JP2015015183A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery which is superior in safety in overcharge, and arranged so that the worsening of high-rate discharge performance can be prevented.SOLUTION: A nonaqueous electrolyte secondary battery comprises a positive electrode including a positive electrode current collector 1, a first positive electrode mixture layer 3, and a second positive electrode mixture layer 2 disposed between the positive electrode current collector 1 and the first positive electrode mixture layer 3. The second positive electrode mixture layer 2 includes lithium complex oxide particles 20. As to the second positive electrode mixture layer 2, the average thickness t thereof, and the average particle diameter dL of the lithium complex oxide particles 20 satisfy the following relational expression: 1.1≤t/dL≤1.8.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery.

  In a non-aqueous electrolyte secondary battery, a positive electrode having a multilayer structure in which another positive electrode mixture layer containing a positive electrode active material is provided on a positive electrode mixture layer containing a specific lithium composite oxide is known. In this multi-layer structure, the resistance of the lower positive electrode mixture layer can be increased during overcharging to make it safe (for example, Patent Documents 1 and 2). In such a technique, the lower positive electrode mixture layer is formed by applying a paste obtained by dispersing or dissolving a lithium composite oxide and other additives (such as a binder resin) in an organic solvent to a positive electrode current collector. It is formed by drying and pressing.

In particular, in Patent Document 2, the lower positive electrode mixture layer contains LiFePO _4, and the content of the conductive auxiliary agent in the lower positive electrode mixture layer is reduced in order to improve overcharge characteristics. In this document, there is a description that it is desirable to make the thickness of the lower positive electrode mixture layer as thin as possible in order to smoothly advance a normal charge / discharge reaction. LiFePO ₄ is a positive electrode active material having a high resistance increase rate during overcharge.

JP 2009-4289 A JP 2007-26676 A

  However, even if the thickness of the lower positive electrode mixture layer is simply reduced, good high rate discharge performance may not be obtained. Further, when the thickness of the lower positive electrode mixture layer is reduced, the resistance increase due to the positive electrode mixture layer may not work.

  An object of this invention is to provide the nonaqueous electrolyte secondary battery which is excellent in the safety | security with respect to an overcharge, and the fall of high rate discharge performance is fully suppressed.

The present invention includes a positive electrode including a positive electrode current collector, a first positive electrode mixture layer, and a second positive electrode mixture layer formed between the positive electrode current collector and the first positive electrode mixture layer. And the second positive electrode mixture layer includes lithium composite oxide particles, and the average thickness t of the second positive electrode mixture layer and the average particle diameter dL of the lithium composite oxide particles are expressed by the following relational expression ( It is a nonaqueous electrolyte secondary battery that satisfies I).
1.1 ≦ t / dL ≦ 1.8 (I)

  In the nonaqueous electrolyte secondary battery of the present invention, the lithium composite oxide particles 20 preferably have an average particle diameter dL in the range of 1 μm to 15 μm.

  In the nonaqueous electrolyte secondary battery of the present invention, the first positive electrode mixture layer includes active material particles, and the ratio between the average particle diameter dL of the lithium composite oxide particles and the average particle diameter dA of the active material particles (DL / dA) is preferably in the range of 0.1-6.

  In the nonaqueous electrolyte secondary battery of the present invention, the lithium composite oxide particles preferably include spinel type lithium manganate.

  The nonaqueous electrolyte secondary battery of the present invention is excellent in safety against overcharging and sufficiently suppresses a decrease in high rate discharge performance.

It is a conceptual diagram which shows the general | schematic expanded cross section for demonstrating one embodiment of the positive electrode of this invention. It is a conceptual diagram which shows the general | schematic expanded cross section of the positive electrode for demonstrating measuring methods, such as the thickness of a 2nd positive mix layer, Comprising: It is the figure which abbreviate | omitted the 1st positive mix layer. It is a schematic sectional drawing of the square-shaped nonaqueous electrolyte secondary battery which is one embodiment of this invention. It is a graph which shows the relationship between t / dL and discharge capacity ratio 1C / 0.2C in the battery manufactured by the Example and the comparative example. It is a graph which shows the relationship between the average thickness t in the battery manufactured by the Example and the comparative example, and discharge capacity ratio 1C / 0.2C.

  Although one embodiment of the nonaqueous electrolyte secondary battery of the present invention is described below, the present invention is not limited to the following description.

(Positive electrode)
The positive electrode of the nonaqueous electrolyte secondary battery of the present invention includes a positive electrode current collector, a first positive electrode mixture layer, and a second electrode formed between the positive electrode current collector and the first positive electrode mixture layer. It includes a positive electrode mixture layer. A schematic enlarged cross-sectional view showing one embodiment of the positive electrode of the present invention is shown in FIG. FIG. 1 is a schematic enlarged cross-sectional view perpendicular to the positive electrode current collector. In FIG. 1, 1 is a positive electrode current collector, 2 is a second positive electrode mixture layer, and 3 is a first positive electrode mixture layer. Show.

