Battery separators melt integrity

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). 

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. 

With a multilayer polypropylene/ polyethylene separator, the impedance rise occurs near the melting point of polyethylene (135 C), and stays high till just past the melting point of polypropylene (165 °C). The rise in impedance corresponds to a collapse in pore structure due to melting of the separator. Laman et al. [5] suggested that at least a thousand-fold increase in impedance is necessary for the separator to stop thermal runaway in a 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 referred to as a loss in melt integrity . 

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