Introduction
Lithium-ion secondary cells, also called rechargeable batteries, (referred to here as “lithium-ion batteries”) are widely used as energy sources for information terminals and consumer electronics, etc. because of their high energy density and cell voltage. Recently, their growing rate of dissemination into areas of general household applications, including hybrid and electric vehicles, is quite evident, and it appears obvious that the demand will further increase in the future.
Because lithium-ion batteries can sometimes become unstable due to short-circuit, over charging and discharging, impact, etc., a variety of protection mechanisms are incorporated at the battery component level to ensure safety.
Of these component parts, the lithium-ion battery separator prevents contact between the positive and negative electrodes, while at the same time playing a role as a spacer which permits the passage of lithium ions. However, it also performs the function of
preventing a rise in battery temperature due to excessive current in the event of a short circuit.
Because the lithium-ion battery separator is set in place so that it comes into contact with the rough surfaces of the positive and negative terminals, high mechanical strength is required. This mechanical strength must be maintained even if there is some rise in temperature, which is common to some degree, for example, during battery charging. Therefore, we conducted piercing and tensile testing measurements of the separator to evaluate changes in strength with respect to changes in temperature. This document introduces actual examples of these tests.
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Regarding the lithium-ion battery separator, previous evaluation examples were also introduced in Application News T146 “Measurement of Separator in Lithium-Ion Battery” and i229 “Multi- Faceted Approach for Evaluating Lithium-Ion Battery Separators.”
Piercing Test
The samples consisted of separators removed from two lithium-ion batteries (cylindrical) used in small electrical devices, and we measured the changes in piercing characteristics due to changes in environmental temperature. Fig. 1 shows an overview of the test conditions, and Table 1 presents details of the test conditions.
Tensile Test
The separators used for the tensile testing were removed from commercially available lithium-ion batteries (square-shaped), so 2 types of samples (below, referred to as samples (1) and (2)) which contained PE (polyethylene) as the principle constituent were used. When conducting the tensile tests, each separator sample (as shown in Fig. 3(a)) was fashioned into dumbbell-shaped specimens oriented in the lengthwise and widthwise directions of each separator, as shown in Fig. 3(b). The total length of all specimen was 35 mm, with the parallel section measuring 10 (L) × 2 (W) mm
Table 3 shows the tensile test conditions that were used.
Fig. 4 and Fig. 5 show the stress – strain curves for the widthwise and lengthwise directions, respectively of sample (1).
Fig. 6 and Fig. 7 show the stress – strain curves for the widthwise and lengthwise directions, respectively of sample (2).
Table 4 shows the test values of the mechanical properties obtained at each temperature.
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