Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-11-10
2002-07-23
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C156S289000, C152S234000
Reexamination Certificate
active
06423446
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrode plate for a secondary battery with a nonaqueous electrolyte (which may be referred to as “electrode plate” hereinafter) represented by a lithium ion secondary battery, and also relates to a process for producing the electrode plate. More particularly, the present invention relates to an electrode plate for a secondary battery with a nonaqueous electrolyte and a producing process thereof providing improved precisions in dimensions and thicknesses of an active material layer and a non-coated portion.
2. Description of the Related Art
In recent years, reduction in size and weight of electronic equipment and communication equipment has been rapidly advanced, and this advance has been also required reduction in size and weight of secondary batteries used as driving power sources for these equipments. For these requests, there have been proposed secondary batteries with a nonaqueous electrolyte having high energy density and high voltage, typically, a lithium ion secondary battery, in place of conventional alkaline batteries.
For both positive and negative electrode plates which give great influence on performance of the secondary battery, it is desired, in order to elongate a charge/discharge cycle life and to realize a high energy density, to make larger an area of the electrode plate disposed in the battery by making thin the electrode plate.
For examples, Japanese Patent Laid-open Publication Nos. SHO 63-10456 and HEI 3-285262 disclose positive electrode plates which are produced by the steps of: dispersing or dissolving an active material powder for the positive electrode plate, which is composed of metallic oxides, sulfides, halides or the like, a conductive material and a binding material (binder) into a suitable wetting agent (referred to as “solvent” hereinafter) to prepare an active material coating solution in a form of paste; and applying the active material coating solution on a surface of a collector as a substrate made of a metallic foil. In the electrode plate thus produced, as the binder, there is used fluororesin such as polyvinylidene fluoride or the like, siliconeacrylic copolymer, or styrene-butadiene copolymer.
On the other hand, a negative electrode plate is produced by adding a material, prepared by dissolving a binder into a suitable wetting agent (solvent), to an active material for the negative electrode such as carbon so as to prepare a coating solution for the active material in the form of paste and applying such active material coating solution on a surface of a collector made of metallic foil. Further, in order to improve the density of a coating film with respect to the collector and improve an adhesive property (adhesion) of the coating film thereto, a press treatment is usually performed.
It is required for the binder for preparing the active material coating solution for the above-mentioned coating type electrode plate to be chemically stable to the nonaqueous electrolyte, to be insoluble against the nonaqueous electrolyte and to be capable of being dissolved by some solvent and being coated thin on the substrate.
Furthermore, it is also required for the active material layer, which is coated and dried to have a sufficient flexibility so as to prevent peeling, chipping, cracking or the like at the assembling process of the battery and to have an excellent adhesion to the collector.
Further, terminals for taking out electric current are mounted to the positive and negative electrode plates, respectively, and both the electrode plates are then wound up together with a separator disposed between them for preventing short-circuit between both the electrode plates, which are then sealed in a container filled up with the nonaqueous electrolyte, thus assembling a secondary battery. In the thus assembled secondary battery, in the case of no capacity balance between the positive and the negative electrodes, there is a fear of causing various problems. For example, in a case where the negative electrode has a less amount of active material and the battery capacity of the negative electrode is smaller than that of the positive electrode, it is not possible to charge, into a space between carbon layers of the negative electrode, all the lithium ions coming out from the positive electrode into the electrolyte at the charging reaction time, and the lithium ions becomes excessive in the electrolyte, which forms lithium metal therein and which may deposit a dendrite (column-shape) on the negative electrode plate. If such deposit grows, the separator interposed between both the electrode plates may be broken, which results in the short-circuit therebetween, and the performance of the battery may be extremely damaged. In order to obviate such problem, the active material for the negative electrode plate is coated much in amount than that for the positive electrode thereby to keep the capacity balance of the positive and the negative electrodes.
Further, the electrode plates are usually provided with non-coated portions to which the active material is not formed such as portions to which the terminals for taking out electric current are mounted and boundary portions between the adjacent active material layers. A pattern of a non-coated (non-coating) portion is optionally determined in accordance with a design of the battery. The methods for preparing such non-coated portion include a method of directly forming a pattern of the coated portion and the non-coated portion by coating the electrode coating solution on the collector while mechanically controlling a coater head and a method of forming a coating film (coated film) entirely on the collector surface and then locally peeling the coated film by mechanical means such as knife thereby to form a non-coated portion.
In the former method, supply-start and supply-stop of the active material coating solution from the coater head are repeated while moving the coater head and/or collector in conformity with the pattern of the coated portion or non-coated portion, or every time the coating working reaches to the boundary portion between the coated portion and the non-coated portion, movement-stop and movement-restart of the coater head and/or collector, separation and re-approach of the coater head with respect to the surface to be coated, and supply-stop and supply-restart of the electrode coating solution are performed repeatedly in a synchronous manner. Intermittent coating workings are thus performed by the mechanical control of the coater head thereby to form an intermediate product of the electrode in which, for example, active material layers each having a length of 600 mm and non-coated portions each having a length of 50 mm are formed alternately repeatedly on the surface of an elongated collector having a predetermined width.
However, as the coating speed is increased, it becomes difficult to perform the mechanical control of the coater head so as to accord with the coating speed, and it becomes impossible to exactly form the pattern in which the coated portions and the non-coated portions are alternately repeatedly formed. Particularly, in a case where it is desired to form the non-coated portions, each having a relatively narrow area, intermittently repeatedly in the portion to be coated, it is extremely difficult to exactly form a pattern of the non-coated portion at a high speed. Furthermore, for the reason that the motion of the coater head cannot follow the high coating speed, local coating amount becomes slightly excessive at the respective supply-start positions, which results in the formation of a built-up (protruded) edge portion of the active material layer. On the other hand, there is a tendency of local coating amount being slightly short at the respective supply-stop positions, and so-called, tailing phenomenon will be caused, and hence, the edge portion of the active material layer inclines. In this inclined portion, the thickness of the active material layer decreases towards the boundary portion
Miyanowaki Shin
Miyazaki Yuichi
Brouillette Gabrielle
Dai Nippon Printing Co. Ltd.
Ladas & Parry
Wills M.
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