G. Z. Tulibaeva

Polymer electrolytes based on poly(ester diacrylate), ethylene carbonate, and LiClO4: a relationship of the conductivity and structure of the polymer according to IR spectroscopy and quantum chemical modeling data

New polymer electrolytes based on poly(ester diacrylate) (PEDA), LiClO4, and additives of ethylene carbonate (EC) have a Li+ ion conductivity comparable with that of liquid electrolytes. The conductivity first decreases by an order of magnitude at an EC content of ∼5 wt.% and then increases by three orders of magnitude at 55 wt.% EC. To understand the nature of this extreme dependence, a comprehensive study using IR spectroscopy and quantum chemical modeling was performed. It was found that the changes in the IR spectra with an increase in the EC content were stepwise to form at final stage the same absorption peaks that were observed for the IR spectra of LiClO4 solutions in EC. The density functional theory studies of the energy and structures of mixed Li+ complexes and LiClO4 with EC and PEDA, which was modeled by oligomers H-((CH2)2COO(CH2)2O)n-CH3 (n ≤ 10) showed a stronger binding of the lithium ion with the polymer matrix in the mixed complexes with one EC molecule at a low content of EC resulting, most likely, in a decrease in the conductivity. Less stable mixed complexes with three EC molecules can be formed with an increase in the EC fraction and they become unstable in EC excess because of the transition of the Li+ ions to solvate complexes containing only EC molecules.

Solvation environment of lithium ion in a LiBF4–propylene carbonate system in the presence of 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid studied by NMR and quantum chemical modeling

Solutions of lithium and 1-ethyl-3-methylimidazolium tetrafluoroborates ([emim][BF4]) in propylene carbonate (PC) were studied by the high-resolution NMR method on 1H, 7Li, 11B, 13C, and 19F nuclei. The degree of solvation of lithium ions was determined by measuring selfdiffusion coefficients by pulse-field-gradient spin echo NMR method on 1H, 7Li, and 19F nuclei. The hydrodynamic radii of solvated Li+ cations were estimated by the Stokes–Einstein equation. The model structures of the solvation complexes of Li+ ion with propylene carbonate molecules and BF4– anion and their associates with ionic liquid components were calculated in terms of the density function theory. The calculated values of the chemical shifts were compared with the experimental data. PC molecules were predominantly bound to the Li+ cation, while LiBF4–[emim][BF4]–PC (1: 4: 4) electrolyte had a maximum conductivity of 9.5 mS cm–1 at 24 °С compared to the compositions of a lower content of the solvent.

Quantum chemical modeling of the degradation of the polymer matrix and solvent molecules in nanocomposite polymer gel electrolytes

Quantum chemical calculations of molecular structures and magnetic shielding constants for H and C nuclei were performed by the density functional method. The structures of the polyester diacrylate degradation products formed under sonication in the presence of TiO2 or Li2TiO3 nanoparticles in the electrolyte polyester diacrylate—LiClO4—ethylene carbonate—nanopowder were proposed based on agreement of the goodness-of-fit with the experimental NMR spectra of the polymer electrolytes. The surface centers on TiO2 and Li2TiO3 under the ultrasonic energy absorption conditions were concluded to catalyze the unexpected metathesis of σ-C—H bonds in the α-positions to the carbonyl group.

Structure of LiBF4 Solvate Complexes in Ethylene Carbonate, Based on High-Resolution NMR and Quantum-Chemical Data

The ion solvation of LiBF4 in ethylene carbonate is studied via high resolution NMR, conductometry, and quantum-chemical simulation. 7Li, 11B, 19F, 13C, and 17O NMR spectra are acquired for LiBF4 solutions in ethylene carbonate, and their conductivity is measured in the concentration range of 0.07–1.77 mol kg–1 at 40°C. Molecular models of solvate complexes of a Li+BF4− ion pair containing n ethylene carbonate molecules are constructed. The calculated 11B chemical shifts are virtually independent of n, which can provide a relationship between 11B experimental shifts and degree of dissociation (α). The α value is estimated from a theoretical change in chemical shift of −0.414 ppm when a BF4− ion transitions from a free state to an associated one of the contact ion pair. The α values are in reasonable agreement with the degree of dissociation for LiBF4 in propylene carbonate, found from the Walden product of the equivalent electrical conductivity of a solution by its viscosity.

Electrolyte systems for primary lithium-fluorocarbon power sources and their working efficiency in a wide temperature range

New compositions of liquid organic electrolytes with working temperatures of up to–50°С were developed for low-temperature primary Li/CFx power sources. Five different compositions of organic electrolytes with a 15-crown-5 (2 vol %) addition and without it were studied on laboratory Li/CFx power sources. 1МLiBF4 (LiPF6) in an ethylene carbonate/dimethyl carbonate/methyl propionate/ethylmethyl carbonate (EC/DMC/MP/EMC) (1: 1: 1: 2) mixture and 1 М LiPF6 in an EC/DMC/EMC (1: 1: 3) mixture each with a 15-crown-5 (2 vol %) addition were found to be the best compositions of organic electrolytes with working temperatures of up to–50°С. The electrochemical tests at 20 and–50°С in the Li/CFx system showed that the 15-crown-5 addition increased the length of the discharge plateau at–50°С three- or fourfold. The mechanisms responsible for the increase in the discharge capacity of the CFx cathode in the presence of a crown ether addition were suggested.

Empirical Formula for the Concentration Dependence of the Conductivity of Organic Electrolytes for Lithium Power Sources in the Vicinity of a Maximum

To optimize the compositions of liquid organic electrolytes for lithium power sources, it is useful to have the dependence of the conductivity on the lithium salt concentration in a convenient analytical form. An empirical formula was suggested on the basis of the modified Kohlrausch equation for the concentration dependence of the conductivity of organic electrolytes in the vicinity of a maximum. The accuracy of this equation was checked on solutions of LiBF4 in propylene carbonate; LiClO4 in ethylene carbonate; and LiPF6 in ethylene carbonate/diethyl carbonate (1: 1), ethylene carbonate/ethylmethyl carbonate (1: 1), and ethylene carbonate/methyl acetate (1: 1) at different temperatures. The calculated data are in good agreement with experiment for all the systems. The new empirical formula allows the determination of the maximum conductivity of organic electrolytes based on a few points with good accuracy, which is very important in choosing the electrolyte salt concentration in practice.