WO2015045719A1 - Positive electrode for stacked lithium ion secondary batteries - Google Patents

Abstract

Provided is a positive electrode for stacked lithium ion secondary batteries, which enables the production of a stacked lithium ion secondary battery having high long-term reliability. A positive electrode for stacked lithium ion secondary batteries, which is used for a stacked lithium ion secondary battery that is provided with a laminate obtained by laminating a plurality of positive electrodes and a plurality of negative electrodes respectively facing each other with separators and a nonaqueous electrolyte solution being interposed therebetween. The positive electrode is provided with a positive electrode active material layer containing positive electrode active material particles, and the ratio of the positive electrode active material particles having a crack among the positive electrode active material particles present in the outermost surface of the positive electrode active material layer is 20% or less.

Description

Stacked type lithium ion secondary battery positive electrode

This embodiment relates to a positive electrode for a stacked lithium ion secondary battery.

A positive electrode for a stacked lithium ion secondary battery is produced, for example, by the following method. After applying the slurry containing the positive electrode active material particles on the metal foil, the slurry is dried by hot air or infrared rays to volatilize the solvent. Then, this is pressed with a roller, the positive electrode active material layer is compressed and densified. By narrowing the interval between the positive electrode active material particles by pressing, the positive electrode resistance value can be lowered, and a positive electrode that can be applied to a stacked lithium ion secondary battery can be produced. In the pressing step, the positive electrode active material layer is pressed at a high pressure in order to increase the density, but the positive electrode active material particles are ruptured by contact between the positive electrode active material particles and the roller, or contact between the positive electrode active material particles. The above force is applied, and cracks may occur in some of the positive electrode active material particles.

Regarding the cracks in the positive electrode active material particles, there are precedent examples that are actively incorporated for the purpose of improving the characteristics of the secondary battery. For example, Patent Document 1 discloses that positive electrode active material particles composed of secondary particles formed by agglomeration of primary particles have cracks, thereby ensuring safety at the time of short-circuiting of the secondary battery. It is described that the elution of is facilitated to develop an internal short circuit safety mechanism. In this technique, the surface layer portion of the positive electrode active material particles has a larger distribution of metals (Mn, Al, Mg, Ca, Zr, B, W, Nb, Ta, In, Mo, and Sn) than the inside. The metal oxide film is formed on the surface to ensure safety at the time of short circuit. Although metal elution is suppressed by this metal oxide film at the time of a short circuit, elution of metal is facilitated by providing a crack.

Patent Document 2 discloses a method for producing positive electrode active material particles made of a nickel-alkali metal-containing composite oxide having cracks on the surface of the primary particles and a secondary battery to which the positive electrode active material particles are applied. By providing cracks in the positive electrode active material particles, volume change of the positive electrode active material particles due to Li in and out of the positive electrode active material particles in the charge / discharge cycle is suppressed, and a decrease in capacity in the charge / discharge cycle is prevented.

JP 2007-18985 A JP 2008-152923 A

Patent Documents 1 and 2 both state that cracks in the positive electrode active material particles are effective for promoting metal elution with respect to overcharge and suppressing volume expansion of the positive electrode active material particles accompanying the charge / discharge cycle.

However, the present inventors recognize that cracks in the positive electrode active material particles reduce the long-term reliability of the lithium ion secondary battery. In some cases, the surface of the positive electrode active material particles is coated with an oxide layer in order to prevent elution of metal elements. In this case, since the positive electrode active material particles have cracks, the inner surface of the positive electrode active material particles without coating is exposed, and the amount of elution of the metal elements constituting the positive electrode active material particles is reduced when the secondary battery is used for a long time. To increase. Further, in the crack, in the manufacturing process of the positive electrode active material particles, OH ions are desorbed from the alkali metal component contained in the positive electrode active material particles, and dissolved in the non-aqueous electrolyte solution to become moisture. Induces a decline. Moisture present in the non-aqueous electrolyte inside the lithium ion secondary battery reacts with a support salt containing Li, for example, LiPF ₆ (LiPF ₆ + H ₂ O → LiPF ₄ O + 2HF). As a result, an acid is generated in the non-aqueous electrolyte, and the metal elements constituting the positive electrode active material particles are eluted, thereby degrading the performance of the secondary battery. For example, when the positive electrode active material particles include lithium manganese oxide, the acid causes elution of Mn. When the eluted Mn adheres to the surface of the negative electrode, the entry and exit of Li in the negative electrode is hindered, and the capacity of the secondary battery is reduced. Also, in the positive electrode, Mn is detached from the crystal structure of the positive electrode active material, so that the crystal structure is broken and the temperature may be significantly increased during operation. In addition, HF can cause dissolution of the metals that make up the current collector.

