Shutdown separator

Shutdown. Separator shutdown is a useful and essential mechanism for limiting temper-... 

Prior work related with shutdown separators also involved application of waxes on membranes." " In these cases, the wax or low melting polymers were coated on the polyolefin separator. The disadvantage of this technique is that the coating can block the pores of the separator and thus can affect the performance by increasing separator resistance. Moreover, the coating level has to be very high to get complete shutdown. 

Figure 10. Typical short-circuit behavior of a 18650 lithium-ion cell with shutdown separator and without PTC (positive temperature coefficient) and CID (current interrupt device). This test simulates an external short circuit of a cell.

The mechanism and characteristics of thermal cutoff devices in several prismatic lithium-ion cells was studied by VenugopaF ° by monitoring the impedance at 1 kHz and the open circuit voltage (OCV) of the cells as a function of temperature. All the cells studied contained PE-based separators with a shutdown temperature between 130 and 135 °C. Within this narrow temperature range, the shutdown separators caused a sharp and irreversible rise in impedance of the cell. Single layer PE separators were effective up to around 145 °C, above which they... 

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). 

G. Samii, A.M. Samii, and D.C. Veno, Shutdown separators with improved properties, US Patent 6949315, September 27,2005. 

Shutdown separator nndergoes a phase change that closes the pores and increases the internal resistance of the cell to reduce/stop current flow to stop the cell operation. 

An abnormal increase in cell temperatnre can occur from internal heating caused by either electrical abuse - overcharge or short circuit - or mechanical abuse - nail penetration or crush. Higher cell temperature also could be a result of external heating. For this reason, battery packs containing Li-Ion cells are designed with safety control circuits that have redundant safety features (PTC, CID, vent, thermal fuse, etc.). Shutdown separators are one of the safety devices inside the cell and act as a last line of defense. The separator shut down is irreversible, which is fine for poly-ethylene-based separators, which melt around 130°C. 

Fig. 20.15 Typical overcharge behavior of a 18,650 lithium-ion ceU with shutdown separator. The PTC (Positive Temperature Coefficient) and CID (Current Interrupt Device) were removed from the ceU header. Reprinted with permission from Chem. Rev. 104 (2004) 4419-4462, Copyright (2004) American Chemical Society...

Figure 20.16 shows the typical nail penetration behavior of a Li-Ion cell with shutdown separator. Clearly, there was a voltage drop from 4.2 to 0.0 V, instantaneously, as the nail penetrates through (when internal short circuit occur) and temperature rose. When the heating rate is low, the cell stops heating when the temperature is close to separator shutdown temperature as shown in Fig. 20.16a. If the heating rate is very high, then the cell continues to heat and fails the nail penetration test as shown in Fig. 20.16b. In this case, the separator shutdown is not fast enough to stop the cell from thermal runaway. Thus a separator only helps to avoid delayed failures in case of internal short circnit as simulated by nail and bar crush tests. Separators with high-temperature melt integrity and good shutdown feature (to avoid delayed failures) are needed to pass internal short-circuit tests.

Fig. 6.12 Typical nail penetration behavior of an 18650 lithium-ion cell with shutdown separator. This test simulates internal short circuit of a cell, (a) Cell passed nail penetration test (b) cell failed nail penetration test...

Shutdown separators are one of the safety devices inside the cell and act as a last fine of defense. The separator shutdown is irreversible, which is fine for... 

Commercial Li-ion cells are manufactured in cylindrical, prismatic metal can and prismatic pouch cell designs. Commercial Li-ion cells are typically fitted with one or more internal protection devices. Some of these are the positive temperature coefficient device (PTC), the current interrupt device (CID) and the shutdown separator. Both the PTC and CID are present in the header of the Li-ion cells as shown in Figure 17.1. 

FIGURE 18.7 Typical nail penetration behavior of a Li-ion cell with shutdown separator, (a) Schematic of a typical nai I test (b) voltage and temperature responses of a cell passing the nail test (c) voltage and temperature responses of a cell that failed the nail test. (For color version of this figure, the reader is referred to the online version of this book.)... 

Gomez-Lahoz, C., J. M. Rodriguez-Maroto, and D. J. Wilson. 1994. Groundwater Cleanup by In-Situ Air Sparging. Vll. Volatile Organic Compounds Concentration Rebound Caused by Diffusion after Shutdown, Separation Science and Technology, vol. 29, no. 12, pp. 1509-1528. 

Shutdown separators. Development of the shutdown separators is yet another method to address the cell safety issues. Shutdown separators consist of multiple layers, where at least two of the layers have different phase transition temperatures. As the cell temperature increases, one of the layers begins to melt and flows into the pore structure of the separator. This blocks the ionic path and, ultimately, stops the electrical current flow and thus prevents further heat buildup. The influence of shutdown separators has been debated in the industry. An example of the typical results can be seen in Fig. 5.9. The abscissa is the power rate at which the cell is being discharged during a potential event and the ordinate is the maximum energy that can be drained from the same cell at those power levels. The region... 

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