Study of Membrane Degradation in High-Power Lithium-Ion Cells

Abstract

Impedance spectra of Celgard ® 2300 membranes that were removed from high-power Li-ion cells showed a significant rise in membrane ionic resistivity for cells that were cycled or stored at elevated temperatures. Atomic force microscopy images revealed dramatic changes in membrane surface morphology. Swelling of the membrane polypropylene fibers and the presence of particles of electrode active material in the membrane pores effectively reduced the membrane porosity, and thereby account for the membrane impedance rise and part of the cell power loss. These results not only indicate that membrane degradation can contribute significantly to Li-ion cell power loss at elevated temperatures, but also reveal mechanisms of membrane degradation.The U.S. Department of Energy's Advanced Technology Development ͑ATD͒ program supports the development of high-power Li-ion batteries for hybrid electric vehicle applications. 1 Included in the ATD program are diagnostic evaluations of Li-ion cells that were aged and/or cycled under various conditions. 2 A primary goal of these diagnostic tests is to determine the mechanisms responsible for the cell power loss that accompanies life tests at elevated temperatures. Among the possible causes of power loss, degradation of the cell membrane and consequent reduced ionic conductivity is a mechanism that should be considered. However, studies of this mechanism in high-power Li-ion cells have not been reported in the literature.Membrane porosity has been shown to be a determining factor for ion transport in batteries and fuel cells, limiting the cell current and reducing the ionic conductivity. [3][4][5] Studies have shown that storage at 75°C for 30 days increases the impedance of composite gel membranes by almost two orders of magnitude and that momentarily raising cell temperatures above the melting point of its polymer membrane produces equivalent effects on the membrane's electrical impedance. 6,7 Swelling of Nafion ® membranes at elevated temperatures has also been documented. 8,9 No comparable studies on Celgard ® 2300 membranes, which are commonly used in Li-ion batteries, have been reported; and none have expressly linked battery power loss with membrane impedance rise and structural instability. In fact, little attention has been paid to the possibility that membrane instability contributes to Li-ion cell power loss. In this study we focus on post-test analysis of Celgard 2300 membranes removed from high-power Li-ion cells which were stored and/or cycled at elevated temperatures. The effect of separator degradation on cell power loss is assessed. ExperimentalThe high-power Li-ion cells included a LiNi 0.8 Co 0.15 Al 0.05 O 2 cathode, a synthetic graphite anode, 1.2 M LiPF 6 ϩ ethylene carbonate ϩ ethylmethylcarbonate ͑EC/EMC͒ electrolyte, and a Celgard 2300 membrane separator. Membrane samples were taken from the following cells; ͑A͒ a fresh cell that was subjected only to formation cycles and initial characterization, ͑B͒ a cell that was at 60% state of charge ͑SOC͒ and shallow c...

Analytical Methods for Investigation of Lithium-Ion Battery Ageing

“…Norin et al [14] showed the influence of the separator degradation on the power loss in LiBs that had been exposed to elevated temperatures. The ionic conductivity of all cells decreased, and atomic force microscopy (AFM) revealed a significant loss in porosity.…”

Probing the Roles of Polymeric Separators in Lithium-Ion Battery Capacity Fade at Elevated Temperatures

“…Norin et al 23,24 have examined the separator membranes (Celgard 2300, PP-PE-PP trilayer structure) that were taken from cells either shallowly (± 1.5% SOC) cycled at 45…”

“…The causes for the membrane impedance increase were found to be a significant loss in porosity that was revealed by atomic force microscopy. 23 They ascribed this porosity loss to the thermal decomposition of the electrolyte (LiPF 6 -EC-EMC electrolyte, the most widely used commercial electrolyte), the product of which precipitates on the separator and clogs the pores in the separator surface. 24 Although Norin et al's studies well established that the porosity loss leads to an increase of the separator impedance, which accounts for 10% of the cell impedance rise, directly resulting in capacity loss, the behavior of separator itself when stored/cycled at elevated temperature is still unknown.…”

Using a Boron-Based Anion Receptor Additive to Improve the Thermal Stability of LiPF[sub 6]-Based Electrolyte for Lithium Batteries

“…LiF precipitation was also found at the surface of electrodes and in the pores of the separator membrane, which may block the transportation of the Li ϩ ion and increase the resistance of cells. 5,6 Highly reactive PF 5 is produced from both the thermal and hydrolysis reactions and can initiate the polymerization of cyclic carbonate to degrade the electrolyte. 1 The instability of LiPF 6 -based electrolyte is a major factor for degradation of lithium batteries operating at elevated temperatures.…”

“…7 Their molecular dynamic simulations show that the average distances between the Li ϩ cation and the F atoms in the PF 6 Ϫ anion in the presence of anion receptors ͑aza ether compounds͒ are much larger than the absence of the aza ether compounds. They postulated that this enlarged distance suppresses the reactivity of the Li ϩ cation and F atoms.…”