover, the chemical compounds formed in the liquid phase are characterized by different mass spectrometry techniques. Reaction mechanisms are then proposed. The detected products are consistent with those of the cycling of Li-based cells. This demonstrates that radiolysis is a versatile and very helpful tool to better understand the phenomena occurring in lithium-ion batteries.

Successive reactions of iron carbonyl cations with dimethyl ether: direct cleavage versus rearrangement

The reaction kinetics of electron impact ionization generated Fe(CO)n+ cations with dimethyl ether (DME) is investigated using a triple-cell Fourier Transform Ion Cyclotron Resonance spectrometer. The primary reaction is rapid substitution of CO by DME. One-step substitution of two CO ligands by one DME molecule also occurs for n = 3–4, and for unrelaxed Fe(CO)2+ ions. Successive substitutions in the Fe(CO)4+/DME system lead mainly to Fe(CO)(DME)2+, in which the last CO remains unsubstituted. This ion is slowly converted to Fe(CO)(DME)3+ and Fe(DME)2+. The latter reaction implies formation of neutral CH3COOCH3 resulting from iron-promoted CO insertion reaction. Further reaction of Fe(DME)+ with DME involves the cleavage of a C–O bond according to two channels, either direct CH3˙ loss or more exothermic CH4 loss, implying a molecular rearrangement. The branching ratio is strongly energy-dependent: the latter channel is observed only for Fe(DME)+ ions having a very low energy content.

Electrolytes Ageing in Lithium‐ion Batteries: A Mechanistic Study from Picosecond to Long Timescales

The ageing phenomena occurring in various diethyl carbonate/LiPF6 solutions are studied using gamma and pulse radiolysis as a tool to generate similar species as the ones occurring in electrolysis of Li-ion batteries (LIBs). According to picosecond pulse radiolysis experiments, the reaction of the electron with (Li(+), PF6(-)) is ultrafast, leading to the formation of fluoride anions that can then precipitate into LiF(s). Moreover, direct radiation-matter interaction with the salt produces reactive fluorine atoms forming HF(g) and C2H5F(g). The strong Lewis acid PF5 is also formed. This species then forms various R(1)R(2)R(3) P=O molecules, where R is mainly -F, -OH, and -OC2H5. Substitution reactions take place and oligomers are slowly formed. Similar results were obtained in the ageing of an electrochemical cell filled with the same model solution. This study demonstrates that radiolysis enables a description of the reactivity in LIBs from the picosecond timescale until a few days.

Identification of Transient Radical Anions (LiClO4)n– (n = 1–3) in THF Solutions: Experimental and Theoretical Investigation on Electron Localization in Oligomers

Picosecond pulse radiolysis measurements of tetrahydrofuran (THF) solutions containing LiClO4 over a wide range of concentration are performed to investigate the formation of transient species. The (35)Cl NMR measurements of these solutions prior to irradiation show that the salt is in the form of (LiClO4)n oligomers. Kinetics and transient absorption spectra of intermediates in each solution are obtained on the time scale from 10 to 3800 ps. A global spectro-kinetic matrix of the data is analyzed by the multicurve resolution alternated least-squares (MCR-ALS) method. It shows the presence of 3 transient species induced by electron pulse, in addition to the solvated electron. A hybrid Monte Carlo/DFT molecular simulation method is elaborated, using the MPW1K functional for the configuration sampling and B3LYP for the spectra calculations. The maximum of the absorption band of the monomer (LiClO4)(-), dimer (LiClO4)2(-), trimer (LiClO4)3(-), and tetramer (LiClO4)4(-) anions are deduced from the simulations. They enable one to label the MCR-ALS spectra (differences are below 0.1 eV) and to interpret the kinetic data. The simulations show also that Li(I) ion catalyzes the reduction of perchlorate by excess electrons. Only the dimer anion, due to its unique structure with a stable Li2(+) core and two nonbridging perchlorates, presents higher stability toward ClO4(-) reduction into ClO3(-). It corresponds to the long-lived species observed in the experiments.

Ultrafast Decay of the Solvated Electron in a Neat Polar Solvent: The Unusual Case of Propylene Carbonate

The behavior of carbonates is critical for a detailed understanding of aging phenomena in Li-ion batteries. Here we study the first reaction stages of propylene carbonate (PC), a cyclical carbonate, by picosecond pulse radiolysis. An absorption band with a maximum around 1360 nm is observed at 20 ps after the electron pulse and is shifted to 1310 nm after 50 ps. This band presents the features of a solvated electron absorption band, the solvation lasting up to 50 ps. Surprisingly, in this polar solvent, the solvated electron follows an ultrafast decay and disappears with a half time of 360 ps. This is attributed to the formation of a radical anion PC(-•). The yield of the solvated electron is low, suggesting that the radical anions are mainly directly produced from presolvated electrons. These results demonstrate that the initial electron transfers mechanisms are strongly different in linear compared with cyclical carbonates.

