O. V. Yarmolenko

Self-diffusion of lithium cations and ionic conductivity in polymer electrolytes based on polyesterdiacrylate

The processes of ionic conductivity are studied in a polymer gel electrolyte synthesized based on polyesterdiacrylate and a low-molecular solvent ethylene carbonate. The self-diffusion coefficients of solvent molecules and Li+ cations are measured by the NMR with the pulsed magnetic field gradient. The Li+ self-diffusion coefficients increase with the increase in the solvent content and are independent of the diffusion time in the interval from 10 to 1600 ms. The latter values imply the absence of limitations for the translational mobility of lithium ions in the spatial range from 10−7 to 10−5 m. Based on the Nernst-Einstein equation, the ionic conductivities are calculated and compared with the experimental conductivities measured by the impedance method. These values coincide for high contents of solvent; for low ethylene carbonate concentrations, the calculated conductivities much exceed the experimental values.

Effect of TiO2 nanoparticle additions on the conductivity of network polymer electrolytes for lithium power sources

Network copolymer electrolytes were synthesized from polyether (polyester) diacrylates with different structures and chain lengths of polyester diacrylate and polyethylene glycol diacrylate. The optimum matrix for ion transport in the electrolyte was formed from only one type of oligomer. The influence of TiO2 nanopowder additions (∼60 nm) on the conductivity of the copolymer electrolyte was studied. The addition of 10 wt % TiO2 led to an increase in the conductivity by an order of magnitude at 30°C; the effective activation energy decreased by 20%. At elevated temperatures, the mobility of polymer chains increased and the contribution of TiO2 nanoparticles in ion transport was only half of the order of magnitude of the conductivity at 100°C. The increase in the conductivity of the polymer electrolyte after the addition of TiO2 was presumably caused by the formation of a more mobile state of the lithium ion near the nanoparticle surface, as shown by pulsed field gradient (PFG) 7Li NMR.

New polymer electrolytes based on polyethylene glycol diacrylate–LiBF4–1-ethyl-3-methylimidazolium tetrafluoroborate with the introduction of alkylene carbonates

Polymer gel electrolytes based on polyethylene glycol diacrylate (PEG DA), salt LiBF4, and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) were synthesized and studied in the presence of propylene carbonate and ethylene carbonate as solvents. The mechanism of ionic transport in the system was studied using electrochemical impedance spectroscopy, liquid mass spectrometry, pulse-field-gradient spin echo NMR spectroscopy. The range of operating temperatures of the gel electrolytes was determined by DSC. The total conductivity at room temperature in these systems is about 10–3 S cm–1. The self-diffusion coefficients on 7Li nuclei in the systems with a solvent attain the values about 10–10 m2 s–1, and in the PEG DA–LiBF4–EMIBF4 system they range from 10–13 to 10–12 m2 s–1. Ternary associates [(EMI)2(BF4)]+ and [Li+(BF4)2]– were found by liquid mass spectrometry to be the main charge carriers.

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.

New solid polymer electrolytes based on polyester diacrylate for lithium power sources

New solid polymer electrolytes are developed for a lithium power source used at the temperatures up to 100°C. Polyester diacrylate (PEDA) based on oligohydroxyethylacrylate and its block copolymers with polyethylene glycol were offered for polymer matrix formation. The salt used was LiClO4. The ionic conductivity of electrolytes was measured in the range of 20 to 100°C using the electrochemical impedance method. It is shown that the maximum conductivity in the whole temperature range is characteristic of the electrolyte based on the PEDA copolymer and polyethylene glycol condensation product (2.8 × 10−6 S cm−1 at 20°C, 1.8 × 10−4 S cm−1 at 95°C).

Effect of solvents on properties of polymer gel-electrolyte based on polyester diacrylate

New polymer gel electrolytes based on polyester diacrylates and LiClO4 salt solutions in organic solvents are developed for lithium ion and lithium polymer batteries with a high ionic conductivity up to 2.7 × 10−3 Ohm−1cm−1 at the room temperature. To choose the optimum liquid electrolyte composition, the dependence is studied of physico-chemical parameters of new gel electrolytes on the composition of the mixture of aprotic organic solvents: ethylene carbonate, propylene carbonate, and λ-butyrolacton. The bulk conductivity of gel electrolytes and exchange currents at the gel electrolyte/Li interface are studied using the electrochemical impedance method in symmetrical cells with two Li electrodes. The glass transition temperature and gel homogeneity are determined using the method of differential scanning calorimetry. It is found that the optimum mixture is that of propylene carbonate and λ-butyrolacton, in which a homogeneous polymer gel is formed in a wide temperature range of −150 to +50°C.

