JP6229333B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

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

  In a nonaqueous 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. It is known that such a configuration makes the lower positive electrode mixture layer resistant to safety when overcharged (for example, Patent Documents 1 and 2). In such a technique, the lower positive electrode mixture layer in the positive electrode is formed by applying a paste in which a lithium composite oxide and a binder resin are dispersed or dissolved in an organic solvent to a current collector, drying, and pressing. ing.

  In particular, Patent Document 1 has an object of providing a battery having excellent safety by including a lower positive electrode mixture layer on a positive electrode and further including a porous insulating layer between the positive electrode and the negative electrode. In the specific example of Patent Document 1, the lower positive electrode mixture layer is not provided with a conductive additive.

JP 2009-4289 A JP 2007-26676 A

  In order to obtain the effect of increasing the resistance at the time of overcharging by the lower positive electrode mixture layer, the amount of the conductive additive contained in the lower positive electrode mixture layer may be reduced. By reducing the amount of the conductive auxiliary agent, the contribution of electric conduction through the conductive auxiliary agent can be reduced, and the increase in resistance due to the specific lithium composite oxide particles can be easily reflected.

  However, in order to improve the safety against overcharge, if a positive electrode with a reduced amount of conductive additive contained in the lower positive electrode mixture layer is used, the high rate discharge characteristics of the nonaqueous electrolyte secondary battery will be reduced. . It was difficult to achieve both safety against overcharge and high rate discharge characteristics by controlling the amount of the conductive additive contained in the lower positive electrode mixture layer.

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery that is excellent in safety against overcharging and prevents deterioration in high-rate discharge characteristics.

  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. A non-aqueous electrolyte secondary battery in which the second positive electrode mixture layer includes lithium composite oxide particles and the coverage of the second positive electrode mixture layer with respect to the positive electrode current collector is 60 to 95% It is.

  When the coverage of the second positive electrode mixture layer is less than 60%, the resistance of the second positive electrode mixture layer is not sufficiently increased during overcharge, and safety against overcharge is reduced. If the coverage exceeds 95%, the high rate discharge characteristics of the nonaqueous electrolyte secondary battery will be significantly reduced.

In the present invention having a predetermined coverage, the ratio (d _A / d _L ) between the average particle diameter d _L of the lithium composite oxide particles and the average particle diameter d _A of the active material particles is 0.1-6. Is preferred.

In the present invention having a predetermined coverage, the maximum value g _M of the gap length between the lithium composite oxide particles on the positive electrode current collector is preferably in the range of 5 μm to 45 μm.

In the present invention having a predetermined coverage, the ratio (d _A / g _M) between the maximum value of the gap length between the lithium composite oxide particles on the positive electrode current collector and the average particle diameter d _A of the active material particles ) Is preferably in the range of 0.07 to 4.

  In the present invention, the lithium composite oxide particles preferably contain spinel type lithium manganate.

  In the nonaqueous electrolyte secondary battery of the present invention, the coverage of the second positive electrode mixture layer with respect to the positive electrode current collector is 60 to 95%, so that it is excellent in safety against overcharge and the high rate discharge characteristic is deteriorated. Is prevented.

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 the measuring method of the coverage with respect to the positive electrode electrical power collector of a 2nd positive mix layer, Comprising: It is the figure which abbreviate | omitted the 1st positive mix layer. It is a conceptual diagram which shows the general | schematic expanded cross section for demonstrating the other embodiment of the positive electrode of this invention. It is a schematic sectional drawing of the square-shaped nonaqueous electrolyte secondary battery which is one embodiment of this invention.

  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 included in the nonaqueous electrolyte secondary battery of the present invention includes a second positive electrode mixture layer formed between the positive electrode current collector and the first positive electrode mixture layer. A schematic cross-sectional view showing one embodiment of the positive electrode of the present invention is shown in FIG. FIG. 1 is a schematic 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. .

