Ken Tasaki

Solubility of Lithium Salts Formed on the Lithium-Ion Battery Negative Electrode Surface in Organic Solvents

The solubility of lithium salts in dimethyl carbonate (DMC) found in solid electrolyte interface (SEI) films was determined. The salt-DMC solutions evaporated, and the salts were transferred into water for ion conductivity measurements. The salts examined included lithium carbonate (Li 2 CO 3 ), lithium oxalate [(LiCO 2 ) 2 ], lithium fluoride (LiF), lithium hydroxide (LiOH), lithium methyl carbonate (LiOCO 2 ,CH 3 ), and lithium ethyl carbonate (LiOCO 2 C 2 H 5 ). The salt molarity in DMC ranged from 9.6 X 10- 4 mol L -1 (LiOCO 2 CH 3 ) to 9 X 10 -5 mol L -1 (Li 2 CO 3 ) in the order of LiOCO 2 CH 3 > LiOCO 2 C 2 H 5 > LiOH > LiF > (LiCO 2 ) 2 > Li 2 CO 3 - X-ray photoelectron spectroscopy measurements on SEI films on the surface of the negative electrode taken from a commercial battery after soaking in DMC for 1 h suggested that the films can dissolve. Separately, the heat of dissolution of the salts was calculated from computer simulations for the same salts, including lithium oxide (Li 2 O), lithium methoxide (LiOCH 3 ), and dilithium ethylene glycol dicarbonate [(CH 2 OCO 2 Li) 2 :LiEDC] in both DMC and ethylene carbonate (EC). The results from the computer simulations suggested that the order in which the salt was likely to dissolve in both DMC and EC was LiEDC > LiOCO 2 CH 3 > LiOH > LiOCO 2 C 2 H 5 > LiOCH 3 > LiF > (LiCO 2 ) 2 > Li 2 CO 3 > Li 2 O. This order agreed with the experiment in DMC within the experimental error. Both experiment and computer simulations showed that the organic salts are more likely to dissolve in DMC than the inorganic salts. The calculations also predicted that the salts dissolve more likely in EC than in DMC in general. Moreover, the results from the study were used to discuss the capacity fading mechanism during the storage of lithium-ion batteries.

Decomposition of LiPF6and Stability of PF 5 in Li-Ion Battery Electrolytes Density Functional Theory and Molecular Dynamics Studies

The decomposition of LiPF 6 and the stability of PF 5 in organic solvents, diethyl carbonate (DEC), dimethyl carbonate (DMC), 7-butyrolactone (GBL), and ethylene carbonate (EC), have been investigated through density functional theory (DFT) calculations, in which solvent was modeled as a dielectric continuum, and also by molecular dynamics (MD) simulations which treated solvents explicitly. Both calculations showed a similar trend in which the decomposition was further promoted in more polar solvents, yet the DFT calculations predicted an endothermic decomposition, while the MD simulations indicated exothermic. This sharp contrast in the results suggests strong solute-solvent interactions, especially for PF 5 , which were not accounted for in the DFT calculations. The specific interaction between PF 5 and solvent was further investigated by DFT calculations for adduct models and also by the MD simulations for solutions. Both calculations suggest a stable formation of a PF 5 -solvent adduct in solution and its stability depends on the solvent. It was found that PF 5 is more stabilized in polar and sterically compact solvents such as EC and GBL than in less polar and bulky, linear carbonates such as DMC and DEC. The reactivity of PF 5 with organic solvents and the difference in the stability of LiPF 6 between organic and aqueous solution are also discussed.

