The invention relates to a method of manufacturing a lithium battery comprising a stack of a negative electrode, a separator, and a positive electrode, which method comprises the steps of applying negative electrode material on a negative current collector so as to form the negative electrode, applying positive electrode material on a positive current collector so as to form the positive electrode, and arranging a separator between the negative electrode and the positive electrode, and which method comprises the following steps:
a) producing a pattern of holes in the negative electrode;
b) producing a pattern of holes in the positive electrode;
c) applying a polymeric material on at least one side of the stack and subjecting the stack and the polymeric material to heat and pressure, so that the polymeric material penetrates the hole, whereby which the negative electrode, the positive electrode, and the separator are stuck and pressed together.
The growing market for lightweight, portable cordless consumer products, such as CD-players, mobile telephones, laptop computers and video cameras, has increased the need for high-density batteries. Specifically, very thin and flexible batteries are required. If an acceptable portability is to be achieved, the batteries contained in said consumer products should provide the necessary amount of energy at the smallest possible weight and volume. Lithium is a very advantageous material for use in batteries in which a high energy density at a minimum weight is required. However, the thinner the battery, the more difficult the application of a pressure needed to maintain a sufficient contact between the respective components of the battery.
A method of manufacturing a lithium battery according to the preamble is known from the International patent application with publication number 00/04601.
The battery obtained by said method has thin and flexible shape and at the same time provides a very high energy density. Moreover, the contact between the electrodes and the separator is obtained and maintained in a very efficient way. The battery can be packed in a thinwalled canister, as the wall of such canister is not needed for maintaining a sufficient pressure on the respective components of the battery. In one of the methods according to the International application 00/04601, a film of a polymeric material is applied to both sides of the stack, and said polymeric film is subjected to heat and pressure. As a result thereof, the polymeric material melts and penetrates into the holes. A battery is obtained by said method with a polymeric film at both sides of the stack as well as polymeric material in each of the holes acting as a plug or rivet and sticking to the respective layers, causing these layers to be bonded together.
It is an object of the invention to provide a method of manufacturing a lithium battery according to the preamble by which an even thinner battery can be created.
To this end, the method of manufacturing a lithium battery according to the preamble is characterized in that the polymeric material comprises bulges which are partially located in the holes of the electrode(s).
By providing such bulges of polymeric material at the ends of the holes in the electrode(s), enough polymeric material is provided to fill the holes and stick together the electrodes and the separator, whereas on the outer sides of the stack, in between the holes, the polymeric material may be very thin or even be absent. The battery thus obtained can provide the same amount of energy as the battery obtained according to the prior art method, but at a smaller volume. Thus, the battery obtained according to the method of the present invention has a higher capacity.
In a particular embodiment, the polymeric material comprises a polymer foil which is provided with bulges.
As was mentioned above, the polymer foil itself may be very thin, while the bulges have to comprise enough polymeric material to fill the holes in the electrodes, thereby sticking together the electrodes and the separator.
In a preferred embodiment, the polymeric material is placed on a carrier foil.
Placement of the polymeric material on a carrier foil facilitates the handling thereof. After stacking of the battery, the carrier foil may either be removed or be left in place.
Advantageously, the carrier foil comprises a polymeric material with a high melting point.
If such carrier foil is provided, the carrier foil itself will not melt during the application of heat, whereas the polymeric material will melt, thereby penetrating the holes of the electrodes.
In a further preferred embodiment, the polymeric material is placed in a patterned arrangement of bulges on the carrier foil.
During the application of heat the bulges will melt and penetrate into the holes in the electrodes. Afterwards the carrier foil can be removed. In the battery thus obtained, the polymeric material which has penetrated the holes in the electrodes is substantially not present at the outer sides of the electrodes.
In another advantageous embodiment, the polymeric material is placed in a patterned arrangement of bulges between portions of a carrier.
Also in this case, after the melting of the polymeric material and penetration thereof into the holes, the carrier foil can be removed from inbetween the openings of the holes in the electrodes. A battery is thereby obtained in which the polymeric material which has penetrated the holes in the electrodes is substantially not present at the outer sides of the electrodes.
In the above methods, use is preferably made of a porous polymeric material for the polymeric material which has to enter the holes in the electrodes, e.g. porous polyethylene. Preferably, the polymeric material is elastic. The carrier foil which may be used preferably comprises a strong foil, such as Mylar(trademark) or Kapton(trademark).
The electrode materials can be made by mixing negative or positive active material, conductive material, and binder material, which are all in the form of powder, in a dry process, or in a wet process in which water or an organic solvent is further added.
The paste-like mixture obtained is then provided over the current collector, dried and compressed. For this purpose, one of the following coating methods can be generally employed: screen printing, roller coating, doctor blade coating, knife coating, extrusion coating, bar coating, dip coating and squeeze coating. The thickness of the coated layer, which is compressed after drying, generally is in the range of 1 to 1000 xcexcm.
Pressing of the stack in the above methods is accomplished by simply pressing it for a short period of time between heated metal plates at a pressure of about 5.104 Pa at about 110 to 150xc2x0 C. The operation may also be carried out using calender rollers. After heating, the stack is cooled down to room temperature. Heating and cooling down may also be carried out in a mold with a particular shape, e.g. the shape of the appliance into which the battery is to be fitted.
In the same way as described above, a multilayer stack of layers can be bonded together in one step, thereby obtaining a battery of increased capacity or voltage.
The lithium battery of the invention can be used in various (cordless) appliances, for example notebook personal computers, portable CD-players, portable telephones, paging equipment, video cameras, electric shavers, electric tools, electric vehicles, and hearing aids. The lithium battery may be used as a primary or as a secondary battery.