Recently, lithium secondary batteries have been used in various fields including portable electronic devices such as mobile phones, personal digital assistants (PDAs), and laptop computers. In particular, in line with growing concerns about environmental issues, research has been actively conducted into lithium secondary batteries having high energy density and discharge voltage as a power source for electric vehicles able to replace fossil fuel-powered vehicles such as gasoline vehicles and diesel vehicles, which are major causes of air pollution, and some of the research is in the commercialization stage.
As devices in which lithium secondary batteries may be used have diversified as described above, lithium secondary batteries have also been diversified to have outputs and capacities suitable for the devices in which the lithium secondary batteries are used.
A pattern coating process is used in which a plurality of coating portions coated with an electrode active material and a plurality of uncoated portions not coated with an electrode active material are formed, such that a battery can be flexibly deformed without causing cracks in an active material coating layer or separation of the coating layer from a current collector even when the battery is bent beyond a certain angle due to the design of various devices.
However, during the pattern coating process, an active material slurry is dragged from one end of the coating portion, and there may be a problem of a drag line, a line along the path on which the active material slurry is dragged, formed on a surface of the uncoated portion. The drag line or drag portion is an abnormal electrode coating portion, which may cause an internal short circuit between a positive electrode and a negative electrode, thereby being a cause of threatening battery stability and increasing a defect rate of the battery.
The drag line may be adjusted by a coating process or physical properties such as viscoelasticity of an active material slurry to be coated, and the inventors of the present invention have found a method of minimizing the drag line in the pattern coating process by controlling the physical properties of the slurry by improving a mixing process of the slurry, thereby completing the present invention.