In this invention, the coverage of the 2nd positive mix layer 2 with respect to the positive electrode electrical power collector 1 is 60 to 95%, Preferably it is 80 to 95%. In this way, the second positive electrode mixture layer 2 does not completely cover the entire surface of the positive electrode current collector 1 but covers a part of the surface of the positive electrode current collector so as to be appropriately exposed. The safety of the battery is excellent and the deterioration of the high rate discharge characteristic is prevented. Although the details of the mechanism for obtaining such an effect are not clear, it is considered to be based on the following mechanism.
Conventionally, when there is a region that does not cover the positive electrode current collector 1 (a gap between the lithium composite oxide particles), the positive electrode current collector 1 and the first positive electrode mixture layer 3 are formed between the lithium composite oxide particles. Since the active material particles 30 may come into contact with each other and a current flows during overcharging, it has been considered that the above resistance increase does not occur effectively as a whole.
However, in the present invention, by setting the coverage of the second positive electrode mixture layer 2 within the above specific range, current is concentrated in the gaps between the lithium composite oxide particles during overcharge. Since the voltage rises due to current concentration in the gaps between the lithium composite oxide particles, it is considered that the safety against overcharge is excellent. Moreover, since the positive electrode current collector 1 and the active material particles 30 of the first positive electrode mixture layer 3 are in contact with each other in the gap between the lithium composite oxide particles, the current flows, so that the high rate discharge characteristics are prevented from being deteriorated. It is thought that.

  In the present invention, the positive electrode preferably has a portion where the active material particles 30 of the first positive electrode mixture layer 3 are in contact with the positive electrode current collector 1. Thereby, a high rate discharge characteristic can be improved.

  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, the coverage of the second positive electrode mixture layer 2 on at least one surface is in the above range. Preferably, the coverage of the second positive electrode mixture layer 2 on both sides is independently within the above range.

The coverage of the second positive electrode mixture layer 2 can be measured by the following method.
An SEM (scanning electron microscope) photograph is taken so that a positive electrode cross section perpendicular to the positive electrode current collector 1 can be seen. In the obtained photograph, first, as shown in FIG. 2, coating with lithium composite oxide particles 20 constituting the second positive electrode mixture layer 2 is performed by a straight line (broken line) perpendicular to the positive electrode current collector 1. Region 21 is defined. Next, the total length of the covering region 21 in each particle 20 is calculated on the photograph. And the ratio of the total length of the coating area | region 21 with respect to the full length 40 of the positive electrode collector 1 surface on which the 1st active material mixture layer 3 was apply | coated is calculated as a coverage. FIG. 2 is a schematic cross-sectional view of the positive electrode perpendicular to the positive electrode current collector 1, in which the first positive electrode mixture layer 3 is omitted.

  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 the surface of the second positive electrode mixture layer 2. It is a layer containing the active material particles 30 formed in the. In the gap between the lithium composite oxide particles on the surface of the positive electrode current collector 1 in the second positive electrode mixture layer 2, the active material particles 30 of the first positive electrode mixture layer 3 are present as shown in FIG. Therefore, the boundary between the second positive electrode mixture layer 2 and the first positive electrode mixture layer 3 cannot be strictly defined. The second positive electrode mixture layer 2 means a region on the surface of the positive electrode current collector where the lithium composite oxide particles 20 are mainly present in a cross section perpendicular to the positive electrode current collector 1. The layer 3 means a region on the second positive electrode mixture layer 2 where the active material particles 30 mainly exist in the same cross section.

In the second positive electrode mixture layer 2, the average particle diameter d _L of the lithium composite oxide particles 20 is not particularly limited, and is usually 1 to 60 μm, preferably 1 to 40 μm.

The average particle diameter d _A of the active material particles 30 in the first positive electrode mixture layer 3 is also not particularly limited, and is usually 0.5 to 60 μm, preferably 1 to 20 μm.

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

The average particle diameter d _A of the active material particles 30 of the first positive electrode mixture layer 3 and the average particle diameter d _L of the lithium composite oxide particles 20 of the second positive electrode mixture layer 2 are energy dispersive X-ray spectroscopy. It can be measured by observing a cross section SEM while discriminating between the lithium composite oxide particles 20 and the active material particles 30 by elemental composition analysis of (EDX). The average particle diameter is a value obtained by calculating an average value of the particle diameters of the respective particles, and the particle diameter is an average value of the longest particle diameter and the shortest particle 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 3.

