Nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, have high energy density, and are therefore widely used as batteries for personal computers, mobile phones, mobile information terminals, and the like. In nonaqueous electrolyte secondary batteries typified by such lithium ion secondary batteries, a separator is ordinarily provided between a cathode and an anode.
Nonaqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, have high energy density. Therefore, in a case where an internal short circuit and/or an external short circuit is/are caused by, for example, breakage of a nonaqueous electrolyte secondary battery or breakage of a device using the nonaqueous electrolyte secondary battery, a high current flows so as to cause intense heat to be generated. This has created demands that nonaqueous electrolyte secondary batteries should prevent greater than a certain level of heat generation to ensure a high level of safety.
Safety of a nonaqueous electrolyte secondary battery is typically ensured by imparting to the nonaqueous electrolyte secondary battery a shutdown function, that is, a function of, in a case where there has been abnormal heat generation, blocking passage of ions between the cathode and the anode with use of a separator to prevent further heat generation. In order to impart a shutdown function to a separator, it is possible to use, as the separator, a porous film made of a material that melts in a case where there is abnormal heat generation. Specifically, according to a battery using such a separator, a porous film melts so as to be non-porous in a case where there has been abnormal heat generation. This blocks passage of ions, and therefore restricts any further heat generation.
Examples of a separator having such a shutdown function encompass a porous film made of polyolefin. A separator, which is made of the polyolefin porous film, melts so as to be non-porous in a case where there has been abnormal heat generation in a battery. This blocks (shuts down) passage of ions, and therefore restricts further heat generation. However, in a case where, for example, heat generation is intense, thermal shrinkage may occur to the separator, which is made of the polyolefin porous film. This may cause a cathode and an anode to come into direct contact, and therefore poses a risk of causing a short circuit. According to a separator which is made of a polyolefin porous film, shape stability at a high temperature is thus insufficient. Therefore, it is sometimes not possible to restrict abnormal heat generation which is caused by a short circuit.
As a separator whose shrinkage at a high temperature is restricted to have excellent shape stability, there has been proposed a separator which includes a porous base material layer containing polyolefin as a main component, the porous base material layer having (i) a filler layer, provided on one surface thereof, which contains an inorganic filler as a main component and (ii) a resin layer, provided on the other surface thereof, which contains resin particles as a main component, the resin particles having a melting point of 100° C. to 130° C. (see Patent Literature 1). Patent Literature 1 discloses that (i) the separator includes the resin layer so that, before a thermal shrinkage temperature of the porous base material layer is reached, the resin particles melt so as to cause the porous base material layer to be shaped into a non-porous film and (ii) the separator includes the filler layer so that, even in a case where the thermal shrinkage temperature of the porous base material layer is reached, the presence of the filler layer prevents a short circuit from occurring between electrodes.