JP2014078341A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of obtaining discharge capacity having less deviation and high rate characteristics even if a positive electrode containing a plurality of kinds of active materials is used.SOLUTION: A nonaqueous electrolyte secondary battery includes: a positive electrode 20 in which a positive electrode mixture layer 5 is formed on a current collector 6; a negative electrode; and a nonaqueous electrolyte. The positive electrode mixture layer includes a first layer 11 containing only lithium manganite as an active material, and a second layer 12 containing only a second active material having higher thermal stability than lithium manganite.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery in which a positive electrode active material has a multi-layer structure.

Research and development related to hybrid electric vehicles, plug-in hybrid electric vehicles, and electric vehicles have been energetically conducted in recent years due to energy strategies for petroleum resources and environmental problems such as air pollution and CO ₂ emissions. Compared to other secondary batteries, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are one of secondary batteries having a high energy density, so that hybrid electric vehicles, plug-in hybrid electric vehicles, etc. It is considered as a potential power source candidate for various electric vehicles.

  Lithium manganate, which is one of the positive electrode active materials in the nonaqueous electrolyte secondary battery, has good characteristics in terms of low cost and high rate characteristics. However, problems remain in terms of capacity and thermal stability. On the other hand, lithium nickel cobalt manganate has good characteristics in terms of capacity and thermal stability. However, problems remain in terms of cost.

  From the above, in consideration of capacity, thermal stability, rate characteristics, and cost, etc., it is necessary to use a mixture of nickel cobalt lithium manganate and lithium manganate for the positive electrode active material. It has been proposed (see Patent Document 1).

JP 2007-234349 A

  However, when nickel cobalt lithium manganate and lithium manganate are mixed and used as the positive electrode active material, it is difficult to uniformly disperse them by kneading because of different particle sizes, particle shapes, and densities. As a result, it is difficult to uniformly distribute the positive electrode active material in the in-plane direction of the positive electrode mixture layer, and there is a problem that it is difficult to obtain a non-aqueous electrolyte secondary battery having a discharge capacity and high rate characteristics with little variation. .

  Accordingly, an object of the present invention is to provide a non-aqueous electrolyte secondary battery that can obtain a discharge capacity and high rate characteristics with little variation even when a positive electrode containing a plurality of types of active materials is used.

  The inventor of the present invention includes a positive electrode mixture layer in which the positive electrode active material is mixed by kneading or the like by constructing the positive electrode mixture layer so as to have a laminated structure including a plurality of layers each including different active materials It has been found that the performance can be improved as compared with the nonaqueous electrolyte secondary battery.

  Based on the above, the present invention provides a nonaqueous electrolyte secondary battery comprising a positive electrode having a positive electrode mixture layer formed on a current collector, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode mixture layer comprises an active material. A first layer containing only lithium manganate, and a second layer containing only a second active material having higher thermal stability than lithium manganate as an active material.

  The second active material may be lithium nickel cobalt manganate.

  The weight ratio of lithium manganate to the second active material in the positive electrode mixture layer may be from 10 to 70 for the second active material with respect to 30 to 90 for the lithium manganate.

  The first layer and the second layer contain a conductive assistant, and the weight ratio of the conductive assistant contained in the first layer to the conductive assistant contained in the second layer is that of the first layer. The conductive aid of the second layer may be 60 or more and 90 or less with respect to 10 or more and 40 or less.

  According to the nonaqueous electrolyte secondary battery of the present invention, even when a positive electrode containing a plurality of types of active materials is used, a discharge capacity and high rate characteristics with little variation can be obtained.

It is a schematic diagram which decomposes | disassembles and shows the nonaqueous electrolyte secondary battery of one Embodiment of this invention. It is a figure which expands and shows the positive electrode of the same nonaqueous electrolyte secondary battery.

An embodiment of the present invention will be described with reference to FIGS. 1 and 2.
FIG. 1 is an exploded schematic view showing the nonaqueous electrolyte secondary battery 50 of the present embodiment. The nonaqueous electrolyte secondary battery 50 includes a positive electrode 20 having the positive electrode mixture layer 5 and the current collector 6, a negative electrode 30 having the negative electrode mixture layer 3 and the current collector 2, and the positive electrode 20 and the negative electrode 30. It is provided with a separator 4 that is interposed and conducts only ions without conducting electrons, and an exterior body 40 that seals them. The outer package 40 is divided into a positive electrode can 9 and a negative electrode can 1, and the positive electrode can 9 and the negative electrode can 1 are fixed in a sealed state with a packing 8 formed of polypropylene or the like. Further, a stainless steel spacer 7 and a web washer 10 may be arranged as necessary for adjusting the gap and the stack pressure. The interior space of the exterior body 40 is filled with an electrolyte solution (not shown) (not shown).

