JP2007250499A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery improved in electrical property such as rate characteristics and cycle characteristics by using a plurality of positive electrode active materials. <P>SOLUTION: When a plurality of kinds of positive electrode active materials are used, lithium insertion and elimination reaction potential of the positive electrode material of a first positive electrode layer 2a on nearer side to the positive electrode current collector 1 is made lower than that of the positive electrode material of a second positive electrode layer 2b which is farther side from the positive electrode current collector 1. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery having an electrode structure suitable for improving electrical characteristics.

  Various non-aqueous electrolyte secondary batteries have been put into practical use, and one of them is a lithium ion secondary battery. Lithium ion secondary batteries are small, light, and have high capacity, so they are also used in portable electronics, communication equipment, electric assist bicycles, electric tools, etc. However, the rate characteristics are not sufficient, and further improvement in characteristics is required for electrical characteristics such as cycle characteristics.

  As the positive electrode active material of this lithium ion secondary battery, lithium cobaltate, lithium spinel manganate, etc. were generally used alone, but recently, these have been mixed to improve the characteristics. It is also considered to be used.

  When these positive electrode active materials are mixed and used as the positive electrode layer, the potential-current profile of the lithium ion secondary battery is the sum of the potential-current profiles of the respective electrode active materials, indicating the overall reaction profile. Conventionally, since the respective electrode active materials are mixed to form a single positive electrode, the electrical characteristics such as the rate characteristics are not sufficiently improved.

  Patent Document 1 discloses a technique for producing a positive electrode by applying a slurry containing different positive electrode active materials with a positive electrode current collector sandwiched between two types of positive electrode active materials.

Further, in Patent Document 2, a plurality of positive electrode materials represented by Li _X Co _1-Y Ni _Y O _Z are stacked on a positive electrode current collector, and Co atoms in a layer of the positive electrode material close to the positive electrode current collector are obtained. A technique is disclosed in which the atomic ratio of Co in the layer of the positive electrode material remote from the positive electrode current collector is larger than the ratio.

JP 2003-257415 A Japanese Patent Laid-Open No. 10-255762

  In Patent Document 1, it is described that the outermost positive electrode active material is improved in safety by using a material having a lower heat generation start temperature by differential scanning calorimetry, but for each side of the positive electrode current collector, Since the first layer and the second layer are provided, the safety is considered only, and the improvement of the electrical characteristics is not sufficient.

  In Patent Document 2, the reaction at the interface between the positive electrode material and the non-aqueous electrolyte is suppressed by increasing the atomic ratio of Co in the positive electrode material layer away from the positive electrode current collector. However, the improvement in electrical characteristics such as rate characteristics was not sufficient.

  The subject of this invention is providing the lithium ion secondary battery which improved the electrical characteristics, such as a rate characteristic and cycling characteristics, using the several positive electrode active material.

  In order to solve the above-mentioned problem, the present invention uses a plurality of positive electrode active materials to form a plurality of positive electrodes on the positive electrode current collector in order from the lowest potential according to the potential at which lithium insertion / extraction reaction occurs. Results of finding that rate characteristics as well as cycle characteristics can be improved by improving the reaction efficiency according to the reaction potential of each positive electrode active material by preparing and coating slurry independently for each active material layer It was made.

  The lithium ion secondary battery of the present invention is a lithium ion secondary battery in which a positive electrode and a negative electrode are laminated via a separator, and has a plurality of positive electrode layers on a positive electrode current collector, and is close to the positive electrode current collector The lithium insertion / release reaction potential of the positive electrode active material of the positive electrode layer on the side is lower than the lithium insertion / release reaction potential of the positive electrode active material of the positive electrode layer on the side far from the positive electrode current collector.

The plurality of positive electrode layers are composed of two layers, and the lithium insertion / release reaction potential of the positive electrode active material of the first positive electrode layer in contact with the positive electrode current collector is a second side far from the positive electrode current collector. The positive electrode active material of the positive electrode layer is lower than the lithium insertion / release reaction potential, and the coating amount of the first positive electrode layer is less than or equal to the coating amount of the second positive electrode layer. LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ is preferably used as the positive electrode active material of the first positive electrode layer, and LiCoO ₂ is preferably used as the positive electrode active material of the second positive electrode layer.

  According to the present invention, in a lithium ion secondary battery using a plurality of types of positive electrode active materials, the positive electrode layer containing the positive electrode active material having a low lithium ion insertion / desorption potential specific to the positive electrode active material is closer to the positive electrode current collector. As a result of improving the reaction efficiency, rate characteristics and cycle characteristics can be improved.

  The lithium ion secondary battery of the present invention is manufactured by laminating a positive electrode and a negative electrode through a separator, storing the battery in a battery outer package, injecting a non-aqueous electrolyte, and then sealing.

