JP6917907B2 - A method for manufacturing and evaluating a positive electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a positive electrode for a lithium ion secondary battery, and a method for manufacturing a lithium ion secondary battery. - Google Patents

Description

The present invention relates to a positive electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method for manufacturing and evaluating a positive electrode for a lithium ion secondary battery.

Lithium-ion secondary batteries have high energy density and excellent charge / discharge cycle characteristics, and are therefore widely used as power sources for small mobile devices such as mobile phones and notebook computers. In recent years, due to growing awareness of environmental issues and energy conservation, large-capacity and long-life storage batteries for vehicles such as electric vehicles and hybrid electric vehicles and power storage systems for household power storage systems are required. Demand for large power sources is also increasing.

In order to improve the characteristics of lithium-ion secondary batteries, various studies have been conducted on their electrodes.

Patent Document 1 describes lithium hydroxide remaining in a lithium transition metal-containing composite oxide constituting a positive electrode active material in a positive electrode composition for a lithium secondary battery containing a positive electrode active material, a conductive agent, a binder and a solvent. A positive electrode composition is described in which the content is 0.4% by weight or less as measured by a neutralization titration method. It is stated that such a composition is stable, and by using it, a lithium secondary battery having excellent discharge capacity and cycle characteristics can be obtained.

Patent Document 2 describes a method for producing a lithium ion secondary battery, which uses a lithium transition metal oxide containing LiOH in a proportion of 0.032% by mass or more and 0.050% by mass or less in addition to the composition as the positive electrode active material. Has been done. According to this manufacturing method, it is described that a lithium ion secondary battery having excellent durability against a charge / discharge cycle can be manufactured.

Patent Document 3 describes a current collector having an aluminum oxide film having a thickness of 0.02 to 1 μm for the purpose of preventing corrosion of the aluminum current collector even when an active material paste using water as a solvent is used. A method for manufacturing a positive electrode for a lithium battery to be used is described.

Patent Document 4 describes a lithium ion battery using an aluminum foil having an anodic oxide film having a thickness of 5 to 1000 nm formed on its surface as a current collector. It is described that the surface of the aluminum foil is imparted with alkali resistance without variation, and the elution of the aluminum foil due to the active material paste is prevented to improve the adhesion between the two.

Patent Document 5 describes an electrode having a mixture containing active material particles capable of reversibly occluding and releasing lithium and a current collector supporting the mixture, and the surface of the current collector has recesses. However, an electrode for a lithium ion secondary battery is described in which the ratio of the area occupied by the recesses to the area of the current collector supported by the mixture is 30%. Further, it is an electrode having the mixture and a current collector carrying the mixture, the surface of the current collector has a recess, and the mixture and the current collector are perpendicular to the electrode surface. Described is an electrode for a lithium ion secondary battery in which the maximum depth of the recess is 1 μm or more in the cut cross section. It is described that an aluminum foil is used as the current collector and a lithium-containing composite oxide is used as the active material. Such an electrode is suitable for achieving both battery safety and output characteristics by improving the adhesion between the mixture and the current collector due to the presence of a plurality of recesses on the surface of the current collector. It is described as.

Japanese Unexamined Patent Publication No. 10-208728 Japanese Unexamined Patent Publication No. 2013-131392 Japanese Unexamined Patent Publication No. 2003-157852 Japanese Unexamined Patent Publication No. 2007-250376 JP-A-2007-59387

An object of the present invention is to provide a lithium ion secondary battery having improved cycle characteristics, a positive electrode suitable for the battery, and a method for producing and evaluating the positive electrode.

According to one aspect of the present invention, an aluminum substrate whose surface is covered with a film and
An electrode containing an active material layer formed on the film of the aluminum base material.
The active material layer contains active material particles and a binder and contains.
The active material particles contain at least particles of a lithium nickel-containing composite oxide having a layered crystal structure.
An electrode for a lithium ion secondary battery that satisfies any of the following conditions A and B is provided.
Condition A: On the surface of the aluminum base material on which the active material layer is formed, the number of recesses having a maximum opening diameter of 5 μm or more in an arbitrary square region of 100 μm on a side does not exceed five.
Condition B: The active material layer contains LiOH, and the content of LiOH is less than 0.5% by mass with respect to the total of the active material particles.

According to another aspect of the present invention, there is provided a lithium ion secondary battery including the positive electrode, the negative electrode, a separator between the negative electrode and the positive electrode, and an electrolytic solution.

According to another aspect of the present invention, the method for producing an electrode for a lithium ion secondary battery including an aluminum base material whose surface is covered with a film and an active material layer formed on the film of the aluminum base material. There,
The process of forming a slurry containing active material particles, a binder, and a solvent,
Including the steps of applying the slurry onto the aluminum substrate, drying and pressing to form an active material layer.
The active material particles contain at least particles of a lithium nickel-containing composite oxide having a layered crystal structure.
On the surface of the aluminum base material on which the active material layer is formed, a product in which the number of recesses having a maximum opening diameter of 5 μm or more does not exceed 5 in an arbitrary square region of 100 μm on a side is determined to be a non-defective product. A method for manufacturing an electrode for a lithium ion secondary battery is provided, which includes a step of making an electrode.