On the other hand, in order to increase the density of the positive electrode active material layer for the purpose of reducing the impedance of the positive electrode for use in a stacked lithium ion secondary battery, the present inventors, for example, It has been experimentally confirmed that it is necessary to press the material particles at such a pressure as to cause cracks. The frequency of occurrence of cracks in the positive electrode active material particles due to the pressing process tends to vary depending on the position in the thickness direction of the positive electrode active material layer. On the surface of the positive electrode active material layer, the frequency of occurrence of cracks in the positive electrode active material particles is higher than that in the positive electrode active material layer. This can be confirmed by observing the surface and cross section of the positive electrode active material layer by SEM (Scanning Electron Microscope). Inside the positive electrode active material layer, there is a gap between the positive electrode active material particles before the pressing step, and the positive electrode active material particles move due to the press pressure. On the other hand, since the positive electrode active material particles present on the surface of the positive electrode active material layer come into contact with the roller in the pressing step, the movement of the positive electrode active material particles is restricted, and the positive electrode active material particles are directly pressed. The surface of the roller is made of metal, and the breaking strength of the positive electrode active material particles is lower than the breaking strength of the metal. Further, the positive electrode active material layer surface before the pressing process has irregularities due to individual positive electrode active material particles, and the positive electrode active material particles protruding in a convex shape are locally subjected to a high pressing pressure and cracks are generated. easy.

An object of the present embodiment is to provide a positive electrode for a laminated lithium ion secondary battery having high long-term reliability when a laminated lithium ion secondary battery is manufactured.

The positive electrode for a stacked lithium ion secondary battery according to this embodiment is a positive electrode for a stacked lithium ion secondary battery that includes a stacked body in which a plurality of positive electrodes and negative electrodes facing each other with a nonaqueous electrolyte and a separator interposed therebetween. There,
The positive electrode comprises a positive electrode active material layer containing positive electrode active material particles,
Of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, the proportion of the positive electrode active material particles having cracks is 20% or less.

The multilayer lithium ion secondary battery according to this embodiment includes the positive electrode for the multilayer lithium ion secondary battery.

The method for producing a laminated lithium ion secondary battery according to this embodiment includes a positive electrode for a laminated lithium ion secondary battery that includes a laminate in which a plurality of positive electrodes and negative electrodes facing each other with a nonaqueous electrolyte and a separator interposed therebetween. A manufacturing method of
After coating and drying a slurry containing positive electrode active material particles on a metal foil that is a positive electrode current collector, including a step of forming a positive electrode active material layer by pressing,
Of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, the proportion of the positive electrode active material particles having cracks is 20% or less.

According to the present embodiment, it is possible to provide a positive electrode for a stacked lithium ion secondary battery that has high long-term reliability when a stacked lithium ion secondary battery is manufactured.

It is sectional drawing which shows an example of the positive electrode for laminated | stacked lithium ion secondary batteries which concerns on this embodiment. It is sectional drawing which shows another example of the positive electrode for laminated lithium ion secondary batteries which concerns on this embodiment. It is sectional drawing which shows another example of the positive electrode for laminated lithium ion secondary batteries which concerns on this embodiment.

[Positive electrode for laminated lithium ion secondary battery]
In the positive electrode active material particles, there is LiOH that is used to form lithium metal oxide in the production process of the positive electrode active material particles, and if there are cracks in the positive electrode active material particles, OH ions are likely to be dissociated from the LiOH. , OH ion desorption increases. In particular, when the positive electrode active material particles are secondary particles formed by agglomeration of primary particles, the primary particles are dispersed by the pressing process, and the amount of desorption of OH ions existing inside the secondary particles is reduced. To increase. Inside the secondary battery, the OH ions become moisture and dissolve in the non-aqueous electrolyte. OH ions become moisture (H ₂ O) by the following reaction, for example. Thereby, the moisture content inside the secondary battery increases.

2OH ⁻ → H ₂ O + (1/2) O ₂ + 2e ⁻
In the case where cracks occur in the positive electrode active material particles due to the pressing process on the positive electrode, the present inventors detect the moisture content measurement (300 ° C.) by the Karl Fischer method of the positive electrode after the pressing process compared to before the pressing process. It has been confirmed that the amount of water to be increased. It has also been confirmed that when the press pressure is changed to increase the frequency of occurrence of cracks in the positive electrode active material particles, the amount of water detected by the water content measurement by the Karl Fischer method of the positive electrode increases.

Furthermore, when cracks are present in the positive electrode active material particles, the surface area of the positive electrode active material particles increases and the moisture adsorption sites increase. In particular, when the positive electrode active material particles are secondary particles formed by agglomeration of primary particles, the surface area of the positive electrode active material particles greatly increases when the primary particles are dispersed by the pressing process. When the surface area increases, the moisture adsorption amount increases when the positive electrode is exposed to the atmosphere when storing the positive electrode. In the case where the positive electrode active material particles have cracks, the present inventors have measured the moisture content by the Karl Fischer method of the positive electrode when the positive electrode is left in the atmosphere as compared to the case where no crack is generated ( It has been confirmed that the amount of water detected at 300 ° C. increases. Cracks in the positive electrode active material particles may occur on the surface and inside the positive electrode active material layer. In particular, when the positive electrode active material particles present on the outermost surface of the positive electrode active material layer have cracks, two cracks are caused by the positive electrode. The contribution to the increase in water content in the secondary battery is large.