In the present invention, the average thickness t of the second positive electrode mixture layer 2 and the average particle diameter dL of the lithium composite oxide particles 20 included in the layer satisfy the following relational expression (I).
1.1 ≦ t / dL ≦ 1.8 (I)

  By making t / dL of the second positive electrode mixture layer 2 within the above range, in the nonaqueous electrolyte secondary battery, safety during overcharge is excellent, and a decrease in high-rate discharge performance is sufficiently suppressed. The Although details for obtaining such an effect are not clear, it is presumed to be based on the following mechanism. When many active material particles 30 of the first positive electrode mixture layer 3 enter the gaps between the lithium composite oxide particles 20 of the second positive electrode mixture layer 2, the active material particles 30 of the first positive electrode mixture layer 3. And the positive electrode current collector come into contact with each other, so that the increase in resistance during overcharging does not work effectively. On the other hand, when the second positive electrode mixture layer 2 is too thick relative to the particle size of the lithium composite oxide particles 20, the resistance of the positive electrode mixture layer as a whole increases and the high-rate discharge performance decreases. In the present invention, by setting the t / dL of the second positive electrode mixture layer 2 within the above range, the active material particles 30 are prevented from entering the gaps between the lithium composite oxide particles 20 and the lithium composite oxide is oxidized. An excessive increase in the thickness of the second positive electrode mixture layer 2 can be suppressed as compared with the particle diameter of the product particles 20. As a result, it is considered that the safety against overcharge is excellent and the reduction of the high rate discharge performance is sufficiently prevented.

  When t / dL of the second positive electrode mixture layer 2 is in a range of less than 1.1, safety against overcharge is reduced. When the t / dL of the second positive electrode mixture layer 2 is in a range exceeding 1.8, the high rate discharge performance of the nonaqueous electrolyte secondary battery is deteriorated.

  The t / dL of the second positive electrode mixture layer 2 is preferably in the range of 1.1 to 1.5. By setting t / dL of the second positive electrode mixture layer 2 in the range of 1.1 to 1.5, the resistance of the second positive electrode active material layer 2 at the time of charge / discharge can be reduced.

  In FIG. 1, the second positive electrode mixture layer 2 and the first positive electrode mixture layer 3 are formed only on one side of the positive electrode current collector 1, but the present invention is not limited to this. 1 may be formed on both sides. When the second positive electrode mixture layer 2 and the first positive electrode mixture layer 3 are formed on both surfaces of the positive electrode current collector 1, t / dL of the second positive electrode mixture layer 2 on at least one surface is The t / dL of the second positive electrode mixture layer 2 on both surfaces is preferably independently in the above range.

The average thickness t of the second positive electrode mixture layer 2 can be measured by the following method.
Take a cross-sectional SEM micrograph. In the obtained photograph, first, as shown in FIG. 2, some lithium composite oxide particles 20 are randomly selected. The contour of the lithium composite oxide particle 20 having the surface of the positive electrode current collector 1 as a reference line La, a line Lb parallel to the reference line La, and the distance from the reference line La is maximum. A parallel line Lb passing through the point P on the line is drawn to each selected particle. Then, the distances between these lines La and Lb are measured as thicknesses t1, t2, and t3, and the average thickness t is calculated. FIG. 2 is a schematic enlarged cross-sectional view of the positive electrode perpendicular to the positive electrode current collector, in which the first positive electrode mixture layer 3 is omitted.

The average particle diameter dL of the lithium composite oxide particles 20 contained in the second positive electrode mixture layer 2 can be measured by the following method.
The average particle diameter dL of the lithium composite oxide particles 20 is a value obtained by calculating the average value of the particle diameters of the lithium composite oxide particles 20 in the SEM photograph. The particle diameter is an average value of the longest particle diameter and the shortest particle diameter in the SEM photograph.

  Regarding the size of the lithium composite oxide particles 20 included in the second positive electrode mixture layer 2 or the positive electrode active material particles 30 included in the first positive electrode mixture layer 3, the primary particles aggregate to form secondary particles. The particle size refers to the particle size of the secondary particles. When the primary particles are not aggregated to form secondary particles, the particle size refers to the particle size of the primary particles.

  The second positive electrode mixture layer 2 is a layer containing lithium composite oxide particles 20 formed on the surface of the positive electrode current collector 1, and the first positive electrode mixture layer 3 is formed on the surface of the second positive electrode mixture layer 2. This is a layer including the formed active material particles 30. As shown in FIG. 1, the boundary between the second positive electrode mixture layer 2 and the positive electrode mixture layer 3 cannot be strictly defined, but the second positive electrode mixture layer 2 is a positive electrode current collector. 1 represents a region on the surface of the positive electrode current collector 1 where the lithium composite oxide particles 20 are mainly present in a cross section perpendicular to 1, and the active material particles 30 are mainly formed in the same cross section as the first positive electrode mixture layer 3. It shall mean the area | region on the 2nd positive mix layer 2 which exists.