Theoretical Studies on the Reductive Decompositions of Solvents and Additives for Lithium-Ion Batteries near Lithium Anodes

The reduction reactions of various solvents and additives for lithium-ion battery electrolytes have been investigated by ab initio calculations in the presence of the lithium anode at the Hartree-Fock level. The solvents and the additives studied included ethylene carbonate, propylene carbonate, vinylene carbonate, vinylethylene carbonate, ethylene sulfite, and tetrahydrofuran. Vinyl vinylene carbonate was also examined as a model compound. A cluster of 15 lithium atoms was used to model a lithium electrode to explicitly include the interactions of the solvent molecules with the lithium metal. The transition state in the reduction reaction was determined for each solvent or additive by searching for the saddle point of the potential energy surface. Internal reaction coordinate calculations were performed both to verify the transition state and also to locate the reactant and the product of the reduction reaction. From the calculations, the activation barrier and the reaction energy were determined. The reaction mechanism was also discussed in terms of the solid electrolyte interface film formation characteristics of each solvent and additive.

Computational Study of Salt Association in Li-Ion Battery Electrolyte

The salt association of LiPF 6 has been investigated through molecular dynamics (MD) simulations in a variety of solvents: ethylene carbonate, propylene carbonate (PC), γ-butyrolactone, dimethyl carbonate, ethyl methyl carbonate (EMC), diethyl carbonate, a 1:1 mixture of PC and 1,2-dimethoxyethane, a 1:1 mixture of PC and methyl propionate, and a 1:1 mixture of PC and EMC. The degree of salt association for each electrolyte system was estimated using the pair distribution function of Li + and the anion calculated from MD simulation trajectories. The predicted values for the salt association factor of LiPF 6 showed a comparable trend to the experimental association constants. The same approach was applied to examine the salt association in the presence of a series of crown ethers: 9-crown-3, 12-crown-4, 13-crown-4, and 15-crown-5 as the Li + trapping agents. The results demonstrated that the crown ethers were effective in separating the Li + ion from the PF 6 anion, and the ability depended on the size of the crown ether; 15-crown-5 had the largest effect on destabilizing the ion association. The method presented here may be used to screen new solvents or new salts for Li rechargeable batteries. The strength of the current method is that it can he applied to any salt, any solvent, and any mix of solvents.

Sustainable conversion of coffee and other crop wastes to biofuels and bioproducts using coupled biochemical and thermochemical processes in a multi-stage biorefinery concept

The environmental impact of agricultural waste from the processing of food and feed crops is an increasing concern worldwide. Concerted efforts are underway to develop sustainable practices for the disposal of residues from the processing of such crops as coffee, sugarcane, or corn. Coffee is crucial to the economies of many countries because its cultivation, processing, trading, and marketing provide employment for millions of people. In coffee-producing countries, improved technology for treatment of the significant amounts of coffee waste is critical to prevent ecological damage. This mini-review discusses a multi-stage biorefinery concept with the potential to convert waste produced at crop processing operations, such as coffee pulping stations, to valuable biofuels and bioproducts using biochemical and thermochemical conversion technologies. The initial bioconversion stage uses a mutant Kluyveromyces marxianus yeast strain to produce bioethanol from sugars. The resulting sugar-depleted solids (mostly protein) can be used in a second stage by the oleaginous yeast Yarrowia lipolytica to produce bio-based ammonia for fertilizer and are further degraded by Y. lipolytica proteases to peptides and free amino acids for animal feed. The lignocellulosic fraction can be ground and treated to release sugars for fermentation in a third stage by a recombinant cellulosic Saccharomyces cerevisiae, which can also be engineered to express valuable peptide products. The residual protein and lignin solids can be jet cooked and passed to a fourth-stage fermenter where Rhodotorula glutinis converts methane into isoprenoid intermediates. The residues can be combined and transferred into pyrocracking and hydroformylation reactions to convert ammonia, protein, isoprenes, lignins, and oils into renewable gas. Any remaining waste can be thermoconverted to biochar as a humus soil enhancer. The integration of multiple technologies for treatment of coffee waste has the potential to contribute to economic and environmental sustainability.