According to another aspect of the present invention, the method for evaluating an electrode for a lithium ion secondary battery including an aluminum base material whose surface is covered with a film and an active material layer formed on the film of the aluminum base material. There,
The active material layer contains active material particles and a binder and contains.
The active material particles contain at least particles of a lithium nickel-containing composite oxide having a layered crystal structure.
On the surface of the aluminum base material on which the active material layer is formed, a product in which the number of recesses having a maximum opening diameter of 5 μm or more does not exceed 5 in an arbitrary square region of 100 μm on a side is determined to be a good product. A method for evaluating an electrode for a lithium ion secondary battery is provided.

According to the embodiment of the present invention, it is possible to provide a lithium ion secondary battery having improved cycle characteristics, a positive electrode suitable for the battery, and a method for producing and evaluating the positive electrode.

It is sectional drawing for demonstrating an example of the lithium ion secondary battery by embodiment of this invention. 6 is an SEM image of the surface of the Al foil from which the active material layer of the positive electrode of Example 1 has been removed. It is an SEM image of the cross section of the positive electrode of Example 1. 6 is an SEM image of the surface of the Al foil from which the active material layer of the positive electrode of Comparative Example 1 has been removed. It is an SEM image of the cross section of the positive electrode of Comparative Example 1.

A lithium ion secondary battery containing a positive electrode using a lithium nickel composite oxide having a layered crystal structure (for example, LiNiO ₂ ) as a positive electrode active material and an aluminum foil (Al foil) as a current collector is particularly charged and discharged. There is a problem that the capacity deteriorates rapidly during the cycle. As a result of paying attention to this problem and diligently examining it, the present inventor considered that this capacity deterioration follows the following mechanism, and completed the present invention.

It is considered that this deterioration of the capacity is caused by the corrosion of the Al foil, and the corrosion is caused by LiOH. Examples of LiOH include those derived from raw materials (residues) contained in the secondary particles (between the primary particles) of the lithium nickel composite oxide. Normally, a thin film (Al ₂ O ₃ passivation film) is formed on the surface of the Al foil, so that the surface is stable. However, when the slurry containing the active material particles is applied onto the Al foil, dried, and then pressed to increase the electrode density, the active material particles dig into the surface of the Al foil, and the film is destroyed at that portion. Al is exposed. As a result, the exposed Al ^{reacts with the OH −} ions dissociated from LiOH to corrode the Al foil, and an insulator is formed. It can be said that such corrosion of the Al foil causes deterioration of the cycle characteristics of the battery.

When the amount of the active material particles embedded in the Al foil is small, no significant decrease in cycle characteristics is observed. On the other hand, when the active material particles are deeply sunk into the Al foil, large recesses are formed on the surface of the Al foil according to the sunk, and when the number is large, the cycle characteristics are significantly deteriorated. The present inventor has found that the recesses formed by the active material particles digging into the surface of the Al foil are related to the cycle characteristics, and have completed the present invention.

The positive electrode according to the embodiment of the present invention includes an aluminum base material whose surface is covered with a film (for example, Al foil) and an active material layer formed on the film of the aluminum base material. Includes active material particles and binder. The active material layer can further contain a conductive additive.

The active material particles include at least particles of a lithium nickel-containing composite oxide having a layered crystal structure. The lithium nickel-containing composite oxide particles can include secondary particles in which a plurality of primary particles of the lithium nickel-containing composite oxide are aggregated.

In order to obtain the effect of improving the cycle characteristics of the lithium ion secondary battery, the surface of the aluminum base material on which the active material layer is formed is formed on the surface of the aluminum base material in an arbitrary square region having a side of 100 μm. It is preferable that the number of recesses having the longest diameter of the opening of the recess having a maximum diameter of 5 μm or more does not exceed 5, and more preferably not more than 3. Further, the ratio of the total opening area of the openings of the recesses (recesses having the longest diameter of 5 μm or more) in the square region to the area of the square region is preferably 10% or less, more preferably 5% or less. 1% or less is more preferable. The depth of the recess having the longest diameter of about 5 μm can be 1 μm or more, 1.5 μm or more, or 2 μm. A recess having such a size can easily break a film having a thickness of about 1 to 10 nm.

For the recesses for counting the number, the recesses where the square area and the opening overlap are counted. Here, the term "overlap" means that the area of the area where the square area and the opening of the recess overlap is two-thirds or more of the area of a circle having a diameter of 5 μm.

On the other hand, if the maximum diameter of the opening of the recess is 1 μm or less, it may be present on the surface of the aluminum base material, and in an arbitrary square region having a side of 100 μm, the longest diameter of the opening of the recess is 0. It is preferable that there is at least one recess of 1 μm or more and 1 μm or less, more preferably at least two (plurality), and there may be five or more. When such a relatively small-sized recess is formed, the effect of improving the adhesion between the active material layer and the aluminum base material can be expected. The formation of such a recess indicates that sufficient pressing has been performed. As the concave portion for counting the number (the concave portion having the longest diameter of the opening of 0.1 μm or more and 1 μm or less), the one in which the entire opening is within the square region can be counted.

Here, the longest diameter of the opening of the recess can be determined from an image obtained by observing the surface of the aluminum base material after removing the active material layer from the electrode with a SEM (scanning electron microscope). In this observation image, the diameter of the circle having the smallest area surrounding the contour corresponding to the opening of the recess is defined as the longest diameter of the opening of the recess.