The positive electrode for a stacked lithium ion secondary battery according to this embodiment is a positive electrode for a stacked lithium ion secondary battery that includes a stacked body in which a plurality of positive electrodes and negative electrodes facing each other with a nonaqueous electrolyte and a separator interposed therebetween. The positive electrode includes a positive electrode active material layer containing positive electrode active material particles, and among the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, a ratio of the positive electrode active material particles having cracks is 20 % Or less. In this embodiment, the following effects are acquired when the ratio of the positive electrode active material particle which has a crack among the positive electrode active material particles which exist in the outermost surface of a positive electrode active material layer is 20% or less.

1st effect can suppress generation | occurrence | production of the water | moisture content by OH ion detach | desorbing from LiOH contained in a positive electrode active material particle. As a result, the moisture content inside the stacked lithium ion secondary battery can be reduced. The second effect is that even when the positive electrode is left in the air, the amount of water adsorbed on the positive electrode active material particles can be reduced. Since the ratio of the positive electrode active material particles having cracks is low, the entire surface area of the positive electrode active material particles is small, and there are few external moisture adsorption sites. Thereby, the amount of moisture adsorbed by the positive electrode active material particles is reduced.

Thus, since the amount of moisture brought into the secondary battery by the positive electrode is reduced, generation of HF due to the reaction of the moisture inside the secondary battery with LiPF ₆ in the non-aqueous electrolyte can be prevented. As a result, it is possible to prevent elution of Mn from the lithium manganese oxide contained in the positive electrode active material particles, which can occur when HF is generated, for example. Moreover, dissolution of the metal used as the current collector of the electrode can be prevented. As a result, when the positive electrode according to the present embodiment is used in a laminated lithium ion secondary battery, an effect of improving long-term reliability such as an improvement in capacity retention rate can be obtained.

In addition, as described above, when the positive electrode active material particles present on the outermost surface of the positive electrode active material layer have cracks, the positive electrode active material particles are exposed to the atmosphere and have a large amount of moisture adsorption. The contribution to the increase in water content in the secondary battery is large. In the present embodiment, the proportion of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer having cracks is 20% or less, so that the amount of water inside the secondary battery can be significantly reduced.

The positive electrode active material particles present inside the positive electrode active material layer are also exposed to the atmosphere, but due to adhesion of the binder around the positive electrode active material particles, contact with other positive electrode active material particles, Compared with the positive electrode active material particles present on the outermost surface, the contact area with the atmosphere is small. In addition, since the gap between the positive electrode active material particles in the positive electrode active material layer is narrow, the air entering the positive electrode active material layer is stagnant compared to the surface of the positive electrode active material layer. Accordingly, the amount of moisture supplied from the atmosphere to the positive electrode active material particles is small, and the increase in the amount of water due to the cracks in the positive electrode active material particles present inside the positive electrode active material layer is This is less than the increase in the amount of water due to the positive electrode active material particles present on the outermost surface having cracks. In the present embodiment, the proportion of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer having cracks is 20% or less, so that the amount of water in the positive electrode is effectively reduced, and the long-term performance of the secondary battery is increased. Reliability can be improved.

An example of a positive electrode for a stacked lithium ion secondary battery according to the present embodiment is shown in FIG. The positive electrode shown in FIG. 1 includes a positive electrode current collector 1 and a positive electrode active material layer 2 formed on the positive electrode current collector 1. The positive electrode active material particles present on the outermost surface of the positive electrode active material layer 2 are positive electrode active material particles 3 having no cracks. On the other hand, the positive electrode active material particles present inside the positive electrode active material layer 2 include positive electrode active material particles 3 having no cracks and positive electrode active material particles 4 having cracks. In the present embodiment, “positive electrode active material particles present on the outermost surface of the positive electrode active material layer” means positive electrode active material particles that can be observed when the positive electrode surface is observed at 2500 times with an electron microscope. Show. The “positive electrode active material particles present inside the positive electrode active material layer” refers to positive electrode active material particles other than the “positive electrode active material particles present on the outermost surface of the positive electrode active material layer”. Further, in this embodiment, “having cracks” indicates a state in which one or more cracks have entered at least a part of the positive electrode active material particles. When the positive electrode active material particles are secondary particles formed by aggregation of primary particles, “having cracks” indicates that the primary particles are separated from each other and that the primary particles themselves have cracks. On the other hand, when the positive electrode active material particles are formed of primary particles, “having cracks” means that the primary particles themselves have cracks. Moreover, although the positive electrode active material particle which exists in the positive electrode active material layer 2 of the positive electrode shown by FIG. 1 contains the positive electrode active material particle 4 which has a crack, positive electrode active material like the positive electrode shown by FIG. The positive electrode active material particles present inside the layer 2 may not include the positive electrode active material particles 4 having cracks.