Computer Simulation of LiPF6 Salt Association in Li-Ion Battery Electrolyte in the Presence of an Anion Trapping Agent

The association of LiPF 6 has been studied through molecular dynamics simulations in ethylene carbonate (EC) in the presence of a series of cyclic and linear aza ethers with the CF 3 SO 2 group attached to the nitrogen atom. The degree of salt association has been calculated according to the method previously developed in order to evaluate the aza ethers ability to promote the salt dissociation. The aza ethers studied in this report generally exhibited a good performance in separating the PF - 6 anion from the Li + cation in the solvent EC, regardless of the size of the aza ether. Examination of the size and the structure effects of the aza ethers on their performance revealed that the larger the size of the aza ether is, the stronger the ability to dissociate the salt becomes up to n = 8, n being the number of the nitrogen atoms in the aza ether. Furthermore, a cyclic aza ether is more effective than the linear counterpart in general since the latter tends to he dissolved into the solvent, thus losing the structural integrity for anion trapping. The correlation between the binding energy of the aza ether with the anion and the electric conductivity of the electrolyte in the presence of the aza ether has also been observed. The binding energy may be used as a screening tool for potential trapping agents.

New Insight into Differences in Cycling Behaviors of a Lithium-ion Battery Cell Between the Ethylene Carbonate- and Propylene Carbonate-Based Electrolytes

Density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations have been performed to gain insight into the difference in cycling behaviors between the ethylene carbonate (EC)-based and the propylene carbonate (PC)-based electrolytes in lithium-ion battery cells. DFT calculations for the ternary graphite intercalation compounds (Li+(S)iCn: S=EC or PC), in which the solvated lithium ion Li+(S)i (i=1~3) was inserted into a graphite cell, suggested that Li+(EC)iCn was more stable than Li+(PC)iCn in general. In addition, MD simulations were carried out to examine the solvation structures at a high salt concentration: 2.45 mol kg-1. The results showed that the solvation structure was significantly interrupted by the counter anions, having a smaller solvation number than that at a lower salt concentration (0.83 mol kg-1). The results from both DFT calculations and MD simulations are consistent with the recent experimental observations.

Hydrogen Cyano Fullerene Derivatives as Acid for Proton Conducting Membranes

The proton conducting nature of hydrogen tricyano fullerene has been characterized for applications in fuel cells. The fullerene alone did not show much proton conductivity itself. However, when mixed in a polymer, it exhibited a high conductivity under low humidity: 10 -2 S cm -1 under the 20% relative humidity. The conductivity was enhanced as the dispersion of the fullerene was improved in the polymer matrix through two fullerene dispersants: C 60 [N(CH 2 CH 2 O) 11 CH 3 ] 2 and C 60 [C 6 H 4 O(CH 2 CH 2 O) 3 CH 3 ] 5 . The computer modeling has been performed to gain insight into the proton conducting mechanism.

Carbonaceous Materials as Proton Conductors I. Fullerenes and Polyaromatic Hydrocarbons

Various levels of electronic structure calculations were performed to examine the proton conductive nature of a series of polyaromatic hydrocarbons (PAHs), with a mixture of hexagon and pentagon rings, including C 60 for which the proton transfer activation barriers were calculated for the first time. The results showed that naphthalene has the lowest proton transfer activation barrier, 0.38 eV at the B3LYP/6-31+G(2d,p)//B3LYP/6-31+G(2d,p) level, among the PAHs investigated, and the calculated barriers leveled off around 1.1 eV once the number of electrons in the system reached 20. Examination of the structures revealed that there is a close relationship between the activation barrier and the dihedral angle of the transition or the equilibrium state of the PAH. Based on the extensive calculations of various sizes of PAHs, our estimated values for the intra- and intermolecular proton-hopping activation barrier of C 60 are less than 1.2 and 0.3 eV, respectively, the latter of which is lower than many known proton-conducting systems, suggesting C 60 to be an excellent proton conductor even without chemical functionalization. The activation barriers are discussed in terms of the structural characteristics of PAHs.