Browsing by Author "Peter S. Fedkiw, Committee Chair"

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Fumed oxide-based nanocomposite polymer electrolytes for rechargeable lithium batteries
(2003-03-18) Zhou, Jian; Peter S. Fedkiw, Committee Chair; Saad A. Khan, Committee Member; Daniel L. Feldheim, Committee Member; John H. van Zanten, Committee Member
Rechargeable lithium batteries are promising power sources for portable electronic devices, implantable medical devices, and electric vehicles due to their high-energy density, low self-discharge rate, and environmentally benign materials of construction. However, the high reactivity of lithium metal limits the choice of electrolytes and impedes the commercialization of rechargeable lithium batteries. One way to tackle this problem is to develop electrolytes that are kinetically stable with lithium. Composite polymer electrolytes (CPEs) based on fumed oxides presented in this work are promising candidates for rechargeable lithium batteries. The effects of fumed oxides (SiO2, Al2O3, TiO2) and binary mixtures of oxides (SiO2/Al2O3) on ionic conductivity of CPEs based on poly(ethylene oxide) (PEO) oligomers (Mw =250, 200, 1000, and 2000) + lithium bis(trifluromethylsulfonyl)imide [LiN(CF3SO2)2] (LiTFSI) (Li:O=1:20) are studied using electrochemical impedance spectroscopy (EIS), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy. Fillers show similar effect on conductivity in all systems: no distinguishable effect is found with filler type, and addition of filler decreases conductivity at temperatures above the melting point (Tm) but increases conductivity at temperatures below. The insulating nature of fillers and stiffening of the polymer solvent (as evidenced by FTIR and DSC data) in the presence of fillers cause a decrease in conductivity at temperatures above Tm, which remains constant upon addition of fillers. The increase in conductivity at temperatures below Tm can be attributed to faster ion transport along the filler surface. Addition of fumed oxides increases electrolyte viscosity (and elasticity) and the extent of enhancement varies with filler type: fumed silica shows the strongest and titania the least. Elastic modulus, yield stress, and normalized viscosity of gel-type composite electrolytes decrease with increasing oligomer Mw when electrolytes are amorphous. The reduction in structure strength may be ascribed to the enhanced interactions between surface hydroxyl groups on fumed oxides and polyether oxygens. Thus, the number of accessible ?OH groups is reduced for interactions among fumed oxide particles, which dictates the strength of solid-like structure. The interfacial stability between electrolyte and lithium is enhanced in the presence of fumed silica. The enhancement in interfacial stability is seen as a decrease in interfacial resistance and cell polarization, and an increase in lithium cycleability and cell capacity. The improved interfacial stability between CPE and lithium is attributed to less lithium corrosion (fillers scavenge water impurities that corrode lithium) and dendrite formation (electrolyte elasiticty inhibits dendrite formation). The extent of the enhancing effect of fumed silica depends on its surface chemistry, with the largest effect seen with hydrophilic fumed silica, which has the largest scavenging capacity and highest elasticity. The effect on cycle capacity is reported of cathode material (metal oxide, carbon, and current collector) in lithium/metal oxide cells cycled with fumed silica-based composite electrolytes. Cells with composite electrolytes show higher capacity and less cell polarization than those with filler-free electrolyte. Among the three active materials studied (LiCoO2, V6O13, and LixMnO2), V6O13 cathodes deliver the highest capacity and LixMnO2 cathodes render the best capacity retention. Discharge capacity of Li/LiCoO2 cells is affected greatly by cathode carbon type and discharge capacity increases with decreasing carbon particle size. Current collector materials also play a significant role in cell cycling performance: Li/V6O13 cells deliver increased capacity using Ni foil and carbon fiber current collectors in comparison to an Al foil. In summary, fumed oxide-based nanocomposite electrolytes are promising candidates for lithium battery applications with high room-temperature conductivity, good mechanical strength, stable interface between lithium metal and electrolytes, and reasonable capacity and capacity retention with optimized cathode compositions.
