Glyme Solvents Research Articles

Analysis of Ion Transport in Glyme-Based Electrolyte Solutions for Li-Air Batteries

In order to elucidate the Li+ ion transport in glyme-based electrolyte solutions for Li-air batteries, LiCF3SO3(LiTfO) or LiN(SO2CF3)2(LiTFSI) dissolved glyme (triglyme(G3), tetra-glyme(G4) and pentaglyme(G5)) electrolytes were prepared, and the self-diffusion coefficients D of Li+ ion (7Li), anion (9F) and glyme (1H) solvent were measured individually by a pulsed-field gradient spin echo (PGSE-)NMR [1-3] in the temperature range of 30oC to 60oC. The viscosity η, density d and ionic conductivity σ were also measured to analyze the mechanism of ion transport in the glyme-based electrolytes. Moreover, molecular dynamics (MD) simulation was applied to reproduce the D values and investigate the dissociation of Li salts and solvation structure of Li+ion in the glyme solvents. As the results, it was found that the glyme with shorter polyethylene glycol (PEG) chains gave lower η (Fig. 1) and higher D values (Fig. 2) and LiTFSI-based electrolytes exhibited higher σ values (Fig. 3) because of high apparent degree of dissociation α app (Fig. 4), which were estimated by using Nernst-Einstein equation, for an increase in the amount of Li+ carriers. The trend was kept in the examined temperature range from 30oC to 60oC. However, the a value for the Li salts was decreased by an increase in the temperature, implying a reduction of the interaction energy between the Li+ ion and glyme solvents to form a metal complex. On the other hand, the a value was increased by the concentration of both Li salts in the electrolytes up to the molar ratio of 1:1 although the viscosity was also increased. Consequently, the highest σ values were attained both for 1 M LiTfO and LiTFSI-based electrolytes. From the results of MD simulation, the above trend of D values for ions and solvents were clearly reproduced and the larger amount of ion pairs of LiTfO was also confirmed than those of LiTFSI in the glyme solvents. The distance between Li+ ion and anion was much expanded for the LiTFSI compared with that for the LiTfO. Based on those results, we further investigated on the other new Li salt, LiN(SO2F)2(LiFSI), and influence of impurity such as H2O from air to the ion transports. The reports will be presented in the meeting. This study was supported by JST “Next Generation Batteries Area in Advanced Low Carbon Technology Research and Development Program (ALCA)” and “A Tenure-track Program” from MEXT, Japan.

Electrochemical Investigations into Blended Electrolytes Containing Ionic Liquids and a Glyme Solvent for Li-O­2 batteries

The Li-O2 battery may theoretically possess practical gravimetric energy densities several times greater than the current state-of-the-art Li-ion batteries.1 This magnitude of development is a requisite for true realization of electric vehicles capable of competing with the traditional combustion engine. However, significant challenges must be addressed before practical application may be considered. These include low efficiencies, low rate capabilities and the parasitic decomposition reactions of electrolyte/electrode materials resulting in very poor rechargeability.2-4 Ionic liquids, ILs, typically display several properties, extremely low vapor pressure and high electrochemical and thermal stability, which make them particularly interesting for Li-O2 battery electrolytes. However, the typically sluggish transport properties generally inhibit rate performance and cells suffer similar inefficiencies during cycling.5 , 6 In addition to the design of new ILs with tailored properties, formulating blended electrolytes using molecular solvents with ILs has been considered to improve their performance.7 , 8 In this work, we will discuss the physical properties vs. the electrochemical performance of a range of formulated electrolytes based on tetraglyme, a benchmark Li-O2 battery electrolyte solvent, and several ILs. The selected ILs are based on the bis{(trifluoromethyl)sulfonyl}imide anion and alkyl/ether functionalized cyclic alkylammonium cations, which exhibit very good stability and moderate viscosity.9 O2 electrochemistry will be investigated in these media using macro and microdisk voltammetry and O2 solubility/diffusivity is quantified as a function of the electrolyte formulation. Furthermore, galvanostatic cycling of selected electrolytes in Li-O2 cells will be discussed to probe their practical electrochemical performance. Finally, the physical characterization of the blended electrolytes will be reported in parallel to further determine structure (or formulation) vs. property relationships and to, therefore, assess the importance of certain electrolyte properties (viscosity, O2supply capability, donor number) on their performance. This work was funded by the EPSRC (EP/L505262/1) and Innovate UK for the Practical Lithium-Air Batteries project (project number: 101577). 1. P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat. Mater., 11, 19 (2012). 2. S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Barde and P. G. Bruce, Angew. Chem., Int. Ed., 50, 8609 (2011). 3. B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov and A. C. Luntz, J. Phys. Chem. Lett., 3, 997 (2012). 4. D. G. Kwabi, T. P. Batcho, C. V. Amanchukwu, N. Ortiz-Vitoriano, P. Hammond, C. V. Thompson and Y. Shao-Horn, J. Phys. Chem. Lett., 5, 2850 (2014). 5. Z. H. Cui, W. G. Fan and X. X. Guo, J. Power Sources, 235, 251 (2013). 6. F. Soavi, S. Monaco and M. Mastragostino, J. Power Sources, 224, 115 (2013). 7. L. Cecchetto, M. Salomon, B. Scrosati and F. Croce, J. Power Sources, 213, 233 (2012). 8. A. Khan and C. Zhao, Electrochem. Commun., 49, 1 (2014). 9. Z. J. Chen, T. Xue and J.-M. Lee, RSC Adv., 2, 10564 (2012).

