JP2013161562A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery capable of ensuring adequate security by suppressing thermorunaway of a battery due to overcharge, even when the charging current or charging voltage changes.SOLUTION: In a lithium ion secondary battery 22 having an electrode group 21 in which a positive electrode 1 and a negative electrode 2 are arranged with separators 3, 4 being interposed therebetween, a battery can 11 housing the electrode group 21, and a nonaqueous electrolyte injected into the battery can 11, the ratio of the weight A of a positive electrode active material contained in the positive electrode and the weight B of a negative electrode active material contained in the negative electrode satisfies a relation 1.90≥A/B≥1.50, and 2.8-3.2 wt% of cyclohexylbenzene is added to the nonaqueous electrolyte. With such an arrangement, adequate security can be ensured during overcharge even if the charging current changes.

Description

  The present invention relates to a lithium ion secondary battery mounted on, for example, a vehicle.

  Since lithium ion secondary batteries have a higher energy density than other secondary batteries, they are now mainly used in portable devices such as digital cameras, notebook computers and mobile phones. In recent years, research and development of large-sized lithium ion secondary batteries for the purpose of electric vehicles and power storage have been actively conducted in order to cope with environmental problems. In particular, in the automobile industry, the development of electric vehicles using a motor as a power source and hybrid electric vehicles using both an internal combustion engine and a motor are underway, some of which have already been put into practical use. Yes.

  While this lithium ion secondary battery has an advantage of high energy density, a countermeasure for safety is required because a non-aqueous electrolyte is used. In particular, a large-sized lithium ion secondary battery for in-vehicle use is required to have higher safety and reliability because the inherent energy is very large. Among them, regarding overcharge, it is important to take sufficient countermeasures since a larger amount of energy is stored in the battery.

  For the purpose of ensuring safety during overcharge, a predetermined substance is added to the non-aqueous electrolyte. As such a substance (additive), for example, cyclohexylbenzene (CHB) is known (Patent Document 1). When cyclohexylbenzene (CHB) is added, a film is formed on the surface of the positive electrode due to a reaction with the positive electrode during overcharge. As a result, the battery resistance is increased and the current value is suppressed, so that the thermal runaway of the battery Is avoided.

  In addition to the above cyclohexylbenzene (CHB), biphenyl (BP), fluorobenzene (FB) and the like are also known as added substances (Patent Documents 2 and 3).

Japanese Patent No. 3247103 Japanese Patent No. 3354057 Japanese Patent No. 3061759

However, it is difficult to ensure sufficient safety with only the above additives in response to various conditions such as charging current and charging voltage.
In view of the above problems, the present invention provides a lithium ion secondary battery that can prevent thermal runaway of a battery due to overcharging and ensure sufficient safety even when a charging current or a charging voltage changes. This is the issue.

  In order to solve the above-mentioned problems, the present invention provides a lithium having an electrode group in which a positive electrode and a negative electrode are arranged via a separator, a battery can containing the electrode group, and a non-aqueous electrolyte injected into the battery can. An ion secondary battery, wherein the ratio of the weight A of the positive electrode active material contained in the positive electrode to the weight B of the negative electrode active material contained in the negative electrode has a relationship of 1.90 ≧ A / B ≧ 1.50, and A feature is that cyclohexylbenzene (CHB) is added to the non-aqueous electrolyte from 2.8 wt% to 3.2 wt%.

  According to the present invention, it is possible to control the thermal runaway of a battery due to overcharging and to ensure safety in response to changes in charging current and charging voltage.

The disassembled perspective view of the flat wound electrode group of the lithium ion secondary battery which is one Embodiment of this invention. The disassembled perspective view of the lithium ion secondary battery which is one Embodiment of this invention. 1 is an external perspective view of a lithium ion secondary battery according to an embodiment of the present invention. The flowchart of the battery preparation process of the lithium ion secondary battery which is one Embodiment of this invention.

  Hereinafter, with reference to FIGS. 1-4, embodiment of the lithium ion secondary battery of this invention is described.

