JP7371186B2 - Positive electrode active material layer and secondary battery - Google Patents

Description

One aspect of the present invention relates to a product, a method, or a manufacturing method. Alternatively, the present invention provides a process,
It relates to machines, manufactures, or compositions of matter.
One embodiment of the present invention relates to a semiconductor device, a display device, a light emitting device, a power storage device, a lighting device, an electronic device, or a manufacturing method thereof. In particular, the present invention relates to a positive electrode active material that can be used in a secondary battery, a secondary battery, and an electronic device having a secondary battery.

Note that in this specification, a power storage device refers to elements and devices in general that have a power storage function. Examples include storage batteries (also referred to as secondary batteries) such as lithium ion secondary batteries, lithium ion capacitors, electric double layer capacitors, and the like.

Further, in this specification, electronic equipment refers to all devices having a power storage device, and an electro-optical device having a power storage device, an information terminal device having a power storage device, etc. are all electronic devices.

In recent years, various power storage devices, such as lithium ion secondary batteries, lithium ion capacitors, and air batteries, have been actively developed. In particular, lithium ion secondary batteries with high output and high energy density are used in mobile information terminals such as mobile phones, smartphones, tablets, and notebook computers, portable music players, digital cameras, medical equipment, and hybrid vehicles (
With the development of the semiconductor industry, the demand for next-generation clean energy vehicles such as HEV), electric vehicles (EV), and plug-in hybrid vehicles (PHEV) is rapidly expanding, and the demand for these vehicles is rapidly expanding as a source of rechargeable energy. It has become indispensable in the information society of Japan.

The characteristics required of lithium-ion secondary batteries include higher energy density,
These include improvements in cycle characteristics, safety in various operating environments, and long-term reliability.

Therefore, improvements to positive electrode active materials are being considered with the aim of improving cycle characteristics and increasing capacity of lithium ion secondary batteries. (Patent Document 1 and Patent Document 2)

Japanese Patent Application Publication No. 2012-018914 Japanese Patent Application Publication No. 2015-201432

Lithium ion secondary batteries and positive electrode active material layers used therein still have room for improvement in various aspects such as charge/discharge characteristics, cycle characteristics, reliability, safety, and cost.

Lithium ion secondary batteries are required to have higher energy density, which further increases the discharge capacity (also referred to as volumetric capacity density) per unit volume of the positive electrode plate. High electrode density is required for positive electrodes to obtain high energy density lithium ion secondary batteries. Further, when the average particle size of the positive electrode active material is large, high electrode density can be obtained. Furthermore, in order to obtain a high electrode density, pressing may be performed. A high electrode density is determined by the tap density or compressed density of the active material, and the compressed density has a particularly high correlation with the electrode density. In this specification, true density means
It is the value obtained by dividing the mass by the volume of the object itself, excluding the surface and internal pores.Tapped density is the bulk density measured by packing powder into a container, vibrating it, and filling it. The density may be lower than that.

On the other hand, when the electrode density was increased by pressing, the battery capacity during charge and discharge cycles tended to decrease.

An object of one embodiment of the present invention is to provide a secondary battery having a positive electrode active material layer with high electrode density.

Alternatively, an object of one embodiment of the present invention is to provide a secondary battery in which a decrease in battery capacity during charge/discharge cycles is suppressed. Alternatively, an object of one embodiment of the present invention is to provide a high-capacity secondary battery. Alternatively, an object of one embodiment of the present invention is to provide a secondary battery with excellent charge/discharge characteristics. Alternatively, an object of one embodiment of the present invention is to provide a secondary battery with high safety or reliability.

Alternatively, an object of one embodiment of the present invention is to provide a novel secondary battery or a method for manufacturing the same.

Note that the description of these issues does not preclude the existence of other issues. Note that one embodiment of the present invention does not need to solve all of these problems. Note that problems other than these can be extracted from the description, drawings, and claims.

In order to achieve the above object, one embodiment of the present invention uses a carbon material with a small specific surface area, specifically, fibers such as carbon fibers with a size of less than 60 m ² /g as a conductive additive to be mixed with a positive electrode active material containing lithium. A shaped material (for example, carbon nanotube (also called CNT)) is used. Since the specific surface area is as small as less than 60 m ² /g, there is little aggregation and friction with itself and the active material is small, so a uniform high electrode density can be obtained after press working. Further, in addition to carbon fiber (specific surface area of less than 60 m ² /g), a conductive additive may be used as a mixture of carbon black having a specific surface area of 60 m ² /g or more. Carbon fiber can disperse pressure and improve strength. Carbon fibers are particularly preferable because they can efficiently form conductive paths even in small amounts, so there is no need to reduce the amount of active material supported.

Further, the carbon fiber having a specific surface area of less than 60 m ² /g is a structure that maintains the shape of the positive electrode active material layer, and also functions as a buffer material in the pressing process. The specific surface area of the positive electrode active material is 60 m ² /
The electrode density of the positive electrode active material layer can be made to be 4.0 g/cm ³ or more by applying a slurry containing carbon fibers of less than 3 g, firing, and pressing at a pressure of 1050 kN/m or more.

Furthermore, by pressing at a pressure of 1450 kN/m or more, the electrode density of the positive electrode active material layer was increased to 4.
It can be 3 g/cm ³ or more.

On the other hand, when only fine particles such as carbon black, typically acetylene black, are used as a conductive additive, agglomeration of the fine particles occurs, making it difficult to obtain a uniform electrode density even when pressed at high press pressure. Have difficulty. The fine particles have a large specific surface area of 60 m ² /g or more and have large friction. Therefore. If the friction is large, it will be less likely to get clogged when pressed. Therefore, it is preferable to use a carbon material having a specific surface area of less than 60 m ² /g and at least a hollow space as the conductive auxiliary agent. 1050kN/
It also functions as a buffer material in the press process at a pressure of m or more, and can reduce the occurrence of cracks in the active material particles. Note that materials with hollow spaces are not limited to carbon fibers, and examples of carbon nanotubes include single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (
MWNT) can be used.

Another embodiment of the present invention is a secondary battery including a positive electrode having the above-mentioned positive electrode active material or a conductive auxiliary agent in contact with the positive electrode active material, and a negative electrode. The positive electrode is manufactured by applying an active material solution containing a conductive additive or binder to one or both sides of a metal foil that functions as a positive electrode current collector, drying it, and then pressing it to increase the electrode density. . Note that the electrode density is the density achieved when the linear pressure under pressing is the same. Moreover, the linear pressure is an index indicating the forming pressure per unit length in the width direction of the roll that performs pressing.

Further, the secondary battery can have various shapes depending on the device used, such as a cylindrical shape, a square shape, a coin shape, a laminate shape (flat plate), and the like.

By mixing a positive electrode active material and carbon fibers with a specific surface area of less than 60 m ² /g and pressing at a pressure of 1050 kN/m or more, the electrode density of the positive electrode active material layer can be made 4.0 g/cm ³ or more. In the measurement of charge and discharge cycles, it is possible to suppress the decrease in charge and discharge cycles to less than 15% at the 100th charge and discharge cycle.

High-density packing is possible to increase the electrode density of the positive electrode active material layer to 4.0 g/cm ³ or more, preferably 4.3 g/cm ³ or more, increasing the battery capacity per unit volume and increasing the battery capacity. A secondary battery can be realized.

Alternatively, it is possible to realize a secondary battery in which a decrease in battery capacity during charge/discharge cycles is suppressed. Alternatively, one embodiment of the present invention can realize a highly safe or reliable secondary battery.

FIG. 2 is a diagram illustrating a peripheral portion of a positive electrode active material showing one embodiment of the present invention. A diagram explaining the sol-gel method. FIG. 3 is a diagram illustrating a method of charging a secondary battery. FIG. 3 is a diagram illustrating a method of charging a secondary battery. FIG. 3 is a diagram illustrating a method of discharging a secondary battery. FIG. 1 is a diagram illustrating an example of an electronic device. FIG. 1 is a diagram illustrating an example of an electronic device. FIG. 1 is a diagram illustrating an example of an electronic device. Cross-sectional SEM photograph. A graph showing the relationship between electrode density and press pressure. Data showing charge/discharge cycle characteristics.

Embodiments of the present invention will be described in detail below with reference to the drawings. However, those skilled in the art will easily understand that the present invention is not limited to the following description, and that its form and details can be changed in various ways. Further, the present invention is not to be interpreted as being limited to the contents described in the embodiments shown below.

In each figure described in this specification, the size and thickness of each component such as a positive electrode, a negative electrode, an active material layer, a separator, and an exterior body are exaggerated for clarity of explanation. There are cases. Therefore, each component is not necessarily limited to its size, nor is it limited to the relative size between each component.

(Embodiment 1)
First, using FIG. 1, a positive electrode active material 100a, which is one embodiment of the present invention, and its surroundings will be described. In FIG. 1, three positive electrode active materials 100a, 100b, and 100c are shown, and carbon fibers 131a, 131b and a binder 130 are shown between them.

