In order to prevent internal shorting in a battery using a non-aqueous electrolytic solution such as a lithium ion battery, a separator having a shutdown function for shutting down a reaction upon temperature thereof exceeding a prescribed temperature is essential. A battery separator typically consists of a microporous membrane. If the temperature increases, the separator shrinks and the fine pores are blocked at around the melting point, which leads to the shutdown of the battery reaction. When an increase in temperature continues further, the microporous membrane switches from shrinking to expanding at a certain temperature, ultimately resulting in membrane puncture (meltdown).
The shutdown temperature can be expressed as the temperature (shutdown starting temperature) at the inflection point of a sample length observed around the melting point in TMA measurement. If the microporous membrane does not have such an inflection point, shutdown and shrinkage progress simultaneously as the temperature increases, which makes it difficult to sufficiently suppress reactions at the time of an abnormality. Therefore, from the perspective of safety, a microporous membrane for a separator preferably has such an inflection point. On the other hand, if the time between the beginning of shutdown and completion of blocking of the pores is short, there is a risk that the energy may be discharged all at once in the event of a meltdown. Therefore, in order to gradually reduce the energy discharged at the time of an abnormality, the shutdown temperature is preferably sufficiently lower than the maximum shrinkage temperature or the meltdown temperature.
In the production of a separator, the strength of the separator is often enhanced by imparting the separator with orientation by means of stretching or the like. A separator having such orientation may exhibit anisotropy with regard to not only strength, but also temperature characteristics such as the shutdown temperature. Here, because separators are ordinarily wound in a state under tension applied in the MD (machine direction), if the difference between the shutdown temperature and the maximum shrinkage temperature in the TD (direction perpendicular to the machine direction; transverse direction), there is a risk that the membrane may shrink rapidly in the TD due to preheating, which may cause shorting at the terminals of the battery. In addition, if the shrinkage rate at the maximum shrinkage temperature (maximum shrinkage rate) is greater than the shrinkage rate at the shutdown temperature (shutdown shrinkage rate), there is also a risk that the membrane may shrink rapidly during shutdown. Therefore, from the perspective of safety, the shutdown temperature in the TD is preferably sufficiently lower than the maximum shrinkage temperature in the TD, and the difference between the shutdown shrinkage rate and the maximum shrinkage rate in the TD is preferably small.
As one method of controlling anisotropy, it is described in Patent Document 1, for example, that the anisotropy of a microporous membrane is controlled by simultaneous biaxial stretching at different ratios. However, although the microporous membrane obtained by this method yields a relatively large difference between the shutdown temperature and the maximum shrinkage temperature, the maximum shrinkage rate tends to be large, and there is a risk that the membrane may shrink rapidly at high temperatures. In addition, with such a stretching method, it is difficult to substantially enhance the strength of the microporous membrane.
In contrast, another method is to enhance the characteristics of a microporous membrane by controlling the components of the microporous membrane. For example, a method of producing a microporous membrane using a mixture of a polyethylene, which has a low melting point and can reduce the shutdown temperature, and a polypropylene, which is advantageous for maintaining the membrane state at high temperatures, is described in Patent Document 2. However, a method of blending a polyethylene and a polypropylene tends to yield a sea-island structure due to the incompatibility of the polyethylene and the polypropylene, and non-uniformity in physical properties may manifest, which makes it difficult to achieve sufficient shutdown characteristics.
In addition, although the permeability of a separator is preferably high from the perspective of battery performance, if enhancing the permeability entails a dramatic reduction in strength, there is a risk that the separator may be easily punctured and that the safety may be diminished. Therefore, a separator for a secondary battery preferably demonstrates both high battery output due to high permeability and a high level of safety due to high strength.