  FIG. 1 is an exploded perspective view of a flat wound electrode group of a lithium ion secondary battery according to an embodiment of the present invention. The flat wound electrode group 21 includes a positive electrode 1 having a constant width positive electrode uncoated portion 1a and a negative electrode 2 having a constant width negative electrode uncoated portion 2a through a separator 3 and a separator 4. It has the structure which arranged so that a mutual uncoated part might be reversed and wound.

<Preparation of positive electrode>
A lithium transition metal composite oxide as a positive electrode active material, scaly graphite as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder are mixed at a weight ratio of 85: 10: 5, and this is mixed with N as a dispersion solvent. -A slurry in which methylpyrrolidone (NMP) was added and kneaded was applied to both sides of an aluminum foil having a thickness of 20 µm. Then, the positive electrode 1 whose width | variety of the positive electrode coating part 1b is 80 mm and whose electrode length is 4 m was obtained by drying, pressing, and cutting. In addition, the positive electrode uncoated part 1a formed continuously was arranged in the one side edge part of the elongate direction of aluminum foil, and the part was used as the positive electrode lead.

<Preparation of negative electrode>
A graphite powder as a negative electrode active material and PVDF as a binder were added, NMP as a dispersion solvent was added thereto, and a kneaded slurry was applied to both surfaces of a rolled copper foil having a thickness of 10 μm. Thereafter, drying, pressing, and cutting were performed to obtain a negative electrode 2 with a negative electrode coating portion 2b having a width of 84 mm and an electrode length of 4.4 m. In addition, the negative electrode uncoated part 2a formed continuously was arranged in the one side edge part of the elongate direction of rolled copper foil, and the part was made into the negative electrode lead.

<Battery assembly>
The produced positive electrode 1 and the negative electrode 2 are wound together with a polyethylene microporous separator 3 and a separator 4 having a width of 90 mm and a thickness of 30 μm so that the two electrodes are not in direct contact with each other, thereby producing a flat wound electrode group 21. did. In the flat wound electrode group, the positive electrode 1, the negative electrode 2, the separator 3, and the separator 4 are stretched by applying a load of 10 N in the longitudinal direction, and meander control is performed so that the electrode end face and the separator end face are in a fixed position. While making. In the center of the flat wound electrode group 21, one or more polypropylene microporous separators 3 and separators 4 are arranged. At this time, the positive electrode uncoated portion 1a and the negative electrode uncoated portion 2a are respectively positioned at opposite ends of the flat wound electrode group 21 (divided into one side and the other side in the winding axis direction). I tried to do it.

  FIG. 2 is an exploded perspective view of a lithium ion secondary battery according to an embodiment of the present invention. A negative electrode external terminal 7 and a positive electrode external terminal 8 are connected in advance to the battery lid 9 provided with the liquid injection hole 10, and the negative electrode external terminal 7 and the negative electrode current collector plate 5 are electrically connected to each other. The electric plate 6 was also made to be electrically conductive. The positive electrode uncoated portion 1a was joined to the positive electrode current collector plate 6 by ultrasonic welding, and the negative electrode uncoated portion 2a and the negative electrode current collector plate 5 were joined in the same manner. Thereafter, the flat wound electrode group 21 to which the battery lid portion was attached was inserted into the battery container 11.

After injecting a predetermined amount of non-aqueous electrolyte that can infiltrate the entire flat wound electrode group 21 into the battery container 11 from the injection hole 10, the injection hole 10 is sealed, whereby the lithium ion secondary battery 22 is sealed. Completed. In the non-aqueous electrolyte, 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) was added to a mixed solution in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1: 2. What was melt | dissolved in the density | concentration of was used. Then, cyclohexylbenzene (CHB) was added as an additive for overcharge countermeasures.

  FIG. 3 is an external perspective view of a lithium ion secondary battery according to an embodiment of the present invention. In the appearance of the lithium ion secondary battery 22 in which the battery container 11 is sealed with the battery lid 9, the negative electrode external terminal 7, the positive electrode external terminal 8, and the liquid injection hole 10 are found.