Picosecond Pulse Radiolysis of Propylene Carbonate as a Solute in Water and as a Solvent.

The ester propylene carbonate (PC) is a solvent with a high static dielectric constant where the charges generated by ionizing radiation are expected to be long-lived at room temperature. Time-resolved optical absorption spectroscopy after picosecond electron pulses reveals the formation of a UV band, within less than two nanoseconds, that is assigned to the radical anion PC(-•), arising from a fast attachment reaction of electrons onto PC. Assignment and reactivity of PC(-•) in neat solvent and solutions are discussed in relation with data obtained in solutions of PC in water under reducing or oxidizing conditions and in solutions in PC of aromatic scavengers with various reduction potentials. The fate of the electrons and the ionization yield in PC are compared with those of other solvents.

Condensation reaction vs. ligand exchange with first-row transition metal cations: a theoretical study of Cu+ heteroleptic model complexes

Analysis of the evolution of the binding energy to a late first-row transition metal cation M+ shows that the third ligand is significantly less strongly bound than the two first. The present study suggests that a new type of cluster-assisted reactions could be induced by the metal center when it is tri-coordinated. Cu+ complexes have been chosen as models for the quantum chemical calculations (density functional and post-Hartree–Fock ab initio). First, an emphasis is made on the analysis of the evolution of the binding energy as the coordination number n varies from 1 to 4 in Cu+(H2O)n and Cu+(CO)n. The evolution of the electronic repulsion between the σ donating orbitals of the ligands and the 3d orbitals is discussed in details. Second, we consider the successive substitution reactions of CO by H2O ligand from Cu+(CO)n (n=1–4). Our calculations suggest that rather than the simple substitution reaction of CO by ROR′ (R, R′=H, alkyl), formation of an acid or ester could occur in the coordination sphere of the metal. Whereas kinetics is likely to be unfavorable in the case of Cu+, such condensation reaction of ROR′ and CO could be observed with late first-row transition metal cations such as Fe+.

Influence de l’énergie interne sur la réactivité de l’ion Fe(CO)2+ avec le diméthyléther, étudiée dans un spectromètre de masse FT–ICR

Abstract The influence of the internal energy on the reactivity of iron carbonyl cations with dimethylether CH 3 OCH 3 (DME) has been studied using a triple cell Fourier Transform Ion Cyclotron Resonance mass spectrometer. The experimental set-up as well as the data analysis are briefly presented before being detailed on the example of the reactivity of Fe(CO) 2 + . The strong energy dependence upon the reactivity of the ion is shown: when working with thermalised ions, the only channels observed are the two successive substitutions of a CO ligand by one DME molecule, whereas other channels are opened up for excited ions (the cleavage of the C–O bond may be homolytic or due to a rearrangement). A reaction mechanism of the C–O bond activation is then proposed.

Observation and Simulation of Transient Anion Oligomers (LiClO4)n– (n = 1–4) in Diethyl Carbonate LiClO4 Solutions

NMR measurements show that diethyl carbonate (DEC, a solvent with a low dielectric constant) solutions of LiClO4 contain (LiClO4)n oligomers. The reduction of these species by solvated and presolvated electrons is followed by picosecond pulse radiolysis measurements. The data analysis shows that several anions absorbing in the near-infrared (NIR) and visible range are formed after the 7 ps electron pulse. In contrast with tetrahydrofuran (THF) solutions of LiClO4, the anionic monomer (LiClO4)- is not observed in DEC solutions. This is due to the fact that DEC is a nonpolar solvent favoring the clustering of monomers in the nonirradiated solution, as shown by NMR results, and also due to the instability of the anionic monomer. The absorption spectra of the anionic dimer (LiClO4)2-, trimer (LiClO4)3-, and tetramer (LiClO4)4- are clearly observed in NIR and visible ranges. Compared to the results obtained for the same system in THF and in agreement with simulated absorption spectra, the experimental results show that the absorption bands are shifted to the blue end of the spectrum when n increases. The kinetics recorded for the molar LiClO4 solution indicates that the solute is only in the form of oligomers (LiClO4)n with a large n value and that the reduced species absorb weakly in the visible region. Lastly, and contrary to what is known for well-separated ions in polar solvents, it is shown that the (LiClO4)n- anions are not stable with respect to self-reduction, leading to the decomposition of perchlorate anions. In this reaction, the perchlorate anion ClO4- is reduced by the Li atom into a chlorate anion ClO3-. This is proved by the presence of ClO3- and chlorinated species detected by mass spectrometry measurements in irradiated DEC solutions containing LiClO4.