Numerous additives are used in electrolytes of lithium‐ion batteries, especially for the formation of efficient solid electrolyte interphase at the surface of the electrodes. The understanding of the degradation processes of these compounds is thus important. They can be obtained through radiolysis. In the case of fluoroethylene carbonate (FEC), picosecond pulse radiolysis experiments evidenced the formation of FEC●‐ . This radical is stabilized in neat FEC, whereas the ring opens to form more stable radical anions when FEC is a solute in other solvents, as confirmed by quantum chemistry calculations. In neat FEC, pre‐solvated electrons primarily undergo attachment compared to solvation. At long timescales, produced gases (H2, CO, and CO2 ) were quantified. A reaction scheme for both the oxidizing and reducing pathways at stake in irradiated FEC was proposed. This work evidences that the nature of the primary species formed in FEC depends on the amount of FEC in the solution.
by Furong Wang, Pascal Pernot, Pierre Archirel, Uli Schmidhammer, Daniel Ortiz, Sophie Le Caër, and Mehran Mostafavia
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.
The reactivity of ethylene carbonate (EC) and of a EC/diethyl carbonate (DEC) mixture was studied under ionizing radiation to mimic the aging phenomena that occur in lithium‐ion batteries. Picosecond‐pulse radiolysis experiments showed that the attachment of the electron to the EC molecule is ultrafast (k(e−EC+EC)=1.3×109 L mol−1 s−1 at 46 °C). This reaction rate is accelerated by a factor of 5.7 compared with the electron attachment to propylene carbonate, which implies that the presence of the methyl group significantly slows the reaction. In a 50:50 EC/DEC mixture, just after the electron pulse the electron is solvated by a mixture of EC and DEC molecules, but its fast decay is attributed exclusively to electron attachment to the EC molecule. Stable products detected after steady‐state irradiation were mainly H2, CH4, CO, and CO2. The evolution of the radiolytic yields with the EC fraction shows that H2 and CH4 did not exhibit linear behavior, whereas CO and CO2 did. Indeed, H2 and CH4 mainly arise from the excited state of DEC, the formation of which is significantly affected by the evolution of the dielectric constant of the mixture and by the electron attachment to EC. CO formation is mainly due to the reactivity of the EC molecule, which is not affected in the mixture, as proven by pulse‐radiolysis experiments.
The behavior under irradiation of neat propylene carbonate (PC), a co-solvent usually used in Li-ion batteries (LIB), and also of Li salt solutions is investigated. The decomposition of neat PC is studied using radiolysis in the pulse and steady state regime and is assigned to the ultrafast formation, in the reducing channel, of the radical anion PC− by electron attachment, followed by the ring cleavage, leading to CO. In the oxidative channel, the PC(-H) radical is formed, generating CO2. The CO2 and CO yields are both close to the ionization yield of PC. The CO2 and CO productions in LiClO4, LiBF4 and LiN(CF3)2(SO2)2 solutions are similar as in neat PC. In contrast, in LiPF6/PC a strong impact on PC degradation is measured with a doubling of the CO2 yield due to the high reactivity of the electron towards PF6− observed in the picosecond range. A small number of oxide phosphine molecules are detected among the various products of the irradiated solutions, suggesting that most of them, observed in carbonate mixtures used in LIBs, arise from linear rather than from cyclical molecules. The similarity between the degradation by radiolysis or electrolysis highlights the interest of radiolysis as an accelerated aging method.
by Daniel Ortiz, Isabel Jiménez Gordon, Jean‐Pierre Baltaze, Oscar Hernandez‐Alba Solène Legand, Vincent Dauvois, Gregory Si Larbi, Uli Schmidhammer, Jean‐Louis Marignier, Jean‐Frédéric Martin, Jacqueline Belloni, Mehran Mostafavi, Sophie Le Caër
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 R1R2R3P=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.