The ratio between the average particle diameter _{d A} of the mean particle diameter _{d L} and the active material particles 30 of the lithium composite oxide particles 20 in the present invention _(d A / _{d L)} is preferably from 0.1 to 6. (D _A / d _L ) is more preferably 0.2 to 3, and further preferably 0.25 to 1.
The average particle diameter d _A of the active material particles 30 with respect to the average particle diameter d _L of the lithium composite oxide particles 20 is preferably in the range of 6 or less. Within this range, it is considered that the active material particles 30 can easily enter the second positive electrode mixture layer 2 and the active material particles 30 can come into contact with the positive electrode current collector 1. More preferably, (d _A / d _L ) is in the range of 3 or less. In the range where (d _A / d _L ) is 3 or less, the active material particles 30 in contact with the positive electrode current collector 1 are considered to increase in the gap between the lithium composite oxide particles 20 on the positive electrode current collector 1. Furthermore, high rate discharge characteristics can be improved.
Further, the ratio (d _A / d _L ) between the average particle diameter d _{A of} the lithium composite oxide particles 20 and the average particle diameter d _L of the active material particles 30 is preferably in the range of 0.1 or more. Since (d _A / d _L ) is in the range of 0.1 or more, it is considered that the active material particles 30 are prevented from entering between the lithium composite oxide particles 20 and the positive electrode current collector 1. In the second positive electrode mixture layer 2, a voltage increase occurs due to current concentration on the active material particles 30 in contact with the positive electrode current collector 1, and safety against overcharging is further improved.

In the present invention, the maximum value g _M of the gap length between the lithium composite oxide particles on the positive electrode current collector 1 is preferably in the range of 5 μm to 45 μm. More preferably, the maximum value g _M of the length of the gap is in the range is 9μm or more 30μm or less. By the maximum value g _M of the gap length between the lithium complex oxide particles and at over 5 [mu] m, since the active material particles 30 can be considered to be able to enter the gap, advantageous in view of high rate discharge property It is. Range Maximum value g _M of the length of the gap is 9μm or, since the active material particles 30 that can enter the gap is considered moderately increases, further preferred. Further, by setting the length g _M of the gap between the lithium composite oxide particles and the range of 45 [mu] m, narrower area of the gap current concentration occurs, it is considered that the voltage rise is sufficiently occur, safety against overcharge It is preferable in terms of excellent properties.

The ratio (d _A / g _M ) between the maximum value g _M of the gap length between the lithium composite oxide particles on the positive electrode current collector 1 and the average particle diameter d _A of the active material particles 30 is 0.07 to 4 It is preferable that it is the range of these. (D _A / g _M ) is more preferably 0.1 or more and 1 or less, and further preferably 0.2 or more and 0.4 or less. The ratio (d _A / g _M ) between the maximum value g _M of the gap length between the lithium composite oxide particles on the positive electrode current collector 1 and the average particle diameter d _A of the active material particles 30 is 4 or less. Thus, if the average particle diameter of the active material particles 30 is small, it is considered that the positive electrode current collector 1 and the active material particles 30 can contact each other, which is advantageous from the viewpoint of high rate discharge characteristics. It is further preferable that the value of (d _A / g _M ) is 1 or less because the active material particles 30 that can be brought into contact with the positive electrode current collector 1 are remarkably increased. Further, the ratio (d _A / g _M ) between the maximum value g _M of the gap length between the lithium composite oxide particles on the positive electrode current collector 1 and the average particle diameter d _A of the active material particles 30 is 0.07. By setting it as the above range, since it is thought that the active material particle 30 suppresses contacting with the positive electrode collector 1, it is preferable at the point which is excellent in the safety | security with respect to an overcharge.

In the present invention, the average value g _L of the gap length between the lithium composite oxide particles on the positive electrode current collector 1 is preferably in the range of 3 μm to 30 μm. From the viewpoint of high rate discharge characteristics, it is considered that the active material particles 30 can enter more by setting the average value g _L of the gaps between the lithium composite oxide particles in the range of 3 μm or more. It is advantageous. In addition, by setting the gap length g _L between the lithium composite oxide particles in a range of 30 μm or less, the active material particle 30 between the lithium composite oxide particles has an average narrow gap area. Since it is considered that the increase in voltage due to current concentration occurs effectively with fewer active material particles 30 entering, it is preferable in terms of excellent safety against overcharging.

