Since alkaline secondary batteries comprising a positive electrode, separator and negative electrode have superior charging and discharging characteristics, superior overcharging and over-discharging characteristics, and can be used repeatedly owing to their long life, they are widely used in electronic equipment having extremely small size and weight. Non-woven fabric used in alkaline battery separators is known to fulfill roles that include separation of the positive and negative electrodes, prevention of short-circuit, retention of electrolyte, and permeation of gas generated by the electrode reactions. Consequently, this non-woven fabric is required to have alkaline resistance, hydrophilicity, liquid retention and oxidation resistance with respect to the electrolyte, and heat resistance with respect to the working temperature. In addition, a non-woven fabric for alkaline battery separators is also required to be provided with running stability when in a wound configuration in addition to mechanical properties such as tensile strength so as to be able to oppose the tension applied in the battery production process.
In recent years, in addition to allowing rapid charging and large current discharge, alkaline secondary batteries have also come to be required to have larger capacities. Increasing battery capacity can be realized by increasing the amounts of positive electrode active material and negative electrode active material. Consequently, attempts have been made to reduce the thickness of the separator by lowering the weighting capacity, namely the basis weight, of the separator non-woven fabric. However, if the thickness of the separator is reduced by lowering the basis weight of the separator non-woven fabric, since the liquid retention of the separator typically decreases, the life of the separator non-woven fabric shortens due to drying of the liquid resulting from repeated charging and discharging. In the case of dry non-woven fabric in particular, lowering of the basis weight causes a considerable loss of non-woven fabric uniformity, thereby increasing the susceptibility to short-circuit between the positive and negative electrodes and lowering leakage resistance. In addition, even in the case of a wet non-woven fabric, there is the risk of being unable to employ a wound configuration due to the significant reduction in tensile strength.
In consideration of the aforementioned reasons, the basis weight of non-woven fabric for alkaline battery separators is typically within the range of 50 to 80 g/m2 and the thickness is typically within the range of 120 to 200 μm, and the capacity of alkaline secondary batteries was unable to be significantly improved.
On the other hand, a non-woven fabric using aliphatic polyamide fibers such as fibers made of Nylon 6 or Nylon 66 has come to be used as a non-woven fabric for alkaline battery separators that has superior hydrophilicity and liquid retention with respect to electrolyte and low electrical resistance when containing electrolyte. Alkaline secondary batteries using this aliphatic polyamide fiber non-woven fabric have superior alkaline resistance, high hydrophilicity and superior electrolyte retention, while also having the characteristic of superior discharge characteristics for large currents. However, this non-woven fabric lacks chemical stability, and has inferior heat resistance as represented with the glass transition temperature as well as inferior oxidation resistance at high temperatures in particular. Consequently, it has the disadvantage of being susceptible to oxidation and decomposition by oxygen gas generated during charging of the alkaline secondary battery, and causes a significant decrease in battery performance when the alkaline secondary battery is used under temperature conditions within the range of 60 to 80° C. Thus, alkaline secondary batteries in which an aliphatic polyamide fiber non-woven fabric is used for the separator non-woven fabric demonstrate large self-discharge caused by decomposition of the non-woven fabric, and particularly in the case of alkaline secondary batteries that undergo repeated charging and discharging at high temperatures, the cycle life is shortened considerably.
On the other hand, polyolefin fiber non-woven fabric has been used in alkaline secondary batteries requiring heat resistance at comparative high temperatures. Although polyolefin fiber non-woven fabric has superior heat resistance, since it is hydrophobic, it is resistant to wetting by electrolyte and has a low electrolyte retention volume. Consequently, this non-woven fabric has high electrical resistance when used as the separator non-woven fabric of an alkaline secondary battery, and is inferior in terms of rapid battery charging and large current discharge as compared with polyamide fiber non-woven fabric. In addition, since there is the risk of electrolyte retained between the fibers being pushed out from inside the separator due to the pressure of oxygen gas generated from the positive electrode during charging, eventually causing the positive electrode to expand due to repeated charging and discharging, there is the risk of the occurrence of dry out in cases in which the liquid retention of the alkaline battery separator non-woven fabric is insufficient.
Therefore, attempts have been made to treat alkaline battery separator non-woven fabric that uses polyolefin fibers with a surfactant. However, there are problems with the stability of the surfactant in electrolyte. In addition, since the surfactant is released when the period while repeated charging and discharging is in progress has elapsed, this has not led to adequate improvement of absorption and retention of electrolyte.
