DOI: 10.35097/1857
title: Impact of Lithium-Ion Battery Separators on Gas Evolution during Temperature Abuse

description: Separators in lithium-ion batteries are considered to be (electro-)chemically inert to typical operating conditions. Yet, temperature abuse tests at elevated temperatures of ca. 60 to 132 °C show that the choice of separator material has a decisive influence on battery behavior and degradation. Using online electrochemical mass spectrometry, we analyzed the evolution of cell voltage and gas products during and after thermal abuse for different separators. Polypropylene and polytetrafluoroethylene seem stable, whereas glass fiber and polyethylene terephthalate separators were reactive and caused additional gas release. Polyethylene terephthalate produced significantly more gas, and the separator failed mechanically, causing unreproducible and drastic performance losses. The CO2 content of polyethylene terephthalate separator is four times higher than that of glass fiber. Yet, a five times higher amount of POF3 was detected for glass fiber. The evolved gas amounts of CO2 or POF3 were modest for polypropylene and polytetrafluoroethylene.
description: Material and Equipment:

A high-temperature test cell (HTT Cell, PAT series test cell type, EL-Cell GmbH) was used for all experiments. Electrodes were purchased from CustomCells Itzehoe GmbH. The negative electrode has an area capacity of 2.2 mAh cm-2 with the following loading: 96 wt% SMG104 graphite active material, 2607SMG104. The capacity of the active material is 350 mAh g-1. It contained further styrene-butadiene-rubber/carboxymethyl cellulose (SBR/CMC) and conductive additives, all applied on copper foil; the positive electrode has an area capacity of 2.0 mAh cm-2, with the following loading: 93.5 wt% NMC 622 active material, K-771, capacity of the active material is 160 mAh g-1. It further contained polyvinylidene fluoride and conductive additives, all applied on aluminum foil. EC:DMC electrolyte (1:1 / v:v,1 M LiPF6, battery grade, Sigma Aldrich) was used. All electrodes were punched out to receive sheets with a uniform diameter of 18 mm. Their weight was measured (XS205, Mettler Toledo), and they were dried overnight at 120 °C under high vacuum before being transferred into an Ar glovebox (water and oxygen content under 0.1 ppm). All capacities were calculated based on the measured weight of the assembled electrodes.
The separator compositions were as follows: borosilicate glass (Whatman GF/A, porosity 91%, thickness 260 mum, m.p. &gt; 300 °C), polyethylene terephthalate/Al2O3 (Viledon FS 3005-25, porosity 55%, thickness 25 mum, m.p. ~250 °C), polypropylene (Celgard 4560, porosity 55%, thickness 110 mum, m.p. ~150 °C), and polytetrafluoroethylene (Omnipore JVWP04700, porosity 88%, thickness 30 mum, m.p. ~300 °C). The HTT test cells contain the four separators mentioned above as sheets with a diameter of 21.6 mm. The cells were assembled in a glovebox under an Ar atmosphere, according to literature.[36] Temperature-stable separator housings and sealing ring materials made of polyether ether ketone (PEEK) were used for the selected temperature range.
Gases used for calibration were purchased from Air Liquide or Westfalen AG. The quantities and purities of the gases were: Ar (99.999%); calibration gas 1: Ar (main component, 99.999%), CH4 (5025 ppm, 99.5%), C2H4 (5025 ppm, 99.5%) and CO2 (5025 ppm, 99.5%); calibration gas 2: Ar (main component, 99.999%), H2 (5005ppm, 99.999%), CO (4991 ppm, 99.0%) and O2 (4700 ppm, 99.999%). Chemicals used for calibration were purchased and used and without further purification: dimethyl carbonate (99% dry, Acros Organics), Lithium hexafluorophosphate (99.99%, Sigma Aldrich), and Lithium peroxide (95%, ABCR). The conductive salt decomposition gas POF3 is calibrated.

Setup for Temperature-induced Stress Test and Electrochemical characterization:

The operando electrochemical and gas analysis setup couples a potentiostat and mass spectrometer. The HTT cell (EL-Cell) connects the lithium-ion battery with quick connectors with a heated transfer line and the mass spectrometer (MS, Omnistar GSD 320, Pfeiffer Vacuum). The parameters of the mass spectrometer are:

Parameter Value
Multiple Ion Detection (MID) m/z: 2, 16, 18, 26, 27, 28, 30, 32, 36, 44, 67, 69, 77, 78, 85, 88, 90, 104, 107, 118, 126, 155
Energy of the electron beam 70 eV
Detector C-SEM
Capillary temperature 200 °C
Input heating 120 °C
Scan rate 500 ms
Ar flow rate 0.65 mL min-1

The heating profile of the HTT cell during temperature-induced stress test are:

Parameter Value
Start temperature 25 °C
Heating rate 2 °C min 1
Test temperature 132 °C
Hold temperature at test temperature while recording the open-circuit voltage 60 min
End temperature 25 °C

All assembled cells were cycled with a Gamry potentiostat (5000 E) in a self-built temperature chamber at 25 °C in the voltage range between 3.0 V and 4.2 V. The cycling procedure was identical for all experiments is as follows:

Parameter Value
Temperature 25 °C
Rest time 6 h
Formation, 1 cycle C/10
Heating step after formation starts at ~3.7 V (~SOC 50%) OCV for 10 h
Cycle after temperature induced stress test C/10
Charge / Discharge CC+CV / CC
U(cut-off) 4.2 V / 3.0 V

For more information about experimental details, see in the Experimental Section in the manuscript and the Supporting Information.