The carbon fibers 131a and 131b are hollow carbon materials, and function as a conductive aid and also as a buffer material. In addition, since it has a smaller specific surface area than AB (acetylene black), which is a carbon material, there is less agglomeration and less friction with itself and the active material, making it possible to obtain a uniform density after pressing. . In particular, even when a high pressing pressure of 1050 kN/m or more is used, it functions as a buffer material, so a high-density electrode can be obtained. Note that the carbon fiber 131a shown in FIG. 1 has a fiber length of 1 μm or more and 20 μm or less, and a fiber diameter of 70 μm.
nm or more. The carbon fiber 131a may have a finer fiber diameter of 10 nm or more and 40 nm or less, or may have a fiber length of 100 μm or more. Note that the carbon fiber 131b is shown in a longitudinal section, and the carbon fiber 131a is cut in the cross-sectional direction, and a circular diameter is shown.

The structure will be explained using one positive electrode active material 100a as an example. The positive electrode active material 100a has a first region 101 inside, and a second region 102 and a third region 103 in the surface layer. As shown in FIG. 1, the second region 102 does not have to cover all of the first region 101.
Similarly, the third region 103 does not have to cover all of the second region 102. Further, a third region 103 may exist in contact with the first region 101.

Furthermore, the second region 102 and the third region 103 may have different thicknesses depending on their locations.

Further, the third region 103 may exist inside the positive electrode active material 100a. For example, when the first region 101 is polycrystalline, the third region 103 may exist near the grain boundary. Further, the third region 103 may exist in a portion of the positive electrode active material 100a having a crystal defect, a crack portion, or in the vicinity thereof. In FIG. 1, some grain boundaries are shown by dotted lines. Note that in this specification and the like, a crystal defect refers to a defect that can be observed in a TEM image or the like, that is, a structure in which another element has entered the crystal, a cavity, etc.

Similarly, as shown in FIG. 1, a second region 102 may exist inside the positive electrode active material 100a. For example, when the first region 101 is polycrystalline, the second region 102 may exist near the grain boundary. Further, the second region 102 may exist in a portion of the positive electrode active material 100a that has a crystal defect, a crack portion, or in the vicinity thereof. Further, the third region 103 and the second region 102 inside the positive electrode active material 100a may overlap.

<First area 101>
The first region 101 includes a composite oxide of lithium and a first transition metal. Further, the first region 101 can be said to include lithium, a first transition metal, and oxygen.

The composite oxide of lithium and the first transition metal preferably has a layered rock salt crystal structure.

As the first transition metal, only cobalt may be used, two types of cobalt and manganese may be used as the first transition metal, or three types of cobalt, manganese, and nickel may be used as the first transition metal.

That is, the first region can include lithium cobalt oxide, lithium manganate, lithium nickel oxide, lithium cobalt oxide in which part of cobalt is replaced with manganese, nickel-manganese-lithium cobalt oxide, and the like. In addition to the transition metal, the first region 101 contains
It may contain metals other than transition metals such as aluminum.

The first region 101 functions as a region of the positive electrode active material 100 that particularly contributes to charge/discharge reactions. In order to increase the capacity when the positive electrode active material 100 is used in a secondary battery, the first region 101 preferably has a larger volume than the second region and the third region.

A material having a layered rock salt type crystal structure is preferable for the first region 101 because it has features such as high discharge capacity and low resistance because lithium can be diffused two-dimensionally. Also, the first
When the region 101 has a layered rock salt type crystal structure, segregation of typical elements such as magnesium, which will be described later, is surprisingly likely to occur.

Note that the first region 101 may be made of single crystal or polycrystal. For example, the first area 10
1 may be a polycrystal with an average crystallite size of 280 nm or more and 630 nm or less. If the material is polycrystalline, grain boundaries may be observed using a TEM or the like. Also, the average grain size is
It can be calculated from the half width of XRD.

Since polycrystals have a clear crystal structure, a sufficient two-dimensional diffusion path for lithium ions is ensured. In addition, it is preferable as the first region 101 because it is easier to produce than single crystal.

Furthermore, the entire first region 101 does not have to have a layered rock salt type crystal structure. For example, the first
A part of the region 101 may be amorphous or may have another crystal structure.

<Second area 102>
The second region 102 includes an oxide of a second transition metal. The second region 102 can be said to include a second transition metal and oxygen.

As the second transition metal, it is preferable to use a non-stoichiometric metal. second area 10
It may be said that it is preferable that 2 has a non-stoichiometric compound. For example, at least one of titanium, vanadium, manganese, iron, chromium, niobium, cobalt, zinc, zirconium, nickel, etc. can be used as the second transition metal. However, it is preferable that the second transition metal is a different element from the first transition metal.

In this specification and the like, a non-stoichiometric metal refers to a metal that can have multiple valences. Furthermore, a non-stoichiometric compound refers to a compound of a metal and another element that can have multiple valences.

Further, the second region 102 preferably has a rock salt crystal structure.

The second area 102 functions as a buffer area that connects the first area 101 and a third area 103, which will be described later. A non-stoichiometric compound is caused by a change in the valence of the metal that the non-stoichiometric compound has.
Interatomic distances can vary. Furthermore, non-stoichiometric compounds often form cation or anion deficiencies or dislocations (so-called Magnelli phase). Therefore, the second region 102 can absorb the strain generated between the first region 101 and the third region 103 as a buffer region.

Further, the second region 102 may contain lithium in addition to the second transition metal and oxygen.
For example, it may contain lithium titanate, lithium manganate, or the like. Furthermore, the second region 102 may have the typical element that the third region 103, which will be described later, has. As a buffer region, it is preferable that the second region 102 contains an element such as lithium that the first region 101 has and an element that the third region 103 has.

That is, the second region 102 can contain lithium titanate, titanium oxide, vanadium oxide, manganese oxide, iron oxide, copper oxide, chromium oxide, niobium oxide, cobalt oxide, zinc oxide, or the like.

Further, the second region 102 may include the first transition metal. For example, the second transition metal may be present in some of the first transition metal sites of the composite oxide containing the first transition metal.

Furthermore, the second region 102 may contain fluorine.

Note that the second region 102 preferably has a rock salt crystal structure;
All of 2 does not need to have a rock salt type crystal structure. For example, the second region 102 may have other crystal structures including a spinel crystal structure, an olivine crystal structure, a corundum crystal structure, and a rutile crystal structure.

Further, as long as a structure in which six oxygen atoms are adjacent to a cation is maintained, the crystal structure may be distorted. Further, a portion of the second region 102 may have cation deficiencies.

Further, a portion of the second region 102 may be amorphous.

If the second region 102 is too thin, its function as a buffer region will be degraded, but if it is too thick, it may lead to a decrease in capacity. Therefore, it is preferable that the second region 102 exists within a depth of 20 nm, more preferably 10 nm from the surface of the positive electrode active material 100.
Further, the second transition metal may have a concentration gradient.

<Third area 103>
The third region 103 has a compound of typical elements. Compounds of typical elements are stoichiometric compounds. As the compound of typical elements, for example, at least one of magnesium oxide, calcium oxide, beryllium oxide, lithium fluoride, and sodium fluoride can be used.

The third region 103 is a region that comes into contact with an electrolyte when the positive electrode active material 100 is used in a secondary battery. Therefore, the material used for the third region 103 is preferably a material that undergoes little electrochemical change during the charging and discharging process and is not easily altered by contact with the electrolytic solution. A compound of typical elements that is electrochemically stable because it is a stoichiometric compound is preferable as the third region 103 . By having the third region 103 in the surface layer of the positive electrode active material 100, stability in charging and discharging the secondary battery can be improved. Here, the high stability of the secondary battery means that, for example, the crystal structure of the composite oxide containing lithium and the first transition metal included in the first region 101 is more stable. Alternatively, it means that the change in capacity of the secondary battery is small even after repeated charging and discharging. Alternatively, it means that a change in the valence of the metal included in the positive electrode active material 100 is suppressed even after repeated charging and discharging.

Furthermore, the third region 103 may contain fluorine. When the third region 103 contains fluorine, some of the anions in the compound of the typical element may be substituted with fluorine.

By partially replacing anions in a compound of a typical element with fluorine, for example, the diffusivity of lithium can be improved. Therefore, even if the third region 103 exists, charging and discharging are hardly hindered. Further, the presence of fluorine in the surface layer of the positive electrode active material particles may improve corrosion resistance against hydrofluoric acid produced by decomposition of the electrolytic solution.

Furthermore, the third region 103 may include lithium, a first transition metal, and a second transition metal.

Further, it is preferable that the compound of the typical element included in the third region 103 has a rock salt type crystal structure. When the third region 103 has a rock salt crystal structure, the crystal orientation tends to match that of the second region 102 . When the crystal orientations of the first region 101, the second region 102, and the third region 103 roughly match, the second region 102 and the third region 103 can function as a more stable covering layer.

However, all of the third region 103 does not have to have a rock salt crystal structure. For example, the third region 103 may have other crystal structures such as a spinel crystal structure, an olivine crystal structure, a corundum crystal structure, and a rutile crystal structure.

Further, as long as a structure in which six oxygen atoms are adjacent to a cation is maintained, the crystal structure may be distorted. Further, a portion of the third region 103 may have cation deficiencies.

Further, a portion of the third region 103 may be amorphous.

If the third region 103 is too thin, the function of improving stability during charging and discharging will be reduced;
If it becomes too thick, the capacity will decrease. Therefore, the thickness of the third region is 0.5 nm or more.
The thickness is preferably 0.5 nm or more and 2 nm or less, more preferably 0.5 nm or more and 2 nm or less.