The depth of the recess is defined as the depth of the recess along the surface of the aluminum base material on the active material layer side in the SEM image of the cross section obtained by cutting the electrode perpendicularly to the surface of the electrode (in the cross-sectional image, the surface of the base material on both sides of the recess is covered. ) Refers to the longest length of the perpendicular line with respect to the line segment A from the line segment A to the bottom of the recess, with reference to the line segment A extending on the recess.

The thickness of the film of the aluminum base material is preferably in the range of 1 to 10 nm. Further, the film preferably contains an oxide, and examples of such an oxide film include an Al ₂ O ₃ passivation film. When the thickness of the film is 1 nm or more, the stability of the surface of the aluminum base material becomes better, and when the thickness of the film is 10 nm or less, the contact resistance can be suppressed.

The lithium nickel-containing composite oxide may contain LiOH as a component other than its composition. _{This LiOH is derived from, for example, a lithium source (LiOH, Li 2} CO ₃ ) used in the process of producing a lithium nickel-containing composite oxide, and has unreacted components, by-products, and reaction products with water. included. LIOH is contained in the secondary particles as a residue between the primary particles.

The content of LiOH in the active material layer can be set in the range of 0.01 to 2% by mass and 0.02 to 2% by mass with respect to the total particles of the active material contained in the active material layer. Can be set.

When the condition A is satisfied, the content of LiOH in the active material layer may be 0.5 to 2% by mass, and further may be 0.5 to 1.5% by mass. When condition A is satisfied and LiOH is contained, an active material (lithium nickel-containing composite oxide) prepared by _{using LiOH as a lithium source and suppressing the amount of Li 2} CO _{3 used can be used, and residual Li 2 can be used.} It is possible to reduce the generation of CO _{2 gas derived from CO 3.} Further, when the LiOH content is within the above range, the volume decrease due to the LiOH content can be suppressed. The LiOH content can be determined by neutralization titration with hydrochloric acid.

On the other hand, when the condition B is satisfied, that is, when the content of LiOH in the active material layer is less than 0.5% by mass with respect to the total amount of the active material particles, capacity deterioration due to the formation of recesses on the surface of the aluminum base material is suppressed. be able to. The LiOH content in this case can be set to 0.01% by mass or more and less than 0.5% by mass, and 0.02% by mass or more and less than 0.5% by mass. When LiOH is contained, an active material (lithium nickel-containing composite oxide) prepared by _{using LiOH as a lithium source and suppressing the amount of Li 2} CO ₃ used can be used, and is derived from _{residual Li 2} CO _3. CO ₂ gas generation can be reduced.

_{The average particle size (D 50} ) of the particles of the active material is preferably 2 to 20 μm, more preferably 2 to 15 μm, still more preferably 2 to 10 μm. When the average particle size is in this range, the formation of recesses due to the embedding of the active material particles into the surface of the aluminum base material can be suppressed, and the size of the formed recesses can be reduced, so that the destruction of the film of the base material can be suppressed. As a result, the cycle characteristics can be further improved. _{Here, the average particle size means the particle size (median diameter: D 50} ) at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction scattering method.

The density of the active material layer (electrode density) is preferably ^{2.5 to 3.5 g / cm 3.} When the electrode density is in such a range, a sufficient volumetric energy density can be obtained, the contact between the active material particles and the contact between the active material and the conductive auxiliary agent are good, and a sufficient porosity can be obtained. Since it can be secured, a sufficient discharge capacity at a high rate can be obtained.

The electrode density can be obtained as follows. First, the thickness and weight of an electrode having a predetermined size (for example, 5 cm × 5 cm) are measured. Since the thickness and weight of the aluminum base material (current collector made of Al foil) are known, they are subtracted from the measured electrode weight and thickness, respectively. The density can be obtained by dividing the weight of only the obtained active material layer by the volume (= thickness × area (5 cm × 5 cm)).

As the aluminum base material, those conventionally used as the electrode base material of the electrochemical element can be used, and a pure aluminum foil (purity of 99.9% by mass or more) or an aluminum alloy foil (for example, Al—Cu alloy, etc.) can be used. Al-Mn alloy, Al-Si alloy, Al-Mg alloy, Al-Mg-Si alloy, Al-Zn-Mg alloy) can be used. The aluminum content in the aluminum base material is preferably 95% by mass or more, more preferably 98% by mass or more, and further preferably 99% by mass or more.

The thickness of the aluminum base material used in the embodiment of the present invention is preferably in the range of 5 μm to 20 μm.

In the method for producing a positive electrode and the evaluation method according to the embodiment of the present invention, a product in which the number of recesses having a maximum opening diameter of 5 μm or more does not exceed 5 in an arbitrary square region of 100 μm on a side is determined to be a non-defective product. Includes steps. Further, a product whose number does not exceed 3 can be determined as a non-defective product. In this determination step, the ratio of the total opening area of the openings of the recesses (recesses having the longest diameter of the opening of 5 μm or more) in the square region to the area of the square region is preferably 10% or less. % Or less is more preferable, and 1% or less is further preferable. As for the recesses for counting the number, the recesses where the square area and the opening overlap are counted in the same manner as described above.