  FIG. 4 is a flowchart of a battery manufacturing process of a lithium ion secondary battery according to an embodiment of the present invention. For electrode preparation, kneading, coating, pressing, and slitting are performed in this order to prepare the electrode stock. For kneading, an active material, a conductive additive and a binder are mixed at a predetermined weight ratio, and a dispersion solvent is added thereto to prepare a slurry adjusted to a predetermined solid content concentration and viscosity. In the coating, the slurry is applied to both surfaces of a metal foil substrate having a predetermined thickness by a predetermined width and a predetermined weight, and then only the solvent is removed by drying, thereby producing an electrode after coating. In the pressing, the post-pressing electrode is produced by compressing the post-coating electrode to a predetermined thickness by a roll press. The slit cuts the post-pressing electrode into a predetermined coated part width and a predetermined uncoated part width to produce an original electrode fabric.

Then, a lithium ion secondary battery is produced through the process of winding, current collector plate welding, can insertion, can welding, and liquid injection using an electrode fabric. In the winding process, the positive electrode and the negative electrode are wound together through a separator so that the two electrodes do not directly contact each other, thereby producing a wound electrode group. In addition, while controlling the meandering so that the electrode end face and the separator end face are in a fixed position, the positive electrode uncoated part and the negative electrode uncoated part are prepared so as to be positioned at the opposite ends of the wound electrode group. To do.
In the current collector plate welding step, the positive electrode current collector plate and the negative electrode current collector plate are ultrasonically welded to the positive electrode uncoated portion and the negative electrode uncoated portion located at the opposite ends of the wound electrode group, respectively. Join. The positive electrode current collector plate and the negative electrode current collector plate are previously connected to the positive electrode external terminal and the negative electrode external terminal in advance in the lid portion.
In the can insertion step and the next can welding step, the wound electrode group with the lid portion including the positive electrode current collector plate and the negative electrode current collector plate attached is inserted into the battery container, and the lid part and the battery container are laser welded. Seal with. In the liquid injection process, a predetermined amount of nonaqueous electrolyte is injected into the battery container from the liquid injection hole provided in the lid, and then the liquid injection hole is sealed by laser welding to produce a lithium ion secondary battery. .

  The present invention is not limited by the above embodiment. Either a square lithium ion secondary battery or a cylindrical lithium ion secondary battery may be used. Moreover, although PVDF was illustrated also as a binder, polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, butyl rubber, nitrile rubber, styrene / butadiene rubber, polysulfide rubber, nitrocellulose, cyanoethyl cellulose, various latexes, acrylonitrile, Polymers such as vinyl fluoride, vinylidene fluoride, propylene fluoride, and chloroprene fluoride, and mixtures thereof may be used.

Further, in the present embodiment, a nonaqueous electrolyte solution in which LiPF ₆ is dissolved in a mixed solution of EC and DMC is exemplified, but a nonaqueous electrolyte solution in which a general lithium salt is used as an electrolyte and this is dissolved in an organic solvent. The present invention is not particularly limited to the lithium salt or organic solvent used. For example, as the electrolyte, LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, or a mixture thereof can be used. Examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, Diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propiontonyl, etc., or a mixed solvent of two or more of these may be used, and the mixing ratio is not limited.

  Next, examples of the prismatic lithium ion secondary battery 22 manufactured according to the above-described embodiment will be described. A comparative example prepared for comparison is also shown.

The specifications of the prismatic lithium ion secondary battery produced as an example are shown in the table below.

Here, the weight ratio is the ratio A / B between the weight A of the positive electrode active material and the weight B of the negative electrode active material, and the amount of electrolyte is (%) is a non-aqueous electrolyte with respect to the total pores of the electrode group 21. The injection amount. Here, the total holes of the electrode group 21 are the total of the holes of the positive electrode 1, the negative electrode 2, and the separators 3 and 4.
The CHB addition amount (wt%) is the CHB addition amount when the amount of the electrolyte solution obtained by removing CHB and other additives from the non-aqueous electrolyte solution is 100%. That is, for example, a CHB addition amount (wt%) of 3 (wt%) means that 3 wt% of CHB was added with the amount of the electrolyte solution obtained by removing CHB and other additives from the nonaqueous electrolyte solution as 100%. And the total is 103%.

The specifications of the prismatic lithium ion secondary battery produced as a comparative example are shown in the table below.

