Battery separators puncture strength

Yamamura, Y. Ooizumi, S. Yamamoto, K. Separator for rechargeable lithium ion batteries with high puncture strength and high melt rupture temperature. Nltto Denko Technol. Rep. (http /Mww.nitto.com/rd/rd6 l.html), 2001, 39, 39. 

Recently Nitto Denko developed a battery separator made by a wet process that had high puncture strength and high heat rupture resistance. They used a polyolefin resin with a high-molecular-weight rubber as its main component materials and cross-linked through oxidation in air. The melt rupture temperature, as measured by thermomechanical analysis, was over 200°C in this material. They also tried cross-linking UHMWPE with electron-beam and ultraviolet irradiation, but this had the... 

However, it has to be conceded that after battery life cycle tests at such temperatures polyethylene separators also reach their limits (although this fact does not yet reflect in failure-mode studies [49]) even in locations with extreme ambient temperatures. The tendency toward using ever-thinner backwebs cannot be continued, however, without seeking protective measures. Suitable provisions have to be made, especially with respect to the separator s oxidative stabihty at elevated temperature. The leading producers of polyethylene separators have recently presented solutions [41, 47] which, even at 150 (xm backweb, provide for oxidative stability and puncture strength in excess of that for the standard product at 250 (xm backweb [41]. 

Currently, 20 pm is the most widely used thickness for Hthium ion battery separators. Single layers can be made as thin as 7 pm or as thick as 40 pm. The thicker the separator, the greater is the mechanical strength and the lower the probability of punctures during cell assembly, but the smaller is the amount of active materials that can be placed in the can. The uniformity of thickness is important so that the jeUyroll (that is, the spirally wound electrodes and separator) will fit into a can." ... 

The force required to create a short through a separator due to mix (electrode material) penetration defines mix penetration strength. In this test, force (with a Vi.-inch diameter ball) is applied on the positive electrode/separator/negative electrode sandwich and the force at which the mix penetrates through the separator and creates an electronic short is called mix penetration force. Mix penetration strength is used to indicate the tendency of separators to allow short circuits during battery assembly. The mix penetration resistance test is more closely related to particle penetration resistance compared to puncture resistance [45]. 

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