This invention relates to a film-type lithium secondary battery, and in particular to an improvement in an electrolyte for use in an electrode or a separator.
In recent years, portable devices such as a portable telephone, a PHS and a small personal computer etc. are undergoing remarkable development in fabrication into small-size and light-weight with a progress of electronics technology. Further, batteries serving as power supplies for use in these portable devices are also required to be built into small-size and light-weight form.
A lithium battery can be mentioned here as an example of a battery to be expected for use in such purpose. In addition to a lithium primary battery already put in practical use, studies have been made on the lithium secondary battery to be put it in practical use, and to achieve its high capacity and long service life.
A major example of such lithium battery is a cylindrical battery utilizing a liquid electrolyte. While, in the lithium primary battery, a film-type battery is also put in practical use by means of a method in which a solid electrolyte is used and a printing technology is applied. Utilizing this technology, many studies have been made to put the film-type battery into practical use in a field of the lithium secondary battery too.
Incidentally, the cylindrical battery is made up by a method in which an electrode group comprising a positive electrode, a negative electrode and a separator is inserted in a cylindrical container and then the liquid electrolyte is filled. In contrast, the film-type lithium secondary battery is made up by a method in which the positive electrode and the negative electrode are opposed each other through a separator comprising a solid-state or gel-state electrolyte composed of the liquid electrolyte and an organic polymer. In the film-type lithium secondary battery, a study is made for improving an initial capacity and a cycle life by maintaining a dissociation of lithium salt in the electrolyte and an ionic conductivity of lithium ion, by using a method for optimizing an organic polymer composing the electrolyte. A polyethylene oxide structure is known as a typical example of the organic polymer. Even if the electrolyte including the polyethylene oxide structure is used, however, a film-type lithium secondary battery has not been put in practical use, which can provide an initial capacity, a high-rate charge/discharge characteristic and a cycle life comparing with those of the cylindrical lithium secondary battery.
The following four points (1) to (4) may be mentioned for reasons of the above defects.
(1) Since the liquid electrolyte is used for the cylindrical battery, a degree of freedom of ion species in the electrolyte is large. Therefore, it is easy in the cylindrical battery to maintain the ionic conductivity of lithium ion in the electrolyte at a level sufficient for functioning as a battery. On the contrary, since the solid-state or gel-state electrolyte is used in the film-type lithium secondary battery, the degree of freedom of ion species in the electrolyte is small and the ionic conductivity of lithium ion in the electrolyte is extremely small as compared with the cylindrical battery using the liquid electrolyte. Accordingly, it is difficult in the film-type lithium secondary battery to obtain an ability equal to the cylindrical lithium secondary battery.
(2) The polyethylene oxide structure forming a typical example of the organic polymer has a good affinity for the liquid electrolyte and a property to restrict the lithium ion. Therefore, the electrolyte including the polyethylene oxide structure is superior in a liquid-holding ability but it decreases a supply of the liquid electrolyte and lithium ion to an active material and causes a lowering of the capacity, especially in a high-rate charge/discharge operation.
(3) The organic polymer having a low affinity for the liquid electrolyte offers a low property to restrict the lithium ion. For this reason, it is easy in the electrolyte including the polymer to supply the lithium ion to the active material. However, it is difficult to use the electrolyte including the polymer as the electrolyte by itself, because the electrolyte is inferior in the liquid-holding ability and can not prevent a leakage of the liquid electrolyte to outside of battery system, thus causing a lack of the liquid electrolyte in the battery system to result in a lowering of its capacity.
(4) It is easy in the cylindrical battery to control electronic isolation of the active material due to swelling of the liquid electrolyte by applying a pressure to the electrode group. On the contrary, it is difficult in the film-type battery to apply the pressure to the electrodes because the positive electrode and the negative electrode are opposed each other through the solid-state or gel-state electrolyte.
On the other hand, a study has hitherto been made in the film-type lithium secondary battery for improving the cycle life by maintaining an electronic conductivity and an ionic conductivity of the active material through means of regulation of a binder mixed in the positive electrode and the negative electrode.