  In the second positive electrode mixture layer 2, the average particle diameter dL of the lithium composite oxide particles 20 is preferably 1 to 15 μm, more preferably 4 to 14 μm, and still more preferably 3 to 10 μm. If the average particle diameter dL of the lithium composite oxide particles 20 is in the range of 15 μm or less, the average thickness t of the first positive electrode mixture layer 3 can be reduced while ensuring safety against overcharge. The above range is preferable from the viewpoint of energy density. For example, when the average particle diameter dL of the lithium composite oxide 20 is 10 μm, according to the relational expression (I), if the average thickness t of the second positive electrode mixture layer 2 is in the range of 11 μm or more, Safety can be secured. If the average particle diameter dL of the lithium composite oxide particles 20 is in the range of 10 μm or less, the average thickness t of the second positive electrode mixture layer 2 can be reduced to 11 μm or less, and the first positive electrode mixture layer 3 The thickness of can be increased. In addition, when the average particle diameter dL of the lithium composite oxide particles 20 is in the range of 1 μm or more, both safety against overcharge and high rate discharge performance can be achieved in a wide range of the average thickness t. If the average particle diameter dL is in the range of 3 μm or more, the range of the average thickness t can be further widened.

  In the first positive electrode mixture layer 3, the average particle diameter dA of the active material particles 30 is usually 0.5 to 60 μm, preferably 1 to 20 μm, more preferably 2 to 8 μm. The average particle diameter dA of the active material particles 30 of the first positive electrode mixture layer 3 is a cross-sectional SEM micrograph similar to the average particle diameter dL of the lithium composite oxide particles 20 except that the measurement target is active material particles. Can be measured.

  The lithium composite oxide particles 20 and the active material particles 30 are discriminated by using EDX elemental composition analysis. Accordingly, the average particle size of the lithium composite oxide particles 20 and the active material particles 30 is determined by determining the type and composition of each particle by EDX elemental composition analysis, as described above, on the SEM image. It can measure by calculating | requiring the average value with a diameter. The lithium composite oxide particles 20 are preferably particles having a larger content ratio of the molar ratio of manganese or phosphorus than the active material particles 30 of the first positive electrode mixture layer.

  In the present invention, the ratio (dL / dA) between the average particle diameter dL of the lithium composite oxide particles 20 and the average particle diameter dA of the active material particles 30 is preferably from the viewpoint of safety against overcharge and high rate discharge performance. 1-6, More preferably, it is 0.5-4, More preferably, it is 1-2. When the value of dL / dA is in the range of 4 or less, safety against overcharging can be further improved. If the value of dL / dA is in the range of 2 or less, it is considered that the active material particles 30 in contact with the positive electrode current collector 1 can be prevented from entering, and the safety against overcharge can be further enhanced. If the value of dL / dA is 0.1 or more, it is considered that the active material particles 30 can contact the positive electrode current collector 1, which is advantageous from the viewpoint of high rate discharge characteristics. A range where the value of dL / dA is 0.5 or more is more advantageous from the viewpoint of high rate discharge characteristics.

  In this invention, the 2nd positive mix layer 2 can be produced by apply | coating the 2nd paste for positive mix layers to the surface of the positive electrode collector 1 with specific thickness, and drying. The second paste for the positive electrode mixture layer is formed by dispersing or dissolving at least the lithium composite oxide particles 20 and the binder in water, and may contain a thickener and a conductive auxiliary agent as necessary.

  The lithium composite oxide particles 20 exhibit an electrochemical behavior in which resistance is increased by extracting lithium from the inside of the oxide during overcharge. The resistance increase is larger than that of the active material particles 30 mainly contained in the first positive electrode mixture layer 3 described later.

  The lithium composite oxide particles 20 preferably have a spinel structure or an olivine structure.

As a specific example of the spinel type lithium composite oxide particles, the general formula:
Li _x M ¹ _p O _4-q
The compound particle | grains represented by these are mentioned.
In the formula, M ¹ is one or more metals including one or more transition metals, and preferably one or more selected from the group consisting of transition metals having an atomic number of 21 to 29 in the long-period periodic table More preferably, it is one or more metals selected from the group consisting of Mn and Ni.
x is 0 <x ≦ 2, preferably 0.8 ≦ x ≦ 1.2.
p is 1.8 ≦ p ≦ 2.2.
q is 0 ≦ q ≦ 0.5.

As a specific example of the olivine-type lithium composite oxide particles, a general formula:
Li _x M ² _r PO _4-q
The compound particle | grains represented by these are mentioned.
In the formula, M ² is one or more metals including one or more transition metals, and preferably one or more selected from the group consisting of transition metals having an atomic number of 21 to 29 in the long-period periodic table More preferably, it is at least one metal selected from the group consisting of Fe, Co, Mn, and Ni.
x is 0 <x ≦ 2, preferably 0.8 ≦ x ≦ 1.2.
r is 0.8 ≦ r ≦ 1.2.
q is 0 ≦ q ≦ 0.5.

