Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1996-12-23
2001-03-13
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
Reexamination Certificate
active
06200703
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a solution of a binder comprising a vinylidene fluoride polymer (i.e., a vinylidene fluoride polymer binder solution) for stably fixing a powdery electrode material (principally comprising an electrode active substance and an optionally added electroconductivity-imparting additive) onto an electrode substrate to form an electrode structure for a non-aqueous battery, particularly a lithium ion battery. The present invention further relates to an electrode-forming composition comprising a mixture of such a binder solution and a powdery electrode material dispersed therein, an electrode structure obtained therefrom, and a non-aqueous-type battery including such an electrode structure.
In recent years, remarkable development has been made in electronic technology, and various apparatus and devices have been reduced in size and weight. Accompanying the reduction in size and weight of electronic apparatus and devices, there has been a remarkably increasing demand for reduction in size and weight of a battery as a power supply for such electronic apparatus and devices. In order to generate a larger energy from a battery of small volume and weight, it is desirable to generate a higher voltage from one battery. From this viewpoint, much attention has been called to a battery using a non-aqueous electrolytic solution in combination with a negative electrode substance comprising, e.g., lithium or a carbonaceous material capable of being doped with lithium ions, and a positive electrode active substance comprising, e.g., a lithium-cobalt oxide.
However, in such a non-aqueous-type battery, the non-aqueous electrolytic solution shows only a low ionic conductivity on the order of 10
−2
-10
−4
S/cm compared with an ordinary level ionic conductivity of ca. 10
−1
S/cm in an aqueous electrolytic solution, so that it becomes essential to use an electrode (layer) in a small thickness of several pm to several hundred &mgr;m and in a large area. As a method of economically obtaining such a thin and large-area electrode, it has been known to disperse a powdery electrode material comprising an electrode active substance in a binder solution obtained by dissolving an organic polymer functioning as a binder for the powdery electrode material to form an electrode-forming composition and applying the composition onto an electroconductive substrate, such as a metal foil or a metal net, followed by drying to form an electrode. As such a binder solution for a non-aqueous-type battery, those obtained by dissolving various grades of vinylidene fluoride polymers in polar solvents, such as N-methyl-2-pyrrolidone, dimethylformamide, N,N-dimethylacetamide have been known as disclosed in Japanese Laid-Open Patent Application (JP-A) 6-93025 and JP-A 6-172452.This is because a vinylidene fluoride polymer is excellent in chemical resistance, weatherability, anti-staining property, etc., is soluble in a polar solvent as described above but is stable against a non-aqueous electrolytic solution while it is swollen to some extent within such a non-aqueous electrolytic solution. Further, a vinylidene fluoride polymer can retain a good adhesion onto a substrate of metal, etc., by copolymerization or modification.
There has been found a problematic phenomenon that a binder solution obtained by dissolving such a vinylidene fluoride polymer in a polar solvent exhibits a remarkably increased solution viscosity depending on a production lot of the solvent used. Such an increased solution viscosity makes it difficult to obtain a uniform thickness of film in the electrode formation step by application and causes gelling at the time of kneading with the active substance so that the film formation per se becomes difficult. Even if the film formation is possible, there has been observed a phenomenon that the binder film after the applying and drying exhibits a large degree of swelling in a non-aqueous electrolytic solution. A large degree of swelling of the binder in a non-aqueous electrolytic solution leads to an increase in contact resistance between the powdery electrode material, particularly the active substance, and the metal foil or metal net, and an increase in contact resistance between the active substance particles themselves, thus resulting in an increased internal resistance in the battery. In the case of a secondary battery capable of repetitive charging and discharging, the increased internal resistance leads to an inferior charge-discharge cycle performance and is liable to result in a shorter battery life.
The gelling during mixing with an active substance is rather remarkably caused in the step of forming a positive electrode-forming slurry composition than in the step of forming a negative electrode-forming slurry composition using carbon as a powdery electrode material. From this fact, it is assumed that the gelling of a vinylidene fluoride polymer in the positive electrode-forming slurry composition is attributable to the function of a lithium-based complex metal oxide as a positive electrode active substance, and it has been also found that this tendency is particularly promoted in the case of adding carbon black as an electro-conductivity-imparting additive.
SUMMARY OF THE INVENTION
Accordingly, a principal object of the present invention is to provide a vinylidene fluoride polymer binder solution which per se is stable without causing a viscosity increase and allows the formation of a battery electrode which is stable and free from excessive swelling in a non-aqueous electrolytic solution, and also an electrode-forming composition formed by dispersing a powdery electrode material in such a binder solution.
Another object of the present invention is to stabilize a positive electrode-forming composition in which the gelling of a vinylidene fluoride polymer is liable to be promoted.
A further object of the present invention is to provide an electrode structure formed from the above-mentioned electrode-forming composition, and a non-aqueous-type battery including such an electrode structure.
According to the present invention, there is provided a vinylidene fluoride polymer binder solution, comprising: a solution of a vinylidene fluoride polymer in an organic solvent, stabilized by addition of an acid. It is preferred that the acid has been added in such an amount as to provide the binder solution, a portion of which will provide a 10-times dilution thereof with deionized water exhibiting a pH of at most 9. The acid may preferably be an organic acid.
According to another aspect of the present invention, there is provided an electrode-forming composition comprising a powdery electrode material dispersed in a vinylidene fluoride polymer binder solution as described above.
According to the present invention, there are further provided an electrode structure, comprising: an electroconductive substrate, and a composite electrode layer disposed on at least one surface of the substrate comprising a powdery electrode material and a vinylidene fluoride polymer stabilized by an organic acid; and a non-aqueous-type battery including such an electrode structure as a positive or a negative electrode.
Some explanation is added regarding the function and effect of the present invention. As a result of our study, it has been found that the above-mentioned abnormal viscosity increase in a vinylidene fluoride polymer binder solution is associated with an acidity or alkalinity of the system. It is difficult to directly measure the acidity or alkalinity of the solution of a vinylidene fluoride polymer in an organic solvent, but a measure may be obtained by diluting a portion thereof with ten times in amount of deionized water and measure a pH of the resultant liquid. As a result of such a measurement, it has been found that the solution system having caused a viscosity increase shows a pH exceeding 9 of such a 10-times dilution liquid. Further knowledge or assumptions have been obtained, such that the viscosity increase is related with dehydr
Horie Katsuo
Kashio Hidetora
Katsurao Takumi
Nagai Aisaku
Shibata Fumio
Chaney Carol
Kureha Kagaku Kogyo Kabushiki Kaisha
Wenderoth , Lind & Ponack, L.L.P.
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