In the present embodiment, of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, the proportion of the positive electrode active material particles having cracks is 20% or less. When the proportion is 20% or less, the amount of water in the positive electrode can be effectively reduced. The ratio is more preferably 15% or less, further preferably 10% or less, and particularly preferably 5% or less. The ratio is preferably small, and may be 0%. As described above, in order to reduce the amount of moisture brought into the secondary battery by the positive electrode, it is desirable that the positive electrode active material particles present on the outermost surface of the positive electrode active material layer have no cracks. However, in order to reduce the resistance value of the positive electrode, it is required to increase the density of the positive electrode active material layer by narrowing the interval between the positive electrode active material particles. In order to realize the electrical resistance required for application to the stacked lithium ion secondary battery, for example, a pressing process can be performed. At this time, in order to narrow the interval between the positive electrode active material particles, the positive electrode active material particles are pressed with a pressure at which cracks are generated in some positive electrode active material particles present on the outermost surface due to contact between the positive electrode active material particles. It is technically difficult to devise a pressing process so that cracks due to collision between the positive electrode active material particles do not occur at all while narrowing the interval between the positive electrode active material particles.

The ratio of the positive electrode active material particles having cracks among the positive electrode active material particles present on the outermost surface of the positive electrode active material layer is a value measured by the following method. The surface of the positive electrode is observed with an electron microscope at 2500 times, and the total number of positive electrode active material particles and the number of positive electrode active material particles with cracks are determined with respect to the positive electrode active material particles present in a region of 48 μm × 36 μm. Count and calculate the ratio.

The positive electrode active material layer is formed, for example, by applying a slurry containing positive electrode active material particles on a metal foil, which is a positive electrode current collector, and drying, followed by pressing. A specific forming method will be described later.

The positive active material particles are Li _x Mn _{2 + y} M1 _z O _{4 + α} (M1 is selected from the group consisting of B, Sn, Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mg and Ga) 1 ≦ x ≦ 1.5, −1 ≦ y ≦ 0, 0 ≦ z ≦ 0.5, −0.1 ≦ α ≦ 0.1), and Li _p Ni ₂₋ Including at least one of _q O _{2 + β} (0 <p ≦ 1, 0 <q ≦ 1, −0.1 ≦ β ≦ 0.1) is preferable from the viewpoint of improving the capacity retention rate. x is preferably 1 ≦ x ≦ 1.3, and more preferably 1 ≦ x ≦ 1.2. y is preferably −0.5 ≦ y ≦ 0, and more preferably −0.3 ≦ y ≦ 0. z is preferably 0 ≦ z ≦ 0.3, and more preferably 0 ≦ z ≦ 0.2. α is preferably −0.07 ≦ α ≦ 0.07, and more preferably −0.05 ≦ α ≦ 0.05. p is preferably 0.03 ≦ p ≦ 1, and more preferably 0.05 ≦ p ≦ 1. q is preferably 0.03 ≦ q ≦ 1, and more preferably 0.05 ≦ q ≦ 1. β is preferably −0.07 ≦ β ≦ 0.07, and more preferably −0.05 ≦ β ≦ 0.05.

The positive electrode active material particles preferably include the Li _x Mn _{2 + y} M1 _z O _{4 + α} and the Li _p Ni _{2 -q} O _{2 + β} from the viewpoint of improving the capacity retention rate. Wherein contained in the positive electrode active material particles _{Li x Mn 2 + y M1 z} O 4 + α and the _Li p _{Ni 2-q} ratio of the _{O 2 + β (Li x Mn} 2 + y M1 z O 4 + α: Li p Ni 2-q O 2 + β) is Although not particularly limited, the mass ratio is preferably 9: 1 to 6: 4.

Some of the positive electrode active material particles present in the positive electrode active material layer may have cracks. In this embodiment, since the crack of the positive electrode active material particles existing on the outermost surface of the positive electrode active material layer contributes greatly to the increase in the amount of moisture, the crack of the positive electrode active material particles existing on the outermost surface of the positive electrode active material layer is reduced. The existing ratio is 20% or less. On the other hand, when performing, for example, a pressing process in order to increase the density of the positive electrode active material layer, the positive electrode active material particles are brought into contact with each other inside the positive electrode active material layer by the press pressure, and the pressure is applied to each other. Such positive electrode active material particles may crack. However, since the crack of the positive electrode active material particles existing inside the positive electrode active material layer has a smaller influence on the increase in the amount of water than the crack of the positive electrode active material particles existing on the outermost surface of the positive electrode active material layer, In the form, the positive electrode active material particles present inside the positive electrode active material layer may have cracks.

Note that the present embodiment includes a case where the positive electrode active material particles present in the positive electrode active material layer have no cracks. In order to suppress an increase in the amount of water inside the secondary battery, it is most effective that the positive electrode active material particles have no cracks in the entire positive electrode active material layer. However, in order to produce such a positive electrode, it is required to increase the density of the positive electrode active material layer and reduce the electrode resistance without generating cracks in the positive electrode active material particles. For example, there are a method in which the press pressure is increased stepwise, a method in which heat is applied and the positive electrode active material layer before pressing is softened, and the like is technically difficult.