Vertically-Aligned Carbon Nanotubes on Aluminum Foil As a Light-Weight Positive Electrode for Lithium/Sulfur Semi-Liquid Batteries

Elemental sulfur is a particularly attractive cathode material for lithium batteries. A high theoretical capacity (1675 mAh/gsulfur), along with widespread availability and low cost, makes Li/S one of the most promising battery technologies.1 Sulfur reduction by lithium involves intermediate species called lithium polysulfides Li2Sn (2 ≤ n ≤ 8). Lithium polysulfides are soluble in commonly used electrolyte solvents like glymes, whereas the final species, S8, Li2S2 and Li2S, precipitate at the end of charge or discharge respectively. This unconventional redox process leads to numerous difficulties. The Li/S cells suffer low practical capacity and short cycle life. These shortcomings mainly result from the solubilizing/precipitating cycles of the active material. This process induces strong mechanical strains to composite-type positive electrodes. Morphological changes occur on the electrode upon cycling, which might lower the specific surface area cycle after cycle, and prevent full utilization of sulfur active material.In our study we use a new approach involving a “catholyte” (i.e. cathode material which is dissolved in the electrolyte) in association with a 3D current collector as positive electrode.2 This catholyte is associated with a current collector that comprises a large specific surface area and which is both flexible and robust to accommodate the precipitating/solubilizing cycles of sulfur. To this purpose, vertically-aligned carbon nanotubes (VACNTs) structures were grown on a light and flexible commonly used 20 µm-thick aluminium foil. The VACNTs were fabricated using a catalyst supported chemical vapor deposition (CVD) method. The conditions of CVD process had to be modified to match the low melting point of aluminium (660 °C) and yielded VACNTs as high as 150 µm. The electrochemical properties of VACNTs-on-aluminium current collectors were investigated in a coin-cell configuration. The catholyte was prepared by mixing metallic lithium, sulfur powder and LiTFSI lithium salt into glyme solvents. The influence of VACNTs height on the electrochemical performance of the cells was studied. Optimization of the catholyte yielded capacities over 1000 mAh/gsulfur during prolonged cycling, using relatively high sulfur loading (>2 mgsulfur/cm2). Remarkable capacity retention was also observed, which was mainly explained by the good mechanical stability and flexibility of VACNT substrates. Last but not least, the combination of multiple characterization techniques, such as in situ and operando X-ray diffraction and electrochemical impedance spectroscopy,3also allowed to take a step forward regarding the understanding of Li/S battery technology, its discharge process and failure mechanisms.Figure 1. SEM picture of 30 µm-high VACNTs grown on a 20 µm-thick aluminium substrate.A. Manthiram, Y. Fu, AND Y.-S. Su, Accounts Chemical Research, 46 (2013) 1125C. Barchasz, F. Mesguich, J. Dijon, J.-C. Leprêtre, S. Patoux, F. Alloin, J. Power Sources, 211 (2012) 19S. Waluś, C. Barchasz, J.-F. Colin, J.-F. Martin, E. Elkaïm, J.-C. Leprêtre, F. Alloin, Chem. Commun., 49 (2013) 7899

Stability of superoxide radicals in glyme solvents for non-aqueous Li–O2 battery electrolytes

Glyme-based electrolytes were studied for the use in lithium-air batteries because of their greater stability towards oxygen reduction reaction intermediates (e.g., superoxide anion radicals (O2˙(-))) produced upon discharge at the cathode compared to previously employed carbonate-based electrolytes. However, contradictory results of glyme stability tests employing KO2 as an O2˙(-) source were reported in the literature. For clarification, we investigated the reaction of KO2 with glymes of various chain lengths qualitatively using (1)H NMR and FTIR spectroscopy as well as more quantitatively using UV-Vis spectroscopy. During our experiments we found a huge impact of small quantities of impurities on the stability of the solvents. Therefore, we studied further the influence of impurities in the glymes on the cycling behavior of Li-O2 cells, demonstrating the large effect of electrolyte impurities on Li-O2 cell performance.