Products judged to be non-defective in this determination step can be selected. By making a judgment at the stage where the negative electrode is manufactured, the negative electrode can be evaluated and selected without manufacturing a secondary battery and performing a cycle test thereof. As a result, the efficiency of evaluation and selection can be improved, and the productivity can be improved. At the time of evaluation, the active material layer of the produced electrode can be removed, and the surface of the aluminum base material on which the active material layer is formed can be observed using a scanning electron microscope (SEM). At that time, an electrode separately manufactured under the same conditions may be evaluated, and the result may be used as an evaluation result of the electrode to be evaluated, or an arbitrary part of the electrode sheet before being processed into a predetermined shape and size may be evaluated. The result may be used as an evaluation result of the electrode after processing.

In this evaluation, it is preferable that the number of recesses having a maximum diameter of 0.1 μm or more and 1 μm or less in an arbitrary square region having a side of 100 μm is at least one on the surface of the aluminum base material. More preferably. As for the recesses for counting the number (the recesses having the longest diameter of the opening of 0.1 μm or more and 1 μm or less), those in which the entire opening is within the square region can be counted in the same manner as described above.

The production method and the evaluation method according to the embodiment of the present invention are used when the content of LiOH in the active material layer of the electrode to be produced is 0.5% by mass or more with respect to the total amount of the active material particles in the active material layer. It is also suitable when the electrode is pressed so that the active material layer has a high density (for example, 2.5 to 3.5 g / cm ^3).

Hereinafter, the positive electrode for a lithium ion secondary battery and its manufacturing method according to the embodiment of the present invention, and the lithium ion secondary battery and its configuration will be described in more detail.

(Lithium-ion secondary battery)
A cross-sectional view of an example (laminated type) of a lithium ion secondary battery is shown in FIG. As shown in FIG. 1, the lithium ion secondary battery of this example has a positive electrode composed of a positive electrode current collector 3 made of an aluminum foil and a positive electrode active material layer 1 containing a positive electrode active material provided on the positive electrode current collector 3. It also has a negative electrode composed of a negative electrode current collector 4 made of a metal such as a copper foil, and a negative electrode active material layer 2 containing a negative electrode active material provided on the negative electrode current collector 4. The positive electrode and the negative electrode are laminated via a separator 5 made of a non-woven fabric, a polypropylene microporous film, or the like so that the positive electrode active material layer 1 and the negative electrode active material layer 2 face each other. The electrode pair is housed in a container made of exterior bodies 6 and 7 made of an aluminum laminated film. A positive electrode tab 9 is connected to the positive electrode current collector 3, a negative electrode tab 8 is connected to the negative electrode current collector 4, and these tabs are pulled out of the container. The electrolytic solution is injected into the container and sealed. It is also possible to have a structure in which a group of electrodes in which a plurality of electrode pairs are laminated is housed in a container.

(Positive electrode)
As the positive electrode active material, a lithium nickel composite oxide having at least a layered crystal structure (layered rock salt type structure) is used. This lithium nickel composite oxide may be used in combination with another positive electrode active material.

As a lithium nickel composite oxide having a layered crystal structure (layered rock salt type structure),
Lithium nickelate (LiNiO ₂ );
At least part of the nickel portion of lithium nickelate is replaced with other metal elements such as aluminum, magnesium, titanium and zinc;
Cobalt-substituted lithium nickelate in which part of the nickel in lithium nickelate is replaced with at least cobalt;
Cobalt Substitution Examples of lithium nickelate in which a part of nickel is replaced with another metal element (for example, at least one of aluminum, magnesium, titanium, zinc, and manganese) can be mentioned.

Other positive electrode active materials include lithium manganate (LiMn ₂ O ₄ ); lithium cobalt oxide (LiCoO ₂ ); at least a part of the manganese and cobalt portions of these lithium compounds is aluminum, magnesium, titanium, zinc and the like. Substituted with metal elements of
Nickel-substituted lithium manganate in which a part of manganese in lithium manganate is replaced with at least nickel;
Nickel-Substituted Lithium Manganese Part of Manganese Substituted With Other Metals (eg Aluminum, Magnesium, Titanium, Zinc); and Lithium Transition Metal Composite Phosphorides with Olivin Structure.

As the lithium nickel-containing composite oxide having a layered crystal structure, a nickel site in which a part of nickel is replaced with another metal can be used. Examples of the metal other than Ni that occupies the nickel site include at least one metal selected from Mn, Co, Al, Mg, Fe, Cr, Ti, and In.

This lithium nickel-containing composite oxide preferably contains Co as a metal other than Ni that occupies nickel sites. Further, the lithium nickel-containing composite oxide more preferably contains Mn or Al in addition to Co, that is, lithium nickel cobalt manganese composite oxide (NCM) having a layered crystal structure, and lithium nickel having a layered crystal structure. Cobalt-aluminum composite oxide (NCA), or a mixture thereof, can be preferably used.

As the lithium nickel-containing composite oxide having a layered crystal structure, for example, one represented by the following formula can be used.
Li _x Ni _2-x-y M1 _y O _{2 + α}
(In the formula, M1 contains at least one of Co, Mn, and Al, and 0 <x ≦ 1, 0 ≦ y <1, −0.1 ≦ α ≦ 0.1).
As for x, 0.6 <x ≦ 1 is preferable, and 0.8 ≦ x ≦ 1 is more preferable. It is more preferable that y is 0 ≦ y <0.5.