[Method for producing positive electrode for laminated lithium ion secondary battery]
The method for producing a positive electrode for a laminated lithium ion secondary battery according to this embodiment is for a laminated lithium ion secondary battery comprising a laminate in which a plurality of positive electrodes and negative electrodes facing each other with a nonaqueous electrolyte and a separator interposed therebetween are laminated. A method for producing a positive electrode comprising: forming a positive electrode active material layer by pressing a slurry containing positive electrode active material particles on a metal foil as a positive electrode current collector; Of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, the proportion of the positive electrode active material particles having cracks is 20% or less. In the method according to the present embodiment, the positive electrode for the laminated lithium ion secondary battery according to the present embodiment can be suitably manufactured.

The method according to the present embodiment includes a step of forming a positive electrode active material layer by applying a slurry containing positive electrode active material particles on a metal foil, which is a positive electrode current collector, and drying, followed by pressing. An aluminum foil or the like can be used as the metal foil that is the positive electrode current collector. The slurry containing the positive electrode active material particles can be prepared, for example, by adding the positive electrode active material particles, the conductive auxiliary agent, and the binder to a solvent and mixing them. Carbon black or the like can be used as the conductive auxiliary agent. As the binder, polyvinylidene fluoride (PVDF) or the like can be used. As the solvent, N-methyl-2-pyrrolidone (NMP) or the like can be used. The thickness of the slurry applied on the metal foil can be set to 100 to 120 μm, for example. The drying method is not particularly limited, and for example, it can be dried with hot air.

The press can be pressed using, for example, a roller press. The pressure at the time of pressing depends on the density of the target positive electrode active material layer, but can be, for example, 100 to 300 MPa. The thickness of the positive electrode active material layer after pressing can be, for example, 70 to 80 μm. The density of the positive electrode active material layer after pressing is preferably 2.5 to 3.5 g / ml. The density can be adjusted by changing the gap length of the roller for pressing. The application, drying, and pressing of the slurry may be performed on one side of the metal foil that is the positive electrode current collector, or may be performed on both sides. In addition, two or more positive electrode active material layers may be formed by performing application, drying, and pressing of the slurry twice or more on one side or both sides of the metal foil. In this case, after applying the slurry, another slurry may be further applied without drying, and then drying and pressing may be performed.

The step of forming the positive electrode active material layer is a step shown below from the viewpoint of setting the ratio of positive electrode active material particles having cracks to 20% or less of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer. It is preferable that

In the method according to the present embodiment, in the step of forming the positive electrode active material layer, it is preferable to perform pressing using a roller made of a material having a lower elastic modulus than the positive electrode active material particles. By using a material having a lower elastic modulus than the positive electrode active material particles as the material of at least the roller surface, that is, the roller surface that is in contact with the surface of the positive electrode active material layer, the positive electrode active material layer has a lower modulus than the positive electrode active material layer surface. The pressure to the inside increases and the collapse of the positive electrode active material particles is suppressed on the outermost surface of the positive electrode active material layer. On the other hand, the pressure inside the positive electrode active material layer becomes high, and cracks may occur in the positive electrode active material particles existing inside the positive electrode active material layer. For example, when the positive electrode active material particles include the Li _x Mn _{2 + y} M1 _z O _{4 + α} and the Li _p Ni _{2 -q} O _{2 + β} , an acrylic resin, rubber, or the like can be used as the material of the roller. The elastic modulus is a value measured by a tensile test.

In the method according to the present embodiment, in the step of forming the positive electrode active material layer, after applying a slurry containing the first positive electrode active material particles on the metal foil that is the positive electrode current collector, It is preferable that a slurry containing second positive electrode active material particles having an average particle diameter smaller than that of the positive electrode active material particles is further applied, dried, and then pressed. In this case, since the average particle diameter of the positive electrode active material particles existing on the outermost surface of the positive electrode active material layer is small, the unevenness generated by the positive electrode active material particles on the surface of the positive electrode active material layer is small before the pressing step, and the positive electrode The surface of the active material layer is smooth. For this reason, in the pressing process, the pressure applied from the roller to the positive electrode active material particles existing on the outermost surface of the positive electrode active material layer becomes uniform, and locally high pressure can be avoided from being applied to the individual positive electrode active material particles. The generation of cracks is suppressed. For example, the average particle diameter of the first positive electrode active material particles can be 5 μm or more, and the average particle diameter of the second positive electrode active material particles can be 2 μm or less. Moreover, the thickness of the positive electrode active material layer after applying the slurry containing the first positive electrode active material particles was applied to the slurry containing the first positive electrode active material particles and the slurry containing the second positive electrode active material particles. It is preferably 80 to 90% of the thickness of the subsequent positive electrode active material layer. The average particle diameter (D50) of the positive electrode active material particles is a median value obtained from a particle size distribution measured by a laser diffraction or scattering method. Also, a slurry containing the first positive electrode active material particles is applied onto the metal foil, dried, and a slurry containing the second positive electrode active material particles having an average particle diameter smaller than that of the first positive electrode active material particles is applied. It is also possible to press after drying. However, in this case, since the adhesion between the two layers is low, peeling may occur at the interface between the two layers.