Further, when the third region 103 contains fluorine, the fluorine is magnesium fluoride (MgF ₂ ).
, lithium fluoride (LiF), and cobalt fluoride (CoF ₂ ). Specifically, when the vicinity of the surface of the positive electrode active material 100 is analyzed by XPS, the peak position of the binding energy of fluorine is preferably 682 eV or more and 685 eV or less, and more preferably about 684.3 eV. This is MgF ₂ , LiF, CoF ₂
This is a binding energy that does not match any of the following.

In this specification, etc., the peak position of the binding energy of a certain element when subjected to XPS analysis refers to the value of the binding energy at which the intensity of the energy spectrum is maximum within the range corresponding to the binding energy of that element. do.

Generally, as positive electrode active materials are repeatedly charged and discharged, side reactions such as first transition metals such as manganese, cobalt, and nickel are eluted into the electrolyte, oxygen is released, and the crystal structure becomes unstable. occurs and deterioration progresses. However, the positive electrode active material 100, which is one embodiment of the present invention,
A second region 102 functioning as a buffer region and an electrochemically stable third region 103
have both. Therefore, elution of the first transition metal is effectively suppressed, and the first region 101
It is possible to make the crystal structure of a composite oxide containing lithium and a transition metal more stable. Therefore, the cycle characteristics of a secondary battery having the positive electrode active material 100 can be significantly improved. Also, voltages exceeding 4.3V (vs. LiLi+), especially 4.5V (
vs. When charging and discharging is performed at a high voltage of LiLi+) or higher, the structure of one embodiment of the present invention exhibits a remarkable effect.

[Particle size]
If the particle size of the positive electrode active materials 100a, 100b, and 100c is too large, it will be difficult to diffuse lithium, and if it is too small, it will be difficult to maintain the crystal structure described below. Therefore, D50
(also referred to as median diameter) is preferably 5 μm or more and 100 μm or less, and 10 μm or more and 70 μm or more.
More preferably, it is not more than μm.

In addition, in order to increase the density of the positive electrode active material layer, large particles (longest part of about 20 μm or more and about 40 μm or less) and small particles (longest part of about 3 μm) are mixed, and the gaps between the large particles are filled with small particles. It is also effective to fill it with Therefore, the particle size distribution may have two or more peaks.

The particle size of the positive electrode active material depends not only on the particle size of the starting material, but also on the lithium contained in the starting material,
It is affected by the ratio of the first transition metal (hereinafter referred to as Li and first transition metal ratio).

When the particle size of the starting material is small, it is necessary to cause grain growth during firing in order to bring the particle size of the positive electrode active material into the above-mentioned preferred range.

In order to promote grain growth during firing, it is effective to make the ratio of the starting material Li to the first transition metal greater than 1, that is, to make the lithium content slightly excessive. For example, when the ratio of Li to the first transition metal is about 1.06, it is easy to obtain a positive electrode active material with a D50 of 15 μm or more. As will be described later, lithium may be lost out of the system during the process of producing the positive electrode active material, so the ratio of lithium to the first transition metal in the finished positive electrode active material is the same as that of the starting material lithium and the first transition metal. may not match the ratio of transition metals.

However, if the amount of lithium is too excessive in order to keep the particle size within a preferable range, there is a risk that the capacity retention rate will decrease when used in a secondary battery.

However, by providing the second region 102 containing the second transition metal in the surface layer, Li
By controlling the first transition metal ratio, a positive electrode active material having a high capacity retention rate can be produced while keeping the particle size within a preferable range.

In the case of a positive electrode active material in which a region containing a second transition metal is provided in the surface layer, the starting material Li and the first
The transition metal ratio is preferably 1.00 or more and 1.07 or less, and 1.03 or more and 1.06 or less.
It is more preferable that it is below.

[Formation of second region]
The second region 102 can be formed by coating particles of a composite oxide containing lithium and a first transition metal with a material containing a second transition metal.

Methods for coating the material containing the second transition metal include liquid phase methods including the sol-gel method, solid phase methods, sputtering methods, vapor deposition methods, CVD (chemical vapor deposition) methods, and PLD (pulsed laser deposition) methods. ) method can be applied. In this embodiment, a case will be described in which a sol-gel method is applied, which can be expected to provide uniform coating and can be processed at atmospheric pressure.

<Sol-gel method>
A method of coating a material containing a second transition metal by applying the sol-gel method will be explained using FIG. 2.

First, a second transition metal alkoxide is dissolved in alcohol.

FIG. 2(A-1) shows the general formula of the second transition metal alkoxide. M2 in the formula of FIG. 2(A-1) represents a second transition metal alkoxide. R represents an alkyl group having 1 to 18 carbon atoms or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms. In addition, Figure 2 (A
-1) shows the general formula when the second transition metal is tetravalent, but one embodiment of the present invention is not limited to this. The second transition metal may be divalent, trivalent, pentavalent, hexavalent or heptavalent. In this case, the alkoxide of the second transition metal has an alkoxy group depending on the valence of the second transition metal.

FIG. 2 (A-2) shows the general formula of titanium alkoxide used when titanium is used as the second transition metal. R in FIG. 2 (A-2) represents an alkyl group having 1 to 18 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 13 carbon atoms.

For example, titanium alkoxides include tetramethoxytitanium, tetraethoxytitanium, tetra-n-propoxytitanium, tetra-i-propoxytitanium (tetraisopropyl orthotitanate, titanium (IV) isopropoxide, titanium tetrasop
ropoxide (IV), TTIP, etc.), tetra-n-butoxytitanium, tetra-i-butoxytitanium, tetra-sec-butoxytitanium, tetra-t-
Butoxytitanium or the like can be used.

FIG. 2(A-3) shows the chemical formula of titanium (IV) isopropoxide (TTIP), which is a type of titanium alkoxide described in the production method described later.

As the solvent for dissolving the second transition metal alkoxide, alcohols are preferable, and for example, methanol, ethanol, propanol, 2-propanol, butanol, 2-butanol, etc. can be used.

Next, particles of a composite oxide containing lithium, a transition metal, magnesium, and fluorine are mixed into an alcoholic solution of a second transition metal alkoxide, and the mixture is stirred in an atmosphere containing water vapor.

By placing it in an atmosphere containing H ₂ O, water and the alkoxide of the second transition metal undergo hydrolysis as shown in FIG. 2(B). Subsequently, dehydration condensation occurs between the products in FIG. 2(B) as shown in FIG. 2(C). By repeating the hydrolysis shown in FIG. 2(B) and the condensation reaction shown in FIG. 2(C), a sol of the second transition metal oxide is generated. This reaction is shown in Figure 2 (D-1) and Figure 2.
As shown in (D-2), this also occurs on the particles 110 of the composite oxide, and a layer containing the second transition metal is formed on the surface of the particles 110.

Thereafter, the particles 110 are collected and the alcohol is vaporized. Details of the manufacturing method will be described later.

Note that in this embodiment, an example is described in which a material containing a second transition metal is coated before coating the positive electrode current collector with particles of a composite oxide containing lithium, a first transition metal, a typical element, and fluorine. However, one embodiment of the present invention is not limited to this. On the positive electrode current collector, lithium, the first
After forming a positive electrode active material layer containing particles of a composite oxide having a transition metal, a typical element, and fluorine, immersing both the positive electrode current collector and the positive electrode active material layer in a second transition metal alkoxide solution,
A material having a second transition metal may be coated.

[Segregation in the third region]
The third region 103 can also be formed by a sputtering method, a solid phase method, a liquid phase method including a sol-gel method, or the like. In particular, typical element sources such as magnesium and fluorine sources are the first
When heated after mixing with the material of the region 101, typical elements are segregated on the surface layer of the positive electrode active material particles, and the third region 103 can be formed. Further, with the third region 103 formed in this manner, the positive electrode active materials 100a, 100b, 100c have excellent cycle characteristics.
becomes.

When forming the third region 103 through heating as described above, the heating is preferably performed after the composite oxide particles are coated with the material containing the second transition metal. Surprisingly, even after coating with the material containing the second transition metal, typical elements such as magnesium segregate on the surface of the particles when heated.

Note that when the typical elements are segregated by heating, when the composite oxide containing lithium and the first transition metal that the first region 101 has is polycrystalline or has crystal defects, lithium Typical elements may also segregate near grain boundaries and near crystal defects of the composite oxide containing the first transition metal and the first transition metal. The typical elements segregated near grain boundaries and near crystal defects can contribute to further stabilizing the crystal structure of the composite oxide containing lithium and the first transition metal, which the first region 101 has.

Furthermore, when the composite oxide containing lithium and the first transition metal included in the first region 101 has cracks, typical elements may also segregate in the cracks due to heating. Furthermore, not only the typical elements but also the second transition metal can be segregated. The crack portion, like the particle surface, is a region in contact with the electrolyte. Therefore, the typical elements and the second transition metal segregate in the crack, and the third region 1
03 and the second region 102, the region that comes into contact with the electrolyte can be made of a chemically stable material. Therefore, a secondary battery with excellent cycle characteristics can be obtained.

When the ratio of the typical element (T) and fluorine (F) in the starting material is within the range of T:F=1:x (1.5≦x≦4) (atomic ratio), the segregation of the typical element can be effectively prevented. This is preferable because it occurs. Also T:F=
More preferably, the ratio is about 1:2 (atomic ratio).

