JP2007335360A - Lithium secondary cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a lithium secondary cell having a small size, high output and high energy density. <P>SOLUTION: The lithium secondary cell with high output and high energy density is provided. In the lithium secondary cell, a negative electrode is a mixture of graphite and amorphous carbon material, where a proportion of the graphite in the mixture is 20 wt.% or more and 80 wt.% or less, the ratio of negative electrode mix density ρ<SB>G</SB>ρ<SB>A</SB>/[ρ<SB>G</SB>(1-X)+ρ<SB>A</SB>X] is 0.55 - 0.70 (where ρ<SB>G</SB>is carbon true density, ρ<SB>A</SB>is amorphous carbon material true density, and X is a proportion of graphite and satisfies 0.2≤X≤0.8), and a total thickness of both sides of a negative electrode mix layer is set to be 50 - 90 μm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery that greatly improves output density and energy density and has a long life.

  For application to fuel cell vehicles, hybrid vehicles, etc., power source devices such as lithium secondary batteries or capacitors have been actively developed. In order to be applied to in-vehicle applications such as fuel cell vehicles and hybrid vehicles, it is important to increase the output, extend the life, and reduce the cost of these power supply devices. In recent years, from the viewpoint of environmental problems such as carbon dioxide reduction, there is an increasing expectation for the practical use of these power supply devices for auxiliary power sources of fuel cell vehicles and hybrid vehicles. For such application to the automobile field, it is important to further increase the output and extend the life of these power supply devices. Furthermore, excellent input characteristics are also required to effectively use energy by regeneration. In order to apply to an auxiliary power source of a fuel cell vehicle, it is desirable that the fuel cell can be driven only by electricity until the fuel cell is started. Requests have also emerged in recent years.

  In order to meet such demands, it is indispensable not only to input and output but also to further improve the energy density, that is, increase the capacity of the battery. For example, Patent Documents 1 and 2 disclose techniques related to increasing the capacity of lithium secondary batteries. These disclosed technologies are technologies for increasing the capacity of lithium secondary batteries, and do not attempt to increase output. However, in order to apply to the automobile field, good load characteristics with a larger current, that is, high input / output is required as battery characteristics. In general, a battery used for a portable device such as a personal computer or a mobile phone is required to have a high capacity characteristic, but a high output is not required. That is, what is required as a load characteristic is at most about 1/3 time rate (3C). On the other hand, in the field of automobiles, a time rate of 1/10 to 1/20 (10C to 20C), that is, a large current 3 to 7 times that of a battery applied to a portable device, is required. Output is required.

  As described above, high-capacity and high-power battery technology is an extremely important issue in the practical application of batteries in the fields of hybrid vehicles, fuel cell vehicles, and the like.

Japanese Patent Laid-Open No. 10-255766 JP 2000-123835 A

  The present invention has been made in view of the background as described above, and provides a high-output, high-energy density, and long-life lithium secondary battery applicable to an auxiliary power source of a hybrid vehicle and a fuel cell vehicle. It is intended.

  The lithium secondary battery according to the present invention includes a positive electrode in which a mixture containing a lithium transition metal composite oxide is formed on both sides of a current collector foil, and a negative electrode mixture containing a negative electrode active material that absorbs and releases Li. In the lithium secondary battery composed of a negative electrode formed on both sides of the battery and a non-aqueous electrolyte containing a lithium salt, the negative electrode mixture is a mixture of graphite, an amorphous carbon material, and an organic binder, The ratio of graphite to the total amount of graphite and amorphous carbon material in the mixture is 20 to 80% by weight.

  The present invention provides a lithium secondary battery with high energy density and high output, and can provide a high output and high capacity lithium secondary battery suitable for hybrid vehicles, fuel cell vehicles, electric vehicles, etc. Can also provide lithium secondary batteries that can be widely applied to fields such as electric tools that require high output and high capacity.

  In order to increase the capacity of a battery, it is important to apply a material capable of high-density filling to the electrode and increase the density of the electrode. Compared to an amorphous carbon material, graphite is a material that can be filled with high density, and by applying graphite, the capacity of the battery can be increased. The voltage of the battery using graphite for the negative electrode is higher than that of the battery using an amorphous carbon material, and the output characteristics tend to be improved. On the other hand, the input characteristics tend to be lowered. is there. Therefore, a battery having a balance between output and input is required for application to automobiles.

