1. Field of the Invention
Aspects of the present invention relate to an electrode plate winding device and rolling method for a cylinder type battery and, more particularly, to an electrode plate winding device and rolling method for cylinder type battery that is to increase the capacity of a battery by rolling the electrode plate in a cylinder-like shape.
2. Description of the Related Art
In general, batteries are divided into primary and secondary batteries. Secondary batteries are rechargeable while primary batteries are not. Secondary batteries are able to charge and discharge and are widely used in the field of cutting-edge electronics such as cellular phones, notebook computers, and camcorders, etc. In particular, a lithium secondary battery is increasingly used since it has high energy density per unit weight and has an operating voltage of 3.6V, which is 3 times the operating voltage of a nickel-cadmium battery or a nickel-metal hydride battery.
The lithium secondary battery uses a lithium based oxide as a cathode active material and a carbon material as an anode active material. In addition, the lithium battery can be made of various shapes, such as cylindrical, rectangular, and pouch-type.
Among them, a cylinder type secondary battery is composed of an electrode assembly, a cylindrical can, which houses the electrode assembly and an electrolytic solution, and a cap assembly that seals the cylindrical can and provides for safety checks in case of high temperatures or pressures. The electrode assembly is generally electrically connected to the cylindrical can and the cap assembly.
The electrode assembly is simply a stack of plates rolled into a cylinder. Here, two rectangular-shaped plate electrodes with a separator arranged between the two electrodes, which prevents a short-circuit between two electrodes, are stacked, laminated, and rolled into a cylinder; and thus, a jelly-roll-type electrode assembly is made.
Each individual electrode plate is attained by applying an active material slurry to a current collector composed of a metallic foil. For example, the positive electrode collector, often a thin aluminum foil, will be coated with a positive electrode active material. However, each electrode generally contains two uncoated parts: one uncoated part on a first-end and one uncoated part on a back-end of each current collector. The first-end of each electrode is located at the outer circumference of the resultant cylindrical electrode assembly and the back-end of each electrode plate is located in the center of the resultant cylindrical electrode assembly. An electrode tap is connected to each electrode plate at one of the uncoated parts. The electrode taps electrically connect one electrode plate to the cylindrical can and the other electrode plate to the cap assembly. The electrode assembly is insulated by the cylindrical can so that the current may only travel through the electrode taps resulting in a cell that is able to be easily inserted into a circuit for charging or discharging. In general, one of the electrode taps is drawn upward from the electrode assembly in the direction of the cap assembly, and the other electrode tap is drawn downward from the electrode assembly to the bottom of the cylindrical can.
In order to form this cylindrical electrode assembly, the positive electrode plate, separator, and negative electrode plate are rolled with a mandrel. The conventional structure of the mandrel of the electrode plate winding device and rolling method are as follows:
The mandrel, which winds the electrode assembly cylindrically, has a generally round cross-section, but is divided into two parts by means of the groove. The back-end of the laminated stack of the positive electrode plate, the separator, the negative electrode plate, and another separator is inserted and fixed into the groove of the mandrel.
Then, the mandrel is rotated. The laminated electrode plates and separators are wound around the outer surface of the mandrel with the uncoated first-end of the stack at the outer circumference of the resultant cylindrical electrode assembly. The electrode assembly is completed by fixing a tape about the outer circumference of the electrode assembly so that the electrode assembly does not unwind.
However, the conventional structure of the mandrel for the electrode plate winding device has the following disadvantages:
In the cylindrical electrode assembly, one of the two electrode taps, which are connected to the uncoated part of the electrode plates, is drawn from the central core inside the electrode assembly. For example, when the number of turns is 18, the uncoated part in which the electrode tap is connected is wound at a first or a second turn.
There is a lower limit for reducing the thickness and width of the electrode taps as each electrode tap requires a certain level of strength and conductivity. The electrode taps are generally of a thickness of about 0.3-0.5 mm. Considering that the thickness of each electrode plate is generally about 150 μm and the thickness of each separator is generally about 20 μm, the thickness of the electrode tap is relatively much thicker.
Because the uncoated part to which an electrode tap, which is of a relatively great thickness and a rectangular plate-shape, is connected is wound around the outer surface of the mandrel at the early stage, the portion of the electrode assembly to which the electrode tap connects has the greater curvature than electrode plate at the same turn number within the electrode assembly. That is, the portion of the electrode plate at which the electrode tap connects in turn 2 of the electrode assembly will have more curvature than another portion of the electrode plate in turn 2 of the same electrode assembly where the electrode tap does not attach resulting in an electrode assembly that is not cylindrically wound. The external shape of the electrode assembly will not have a cylindrical shape because of the placement of the electrical tap.
Because of the noncylindrical resultant shape of the electrode assembly, the battery capacity per volume is reduced as the number of turns achievable in the same volume is fewer. Furthermore, with the noncylindrical electrode assembly the insertion of the electrode assembly into the cylindrical can is more difficult and requires more volume.