In the method according to the present embodiment, in the step of forming the positive electrode active material layer, after applying a slurry containing the first positive electrode active material particles on the metal foil that is the positive electrode current collector, It is preferable that a slurry containing second positive electrode active material particles having a compressive strength larger than that of the positive electrode active material particles is further applied, dried, and then pressed. In this case, since the compressive strength of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer is high, cracks are unlikely to occur in the positive electrode active material particles present on the outermost surface of the positive electrode active material in contact with the roller in the pressing step. . In addition, since the positive electrode active material particles having high compressive strength have a high density, the permeability of the electrolytic solution into the positive electrode active material particles is low, and the efficiency of Li ion entry / exit into the positive electrode active material particles may be reduced. is there. However, in this embodiment, since the compressive strength of the positive electrode active material particles existing on the outermost surface of the positive electrode active material layer is increased, the influence is small. For example, the compressive strength of the first positive electrode active material particles can be 30 MPa or less, and the average particle diameter of the second positive electrode active material particles can be 40 MPa or more. The compressive strength of the positive electrode active material particles is a value calculated from a value measured by a compression tester.

[Stacked lithium ion secondary battery]
The stacked lithium ion secondary battery according to the present embodiment includes the positive electrode for the stacked lithium ion secondary battery according to the present embodiment. The laminated lithium ion secondary battery according to the present embodiment is a laminate in which a plurality of laminated lithium ion secondary battery positive electrodes and negative electrodes according to the present embodiment are opposed to each other with a nonaqueous electrolyte and a separator interposed therebetween. Prepare the body.

As the non-aqueous electrolyte, for example, EC (ethylene carbonate), DEC (diethylene carbonate), DMC (dimethylene carbonate) and the like can be used. These may use 1 type and may use 2 or more types together. The non-aqueous electrolyte can contain a lithium salt such as LiPF ₆ as a supporting salt. As a material for the separator, for example, polypropylene or the like can be used.

As the negative electrode, for example, a negative electrode having a negative electrode active material layer formed on a negative electrode current collector can be used. As the negative electrode current collector, for example, a copper foil or the like can be used. Carbon or the like can be used as the negative electrode active material contained in the negative electrode active material layer. The negative electrode can be obtained, for example, by forming a negative electrode active material layer containing a negative electrode active material and a binder on a negative electrode current collector. As the binder, for example, polyvinylidene fluoride (PVDF) can be used.

In the laminated lithium ion secondary battery according to the present embodiment, for example, a separator is disposed between the positive electrode for a laminated lithium ion secondary battery according to the present embodiment and the negative electrode, and a plurality of the separators are laminated to form a laminated body. Then, the laminate is inserted into the exterior body, a nonaqueous electrolytic solution containing a supporting salt is injected into the exterior body, and the exterior body is sealed under reduced pressure. As the exterior body, for example, a laminate film can be used.

Hereinafter, examples of the present embodiment will be described, but the present embodiment is not limited thereto. The moisture content of the positive electrode was measured at 300 ° C. by the Karl Fischer method. The elastic modulus was measured by a tensile test. The average particle diameter (D50) of the positive electrode active material particles was measured by laser diffraction and scattering methods. The compressive strength of the positive electrode active material particles was measured with a compression tester. The ratio of the positive electrode active material particles having cracks among the positive electrode active material particles present on the outermost surface of the positive electrode active material layer was measured by the following method. The surface of the positive electrode produced by an electron microscope is observed at a magnification of 2500, and the total number of positive electrode active material particles and the number of positive electrode active material particles with cracks are present with respect to the positive electrode active material particles present in a 48 μm × 36 μm region. And the ratio was calculated.

[Example 1]
As the positive electrode active material particles, a mixture of LiMn ₂ O ₄ and LiNiO ₂ (LiMn ₂ O ₄ : LiNiO ₂ = 8: 2 (mass ratio)) was used. To this mixture, carbon black as a conductive auxiliary agent and polyvinylidene fluoride (PVDF) as a binder were added and stirred with a mixer and mixed with N-methyl-2-pyrrolidone (NMP) to prepare a slurry. . The slurry was applied to the surface of a positive electrode current collector, which was an aluminum (Al) foil having a thickness of 20 μm, to a thickness of 100 μm and dried with hot air. The slurry application and drying steps were performed twice on both sides of the positive electrode current collector. The positive electrode active material layer formed on both surfaces of the positive electrode current collector was pressed by a roller press (pressing pressure: 200 MPa), the thickness of the positive electrode active material layer was 80 μm, and the positive electrode active material layer was densified. This obtained the positive electrode in a present Example.

An acrylic resin having a lower elastic modulus than that of the positive electrode active material particles was used as a material constituting the surface of the roller used in the press by the roller press in contact with the positive electrode active material layer. As a result, the generation of cracks in the positive electrode active material particles present on the outermost surface of the positive electrode active material layer due to pressing is suppressed, and among the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, positive electrode active material particles having cracks The percentage of was 19%. On the other hand, the positive electrode active material layer has a pressure necessary for realizing high density inside, so that the positive electrode active material particles having the same mechanical strength are brought into contact with each other and compressed, and the positive electrode active material layer Cracks occurred in some of the positive electrode active material particles present in the interior of the glass.