In the present invention, the ratio (d _A / g _L ) between the average value g _L of the gap length between the lithium composite oxide particles and the average particle diameter d _A of the active material particles 30 is in the range of 0.1 to 8. Preferably there is. By setting the ratio (d _A / g _L ) between the average value of the gap length between the lithium composite oxide particles and the average particle diameter d _A of the active material particles 30 to 8 or less, the positive electrode current collector 1 This is advantageous from the viewpoint of high-rate discharge characteristics. Further, the ratio (d _A / g _L ) between the average value of the length of the gap between the lithium composite oxide particles and the average particle diameter d _A of the active material particles 30 is in the range of 0.1 or more, so that the positive electrode It is considered that the entrance of the active material particles 30 that come into contact with the current collector 1 is suppressed, which is preferable in terms of excellent safety against overcharging.

The average value g _L and the maximum value g _M of the gap length between the lithium composite oxide particles on the positive electrode current collector are measured by the following method.
A cross-sectional SEM micrograph is taken by the method described above. In the obtained photograph, first, the distance between the lithium composite oxide particles 30 and the adjacent lithium composite oxide particles 30 is defined as the gap length between the lithium composite oxide particles. When it is in contact with the adjacent lithium composite oxide particles 30, it is not considered as a gap between the lithium composite oxide particles. The average value g _L of the gaps between the lithium composite oxide particles is the average value of the gap lengths between the lithium composite oxide particles. Maximum value g _M of the length of the gap between the lithium composite oxide particles is the average value of the maximum length of the gap between the lithium composite oxide particles in a particular range.

  The second positive electrode mixture layer 2 can be produced by applying a specific second positive electrode mixture layer paste to the surface of the positive electrode current collector 1 with a specific thickness and drying.

  The second paste for the positive electrode mixture layer is formed by dispersing at least the lithium composite oxide particles 20 and the binder in water, and may contain a thickener and a conductive aid, if necessary. When coating an aluminum foil with an organic solvent instead of water, or using an organic solvent with water, the effect of the aluminum foil repelling the paste is suppressed, so the particles are aligned on the surface of the positive electrode current collector. It becomes easy and there is a possibility that the coverage becomes too large.

  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 may contain elements such as Al, Zn, Cr, Ti, P, and B in an amount within a range in which the effects of the present application 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 preferably include spinel type lithium composite oxide particles from the viewpoint of paste coatability among the lithium composite oxide particles described above, and in particular, spinel type lithium manganate. More preferably, it contains particles. Compared to olivine-type lithium composite oxide particles such as olivine-type lithium iron phosphate, spinel-type lithium composite oxide particles have a small specific surface area, and in particular, spinel-type lithium manganate particles have a small specific surface area. Evaporation can be suppressed.

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, a styrene butadiene rubber, an olefin polymer (for example, polyethylene (PE)), a vinyl polymer, and a fluorine polymer can be used. From the viewpoint of high voltage resistance, an acrylic polymer dispersion is preferred. 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 auxiliary agent is present in the second positive electrode mixture layer in the form of particles or fibers, but the particle size is extremely smaller than that of the lithium composite oxide particle 20, so that the lithium composite oxidation in the second positive electrode mixture layer is performed. The material particles 20 and the active material particles 30 in the first positive electrode mixture layer can be clearly distinguished.

The second positive electrode mixture layer 2 preferably contains a conductive additive from the viewpoint of high rate discharge characteristics.
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 mass parts, Preferably it is 0.6-2 mass parts. When the ratio of the conductive additive in the second positive electrode mixture layer is in the range of 0.6 parts by mass or more, more sufficient high rate discharge characteristics can be obtained. When the proportion of the conductive additive in the second positive electrode mixture layer is 2 parts by mass or less, it is further advantageous in terms of safety during overcharge.

  The coverage of the second positive electrode mixture layer 2 is controlled by adjusting the amount of water to which solid components such as the lithium composite oxide particles 20, the binder, the thickener, and the conductive additive are added. Can do. When the solid content concentration is too low, the coverage of the second positive electrode mixture layer 2 becomes too small.

  The coverage of the second positive electrode material mixture layer 2 can be controlled by selecting or adjusting the gap between the applicators when the applicator is used. When the gap of the applicator is too large, the coverage of the second positive electrode mixture layer 2 becomes too large.

  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. Thereafter, it can be pressed to a predetermined density. Only the second positive electrode mixture layer 2 may be pressed first, and then the first positive electrode mixture layer paste may be applied onto the second positive electrode mixture layer 2.