FIG. 2 is an enlarged view showing the positive electrode 20. The positive electrode mixture layer 5 includes a first layer 11 containing only lithium manganate as an active material, and a nickel cobalt lithium manganate (second active material) superior in thermal stability to lithium manganate as an active material. And the second layer 12 containing only. Thereby, the positive mix layer 5 is made into the two-layer structure from which the active material contained in each layer differs.
As the current collector 6, various known materials such as aluminum foil can be used.

The second layer 12 is a slurry obtained by adding nickel cobalt lithium manganate and a conductive additive to a binder solution containing NMP (N-methyl-2-pyrrolidone) and mixing them. It is formed by applying and drying on the electric body 6. There is no restriction | limiting in particular in the method of application | coating and drying, A well-known various method can be used.
The mixing ratio of the nickel cobalt lithium manganate, the conductive additive, and the binder in the second layer 12 is 70 to 93 wt% for nickel cobalt lithium manganate, 5 to 20 wt% for carbon particles, and 2 to 10 wt% for the binder. % Can be set as appropriate within the range of%.

  As a 2nd active material contained in the 2nd layer 12, nickel cobalt lithium aluminum oxide etc. other than nickel cobalt lithium manganate can be used. The ratio of lithium manganate to the second active material in the positive electrode mixture layer is appropriately set in consideration of various factors such as capacity, thermal stability, rate characteristics, and cost required for the non-aqueous electrolyte secondary battery. However, it is preferable to adjust the weight ratio so that the second active material is 10 to 70 with respect to the lithium manganate 30 to 90.

The first layer 11 is formed by applying and drying a slurry obtained by adding lithium manganate and a conductive additive to a binder solution containing NMP and mixing them on the second layer 12 and drying. There is no restriction | limiting in particular in the method of application | coating and drying, A well-known various method can be used. Moreover, as a conductive support agent and a binder, the thing similar to a 2nd layer can be used.
The mixing ratio of the lithium manganate, the conductive additive, and the binder in the first layer 11 is 85 to 93% by weight of lithium manganate, 2 to 10% by weight of carbon particles, and 5 to 10% by weight of binder.

  The conductive aid used for forming the first layer 11 and the second layer 12 is used to improve the electrical conductivity of the positive electrode 20, and includes carbon black, natural graphite, artificial graphite, titanium oxide, ruthenium oxide, and the like. Metal oxides, metal fibers, and the like can be used. Among them, carbon black having a structure structure is preferable, and furnace black, ketjen black, acetylene black, etc., which are one of them, are particularly preferably used. A mixed system of carbon black and other conductive agent such as graphite is also preferably used.

  As the binder, polyvinylidene fluoride which is a non-aqueous binder can be suitably used. Although an aqueous binder such as styrene butadiene latex can be used, in that case, it is preferable to use carboxymethyl cellulose or the like as a dispersant.

The negative electrode mixture layer 3 is formed by adding a negative electrode active material, a binder, and a dispersant to water, and applying and drying a slurry obtained by stirring on the current collector 2.
As the negative electrode active material, various carbon particles such as natural graphite, coke, graphitized carbon, carbon fiber, spherical carbon, artificial graphite, and amorphous graphite can be used. As the current collector 2 of the negative electrode 30, for example, a copper foil or the like can be used.
The mixing ratio of the negative electrode active material, the binder, and the dispersant in the negative electrode mixture layer 3 is, for example, 96 to 99 wt% for the negative electrode active material, 0.1 to 2 wt% for the binder, and 0.1 to 2 wt% for the dispersant. %. In general, the carbon particles exhibit both functions of the active material and the conductive additive, but the active material and the conductive additive may be different carbon particles.

Solvents for the electrolyte solution that fills the interior of the outer package include low-viscosity chain carbonates such as dimethyl carbonate and diethyl carbonate, and high-permittivity cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, and γ-butyrolactone. 1,2-dimethoxyethane, tetrahydrofuran, 4-trifluoromethyl-1,3-dioxolan-2-one methyl acetate, methyl propionate, vinylene carbonate, dimethylformamide, sulfolane, and mixed solvents thereof. it can.
Further, the electrolyte contained in the electrolyte solution, not particularly _{limited, LiClO 4, LiBF 4, LiAsF} 6, LiPF 6, LiCF 3 SO 3, LiN (CF 3 SO 2) 2, LiI, LiAlCl 4 , etc. and their A mixture etc. are mentioned. Preferably, a lithium salt obtained by mixing one or more of LiBF ₄ and LiPF ₆ is preferable.

