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Nonaqueous electrolyte secondary batteryRelated Patent Categories: Chemistry: Electrical Current Producing Apparatus, Product, And Process, Current Producing Cell, Elements, Subcombinations And Compositions For Use Therewith And Adjuncts, ElectrodeNonaqueous electrolyte secondary battery description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060051669, Nonaqueous electrolyte secondary battery. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This is a Continuation Application of PCT Application No. PCT/JP03/04636, filed Apr. 11, 2003, which was not published under PCT Article 21(2) in English. [0002] This application is based upon and claims the benefit of priority from prior Japanese Patent Applications No. 2002-110851, filed Apr. 12, 2002; and No. 2003-083135, filed Mar. 25, 2003, the entire contents of both of which are incorporated herein by reference. BACKGROUND OF THE INVENTION [0003] 1. Field of the Invention [0004] The present invention relates to a nonaqueous electrolyte secondary battery. [0005] 2. Description of the Related Art [0006] As a lithium ion secondary battery, a battery having an arrangement using a lithium composite oxide as a positive electrode active material and a carbonaceous material, which allows doping and undoping of lithium ions, as a negative electrode active material is known. This lithium ion secondary battery is expected to be used as a superior power supply because the battery is small and light, the voltage of a single cell is high, and a high energy density can be obtained. [0007] The conventional lithium ion secondary battery realizes a high energy density by using highly densified positive and negative electrodes. However, in this lithium ion secondary battery including the highly densified positive and negative electrodes, the initial characteristics, particularly, the discharge characteristics when a large electric current is discharged are unstable, and the charge/discharge cycle characteristics deteriorate. [0008] Jpn. Pat. Appln. KOKAI Publication No. 11-86870 discloses a positive electrode in which linear grooves are formed in a depolarizing mix layer formed on the main surface of a current collector and containing a material which participates in an electrochemical reaction, such that the two ends of each groove open at the edges of the depolarizing mix layer. The principal object of this invention is, if the internal temperature of the battery rises due to a shortcircuit or an abnormal electric current and the electrode material or electrolyte gasifies, to rapidly move the gas outside the electrode through the grooves. A reduction in liquid injection time is also described as a secondary effect of the grooves. However, this invention does not mention, e.g., the selection of the form of the grooves in relation to the battery characteristics such as the large-current discharge characteristics or cycle life characteristics. BRIEF SUMMARY OF THE INVENTION [0009] The present inventors made extensive studies on factors by which the initial characteristics, particularly, the discharge characteristics when a large electric current is discharged become unstable in a high-energy-density, nonaqueous electrolyte secondary battery including high-density positive and negative electrodes, i.e., low-void positive and negative electrodes. As result, the present inventors have found that there is a large difference between the values of permeability of a nonaqueous electrolyte to the high-density positive electrode and the high-density negative electrode (more specifically, the permeation speed of an electrolyte at the negative electrode is much higher than that of the electrolyte at the positive electrode), and this causes a nonuniform nonaqueous electrolyte permeation distribution between the positive and negative electrodes. [0010] The present inventors also made extensive studies on deterioration of the charge/discharge cycle characteristics of a high-energy-density, nonaqueous electrolyte secondary battery including high-density positive and negative electrodes, i.e., low-void positive and negative electrodes. As result, the present inventors have found that the negative electrode has a volume change larger than that of the positive electrode during a charge/discharge cycle and hence easily swells, so the electrolyte often moves toward the negative electrode when the charge/discharge cycle is repeated, and this prevents sufficient supply of the electrolyte to the positive electrode at the charge/discharge cycle progresses. [0011] On the basis of these findings, the present inventors made further extensive studies, and made it possible, by defining the specific surface area per unit area of a positive electrode active material of the positive electrode to 0.5 to 1.0 times the specific surface area per unit area of a negative electrode active material of the negative electrode which opposes the positive electrode with a separator sandwiched between them, to balance the speeds of permeation of the nonaqueous electrolyte to the positive and negative electrodes, to make uniform the nonaqueous electrolyte permeation distribution between the positive and negative electrodes, and to impregnate the positive electrode with a sufficient amount of the nonaqueous electrolyte, when an electrode assembly having the positive and negative electrodes and the separator was accommodated in an outer packaging member such as a metal can, and the nonaqueous electrolyte was supplied to the outer packaging member. Consequently, the present inventors could obtain a high-energy-density, nonaqueous electrolyte secondary battery in which the initial characteristics, particularly, the charge characteristics when a large electric current was discharged stabilized and the charge/discharge cycle characteristics improved, and completed the present invention. [0012] Also, the present inventors made it possible, by forming grooves having a specific form in a positive electrode active material formation surface of a positive electrode, to balance the speeds of permeation of a nonaqueous electrolyte to the positive and negative electrodes, to make uniform the nonaqueous electrolyte permeation distribution between the positive and negative electrodes, and to impregnate the positive electrode with a sufficient amount of the nonaqueous electrolyte, when an electrode assembly having the positive and negative electrodes and the separator was accommodated in an outer packaging member such as a metal can, and the nonaqueous electrolyte was supplied to the outer packaging member. Consequently, the present inventors could obtain a high-energy-density, nonaqueous electrolyte secondary battery in which the initial characteristics, particularly, the charge characteristics when a large electric current was discharged stabilized and the charge/discharge cycle characteristics improved, and completed the present invention. [0013] According to the present invention, there is provided a nonaqueous electrolyte secondary battery comprising an electrode assembly having a high-density positive electrode in which a positive electrode active material layer is formed on at least one surface of a positive electrode current collector, a high-density negative electrode in which a negative electrode active material layer is formed on at least one surface of a negative electrode current collector, and a separator interposed between the positive and negative electrodes, and having a structure in which the electrode assembly is impregnated with a nonaqueous electrolyte, [0014] wherein a specific surface area per unit area of the positive electrode active material layer of the positive electrode is 0.5 to 1.0 times a specific surface area per unit area of the negative electrode active material layer of the negative electrode which opposes the positive electrode with the separator sandwiched between them. [0015] According to the present invention, there is provided a nonaqueous electrolyte secondary battery comprising an electrode assembly having a high-density positive electrode in which a positive electrode active material layer is formed on at least one surface of a positive electrode current collector, a high-density negative electrode in which a negative electrode active material layer is formed on at least one surface of a negative electrode current collector, and a separator interposed between the positive and negative electrodes, and having a structure in which the electrode assembly is impregnated with a nonaqueous electrolyte, [0016] wherein a plurality of grooves are formed in a formation surface of the positive electrode active material layer of the positive electrode, such that end portions of the grooves open at edges of the positive electrode active material layer, and [0017] the grooves are formed at a frequency of 1 to 10 grooves per mm of the positive electrode active material layer, and a sectional area of the grooves account for a ratio of 1 to 20% of a sectional area of the positive electrode active material layer. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING [0018] FIG. 1 is a partial sectional view showing a cylindrical type-lithium ion secondary battery as a form of a nonaqueous electrolyte secondary battery according to the present invention; [0019] FIG. 2 is a partially cutaway perspective view showing a square type-lithium ion secondary battery as another form of the nonaqueous electrolyte secondary battery according to the present invention; [0020] FIG. 3 is a perspective view showing a thin type-lithium ion secondary battery as still another form of the nonaqueous electrolyte secondary battery according to the present invention; Continue reading about Nonaqueous electrolyte secondary battery... 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