In order to solve the problem of hydrophobicity of alkaline battery separator non-woven fabric composed of a polyolefin fiber non-woven fabric, numerous methods have been proposed for improving absorption or retention of electrolyte by imparting hydrophilicity to the polyolefin fibers. For example, sulfonation treatment consisting of treatment with hot conc. sulfuric acid, fuming sulfuric acid or chlorosulfuric acid is disclosed in Japanese Unexamined Patent Publication No. Sho. 56-3973 and Japanese Unexamined Patent Publication No. Sho. 58-175256, while a method in which the structural surface of non-woven fabric is modified by fluorine treatment by treating with a gas containing fluorine, acrylic acid graft polymerization treatment in which groups having a hydrophilic group such as in acrylic acid or methacrylic acid are graft polymerized, corona discharge treatment or reducing fiber diameter and so forth is disclosed in Japanese Unexamined Patent Publication No. Hei. 1-132042. However, since the hydrophilic treatment methods described in these examples of the prior art cause a considerable decrease in strength in the alkaline battery separator non-woven fabric, cause deterioration of the appearance or attempt to reduce thickness by lowering the basis weight, they have problems including difficulty in enabling stable industrial production.
Therefore, inventions that use aromatic polyamide fibers or completely aromatic polyamide fibers for the alkaline battery separator non-woven fabric instead of aliphatic polyamide fibers are disclosed in, for example, Japanese Unexamined Patent Publications Nos. Hei. 5-283054, Sho. 53-58636 and Sho. 58-147956. Non-woven fabric composed of aromatic polyamide fibers or completely aromatic polyamide fibers typically have superior hydrophilicity as well as superior alkaline resistance and oxidation resistance. However, due to their high heat resistance, the adhesiveness itself of a non-woven fiber formed only of these fibers is low, and since the adhesiveness with typical thermoplastic binder fibers is particularly low, the non-woven fabric strength is inadequate. Although methods that use an adhesive resin have been considered for improving adhesiveness, when a non-woven fabric adhered according to these methods is used as a battery separator, there is the risk of the adhesive resin dissolving in the battery electrolyte.
So-called semi-aromatic polyamide fibers have been proposed to improve on the problem of adhesiveness. An alkaline battery separator that uses a semi-aromatic polyamide fiber (MXD-6 fiber) non-woven fabric formed from aromatic diamine and aliphatic dicarboxylic acid has inferior oxidation resistance at high temperatures, and may deteriorate as a result of being oxidized by oxygen gas generated during charging. On the other hand, semi-aromatic polyamide fibers formed from aliphatic diamine and aromatic dicarboxylic acid being able to be preferably used in battery separators as fibers having hydrophilicity, alkaline resistance and oxidation resistance is disclosed in, for example, Japanese Unexamined Patent Publications Nos. Hei. 9-259856 and 2002-151041.
However, semi-aromatic polyamide fibers formed from aliphatic diamine and aromatic dicarboxylic acid have low fiber strength and leakage resistance cannot be said to be adequate. In addition, since they also have high heat resistance in the same manner as aromatic polyamide fibers, it is necessary to mix them with thermoplastic binder resin fibers such as polyolefin fibers or others to increase adhesiveness between fibers and enhance the non-woven fiber strength. However, since the melting point of the thermoplastic binder resin is lower than the temperature environment in alkaline secondary batteries are used in large equipment (160° C. or higher), there is the risk of having a detrimental effect on long-term stability of the battery separator.
In this manner, semi-aromatic polyamide fiber non-woven fabric is promising as a non-woven fabric for alkaline battery separators. However, problems still remain with respect to adhesiveness with binder resin fibers, leakage resistance, further inhibition of self-discharge phenomena, and improving yield during alkaline secondary battery production in the case of reducing thickness by lowering the basis weight of the separator non-woven fabric. Moreover, this non-woven fabric is also unable to effectively respond to severe requirements for increasing battery capacity on the premise of rapid charging and large current discharge. Therefore, the object of the present invention is to provide a non-woven fabric for an alkaline battery separator, which is based on the use of a semi-aromatic polyamide fiber non-woven fabric, allows rapid charging and large current discharge, enables thickness to be reduced for higher battery capacity, and has superior alkaline resistance and a method for producing the same.