Since the third region 103 formed by segregation is formed by epitaxial growth, the crystal orientations of the second region 102 and the third region 103 may partially match. In other words, the second region 102 and the third region 103 may form a topography. When the crystal orientations of the second region 102 and the third region 103 are approximately the same, they can function as a better covering layer.

However, all of the typical elements such as magnesium added as starting materials are in the third region 10.
It is not necessary to segregate into 3. For example, the first region 101 may contain a small amount of a typical element such as magnesium.

[Method for producing positive electrode active material]
Next, an example of a method for manufacturing the positive electrode active materials 100a, 100b, and 100c will be described.

<Step 11: Preparation of starting materials>
First, starting materials are prepared. From the raw materials prepared in this step, the first area 10 is finally
First and third regions 103 are formed.

As raw materials for lithium and the first transition metal that the first region 101 has, a lithium source and a first
A transition metal source is prepared. Further, a typical element source is prepared as a raw material for the compound of the typical element that the third region 103 has.

In addition to these, it is preferable to prepare a fluorine source. By adding fluorine to raw materials,
In a later step, the typical elements of the third region 103 are changed to positive electrode active materials 100a, 100b, 100.
This has the effect of promoting segregation on the surface of c.

As the lithium source, for example, lithium carbonate or lithium fluoride can be used. 1st
As the transition metal source, for example, an oxide of the first transition metal can be used. As the typical element source, for example, an oxide of a typical element included in the third region, a fluoride of a typical element included in the third region, etc. can be used.

As the fluorine source, for example, lithium fluoride, a fluoride of a typical element included in the third region, or the like can be used. That is, lithium fluoride can be used both as a lithium source and as a fluorine source.

The fluorine contained in the fluorine source is preferably 1.0 times or more and 4 times or less (atomic ratio), and 1.5 times or more and 3 times or less (atomic ratio) of the typical element contained in the typical element source. It is more preferable that

<Step 12: Mixing of starting materials>
Next, the lithium source, the first transition metal source, and the typical element source are mixed. It is preferable to further add a fluorine source. For example, a ball mill or a bead mill can be used for mixing.

<Step 13: First heating>
Next, in step 12, the mixed materials are heated. This step may be called firing or first heating. Heating is preferably performed at a temperature of 800°C or more and 1100°C or less, and 900°C
It is more preferable to carry out the heating at a temperature of 1000° C. or lower. The heating time is preferably 2 hours or more and 20 hours or less. The firing is preferably performed in a dry atmosphere such as dry air. The dry atmosphere preferably has a dew point of, for example, -50°C or lower, more preferably -100°C or lower. In this embodiment, heating is performed at 1000°C for 10 hours, and the temperature rise is 200°C.
℃/h, and dry air with a dew point of -109℃ is flowed at 10 L/min. The heated material is then cooled to room temperature.

By heating in step 13, a composite oxide of lithium and the first transition metal having a layered rock salt type crystal structure can be synthesized. At this point, the typical elements and fluorine contained in the starting material are in solid solution in the composite oxide. However, some of the typical elements may already be unevenly distributed on the surface of the composite oxide.

Furthermore, particles of a composite oxide containing lithium, cobalt, fluorine, and magnesium synthesized in advance may be used as a starting material. In this case, steps 12 and 13 can be omitted. For example, lithium cobalt oxide particles (trade name: C-20F) manufactured by Nihon Kagaku Kogyo Co., Ltd. can be used as one of the starting materials. This particle size is about 20
It is a lithium cobalt oxide particle that has a particle size of μm and contains fluorine, magnesium, calcium, sodium, silicon, sulfur, and phosphorus in a region that can be analyzed by XPS from the surface.

<Step 14: Coating with second transition metal>
Next, the composite oxide of lithium and the first transition metal is cooled to room temperature. Then, the surface of the composite oxide particles of lithium and the first transition metal is coated with a material containing the second transition metal. In this production method example, a sol-gel method is applied.

First, a second transition metal alkoxide dissolved in alcohol and composite oxide particles of lithium and the first transition metal are mixed.

For example, when titanium is used as the second transition metal, TTIP can be used as the alkoxide of the second transition metal. Further, as the alcohol, for example, isopropanol can be used.

Next, the liquid mixture is stirred in an atmosphere containing water vapor. Stirring can be performed, for example, with a magnetic stirrer. The stirring time may be sufficient as long as the water in the atmosphere and TTIP undergo hydrolysis and polycondensation reactions, for example, 4 hours at 25°C and 90% RH.
(Relative Humidity).

As described above, by reacting water in the atmosphere with TTIP, the sol-gel reaction can proceed more slowly than when adding liquid water. Furthermore, by reacting titanium alkoxide and water at room temperature, the sol-gel reaction can proceed more slowly than, for example, when heating is performed at a temperature exceeding the boiling point of alcohol as a solvent. By proceeding with the sol-gel reaction slowly, a coating layer containing titanium with a uniform thickness and high quality can be formed.

A precipitate is collected from the mixed liquid after the above treatment. As a recovery method, filtration, centrifugation, evaporation to dryness, etc. can be applied. In this embodiment, recovery is performed by filtration. A paper filter will be used for filtration, and the residue will be washed with the same alcohol as the solvent in which the titanium alkoxide was dissolved.

Next, the collected residue is dried. In this embodiment, vacuum drying is performed at 70° C. for 1 hour.

<Step 15: Second heating>
Next, the composite oxide particles prepared in step 14 and coated with the material containing the second transition metal are heated. This step may be referred to as second heating. The heating time is preferably 50 hours or less, more preferably 2 hours or more and 10 hours or less, and even more preferably 1 hour or more and 3 hours or less. If the heating time is too short, segregation of typical elements may not occur, but if it is too long, the second transition metal may diffuse too much and a good second region 102 may not be formed.

The specified temperature is preferably 500°C or more and 1200°C or less, more preferably 800°C or more and 1000°C or less. If the specified temperature is too low, segregation of the typical element and the second transition metal may not occur. However, if the temperature is too high, the first transition metal in the composite oxide particles will be reduced and the composite oxide particles will decompose, and the layered structure of lithium and the first transition metal in the composite oxide particles will not be maintained. , etc.

In this embodiment, the specified temperature is set to 800°C and held for 2 hours, and the temperature is increased to 200°C.
/h, and the flow rate of dry air is 10 L/min.

By heating in step 15, the composite oxide of lithium and the first transition metal and the oxide of the second transition metal coated thereon become topotoxy. In other words, the first region 101 and the second region 102 form a topography.

Further, by heating in step 15, the typical elements that were solidly dissolved inside the composite oxide particles of lithium and the first transition metal are unevenly distributed on the surface and become solid solution, that is, segregated, and become a compound of the typical elements.
A third region 103 is formed. At this time, the compound of the typical element grows heteroepitaxially from the second region 102. In other words, the second region 102 and the third region 103 become topotaxies.

The crystal orientations of the second region 102 and the third region 103 approximately match, and the crystal orientation of the first region 101
Therefore, when the positive electrode active materials 100a, 100b, and 100c are used in a secondary battery, changes in the crystal structure of the first region 101 caused by charging and discharging can be effectively suppressed. Furthermore, even if lithium is removed from the first region 101 due to charging, the surface layer portion with stable bonds suppresses the release of the first transition metal such as cobalt and oxygen from the first region 101. can do. Furthermore, the area in contact with the electrolyte can be made of a chemically stable material. Therefore, a secondary battery with excellent cycle characteristics can be obtained.

Note that a portion of the first region 101 and the second region 102 only need to be topotaxy, and it is not necessary that all of the first region 101 and the second region 102 be topotaxy. Furthermore, it is sufficient that a portion of the second region 102 and the third region 103 are topotaxy, and it is not necessary that all of the second region 102 and the third region 103 are topotaxy.

Further, when the compound of the typical element included in the third region contains oxygen, it is preferable to perform the heating in step 15 in an atmosphere containing oxygen. By heating in an atmosphere containing oxygen, the third
The formation of the region 103 is promoted.

Furthermore, fluorine contained in the starting material promotes segregation of typical elements.

As described above, in the method for manufacturing a positive electrode active material that is one embodiment of the present invention, after coating the element forming the second region 102, heating is performed to form the third region 103, and the positive electrode active material 100
It becomes possible to form two types of regions on the surfaces of a, 100b, and 100c. In other words, normally two coating steps are required to provide two types of regions on the surface layer, but the method for producing a positive electrode active material, which is an embodiment of the present invention, requires one coating step (sol-gel process). ), it is a highly productive manufacturing method.

<Step 16: Cooling>
Next, the particles heated in step 15 are cooled to room temperature. It is preferable to take a long time to cool down the temperature because it facilitates topotaxy. For example, it is preferable that the time for lowering the temperature from the holding temperature to the room temperature is the same as or longer than the time for raising the temperature, specifically, 10 hours or more and 50 hours or less.

<Step 17: Collection>
The cooled particles are then collected. Furthermore, it is preferred to sieve the particles. Through the above steps, positive electrode active materials 100a, 100b, and 100c having the first region 101, the second region 102, and the third region 103 can be manufactured.

