Transport Properties of Lithium Bis(Oxalato)Borate-based Electrolyte for Lithium-ion Cells

Abstract

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.