Hung Sui Lee

The Compatibility of a Boron-Based Anion Receptor with the Carbon Anode in Lithium-Ion Batteries

The electrochemical stability of the boron anion receptor, tris(pentafluorophenyl) borane (TPFPB), on a carbonaceous mesocarbon microbeads (MCMB) electrode and its compatibility with the passivated solid electrolyte interphase (SEI) layer on the carbon anode were investigated. Comparison of the irreversible capacity loss of the MCMB electrode during initial galvanostatic cycling using electrolyte with and without the TPFPB additive indicates excellent electrochemical stability of TPFPB on the carbonaceous electrode. Cyclic voltammetry studies show that the SEI layer on the surface of the carbon electrode can be formed through the decomposition of ethylene carbonate in the presence of TPFPB. Prolonged cycling test verifies the long-term stability of the SEI layer on carbon in the presence of the TPFPB additive. The SEI layer is not dissolved by TPFPB even after heat-treatment under conditions which dissolve LiF salt. This suggested a cross-linked structure for the SEI layer on carbon electrode. A Li-ion cell using an electrolyte containing TPFPB displays better cycling performance than a cell without TPFPB under the same conditions.

Temperature dependence of ion pairing of a potassium salt in nonaqueous liquid and polymer electrolytes: X‐ray absorption studies

Near‐edge x‐ray absorption fine structure spectroscopy was used to study the effect of temperature on ion pairing of a potassium salt in a modified carbonate (MC3) solution and in a poly(ethylene oxide)‐potassium salt complex that used MC3 as a plasticizer. The modified carbonate was made by attaching three ethylene oxide units to the four position of ethylene carbonate. Spectra were obtained, at the K edge of potassium, over the temperature range of 25–110u2009°C. Studies of reference systems showed a correlation between ion pairing and white line splitting in the near‐edge region of the spectra. The degree of white line splitting was used as a qualitative indicator of the degree of ion pairing as a function of temperature. The results indicate that, in both systems, the number of ion pairs increases with increasing temperature.

Studies of LiMn2O4 Capacity Fading at Elevated Temperature Using In Situ Synchrotron X-Ray Diffraction

Using in situ X-ray diffraction, the capacity fading mechanism of LiMn{sub 2}O{sub 4} under various conditions has been studied. The capacity fading can be monitored by the structural changes during cycling. At the beginning of cycling, the structural changes closely follow the charge-discharge curve: the main structure starts in cubic I, transfers to cubic II, and ends as cubic III during charge, and reverses the course during discharge. These phase transitions track the charging and discharging curves with small hysteresis. However, with an increased amount of capacity fading during cycling, the structural changes deviate from the charge-discharge curve: Bragg peaks representing cubic I and II remain present at the end of charge, when the whole cathode should have been transferred to cubic III. Similarly, the residues of cubic II and III were observed at the end of discharge. The amount of residues increases with increasing capacity fading by cycling at 55 C, overcharging to 5.2 V, or after multiple cycling at normal condition. Adding a tris(pentafluorophenyl) borane (TPFPB) compound in electrolyte can reduce the residues and washing the cathode with solvent can partially restore the lost capacity in the subsequent cycling, showing the important role of electrolyte decomposition in capacitymorexa0» fading.«xa0less

X-Ray Absorption Studies of Poly(vinylferrocene) Polymers for Anion Separation

X-ray absorption spectroscopy (XAS) was used to study the electrochemical incorporation of perrhenate anion, from a 0.1 M NH 4 ReO 4 solution, into poly(vinylferrocene) (PVFc) and a modified PVFc (a copolymer of 30% t-butyl acetylferrocene and 70% t-butyl divinyl ferrocene). The polymers were deposited on a carbon cloth current collector from a solution of the polymers in CH 2 Cl 2 . In situ XAS measurements were done at the Fe K edge on the reduced polymers and at 0.9 V vs. Ag/AgCl. Ex situ XAS was done at the Re L 3 edge after oxidation of the polymers at 0.9 V. The oxidized electrodes were washed in water to remove dissolved NH4ReO4 in the electrode pores. XAS was done both on wet-washed electrodes and on electrodes that were dried. XAS showed that at 0.9 V the Fe was oxidized from a ferrocene to a ferrocenium moiety and the Fe--C bond distance increased from 2.05 to 2.08 A. Both the X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure (EXAFS) results are consistent with having >75% of the ferrocene moieties in the polymer oxidized at 0.9 V. The Fe K-edge EXAFS showed no direct indication of interaction of Fe with ReO - 4 . At the Re L 3 edge the only indication of interaction of ReO - 4 with the polymers was a slight change in the XANES features.