  The present inventor can balance input and output by setting the ratio of graphite and amorphous carbon within a suitable range. It has been found that a high-capacity and high-power lithium secondary battery can be provided by using a mixture of graphite and amorphous carbon material for the negative electrode and setting the proportion of graphite in the mixture to 20 to 80% by weight. .

It is possible to increase the capacity of the battery by applying a high-density electrode, but as the density increases, the amount of electrolyte that can be held by the electrode decreases, and the surface is formed by the electrolyte and the negative electrode active material surface. There is concern about the inhibition of the electrode reaction occurring at the electrode reaction interface. When such an adverse effect on the electrode reaction occurs, the input / output is lowered, and the input / output required in the automobile field cannot be secured. In order to maintain an electrode reaction accompanying a high load, it is important that the electrode can hold an amount of electrolyte sufficient to secure an electrode reaction interface that can handle this electrode reaction. Ensuring an electrode structure capable of holding such an amount of electrolyte is a technical point. By setting the electrode density within a suitable range, a lithium secondary battery with high capacity and high output can be provided. That is, graphite, amorphous carbon material, and the ratio of the negative electrode mixture density consisting binder ρ _G ρ _A / _{[ρ G (1-X) +} ρ A X ] (where, [rho _G = graphite true density, ρ _A = true density of amorphous carbon material, X = ratio of graphite.) By setting 0.55 to 0.70, a high capacity and high output lithium secondary battery can be provided.

  As described above, it is possible to increase the capacity by increasing the density of the electrode, but it is also possible to increase the capacity by increasing the thickness of the electrode and filling more active material. is there. However, there is a concern that the electrode resistance increases and the input / output decreases as the electrode becomes thicker. An electrode having a thickness capable of ensuring a high output without increasing the electrode resistance is technically important in order to apply the battery to the automobile field. By making the total thickness of both surfaces of the negative electrode mixture layer provided on both surfaces of the current collector foil be 50 to 90 μm, it is possible to achieve both high capacity and high output. That is, by setting the thickness of the negative electrode mixture layer to 50 to 90 μm, a high-capacity and high-power lithium secondary battery can be provided.

  A lithium transition metal composite oxide can be used for the positive electrode active material of the lithium secondary battery of the present invention. A part of a positive electrode active material such as lithium nickelate or lithium cobaltate, such as Ni or Co, can be substituted with one or more transition metals.

  Examples of the binder include polyvinylidene fluoride (PVDF) and styrene butadiene rubber (SBR).

As the electrolyte, for example, a non-aqueous solvent selected from at least one selected from propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, tetrahydrofuran, 1,2-diethoxyethane, For example, carbon such as an electrolytic solution in which at least one lithium / transition metal salt selected from LiClO ₄ , LiBF ₄ , LiPF ₆ or the like is dissolved, a solid electrolyte having lithium ion conductivity, a gel electrolyte, or a molten salt is generally used. A known electrolyte used in a battery using a system material or the like as a negative electrode active material can be used. Moreover, even if a microporous separator is used according to the structural requirements of the battery, the effect of the present invention is not impaired at all.

  Both the positive electrode mixture and the negative electrode mixture are coated, dried and rolled on both sides of a current collector foil such as aluminum and shaped to a predetermined thickness. The preferable thickness of the negative electrode mixture is 50 to 90 μm in total on both sides. The average particle size of the secondary particles of graphite is preferably 10 to 20 μm. The average particle size of the amorphous carbon powder is preferably 5 to 15 μm. The density of the negative electrode mixture is preferably in the range of 0.89 to 1.42. The binder composition of the negative electrode mixture is preferably 3 to 15% by weight of the mixture weight.

  The lithium secondary battery of the present invention can be applied to a hybrid vehicle, a fuel cell vehicle, an electric vehicle, and the like, and further to a power source such as an electric tool that requires a high output.