  Various known materials can be used as the separator 4. Further, the basic structure of the non-aqueous electrolyte secondary battery 50 is not particularly limited, and can be appropriately selected depending on the application, such as a cylindrical shape or a rectangular shape. The number of layers of the positive electrode 20 and the negative electrode 30 may be a single layer or a plurality of layers. Further, the method of stacking a plurality of layers may be a stacked type or a wound type.

  In the nonaqueous electrolyte secondary battery 50 of the present embodiment configured as described above, the positive electrode mixture layer 5 includes the first layer 11 and the second layer 12, and each layer has one type of active material different from each other. Contains only substances. Therefore, since the slurry for forming each layer can be prepared separately, the coating film to which the slurry is applied rather than the case where the positive electrode mixture layer having a single layer structure is formed with the slurry in which two different types of positive electrode active materials are mixed. The distribution variation of each active material in the inside, particularly the distribution variation in the in-plane direction (thickness direction of the coating film) can be suppressed. As a result, the active material can be uniformly dispersed in both the first layer 11 and the second layer 12 to form a positive electrode mixture layer, and the discharge capacity varies while using multiple types of active materials. Can be suppressed.

Further, by using a material having better thermal stability than lithium manganate as the second layer active material, a positive electrode having a good balance between cost and thermal stability can be formed.
Here, when nickel cobalt lithium manganate is used as the active material of the second layer, the electrical resistance of the second layer can be efficiently reduced by mixing the conductive additive into the slurry at a higher mixing ratio than the first layer 11. It is possible to improve the performance of the whole positive electrode by reducing the efficiency. The ratio (weight ratio) between the conductive auxiliary agent contained in the first layer 11 and the conductive auxiliary agent contained in the second layer 12 is in the second layer 12 with respect to the conductive auxiliary agent 10 to 40 in the first layer 11. It is preferable to adjust appropriately in the range which is set to 60 or more and 90 or less. Note that the conductive aid for the first layer and the conductive aid for the second layer may be the same or different.

The nonaqueous electrolyte secondary battery of this embodiment will be described in more detail using examples.
Example 1
An aluminum foil having a thickness of 15 μm was used as a positive electrode current collector. A slurry using nickel cobalt lithium manganate as the positive electrode active material was applied onto the current collector with a doctor blade so as to have a basis weight of 6.1 mg / cm ² , dried by heating at 120 ° C. for 30 minutes. Two layers were formed. Next, a slurry using lithium manganate as the positive electrode active material was applied on the second layer so as to have a basis weight of 14.2 mg / cm ² , and dried by heating at 120 ° C. for 30 minutes. A layer was formed. This was pressed to a density of 2.4 g / cm ³ to form a positive electrode mixture layer having a first layer and a second layer, and a positive electrode was produced.

The slurry for forming the second layer is a mixture of 92.6% by weight of nickel cobalt lithium manganate, 4.6% by weight of acetylene black as a conductive additive, and 2.8% by weight of polyvinylidene fluoride as a binder, NMP was added to prepare a solid content of 60%.
The slurry for forming the first layer was mixed with 92.6% by weight of lithium manganate, 4.6% by weight of acetylene black and 2.8% by weight of polyvinylidene fluoride, and NMP was added to obtain a solid content of 60%. It prepared so that it might become.

The obtained positive electrode was punched into a circle having a diameter of 15 mm, and a coin cell was manufactured using a Li foil punched into a disk having a diameter of 16 mm as a negative electrode, whereby a nonaqueous electrolyte secondary battery of Example 1 was obtained. As the separator, model number 2200 manufactured by Celgard was used, and the electrolyte was a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) 3: 7 (v / v), and LiPF ₆ was 1M. What was added was used.
In the positive electrode of Example 1, the weight ratio of the active material was the second layer nickel cobalt lithium manganate 30 to the first layer lithium manganate 70.

(Example 2)
The slurry for forming the second layer was prepared by mixing 75.5% by weight of nickel cobalt lithium manganate, 15.1% by weight of acetylene black, and 9.4% by weight of polyvinylidene fluoride, adding NMP, It was prepared and prepared so as to be 60%. The same slurry as in Example 1 was used as the slurry for forming the first layer.

A slurry for forming the second layer was applied on the same current collector as in Example 1 so that the basis weight was 7.0 mg / cm ² , and dried by heating at 120 ° C. for 30 minutes. A layer was formed. Next, a slurry for forming the first layer is applied on the second layer so as to have a basis weight of 13.3 mg / cm ² , and is heated and dried at 120 ° C. for 30 minutes to become the first layer. Formed.