About the produced battery, an overcharge test result (charging current change) and the energy density of the battery are shown in the table below.

As is clear from this result, the lithium ion secondary battery of the present invention has a condition (1) the ratio of the positive electrode active material weight A to the negative electrode active material weight B is 1.90 ≧ A / B ≧ 1.50. And (2) cyclohexyl benzene (CHB) is preferably added to the non-aqueous electrolyte from 2.8 wt% to 3.2 wt%.
As for the former condition (1), when the filling amount of the negative electrode active material is not more than a certain level with respect to the filling amount of the positive electrode active material, lithium ions released from the positive electrode during overcharge are metal on the negative electrode. This is considered to be because the risk of precipitation as lithium and short circuiting increases. The latter condition (2) assumes an error of 0.2% by weight with respect to the added amount of 3.0% by weight.

In Comparative Examples 1 and 2, the ratio A / B between the weight A of the positive electrode active material and the weight B of the negative electrode active material is 2.10 and 2.00, which is the upper limit of the weight ratio (A / B). The value is larger than 90. Therefore, in Comparative Example 1, the test results resulted in rupture and ignition at both charging currents 30A and 150A, and in Comparative Example 2, the test result resulted in rupture and ignition at the charging current 30A.
In Comparative Examples 3 and 4, the addition amount of cyclohexylbenzene (CHB) is 2.0 and 1.0, which is smaller than the lower limit value of 2.8% by weight of the CHB addition amount. Therefore, in Comparative Examples 3 and 4, the test results that resulted in rupture and ignition at both charging currents of 150A.
Thus, in order to ensure sufficient safety against fluctuations in charging current, the ratio A / B of the weight A of the positive electrode active material to the weight B of the negative electrode active material and the amount of cyclohexylbenzene (CHB) added It is necessary to make both appropriate, and by setting these to the above-mentioned conditions (1) and (2), sufficient safety in overcharging can be ensured even when the charging current changes.

  Furthermore, in the lithium ion secondary battery of the present invention, it is preferable that the injection amount of the non-aqueous electrolyte is 140% or more and 160% or less with respect to the total vacancies of the electrode group. When the injection amount of the non-aqueous electrolyte was small, as seen in Example 7, the test result at a charging current of 150 A resulted in smoke generation. This is because the amount of cyclohexylbenzene (CHB) is insufficient due to the small injection amount, and the effect of heat absorption by the non-aqueous electrolyte is reduced. Moreover, when there is much injection amount, a battery weight will increase and an energy density will fall. In Examples 1 to 6, the battery temperature was 100 ° C. to 130 ° C., and in Example 7, the battery temperature was 300 ° C.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various designs can be made without departing from the spirit of the present invention described in the claims. It can be changed. For example, the above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to one having all the configurations described. Further, a part of the configuration of an embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of an embodiment. Furthermore, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

DESCRIPTION OF SYMBOLS 1 Positive electrode 1a Positive electrode uncoated part 1b Positive electrode coated part 2 Negative electrode 2a Negative electrode uncoated part 2b Negative electrode coated part 3 Separator 4 Separator 5 Negative electrode current collector plate 6 Positive electrode current collector plate 7 Negative electrode external terminal 8 Positive electrode external terminal 9 Battery lid 10 Injection hole 11 Battery container 21 Flat wound electrode group 22 Lithium ion secondary battery

Claims (2)

A lithium ion secondary battery having an electrode group in which a positive electrode and a negative electrode are arranged via a separator, a battery can containing the electrode group, and a non-aqueous electrolyte injected into the battery can,
The ratio of the weight A of the positive electrode active material contained in the positive electrode to the weight B of the negative electrode active material contained in the negative electrode is in a relationship of 1.90 ≧ A / B ≧ 1.50, and the non-aqueous electrolyte contains A lithium ion secondary battery, wherein cyclohexylbenzene is added in an amount of 2.8 wt% to 3.2 wt%.   2. The lithium ion secondary battery according to claim 1, wherein an injection amount of the non-aqueous electrolyte is 140% or more and 160% or less with respect to a total number of vacancies of the electrode group.

Images