  The lithium composite oxide particles 20 may contain elements such as Al, Zn, Cr, Ti, P, and B in amounts within a range where the effects of the present invention can be achieved. The second positive electrode mixture layer 2 may include two or more lithium composite oxide particles 20 having different compositions.

  In the present invention, the lithium composite oxide particles 20 include spinel-type lithium composite oxide particles, particularly spinel-type lithium manganate particles, from the viewpoint of paste coating properties, among the lithium composite oxide particles described above. Is more preferable. Compared to olivine-type lithium composite oxide particles such as olivine-type lithium iron phosphate, spinel-type lithium composite oxide particles, especially spinel-type lithium manganate particles, have a small specific surface area, so that the solvent evaporation of the paste is suppressed. Can do.

As a specific example of spinel type lithium manganate particles, a general formula:
Li _x Mn _s M ³ _t O _4-q
The compound particle | grains represented by these are mentioned.
In the formula, M ³ is one or more metals (other than Mn) including one or more transition metals, preferably selected from the group consisting of transition metals having an atomic number of 21 to 29 in the long-period periodic table. One or more metals, more preferably Ni.
x is 0 <x ≦ 2, preferably 0.8 ≦ x ≦ 1.2.
t is 0 ≦ t ≦ 0.5, preferably 0.
s is 1.8−t ≦ s ≦ 2.2−t.
q is 0 ≦ q ≦ 0.5.

  As the binder for the second positive electrode mixture layer, an aqueous dispersion such as an acrylic polymer, styrene butadiene rubber (SBR), a polyolefin polymer, a vinyl polymer, or a fluorine polymer can be used. The polymer aqueous dispersion is obtained by emulsifying and dispersing polymer particles in water, and may contain a surfactant as an emulsifier, if necessary. When an organic solvent is used, polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, or the like can be used.

  As the second thickener for the positive electrode mixture layer, water-soluble polymers such as acrylic and cellulose can be used. From the viewpoint of dispersibility and thickening, a cellulosic thickener is preferred. Specific examples of the cellulose-based thickener include carboxymethylcellulose (CMC), methylcellulose (MC), hydroxypropylmethylcellulose (HPMC), etc. Among them, CMC is particularly preferable.

  As the second conductive additive for the positive electrode mixture layer, carbon black such as acetylene black, ketjen black and furnace black, graphite, conductive fibers such as carbon fiber and metal fiber, metal powder, and the like can be used. The conductive additive is present in a granular form in the second positive electrode mixture layer. Since the particle size is extremely smaller than the average particle size of the lithium composite oxide particles 20, both the lithium composite oxide particles 20 of the second positive electrode mixture layer 2 and the active material particles 30 of the first positive electrode mixture layer 3 are used. Are clearly distinguishable.

  The addition amount of the second conductive additive for the positive electrode mixture layer is usually 0.1% with respect to 100 parts by weight of the lithium composite oxide particles, from the viewpoint of further improvement of safety against overcharge and high rate discharge characteristics. It is -4.0 mass part, Preferably it is 0.6-2.0 mass part.

  The average thickness t of the second positive electrode mixture layer 2 is preferably 4 to 20 μm, more preferably 5 to 10 μm. If the average thickness t of the second positive electrode mixture layer is too large, the thickness of the first positive electrode mixture layer is limited, so that the energy density is lowered. The thickness t of the second positive electrode mixture layer is the thickness of the second positive electrode mixture layer 2 that has undergone the pressing process. The thickness t of the second positive electrode mixture layer can be controlled by selecting or adjusting the gap and the solid content concentration of the applicator when applied using an applicator. The t / dL of the second positive electrode mixture layer 2 can be adjusted by controlling the thickness t of the second positive electrode mixture layer with respect to the average particle diameter dL of the lithium composite oxide 20.

  As the positive electrode current collector 1, for example, a foil made of aluminum or an aluminum alloy can be used. The thickness of the positive electrode current collector 1 is usually 5 to 30 μm.

  The 1st positive mix layer 3 can be produced by apply | coating the 1st positive mix layer paste on the 2nd positive mix layer 2, and drying. Then, a positive electrode plate can be produced by pressing to a predetermined density.

  The first paste for the positive electrode mixture layer is formed by dispersing or dissolving at least a positive electrode active material and a binder in a solvent, and may contain a thickener and a conductive auxiliary agent as necessary.

As the active material particles 30 of the first positive electrode mixture layer 3, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiCoNiO ₂ , LiCoMO ₂ , LiNiMO ₂ (in this paragraph, M is Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb and B). Furthermore, a part of these positive electrode active materials may be replaced with a different element. From the viewpoint of energy density, a ternary layered oxide of Co, Ni and Mn is preferable.