In other respects, a coin cell was produced in the same procedure as in Example 1, and the nonaqueous electrolyte secondary battery in Example 2 was obtained.
In the positive electrode of Example 2, the weight ratio of the active material was the same as that of Example 1. The weight ratio of the conductive assistant contained in each layer was the first layer 37 with respect to the second layer 63.

(Comparative example)
The nonaqueous electrolyte secondary battery of the comparative example was produced by making the positive electrode mixture layer into a single layer structure containing two types of active materials.
The slurry for forming the positive electrode mixture layer is a mixture of 64.8% by weight of lithium manganate, 27.8% by weight of nickel cobalt lithium manganate, 4.6% by weight of acetylene black, and 2.8% by weight of polyvinylidene fluoride. NMP was added to prepare a solid content of 60%.
This slurry was applied onto an aluminum foil so as to have a basis weight of 20.3 mg / cm ² on the same current collector as in Example 1, and dried by heating at 120 ° C. for 30 minutes. After drying by heating, the mixture was pressed to a density of 2.4 g / cm ³ to form a positive electrode mixture layer.
In other respects, a coin cell was produced in the same procedure as in Example 1, and a non-aqueous electrolyte secondary battery of a comparative example was obtained. In the positive electrode of the comparative example, the weight ratio of the active material was the same as in Example 1.

(Charge / discharge test)
Charging / discharging was performed in a cycle of 3.0 V to 4.25 V, and the discharge capacity was evaluated at four different current densities (0.2 C, 1.0 C, 2.0 C, 4.0 C).
The results are shown in Table 1. In addition, the numerical value in Table 1 is an average value, and the number in parenthesis shows a standard deviation.

  As shown in Table 1, the battery of Example 1 had a discharge capacity at 0.2C of 115.9 mAh / g, whereas the battery of the comparative example had a discharge capacity of 0.2. It was 2 mAh / g, and no significant difference was observed. However, the standard deviation was 0.56 in Example 1 and 1.11 in Comparative Example 1. Thus, it was confirmed that the present invention is effective in suppressing variation in discharge capacity.

In the comparative example, the discharge capacity was greatly reduced to 62.8 mAh / g when the high rate was 4.0 C, whereas in Examples 1 and 2, the discharge capacity was in the order of 90 mAh / g even at 4.0 C. Was maintained and showed excellent rate characteristics.
Here, in the battery of Example 2, the discharge capacity at 4.0 C was 95.8 mAh / g, whereas in the battery of Example 1, the discharge capacity at 4.0 C was 91.9 mAh / g. Met. The standard deviation was 3.53 in Example 2 and 4.52 in Example 1. From this, in the present invention, it was confirmed that the rate characteristics can be further improved by mixing more conductive assistant in the second layer than in the first layer.

  As mentioned above, although this invention was demonstrated using embodiment and an Example, the technical scope of this invention is not limited to the content of the said embodiment and Example, In the range which does not deviate from the meaning of this invention, Various changes can be added to or deleted from the components.

For example, in the present invention, any of the first layer and the second layer of the positive electrode mixture layer may be formed on the current collector. That is, the first layer may be formed on the current collector and the second layer may be formed on the first layer.
The positive electrode mixture layer may be formed on both sides of the current collector.

  Further, the positive electrode mixture layer may be configured to include a third layer containing only one type of active material different from both the first layer and the second layer.

1 Negative electrode can 2 Current collector (of negative electrode)
3 Negative electrode mixture layer 4 Separator 5 Positive electrode mixture layer 6 Current collector (for positive electrode)
7 Stainless steel spacer 8 Packing 9 Positive electrode can 10 Web washer 11 First layer 12 Second layer 20 Positive electrode 30 Negative electrode 50 Nonaqueous electrolyte secondary battery

Claims (4)

In a non-aqueous electrolyte secondary battery comprising a positive electrode having a positive electrode mixture layer formed on a current collector, a negative electrode, and a non-aqueous electrolyte,
The positive electrode mixture layer includes a first layer containing only lithium manganate as an active material, and a second layer containing only a second active material having higher thermal stability than lithium manganate as an active material. A non-aqueous electrolyte secondary battery.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the second active material is nickel cobalt lithium manganate.   The weight ratio of lithium manganate to the second active material in the positive electrode mixture layer is 10 to 70 for the second active material with respect to 30 to 90 for lithium manganate. Item 3. The nonaqueous electrolyte secondary battery according to Item 1 or 2.   The first layer and the second layer contain a conductive assistant, and the weight ratio of the conductive assistant contained in the first layer to the conductive assistant contained in the second layer is that of the first layer. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the second layer of the conductive auxiliary is 60 to 90 with respect to the conductive auxiliary of 10 to 40. 5.

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