The following examples illustrate the invention. In addition, this invention is not limited to the Example described below. The average particle size of graphite used in the following examples was 15 μm, and the average particle size of amorphous carbon was 10 μm.
Example 1
LiNi _0.8 Co _0.2 O ₂ was used as the positive electrode active material, and the positive electrode active material, the conductive agent graphite, and the binder polyvinylidene fluoride were mixed at a weight ratio of 85: 10: 5 using a kneader. The mixture was kneaded for a minute to obtain a positive electrode mixture. The positive electrode mixture was applied to an aluminum foil having a thickness of 20 μm. On the other hand, a mixture of an amorphous carbon material and natural graphite was used as the negative electrode active material, and polyvinylidene fluoride was used as the binder, and the mixture was kneaded at a weight ratio of negative electrode active material: binder = 90: 10. . The obtained negative electrode mixture was applied to a copper foil having a thickness of 10 μm. Each of the produced positive and negative electrodes was roll-formed with a press machine and then vacuum-dried at 150 ° C. for 5 hours. _A ratio of the amount of graphite of the mixed negative electrode of amorphous carbon material and graphite, the thickness of the mixture layer on both sides, the density of the negative electrode mixture, and the density of the negative electrode mixture ρ _G ρ _A / [ρ _G (1-X) + ρ _A Table 1 shows the ratio to X] (where ρ _G = graphite true density).

After drying, the positive electrode plate and the negative electrode plate were wound through the separator 3 and inserted into the battery can 4. The negative electrode current collecting lead piece 6 was collected on the nickel negative electrode current collecting lead portion 8 and ultrasonically welded, and the current collecting lead portion was welded to the bottom of the can. On the other hand, the positive electrode current collecting lead piece 5 was ultrasonically welded to the aluminum current collecting lead portion 7 and then the aluminum lead portion was resistance welded to the lid 9 with the positive electrode terminal portion 11 interposed therebetween. After injecting an electrolytic solution (1M LiPF ₆ / EC: DEC = 1: 1), the lid was sealed with caulking of the can 4 to obtain a battery. A gasket 12 was inserted between the upper end of the can and the lid. A schematic diagram of the battery thus manufactured is shown in FIG.

The battery capacity was determined by charging and discharging at a charge end voltage of 4.2 V, a discharge end voltage of 2.7 V, and a charge / discharge rate of 1 C (1 hour rate of the rated electric capacity). In the state of SOC (state of charge) 50%, currents of 1C, 5C, 10C, and 20C were applied for 10 seconds, the voltage at the 10th second at each current value was measured, and the input / output performance was examined. Using battery discharge end voltage (V _D) and the current value when the extrapolated straight line of the current-voltage characteristic to a final discharge voltage (I _D), was determined output from the equation _{P O = I D × V D} . On the other hand, the input is obtained from the formula P _I = I _C × V _C using the current value (I _C ) obtained by extrapolating the battery end-of-charge voltage (V _C ) and the current-voltage characteristic line to the end-of-charge voltage. It was. Table 2 shows the input / output measurement results.

As the amount of graphite increased, both energy density and power density tended to increase. When the amount of graphite was 20% or more, the energy density exceeded 80 Wh / kg, and the power density became 3000 W / kg or more. On the other hand, the input density tends to decrease as the amount of graphite increases. When the amount of graphite is 90%, the input density is less than 2000 W / kg.
(Example 2)
LiMn _1/3 Ni _1/3 Co _1/3 O ₂ was used as the positive electrode active material, and a mixture of an amorphous carbon material and artificial graphite was used as the negative electrode active material. In this example, the amount of graphite was 50%. A battery was produced in the same manner as in Example 1. Negative electrode mixture ratio of density ^{0.92g / cm 3, 1.02g / cm} 3, 1.10g / cm 3, 1.17g / cm 3, 1.24g / cm 3, and at 1.34 g / cm ³ There, the ratio of the negative electrode mixture density ρ _G ρ _a / _{[ρ G (1-X) +} ρ a X ] is 0.51,0.56,0.61,0.65,0.69, and 0.74 Met. In addition, the thickness of the negative mix layer was 86 micrometers, 78 micrometers, 72 micrometers, 69 micrometers, 64 micrometers, and 60 micrometers. A battery capacity test and an input / output test were conducted in the same manner as in Example 1 to obtain battery performance. Test results along with the negative electrode mixture density ratio _{ρ G ρ A / (ρ G} (1-X) + ρ A X) shown in Table 3.

