Ionic liquids consisting of bis(fluorosulfonyl)imide (FSI − ) anion show promise as electrolytes for Li-ion-based electric storage devices, as they exhibit relatively low viscosity, high chemical stability, and form robust solid−electrolyte interphase (SEI) protecting liquid electrolyte from further breakdown on the electrode. These ionic liquids have been reported to inhibit dendrite formation on lithium metal and lithiated graphite electrodes, which also relates to the unusual SEI properties. In this study, we examine the chemistry aspects that may account for this behavior. Radiolysis was used to induce redox reactions of FSI − anions in model systems, and matrix isolation electron paramagnetic resonance was used to identify radical (ion) intermediates generated in these reactions. Our results suggest that qualitative differences between such ionic liquid electrolytes versus common carbonate electrolytes reflect ease of mineralization of the reduced anion without the concurrent generation of organic radicals and/or elimination of gaseous products in side reactions of the corresponding radical intermediates.
Room temperature ionic liquids (IL) find increasing use for the replacement of organic solvents in practical applications, including their use in solar cells and electrolytes for metal deposition, and as extraction solvents for the reprocessing of spent nuclear fuel. The radiation stability of ILs is an important concern for some of these applications, as previous studies suggested extensive fragmentation of the constituent ions upon irradiation. In the present study, electron paramagnetic resonance (EPR) spectroscopy has been used to identify fragmentation pathways for constituent anions in ammonium, phosphonium, and imidazolium ILs. Many of these detrimental reactions are initiated by radiation-induced redox processes involving these anions. Scission of the oxidized anions is the main fragmentation pathway for the majority of the practically important anions; (internal) proton transfer involving the aliphatic arms of these anions is a competing reaction. For perfluorinated anions, fluoride loss following dissociative electron attachment to the anion can be even more prominent than this oxidative fragmentation. Bond scission in the anion was also observed for NO(3)(-) and B(CN)(4)(-) anions and indirectly implicated for BF(4)(-) and PF(6)(-) anions. Among small anions, CF(3)SO(3)(-) and N(CN)(2)(-) are the most stable. Among larger anions, the derivatives of benzoate and imide anions were found to be relatively stable. This stability is due to suppression of the oxidative fragmentation. For benzoates, this is a consequence of the extensive sharing of unpaired electron density by the π-system in the corresponding neutral radical; for the imides, this stability could be the consequence of N-N σ(2)σ(*1) bond formation involving the parent anion. While fragmentation does not occur for these "exceptional" anions, H atom addition and electron attachment are prominent. Among the typically used constituent anions, aliphatic carboxylates were found to be the least resistant to oxidative fragmentation, followed by (di)alkyl phosphates and alkanesulfonates. The discussion of the radiation stability of ILs is continued in the second part of this study, which examines the fate of organic cations in such liquids.
Whereas there are numerous experimental and computational studies of electrochemical reduction leading to the formation of solid-electrolyte interface (SEI) in lithium-ion batteries, so far there have been no direct spectroscopic observations of radical intermediates involved in the SEI formation. In Part 1 of this series, radiolysis and laser photoionization of carbonate electrolytes are used to observe and identify these reaction intermediates using electron paramagnetic resonance spectroscopy. Our study indicates that the suggested scenarios for electrolyte reduction require elaboration. In particular, we establish the occurrence of efficient H abstraction and 1,2-migration involving radicals generated through the reductive ring-opening. Instead of the primary radicals postulated in the current models, secondary and tertiary radicals are generated, biasing the subsequent chemistry to radical disproportionation. The consequences of this bias for radical and anionic polymerization are examined, and it is suggested that branching and the formation of a polymer network is favored. We argue that this chemistry accounts for some of the heretofore unexplained properties of SEI, including the dramatic difference in solvent permeability for SEIs derived from ethylene carbonate and propylene carbonate.
Spectra of the hydrated electron in pressurized light and heavy water at temperatures up to and beyond the critical temperature are reported, for wavelengths between 0.4 and 1.7 microm. In agreement with previous work, spectra can be approximately represented by a Gaussian function on the low-energy side, and a Lorentzian function on the high-energy side in subcritical water, but deviations from this form are very clear above 200 degrees C. The spectrum shifts strongly to the red as temperature rises. At supercritical temperatures, the spectrum shifts slightly to the red as density decreases, and the Gaussian-Lorentzian form is a very poor description. Application of spectral moment theory allows one to make an estimate of the average size of the electron wave function and of its kinetic energy. It appears that for water densities below about 0.6 g/cc, and down to below 0.1 g/cc, the average radius of gyration for the electron remains constant at around 3.4 angstroms, and its absorption maximum is near 0.9 eV. For higher densities, the electron is squeezed into a smaller cavity and the spectrum is shifted to the blue. The enthalpy and free energy of electron hydration are derived as a function of temperature on the basis of existing equilibrium data and absolute proton hydration energies derived from the cluster-based common point method. In a discussion, we compare the effective "size" of the hydrated electron derived from both methods.