A negative electrode active material layer containing a negative electrode active material mainly composed of carbon and PVDF as a binder was formed on a copper foil as a negative electrode current collector to obtain a negative electrode. A separator made of polypropylene was disposed between the positive electrode and the negative electrode, and the negative electrode, the separator, and the unit layer of the positive electrode were laminated several times. The obtained laminate was inserted into an exterior body made of a laminate film, and a solution of LiPF ₆ as a supporting salt dissolved in a nonaqueous electrolytic solution made of EC (ethylene carbonate) and DEC (diethylene carbonate) was injected. The inside of the body was sealed in a vacuum state. As a result, a stacked lithium ion secondary battery in this example was obtained.

In order to investigate the long-term reliability of the laminated lithium ion secondary battery, a charge / discharge cycle test was conducted. The charge / discharge cycle test was conducted by the following method. A constant current charge / discharge cycle of 1 C was performed at an upper limit voltage of 4.1 V and a lower limit voltage of 3 V. The results are shown in Table 1. In Table 1, the moisture content and the capacity retention rate after 1000 cycles are the values of Example 1, with the measured value of Comparative Example 1 described later normalized to “1”.

In this example, since the generation of cracks in the positive electrode active material particles present on the outermost surface of the positive electrode active material layer is suppressed, the amount of water generated by the positive electrode is reduced inside the stacked lithium ion secondary battery. Thereby, the performance fall of the secondary battery by water was prevented, and the long-term reliability of the secondary battery improved.

[Example 2]
Example 1 except that a mixture of LiMn ₂ O ₄ and LiNiO ₂ (LiMn ₂ O ₄ : LiNiO ₂ = 8: 2 (mass ratio), D50 = 5 to 30 μm) was used as the first positive electrode active material particles. A slurry 1 was prepared in the same manner as in 1. Further, Examples were used except that a mixture of LiMn ₂ O ₄ and LiNiO ₂ (LiMn ₂ O ₄ : LiNiO ₂ = 8: 2 (mass ratio), D50 ≦ 1 μm) was used as the second positive electrode active material particles. A slurry 2 was prepared in the same manner as in 1. Slurry 1 was applied to the surface of a positive electrode current collector, which is an aluminum (Al) foil having a thickness of 20 μm, to a thickness of 30 μm, and then slurry 2 was applied to a thickness of 70 μm and dried with hot air. At this time, as shown in FIG. 3, the positive electrode active material layer includes a first positive electrode active material layer 5 and a second positive electrode active material layer 6. The steps of applying and drying the slurries 1 and 2 were also performed on the other surface of the positive electrode current collector. The positive electrode active material layer formed on both surfaces of the positive electrode current collector was pressed by a roller press, so that the thickness of the positive electrode active material layer was 80 μm and the positive electrode active material layer was densified. This obtained the positive electrode in a present Example. Of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, the proportion of positive electrode active material particles having cracks was 19%. In addition, the positive electrode active material particles present inside the positive electrode active material layer were cracked in part due to contact between the positive electrode active material particles during pressing. Thereafter, a stacked lithium ion secondary battery was obtained in the same manner as in Example 1.

In this example, the average particle size of the second positive electrode active material particles present on the outermost surface of the positive electrode active material layer is larger than the average particle size of the first positive electrode active material particles present inside the positive electrode active material layer. Since it was small, the unevenness | corrugation by the positive electrode active material particle of the positive electrode active material layer surface became small. Therefore, when pressing with a roller press, the pressure applied to the surface of the positive electrode active material layer from the roller becomes uniform, and the occurrence of cracks in the second positive electrode active material particles present on the outermost surface of the positive electrode active material layer can be suppressed. It was. As a result, the amount of moisture generated by the positive electrode inside the stacked lithium ion secondary battery is reduced, and the performance of the secondary battery can be prevented from being deteriorated by water, so the long-term reliability of the secondary battery is improved.

[Example 3]
Example 1 except that a mixture of LiMn ₂ O ₄ and LiNiO ₂ (LiMn ₂ O ₄ : LiNiO ₂ = 8: 2 (mass ratio), compressive strength: 20 MPa or less) was used as the first positive electrode active material particles. A slurry 1 was prepared in the same manner as in 1. Further, as the second positive electrode active material particles, a mixture of LiMn ₂ O ₄ and LiNiO ₂ (LiMn ₂ O ₄ : LiNiO ₂ = 8: 2 (mass ratio), compressive strength: 50 MPa or more) was used. A slurry 2 was prepared in the same manner as in Example 1. A laminated lithium ion secondary battery including a positive electrode and the positive electrode was produced in the same manner as in Example 2 except that these slurries 1 and 2 were used. Of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, the proportion of positive electrode active material particles having cracks was 19%. In addition, the positive electrode active material particles present inside the positive electrode active material layer were cracked in part due to contact between the positive electrode active material particles during pressing.