  As the binder for the first positive electrode mixture layer, polyvinylidene fluoride, polyethylene, polytetrafluoroethylene, acrylic polymer, styrene butadiene rubber, carboxymethyl cellulose (CMC), or the like can be used.

  As the first thickener for the positive electrode mixture layer, the same compound as the thickener used for the second positive electrode mixture layer paste when an aqueous binder is used can be used. A cellulosic thickener is preferred from the viewpoint of thickening.

  As the first conductive additive for the positive electrode mixture layer, the same material as the conductive additive used for the second positive electrode mixture layer paste can be used. The amount of the first conductive additive for the positive electrode mixture layer added is usually 0.5 to 10 parts by weight, preferably 1 to 100 parts by weight with respect to 100 parts by weight of the positive electrode active material particles from the viewpoint of charge / discharge efficiency. 5 parts by mass.

  The solvent of the first positive electrode mixture layer paste is not particularly limited, and examples thereof include organic solvents such as N-methyl-2-pyrrolidone, water, and mixtures thereof.

The ratio of the thickness of the first positive electrode mixture layer 3 to the average thickness t of the second positive electrode mixture layer 2 is preferably 3 or more and 20 or less from the viewpoint of improving the energy density of the battery. The thickness of the 1st positive mix layer 3 is the thickness of the 1st positive mix layer 3 which passed through the press process, Comprising: The thickness of the 2nd positive mix layer 2 is not included.
The thickness of the first positive electrode mixture layer 3 is the maximum thickness t1, t2 of some lithium composite oxide particles 20 randomly selected in the method of measuring the average thickness t of the second positive electrode mixture layer 2. , T3 and the like are average values of the thicknesses of the first positive electrode mixture layer 3 immediately above the maximum thickness.

(Negative electrode)
The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on one side or both sides thereof. The negative electrode can be prepared by applying a negative electrode paste to the surface of a negative electrode current collector made of copper or a copper alloy and drying, and then pressing the formed negative electrode mixture layer to a predetermined density. it can. The negative electrode paste includes a negative electrode active material and a binder, and may further include a thickener, a conductive aid, and the like as necessary.

Examples of the negative electrode active material include graphite (graphite), non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, amorphous carbon, and other carbonaceous materials, metal oxides, and lithium metal oxides (Li ₄ Ti ₆ O ₁₂ etc.), polyphosphoric acid compounds and the like can be used alone or in combination. As the binder, polyvinylidene fluoride or styrene butadiene rubber (SBR) can be used. As a thickener, the same compound as the thickener which can be used for the 2nd positive mix layer paste of a positive electrode can be used.

(Nonaqueous electrolyte)
The non-aqueous electrolyte is a solution obtained by dissolving an electrolyte salt in an organic solvent. The organic solvent constituting the nonaqueous electrolyte is not particularly limited as long as it is used for a nonaqueous electrolyte secondary battery. Specific examples include cyclic carbonates such as propylene carbonate, ethylene carbonate, and butylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, or a mixture of two or more thereof.

The electrolyte salt constituting the non-aqueous electrolyte is not particularly limited as long as it is used for a non-aqueous electrolyte secondary battery. Specific examples include LiBF ₄ , LiPF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ), (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ or the like may be used alone or in admixture of two or more.

(Separator)
As the separator, a microporous film, a nonwoven fabric, or the like can be used alone or in combination. Specific examples include olefin-based resins such as polyethylene and polypropylene, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, and the like, and olefin-based resins are preferred.

(Non-aqueous electrolyte secondary battery)
The configuration of the non-aqueous electrolyte secondary battery of the present invention is not particularly limited as long as it has the above-described positive electrode, and usually further includes the above-described negative electrode, non-aqueous electrolyte, and separator together with the positive electrode. .

Hereinafter, a prismatic nonaqueous electrolyte secondary battery according to an embodiment of the present invention will be briefly described with reference to FIG.
The nonaqueous electrolyte secondary battery 50 is configured by accommodating an electrode group 52 and a nonaqueous electrolyte in a battery case 56 sealed with a case lid 57. The case lid 57 has a safety valve 58 and a negative electrode terminal 59, and is joined to the opening of the battery case 56 by laser welding. The electrode group 52 includes a negative electrode 53, a positive electrode 54, and a separator 55. Specifically, the electrode group 52 is produced by laminating and winding the negative electrode 53 and the positive electrode 54 via the separator 55. In this case, the positive electrode 54 is the positive electrode of the present invention in which the second positive electrode mixture layer 2 and the first positive electrode mixture layer 3 are formed on both surfaces of the positive electrode current collector. The negative electrode 53 is the negative electrode in which a negative electrode mixture layer is formed on both surfaces of the negative electrode current collector. The negative electrode terminal 59 is connected to the negative electrode 53 via the negative electrode lead 60, and the positive electrode 54 is connected to the inner surface of the battery case 56.

