1. Field of the Invention
The present invention relates to a method of manufacturing a lithium secondary battery, and more particularly, to a method of manufacturing a lithium secondary battery having a high ionic conductivity, good mechanical properties, a stable interface characteristic, a good discharging characteristic at a high and low temperature and an efficient discharging characteristic.
2. Description of the Prior Art
There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as cellular communication, satellites, portable computers and electric vehicles. In particular, great effort has been dedicated to the development of a lithium ion battery having a cathode including lithium, an anode including lithium or carbon and a non-aqueous electrolyte, because of its higher energy density than that of a lead storage battery or nickel-cadmium battery having an aqueous electrolyte.
However, recently, the widely used lithium ion battery which has a satisfactory ionic conductivity uses a liquid electrolyte, however, the leakage of the liquid electrolyte occurs freuently. Moreover, any leakage in the cell lessens the performance of the battery. Accordingly, lithium ion batteries are packaged with an aluminum can and are provided with various protective devices, thereby enlarging the volume of the cell, and reducing the energy density to an undesirable degree. Furthermore, such a lithium ion battery is not applicable to a battery having thickness of 3 mm or less.
In contrast, lithium ion polymer batteries which utilize polymer electrolyte instead of the liquid electrolyte, are free from problems of leakage, have an improved stability and have a reduced volume because the electrolyte is impregnated into a polymer matrix. In addition, since the polymer electrolyte and electrodes can be attached together, a stack-type battery as well as a winding-type battery can be manufactured. However, they tend to exhibit inferior properties compared to the liquid electrolytes. This is attributed by the fact that ionic conductivities for the solid electrolytes are often 5-100 times inferior to that of the liquid electrolytes.
In general, a polymer lithium secondary battery includes an anode, a polymer electrolyte and a cathode. The components are selected to satisfy various conditions of the secondary battery such as lifetime, capacity, temperature characteristic, stability, etc.
As for the components of the cathode applied to the lithium ion polymer battery, lithium oxide complex (LiCoO2, LiMn2O4, LiNiO2) which has a laminated structure and lithium ion can be inserted between layers or separated from layers, can be used. As of the components for the anode, carbon compounds such as graphite compounds or coke can be used and these are examples of which include mesocarbon microbeads (MCMB) and mesophase carbon fiber (MPCF).
A polymer electrolyte which is widely used as a main component of the lithium battery is free from the leakage problem. The manufacturing of the battery using the polymer electrolyte does have some advantages, however, the polymer electrolyte is required to have a good ionic conductivity, a thermal and electrochemical stability, a good mechanical strength and a good adhesiveness to the electrodes.
The polymer electrolytes which is currently used or under development include a main liquid-type organic solvent such as ethylene carbonate and propylene carbonate, a vice liquid-type organic solvent such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, and polyvinylidene difluoride-based compounds (PVdF), polyacrylonitriles (PAN), polyethylene oxides, a copolymer thereof or a mixture thereof, which can accept lithium salts such as LiPF6 and LiAsF6.
The polymer electrolyte including the polyvinylidene fluoride compounds has a good mechanical strength. However, the adhesiveness to the electrodes is not sufficient and this requires an adhering process using heat or pressure. The solvent might evaporate during the adhering process of the electrodes to the electrolyte. Thus, films which do not contain the electrolyte are adhered to the electrodes and then additional impregnation process is implemented to the solvent.
When polyacrylate polymer electrolytes having good affinity to the solvent are used in order to increase the adhesiveness to the electrodes, it can be accomplished, however, the mechanical intensity of the electrolyte is not good.
One class of polymer electrolytes, specifically gel electrolytes in which liquid electrolyte is dispersed in a polymer matrix, includes a significant fraction of solvents in addition to the salt contained in the polymer matrix.
A method for preparing the gel electrolytes is disclosed in U.S. Pat. No. 5,456,000. A cell is assembled by integrating a polymer film with a cathode and an anode and then, laminating. Thereafter, the solvent and the electrolyte salt may be introduced to the polymer film in order to swell the battery. This battery has an advantage of allowing the cell to be fabricated in a non-dry environment. However, a film is formed from a polymer containing a plasticizer in order to facilitate the impregnation of the polymer film with the solvent. As a result, the battery is assembled, the plasticizer is then extracted out to form a micro-porous film and the solvent used for the extraction is evaporated. Such a process requires homogeneous impregnation of the polymer with the solvent and also requires many hours to lengthen the processing time.