Then, an active material layer containing the obtained positive electrode active materials 100a, 100b, and 100c, vapor-grown carbon fibers 131a and 131b as conductive aids, and a binder 130 is formed on the positive electrode current collector. Positive electrode active materials 100a, 100b, 100c, vapor grown carbon fibers 131a, 131
A positive electrode paste containing 1.b and a binder 130 is prepared, applied to a positive electrode current collector, dried, and pressure-molded using a roll or the like. In order to obtain a high electrode density, the electrode density can be increased to 4.0 g/cm ³ or more by pressing at a pressure of 1050 kN/m or more. FIG. 1 shows an example of a cross section of an active material layer formed on a positive electrode current collector (not shown).

Note that examples of the binder 130 include styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber,
It is preferable to use a rubber material such as an ethylene-propylene-diene copolymer. Furthermore, fluororubber can be used as the binder.

Further, as the binder, it is preferable to use, for example, a water-soluble polymer. As the water-soluble polymer, for example, polysaccharides can be used. As the polysaccharide, cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, starch, and the like can be used. Further, it is more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

Or, as a binder, polystyrene, polymethyl acrylate, polymethyl methacrylate (PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride,
It is preferable to use materials such as polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

The binder may be used in combination of two or more of the above binders.

<Positive electrode current collector>
As the positive electrode current collector, highly conductive materials such as metals such as stainless steel, gold, platinum, aluminum, and titanium, and alloys thereof can be used. Further, it is preferable that the material used for the positive electrode current collector does not elute at the potential of the positive electrode. Furthermore, an aluminum alloy to which an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum is added, can be used. Alternatively, it may be formed of a metal element that reacts with silicon to form silicide. Examples of metal elements that react with silicon to form silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector may have a foil shape, a plate shape (sheet shape), a net shape, a punched metal shape, an expanded metal shape, or the like as appropriate. The current collector preferably has a thickness of 5 μm or more and 30 μm or less.

(Embodiment 2)
In Embodiment 1, an example of manufacturing a positive electrode was shown, but in this embodiment, a secondary battery in which a positive electrode, a negative electrode, and an electrolyte are wrapped in an exterior body will be described as an example.

[Negative electrode]
The negative electrode has a negative electrode active material layer and a negative electrode current collector. Further, the negative electrode active material layer may include a conductive additive and a binder.

<Negative electrode active material>
As the negative electrode active material, for example, alloy-based materials, carbon-based materials, etc. can be used.

As the negative electrode active material, an element capable of performing a charge/discharge reaction through an alloying/dealloying reaction with lithium can be used. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. These elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh/g. For this reason, it is preferable to use silicon as the negative electrode active material. Further, compounds having these elements may also be used. For example, SiO, _Mg2Si , _Mg2Ge , SnO, _SnO2 , _Mg2Sn , _SnS2 , V
_{2Sn3 , FeSn2 , CoSn2 , Ni3Sn2} , _Cu6Sn5 , _Ag3Sn , _{Ag3
Sb , Ni2MnSb} , _CeSb3 , _LaSn3 , _La3Co2Sn7 , _CoSb3 , I
There are nSb, SbSn, etc. Here, an element that can perform a charge/discharge reaction by alloying/dealloying reaction with lithium, a compound having the element, etc. may be referred to as an alloy-based material.

In this specification and the like, SiO refers to silicon monoxide, for example. Or SiO is SiO
It can also be expressed as _x . Here, x preferably has a value near 1. For example, x is 0
．． It is preferably 2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less.

As the carbon-based material, graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, carbon black, etc. may be used.

Examples of graphite include artificial graphite and natural graphite. Examples of the artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape, which is preferred. Furthermore, it is relatively easy to reduce the surface area of MCMB, which may be preferable. Examples of natural graphite include:
Examples include flaky graphite and spheroidized natural graphite.

Graphite exhibits a potential as low as that of lithium metal when lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed) (0.05V or more and 0.3V or less vs. Li/L)
i ⁺ ). This allows the lithium ion secondary battery to exhibit a high operating voltage. Furthermore, graphite is preferable because it has advantages such as a relatively high capacity per unit volume, a relatively small volumetric expansion, low cost, and higher safety than lithium metal.

Further, as negative electrode active materials, titanium dioxide (TiO ₂ ), lithium titanium oxide (Li ₄ T
i ₅ O ₁₂ ), lithium-graphite intercalation compound (Li _x C ₆ ), niobium pentoxide (Nb ₂ O ₅ )
, tungsten oxide (WO ₂ ), molybdenum oxide (MoO ₂ ), and other oxides can be used.

Further, as the negative electrode active material, Li _3-x M _x N (M=Co, Ni, Cu) having a Li ₃ N type structure, which is a double nitride of lithium and a transition metal, can be used. For example, Li _2.6
Co _0.4 N ₃ is preferable because it exhibits a large charge/discharge capacity (900 mAh/g, 1890 mAh/cm ³ ).

When a double nitride of lithium and a transition metal is used, since the negative electrode active material contains lithium ions,
It is preferable that it can be combined with materials such as V ₂ O ₅ and Cr ₃ O ₈ that do not contain lithium ions as the positive electrode active material. Note that even when a material containing lithium ions is used as the positive electrode active material, a double nitride of lithium and a transition metal can be used as the negative electrode active material by removing the lithium ions contained in the positive electrode active material in advance.

Furthermore, a material that causes a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used as the negative electrode active material. Materials that cause conversion reactions include oxides such as Fe ₂ O ₃ , CuO, Cu ₂ O, RuO ₂ , and Cr ₂ O ₃ , sulfides such as CoS _0.89 , NiS, and CuS, and Zn ₃ N ₂ , _Cu3N , _Ge3
It also occurs with nitrides such as _N4 , phosphides such as _NiP2 , _FeP2 , _CoP3 , and fluorides such as _FeF3 , _BiF3 .

As the conductive agent and binder that the negative electrode active material layer can have, the same materials as the conductive agent and binder that the positive electrode active material layer can have can be used.

<Negative electrode current collector>
The same material as the positive electrode current collector can be used for the negative electrode current collector. Note that it is preferable to use a material that does not form an alloy with carrier ions such as lithium for the negative electrode current collector.

[Electrolyte]
The electrolytic solution includes a solvent and an electrolyte. As the solvent for the electrolytic solution, an aprotic organic solvent is preferable, such as ethylene carbonate (EC), fluoroethylene carbonate (FEC), etc.
), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (E
MC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether , methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc., or two or more of these may be used in any combination and ratio.

In addition, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as a solvent for the electrolyte, the internal temperature of the secondary battery will not rise due to internal short circuits or overcharging. This can prevent the secondary battery from exploding or catching fire. Ionic liquids are composed of cations and anions, and include organic cations and anions. Examples of the organic cation used in the electrolytic solution include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. In addition, examples of anions used in the electrolytic solution include monovalent amide anions, monovalent methide anions, fluorosulfonic acid anions, perfluoroalkylsulfonic acid anions, tetrafluoroborate anions, perfluoroalkylborate anions, and hexafluorophosphate anions. , or perfluoroalkyl phosphate anion.

Further, as the electrolyte to be dissolved in the above solvent, for example, LiPF ₆ , LiClO ₄ , Li
AsF ₆ , LiBF ₄ , LiAlCl ₄ , LiSCN, LiBr, LiI, Li ₂ SO ₄
, Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO
₃ , LiC _{( CF3SO2} ) _{3 ,} LiC( _{C2F5SO2 ) 3} , LiN( _{CF3SO2 )
2} , one type of lithium salt such as LiN( _{C4F9SO2 )} ( _{CF3SO2 )} , LiN( _{C2F5SO2 ) 2 ,} or two or more of these in any combination and ratio _. Can be used.

As the electrolytic solution used in the secondary battery, it is preferable to use a highly purified electrolytic solution that has a low content of particulate dust and elements other than the constituent elements of the electrolytic solution (hereinafter also simply referred to as "impurities"). Specifically, it is preferable that the weight ratio of impurities to the electrolytic solution is 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

Additionally, additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), LiBOB, and dinitrile compounds such as succinonitrile and adiponitrile may be added to the electrolyte. good. The concentration of the added material may be, for example, 0.1 wt% or more and 5 wt% or less based on the entire solvent.

Alternatively, a polymer gel electrolyte in which a polymer is swollen with an electrolytic solution may be used.

By using a polymer gel electrolyte, safety against leakage and the like is increased. Further, it is possible to make the secondary battery thinner and lighter.

Polymers to be gelled include silicone gel, acrylic gel, acrylonitrile gel,
Polyethylene oxide gel, polypropylene oxide gel, fluoropolymer gel, etc. can be used.

As the polymer, for example, a polymer having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing them can be used. For example, PVDF and hexafluoropropylene (H
PVDF-HFP, which is a copolymer of FP), can be used. The polymer formed may also have a porous shape.

In addition, instead of an electrolyte, solid electrolytes containing inorganic materials such as sulfides and oxides, and P
A solid electrolyte containing a polymeric material such as EO (polyethylene oxide) can be used. When using a solid electrolyte, there is no need to install separators or spacers. Additionally, since the entire battery can be solidified, there is no risk of leakage, dramatically improving safety.

[Separator]
Moreover, it is preferable that the secondary battery has a separator. As the separator, for example, one made of paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, polyurethane, etc. can be used. I can do it. It is preferable that the separator is processed into an envelope shape and arranged so as to surround either the positive electrode or the negative electrode.

