The decrease of ionic conductivity of polymeric separators in high-power Li-ion cells, which were cycled or stored at elevated temperatures, was accompanied by dramatic changes in separator surface morphology. The source and nature of polymer separator degradation in high-power Li-ion batteries have been studied. We attributed the observed porosity loss to a deposit, which precipitated onto the separator surface from the electrolyte and clogged separator pores. This deposit resulted from a homogenous decomposition process of the LiPF 6 -ethylene carbonate-ethyl-methyl carbonate electrolyte, which was significantly accelerated at elevated temperatures. The electrolyte decomposition products consisted of lithium halophosphates and displayed very strong fluorescence. They contributed to the solid electrolyte interphase layers on both cathode and anode where they underwent further oxidation or reduction reactions, respectively.

AC alternating current AFM atomic force microscopy ANL Argonne National Laboratory ASI area-specific impedance (ohm-cm 2) ATD Advanced Technology Development ATR attenuated total reflection BNL Brookhaven National Laboratory BSF battery scaling factor CSAFM current-sensing atomic force microscopy CE capillary electrophoresis CF capacity fade DEC diethyl carbonate DEDOHC diethyl-2,5-dioxahexane carboxylate DMC dimethyl carbonate DMDOHC dimethyl-2,5-dioxahexane carboxylate DME dimethoxy ethane DMF dimethyl formamide DOE U.S. Department of Energy EC ethylene carbonate EDX energy dispersive X-ray analysis EELS electron energy loss spectroscopy EMC ethyl methyl carbonate EMDOHC ethyl methyl-2,5-dioxahexane carboxylate EOC end of charge EOL end of life EXAFS extended X-ray absorption fine structure EY electron yield FFT fast Fourier transform FID flame ionization detector FTIR Fourier transform infrared FY fluorescence yield GC gas chromatography GC-MS gas chromatography mass spectrometry GPC gel permeation chromatography Gen 1 (+) LiNi 0.8 Co 0.2 O 2 cathode Gen 1 (-) MCMB:SFG-6 (82:18) Gen 1 electrolyte 1.2 M LiPF 6 EC:DEC (1:1 by wt) Gen 2 cells ATD Generation 2 baseline cells Gen 2 (+) LiNi 0.8 Co 0.15 Al 0.05 O 2 cathode Gen 2 (-) MAG-10 synthetic graphite anode Gen 2 electrolyte 1.2 M LiPF 6 EC:EMC (3:7 by wt) HEV hybrid electric vehicle HF hydrogen fluoride (hydrofluoric acid) DIAGNOSTIC EXAMINATION OF GENERATION 2 LITHIUM-ION CELLS AND ASSESSMENT OF PERFORMANCE DEGRADATION MECHANISMS

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...