As the lithium nickel manganese-containing composite oxide having a spinel structure, for example, one represented by the following formula can be used.
Li _{1 + x2} Mn _2-x2-y2 M2 _y2 O _{4 + β}
(In the formula, M2 contains at least one selected from B, Sn, Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mg, Ga and Li, and 0 ≦ x2 ≦ 0. 5, 0 ≦ y2 ≦ 0.5, −0.1 ≦ β ≦ 0.1)

The average particle size of the positive electrode active material is preferably, for example, 0.1 to 50 μm, more preferably 1 to 30 μm, and even more preferably 2 to 25 μm from the viewpoint of reactivity with the electrolytic solution, rate characteristics, and the like. Further, from the viewpoint of suppressing the formation of recesses due to the embedding of the active material particles on the surface of the aluminum base material, the average particle size of the positive electrode active material is preferably 2 to 20 μm, more preferably 2 to 15 μm, still more preferably 2 to 10 μm. .. _{Here, the average particle size means the particle size (median diameter: D 50} ) at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction scattering method.

The positive electrode is composed of a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector. The positive electrode is arranged so that the active material layer faces the negative electrode active material layer on the negative electrode current collector via a separator.

The positive electrode active material layer can be formed as follows. First, a slurry containing a positive electrode active material, a binder and a solvent (and, if necessary, a conductive auxiliary agent) can be prepared, applied onto a positive electrode current collector, dried, and pressed. N-methyl-2-pyrrolidone (NMP) can be used as the slurry solvent used when producing the positive electrode.

As the binder, those usually used as a binder for positive electrodes such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) can be used.

The positive electrode active material layer can contain a conductive auxiliary agent in addition to the positive electrode active material and the binder. The conductive auxiliary agent is not particularly limited, and a conductive material usually used as a conductive auxiliary agent for a positive electrode such as carbon black, acetylene black, natural graphite, artificial graphite, carbon fiber and the like can be used.

A large proportion of the positive electrode active material in the positive electrode active material layer is preferable because the capacity per mass is large. However, from the viewpoint of lowering the resistance of the electrode, it is preferable to add a conductive auxiliary agent, and from the viewpoint of electrode strength. It is preferable to add a binder. If the proportion of the conductive auxiliary agent is too small, it becomes difficult to maintain sufficient conductivity, which tends to lead to an increase in electrode resistance. If the proportion of the binder is too small, it becomes difficult to maintain the adhesive force with the current collector, the active material, and the conductive auxiliary agent, and electrode peeling may occur. From the above points, the content of the conductive additive in the active material layer is preferably 1 to 10% by mass, and the content of the binder in the active material layer is preferably 1 to 10% by mass.

The porosity of the positive electrode active material layer (not including the current collector) is preferably 10 to 30%, more preferably 20 to 25%. When the porosity of the positive electrode active material layer is set to the above value, the discharge capacity at the time of use at a high discharge rate is improved, which is preferable.

(Negative electrode)
A carbonaceous material can be used as the negative electrode active material. Examples of the carbonaceous material include graphite, amorphous carbon (for example, graphitizable carbon and non-graphitizable carbon), diamond-like carbon, fullerene, carbon nanotubes, carbon nanohorns and the like. As the graphite, natural graphite and artificial graphite can be used, and inexpensive natural graphite is preferable from the viewpoint of material cost. Examples of the amorphous carbon include those obtained by heat-treating coal pitch coke, petroleum pitch coke, acetylene pitch coke and the like.

The average particle size of the negative electrode active material is preferably 2 μm or more, more preferably 5 μm or more, from the viewpoint of suppressing side reactions during charging / discharging and suppressing a decrease in charging / discharging efficiency, and from the viewpoint of input / output characteristics and electrode fabrication. From the viewpoint of (smoothness of the electrode surface, etc.), 40 μm or less is preferable, and 30 μm or less is more preferable. _{Here, the average particle size means the particle size (median diameter: D 50} ) at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction / scattering method.

To prepare the negative electrode, a slurry containing the negative electrode active material, a binder, a solvent, and a conductive auxiliary agent if necessary is applied onto the negative electrode current collector, dried, and pressed as necessary to form a negative electrode active material layer. By forming, a negative electrode (a current collector and a negative electrode active material layer on the current collector) can be obtained. Examples of the method for applying the negative electrode slurry include a doctor blade method, a die coater method, and a dip coating method. Additives such as antifoaming agents and surfactants may be added to the slurry, if necessary.

The content of the binder in the negative electrode active material layer is preferably in the range of 0.5 to 30% by mass as the content of the binder with respect to the negative electrode active material from the viewpoint of the binding force and the energy density, which are in a trade-off relationship. , 0.5 to 25% by mass is more preferable, and 1 to 20% by mass is even more preferable.