The separator may have a multilayer structure. For example, a film of an organic material such as polypropylene or polyethylene can be coated with a ceramic material, a fluorine material, a polyamide material, or a mixture thereof. As the ceramic material, for example, aluminum oxide particles, silicon oxide particles, etc. can be used. As the fluorine-based material, for example, PVDF, polytetrafluoroethylene, etc. can be used. As the polyamide material, for example, nylon, aramid (meta-aramid, para-aramid), etc. can be used.

Coating with a ceramic material improves oxidation resistance, so it is possible to suppress deterioration of the separator during high voltage charging and discharging and improve the reliability of the secondary battery. Furthermore, coating with a fluorine-based material makes it easier for the separator and electrode to come into close contact with each other, thereby improving output characteristics.
Coating with a polyamide-based material, especially aramid, improves heat resistance, thereby improving the safety of the secondary battery.

For example, a polypropylene film may be coated on both sides with a mixed material of aluminum oxide and aramid. Alternatively, the surface of the polypropylene film in contact with the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface in contact with the negative electrode may be coated with a fluorine-based material.

When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, so that the capacity per volume of the secondary battery can be increased.

[Exterior body]
As the exterior body of the secondary battery, a metal material such as aluminum or a resin material can be used, for example. Moreover, a film-like exterior body can also be used. As a film,
For example, a highly flexible metal thin film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and then the outer surface of the exterior body is formed on the thin metal film. A three-layered film provided with an insulating synthetic resin film such as polyamide resin or polyester resin can be used.

A secondary battery is assembled using the negative electrode, negative electrode active material, negative electrode current collector, electrolyte, separator, and exterior body shown above, and the positive electrode, positive electrode active material, and positive electrode current collector shown in Embodiment 1. be able to. For example, if the exterior body is a metal can and is circular, a coin-shaped (single-layer flat type) secondary battery can be manufactured by a known manufacturing method.

Furthermore, if a metal can with a hollow cylindrical exterior body is used, a cylindrical secondary battery can be manufactured by a known manufacturing method.

In addition, by storing the above-mentioned wound body in a space formed by bonding a film-like exterior body and a film having recesses together by thermocompression bonding, etc., a laminate type secondary battery can be manufactured by a known manufacturing method. It can be made.

[Charging and discharging method]
Note that charging and discharging of the secondary battery can be performed, for example, as follows.

≪CC charging≫
First, CC charging will be explained as one of the charging methods. CC charging is a charging method in which a constant current is passed through the secondary battery during the entire charging period, and charging is stopped when a predetermined voltage is reached. The secondary battery is assumed to be an equivalent circuit of internal resistance R and secondary battery capacity C as shown in FIG. 3(A). In this case, the secondary battery voltage V _B is the sum of the voltage V _R applied to the internal resistance R and the voltage V _C applied to the secondary battery capacity C.

During CC charging, as shown in FIG. 3(A), the switch is turned on and a constant current I flows through the secondary battery. During this time, since the current I is constant, the voltage V _R applied to the internal resistance R is also constant according to Ohm's law of V _R =R×I. On the other hand, the voltage V _C applied to the secondary battery capacity C increases with the passage of time. Therefore, the secondary battery voltage _VB increases over time.

Then, when the secondary battery voltage _VB reaches a predetermined voltage, for example 4.3V, charging is stopped. When CC charging is stopped, the switch is turned off and the current I = 0, as shown in Figure 3(B).
becomes. Therefore, the voltage VR applied to the internal resistance _R becomes 0V. Therefore, the secondary battery voltage _VB decreases by the amount that the voltage drop across the internal resistance R disappears.

FIG. 3C shows an example of the secondary battery voltage _VB and charging current during CC charging and after CC charging is stopped. The secondary battery voltage _VB, which had increased while CC charging was being performed, is now C
It is shown that the voltage decreases slightly after C charging is stopped.

≪CCCV charging≫
Next, CCCV charging, which is a charging method different from the above, will be explained. CCCV charging is a charging method in which the battery is first charged to a predetermined voltage by CC charging, and then charged by CV (constant voltage) charging until the current flowing decreases, specifically, until the final current value is reached. .

During CC charging, as shown in FIG. 4A, the constant current power source is switched on, the constant voltage power source is switched off, and a constant current I flows through the secondary battery. During this time, since the current I is constant, the voltage V _R applied to the internal resistance R is also constant according to Ohm's law of V _R =R×I. On the other hand, the voltage V _C applied to the secondary battery capacity C increases with the passage of time. Therefore, the secondary battery voltage _VB increases over time.

Then, when the secondary battery voltage _VB reaches a predetermined voltage, for example 4.3V, the
Switch to V charging. During CV charging, as shown in FIG. 4(B), the constant voltage power source is switched on, the constant current power source is switched off, and the secondary battery voltage _VB remains constant. On the other hand, the voltage V _C applied to the secondary battery capacity C increases with the passage of time. _VB = _VR
+V _C , the voltage V _R applied to the internal resistance R decreases over time. As the voltage V _R applied to the internal resistance R decreases, the current I flowing through the secondary battery also decreases according to Ohm's law of V _R =R×I.

When the current I flowing through the secondary battery reaches a predetermined current, for example, a current equivalent to 0.01C, charging is stopped. When CCCV charging is stopped, all the switches are turned off and the current I becomes 0, as shown in FIG. 4(C). Therefore, the voltage VR applied to the internal resistance _R becomes 0V. However, since the voltage V _R applied to the internal resistor R has become sufficiently small due to CV charging, even if the voltage drop across the internal resistor R disappears, the secondary battery voltage V _B hardly drops.

FIG. 4(D) shows an example of the secondary battery voltage _VB and charging current during CCCV charging and after CCCV charging is stopped. It is shown that even if CCCV charging is stopped, the secondary battery voltage _VB hardly drops.

≪CC discharge≫
Next, CC discharge, which is one of the discharge methods, will be explained. CC discharge is a discharging method in which a constant current flows from the secondary battery throughout the discharge period, and discharging is stopped when the secondary battery voltage _VB reaches a predetermined voltage, for example, 2.5V.

FIG. 5 shows an example of the secondary battery voltage _VB and discharge current during CC discharge. It is shown that the secondary battery voltage _VB decreases as the discharge progresses.

Next, the discharge rate and charge rate will be explained. The discharge rate is the relative ratio of current during discharge to battery capacity, and is expressed in units of C. In a battery with a rated capacity of X (Ah), the current equivalent to 1C is X (A). When discharging with a current of 2X (A), 2C
It is said that when the battery was discharged at a current of X/5 (A), it was discharged at 0.2C. Also, the charging rate is the same; when charging with a current of 2X (A), it is said to be charged at 2C, and when charging with a current of X/5 (A), it is said to be charged at 0.2C. It is said that

(Embodiment 3)
In this embodiment, an example in which a secondary battery, which is one embodiment of the present invention, is mounted in an electronic device will be described.

By using the secondary battery of one embodiment of the present invention as a secondary battery in everyday electronic devices, a product with a long life can be provided. For example, everyday electronic devices include electric toothbrushes, electric shavers, electric beauty devices, etc., and the secondary batteries for these products are small, lightweight, and stick-shaped to make them easier for users to hold. , and a high capacity secondary battery is desired.

FIGS. 6(A) and 6(B) show an example of a tablet-type terminal that can be folded into two. Figure 6 (
The tablet terminal 9600 shown in FIG. 6A) and FIG. 6B includes a housing 9630a and a housing 963.
0b, a movable part 9640 that connects the housings 9630a and 9630b, a display part 9631 having a display part 9631a and a display part 9631b, a display mode changeover switch 9626, a power switch 9627, a power saving mode changeover switch 9625, a fastener 9629, It has an operation switch 9628. By using a flexible panel for the display portion 9631,
The tablet terminal can have a wider display area. 6(A) shows the tablet terminal 9600 in an open state, and FIG. 6(B) shows the tablet terminal 9600 in a closed state.

Furthermore, the tablet terminal 9600 has a power storage body 9635 inside the housing 9630a and the housing 9630b. The power storage body 9635 is provided passing through the movable part 9640 and spanning the housings 9630a and 9630b.

A portion of the display portion 9631a can be a touch panel area 9632a, and data can be input by touching the displayed operation keys 9638. Note that the display section 963
1a shows, as an example, a configuration in which half of the area has a display-only function and the other half has a touch panel function, but the configuration is not limited to this. Display section 963
The entire area of 1a may have a touch panel function. For example, the display section 96
The entire surface of 31a can be used as a touch panel by displaying keyboard buttons, and the display section 9631b can be used as a display screen.

Further, in the display portion 9631b, a part of the display portion 9631b can be used as a touch panel area 9632b, similarly to the display portion 9631a. Further, by touching the position where the keyboard display switching button 9639 of the touch panel is displayed with a finger, stylus, etc., the keyboard button can be displayed on the display portion 9631b.

Further, touch input can be performed simultaneously on the touch panel area 9632a and the touch panel area 9632b.

Further, a display mode changeover switch 9626 can switch the display orientation such as portrait display or landscape display, and can select switching between black and white display or color display. The power saving mode changeover switch 9625 can optimize the display brightness according to the amount of external light during use, which is detected by an optical sensor built into the tablet terminal 9600. The tablet terminal may include not only a light sensor but also other detection devices such as a sensor that detects tilt, such as a gyro or an acceleration sensor.