In part 1 of this study, radiolytic degradation of constituent anions in ionic liquids (ILs) was examined. The present study continues the themes addressed in part 1 and examines the radiation chemistry of 1,3-dialkyl substituted imidazolium cations, which currently comprise the most practically important and versatile class of ionic liquid cations. For comparison, we also examined 1,3-dimethoxy- and 2-methyl-substituted imidazolium and 1-butyl-4-methylpyridinium cations. In addition to identification of radicals using electron paramagnetic resonance spectroscopy (EPR) and selective deuterium substitution, we analyzed stable radiolytic products using (1)H and (13)C nuclear magnetic resonance (NMR) and tandem electrospray ionization mass spectrometry (ESMS). Our EPR studies reveal rich chemistry initiated through "ionization of the ions": oxidation and the formation of radical dications in the aliphatic arms of the parent cations (leading to deprotonation and the formation of alkyl radicals in these arms) and reduction of the parent cation, yielding 2-imidazolyl radicals. The subsequent reactions of these radicals depend on the nature of the IL. If the cation is 2-substituted, the resulting 2-imidazolyl radical is relatively stable. If there is no substitution at C(2), the radical then either is protonated or reacts with the parent cation forming a C(2)-C(2) σσ*-bound dimer radical cation. In addition to these reactions, when methoxy or C(α)-substituted alkyl groups occupy the N(1,3) positions, their elimination is observed. The elimination of methyl groups from N(1,3) was not observed. Product analyses of imidazolium liquids irradiated in the very-high-dose regime (6.7 MGy) reveal several detrimental processes, including volatilization, acidification, and oligomerization. The latter yields a polymer with m/z of 650 ± 300 whose radiolytic yield increases with dose (~0.23 monomer units per 100 eV for 1-methyl-3-butylimidazolium trifluorosulfonate). Gradual generation of this polymer accounts for the steady increase in the viscosity of the ILs upon irradiation. Previous studies at lower dose have missed this species due to its wide mass distribution (stretching out to m/z 1600) and broad NMR lines, which make it harder to detect at lower concentrations. Among other observed changes is the formation of water immiscible fractions in hydrophilic ILs and water miscible fractions in hydrophobic ILs. The latter is due to anion fragmentation. The import of these observations for use of ILs as extraction solvents in nuclear cycle separations is discussed.
Improving the stability of Li ion electricity storage devices is important for practical applications, including the design of rechargeable automotive batteries. Many promising designs for such batteries involve positive electrodes that are complex oxides of transition metals, including manganese. Deposition of this Mn on the graphite negative electrode is known to correlate with gradual capacity fade [by increasing retention of lithium cations in the solid electrolyte interphase (SEI)] in Li ion batteries. This SEI contains partially reduced and fully mineralized electrolyte, in the outer (organic) and inner (mineral) layers. In this study, we explore structural aspects of this Mn deposition via a combination of electrochemical, X-ray absorption, and electron paramagnetic resonance experiments. We confirm previous observations that suggest that on a delithiated graphite electrode Mn is present as Mn2+ ion. We show that these Mn2+ ions are dispersed: there are no Mn-containing phases, such as MnF2, MnO, or MnCO3. These isolated Mn2+ ions reside at the surface of lithium carbonate crystallites in the inner SEI layer. For a lithiated graphite electrode, there is reduction of these Mn2+ ions to an unidentified species different from atomic, nanometer scale or mesoscale Mn(0) clusters. We suggest that Mn2+ ions are transported from the positive electrode to the graphite electrode as complexes in which the cation is chelated by carboxylate groups that are products of electrolytic breakdown of the carbonate solvent. This complex is sufficiently strongly bound to avoid cation exchange in the outer SEI and thereby reaches the inner (mineral) layer, where the Mn2+ ion is chemisorbed at the surface of the carbonate crystallites. We conjecture that stronger chelation can prevent deposition of Mn2+ ions and in this way retard capacity fade. This action might account for the protective properties of certain battery additives.
Extensive polymerization of ethylene carbonate (EC) leading to the formation of oligomers with masses up to 1 to 2 kDa in electron beam radiolysis is demonstrated using electrospray ionization mass spectrometry and nuclear magnetic resonance. This polymer has a different structure and morphology than the linear chain copolymer of ethylene oxide and EC that is generated in anionic polymerization of intact EC molecules. This radiolytically generated polymer exhibits chain branching and pendant carbonate groups, and it can form a 3D organic network that is additionally cross-linked through lithium ions. Such a morphology is consistent with the occurrence of anionic and radical polymerization that involve the products of recombination and disproportionation of secondary radicals generated in one-electron reduction of EC. Our examination of this chemistry suggests that the same polymer is likely to occur in electrochemical reduction of EC. The formation of this polymeric network qualitatively accounts for some of unexplained properties of the solid-electrolyte interface (SEI) occurring in electrochemical cells with EC-based electrolyte, including common lithium batteries.
Photoirradiated metal oxide semiconductors are known to reduce carbon dioxide to methane. This multistep reaction is commonly represented as a sequence of proton-coupled two-electron reactions leading from carbon dioxide to formate to formaldehyde to methanol and to methane. We suggest that the actual reaction mechanism is more complex, as it involves two-carbon molecules and radicals in addition to these onecarbon species. The ″stepping stone″ of this mechanism for carbon dioxide fixation could be glyoxal, which is the product of recombination of two formyl radicals, or glycolaldehyde, which is its reduced form. We demonstrate the main steps of this reduction chain and suggest a catalytic cycle integrating these steps and the radical chemistry. In addition to methane, this cycle generates complex organic molecules, such as glycolaldehyde, acetaldehyde, and methylformate, which were observed in product analyses. This cycle can be regarded as one of the simplest realizations of multistep, photosynthetic fixation of atmospheric carbon in prebiotic nature.
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