As the solvent, an organic solvent such as N-methyl-2-pyrrolidone (NMP) or water can be used. When an organic solvent is used as the solvent, a binder for an organic solvent such as polyvinylidene fluoride (PVDF) can be used. When water is used as the solvent, a rubber-based binder (for example, SBR (styrene-butadiene rubber)) or an acrylic binder can be used. Such an aqueous binder can be used in the form of an emulsion. When water is used as the solvent, it is preferable to use a water-based binder and a thickener such as CMC (carboxymethyl cellulose) in combination.

The negative electrode active material layer may contain a conductive auxiliary agent, if necessary. As the conductive auxiliary agent, a conductive material generally used as a conductive auxiliary agent for the negative electrode, such as a carbonaceous material such as carbon black, Ketjen black, and acetylene black, can be used. The content of the conductive auxiliary agent in the negative electrode active material layer is preferably in the range of 0.1 to 3.0% by mass as the content ratio with respect to the negative electrode active material. The content of the conductive auxiliary agent in the negative electrode active material is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and is caused by excessive addition of the conductive auxiliary agent from the viewpoint of forming a sufficient conductive path. 3.0% by mass or less is preferable, and 1.0% by mass or less is more preferable, from the viewpoint of suppressing gas generation and deterioration of peeling strength due to electrolyte decomposition.

The average particle size (primary particle size) of the conductive auxiliary agent is preferably in the range of 10 to 100 nm. The average particle size (primary particle size) of the conductive auxiliary agent is preferably 10 nm or more, more preferably 30 nm or more, and a sufficient number of contact points from the viewpoint of suppressing excessive aggregation of the conductive auxiliary agent and uniformly dispersing the conductive auxiliary agent in the negative electrode. Is preferably 100 nm or less, and more preferably 80 nm or less from the viewpoint of forming a good conductive path. When the conductive auxiliary agent is in the form of fibers, those having an average diameter of 2 to 200 nm and an average fiber length of 0.1 to 20 μm can be mentioned.

Here, the average particle size of the conductive auxiliary agent is the median diameter (D ₅₀ ), which means the particle size at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction / scattering method.

As the negative electrode current collector, copper, stainless steel, nickel, titanium or an alloy thereof can be used. Examples of the shape include a foil, a flat plate, and a mesh.

(Electrolytic solution)
As the electrolytic solution, a non-aqueous electrolytic solution in which a lithium salt is dissolved in one or more kinds of non-aqueous solvents can be used.

Examples of the non-aqueous solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC); dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate. Chain carbonates such as (EMC) and dipropyl carbonate (DPC); aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate; γ-lactones such as γ-butylolactone; 1,2-ethoxy Chain ethers such as ethane (DEE) and ethoxymethoxyethane (EME); cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran can be mentioned. One of these non-aqueous solvents can be used alone or a mixture of two or more.

Examples of the lithium salt dissolved in the nonaqueous solvent, is not particularly limited, for example _{LiPF 6, LiAsF 6, LiAlCl 4} , LiClO 4, LiBF 4, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li Examples thereof include (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , and lithium bisoxalatoborate. These lithium salts can be used alone or in combination of two or more. Further, a polymer component may be contained as a non-aqueous electrolyte. The concentration of the lithium salt can be set in the range of 0.8 to 1.2 mol / L, preferably 0.9 to 1.1 mol / L.

(Additive)
The electrolytic solution preferably contains a compound usually used as an additive for a non-aqueous electrolytic solution. For example, carbonate compounds such as vinylene carbonate and fluoroethylene carbonate; acid anhydrides such as maleic acid anhydride; boron additives such as boronic acid ester; sulfite compounds such as ethylene sulfide; 1,3-propanesulton. , 1,2-Propane sulton, 1,4-butane sulton, 1,2-butan sulton, 1,3-butan sulton, 2,4-butan sulton, 1,3-pentane sulton and other cyclic monosulfonic acid esters; methylenemethane disulfonic acid Examples thereof include cyclic disulfonic acid ester compounds such as esters (1,5,2,4-dioxadithian-2,2,4,4-tetraoxide) and ethylenemethanedisulfonic acid esters. These additives may be used alone or in combination of two or more. In particular, a cyclic sulfonic acid ester compound is preferable, and a cyclic disulfonic acid compound is preferable, from the viewpoint that a film can be more effectively formed on the surface of the positive electrode and the battery characteristics can be improved.

The content of the additive in the electrolytic solution is preferably 0.01 to 10% by mass, preferably 0.1 to 5% by mass, from the viewpoint of obtaining a sufficient addition effect while suppressing an increase in viscosity and resistance of the electrolytic solution. More preferably, it is by mass%.

(Separator)
As the separator, a resin-made porous membrane, woven fabric, non-woven fabric or the like can be used. Examples of the resin constituting the porous film include polyolefin resins such as polypropylene and polyethylene, polyester resins, acrylic resins, styrene resins, nylon resins and the like. In particular, a polyolefin-based microporous membrane is preferable because it is excellent in ion permeability and the ability to physically separate the positive electrode and the negative electrode. Further, if necessary, a layer containing inorganic particles may be formed on the separator, and examples of the inorganic particles include insulating oxides, nitrides, sulfides, and carbides, and among them, TIO. It is preferable to contain ₂ or Al ₂ O _3.

(Outer container)
A case made of a flexible film, a can case, or the like can be used for the outer container, and it is preferable to use the flexible film from the viewpoint of reducing the weight of the battery.