Further, although FIG. 6A shows an example in which the display area of the display portion 9631b and the display portion 9631a are the same, this is not particularly limited, and the size of one and the other may be different, and the quality of the display may also be affected. May be different. For example, one display panel may be capable of displaying higher definition than the other.

FIG. 6(B) shows a closed state, and the tablet terminal has a housing 9630, a solar battery 963,
3. It has a charge/discharge control circuit 9634 including a DCDC converter 9636. In addition, the power storage body 9
As 635, a power storage body according to one embodiment of the present invention is used.

Note that since the tablet terminal 9600 can be folded into two, it can be folded so that the housing 9630a and the housing 9630b are overlapped when not in use. By folding,
Since the display portions 9631a and 9631b can be protected, the durability of the tablet terminal 9600 can be increased. Further, since the power storage body 9635 using the secondary battery of one embodiment of the present invention has high capacity and good cycle characteristics, it is possible to provide the tablet terminal 9600 that can be used for a long time.

In addition, the tablet terminals shown in Figures 6(A) and 6(B) have the ability to display various information (still images, videos, text images, etc.), calendars, dates, and times, etc. It can have a function of displaying on the display, a touch input function of performing a touch input operation or editing the information displayed on the display, a function of controlling processing by various software (programs), and the like.

The solar cell 9633 attached to the surface of the tablet device provides power to the touch panel,
It can be supplied to a display unit, a video signal processing unit, or the like. Note that the solar cell 9633 can be provided on one or both sides of the housing 9630, and a configuration can be provided in which the power storage body 9635 is efficiently charged. Note that if a lithium ion battery is used as the power storage body 9635,
It has advantages such as being able to be made smaller.

Further, FIG. 6(C) shows the configuration and operation of the charge/discharge control circuit 9634 shown in FIG. 6(B).
A block diagram is shown and explained below. In FIG. 6(C), a solar cell 9633, a power storage body 9635, and a D
CDC converter 9636, converter 9637, switches SW1 to SW3, display section 9
631, and a power storage body 9635, a DC/DC converter 9636, a converter 9637, and switches SW1 to SW3 correspond to the charging/discharging control circuit 9634 shown in FIG. 6(B).

First, an example of the operation when the solar cell 9633 generates power using external light will be described.
The power generated by the solar cell is boosted or stepped down by a DC/DC converter 9636 so that it becomes a voltage for charging the power storage body 9635. When the power from the solar cell 9633 is used to operate the display section 9631, the switch SW1 is turned on and the converter 9637
Then, the voltage required for the display portion 9631 is increased or decreased. In addition, the display section 9631
When the display is not performed, SW1 may be turned off and SW2 may be turned on to charge the power storage body 9635.

Although the solar cell 9633 is shown as an example of a power generation means, it is not particularly limited, and the power storage body 9635 may be charged by other power generation means such as a piezoelectric element (piezo element) or a thermoelectric conversion element (Peltier element). It may be. For example, a non-contact power transmission module that wirelessly (contactlessly) transmits and receives power for charging, or a combination of other charging means may be used.

FIG. 7 shows an example of another electronic device. In FIG. 7, a display device 8000 is an example of an electronic device using a secondary battery 8004 according to one embodiment of the present invention. Specifically, the display device 8000 corresponds to a display device for receiving TV broadcasting, and includes a housing 8001, a display section 8002, and a speaker section 80.
03, has a secondary battery 8004, etc. A secondary battery 8004 according to one embodiment of the present invention has a housing 8
It is provided inside 001. The display device 8000 can receive power from a commercial power source, or can use power stored in the secondary battery 8004. Therefore, even when power cannot be supplied from a commercial power source due to a power outage or the like, the display device 8000 can be used by using the secondary battery 8004 according to one embodiment of the present invention as an uninterruptible power source.

The display portion 8002 includes a liquid crystal display device, a light emitting device in which each pixel is equipped with a light emitting element such as an organic EL element, an electrophoretic display device, and a DMD (Digital Micromirror Device).
ce), PDP (Plasma Display Panel), FED (Field
A semiconductor display device such as an emission display (emission display) can be used.

Note that display devices include all information display devices, such as those for receiving TV broadcasts, personal computers, and advertisement display.

In FIG. 7, a stationary lighting device 8100 includes a secondary battery 810 according to one embodiment of the present invention.
This is an example of an electronic device using 3. Specifically, the lighting device 8100 includes a housing 8101, a light source 8102, a secondary battery 8103, and the like. In FIG. 7, a secondary battery 8103 is housed in a housing 8101.
Although a case is illustrated in which the light source 8102 is provided inside the ceiling 8104, the secondary battery 8103 may be provided inside the housing 8101. Lighting device 8
100 can receive power from a commercial power source or can use power stored in the secondary battery 8103. Therefore, even when power cannot be supplied from a commercial power source due to a power outage or the like, the lighting device 8100 can be used by using the secondary battery 8103 according to one embodiment of the present invention as an uninterruptible power source.

Note that although FIG. 7 illustrates a fixed lighting device 8100 provided on a ceiling 8104, a secondary battery according to one embodiment of the present invention can be installed on a wall other than the ceiling 8104, for example, a side wall 8105, a floor 81
06, it can be used for a stationary lighting device installed in a window 8107, etc., or it can be used for a tabletop lighting device, etc.

Further, as the light source 8102, an artificial light source that obtains light artificially using electric power can be used. Specifically, examples of the artificial light source include discharge lamps such as incandescent lamps and fluorescent lamps, and light emitting elements such as LEDs and organic EL elements.

In FIG. 7, an air conditioner having an indoor unit 8200 and an outdoor unit 8204 is an example of an electronic device using a secondary battery 8203 according to one embodiment of the present invention. Specifically, the indoor unit 8200 includes a housing 8201, an air outlet 8202, a secondary battery 8203, and the like. In Figure 7,
Although the case where the secondary battery 8203 is provided in the indoor unit 8200 is illustrated, the secondary battery 8203 may be provided in the outdoor unit 8204. Or the indoor unit 8200 and the outdoor unit 8
204 may be provided with a secondary battery 8203. The air conditioner is
Power can be supplied from a commercial power source, or power stored in the secondary battery 8203 can be used. In particular, the secondary battery 8203 is installed in both the indoor unit 8200 and the outdoor unit 8204.
is installed, even when power cannot be supplied from the commercial power source due to a power outage, etc.
By using the secondary battery 8203 according to one embodiment of the present invention as an uninterruptible power source, it is possible to use an air conditioner.

Although Fig. 7 shows an example of a separate air conditioner consisting of an indoor unit and an outdoor unit, it is possible to create an integrated air conditioner that has the functions of an indoor unit and an outdoor unit in one housing. , a secondary battery according to one embodiment of the present invention can also be used.

In FIG. 7, an electric refrigerator-freezer 8300 is an example of an electronic device using a secondary battery 8304 according to one embodiment of the present invention. Specifically, the electric refrigerator-freezer 8300 includes a housing 8301, a refrigerator door 8302, a freezer door 8303, a secondary battery 8304, and the like. In FIG. 7, a secondary battery 8304 is provided inside a housing 8301. The electric refrigerator-freezer 8300 can receive power from a commercial power source, or can use power stored in the secondary battery 8304. Therefore, even when power cannot be supplied from a commercial power source due to a power outage or the like, the electric refrigerator-freezer 8300 can be used by using the secondary battery 8304 according to one embodiment of the present invention as an uninterruptible power source.

Note that among the above-mentioned electronic devices, electronic devices such as high-frequency heating devices such as microwave ovens and electric rice cookers require high power for a short time. Therefore, by using the secondary battery according to one embodiment of the present invention as an auxiliary power source to supplement power that cannot be supplied by the commercial power source, it is possible to prevent the breaker of the commercial power source from tripping when using electronic equipment. .

In addition, during times when electronic devices are not in use, especially during times when the proportion of the amount of electricity actually used (referred to as the power usage rate) out of the total amount of electricity that can be supplied by the commercial power supply source is low, By storing power in the secondary battery, it is possible to suppress an increase in the power usage rate outside of the above-mentioned time period. For example, in the case of the electric refrigerator-freezer 8300, the temperature is low and the refrigerator compartment door 830
2. Power is stored in the secondary battery 8304 at night when the freezer door 8303 is not opened or closed. By using the secondary battery 8304 as an auxiliary power source during the daytime when the temperature is high and the refrigerator compartment door 8302 and the freezer compartment door 8303 are opened and closed, the power usage rate during the daytime can be kept low.

According to one embodiment of the present invention, the cycle characteristics of a secondary battery can be improved, and reliability can be improved. Further, according to one aspect of the present invention, a high-capacity secondary battery can be obtained, the characteristics of the secondary battery can be improved, and the secondary battery itself can be made smaller and lighter. can. Therefore, by mounting a secondary battery, which is one embodiment of the present invention, in the electronic device described in this embodiment, the electronic device can have a longer life and be lighter. This embodiment can be implemented in combination with other embodiments as appropriate.

(Embodiment 4)
In this embodiment, an example is shown in which a secondary battery, which is one embodiment of the present invention, is mounted on a vehicle.

