A nonaqueous secondary battery, which is represented by a lithium ion secondary battery, has a high energy density and is widely used as a main electric power source of a portable electronic equipment, such as a portable phone and a notebook computer. The lithium ion secondary battery is demanded to attain a further high energy density, but has a technical issue on assuring safety.
A separator plays an important role on assuring safety of a lithium ion secondary battery, and under the current situation, a polyethylene microporous membrane is used since it has a high strength and shutdown function. The shutdown function referred herein means a function of shutting down an electric current by closing the pores of the microporous membrane when the temperature of the battery is increased, and the battery is suppressed from generating heat by the function, thereby preventing the battery from suffering thermal runaway.
The energy density of the lithium ion secondary battery is being increased year by year, and for assuring safety, heat resistance is demanded in addition to the shutdown function. However, the shutdown function contradicts the heat resistance since the operation mechanism thereof depends on closure of the pores through melting of polyethylene. There have been proposals on improvement in heat resistance with the molecular weight of polyethylene, the crystalline structure or the like, but sufficient heat resistance has not yet been attained. Such techniques have been proposed that polypropylene is blended or laminated, but under the current situation, these systems fail to attain sufficient heat resistance. Furthermore, for enhancing the heat resistance with the shutdown function attained simultaneously, such techniques have been proposed that heat resistant porous layers are coated on both front and back surfaces of a polyethylene microporous membrane, and nonwoven fabrics containing heat resistant fibers are laminated thereon.
It is an important factor of a separator for assuring safety of a nonaqueous secondary battery that the separator has shutdown function and heat resistance, and furthermore, it is also important that the separator has flame retardancy from the standpoint of ignition. The currently available separator for a nonaqueous secondary battery as described above uses a polyethylene microporous membrane in consideration of shutdown characteristics, and there are many techniques for enhancing heat resistance mainly with the polyethylene microporous membrane. Polyethylene is a polymer that is highly combustible, and in consideration of the property, cannot be considered as having high safety.
Such a separator has been known that has a polyethylene microporous membrane and a heat resistant porous layer having an oxygen index of 26 or more, which are laminated on each other (see Patent Document 1). However, a polyethylene microporous membrane is still combustible even though it is coated with a layer having a high oxygen index, and it is not effective from the standpoint of flame retardancy.
Such a separator has been also known that has a polyethylene microporous membrane and a heat resistant porous layer laminated on each other, in which ceramic powder is mixed in the heat resistant porous layer (see Patent Document 2). In Patent Document 2, the ceramic powder is mixed for the purpose of improving the ion permeability. However, there is no effect in flame retardancy by adding ceramic powder, which is represented by a so-called metallic oxide. Furthermore, the separator has a handling problem, in which an equipment is severely abraded due to the ceramic particles, which are generally hard. In the case where the equipment is abraded, metallic powder and the like are attached to the separator and may cause decrease in capability of the battery.
In addition, techniques for imparting flame retardant effect to the separator by adding a flame retarder thereto (see Patent Documents 3 to 6). Patent Document 3 discloses examples of utilizing a halogen flame retarder and barium sulfate in the form of solid particles. Patent Documents 4 to 6 disclose examples of adding a polymer flame retarder to a separator. The proposals contribute to flame retardancy of a separator, but cannot enhance the heat resistance sufficiently, and thus it is difficult to assure safety of a battery.    Patent Document 1: JP-A-2006-269359    Patent Document 2: Japanese Patent No. 3,175,730    Patent Document 3: JP-A-7-272762    Patent Document 4: JP-A-2006-351316    Patent Document 5: JP-A-2005-149881    Patent Document 6: JP-A-2001-210314