I do not see any justification for the additional time. Ray On 11/19/15, 11:14 AM, "[email protected] on behalf of [email protected]" <[email protected] on behalf of [email protected]> wrote:
Hello,
A change in allocation has been requested:
Requester: redfern (Paul Redfern) Project: Redoxshuttles Title: Redox Shuttles and Additives for Lithium-Ion Batteries Description: Over the past year we have calculated solvation free energies of molecular LiO2 and Li2O2 in various solvents were calculated using various explicit and implicit solvent models, as well as ab initio molecular dynamics (AIMD) methods. Best estimates for the solvation energies from these calculations along with calculated lattice energies of Li2O2 and LiO2 were used to determine the solubility of bulk LiO2 and Li2O2. The solubility of LiO2 was found to be about 17 orders higher than that of Li2O2. The effect of finite crystal size of LiO2 and Li2O2 on the solubility was also considered and was found to increase the solubility, although LiO2 is still much more soluble. The difference in solubilities between LiO2 and Li2O2 will likely affect the growth mechanism and resulting morphologies of the products formed during battery discharge, affecting the performance of the battery cell. We also examined production of a lithium superoxide discharge product on the cathod e in a Li-O2 battery. DFT was used to obtain values of material properties not available experimentally such as LiO2 solubility in DME and elastic properties of the particles. The mesoscale model predicts that coarsening, in which large particles grow and small ones disappear, has a substantial effect on the size distribution of the LiO2 particles during the discharge process. The size evolution during discharge is the result of an interplay between this coarsening process as well as growth. The growth through continued deposition of LiO2 has the effect of causing large particles to grow faster and delays the dissolution of small particles. The predicted size evolution is consistent with experimental results for a previously reported cathode material based on activated carbon during discharge and when it is at rest. The model, even without complex microstructure, can capture size and number of LiO2 particles, but not the shape, e.g. toroid formation. Best estimates for the L iO2 solvation energy from these calculations along with a calculated lattice energy of LiO2 were used to determine the solubility of bulk LiO2. The best estimate for the solubility of LiO2 is about 1 mM, although this has very large uncertainties because small differences in the calculated solvation energy leads to large differences in the solubility due to the exponential. For example, a difference of 2 kcal/mol in solvation energy translates to more than 2 orders of magnitude difference in solubility. The solvation energies are difficult to calculate even to 2 kcal/mol in accuracy. This value from density functional calculations is used initially in the model and subsequently a value for solubility is derived from fits to reproduce experimental data. The solubility from these fits is similar to that predicted from the DFT calculations.
We intend to evaluate at least 200 electrolytes for Lithium-O2 batteries. The most important challenge in Lithium-Air battery research is finding stable non-aqueous electrolytes which permit reversible cycling of the cell. A solvent should have a high dielectric constant in order to dissolve Li+, low viscosity for fast Li+ transport, high stability, low vapor pressure, high oxygen solubility, high O2 and superoxide transport properties, and low toxicity. Solvents should partially dissolve lithium oxide species in order to reduce clogging and increase charge current rate. Quantum chemical calculations can help identify reaction pathways which affect reversibility. So far organic solvents such as ethers, amides, lactams, oxazolidinones, phosphorus compounds and nitriles have been screened for Lithium-Air batteries based on susceptibility to attack by superoxide anion and pKa. No correlation between simple molecular descriptors and activation free energies was found. Low electro philicity has also been used to identify potential candidates, since they would be less susceptible to attack by superoxide or other anion. Other solvents such as sulfones, sulfoxides and ionic liquids have been examined. Lithium salts like LiTFSI can decompose at the lithium electrode and the resulting CF3 radicals can abstract hydrogens from solvents like ethers. Hydrogens on carbons adjacent to heteroatoms (e.g. O, N, F) are more labile due to the anomeric effect. Superoxide anion can abstract protons from proton source impurities (e.g. water) leading to formation of the strong base HOO- via disproportionation which can then react as a nucleophile with the solvent. Superoxide was found not to react with TEGDME ether (Nazar). We will use hydrogen and proton abstraction from and nucleophilic attack by a strong base on the electrolyte along with advanced descriptors (e.g. Fukui functions, hardness and chemical potential) to screen for stable solvents.
Oxygen reduction in the aprotic Li-O2 battery takes place at the boundary between the electrolyte and the cathode so oxygen must dissolve and diffuse in the electrolyte. Unfortunately, aprotic solvents are inadequate in this regard so oxygen enriching materials must be developed. Oxygen enriching materials that have been examined include cobalt porphyrin, iron phthalocyanine, artificial hemoglobin, membranes and perfluorochemicals (PFCs). PFCs have high oxygen solubility, low surface tension and viscosity to enhance O2 diffusivity and Li+ transference number as well as wettability of the cathode surface, high chemical and thermal stability, hydrophobicity and low flammability. In the Li-O2 battery the PFC must be miscible with the polar organic solvent and resistant to nucleophilic attack by superoxide anion radical or HOO-. Miscibility of PFCs with polar organic solvents is increased if LiPFOS is used instead of LiTFSI. Increasing O2 pressure increases O2 diffusivity as well as solubility. We will examine many PFCs for susceptibility to nucleophilic attack by superoxide anion radical and HOO- using DFT calculations. Ab initio calculations will also be done to improve our understanding of interactions between PFCs and O2. Other oxygen enriching materials include reversible oxygen carriers like perfluoro cryptands and crown ethers which are known to have high oxygen carrying capacities, while acyclic perfluoro ethers do not bind O2. DFT calculations will be used to examine perfluoro cryptands and crown ethers. Hybrid ionic liquid fluoro-organic solvent mixtures are also promising as they combine the stability to anion attack of ionic liquids with the high oxygen transport and safety of nonflammable fluoro-organics. We intend to use Gaussian 09 for quantum chemical calculations. In Gaussian 09 the DFT calculations scale well up to 64 processors, however the high-level ab initio methods scale only up to 16 processors. Therefore these calculations do require a longer execution time.
Current: undetermined amount Justification:
Requested: 100000
A specific reason has been given: ran out of time
This needs to be approved and the final allocation amount decided upon.
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