Introduction
Separators play a crucial role in lithium-ion batteries as they physically separate the anode and cathode while allowing the passage of lithium ions between the electrodes. Different materials and technologies are used for separators to meet different safety, performance and cost requirements. One commonly used group of separators in lithium-ion batteries are polyolefin separators, as they are chemically resistant to electrolytes, easy to produce and relatively cost effective.
In the case of competitive studies, the characterization and identification of separators can be essential in ensuring quality and improving the performance of a battery.
Two different separator foils were investigated by the TGA-FT-IR technique to determine the Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition behavior and to identify the composition.
Measurement and Discussion
The measurement conditions are detailed in table 1.
Table 1: Measurement conditions
| Instrument | NETZSCH TG Libra® coupled to the Bruker FT-IR INVENIO |
|---|---|
| Temperature program | RT to 850°C |
| Heating rate | 10 K/min |
| Purge gas | Nitrogen, 40 ml/min |
| Crucibles | Al2O3, 85 μl, open |
Figure 1 shows a comparison of the TGA curves (sample A: green; sample B: red) of two separator foils. Both samples were heated to 850°C under an inert atmosphere, resulting in complete PyrolysisPyrolysis is the thermal decomposition of organic compounds in an inert atmosphere.pyrolysis. As a result, no Pyrolytic CarbonPyrolytic carbon is carbon which is generated by the pyrolysis of organic matter in an oxygen-free atmosphere. pyrolytic carbon or Ash ContentThe ash is a measure of the mineral oxide content on a weight basis. Thermogravimetric analysis (TGA) in an oxidative atmosphere is a well-proven method to determine the inorganic residue, commonly referred to ash, in organic materials such as polymers, rubbers, etc. Therefore, the TGA measurement will identify if a material is filled and calculates the total filler content.ash content could be identified. However, a slight deviation in the Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition onset temperature (437°C for sample A vs. 447°C for sample B) was observed. It is thus likely that two different materials were used for these separator foils. With the help of the c-DTA®® function, the melting points of these two samples could also be determined. Again, a significant difference of 116°C vs. 168°C was detected.
The melting of a separator foil is an important safety feature for batteries. In modern batteries, many separators have a so-called “shutdown function”. This means that when overheated, the separator melts or closes its pores, stopping the current flow and thus protecting the battery before dangerous Thermal runawayA thermal runaway is the situation where a chemical reactor is out of control with respect to temperature and/or pressure production caused by the chemical reaction itself. Simulation of a thermal runaway is usually carried out using a calorimeter device according to accelerated rate calorimetry (ARC®).thermal runaway occurs.
The Gram Schmidt curves show the total IR intensities. They are in good correlation with the TGA and DTG curves.
The identification of the separator material is exemplarily executed for sample A. The Identify feature of Proteus® contains several thousands of measured data sets for different thermal analysis methods and different classes of materials which can be compared with the current data. Here, the TGA curve and the Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting point determined by c-DTA® of sample A are compared with the polymer library data. They show high similarity to polypropylene (pink curves); see figure 2.
As additional proof, the gas phase spectra detected by FT-IR at 462°C were compared with those in the TGAFT- IR database of polymers, which contains the PyrolysisPyrolysis is the thermal decomposition of organic compounds in an inert atmosphere.pyrolysis spectra of all typical polymers. Again, high similarity to polypropylene was found; see figure 3.
Prior to a TGA-FT-IR measurement, an ATR-IR spectrum may also be helpful for identification. The separator foil was put on the ATR diamond crystal and an IR spectrum was taken of the solid material; see figure 4. Comparison of the spectrum with the library also yielded high similarity to polypropylene, as shown in figure 5.
Summary
The combination of a thermobalance (TGA) and an FT-IR system provides a complementary set of measurement data – such as Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting point, Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition behavior, Ash ContentThe ash is a measure of the mineral oxide content on a weight basis. Thermogravimetric analysis (TGA) in an oxidative atmosphere is a well-proven method to determine the inorganic residue, commonly referred to ash, in organic materials such as polymers, rubbers, etc. Therefore, the TGA measurement will identify if a material is filled and calculates the total filler content.ash content, filler content and the identification of the gases released – from just one sample measurement. In this example, the Thermal StabilityA material is thermally stable if it does not decompose under the influence of temperature. One way to determine the thermal stability of a substance is to use a TGA (thermogravimetric analyzer). thermal stability and the material of separator foils could be identified using the Identify library, the TGA-FT-IR database of polymers, and the ATR spectrum of the solid compound with only one instrument setup.