Dechlorination of polychlorobiphenyls using NaBH 4 and NaBH 4/LiCl at 120–310°C in glyme solvents

High temperature PCB dechlorination (Aroclor 1016) occurred using NaBH 4 alone in tetraglyme at 290–310°C within 2 h in a sealed tube. Aroclor 1016 dechlorination was also quantitatively achieved using NaBH 4/LiCl/glyme solvents (di-, tri-, or tetraglyme) at 125–135°C. The best results were obtained by prestirring NaBH 4, LiCl and the glyme solvent at room temperature before heating at 125–135°C. At equivalent conditions, PCB dechlorination rates were found to depend on solvent in the order: tetraglyme>triglyme>diglyme. At 130°C, Aroclor 1016 can be dechlorinated in NaBH 4/LiCl/tetraglyme in 4 h. 2-Chlorobiphenyl and 2,2′-dichlorobiphenyl were the least reactive congeners in dechlorinations with NaBH 4/LiCl in diglyme. Competitive dechlorinations with NaBH 4/LiCl in diglyme showed 3-chloro- and 4-chlorobiphenyl reacted faster than 2-chlorobiphenyl at 130°C. The reactions were clean with no solvent decomposition in the range of 120–162°C.

Solution Synthesis and Characterization of Quantum Confined Ge Nanoparticles

A solution synthesis of crystalline Ge nanoparticles (nc-Ge ) is reported. The metathesis reaction of NaGe with excess GeCl4 in glyme solvents produces nc-Ge. Metathesis reactions between KGe and excess GeCl4 or GeCl2:(dioxane) in glyme and Mg2Ge and excess GeCl4 in diglyme and triglyme were also investigated. The surface of these nanoparticles is terminated with alkyl groups by reaction with alkyl Li and Grignard reagents. The alkyl-terminated crystalline Ge nanoparticles (R-nc-Ge) were characterized by Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, powder X-ray diffraction, UV−vis absorption spectroscopy, photoluminescence, and photoluminescence excitation spectroscopy. The optical properties of R-nc-Ge made by this method agree with predictions from quantum confinement models.

The study of electrolyte solutions based on solvents from the “glyme” family (linear polyethers) for secondary Li battery systems

The behaviour of Li electrodes was studied in solutions of dimethoxy ethane (DME), ethyl glyme (EG, CH 3CH 2OCH 2CH 2OCH 2CH 3) and diglyme (DG, CH 3OCH 2CH 2OCH 2CH 2OCH 3). Salts from the LiAsF 6, LiClO 4, LiSO 3CF 3, LiN(SO 2CF 3) 2, LiBF 4, LiBr and LiI list were used as electrolytes and 1–3 Dioxolane (DN), propylene, ethylene and dimethyl carbonates were used as co-solvents. The correlation amongst the surface chemistry, morphology and cycling efficiency of Li electrodes in these solutions was studied using surface sensitive FTIR spectroscopy, electron microscopy (SEM) and chronopotentiometry. Basically, solutions of the above salts in the three glyme solvents tested, as well as those tested in solvent mixtures containing DN or alkyl carbonates with any one of the glyme solvents, do not seem suitable for use in rechargeable Li battery systems having Li metal anodes because the rough morphology of Li upon deposition-dissolution cycling in them leads to low Li cycling efficiency. The key factor in this respect seems to be the surface chemistry developed on Li in these solvents. This point is discussed thoroughly in this paper.

Zur darstellung von permethylierten cyclosilanylalkalimetall-derivaten

A simple way for the preparation of undecamethylcyclohexasilanylalkalimetal compounds is described by the reaction of dodecamethylcyclohexasilane with alkalimetal alcoholats in glyme solvents. Reactions of other cyclosilanes are also discussed. Some reactions of undecamethylcyclohexasilanylpotassium with element-halogen derivates and inorganic acids are described.

Preparation and methylation of alkali metal phosphides

A sodium phosphide characterized as Na 3P by earlier workers has been demonstrated to be the major product resulting from the reaction of white phosphorus with sodium dispersion in inert organic media. Sodium phosphide and the mixed phosphide formed by reacting phosphorus with sodium-potassium alloy react readily with methyl halides in glyme solvents to give varying mixtures of tetramethylphosphonium- and trimethylphosphonium halides. These phosphonium salts have been converted to trimethylphosphine oxide and trimethylphosphine, respectively, to give overall yields of ca. 60 per cent of organophosphorus products under optimum conditions.