Battery separators thermal runaway

The shutdown property of separators is measured by measuring the impedance of a separator while the temperature is linearly increased. Figure 7 shows the actual measurement for Celgard 2325 membrane. The heating rate was around 60 °C/min, and the impedance was measured at 1 kHz. The rise in impedance corresponds to a collapse in pore structure due to melting of the separator. A 1000-fold increase in impedance is necessary for the separator to stop thermal runaway in the battery. The drop in impedance corresponds to opening of the separator due to coalescence of the polymer and/or to penetration of the separator by the electrodes this phenomenon is... 

In lithium ion rechargeable batteries, shutdown separators are used as part of the overall battery safety system. These devices prevent, or substantially reduce the likelihood of thermal runaway, which may arise from short circuiting caused by physical damage, internal defect, or overcharging. The shutdown separators, will shutdown by a sufficient pore closure to substantially stop ion or current flow within the cell (37). 

The contrasting structure of the plates and the separators is also relevant to the functioning of the battery. For example, the capillary pressures dictate that electrolyte fills the plates preferentially. This preferential filling appears to be the ideal situation since it can best support the electrochemical reaction, i.e., it leaves the separator partially saturated so that movement of electrolyte can provide pathways for gas transport. If, however, the overall saturation is too low or there is excessive loss of water, the separator will dry out and give rise to an increase in the internal resistance of the battery and the possibility of thermal runaway. An increase in internal resistance, and consequent low service-life, can also result if the compression between separators and battery plates relaxes over a period of time. Overcompression may cause fibres to fracture with a loss of resilience, i.e., the separators lose the ability to return to original thickness after a high pressure is applied and... 

The separators nsed in Li-Ion batteries shonld have high-temperature melt integrity. The separator shonld maintain its melt integrity after shut down so that the electrodes do not touch and create a short. This helps to avoid the thermal runaway even when the cell is exposed to high temperatures. Thermal mechanical analysis (TMA) is a very good technique to measure the high-temperature melt integrity of separators. 

Figure 20.14 shows a typical short-circuit curve for an 18,650 Li-Ion cell with a shntdown separator. The cell does not have other safety devices (e.g., CID, PTC), which nsnally work before separator shnt down. As soon as the cell is short circnited externally through a very small shunt resistor, the cell starts heating because of the large current drained through the cell. The shut down of the separator, which occnrs aronnd 130°C, stops the cell from heating further. The current decrease is caused by an increase of battery internal resistance dne to separator shutdown. The separator shutdown helps to avoid the thermal runaway of the cell. 

The separator in a Li-ion battery is typically a thin (15/xm) microporous polypropylene film. It prevents the electrodes from shorting directly or through Li microdendrite growth on overcharge, and it also serves as a thermal shut-down safety device. When heated above ISO C (for example due to an internal short in a cell) the separator melts and its pores close, thus preventing current flow and thermal runaway. It is common to investigate the shut-down behavior of separators by measurement of cell impedance at selected frequencies, such as 1 kHz, dependent on temperature (Uchida [2003]). 

This product acts as a separator or mass transport barrier between the cathode and the anode to limit electrochemical self-discharge. If the integrity of this separator is breached, the battery can experience a thermal runaway condition, whereby the active electrochemical components are chemically consumed with accompanying generation of large amounts of excess heat. At the same time, if battery conditions are such that alloy formation exceeds usage, the excess alloy can cause periodic shorting, the alloy noise sometimes seen in cold-stored batteries. 

The most common separator for these batteries is the PP separator. The separator s properties for these batteries are not very stringent because there are no undesirable electrochemical deposits (e.g., dendrites), and electrodes are very smooth. Thermal runaway is not a major concern because it does not occur unless the temperature reaches 180°C. Thus, a simple separator with low electric resistance, high strength, and low shrinkage is adequate. Of course, the separators need to be thin and defect free, and should have desirable pore size distribution. 

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