In order to overcome the above-described problem, U.S. Pat. No. 5,219,679 discloses a method of preparing a polymer electrolyte after mixing the polymer and liquid electrolyte. In this method, the solvent is already homogeneously dispersed into the polymer prior to the assembling of a battery. An additional process of extracting a plasticizer or drying is not necessary, however, the preparation of the polymer electrolyte and the assembling of the cell should be implemented under a dry condition. Furthermore, if the polymer electrolyte contains a large amount of solvent, the mechanical strength is poor. This makes a continuous processing being difficult and an electrical short being liable to generate.
U.S. Pat. Nos. 5,585,039, 5,639,573, 5,716,421 and 5,688,293 disclose polymer electrolytes prepared by filling polymer electrolytes into porous films which is good enough to overcome the problems of the mechanical strength. According to the method introduced in these patents, a filling process or a coating process of the electrolyte into or onto the porous film is additionally implemented, thus complicating the manufacturing process of the battery.
Therefore, it is preferred that a gel polymer electrolyte containing a polymer and a solvent is prepared, then an anode, a cathode and a polymer electrolyte thus obtained are integrated to manufacture the battery, which simplifies the manufacturing process of the battery.
In this case, since only one coating process is necessary for preparing the polymer electrolyte, the ionic conductivity, mechanical strength of the polymer and the solvent mixture and the interface adhesiveness to the electrodes are anticipated to exhibit good qualities. Since the polymer electrolyte impregnated with the solvent is integrated, a lamination method at a high temperature cannot be used. Accordingly, the polymer electrolyte should have good interface adhesiveness to the electrodes.
U.S. Pat. No. 5,849,433 discloses a method for preparing a polymer electrolyte using a material having a good mechanical strength and adhesiveness in order to improve the mechanical properties. According to the method, the polymer electrolyte is prepared by forming a film from a mixture of materials having a good mechanical strength and adhesiveness to obtain a desired polymer electrolyte and by impregnating the film with a liquid electrolyte. However, in this method, additional impregnation process of the polymer film with the liquid electrolyte is necessary to manufacture the polymer electrolyte.
It is an object in the present invention to provide a method of manufacturing a lithium secondary battery by directly coating a polymer electrolyte composition having a good mechanical strength and a good adhesiveness onto an electrode.
To accomplish the object, a method of manufacturing a lithium secondary battery is provided in the present invention. A polymer mixture including a) a polymer mixture which includes polyvinylidene fluoride-based polymer and b) at least one polymer selected from the group consisting of polyacrylonitrile and polymethyl methacrylate is mixed with a solvent in which a lithium salt is dissolved. The mixing ratio of the polymer mixture and the solvent is about 1:3-10. Thus obtained first mixture is heated to obtain a polymer electrolyte composition. Thus obtained polymer electrolyte composition is coated onto a first electrode which is one of an anode and a cathode, and then dried to obtain a polymer electrolyte layer. A second electrode which is a remaining one of said anode and cathode is attached onto the polymer electrolyte layer.
Polyvinylidene fluoride-based polymer includes a large amount of electrolyte and lithium salts and provides a good mechanical strength. Polymethyl methacrylate polymer has a good affinity to the solvent which strongly adheres the electrolyte to the electrodes. Polyacrylonitrile polymer has a good adhesiveness to the electrolyte, thus it improves the adhesiveness of the electrolyte to the electrodes without deteriorating the excellent mechanical properties of the polyvinylidene fluoride-based polymer.
According to the present invention, as a lithium secondary battery is manufactured by directly coating a polymer electrolyte composition having a good mechanical strength and a good affinity with the solvent. As a result, a lithium secondary battery having a minimized leakage and evaporation of the solvent in the polymer electrolyte, a stable charge/discharge characteristic and a high capacitance can be manufactured.