The present invention relates to a nonaqueous electrolyte secondary battery, and in particular, to a nonaqueous electrolyte secondary battery with a rectangular type battery case.
Various types of batteries are now used as power source for small-size electronic devices. As power source for the devices such as cellphone (handy phone), notebook-sized personal computer, cam-corder, etc., nonaqueous electrolyte secondary battery is used. These nonaqueous electrolyte batteries can be cylindrical type batteries or rectangular type batteries.
To prepare a lithium ion battery as a power source for small-size electronic devices, a positive electrode and a negative electrode are produced by coating active components on a positive electrode current collector and a negative electrode current collector, respectively. These electrodes are laminated on each other with a separator interposed between them, and they are wound up, and a battery element is prepared. This battery element is accommodated in a battery case and is sealed to be used as a sealed battery.
These batteries can be cylindrical type batteries and rectangular type batteries. A cylindrical type battery is produced by accommodating a battery element wound up in cylindrical shape into a cylindrical battery case. The rectangular type battery is produced by accommodating the battery element into a battery case in the shape other than cylindrical shape, e.g. in from of rectangular parallelepiped.
In the cylindrical type battery, the battery element is designed in cylindrical shape. When it is accommodated in the battery case, the force is evenly applied on the battery element, and battery reaction also occurs evenly without being deviated to a specific point.
However, the devices such as notebook-size personal computer, cellphone, etc. where the batteries are used, are usually designed in form of rectangular parallelepiped. In this type of battery accommodating compartment, there is a problem in that ineffective or useless volume is increased in the cylindrical battery because the diameter of the cylindrical type battery is limited by the thickness of the battery accommodating compartment. Problem may arise in that it is not possible to use a cylindrical type battery of large capacity for a thin-walled type devices. In this respect, for small-size or thin-walled type devices, a rectangular type battery with smaller thickness is used instead of a cylindrical type battery.
FIG. 3 shows a conventional type nonaqueous electrolyte secondary battery. FIG. 3(A) is a side view of a battery element for a rectangular type battery, and FIG. 3(B) is a plan view of a battery element for a rectangular type battery. FIG. 3(C) is a plan view of a rectangular type battery case with the battery element accommodated in it as seen from above.
On a material containing iron as main component, a nickel-plated layer is formed, and this is used as a battery case of a nonaqueous electrolyte secondary battery 1. In the battery case 2, a battery element 3 is accommodated, which is produced by winding up a positive electrode with a positive electrode active component coated on it and a negative electrode with negative electrode active component coated on it with a separator interposed between them. A positive electrode tab 4 is mounted on the positive electrode, and a negative electrode tab 5 is mounted on the negative electrode.
In general, belt-like members are laminated on each other in the order of positive electrodexe2x80x94separatorxe2x80x94negative electrodexe2x80x94separator. Using a winding device, the negative electrode is wound up on outer side, and a separationxe2x80x94preventive tape 7 is used to fix the separator surface on the outermost periphery and to prevent partial separation of the battery element. Then, the battery element is accommodated in the battery case 2.
The separator on the outermost periphery protects the battery element. It is also used for the purpose of improving the sliding performance between the battery element and inner surface of metal material such as aluminum, stainless steel, or nickel-plated soft steel used as the battery case. Further, it is used for the purpose of facilitating the insertion of the battery element or of preventing deformation of the wound-up battery element, just as the separation of outer shell of a bamboo shoot, due to the friction with the inner surface of the battery case.
However, the separator on the outermost layer is originally not required, and, when it is used, it adds additional ineffective thickness to the battery element. In the thin-walled battery used in recent years, the thickness of the separator on battery capacity gives increasingly more serious influence.
For instance, in case of a lithium ion secondary battery of 4 mm in thickness, a battery element of about 3.2 mm in thickness is used. The separators each of 0.05 mm in thickness arranged on each side on outer periphery occupy about 3% of the total volume of the battery element
The rectangular type lithium ion secondary batteries are also used in small-size devices, and there are now strong demands on the reduction of battery volume and on the increase of battery capacity. Therefore, the reduction or elimination of the portions not contributing to the battery capacity in the battery is very important to increase the battery capacity.
It is an object of the present invention to provide a rectangular type lithium ion secondary battery, by which it is possible to reduce or eliminate the portions not contributing to the battery reaction of the battery element, and to provide a rectangular type nonaqueous electrolyte secondary battery, which has smaller thickness and larger capacity and has a battery element stable in form and easy to assemble.
The present invention provides a nonaqueous electrolyte secondary battery, comprising a battery element produced by laminating and winding up a positive electrode and a negative electrode with a separator interposed therebetween, a separator is not provided on the outermost periphery of the battery element, a current collector surface of one of the positive electrode or the negative electrode is exposed to outside, and a separation-preventive tape is attached on the battery element except a part of one of the longer sides on the outermost periphery.