Fluoride group polymers such as polyethylene tetrafluoride and polyvinylidene fluoride etc. are used for a first example of the binder. A positive electrode using this binder is made up as follows. The binder is dissolved in a volatile solvent and mixed together with a positive active material, a conductive agent and an electrolyte to get a cathode composite. The cathode composite is applied to the positive current collector. Thereafter, the volatile solvent is evaporated. The same method is applied to the negative electrode too.
An organic monomer having a polymeric functional group at its molecular chain end such as polyethylene oxide diacrylate etc. for example, is used for a second example of the binder. A positive electrode using this binder is made up as follows. The binder is dissolved in a liquid electrolyte and mixed together with a positive active material and a conductive agent to get a cathode composite. This cathode composite is applied to the positive current collector. Thereafter, the organic monomer forming the binder is polymerized. In this positive electrode, the solid-state or gel-state electrolyte formed by polymerizing the organic monomer functions also for the binder as it is. The same method is applied to the negative electrode too.
However, the film-type lithium secondary battery using the binder of first example includes the following problem. It is inevitable to completely remove the volatile solvent in order to maintain the battery performance at a level sufficient for functioning as a battery, but on the other hand it is required to avoid evaporation of a plasticizer etc. contained in the electrolyte. Therefore, difficult problems are remaining in a manufacturing process.
The film-type lithium secondary battery using the binder of second example includes the following problems: (i) The polyethylene oxide structure forming the skeleton of organic polymer has a high affinity for the liquid electrolyte and an ability to restrict the lithium ion. For this reason, the electrolyte including the polyethylene oxide structure is superior in the liquid-holding ability, but it decreases a supply of the liquid electrolyte and lithium ion to an active material and causes a decrease in the capacity especially in a high-rate charge/discharge operation. (ii) The electrolyte containing the polyethylene oxide structure has a large degree of swelling against the liquid electrolyte. Accordingly, in the electrode containing the electrolyte, an electrode composite is swelled by the liquid electrolyte at time of initial charging operation, so that the active material in the electrode is electronically isolated. Consequently, an abrupt lowering of capacity occurs with a progress of cycle. (iii) The organic polymer having a structure of low affinity for the liquid electrolyte has a low property to restrict the lithium ion. For this reason, it is easy in the electrolyte containing the polymer to supply the lithium ion to the active material. However, it is difficult to use the electrolyte including the polymer as the electrolyte by itself, because the electrolyte is inferior in the liquid-holding ability and can not prevent a leakage of the liquid electrolyte to outside of battery system, thus causing a lack of the liquid electrolyte in the battery system to result in a lowering of its capacity.
This invention is made in consideration of the above-mentioned problems, and an object of this invention is to provide a film-type lithium secondary battery which can maintain an ionic conductivity of lithium ion in an electrolyte for use in an electrode or a separator, at a level sufficient for functioning as a battery, and which is superior in all of an initial capacity, a high-rate charge/discharge characteristic and a cycle life.
This invention provides a film-type lithium secondary battery in which a positive electrode and a negative electrode are opposed each other through a separator; characterized in that at least one of the positive electrode, the negative electrode and the separator contains an electrolyte having a specified structure, the electrolyte having the above specified structure is composed of a liquid electrolyte and an organic polymer, the organic polymer is formed by polymerizing an organic monomer having a polymeric functional group at its molecular chain end, the organic polymer contains in its molecule a first chemical structure and a second chemical structure, the first chemical structure is at least one of an ethylene oxide structure and a propylene oxide structure, and the second chemical structure is at least one kind selected from among an alkyl structure, a fluoroalkyl structure, a benzene structure, an ether group and an ester group.
