Melt integrity

Thermal-mechanical analysis (TMA) has proven a more reproducible measure of melt integrity [20]. The TMA test involves measuring the shape change of a separator under load while the temperature is linearly increased. Typically, separators show some shrinkage, then start to elongate, and finally break (see Fig. 5). 

Nonwoven materials such as cellulosic fibers have never been successfully used in lithium batteries. This lack of interest is related to the hygroscopic nature of cellulosic papers and films, their tendency to degrade in contact with lithium metal, and their susceptibility to pinhole formation at thickness of less than 100 fjim. For future applications, such as electric vehicles and load leveling systems at electric power plants, cellulosic separators may find a place because of their stability at higher temperatures when compared to polyolefins. They may be laminated with polyolefin separators to provide high-temperature melt integrity. 

Conventional shutdown temperatures are around 130°C. However, a microporous polyolefin battery separator with a shutdown temperature of 95-110°C and a melt integrity of more than 165°C can be made from a basic UHMWPE formulation (38). 

Figure 20.4 shows the SEM of the Nitto Denko separator that resembles more of a nonwoven texture similar to the SEM of a separator sample made through a wet process. The Nitto Denko separator has more shrinkage in both machine and transverse directions. The separator shows the shutdown behavior when heated close to melting temperature of the polyolefin, but the melt integrity is poor. 

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. 

Overcharge Shutdown behavior, high-temperature melt integrity Separator should completely shut down and then meiintain its melt integrity at high temperatures... 

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.

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