The positive electrode active material that can be used in the lithium ion secondary battery of the present invention is a composite oxide that is LiMO ₂ (where M represents at least one transition metal), such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _4. LiCo _1/3 Ni _1/3 Mn _1/3 O _{2 and the} like. For example, a plurality of positive electrode layers are formed on a positive electrode current collector made of an aluminum foil. The positive electrode layer on the side close to the positive electrode current collector is a positive electrode active material having a low lithium insertion / release reaction potential, for example, LiCo _1/3 Ni Dispersing _1/3 Mn _1/3 O ₂ in N-methyl-2-pyrrolidone (NMP) with a conductive aid such as carbon black and a binder such as polyvinylidene fluoride (PVdF) The slurry is prepared and coated, for example, by applying it on an aluminum foil and dried to form. After forming on one surface of the positive electrode current collector, the same is formed on the back surface, and a positive electrode layer is formed on both surfaces. Thereafter, a positive electrode active material having a high lithium insertion / elimination reaction potential, such as LiCoO ₂ , is dispersed in NMP with a conductive aid such as carbon black and a binder such as PVdF, and a slurry is prepared. By coating and drying on the positive electrode layer on the side close to the electric body, the positive electrode layer on the side far from the positive electrode current collector is formed on both sides and compressed to produce a positive electrode. At this time, it is better that the coating amount of the positive electrode layer closer to the positive electrode current collector is less than or equal to the coating amount of the positive electrode layer farther from the positive electrode current collector. The coating amount is the weight of the solid component (electrode active material, conductive additive, binder) of the active material per unit area. Further, in addition to LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ and LiCoO ₂ used here, lithium insertion / extraction of the positive electrode active material of the positive electrode layer closer to the positive electrode current collector for a plurality of positive electrode layers. There is no particular limitation as long as the separation reaction potential is lower than the lithium insertion / elimination reaction potential of the cathode active material farther from the cathode current collector.

  In the lithium ion secondary battery of the present invention, natural graphite, artificial graphite or the like can be used as the negative electrode active material. For example, the negative electrode is formed on a negative electrode current collector made of copper foil, the negative electrode active material is a conductive additive such as carbon black, and a binder such as polyvinylidene fluoride (PVdF) is N-methyl-2-pyrrolidone (NMP). A slurry is prepared by dispersing, and the slurry is coated, for example, by applying on a copper foil, dried and compressed.

  As the separator, a polyolefin resin such as polypropylene or polyethylene, a porous film such as a fluororesin, or the like can be used.

Examples of the electrolyte include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, and vinylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, methyl formate, and methyl acetate. , Fatty carboxylic acid esters such as propionate ether, γ-lactones such as γ-butyrolactone, chain ethers such as 1,2-diethoxyethane and ethoxymethoxyethane, and cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran Dimethyl sulfoxide, 1,3-dioxirane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, Ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, diethyl ether One kind of aprotic solvent such as the above or a mixture of two or more kinds is used to dissolve the lithium salt dissolved in these organic solvents. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , Li (CF ₃ SO ₂ ) ₂ , LiBr, LiCl, low fatty acid carboxylic acid lithium, and imides. Further, a polymer electrolyte may be used instead of the electrolytic solution.

  Hereinafter, the present invention will be described more specifically based on examples with reference to the drawings.

Example 1
FIG. 1 is a cross-sectional view illustrating the configuration of the positive electrode of the lithium ion secondary battery of the present invention. The slurry was prepared by mixing 95 parts by weight of the positive electrode active material, 2 parts by weight of carbon black as a conductive additive, and 3 parts by weight of PVdF as a binder, and mixing this with an NMP solution to prepare a slurry. Was applied to one side of an aluminum current collector with a thickness of 15 μm by a doctor blade method to form an active material layer.

As an actual procedure, as the first positive electrode layer 2 a in contact with the positive electrode current collector 1, a layer containing LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ as an active material is far from the positive electrode current collector 1. As the second positive electrode layer 2b on the side, a layer containing LiCoO ₂ as an active material was sequentially applied onto the positive electrode current collector 1 made of aluminum and dried to prepare a positive electrode layer of a lithium ion secondary battery. The positive electrode layer was produced by setting the coating amount of the first positive electrode layer 2a to the entire positive electrode layer to 25%.

  Artificial graphite was used as the negative electrode active material. A slurry is prepared by mixing 95 parts by weight of the negative electrode active material, 2 parts by weight of carbon black as a conductive assistant, and 3 parts by weight of PVdF as a binder, and mixing this with an NMP solution. Was applied to both sides of a copper current collector with a thickness of 20 μm by the doctor blade method to obtain a negative electrode active material layer.

