Browsing by Author "Azeez, Fadhel Abbas"

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    Lithium bis(Oxalato)Borate-Based Electrolyte for Lithium-Ion Cells
    (2010-01-12) Azeez, Fadhel Abbas; Professor Orlin D. Velev, Committee Member; Professor Xiangwu Zhang, Committee Member; Professor Saad A. Khan, Committee Member; Professor Peter S. Fedkiw, Committee Chair
    Compact, light weight rechargeable batteries offering high-energy densities has become necessary in the 21st century especially for application such as portable electronics devices, hybrid electric vehicles, and load leveling in electric power generation/distribution. Among rechargeable batteries, lithium-based systems seem able to fill these needs. The state-of-art electrolyte for Li-ion batteries of LiPF6 dissolved in organic-carbonate solvents has disadvantages in low- and high-temperature environments. At high temperature, the thermal instability of LiPF6 is believed to be the main cause for poor performance of lithium-ion batteries. At low temperature, the high viscosity of ethylene carbonate, which is a major component in the solvent mixture, restricts use to above -20 oC. These factors limit the operation of lithium-ion batteries to be between -20 and 60 oC. In an attempt to improve the performance, enhance the safety, and lower the cost of lithium-ion cells, we use a stable salt at high temperature, lithium bis(oxalato)borate (LiBOB), and dissolve it in mixtures of -butyrolactone (GBL), ethyl acetate (EA), and ethylene carbonate (EC), with and without fumed silica nano particulates as a gelling agent. Conductivity, cycling studies of cathode half-cells, rheology, and FTIR measurements are performed for LiBOB in such mixtures as a function of salt concentration, solvent composition, temperature, and fumed silica content and type. Three types of cathodes are used, LiCoO2, LiMn2O4, and LiFePO4, for the half-cell cycling measurements. We find that LiBOB in a mixture of GBL:EA:EC yields a technologically acceptable conductivity, and LiBOB in GBL:EA:EC is a potential 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 ~ 6.0 and 11.1 mS cm-1 at -3 at 25 oC, respectively, and at 1 M LiBOB in solvent composition of 1:1:0.1, the conductivity is ~10.8 and 20.0 mS cm-1 at 25 and 60 oC, respectively. These conductivities are higher than that of the state-of-art electrolyte, which is 9.5 mS cm-1 at 25 oC. The product of conductivity with viscosity, which an indication for ion disassociation, is essentially independent of temperature. Although LiBOB in GBL:EA:EC (1:1:1) has the highest product value, it’s conductivity is the lowest. This indicates that our susyem is viscosity dominated. Adding fumed silica to a LiBOB-based electrolyte yields a mixture with an elastic modulus independent of frequency and larger than the viscous modulus in a dynamic rheology experiment, which indicates formation of a 3-D gel structure. Fumed silica enhanced the mechanical properties of the electrolyte without sacrificing its conductivity. The surface chemistry of fumed silica (native silanol vs octyl-modified) has no effect on conductivity but a significant effect on rheological properties of the mixture. Using a gel electrolyte is anticipated to enhance the safety of lithium-ion batteries by eliminating leakage problems associated with a liquid electrolyte. Cathode half-cells using a LiBOB-based electrolyte give good performance and in the case of LiMn2O4 half-cells, the performance is better than that using state-of-art electrolyte. It is expected that LiMn2O4 cathodes will lower the cost of lithium-ion batteries based on material cost. The performance of LiFePO4 and LiCoO2 half-cells using the gel electrolyte is comparable to half-cells using state-of-art electrolyte. In addition, by using the gel electrolyte with high concentration of fumed silica, we are able to eliminate the need for a CelgardTM separator in the cell, which should also lower the cost of lithium-ion batteries. Results reported in this study show that using 1 M LiBOB in GBL:EA:EC + 20 % R805 can prevent contact between the cathode and the anode without Celgard and give a better performance than cells using the separator. Results obtained in this dissertation warrant further study of LiBOB-based gel electrolyte as a potential replacement for the state-of-art electrolyte for use in lithium-ion batteries.
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    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.