  The first paste for the positive electrode mixture layer is formed by dispersing or dissolving at least the active material particles 30 and the 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, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiCoNiO ₂ , LiCoMO ₂ , LiNiMO ₂ (in this paragraph, M is commonly Na, Mg, Sc, Y). , Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb and B). Furthermore, a part of elements of these positive electrode active materials may be replaced with a different metal. From the viewpoint of energy density, a ternary layered oxide containing Co, Ni and Mn is preferable.

Active material particles 30 is preferably an aspect ratio (average value of the longest diameter _{d M} / shortest diameter _{d m)} is 1 to 1.4. When the aspect ratio is within this range, the active material particles 30 can easily pass through the gaps of the lithium composite oxide particles, which is advantageous from the viewpoint of high rate discharge characteristics. More preferably, the aspect ratio of the active material particles 30 is 1 or more and 1.25 or less. This range is further advantageous from the viewpoint of high rate discharge characteristics.

The aspect ratio of the first active material particles 30 of the positive electrode mixture layer 3, by using a SEM micrograph of a cross-section, measuring the longest particle diameter d _M and the shortest diameter d _m of the active material particles 30, calculates can do. Maximum grain size d _M is the length between the most distant two points on the outline of the particle. The midpoint of the two points that are farthest apart from each other on the particle outline is taken as the center point. Shortest diameter d _m is the shortest diameter passing through the center point.

  As the binder for the first positive electrode mixture layer, polyvinylidene fluoride, polytetrafluoroethylene, acrylic polymer, styrene butadiene rubber, polyethylene (PE), 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 contained in the second positive electrode mixture layer paste can be used when an aqueous binder is used.

  As the first conductive additive for the positive electrode mixture layer, the same material as the conductive additive contained in the second positive electrode mixture layer paste can be used. The addition amount of the first conductive additive for the positive electrode mixture layer is usually 0.5 to 10 parts by mass, 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 (t _A / t _L ) of the thickness t _A of the first positive electrode mixture layer 3 to the thickness t _L of the second positive electrode mixture layer 2 is from the viewpoint of safety against overcharge and improvement of the energy density of the battery. 3 to 20 are preferable. The thickness of the first positive electrode mixture layer 3 is a thickness that does not include the thickness of the second positive electrode mixture layer 2.
The thickness t _L of the second positive electrode mixture layer 2 is an average value of the maximum thicknesses of the layers in the SEM image described above.
The thickness t _A of the first positive electrode mixture layer 3 is an average value of the thicknesses of the first positive electrode mixture layer 3 immediately above the maximum thickness of the second positive electrode mixture layer 2.

In the present invention, in the positive electrode, the average particle diameter d _A of the active material particles 30 of the first positive electrode mixture layer 3 is smaller than the average particle diameter d _L of the lithium composite oxide particles 20 of the second positive electrode mixture layer 2. Although it demonstrated using FIG. 1 which shows an embodiment, it is not limited to this. For example, as shown in FIG. 3, the positive electrode of the present invention has an average particle diameter d _A of active material particles 30 of the first positive electrode mixture layer 3, and lithium composite oxide particles of the second positive electrode mixture layer 2. it may be larger than the average particle diameter _{d L} of 20. The embodiment shown in FIG. 3 is the same as the embodiment shown in FIG. 1 except that the size relationship between the lithium composite oxide particles 20 and the active material particles 30 is different. Description of the aspect is omitted. In FIG. 3, even when the average particle diameter d _A of the active material particles 30 is larger than the average particle diameter d _L of the lithium composite oxide particles 20, the active material particles 30 and the positive electrode current collector 1 may be in contact with each other. I understand. The average particle diameter d _A of the active material particles 30 of the first positive electrode mixture layer 3 and the average particle diameter d _L of the lithium composite oxide particles 20 of the second positive electrode mixture layer 2 may be equal.

(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 the thickener, the same compound as the thickener used for the second positive electrode mixture layer paste of the 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 rectangular 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 1, and the negative electrode 53 is the current collector. It is the said negative electrode by which the negative mix layer was formed in both surfaces. 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, LiMn ₂ O ₄ 100 parts by mass, 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 the positive electrode current collector 1, using an applicator having a gap of 30 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.
As a positive electrode active material, a ternary layered oxide (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ) 100 parts by mass, 4.3 parts by mass AB, and 4.3 parts by mass polyvinylidene fluoride ( PVdF) was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare paste II having a solid content of 70% 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. It was 95% when the coverage of the 2nd positive mix layer 2 was measured by the method mentioned later. The average particle diameter d _A of the positive electrode active material 30 in the first positive electrode mixture layer 3 was 3.2 μm. The aspect ratio of the active material particles 30 in the first positive electrode mixture layer 3 was 1.18. The maximum value g _M of the gaps between the lithium composite oxide particles was 9.8 μm, and the average value g _L of the gaps between the lithium composite oxide particles was 7.2 μm. From the cross-sectional SEM photograph, the ratio of the thickness of the first positive electrode mixture layer 3 to the thickness of the second positive electrode mixture layer 2 was calculated to be about 4.5.