A secondary battery was assembled using the above-described positive electrode layer, negative electrode layer, and separator, and an electrolyte was injected to produce a lithium ion battery. The electrolyte was used by dissolving LiPF ₆ at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (mixing volume ratio: EC / DEC = 30/70).

(Example 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the amount of the first positive electrode layer applied to the entire positive electrode layer was 50%.

(Comparative Example 1)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the coating amount of the first positive electrode layer with respect to the entire positive electrode layer was 60%.

(Comparative Example 2)
As a first positive electrode layer in contact with the positive electrode current collector, a coating amount of a layer containing LiCoO ₂ as an active material is 75%, and as a second positive electrode layer far from the positive electrode current collector, LiCo _1/3 Ni _{1 is used. / 3 A} lithium ion secondary battery was fabricated in the same manner as in Example 1 except that the coating amount of the layer containing Mn _1/3 O ₂ as an active material was 25%.

(Comparative Example 3)
FIG. 2 is a cross-sectional view illustrating the configuration of the positive electrode of a conventional lithium ion battery. As a positive electrode active material, 75 parts by weight of LiCoO ₂ and 25 parts by weight of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ are mixed to prepare a slurry, which is applied onto the positive electrode current collector 1, A lithium ion secondary battery was produced in the same manner as in Example 1 except that the layer 2 was formed.

  The rate characteristics and capacity retention rate of the lithium ion secondary battery obtained as described above were measured. The rate characteristic is 1.0 C relative to the discharge capacity obtained by charging the battery voltage to 4.2 V (charging condition: current 0.2 C, 2.5 hours, 20 ° C.) and discharging to 0.2 V battery voltage at 0.2 C. The ratio of the discharge capacity obtained by discharging the battery voltage to 3.0V was expressed as Comparative Example 3 as 100%, and the capacity maintenance rate was charged (upper limit voltage 4.2V, current 0.2C) as the charge / discharge cycle conditions. , 2.5 hours, 20 ° C.) The ratio of the discharge capacity at the 500th cycle to the discharge capacity at the first cycle when the discharge (lower limit voltage is 3.0 V, current 0.2 C, 20 ° C.) is set to 100 in Comparative Example 3. Expressed as%. The rate characteristics and capacity retention rates of Example 1, Example 2, and Comparative Examples 1 to 3 are shown in Table 1. The rate characteristics and the ratio of the coating amount of the first positive electrode layer to the total coating amount of the positive electrode layer The relationship is shown in FIG.

  From these results, compared to Comparative Example 3 in which the positive electrode layer is one layer, the rate characteristic is improved by 4% in Example 1 and the capacity retention rate in the cycle is improved by 10%. In Example 2, the rate characteristic is increased by 1% and in the cycle. It can be seen that the capacity retention rate improved by 9%.

  Further, in Comparative Example 1, the rate characteristic is the same as that in Comparative Example 3, but the capacity retention rate is improved by 3%, and in Comparative Example 2, the rate characteristic is deteriorated by 2% and the capacity maintenance ratio is deteriorated by 3%. Recognize.

In this example, LiCoO ₂ and LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ are shown as examples. However, if two or more kinds are selected and arranged in order from the lowest reaction potential, the present example is used. The effects of the present invention are not limited.

Sectional drawing explaining the structure of the positive electrode of the lithium ion secondary battery of this invention. Sectional drawing explaining the structure of the positive electrode of the conventional lithium ion secondary battery. The figure which shows the ratio with respect to the coating amount of the whole positive electrode layer of a rate characteristic and the coating amount of a 1st positive electrode layer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode collector 2a 1st positive electrode layer 2b 2nd positive electrode layer 2 Positive electrode layer

Claims (4)

  In a lithium ion battery in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, a plurality of positive electrode layers are provided on a positive electrode current collector, and lithium is inserted into the positive electrode active material of the positive electrode layer on the side close to the positive electrode current collector A lithium ion secondary battery, wherein a desorption reaction potential is lower than a lithium insertion desorption reaction potential of a positive electrode active material of the positive electrode layer far from the positive electrode current collector.   In a lithium ion battery in which a positive electrode and a negative electrode are laminated via a separator, the positive electrode active material lithium of the first positive electrode layer having two positive electrode layers on the positive electrode current collector and in contact with the positive electrode current collector The insertion / desorption reaction potential is lower than the lithium insertion / release reaction potential of the positive electrode active material of the second positive electrode layer far from the positive electrode current collector, and the coating amount of the first positive electrode layer is the second positive electrode layer The lithium ion secondary battery according to claim 1, wherein the amount is less than or equal to the amount of coating. 3. The lithium ion secondary battery according to claim 2, wherein the positive electrode active material of the first positive electrode layer is LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ . 4. The lithium ion secondary battery according to claim ₂ , wherein the positive electrode active material of the second positive electrode layer is LiCoO _{2. 5} .

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