(1) Method for producing positive electrode plate (Example 1)
The positive electrode plate was manufactured as follows.
Lithium composite oxide, 100 parts by mass of LiMn ₂ O ₄ , 1.1 parts by mass of acetylene black (AB), 2.6 parts by mass (in terms of solid content) of an acrylic binder aqueous dispersion and 1.6 parts by mass Of carboxymethylcellulose (CMC) was dispersed in pure water to prepare paste I having a solid content of 50% by mass. This paste I was applied onto an aluminum (Al) foil having a thickness of 20 μm, which is a positive electrode current collector, using an applicator having a gap of 20 μm. Next, the applied Al foil was dried at 100 ° C. to evaporate pure water, and an Al foil provided with the second positive electrode mixture layer 2 was produced.
100 parts by mass of ternary lithium layered oxide (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ), 4.3 parts by mass of AB, and 4.3 parts by mass of polyvinylidene fluoride as active material particles (PVdF) was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a paste II having a solid concentration of 64% by mass. This paste II was applied onto the second positive electrode mixture layer 2 of the Al foil and dried at 120 ° C. to evaporate NMP, thereby forming a first positive electrode mixture layer 3.
After performing the above operation on one side of the Al foil, compression molding was performed with a roll press to obtain a positive electrode plate A1.
When measured by the following method, the average thickness t of the second positive electrode mixture layer 2 is 15.2 μm, and the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 13.4 μm. Yes, t / dL was 1.1. The thickness of the 1st positive mix layer 3 was 65 micrometers, when it measured with the cross-sectional photograph of SEM.

In the examples and comparative examples of the present invention, the average particle diameter d _A of the positive electrode active material particles 30 in the first positive electrode mixture layer 3 was 2.6 μm. Table 1 shows the average particle diameter d _A of the positive electrode active material particles 30.

(Example 2)
A positive electrode plate A2 was obtained in the same manner as in Example 1 except that an applicator with a gap of 80 μm was used when forming the second positive electrode mixture layer 2. The average thickness t of the second positive electrode mixture layer 2 is 19.6 μm, the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 13.5 μm, and t / dL is 1. It was 5.

Example 3
A positive electrode plate A3 was obtained in the same manner as in Example 1 except that an applicator with a gap of 100 μm was used when forming the second positive electrode mixture layer 2. The average thickness t of the second positive electrode mixture layer 2 is 21.0 μm, the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 13.4 μm, and t / dL is 1. 6.

Example 4
In the formation of the second positive electrode mixture layer 2, the same method as in Example 1 was used except that LiMn ₂ O ₄ having an average particle size smaller than that of Example 1 was used and an applicator having a gap of 40 μm was used. A positive electrode plate A4 was obtained. The average thickness t of the second positive electrode mixture layer 2 is 7.9 μm, the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 5.0 μm, and t / dL is 1. It was 8.

(Comparative Example 1)
A positive electrode plate B1 was obtained in the same manner as in Example 1 except that an applicator with a gap of 40 μm was used when forming the second positive electrode mixture layer 2. The average thickness t of the second positive electrode mixture layer 2 is 13.6 μm, the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 13.4 μm, and t / dL is 1. 0.

(Comparative Example 2)
In the formation of the second positive electrode mixture layer 2, the same method as in Example 1 was used except that LiMn ₂ O ₄ having an average particle size smaller than that of Example 1 was used and an applicator having a gap of 50 μm was used. A positive electrode plate B2 was obtained. The average thickness t of the second positive electrode mixture layer 2 is 10.4 μm, the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 5.1 μm, and t / dL is 2. 1

(Comparative Example 3)
In the formation of the second positive electrode mixture layer 2, except that LiMn ₂ O ₄ having an average particle size smaller than that of Example 1 was used and an applicator having a gap of 70 μm was used, the same method as in Example 1 was used. A positive electrode plate B3 was obtained. The average thickness t of the second positive electrode mixture layer 2 is 14.2 μm, the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 5.2 μm, and t / dL is 2. It was 8.

(Comparative Example 4)
In the formation of the second positive electrode mixture layer 2, except that LiMn ₂ O ₄ having an average particle size smaller than that of Example 1 was used and an applicator having a gap of 80 μm was used, the same method as in Example 1 was used. A positive electrode plate B4 was obtained. The average thickness t of the second positive electrode mixture layer 2 is 15.1 μm, the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 4.9 μm, and t / dL is 3. 1

(Example 5)
A positive electrode plate A5 was obtained in the same manner as in Example 1 except that the second positive electrode mixture layer 2 was coated with a gap of 61 μm using a comma reverse instead of an applicator. The average thickness t of the second positive electrode mixture layer 2 is 15.0 μm, the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 13.3 μm, and t / dL is 1. 1

(Example 6)
In the formation of the second positive electrode mixture layer 2, it was carried out except that LiMn ₂ O ₄ having an average particle size smaller than that of Example 1 was used and coating was performed with a gap of 45 μm using a comma reverse instead of the applicator. In the same manner as in Example 1, positive electrode plate A6 was obtained. The average thickness t of the second positive electrode mixture layer 2 is 8.0 μm, the average particle diameter dL of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 is 5.1 μm, and t / dL is 1. It was 8.