Hectorite-Based Nanocomposite Electrolytes for Lithium-Ion Batteries
(2002-04-29) Riley, Michael William; Benny D. Freeman, Committee Member; Robert A. Osteryoung, Committee Member; Peter S. Fedkiw, Committee Chair; Saad A. Khan, Committee Co-Chair
Rechargeable lithium-ion batteries are becoming an increasingly important technology for energy storage due to their high-energy density and low self-discharge rates compared to batteries. However, issues of reliability, safety, and cycle life among others hamper their acceptance as an energy storage medium for applications beyond portable electronics. Electrolyte concentration polarization becomes a problem at high-discharge rates, making them unsuitable for applications requiring high power such as electric vehicles. Hectorite clay is presented in this work as a promising component for electrolytes for lithium-ion batteries. This negatively-charged, plate-shaped (250 nm diameter by 1 nm thickness) clay has exchangeable cations for which lithium may be substituted. When properly dispersed in high-dielectric solvents such as the carbonates (ethylene carbonate and propylene carbonate) typically used in lithium-ion cells, a shear-thinning physical gel is created possessing a good conductivity (as high as 2×10-4 S⁄cm at room temperature has been measured) with near unity lithium-ion transference numbers. As a result, electrolytes designed around the clay could drastically reduce concentration polarization and possibly present an inherently safer electrolyte as toxic salts such as LiPF6 that are typically used could be eliminated. Hectorite clay dispersions in aqueous and non-aqueous (1:1 (v:v) ethylene carbonate: poly(ethylene)glycol dimethyl ether 250 MW) solvents have been studied using dynamic and steady rheology, conductivity, and TEM imaging to examine their microstructures and recovery after shear deformation. Two different particle size clays (25 nm and 250 nm) were included in the study. The aqueous dispersions show a highly-exfoliated microstructure (fractal dimension, Dƒ=1.6) created primarily through electrostatic repulsive forces which recovers after shear deformation through reorientation of the clay platelets. The nonaqueous dispersions form gel structures at higher concentrations than the aqueous dispersions with a much higher degree of aggregation (Dƒ= 2.5), and recovery after shear deformation appears to be an aggregation controlled process as well. The use of two different particle size clays (25 and 250 nm diameter) reveals that particle size of the clay platelets does not have a significant impact on the gel modulus, fractal dimension, or recovery after shear deformation, although conductivity measurements indicate a higher degree of aggregation with the smaller clay platelets. TEM imaging of non-aqueous clay dispersions at low magnification shows the clay to be uniformly distributed, while high magnification shows that the platelets exist in aggregates of approximately 5 layers. Use of the single-ion conducting hectorite-based electrolytes in lithium-ion cells requires an electrode that contains a single-ion conductor in the typically porous structure. Cathodes based on LiCoO2 that contain various lithium-conducting species (lithium hectorite, lithium Laponite®, and lithium-exchanged NAFION®) have been studied in conjunction with lithium metal anodes. Performance was compared to that of cells with a standard liquid electrolyte (i.e., LiPF6 + 1:1 w⁄w ethylene carbonate:ethylmethyl carbonate). Effects on cathode capacity were examined for these variables: hot-press force used in construction of the porous cathode, carbon type (graphite vs. carbon black), and clay particle size. AC impedance spectroscopy was used to probe the cells and equivalent circuits were used to model the physical processes that occur. Cathodes containing 4 wt. % lithium hectorite + 3 wt. % lithium-exchanged NAFION® + 3 wt. % carbon black exhibit capacities approximately 90 mAh⁄g LiCoO2 compared to that observed in a standard cell of 110 mAh⁄g LiCoO2. These hectorite-based electrolytes and clay-containing cathodes are potentially attractive for use in single-ion conducting lithium-ion batteries designed for high-discharge applications.
Transport Properties of Lithium Bis(Oxalato)Borate-based Electrolyte for Lithium-ion Cells
(2005-11-18) Azeez, Fadhel Abbas; Richard J. Spontak, Committee Member; Saad A. Khan, Committee Member; Peter S. Fedkiw, Committee Chair
The need for compact, light weight rechargeable batteries offering high-energy densities has become necessary in the 21[superscript st] century especially for portable electronics devices, hybrid electric vehicles, and load leveling in electric power generation/distribution. Among rechargeable batteries, lithium-based systems seem to be able to fulfill these needs. The current state-of-art electrolyte of LiPF₆ dissolved in organic-carbonate solvents has disadvantages in low-temperature and high-temperature environments. At high temperature, the thermal instability of LiPF₆ is believed to be the main cause for the poor performance of lithium-ion batteries. At low temperature, the high viscosity of ethylene carbonate, which is a major component in the solvent mixture of state-of-art electrolyte, restricts the use of electrolyte to above -20 °C. These factors restrict the operation of lithium-ion batteries to be between -20 and 60 °C. In an attempt to improve the performance of lithium-ion cells, we use a stable salt at high temperature, Lithium bis(oxalato)borate (LiBOB), and dissolve it in mixtures of γ-butyrolactone (GBL), ethyl acetate (EA), and ethylene carbonate(EC). The conductivity and viscosity are measured for LiBOB in such mixtures as function of salt concentration, solvent composition, and temperature. We find that LiBOB in a mixture of GBL + EA + EC yields a technologically acceptable conductivity, and it is an acceptable candidate for lithium-ion cells. For example, LiBOB based-electrolyte with a salt concentration of 0.7 M LiBOB in a GBL: EA: EC (wt ) composition of 1:1:0 has a conductivity ~6 mS cm⁻185; at -3 °C, and at 1 M LiBOB in solvent composition of 1:1:0.1, the conductivity is ~22 mS cm⁻¹ at 74 °C. The product of conductivity with viscosity was essentially independent of temperature but was dependent on solvent composition. Results from this study encourage us to examine in future studies the performance of full and half cells using LiBOB-based electrolyte to see if it can be used in lithium-ion cells.