In the examples and comparative examples of the present invention, the average particle diameter d _L of the lithium composite oxide particles 20 in the second positive electrode mixture layer 2 was 10.4 μm. Table 1 shows the average particle diameter d _L of the lithium composite oxide particles 20.

(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 24 μm was used when forming the second positive electrode mixture layer 2. When the coverage of the second positive electrode mixture layer 2 was measured, it was 80%. The average particle diameter d _A of the active material particles 30 in the first positive electrode mixture layer 3 was 3.1 μm. The aspect ratio of the active material particles 30 in the first positive electrode mixture layer 3 was 1.23. The maximum value g _M of the gap length between the lithium composite oxide particles was 15 μm, and the average value g _L of the gap length between the lithium composite oxide particles was 11 μm.

(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 15 μm was used when forming the second positive electrode mixture layer 2. The coverage of the second positive electrode mixture layer 2 was measured and found to be 60%. The average particle diameter d _A of the active material particles 30 in the first positive electrode mixture layer 3 was 3.0 μm. The aspect ratio of the active material particles 30 in the first positive electrode mixture layer 3 was 1.23. The maximum value g _M of the gaps between the lithium composite oxide particles was 23 μm, and the average value g _L of the gaps between the lithium composite oxide particles was 16 μm.

Example 4
In the formation of the second positive electrode mixture layer 2, the solid content concentration of the paste I was controlled to a high value of 60%, and an applicator with a gap of 25 μm was used. A positive electrode plate A4 was obtained.

(Example 5)
In forming the second positive electrode mixture layer 2, the same paste I as in Example 1 was applied by the same method as in Example 1 using a comma reverse with a gap of 46 μm instead of an applicator, and the positive electrode plate A 5 was applied. Obtained.

(Example 6)
When forming the second positive electrode mixture layer 2, the same paste I as in Example 1 was applied by a method similar to Example 1 using a comma direct with a gap of 30 μm instead of an applicator, and the positive electrode plate A 6 was applied. Obtained.

(Example 7)
The same method as in Example 1 except that the paste I is 100 parts by mass of LiMn ₂ O ₄ , 3.2 parts by mass of AB, 2.7 parts by mass of an acrylic binder and 1.7 parts by mass of CMC. Was applied to obtain a positive electrode plate A7. The coverage of the second positive electrode mixture layer 2 was measured and found to be 95%.

(Example 8)
Except that the paste I was 100 parts by mass of LiMn ₂ O ₄ and 0.5 parts by mass of AB, it was applied in the same manner as in Example 1 to obtain a positive electrode plate A8. The coverage of the second positive electrode mixture layer 2 was measured and found to be 95%.

(Comparative Example 1)
In the formation of the second positive electrode mixture layer 2, the solid content concentration of the paste I was controlled to 60 mass% by adjusting the amount of pure water, and an applicator with a gap of 58 μm was used. A positive electrode plate B1 was obtained by the same method. When the coverage of the second positive electrode mixture layer 2 was measured, it was 100%. The average particle diameter d _A of the active material particles 30 in the first positive electrode mixture layer 3 was 3.0 μm. The aspect ratio of the active material particles 30 in the first positive electrode mixture layer 3 was 1.22.

(Comparative Example 2)
In the formation of the second positive electrode mixture layer 2, the solid content concentration of the paste I was controlled to 40 mass% by adjusting the amount of pure water, and an applicator with a gap of 40 μm was used. A positive electrode plate B2 was obtained by the same method. The coverage of the second positive electrode mixture layer 2 was measured and found to be 55%. The average particle diameter d _A of the active material particles 30 in the first positive electrode mixture layer 3 was 3.0 μm. The aspect ratio of the active material particles 30 in the first positive electrode mixture layer 3 was 1.24. The maximum value g _M of the gap between the lithium composite oxide particles was 47 μm, and the average value g _L of the gap between the lithium composite oxide particles was 37 μm.