(Comparative Example 5)
A positive electrode plate B5 was obtained in the same manner as in Example 1 except that the second positive electrode mixture layer 2 was not formed.

(2) Manufacturing method of negative electrode plate The negative electrode plate was manufactured as follows.
A paste was made by dispersing graphite, SBR, and CMC in pure water. This paste was applied on a copper foil having a thickness of 15 μm, and then dried at 100 ° C. to evaporate pure water, thereby forming a negative electrode mixture layer.
After performing the above operation on one side of the copper foil, it was compression molded with a roll press to obtain a negative electrode plate.

(3) Test Battery Production Method A test battery was produced using Tom Cell (manufactured by Nippon Tom Cell Co., Ltd.), the positive electrode plate and the negative electrode plate, and the lower lid, separator, disk, leaf spring and upper lid. A microporous film made of polyethylene having a thickness of 30 μm was used as the separator. As the non-aqueous electrolyte, 1M LiPF6 salt in which ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) were mixed at a volume ratio of 30:35:35 was used.
The detailed manufacturing method of the battery is as follows.
Inside the packing on the bottom lid made of Al, a positive electrode plate (circular with a diameter of about 1.4 cm), a separator (circular with a diameter of 1.6 cm), and a negative electrode plate (with a diameter of 1.5 cm) previously immersed in an electrolyte solution Of the positive electrode plate and the negative electrode plate so that the active material coated surfaces of the positive electrode plate and the negative electrode plate face each other. Thereafter, in order to uniformly press the positive electrode plate and the negative electrode plate, a SUS disk and a leaf spring were placed, and finally, a SUS upper lid was placed, and then a nut was used to tighten evenly.

(4) Average thickness t and average particle diameters dL and dA
The average thickness t of the second positive electrode mixture layer 2 was measured by the following method.
The positive electrode was impregnated with an epoxy resin, dried for 1 day and solidified, and then the positive electrode cross section was polished so that a positive electrode cross section perpendicular to the positive electrode current collector could be seen. 20 SEM micrographs of the polished cross section were taken at random with a measurement magnification of 500 times. In each of the obtained photographs, first, as shown in FIG. 2, lithium composite oxide particles 20 were selected every 30 μm in the direction parallel to the surface of the positive electrode current collector 1, and a total of 100 particles were extracted. . The surface of the positive electrode current collector 1 is the reference line La, the lithium composite oxide particle 20 is a line Lb parallel to the reference line La, and the distance from the reference line La is maximum. A parallel line Lb passing through the point P on the contour line of was drawn on each selected particle. The distances between these lines La and Lb were defined as thicknesses t1, t2, and t3. As the average thickness t, an average value of a total of 60 measurement values obtained by excluding 20 values from the maximum value and the minimum value among 100 measurement values in the SEM photograph was used. When the surface of the positive electrode current collector has irregularities, the reference line La is a line indicating the surface with the irregularities smoothed. When the surface of the positive electrode current collector is curved as a whole, the thickness t is the thickness when the tangent to the curved surface of the positive electrode current collector in the SEM photograph is used as a reference line.

The average particle diameter dL of the lithium composite oxide particles 20 included in the second positive electrode mixture layer 2 and the average particle diameter dA of the active material particles 30 included in the first positive electrode mixture layer 3 are the elemental composition analysis of EDX. The SEM image was measured by the following method while discriminating the type and composition of each particle.
The average particle diameter dL of the lithium composite oxide particles 20 was determined as an average value of particle diameters as shown in FIG. 2 in the above SEM photograph. The particle diameter is an average value of the longest particle diameter d1 and the shortest particle diameter d2. The center point M was defined as the midpoint between the two most distant points on the particle outline in the SEM photograph. The length between two points on a straight line connecting the two most distant points on the contour was defined as the longest particle diameter d1. The shortest particle diameter d2 is the length between the two points on a straight line passing through the center point M and having the shortest length between the two points where the straight line intersects the particle outline. For the lithium composite oxide particles 20, 100 lithium composite oxide particles 20 adjacent to each other in the second positive electrode mixture layer 2 were targeted in order to prevent artificial selection. When there was no adjacent particle, the closest particle was targeted. In addition, when the average value of the particle diameters is arranged from the maximum value to the minimum value in the order of the size, among these values, 20 values from the maximum value and 20 values from the minimum value are obtained. The average value of a total of 60 excluded values was used.
The average particle diameter dA of the active material particles 30 was measured using a cross-sectional SEM micrograph in the same manner as the average particle diameter dL of the lithium composite oxide particles 20 except that the measurement target was active material particles.

