Porous polymer bodies having many open micropores are used in various fields as separator membranes for use in ultrapure water production, purification of chemical solutions, water treatment, etc.; waterproof moisture-permeable films for use in clothing, sanitary supplies, etc.; and battery separators for use in secondary batteries, etc.
Secondary batteries are widely used as power supplies for portable instruments, such as OA, FA, electric appliances for home use, communication appliances, etc. In particular, portable instruments using lithium ion secondary batteries are becoming widespread because, when mounted on such instruments, the lithium ion secondary batteries have high volumetric efficiency and therefore can reduce the size and the weight of the instruments.
On the other hand, large-size secondary batteries are under research and development in many fields related to energy and environmental issues, including load-leveling, UPSs and electric vehicles. Among these, lithium ion secondary batteries that belong to one type of nonaqueous electrolyte secondary batteries are becoming used in various applications for the reasons that the batteries are excellent in large capacity, high output power, high voltage and long-term storage stability.
Lithium ion secondary batteries are generally so designed as to have a highest working voltage falling in a range of from 4.1 to 4.2 V. Aqueous solutions are electrolyzed at such a high voltage and could not be used as electrolyte solutions. Consequently, so-called nonaqueous electrolytes, which contain organic solvents, are used as electrolyte solutions that can withstand high voltages. High-permittivity organic solvents, which can dissolve a larger amount of lithium ions, are used as solvents for nonaqueous electrolytes. Organic carbonate compounds, such as propylene carbonate, ethylene carbonate, etc., are mainly used as high-permittivity organic solvents. A highly-reactive electrolyte such as lithium hexafluorophosphate or the like is dissolved in a solvent and is used as a supporting electrolyte to serve as a lithium ion source in the solvent.
A lithium ion secondary battery comprises a separator arranged between a positive electrode and a negative electrode in order to prevent internal short-circuits. From the nature of the system, the separator is naturally required to have insulating properties. In addition, the separator must have a microporous structure in order to achieve high permeability for passage of lithium ions therethrough and to diffuse and retain an electrolyte solution therein. To satisfy these requirements, porous films are used for separators.
The recent tendency toward a rise in battery capacity has resulted in the increase in the importance in battery safety. The characteristics of battery separators that contribute to safety include shutdown characteristics (hereinafter referred to as “SD characteristics”). The SD characteristics have such a function that micropores are closed at a high temperature in a range of approximately from 100° C. to 150° C. to thereby intercept ionic conduction in a battery and prevent a subsequent temperature rise in the battery. The lowest temperature at which micropores of a porous film are closed is referred to as a shutdown temperature (hereinafter referred to as “SD temperature”). Porous films to be used as battery separators need to have the SD characteristics.
However, because of recent increases in energy density and capacity of lithium ion secondary batteries, there have been accidents in which the ordinary shutdown characteristics could not sufficiently function so that the internal temperature of batteries may exceed over the melting point, approximately 130° C., of a polyethylene that is used as a material of already-existing separators, and as a result, this may cause thermal shrinkage and rupture of the separator and a short-circuit between electrodes. Given the situation and in order to ensure battery safety, there is a demand for separators having higher heat resistance than that for the present SD characteristics.
To satisfy the requirement, a multilayer porous film has been proposed that comprises, as arranged on at least one surface of a polyolefinic resin porous film, a coating layer formed of a porous layer that contains a metal oxide and a resin binder (PTLs 1 to 5). These documents say that, in these inventions, the proposed method is excellent in safety in that a coating film filled with a large amount of inorganic fine particles of α-alumina or the like is provided on a porous film and can therefore prevent short-circuits between electrodes even in an emergency of abnormal heating and continuous temperature increasing over the SD temperature.