(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 LiPF ₆ 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) Measuring method Each of the obtained positive electrode and battery was measured by the following method.

(Measurement method of coverage)
The coverage 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 one 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 1 could be seen. An SEM (scanning electron microscope) photograph of the polished cross section was taken at a measurement magnification of 500 times. In the photograph obtained while discriminating the type and composition of each particle by elemental composition analysis of EDX, first, as shown in FIG. 2, a second line is shown by a straight line (broken line) perpendicular to the positive electrode current collector 1. The covering region 21 by the lithium composite oxide particles 20 constituting the positive electrode mixture layer 2 was defined. Next, the total length of the covering region 21 in each particle 20 was calculated on the photograph. And the ratio of the total length of the coating area | region 21 with respect to the full length 40 of the positive electrode collector 1 surface was computed as a coverage. The total length of the surface of the positive electrode current collector 1 was an area where the first active material mixture layer 3 was applied. The measurement is intended for all lithium composite oxide particles 20 adjacent to each other in a region where the total length 40 is 500 μm in the SEM micrograph, and the total length 40 on the surface of the positive electrode current collector is the lithium composite oxide particles 20a at both ends. It was set as the length on the said surface including the coating | coated area | regions 21a and 21b in 20b at both ends. When the surface of the positive electrode current collector 1 has irregularities, the vertical straight line is a straight line that is perpendicular to the uneven surface, and the surface of the positive electrode current collector 1 is curved as a whole. Is a straight line perpendicular to the tangent of the curved surface in cross section. FIG. 2 is a schematic cross-sectional view of the positive electrode perpendicular to the positive electrode current collector 1, in which the first positive electrode mixture layer 3 is omitted.

(Measuring method of particle size)
The average particle diameter d _L of the lithium composite oxide particles 20 contained in the second positive electrode mixture layer 2 and the average particle diameter d _A of the active material particles 30 contained in the first positive electrode mixture layer 3 are elements of EDX While discriminating the type and composition of each particle by composition analysis, the SEM image was measured by the following method.
In the above SEM photograph, the average particle diameter d _L of the lithium composite oxide particles 20 is 100 adjacent to each other in the second positive electrode mixture layer 2 in order to prevent artificial selection, as shown in FIGS. For each lithium composite oxide particle 20, the average value of the particle size of each particle was determined. Particle size is the average value of the longest diameter d ₁ and the shortest diameter d _2. 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 at its farthest straight line connecting two points on the contour and the longest diameter d _1. A straight line passing through the center point M, and the length between the two points in the shortest straight line distance between two points and the contour of the straight line and the particle crosses the shortest diameter d _2. The average particle diameter d _L is a value obtained by arranging the particle diameter values of each particle from the maximum value to the minimum value in order of size, and among these values, 20 values from the maximum value and 20 values from the minimum value. The average value of a total of 60 values excluding the value of was used.
The average particle diameter d _A of the active material particles 30 was measured using a cross-sectional SEM micrograph in the same manner as the average particle diameter d _{L of} the lithium composite oxide particles 20, except that the measurement target was active material particles. .

(Gap measurement method)
The average value g _L and the maximum value g _M of the gap length between the lithium composite oxide particles on the positive electrode current collector were measured by the following methods.
SEM micrographs were taken at a measurement magnification of 500 times by the method described above. In the obtained photograph, first, the distance (g in FIG. 2) on the surface of the positive electrode current collector between the lithium composite oxide particles 20 and the adjacent lithium composite oxide particles 20 is determined between the lithium composite oxide particles. It was defined as the gap length. The maximum value of the gap length between the lithium composite oxide particles is the maximum length among the gap lengths between the adjacent lithium composite oxide particles in the obtained photograph. Maximum value g _M of the length of the gap is determined by measuring the maximum length of a gap in the range length of the positive electrode current collector is 100 [mu] m, and the average maximum length of each of the ranges.
The average value of the gap length between the lithium composite oxide particles is an average length of the gap lengths between the adjacent lithium composite oxide particles in the obtained photograph. The average length g _L of the gaps was obtained by measuring the average length of the gaps in the range where the length of the positive electrode current collector was 100 μm and averaging the average lengths of the respective ranges.