(5) Charge / Discharge Test The charge / discharge test was performed by the following method.
A constant current and constant voltage charge with an upper limit voltage of 4.2 V was performed at room temperature for 8 hours at a charge current value (current density of about 0.5 mA / cm ² ) that was 0.2 times the design capacity of the battery. Thereafter, the battery was discharged to 2.75 V with a current value 0.2 times the design capacity, and the 0.2 C capacity was measured. Next, the battery was charged in the same manner, discharged to 2.75 V at a current value that was one time the design capacity, and the 1C capacity was measured.
The discharge rate characteristic of each battery was calculated as 1 C capacity / 0.2 C capacity. A large ratio means that high rate discharge characteristics are good.

(6) Measurement of resistance of positive electrode plate The resistance value of the produced positive electrode plate was measured by the following method.
The positive electrode plate was sandwiched between metal jigs (circular with a diameter of 1.0 cm), pressed with a load of 0.18 kg / cm ² , and resistance was measured with a DC resistance meter.

(7) Overcharge test The overcharge test was conducted by the following method.
A constant current and constant voltage charge with an upper limit voltage of 8.4 V was performed at room temperature for 1.5 hours at a charging current value current density of about 5 mA / cm ² which is twice the design capacity of the battery. The SOC (charged state) (%) when 6.5V was reached was calculated. The smaller the SOC when reaching 6.5V, the higher the safety. A battery whose voltage has reached 6.5 V during overcharge indicates that the second positive electrode mixture layer 2 has increased resistance, and can interrupt the current during overcharge. In this test, a battery with high heat dissipation was used, so the voltage did not increase due to the shutdown of the separator.
Table 1 shows the discharge rate characteristics of A1 to A6 and B1 to B4, the SOC when the overcharge test reaches 6.5 V, and safety.

(Evaluation)
Compared with the positive electrode plate of A3, the positive electrode plate of B2 did not have good discharge rate characteristics although the average thickness t of the second positive electrode mixture layer 2 was thin. With the A4 electrode plate in which the t / dL of the second positive electrode mixture layer 2 was in the range of 1.8 or less, good discharge rate characteristics were obtained.

When the relationship between t / dL and discharge rate characteristics (1C / 0.2C) in each battery was plotted (FIG. 4), a correlation was observed between them. Specifically, when t / dL is 1.8 or less, the discharge rate characteristics are good and a relatively high rate discharge performance is maintained. However, when t / dL is about 2.1 or more, the high rate discharge performance is remarkable. When the relationship between the average thickness t and the discharge rate characteristic (1C / 0.2C) in each reduced battery was plotted (FIG. 5), no correlation was found between them.

  A battery that reached 6.5 V, which is equal to or higher than the decomposition potential of the electrolytic solution, was rated as “good” when overcharged, and “x” when not reached. Although the average thickness t of the second positive electrode mixture layer 2 was larger in the positive electrode plate B1 than in the A4 positive electrode plate, the voltage did not reach 6.5 V in the overcharge test. The A1 positive electrode plate in which the t / dL of the second positive electrode mixture layer 2 was 1.1 or more improved the safety against overcharge. The A2 positive electrode plate in which the t / dL of the second positive electrode mixture layer 2 was in the range of 1.5 or more further improved the safety against overcharge.

  A5 and A6 positive plates prepared using other coating methods are also safe for overcharge in batteries in which the t / dL of the second positive electrode mixture layer 2 is in the range of 1.1 to 1.8. The discharge rate characteristics were excellent.

1: Positive current collector 2: Second positive electrode mixture layer 3: First positive electrode mixture layer 20: Lithium composite oxide particles 30: Active material particles 50: Non-aqueous electrolyte secondary battery 52: Electrode group 53: Negative electrode 54: Positive electrode 55: Separator 56: Battery case 57: Case lid 58: Safety valve 59: Negative electrode terminal 60: Negative electrode lead

Claims (4)

A positive electrode including a positive electrode current collector, a first positive electrode mixture layer, and a second positive electrode mixture layer formed between the positive electrode current collector and the first positive electrode mixture layer;
The second positive electrode mixture layer includes lithium composite oxide particles,
A nonaqueous electrolyte secondary battery in which an average thickness t of the second positive electrode mixture layer and an average particle diameter dL of the lithium composite oxide particles satisfy the following relational expression (I).
1.1 ≦ t / dL ≦ 1.8 (I)   The nonaqueous electrolyte secondary battery according to claim 1, wherein an average particle diameter dL of the lithium composite oxide particles is in a range of 1 μm to 15 μm.   The first positive electrode mixture layer includes active material particles, and the ratio (dL / dA) of the average particle diameter dL of the lithium composite oxide particles to the average particle diameter dA of the active material particles is 0.1 to 6 The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the nonaqueous electrolyte secondary battery is in a range of   The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the lithium composite oxide particles include spinel type lithium manganate.

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