There was a tendency for the energy density to improve as the electrode density increased. The negative electrode mixture of the density ratio ρ _G ρ _A / _{[ρ G (1-X) +} ρ A X ] is a value less than the battery 2-1 in 78Wh / kg and 80 Wh / kg of 0.51. The power density tended to increase as the ratio was increased, but the battery 2-6 having the highest density and a ratio of 0.74 tended to decrease slightly.

  Next, a constant current pulse cycle test was performed. The charge / discharge current value of input / output (charge / discharge) was 10 C (1/10 hour rate), the input / output time was 20 seconds, and the rest time was 30 seconds. The pulse cycle test was conducted 100,000 times at 50% SOC, and the battery internal resistance before and after the test was determined from the linear slope of the current-voltage characteristics of the output test. The results are shown in Table 4.

Negative electrode mixture density is low, the small battery 2-1 ratio of the negative electrode mixture density _{ρ G ρ A / (ρ G} (1-X) + ρ A X) is 0.51, the negative electrode mixture density is high, the ratio The battery 2-6 having a large resistance of 0.74 was a large resistance increase with a resistance increase rate exceeding 10%.
(Example 3)
LiMn _0.5 Ni _0.5 O ₂ was used as the positive electrode active material, and the same mixture of graphite and amorphous carbon material as in Example 2 was used as the negative electrode active material. An electrode was produced in the same manner as in Example 1 to produce a battery. The negative electrode mixture density was 1.17 g / cm ³ , and the ratio of the negative electrode mixture density to ρ _G ρ _A / (ρ _G (1−X) + ρ _A X) was 0.65. The thickness of the negative electrode mixture layer was 41 μm, 53 μm, 65 μm, 78 μm, 87 μm, and 99 μm. A capacity test and an input / output test were conducted in the same manner as in Example 1 to obtain battery performance. The results are shown in Table 5.

  As the negative electrode mixture layer was thickened, the energy density was improved. However, the battery 3-1 having the thinnest mixture layer has a small energy density of 71 Wh / kg. Next, a constant current pulse cycle test was performed. The charge / discharge current value of input / output (charge / discharge) was 10 C (1/10 hour rate), the input / output time was 20 seconds, and the rest time was 30 seconds. The pulse cycle test was conducted 100,000 times at 50% SOC, and the battery internal resistance before and after the test was determined from the linear slope of the current-voltage characteristics of the output test. The results are shown in Table 6.

  In the battery 3-6 having a mixture layer thickness of 99 μm, the resistance increase was a large value exceeding 10 and the resistance increase was large.

1 is a side sectional view showing a lithium secondary battery according to the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Positive electrode, 2 ... Negative electrode, 3 ... Separator, 4 ... Battery can, 5 ... Positive electrode current collection lead piece, 6 ... Negative electrode current collection lead piece, 7 ... Positive electrode current collection lead part, 8 ... Negative electrode current collection lead part, 9 ... Battery cover, 10 ... Rupture valve, 11 ... Positive electrode terminal, 12 ... Gasket.

Claims (3)

  A positive electrode in which a mixture containing a lithium transition metal composite oxide is formed on both sides of the current collector foil, a negative electrode in which a negative electrode mixture containing a negative electrode active material that absorbs and releases Li is formed on both sides of the current collector foil, and lithium In a lithium secondary battery composed of a non-aqueous electrolyte containing salt, the negative electrode mixture is a mixture of graphite, an amorphous carbon material and a binder, and the graphite and the amorphous carbon material in the mixture A lithium secondary battery, wherein a ratio of graphite to a total amount is 20 to 80% by weight. 2. The lithium secondary battery according to claim 1, wherein a ratio of negative electrode mixture density composed of graphite, an amorphous carbon material, and a binder ρ _G ρ _A / [ρ _G (1−X) + ρ _A X] ( Here, ρ _G = graphite true density, ρ _A = amorphous carbon material true density, X = ratio of graphite, 0.2 ≦ X ≦ 0.8) is 0.55 to 0.70. A lithium secondary battery characterized by that.   The lithium secondary battery according to claim 1, wherein the total thickness of both surfaces of the negative electrode mixture layer provided on both surfaces of the current collector foil is 50 to 90 μm.

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