The above-mentioned xe2x80x9cfirst chemical structurexe2x80x9d has a high affinity for the liquid electrolyte, and the above-mentioned xe2x80x9csecond chemical structurexe2x80x9d has a low affinity for the liquid electrolyte. Since the above both chemical structures are permitted to coexist in the organic polymer composing the electrolyte in the present invention, the respective chemical structures are subjected to phase separation into micro scale, as a whole the electrolyte is brought into a state where a liquid-holding ability is maintained and transfer of lithium ion is not impeded. As the result, an ionic conductivity can be obtained to such a level as sufficient for functioning as a battery in the electrolyte Therefore, a film-type lithium secondary battery superior in all of the initial capacity, the high-rate charge/discharge characteristic and the cycle life, can be obtained.
Consequently, according to the present invention, a film-type lithium secondary battery which can maintain the ionic conductivity of lithium ion of the electrolyte for use in the electrode or the separator, at a level sufficient for functioning as a battery, and which is superior in the initial capacity, the high-rate charge/discharge characteristic and the cycle life.
In the present invention, references are made mainly to a case where only the separator contains the electrolyte having the above specified structure and a case where only at least one of the positive electrode and the negative electrode contains the electrolyte having the above specified structure. However, references can also be made to a case where the positive electrode or the negative electrode and the separator contain the electrolyte having the above specified structure and a case where all of the positive electrode, the negative electrode and the separator contain the electrolyte having the above specified structure. Therefore, since the electrolyte having the above specified structure can be contained in a voluntary composition material, a workability in manufacture can be improved and a manufacturing cost can be reduced.
For the above organic monomer, reference can be made to (a) a monomer containing in its molecule the first chemical structure and the second chemical structure, (b) a mixture of an organic monomer containing the first chemical structure and an organic monomer containing the second chemical structure, and (c) a mixture including two or more kinds of a compound expressed by equation (I), a compound expressed by equation (II) and a compound expressed by equation (III). In concrete, the compound expressed by equation (I) may be mentioned for the above organic monomer of the article (a). Incidentally, symbols X in the equations (I) through (III) represent a j-valent connecting group, and a structure represented by equation (IV) for example may be mentioned therefor. However, the X is not limited to these structures. Therefore, allowance for selecting the organic monomer i.e. a possibility of use of this invention can be improved. 
X: j-valent connecting group
R1:xe2x80x94CpH2pxe2x80x94 p=0xcx9c10
R2:xe2x80x94CqH2qxe2x80x94 q=0xcx9c10
R3:xe2x80x94CtH2txe2x80x94 t=0xcx9c10
R4:CvH2v+1 v=0xcx9c5
r, s: s=1 when r=0, s=0 when r=1
J=1xcx9c6 m=0xcx9c3 k=1xcx9c500 n=0xcx9c20 
X: j-valent connecting group 
j=1xcx9c6 m=0xcx9c500 n=4xcx9c500
X: j-valent connecting group
R1:xe2x80x94CpH2pxe2x80x94 p=10
R2:xe2x80x94CqH2qxe2x80x94 q=10
R3:xe2x80x94CtH2txe2x80x94 t=10
r, s: s=1 when r=0, s=0 when r=1
J=1xcx9c6 k=0xcx9c3 m=0xcx9c20
xe2x80x83Example of X (j-valent connecting group)xe2x80x83xe2x80x83(IV)
CxHyFzxe2x80x94 x=1xcx9c10, y+z=2x+1
xe2x80x94CxHyFzxe2x80x94 x=1xcx9c10, y+z=2x
xe2x80x94CxHyFz less than  x=1xcx9c10, y+z=2xxe2x88x921
 greater than CxHyFz less than  x=1xcx9c10, y+z=2xxe2x88x922
xe2x80x94CH2Oxe2x80x94
The above-mentioned organic polymer preferably contains the first chemical structure in its molecule at a rate smaller than or equal to 75 wt %. In particular, it is preferable to use a containing rate ranging from 25 to 60 wt %. A containing rate of the first chemical structure larger than 75 wt % is not preferable because the whole property of the organic polymer becomes to be governed by the property represented by the first chemical structure.
The organic polymer is preferably formed by polymerizing the organic monomer with irradiation of ionizing radiation.