The present disclosure relates to a separator, a battery using the same, and a method for producing a separator, and also to a microporous membrane and a method for producing a microporous membrane. More specifically, it relates to a high-performance separator with excellent safety, a battery using the same, and a method for producing a separator, and also to a microporous membrane and a method for producing a microporous membrane.
With the recent progress in the technology of portable electronic devices, higher-performance mobile phones or notebook computers have been developed. In order to support such development, there is a need for excellent drive power supplies. Electronic devices are often required to operate for a long period of time, and they are also required to be lightweight and small. Accordingly, there is a demand for a power supply with high energy density. As a power supply that meets the demand, a lithium-ion secondary battery that achieves a high energy density has been widely used.
Such a lithium-ion secondary battery has extremely high energy density and uses a flammable organic solvent as the electrolytic solution, and, therefore, high safety is required. For this reason, various measures have been taken on lithium-ion secondary batteries so as to ensure safety even in the event of an abnormality.
For example, in order to provide double or triple protection against short-circuiting, a lithium-ion secondary battery is designed so that the current is stopped when a short circuit occurs therein, thereby ensuring safety. For example, in the case where an electrically conductive substance is mixed into the battery, whereby an internal short circuit occurs due to the formation of lithium dendrites, a safety circuit in the lithium-ion secondary battery performs the current cutoff function. In the case where the abnormal reaction is not terminated by the cutoff but is accelerated, and heat is thus abnormally generated, a porous membrane inside the battery melts to close the pores thereof. As a result, ion permeation is suppressed, thereby suppressing the abnormal reaction.
Such a lithium-ion secondary battery is expected to find wider applications in the fields of automobiles, home appliances, etc., and thus is required to have even higher safety, higher capacity, and a lighter weight and smaller size. In particular, assuming harsh conditions including crushing or like deformation due to possible external pressure upon loading on a movable body, puncture with a nail or like electrically conductive projection, etc., even more safety measures are required.
In order to meet such a demand for safety measures, a method for preventing a short circuit between the positive electrode and the negative electrode by covering the electrodes with an insulator has been proposed. Also, a technique to further improve the performance of a separator while maintaining the original performance has been proposed.
For example, JP-A-10-241657 (Patent Document 1) and Japanese Patent No. 3253632 (Patent Document 2) propose a technique in which an insulating material particle aggregate layer made of insulating material particles is placed between a positive electrode and a negative electrode. JP-A-2001-319634 (Patent Document 3) proposes a technique in which a ceramic complex layer including a matrix material, such as polyvinylidene fluoride, and inorganic particles is disposed on polyethylene. In addition, a separator (Celgard) formed by laminating polyethylene and polypropylene is commercially available.
These techniques are some examples of techniques to improve heat resistance, which has been a problem in known separators formed solely of a polymer film such as a polyolefin film. That is, in the case where pores are closed to suppress ion permeation, but heat generation cannot be suppressed and the temperature rises, because a separator made of a polyolefine has poor heat resistance, this may cause a meltdown of the separator, resulting in an internal short circuit. According to these techniques, even when a separator substrate undergoes a meltdown, an internal short circuit can be suppressed.
However, according to these techniques, an electrode is provided with a short-circuit prevention layer, while a separator is provided with a functional layer made of inorganic or organic components. As a result, the electrodes and the separator have an increased thickness, and this is disadvantageous in improving capacity. In particular, the formation of a functional layer has the problems of difficulty in selecting the material and complexity of the process.
In order to solve these problems, the following patent documents propose techniques to cover a separator with a thin inorganic membrane.
That is, JP-A-2004-14127 (Patent Document 4) proposes a technique to form an inorganic oxide porous membrane on an organic porous film by the sol-gel method. Japanese Patent No. 3771314 (Patent Document 5) describes a separator including a polyolefine porous film and an inorganic thin film formed on the cavity surface of the polyolefine porous film, the cavity surface having been subjected to an easy-adhesion treatment. Japanese Patent No. 3797729 (Patent Document 6) proposes a technique to cover a plastic film having poor heat resistance with a ceramic made of a SiO2 membrane.
According to the techniques of these three patent documents, a silicon organic compound or the like is applied, and the organic substances are removed to form an inorganic membrane. However, these techniques are problematic in that application and drying are necessary, making it difficult to form an inorganic membrane at low cost.
In order so solve these problems, JP-A-2005-196999 (Patent Document 7) proposes a technique to form an inorganic membrane on the surface of a separator substrate by deposition and sputtering. This technique is advantageous in that an inorganic membrane can be easily formed.