As the flexible film, a flexible film in which resin layers are provided on the front and back surfaces of a metal layer as a base material can be used. As the metal layer, one having a barrier property such as preventing leakage of an electrolytic solution and infiltration of water from the outside can be selected, and aluminum, stainless steel, or the like can be used. A heat-sealing resin layer such as modified polyolefin is provided on at least one surface of the metal layer. The outer container is formed by making the heat-sealing resin layers of the flexible film face each other and heat-sealing the periphery of the portion accommodating the electrode laminate. A resin layer such as a nylon film or a polyester film can be provided on the surface of the exterior body, which is the surface opposite to the surface on which the heat-sealing resin layer is formed.

(Example 1)
The positive particle active material has an average particle size (D ₅₀ ) of 9 μm and a crystal structure of spinel-type lithium manganate (LMO), and an average particle size (D ₅₀ ) of 6 μm and a layered crystal structure of lithium nickelate (LNO). The mixed positive electrode active material (the content of LiOH with respect to the entire positive electrode active material is 0.65% by mass, and the average particle size (D ₅₀ ) of the mixed positive electrode active material particles is 8.3 μm. ) Was dispersed in NMP in which PVDF was dissolved. This slurry is applied and dried on an aluminum foil (Al foil) having a thickness of 20 μm, and similarly, the slurry is applied and dried on the other surface, and the coating layer formed on both sides of the Al foil is compressed (pressed). ), And processed into a predetermined shape to obtain a positive electrode.

On the other hand, a slurry in which graphite was dispersed in NMP in which PVDF was dissolved was prepared. This slurry is applied and dried on a copper foil having a thickness of 10 μm, and similarly, the slurry is applied and dried on the other surface, and the coating layer formed on both sides of the copper foil is compressed into a predetermined shape. It was processed to obtain a negative electrode.

These five positive electrodes and six negative electrodes were alternately laminated via a separator and sealed in a laminated exterior material together with an electrolytic solution to form a secondary battery. As the electrolytic solution, an electrolytic solution in which a lithium salt was dissolved in a mixed solvent of EC and DEC (volume ratio EC: DEC = 3: 7, LiPF ₆ concentration 1 mol / L) was used.

Five of these batteries were prepared, and the cycle characteristics of each battery were evaluated. The results are shown in Table 1.

On the other hand, apart from the production of these batteries, five compressed positive electrodes before assembling the batteries were produced in the same manner. For each positive electrode, the surface of the Al foil from which the active material layer was removed and the cross section of the positive electrode were observed with a scanning electron microscope (SEM). At any three locations on the surface of the Al foil from which the active material layer has been removed, there are a plurality of recesses having a maximum diameter of 0.1 μm or more and 1 μm or less per 100 μm square region. No recesses with a maximum diameter of 5 μm or more were observed.

FIG. 2A shows an SEM image of the surface of the Al foil from which the active material layer has been removed, and FIG. 2B shows an SEM image of a cross section of the positive electrode. From these images, it can be seen that small recesses are formed on the surface of the Al foil, but large recesses having an opening having a maximum diameter of 5 μm or more are hardly formed.

(Example 2)
A positive electrode and a battery were produced in the same manner as in Example 1 except that the positive electrode active material shown in Table 1 was used, and the cycle characteristics of the battery were evaluated. The results are shown in Table 1.

(Example 3)
A positive electrode and a battery were produced in the same manner as in Example 1 except that the positive electrode active material used was shown in Table 1, and the cycle characteristics of the battery were evaluated. The results are shown in Table 1.

(Comparative Example 1)
A positive electrode and a battery were produced in the same manner as in Example 1 except that the positive electrode active material shown in Table 1 was used, and the cycle characteristics of the battery were evaluated. The results are shown in Table 1.

Further, with respect to the prepared positive electrode, the surface of the Al foil from which the active material layer was removed and the cross section of the positive electrode were observed by SEM. At any three locations on the surface of the Al foil, the number of recesses having the longest diameter of the opening of 5 μm or more per 100 μm square region exceeded five.

FIG. 3A shows an SEM image of the surface of the Al foil from which the active material layer has been removed, and FIG. 3B shows an SEM image of a cross section of the positive electrode. From the image of FIG. 3A, it can be seen that a large number of large recesses having an opening having a maximum diameter of 5 μm or more are formed on the surface of the Al foil. From the image of FIG. 3B, it can be seen that recesses deeper than the thickness (1 to 10 nm) of the oxide film on the Al foil are formed.

(Comparative Example 2)
A positive electrode and a battery were produced in the same manner as in Example 1 except that the positive electrode active material shown in Table 1 was used, and the cycle characteristics of the battery were evaluated. The results are shown in Table 1.

Further, with respect to the prepared positive electrode, the surface of the Al foil from which the active material layer was removed was observed by SEM. At any three locations on the surface of the Al foil, the number of recesses having the longest diameter of the opening of 5 μm or more per 100 μm square region exceeded five.

(Evaluation of cycle characteristics)
The obtained secondary battery was subjected to a cycle test under the following conditions.
CC-CV charging (upper limit voltage 4.15V, current 1C, CV time 3 hours) and CC discharging (lower limit voltage 2.5V, current 1C) were performed for 500 cycles at an ambient temperature of 45 ° C.
The ratio of the discharge capacity of the 500th cycle to the discharge capacity of the 1st cycle is defined as the capacity retention rate (%), the capacity retention rate of 70% or more is defined as good cycle characteristics: ○, and less than 70% is defined as poor cycle characteristics: ×. And said.