Nanocomposite network polymer gel-electrolytes: TiO2- and Li2TiO3-nanoparticle effects on their structure and properties

The effects of TiO2-(∼60 nm) and Li2TiO3-(∼20 nm) nanoparticles on the conductivity, structure, and mechanical strength of (polyether diacrylate-LiClO4-ethylene carbonate)-based polymer gelelectrolytes are studied. When the gel-electrolytes are synthesized with the TiO2- and Li2TiO3-nanoparticles ultrasonic pretreatment, both polyether diacrylate and ethylene carbonate are partially decomposed in the solution; this is evidenced by the appearance of -CH3-group signal at 1.2 ppm in 1H-NMR-spectra, as well, as by the peak area analysis. The gel-electrolyte matrix partial decomposition was shown not to affect the network polymer electrolyte conductivity and mechanical properties. Analysis of NMR spectra for 7Li nuclei, taken with nanocomposite polymer electrolyte rotating under magic angle, revealed two Li+ ion environments: with the nanoparticles and the polymer matrix. Upon the adding of TiO2 nanoparticles (10 mass %) the polymer electrolyte conductivity increased by order of magnitude (up to 1.8 × 10−3 S/cm at 20°C); upon the adding of Li2TiO3, by a factor of 2 only (up to 7.0 × 10−4 S/cm at 20°C). The electrolyte-solution ultrasonic treatment increased the films’ mechanical strength; the larger effect occurred with Li2TiO3 (the modulus of elasticity is 15 MPa).

New network-gel-electrolytes consisting of polyethylene glycol diacrylate, LiBF4, and 1-buthyl-3-methylimidazolium tetrafluoroborate, added with alkylene carbonates: the ion transfer mechanism and properties

Polymer gel-electrolytes based on polyethylene glycol diacrylate, LiBF4 salt, ethylene carbonate, propylene carbonate, and 1-buthyl-3-methylimidazolium tetrafluoroborate ionic liquid, are synthesized by using radical polymerization. For the optimal composition (polyethylene glycol diacrylate, 19 wt %; LiBF4, 10 wt %; 1-buthyl-3-methylimidazolium tetrafluoroborate, 44 wt %; ethylene carbonate, 27 wt %) the conductivity is maximal: 2.5 × 10−3 S/cm at 20°C; 1.1 × 10−2 S/cm at 100°C. The system remains thermostable up to 107°C. It was shown, by using NMR with the pulsed magnetic field gradient, that the polymer matrix is solely involved in the Li+ ion solvation in the electrolyte composed of polyethylene glycol diacrylate, LiBF4, and 1-buthyl-3-methylimidazolium tetrafluoroborate; and the Li+ self-diffusion coefficients do not depend on the ionic liquid content. When the electrolyte is added with alkylene carbonates, both the polymer matrix and the solvent are involved in the Li+ ion solvation. The highest conductivity of 10−3 S/cm (at 20°C) was reached when 27 wt % of ethylene carbonate have been added; here Li+ ions are entirely solvated by ethylene carbonate molecules, the diffusion coefficient being equal to 10−11 m2/s.

Effect of 15-crown-5 on the charge transfer resistance at the polymer electrolyte/modified Li-electrode interface

The effect of 15-crown-5, which is applied immediately to pure and modified surface of a lithium electrode, on the charge transfer resistance at the electrode/polymer electrolyte interface is studied. The polymer electrolyte consists of a 1: 1 mixture of oligourethan dimethacrylate and polypropylene glycol monomethacrylate (20 wt %), an initiator (azobisisobutyronitrile) (2 wt %), and a 1 M LiClO4 solution in gamma-butyrolactone (78 wt %). The conductivity of this gel electrolyte is 3 × 10−3 S cm−1. The temperature dependence of the impedance of the Li/gel electrolyte/Li electrochemical cells is measured for electrodes of four types. The activation energies for the charge transfer at the Li/electrolyte interface are calculated. It is found that, after treating the test lithium electrodes with 15-crown-5, the charge transfer resistance decreases, and in the case of the modified lithium surface, the activation energy for the process decreases by 1.8 times.

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.