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Pasadena, CA, United States

Superoxide is a strong base that can induce base-catalyzed autoxidation of weakly acidic solvents. We report on the performance of several computational protocols for predicting pKa values for a wide range of aliphatic C-H acids in DMSO. Calculations at the MP2/CBS level with CCSD(T)/aug-cc-pVDZ corrections and solvent effects calculated using the SVPE model provide the best overall performance (rms deviation is 0.65 pKa). The B3LYP, M06, and M06-2X functionals can also achieve high accuracy (<1 pKa) by employing empirical corrections to fit the experimental data. Computational results provide a convenient means of screening for suitable solvents in Li-air batteries. © 2012 Elsevier B.V. All rights reserved.

Islam M.M.,Pennsylvania State University | Bryantsev V.S.,Liox Power Inc. | Van Duin A.C.T.,Pennsylvania State University
Journal of the Electrochemical Society | Year: 2014

Lithium-sulfur batteries are amongst the most appealing choices for the next generation large-scale energy storage applications. However, these batteries still suffer several formidable performance degradation issues that impede its commercialization. The lithium negative electrode yields high anodic capacity, but it causes dendrite formation and raises safety concerns. Furthermore, the high reactivity of lithium is accountable for electrolyte decomposition. To investigate these issues and possible countermeasures, we used ReaxFF reactive molecular dynamics simulations to elucidate anode-electrolyte interfacial chemistry and utilized an ex-situ anode surface treatment with Teflon coating. In this study, we employed Li/SWCNT (single-wall carbon nanotube) composite anode instead of lithium metal and tetra(ethylene glycol) dimethyl ether (TEGDME) as electrolyte.We find that at lithium rich environment at the anode-electrolyte interface, electrolyte dissociates and generates ethylene gas as a major reaction product, while utilization of Teflon layer suppresses the lithium reactivity and reduces electrolyte decomposition. Lithium discharge from the negative electrode is an exothermic event that creates local hot spots at the interfacial region and expedites electrolyte dissociation reaction kinetics. Usage of Teflon dampens initial heat flow and effectively reduces lithium reactivity with the electrolyte. © 2014 The Electrochemical Society.

Solvation effects play a major role in determining the cycling characteristics of the non-aqueous rechargeable Li-air battery. We use a mixed cluster/continuum solvent model with varying number of explicit solvent molecules (n = 4-10) to calculate the solvation free energies (ΔG solv *) of Li + and O 2 - ions and neutral LiO 2, Li 2O 2, LiO, and Li 2O species in acetonitrile solvent. Calculations for complexes with the full first solvation shell around Li + (n = 4) and O 2 - (n = 8) show excellent agreement with the solvation free energies obtained using the cluster pair approximation (the error is below 2.0 kcal/ mol). The use of the pure continuum model fitted to reproduce the experimental values of ΔG solv *(Li +) and ΔG solv *(O 2 -) gives the solvation free energies of various lithium-oxygen species (LixOy; x, y = 1, 2) that are in excellent agreement with the results obtained using mixed cluster/continuum models (n ≥ 8). This provides a theoretical framework for including solvent effects in the theoretical models of oxygen reduction and evolution reactions in the aprotic Li-air battery. © Springer-Verlag 2012.

High capacity alkali metal/oxygen batteries, e.g. Li/O

Bryantsev V.S.,Liox Power Inc. | Faglioni F.,Liox Power Inc. | Faglioni F.,University of Modena and Reggio Emilia
Journal of Physical Chemistry A | Year: 2012

Finding suitable solvents remains one of the most elusive challenges in rechargeable, nonaqueous Li-air battery technology. Although ether and amides are identified as stable classes of aprotic solvents against nucleophilic attack by superoxide, many of them are prone to autoxidation under oxygen atmosphere. In this work, we use density functional theory calculations coupled with an implicit solvent model to investigate the autoxidative stability of ether- and N,N-dialkylamide-based solvents. The change in the activation free energy for the C-H bond cleavage by O2 is consistent with the extent of peroxide production for each class of solvent. Conversely, the thermodynamic stability alone is not sufficient to account for the observed variation in solvent reactivity toward O2. A detailed understanding of the factors influencing the autoxidative stability provides several strategies for designing molecules with enhanced air/O2 stability, comparable or superior to that of structurally related hydrocarbons. The mechanism of superoxide-mediated oxidation of hydroperoxides derived from ethers and amides is presented. The degradation mechanism accounts for the primary decomposition products (esters and carboxylates) observed in the Li-air battery with ether-based electrolytes. The identification of solvents having resistance to autoxidation is critical for the development of rechargeable Li-air batteries with long cycle life. © 2012 American Chemical Society.

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