(Aspect ratio measurement method)
The aspect ratio of the first active material particles 30 of the positive electrode mixture layer 3, by using a SEM micrograph of a cross-section, measuring the longest particle diameter d _M and the shortest diameter d _m of the active material particles 30, calculates did. Maximum grain size d _M and the minimum diameter d _m of the active material particles 30, except that the measurement target and the active material particles, similar to the maximum diameter d ₁ and the shortest diameter d ₂ of the lithium composite oxide particles 20 It was measured by the method. Aspect ratio, and ten active material particles 30 adjacent to each other, and the average value of the ratio of the longest diameter _{d M} to the shortest diameter _{d m (d M / d m} ).

(5) Charge / Discharge Test The charge / discharge test was performed by the following method.
A constant current / constant voltage charge with a power supply 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. The larger the ratio, the better the high rate discharge characteristics.

(6) 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 1.25 hours at a charge current value (current density of about 5 mA / cm ² ) twice the design capacity of the battery. The SOC (state of charge) (%) when the voltage reached 7V was calculated. A battery whose voltage has reached 7 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.

(Evaluation)
Table 2 shows the discharge capacity ratios from A1 to A3, A7, A8, B1 and B2, the SOC when the overcharge test reaches 7V, and the safety. A discharge capacity ratio of 1 C / 0.2 C was calculated. The larger the ratio, the better the high rate discharge characteristics. The smaller the SOC when 7V is reached, the higher the safety. The battery that has not reached 7V is indicated by safety. A battery having an SOC of 230% or less and having reached 7 V is indicated as “◯” because the safety is better. A battery having an SOC exceeding 230% and reaching 7 V is indicated by Δ.

  In the positive electrode plates A1 to A3 of the example in which the coverage of the second positive electrode mixture layer 2 is 60 to 95%, good high rate discharge characteristics are obtained, the voltage reaches 7V, and the safety against overcharge is high. It can be seen that it is secured.

  A4 can produce a positive electrode plate having the same coverage as A1, although the solid content of the paste at the time of application and the gap between the applicators are different. The result of the overcharge test using the electrode plate is the same as that of A1, and the coverage is not determined only by the solid content concentration at the time of application.

  In A5 and A6, although the apparatus used at the time of application is not an applicator, an electrode plate having the same coverage as A1 can be produced. The result of the overcharge test using the electrode plate is the same as that of A1, and does not depend on the device used at the time of application.

1: Positive current collector 2: Second positive electrode mixture layer 3: First positive electrode mixture layer 20: Lithium composite oxide particles 21: Covering region 30: Active material particles 40: Full length of measurement region 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 first positive electrode mixture layer is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiCoNiO ₂ , LiCoMO ₂ and LiNiMO ₂ (M is commonly Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu , Zn, Al, Cr, Pb, Sb and B), and those in which some elements of these positive electrode active materials are substituted with different elements, and ternary layered layers containing Co, Ni and Mn Including active material particles selected from the group consisting of oxides,
The active material particles have an average particle size of 0.5 to 60 μm,
The second positive electrode mixture layer includes lithium composite oxide particles and a conductive additive,
The lithium composite oxide particles are spinel type or olivine type lithium composite oxide particles,
The amount of the conductive auxiliary agent contained in the second positive electrode mixture layer is 0.1 to 4 parts by mass with respect to 100 parts by mass of the lithium composite oxide particles,
The second positive-electrode mixture lithium composite oxide particles contained in the adhesive layer is the cathode current region covering the conductor occupies the positive electrode mixture application area coverage Ri 60% to 95% der,
_A ratio (d _A / g _M ) between a maximum value g _M of a gap length between the lithium composite oxide particles on the positive electrode current collector and an average particle diameter d _A of the active material particles is 0.1 or more A nonaqueous electrolyte secondary battery having a range of 1 or less . The nonaqueous electrolyte according to claim 1, wherein a ratio (d _A / d _L ) between an average particle diameter d _L of the lithium composite oxide particles and an average particle diameter d _A of the active material particles is 0.1 to 6. Secondary battery. The non-aqueous electrolyte secondary battery of lithium described maximum value g _M of the length of the gap between the composite oxide particles is from 5μm to claim 1 or 2 in the range of 45μm on the positive electrode current collector. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the lithium composite oxide particles comprising spinel-type lithium manganate.

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