Preferential Solvation of Lithium Cations and Impacts on Oxygen Reduction in Lithium-Air Batteries.

The solvation of Li+ with 11 nonaqueous solvents commonly used as electrolytes for lithium batteries was studied. The solvation preferences of different solvents were compared by means of electrospray mass spectrometry and collision-induced dissociation. The relative strength of the solvent for the solvation of Li+ was determined. The Lewis acidity of the solvated Li+ cations was determined by the preferential solvation of the solvent in the solvation shell. The kinetics of the catalytic disproportionation of the O2•- depends on the relative Lewis acidity of the solvated Li+ ion. The impact of the solvated Li+ cation on the O2 redox reaction was also investigated.

Studies of LiMn[sub 2]O[sub 4] Capacity Fading at Elevated Temperature Using In Situ Synchrotron X-Ray Diffraction

Using in situ X-ray diffraction, the capacity fading mechanism of LiMn{sub 2}O{sub 4} under various conditions has been studied. The capacity fading can be monitored by the structural changes during cycling. At the beginning of cycling, the structural changes closely follow the charge-discharge curve: the main structure starts in cubic I, transfers to cubic II, and ends as cubic III during charge, and reverses the course during discharge. These phase transitions track the charging and discharging curves with small hysteresis. However, with an increased amount of capacity fading during cycling, the structural changes deviate from the charge-discharge curve: Bragg peaks representing cubic I and II remain present at the end of charge, when the whole cathode should have been transferred to cubic III. Similarly, the residues of cubic II and III were observed at the end of discharge. The amount of residues increases with increasing capacity fading by cycling at 55 C, overcharging to 5.2 V, or after multiple cycling at normal condition. Adding a tris(pentafluorophenyl) borane (TPFPB) compound in electrolyte can reduce the residues and washing the cathode with solvent can partially restore the lost capacity in the subsequent cycling, showing the important role of electrolyte decomposition in capacitymorexa0» fading.«xa0less

A study of the Zn/V{sub 6}O{sub 13} secondary battery

Galvanostatic measurements, intermittent titration experiments, and X-ray diffraction analysis indicate that the cathode reaction mechanism in a nonaqueous Zn/V{sub 6}O{sub 13} secondary battery is an intercalation process. The measurements also yielded kinetic parameters such as the chemical diffusion coefficient and mobility of Zn in V{sub 6}O{sub 13} as well as the partial conductivities and mobilities of electrons and ions. Cycling tests confirmed that the reversibility and stability of the cell were good.

X-ray absorption studies on organo-disulfide redox cathodes

We have measured the near-edge x-ray absorption fine structure (NEXAFS) spectra of the K-edge of sulfur in organo-disulfide redox polymeric electrodes in both charged and discharged states. The formation and scission of S-S bonding during the charge-discharge cycle were observed through NEXAFS spectroscopy. 4 refs., 2 figs.

Nickel-metal hydride and lithium-ion batteries for hev applications: performance, limitations and recommendations for improvement

Nickel-metal hydride and lithium-ion batteries are the two contenders for hybrid electric vehicle HEV applications. Early entrants such as the Honda Insight and the Toyota Prius use nickel-metal hydride batteries. Lithium-ion batteries with either modified LiNiO2 or LiMn2O4 spinel cathodes are being considered for the next generation HEV. The main advantage of metal hydride batteries is that there is a built in electrochemical mechanism for balancing cells in series and accommodating overcharge. Other advantages are safety and ease of scale-up to large sizes. The major drawback is limited cycle life due to pulverization and corrosion of the metal hydride electrodes. Lithium-ion batteries with the required power for HEV can be designed and built. However, the batteries loose power capability, particularly when cycled or stored at elevated temperatures. This appears to be mostly due to the buildup of electrolyte decomposition products on the surface of the cathodes.

Langmuir-Blodgett films of electroactive polymers

Electroactive Langmuir-Blodgett (LB) films have been assembled from mixtures of poly(3-alkyl thiophenes) and stearic acid. Stable monolayers and highly ordered multilayers were formed with varying alkyl chain lengths from four carbons to eighteen carbons. X-ray diffraction revealed that the multilayers form well-defined two-dimensional structures. X-ray absorption experiments revealed that the polythiophene backbone is disordered in a matrix of highly ordered and vertically oriented stearic acid molecules. With long (eighteen carbon) alkyl chains, the chains were oriented along the stearic acid molecules.<<ETX>>