In comparison with a primary battery having a discharge function only, a secondary battery such as a capacitor having both charge and discharge functions is fabricated by various methods for electrically connecting an internal current source to an external resistor. As a result, such connection methods significantly affect not only resistance and efficiency of the secondary battery, but also productivity of the secondary battery itself and use convenience thereof.
Therefore, there is a strong need for a terminal connection method capable of increasing electric capacity and reducing internal resistance, and an electrical energy storage device functioning as a secondary battery and fabricated by the terminal connection method.
FIG. 1 is a perspective view of a conventional cylindrical electrical energy storage device, FIG. 2 is a plan view of the cylindrical electrical energy storage device shown in FIG. 1, FIG. 3 is a cross-sectional view taken along the line I-I′ of the cylindrical electrical energy storage device shown in FIG. 1, and FIG. 5 is a plan view of an electrode winding body included in the conventional cylindrical electrical energy storage device shown in FIG. 1.
Referring to FIGS. 1 to 3 and 5, the conventional cylindrical electrical energy storage device includes an electrode winding body 10 for generating charge transfer by oxidation and reduction reactions with electrolyte, a terminal plate 20 electrically connecting the electrode winding body 10 to an external resistor, and a can 30 for fixing the terminal plate 20 to the electrode winding body 10 and sealing the electrolyte and the electrode winding body 10 from the outside.
The electrode winding body 10 has a cylindrical shape in which a positive electrode 16 generating electrons by an oxidation reaction, a negative electrode 18 absorbing the generated electrons to cause a reduction reaction, and separation layers 14 physically separating the negative electrode 16 from the positive electrode 18 and isolating the places where the oxidation and reduction occur, which are sequentially wound around a winding core 12. A plurality of positive electrode leads A formed by a positive electrode collector and a plurality of negative electrode leads B formed by a negative electrode collector project separately from one end of the winding body 10 to form a substantially cylindrical shape.
The terminal plate 20 includes positive and negative electrode terminals 24 and 28, positive and negative electrode connection plates 22 and 26 connecting the positive and negative electrode leads A and B to the positive and negative electrode terminals 24 and 28, and a coupling member 21 to which the positive and negative electrode terminals 24 and 28 and the positive and negative electrode connection plates 22 and 26 are fixed. The positive electrode connection plate 22 is in contact with the positive electrode lead A by a positive electrode lead connection part 22a, and the negative electrode connection plate 26 is in contact with the negative electrode lead B by a negative electrode lead connection part 26a. 
The positive and negative electrode connection plates 22 and 26 are integrally formed with the body, the lead connection parts 22a and 26a, and the terminals 24 and 28 to form a disc shape. The positive and negative electrode connection plates 22 and 26 may be integrally formed by die-casting or casting, or the lead connection parts 22a and 26a and the positive and negative electrode terminals 24 and 28 may be connected to the body by any one of welding, soldering, and brazing. A projection 21a is formed in the center of the terminal plate 20 to be inserted into the winding core 12 during manufacture of a battery, thereby positioning the connection plates 22 and 26.
The can 30 has a cylindrical structure with one open end and accommodates the electrode winding body 10. After accommodating the electrode winding body 10, the terminal plate 20 is fixed such that the leads A and B formed at an upper end of the electrode winding body 10 are in contact with the lead connection parts 22a and 26a and seals the can 30. Here, in order to increase the sealing effect, a sealing material 29 such as rubber may be used. The can 30 may be formed of a metal material such as aluminum, stainless steel, tin-plated steel, and the like, or formed of a resin material such as PE, PP, PPS, PEEK, PTEE, ESD, and the like. The material for the can 30 may be different depending on the kind of electrolyte.
After accommodating the electrode winding body 10 in the can 30 and sealing the can 30 using the terminal plate 20, the electrolyte is injected into the can 30 through an injection hole H to complete the conventional electrical energy storage device 90.
However, the above-described conventional electrical energy storage device 90 has the following problems.
First, it is not easy to connect the terminal plate 20 of the electrical energy storage device to the polarity-leads A and B of the electrode winding body.
In particular, the polarity-leads A and B may not be securely fixed to the terminal plate 20 when they are joined to the terminal plate 20 by welding or soldering.
Moreover, the welding or soldering may cause a contact failure between the polarity-leads A and B and an interconnecting member 40, which increases the entire internal electrical equivalent resistance.
During the welding process, the electrical energy storage device is joined by laser welding or ultrasonic welding. The laser welding uses a beam having high energy produced by stimulated radiation between energy levels of atoms or molecules.
Here, since the laser beam provides a concentrated heat source having a high energy density, it has little thermal influence on the materials and causes little thermal deformation, and thus it is used for precise welding, cutting, etc.
The laser welding has the advantage of easy operation because it can be done in the air and the laser beam can be easily directed to a place away from a laser generator. However, conventionally, only if the terminal plate has a thickness greater than a predetermined value, it is possible to cope with an increase in internal pressure. Therefore, when the lead connection parts of the electrode winding body having a very small thickness and the terminal plate having a large thickness are directly joined together by laser welding, there is a significant difference in thermal effects between the terminal plate and the lead connection parts, and thus the welding is not properly performed.
Meanwhile, the ultrasonic welding is a kind of pressure welding, in which the objects to be welded are stacked together and inserted between a welding chip and a pressure receiving portion and then ultrasonic vibration is applied from the welding chip to the objects while a low static pressure is applied thereto. In this method, the friction on the bonding surfaces caused by the vibration destroys any oxide film on the surfaces and causes local plastic deformation such that newly exposed metal surfaces are closely adhered to each other. Moreover, a local increase in temperature caused by the frictional heat accelerates diffusion and re-crystallization of atoms, which results in the formation of a strong pressure-welded portion. By means of this method, the polarity-leads and the terminal plate may be joined together. However, when the leads of the electrode winding body having a very small thickness are directly ultrasonic-welded to the terminal plate having a large thickness, there is a significant difference in thermal effects between the terminal plate and the leads, and thus the welding is not properly performed
Due to this instability of welding, in the case that polarity-leads of an aluminum electrode winding body (collector) having a thickness of 10 to 40 μm are directly welded to a terminal plate having a thickness of 0.5 to 1.0 mm, for example, a high laser energy sufficient to basically melt the thick terminal plate is used, and thus the polarity-leads are significantly affected by a minute change in the shape of the objects to be welded and by a change in contact state. As a result, the welding may not be properly performed or the objects may be excessively melted. The improper welding increases the internal resistance of a capacitor, and when the objects are excessively melted, the insulating paper may be burned to cause an insulation failure between the two electrodes.