Cell electrode with thick tip portion

Adhesive bonding and miscellaneous chemical manufacture – Surface bonding means and/or assembly means therefor – With coating means for work

Reexamination Certificate

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Details

C156S345420, C156S519000, C156S542000, C156S583200, C118S066000

Reexamination Certificate

active

06234225

ABSTRACT:

BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to an electrode capable of achieving a good cell performance, and more particularly to an electrode having good repetitive charge/discharge characteristics and to a non-aqueous electrolyte secondary cell using such an electrode.
b) Description of the Related Art
Secondary cells are widely used as power sources of electronic apparatuses of high quality and compact portable size. Of these secondary cells, recently developed lithium cells have a large capacity and a large output so that demands for these cells as power sources of portable electronic apparatuses are rapidly expanding. It is known, however, that the capacity of a secondary cell gradually lowers after charge and discharge are repeated.
The invention has been made in consideration of such circumstances and is based upon the finding that the tip portion of an electrode of the secondary cell has a high potential and because of this the capacity of the cell gradually lowers after repetitive charge/discharge cycles.
The electric conductivity of non-aqueous electrolyte used by a non-aqueous secondary cell is lower than that of aqueous or water-containing electrolyte. It is therefore necessary to broaden an electrode area. From this reason, a usual cylindrical non-aqueous secondary cell uses an electrode sheet which is wound in a cylindrical cell can. This electrode sheet has a thin film coated on one or both surfaces of a conductive support sheet, the thin film being made of a depolarizing mix (conductive materials of positive electrode active material, negative electrode material, conductivity-imparting material, binding material, and the like).
The electrode sheet is required to be electrically connected via a positive or negative electrode lead plate to a positive or negative electrode terminal of a cell so that a partial area of the conductive support sheet (usually one or both ends thereof) must be exposed for such electrical connection. In a usual electrode sheet, an exposed area not covered with the depolarizing mix layer is therefore formed to expose the conductive sheet.
A number of non-aqueous secondary cells are required to be mass produced with low cost in order to follow a recent expansion of cell application fields. To meet such requirements, electrode sheets are generally manufactured by coating a depolarizing mix layer of electrode materials on the surface of a stripe conductive sheet and thereafter cutting the sheets into separate electrode sheet pieces having a predetermined length. Even with such a electrode sheet manufacture method using the stripe conductive sheet and cutting it, it is obvious that the exposed area is required to be formed on the conductive sheet. Conventionally, the depolarizing mix layer is peeled off from the end portion of a cut electrode sheet. However, a work efficiency of this method is very bad and various other methods have been proposed.
According to a method described in JP-B-SHOU-55-31989, an adhesive tape is bonded to a partial area of a conductive sheet piece and a depolarizing mix layer is formed on the conductive sheet piece. Thereafter, the adhesive tape is peeled off to thereby form a partial area of the electrode sheet not covered with the depolarizing mix layer (an exposed area of the surface of the conductive sheet). This method is an improved method which mechanically peels off the depolarizing mix layer from the electrode sheet. However, this method is applied to each separate electrode sheet piece so that it is not suitable for mass production of electrode sheets.
According to JP-A-HEI-1-184069 and JP-A-HEI-1-194265, a doctor blade method is disclosed which has a shutter capable of controlling to intermittently supply coating liquid for forming an electrode depolarizing mix layer. Namely, with this doctor blade method, a stripe conductive sheet is coated with depolarizing mix layer pieces at an interval which leaves the non-coated areas between adjacent depolarizing mix layer pieces. These areas of the conductive sheet not coated with the depolarizing mix layer are used for electrical connection to positive or negative electrode plates.
This method is supposed to be theoretically suitable for mass production of electrode sheets. However, according to the teachings disclosed in JP-A-HEI-8-45501, boundaries between the areas of the electrode sheet covered with the depolarizing mix layer and the areas not covered with this layer become likely to be irregular, and in addition to this the thickness of the depolarizing mix layer becomes likely to be irregular.
This JP-A-HEI-8-45501 also discloses a method of coating a stripe conductive sheet with depolarizing mix layers at an interval which leaves the non-coated areas between adjacent depolarizing mix layers. In this method, liquid for forming a depolarizing mix layer on the surface of a stripe conductive sheet at a predetermined interval is supplied with an extrusion type injector having a slot nozzle. It is described in JP-A-HEI-8-45501 that use of such an injector can suppress irregularity of boundaries between the area covered with the depolarizing mix layer and the area not covered therewith and improve the uniformity of thickness of the depolarizing mix layer.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an electrode having good repetitive charge/discharge characteristics or a non-aqueous electrolyte secondary cell using such an electrode excellent in charge/discharge cycle characteristics.
It is another object of the present invention to provide a method of mass producing an electrode sheet with a depolarizing mix layer having a high uniformity of layer thickness, particularly of mass producing a stripe electrode sheet suitable for non-aqueous secondary cells, and manufacture apparatuses suitable for use with such a manufacture method.
According to one aspect of the invention, there is provided a cell electrode of a stripe rectangle shape having at least one electrode depolarizing mix layer on a current collector, wherein a thickness of the electrode depolarizing mix layer at one end portion is thicker than an average thickness of the electrode depolarizing mix layer by 2 to 25%.
Use of the depolarizing mix layer having a thick tip portion can provide a non-aqueous secondary cell having excellent charge/discharge cycle characteristics and high productivity.
According to another aspect of the invention, there is provided a secondary cell comprising: a first electrode of a rectangle shape having an electrode depolarizing mix layer on a current collector, a thickness of the electrode depolarizing mix layer at least one end portion being thicker than an average thickness of the electrode depolarizing mix layer by 2 to 25%; a second electrode of a rectangle shape having an electrode depolarizing mix layer on a current collector and an opposite polarity to the first electrode; a separator provided between the first and second electrodes; a container for housing the first and second electrodes and the separator; and electrolyte poured into the container.
According to a further aspect of the invention, there is provided a method of manufacturing an electrode sheet comprising the steps of: (a) sequentially bonding adhesive tape pieces to a strip conductive sheet on at least one surface thereof in the traverse direction of the sheet at a predetermined interval therebetween in the longitudinal direction of the sheet; (b) coating electrode depolarizing mix solution on the surface of the stripe conductive sheet where the adhesive tape piece was bonded, so as to expose at least one end portion of the adhesive tape sheet along the traverse direction of the sheet, and heating and drying the electrode depolarizing mix solution; and holding the exposed end portion of the adhesive tape piece and pulling up the exposed end portion along the traverse direction of the sheet to peel off the adhesive tape piece.
According to a still further aspect of the invention, there is provided an electrode sheet manufacturing sys

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