In Comparative Examples 1 and 2, the average particle size (D ₅₀ ) of the active material particles (overall) was large (larger than 10 μm), many large recesses (the longest diameter of the opening was 5 μm or more) were formed on the Al foil surface, and Due to the large amount of LiOH (0.5% by mass or more), the cycle characteristics were low.

On the other hand, in Example 1, although the amount of LiOH is large (0.5% by mass or more), the average particle size (D ₅₀ ) of the active material particles (overall) is as small as 10 μm or less, so that a large recess (opening) is formed on the surface of the Al foil. The longest diameter of 5 μm or more) was hardly formed, and as a result, the cycle characteristics were good.

Further, in Examples 2 and 3, the average particle size (D ₅₀ ) of the active material particles (overall) is large (larger than 10 μm), and a large recess may be formed on the Al foil surface, but the amount of LiOH is large. Since it was less than 0.5% by mass, the cycle characteristics were good.

Although the present invention has been described above with reference to the embodiments and examples, the present invention is not limited to the above embodiments and examples. Various changes that can be understood by those skilled in the art can be made to the structure and details of the present invention within the scope of the present invention.

This application claims priority on the basis of Japanese application Japanese Patent Application No. 2016-024969 filed on February 12, 2016 and incorporates all of its disclosures herein.

1 Positive electrode active material layer 2 Negative electrode active material layer 3 Positive electrode current collector 4 Negative electrode current collector 5 Separator 6 Laminated exterior body 7 Laminated exterior body 8 Negative electrode tab 9 Positive electrode tab

Claims (12)

An aluminum base material whose surface is covered with a film,
An electrode containing an active material layer formed on the film of the aluminum base material.
The active material layer contains active material particles and a binder and contains.
The active material particles contain at least particles of a lithium nickel-containing composite oxide having a layered crystal structure, and _{the average particle size (D 50} ) of the active material particles is 2 to 10 μm.
The active material layer contains LiOH, and the content of LiOH in the active material layer is 0.5% by mass or more and 2% by mass or less with respect to the whole of the active material particles.
The density of the active material layer is 2.5 to 3.5 g / cm ³ .
A plurality of recesses having a maximum opening diameter of 0.1 μm or more and 1 μm or less are formed in a square region having an arbitrary side of 100 μm on the surface of the aluminum base material on which the active material layer is formed.
An electrode for a lithium ion secondary battery that satisfies the following condition A.
Condition A: In the above surface of the active material layer formed aluminum substrate, in the area of a square of an arbitrary one side 100 [mu] m, the number of recesses longest diameter is not less than 5μm openings 5 pieces of not exceeding. The electrode according to claim 1, wherein the film has a thickness of 1 to 10 nm. The electrode according to claim 1 or 2, wherein the film contains an oxide. The electrode according to any one of claims 1 to 3 , wherein the active material particles further include particles of a lithium manganese-containing composite oxide having a spinel crystal structure. A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator between the negative electrode and the positive electrode, and an electrolytic solution, which are the electrodes according to any one of claims 1 to 4. A method for manufacturing an electrode for a lithium ion secondary battery, which comprises an aluminum base material whose surface is covered with a film and an active material layer formed on the film of the aluminum base material.
The process of forming a slurry containing active material particles, a binder, and a solvent,
Including the steps of applying the slurry onto the aluminum substrate, drying and pressing to form an active material layer.
The active material particles contain at least particles of a lithium nickel-containing composite oxide having a layered crystal structure.
On the surface of the aluminum base material on which the active material layer is formed, a product in which the number of recesses having a maximum opening diameter of 5 μm or more does not exceed 5 in an arbitrary square region of 100 μm on a side is determined to be a good product. A method for manufacturing an electrode for a lithium ion secondary battery, which includes a step of making an electrode. The production method according to claim 6 , wherein the film has a thickness of 1 to 10 nm. The production method according to claim 6 or 7 , wherein the film contains an oxide. A method for evaluating an electrode for a lithium ion secondary battery, which includes an aluminum base material whose surface is covered with a film and an active material layer formed on the film of the aluminum base material.
The active material layer contains active material particles and a binder and contains.
The active material particles contain at least particles of a lithium nickel-containing composite oxide having a layered crystal structure.
On the surface of the aluminum base material on which the active material layer is formed, a product in which the number of recesses having a maximum opening diameter of 5 μm or more does not exceed 5 in an arbitrary square region of 100 μm on a side is determined to be a good product. An evaluation method for electrodes for lithium-ion secondary batteries. The evaluation method according to claim 9 , wherein the thickness of the film is 1 to 10 nm. The evaluation method according to claim 9 or 10 , wherein the film contains an oxide. A method for manufacturing a lithium ion secondary battery containing a positive electrode, a negative electrode, a separator between the negative electrode and the positive electrode, and an electrolytic solution.
A method for manufacturing a lithium ion secondary battery, which comprises a step of forming a positive electrode by the manufacturing method according to any one of claims 6 to 8.

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