Lithium bis(Oxalato)Borate-Based Electrolyte for Lithium-Ion Cells

Abstract

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.