In this example, since positive electrode active material particles having high mechanical strength were used as the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, generation of cracks in the positive electrode active material particles was suppressed in the pressing step. I was able to. As a result, the amount of moisture generated by the positive electrode inside the stacked lithium ion secondary battery is reduced, and the performance of the secondary battery can be prevented from being deteriorated by water, so the long-term reliability of the secondary battery is improved.

[Comparative Example 1]
The same as in Example 1 except that a material having a higher elastic modulus than the positive electrode active material particles (SUS) is used as the material constituting the surface of the roller used in the press by the roller press in contact with the positive electrode active material layer. A positive electrode was prepared. Of the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, the proportion of positive electrode active material particles having cracks was 40%. In addition, the positive electrode active material particles present inside the positive electrode active material layer were cracked in part due to contact between the positive electrode active material particles during pressing. Thereafter, a stacked lithium ion secondary battery was obtained in the same manner as in Example 1, and a charge / discharge cycle test was performed. The results are shown in Table 1.

In this comparative example, since many cracks occurred in the positive electrode active material particles present on the outermost surface of the positive electrode active material layer, the amount of water generated by the positive electrode increased in the laminated lithium ion secondary battery. Thereby, the performance of the secondary battery was lowered by water, and the long-term reliability of the secondary battery was lowered.

This application claims priority based on Japanese Patent Application No. 2013-199444 filed on September 26, 2013, the entire disclosure of which is incorporated herein.

As mentioned above, although this invention was demonstrated with reference to embodiment and an Example, this invention is not limited to the said embodiment and Example. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

The multilayer lithium ion secondary battery according to the present embodiment can be used for multilayer lithium ion secondary batteries that require long-term reliability because performance degradation in charge / discharge cycles is suppressed.

DESCRIPTION OF SYMBOLS 1 Positive electrode collector 2 Positive electrode active material layer 3 Positive electrode active material particle which does not have a crack 4 Positive electrode active material particle 5 which has a crack 1st positive electrode active material layer 6 2nd positive electrode active material layer

Claims (9)

A positive electrode for a laminated lithium ion secondary battery comprising a laminate in which a plurality of positive electrodes and negative electrodes facing each other with a non-aqueous electrolyte and a separator interposed therebetween,
The positive electrode comprises a positive electrode active material layer containing positive electrode active material particles,
A positive electrode for a stacked lithium ion secondary battery, wherein a ratio of the positive electrode active material particles having cracks to the positive electrode active material particles present on the outermost surface of the positive electrode active material layer is 20% or less. 2. The stacked lithium ion secondary according to claim 1, wherein the positive electrode active material layer is formed by applying a slurry containing positive electrode active material particles on a metal foil that is a positive electrode current collector, drying, and then pressing the slurry. Battery positive electrode. 3. The positive electrode for a stacked lithium ion secondary battery according to claim 1, wherein some of the positive electrode active material particles present inside the positive electrode active material layer have cracks. The positive electrode active material particles are selected from the group consisting of Li _x Mn _{2 + y} M1 _z O _{4 + α} (M1 is B, Sn, Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mg, and Ga). 1 ≦ x ≦ 1.5, −1 ≦ y ≦ 0, 0 ≦ z ≦ 0.5, −0.1 ≦ α ≦ 0.1), and Li _p Ni ₂₋ 4. The stacked lithium according to claim 1, comprising at least one of _q O _{2 + β} (0 <p ≦ 1, 0 <q ≦ 1, −0.1 ≦ β ≦ 0.1). Positive electrode for ion secondary battery. A multilayer lithium ion secondary battery comprising the positive electrode for the multilayer lithium ion secondary battery according to any one of claims 1 to 4. A method for producing a positive electrode for a laminated lithium ion secondary battery comprising a laminate in which a plurality of positive electrodes and negative electrodes facing each other with a non-aqueous electrolyte and a separator interposed therebetween,
After coating and drying a slurry containing positive electrode active material particles on a metal foil that is a positive electrode current collector, including a step of forming a positive electrode active material layer by pressing,
The manufacturing method of the positive electrode for laminated lithium ion secondary batteries whose ratio of the said positive electrode active material particle which has a crack among the said positive electrode active material particles which exist in the outermost surface of the said positive electrode active material layer is 20% or less. The positive electrode for a stacked lithium ion secondary battery according to claim 6, wherein in the step of forming the positive electrode active material layer, pressing is performed using a roller composed of a material having a lower elastic modulus than the positive electrode active material particles. Production method. In the step of forming the positive electrode active material layer, after applying a slurry containing the first positive electrode active material particles on the metal foil that is the positive electrode current collector, the average particle diameter is larger than that of the first positive electrode active material particles. The method for producing a positive electrode for a stacked lithium ion secondary battery according to claim 6, wherein a slurry containing small second positive electrode active material particles is further applied, dried and then pressed. In the step of forming the positive electrode active material layer, after applying a slurry containing the first positive electrode active material particles on the metal foil as the positive electrode current collector, the compressive strength is larger than that of the first positive electrode active material particles The method for producing a positive electrode for a stacked lithium ion secondary battery according to claim 6, wherein the slurry containing the second positive electrode active material particles is further applied, dried, and then pressed.

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