When a secondary battery is installed in a vehicle, next-generation clean energy vehicles such as hybrid vehicles (HEVs), electric vehicles (EVs), or plug-in hybrid vehicles (PHEVs) can be realized.

FIG. 8 illustrates a vehicle using a secondary battery, which is one embodiment of the present invention. A car 8400 shown in FIG. 8A is an electric car that uses an electric motor as a power source for driving. Alternatively, it is a hybrid vehicle that can appropriately select and use an electric motor and an engine as a power source for driving. By using one embodiment of the present invention, a vehicle with a long cruising distance can be realized. Further, the automobile 8400 has a secondary battery. A plurality of secondary battery modules may be used by arranging them on the floor of the vehicle. Further, a battery pack including a plurality of secondary batteries may be installed on the floor of the vehicle. The secondary battery not only drives the electric motor 8406 but also can supply power to light emitting devices such as the headlight 8401 and a room light (not shown).

Further, the secondary battery can supply power to display devices such as a speedometer and a tachometer that the automobile 8400 has. Further, the secondary battery can supply power to a semiconductor device such as a navigation system included in the automobile 8400.

The automobile 8500 shown in FIG. 8(B) can be charged by receiving power from an external charging facility using a plug-in method, a non-contact power supply method, or the like to a secondary battery of the automobile 8500. FIG. 8B shows a state in which a ground-mounted charging device 8021 is charging a secondary battery 8024 mounted on an automobile 8500 via a cable 8022. When charging, a predetermined method such as CHAdeMO (registered trademark) or Combo may be used for the charging method and the connector standard. The charging device 8021 may be a charging station provided at a commercial facility, or may be a home power source. For example, using plug-in technology, the secondary battery 8024 mounted on the automobile 8500 can be charged by external power supply. Charging can be performed by converting AC power into DC power via a conversion device such as an ACDC converter.

Although not shown, a power receiving device can be mounted on a vehicle and electrical power can be supplied from a ground power transmitting device in a non-contact manner for charging. In the case of this contactless power supply method, by incorporating a power transmission device into the road or outside wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is running. Further, electric power may be transmitted and received between vehicles using this non-contact power supply method. Furthermore, a solar cell may be provided on the exterior of the vehicle to charge the secondary battery when the vehicle is stopped or traveling. For such non-contact power supply, an electromagnetic induction method or a magnetic resonance method can be used.

Further, FIG. 8(C) is an example of a two-wheeled vehicle using the secondary battery of one embodiment of the present invention. Figure 8 (C
) is equipped with a secondary battery 8602, a side mirror 8601, and a direction indicator light 8.
603. The secondary battery 8602 can supply electricity to the direction indicator light 8603.

In addition, the scooter 8600 shown in FIG. 8(C) has a secondary battery 8602 in the under-seat storage 8604.
can be stored. Even if the under-seat storage 8604 is small, the secondary battery 8602 is
It can be stored in the under-seat storage 8604. The secondary battery 8602 is removable, and when charging, the secondary battery 8602 can be carried indoors, charged, and stored before driving.

According to one aspect of the present invention, the cycle characteristics of a secondary battery are improved, and the capacity of the secondary battery can be increased. Therefore, the secondary battery itself can be made smaller and lighter. If the secondary battery itself can be made smaller and lighter, it will contribute to reducing the weight of the vehicle and improve its cruising distance. Further, a secondary battery mounted on a vehicle can also be used as a power supply source other than the vehicle.

This embodiment can be implemented in combination with other embodiments as appropriate.

In this example, a positive electrode containing a positive electrode active material that is one embodiment of the present invention was manufactured, and the results of observing a cross section around the positive electrode active material will be described. Furthermore, the results of evaluating the characteristics of a secondary battery using a positive electrode containing the positive electrode active material will be explained.

[Preparation of positive electrode active material]
≪Sample 01≫
In this example, as the positive electrode active material of the sample, the first region has lithium cobalt oxide as a composite oxide of lithium and the first transition metal, and the second region has lithium cobalt oxide. An oxide having lithium titanate as an oxide and having magnesium oxide as an oxide of a typical element included in the third region were prepared.

In this example, lithium cobalt oxide particles (manufactured by Nihon Kagaku Kogyo Co., Ltd., trade name: C-20F) were used as the starting material. Therefore, in this embodiment, step 1 explained in Embodiment 1 is
2 and step 13 were omitted. The above lithium cobalt oxide particles have a particle size of approximately 20 μm.
m, and the region that can be analyzed by XPS contains fluorine, magnesium, calcium, sodium,
Lithium cobalt oxide particles containing silicon, sulfur, and phosphorus.

Next, in step 14, lithium cobalt oxide particles containing magnesium and fluorine were coated with a material containing titanium by a sol-gel method. Specifically, TTIP was dissolved in isopropanol to prepare an isopropanol solution of TTIP. Then, lithium cobalt oxide particles were mixed into the solution. The mixture was mixed so that TTIP was 0.04 ml/g with respect to lithium cobalt oxide containing magnesium and fluorine.

The above liquid mixture was stirred with a magnetic stirrer for 4 hours under conditions of 25° C. and 90% RH. Through this treatment, a hydrolysis and polycondensation reaction occurred with water in the atmosphere and TTIP, and a layer containing titanium was formed on the surface of the lithium cobalt oxide particles containing magnesium and fluorine.

After the above treatment, the mixed solution was centrifuged, the supernatant of the solvent was removed, and the residue was collected. The centrifugation conditions were 3000 rpm and 1 minute.

The collected residue was vacuum dried at 70°C for 1 hour.

Next, the lithium cobalt oxide particles coated with the titanium-containing material were heated. Heating was performed using a muffle furnace at an oxygen flow rate of 10 L/min at 800° C. (temperature increase: 200° C./hour) for a holding time of 2 hours.

The heated particles were then cooled to room temperature. Cooling time from holding temperature to room temperature is 10 to 1
It was set as 5 hours. Thereafter, a crushing process was performed. The crushing treatment was performed by passing through a sieve, and the sieve used had an opening of 53 μm.

Finally, the cooled particles were collected to obtain a positive electrode active material of sample 01.

[Preparation of secondary battery]
CR2032 type (diameter 20 mm) using the positive electrode active material of sample 01 prepared above.
A coin-shaped secondary battery with a height of 3.2 mm was fabricated.

For the positive electrode, a positive electrode active material (LCO), carbon nanotubes (VGCF (registered trademark)), and polyvinylidene fluoride (PVDF) were used. LCO: carbon nanotube: PVDF = 96
A current collector (aluminum foil) was coated with a slurry mixed at a ratio of 2:2 (weight ratio). Cross-sectional SEM photographs are shown in FIGS. 9(A) and 9(B). Three samples of the same sample 01 were prepared, and the press pressures were: no press, 210 kN/m, 713 kN/m, and 12
The electrode density was measured under different conditions: 16 kN/m and 1467 kN/m. The pressing temperature was 120°C. In this example, an example was shown in which the weight ratio of the positive electrode active material, carbon fiber, and polyvinylidene fluoride in the positive electrode was 96:2:2, but there is no particular limitation. For example, the weight ratio may be 95:3. :2 may be used.

Table 1 shows the results of measuring the electrode density.

As shown in Table 1, the electrode density of the positive electrode active material layer obtained under the two conditions of 1216 kN/m and 1467 kN/m was 4.6 g/cm ³ .

Vapor-Grown Carbon Fiber (VGCF (registered trademark)) used
Typical values for fiber diameter are 150 nm, fiber length 10 μm or more and 20 μm or less, true density 2.1 g/cm ³ , and specific surface area 13 m ² /g. Note that the fiber diameter refers to the diameter of a perfect circle circumscribed to a cross section perpendicular to the fiber axis from an image taken two-dimensionally by SEM. . In addition, true density refers to density in which only the volume occupied by the substance itself is used as the volume for density calculation. Further, the specific surface area refers to the surface area per unit mass or the surface area per unit volume of an object.

Lithium metal was used as the counter electrode.

The electrolyte contains 1 mol/L lithium hexafluorophosphate (LiPF ₆ ), and the electrolyte contains ethylene carbonate (EC) and diethyl carbonate (DEC).
A mixture of C:DEC=3:7 (volume ratio) and vinylene carbonate (VC) at 2% by weight was used. Polypropylene was used for the separator.

The positive electrode can and the negative electrode can were made of stainless steel (SUS).

Also, for comparison, sample 0 using AB instead of carbon nanotubes (VGCF)
2 was prepared, three samples were made in the same process, and the press pressure was no press (0 kN/m).
, 210kN/m, 713kN/m, 1216kN/m, and 1467kN/m, respectively. 4.0 g/cm ³ was obtained only under one condition of 1467 kN/m. From these results, when only AB is used, the electrode density is 4.0 g/cm with good uniformity even if the press pressure is increased.
³ , and it is difficult to obtain an electrode density of 4.2 g/cm ³ or more.

Further, FIG. 10 shows the results of creating a graph based on the average value of electrode density. The vertical axis shows the electrode density and the horizontal axis shows the press pressure.

[Evaluation of cycle characteristics]
Next, the charging and discharging characteristics of the secondary battery of sample 01 (press pressure condition: 1467 kN/m sample) produced above were evaluated. The measurement temperature was 45°C. Charging is 4.55V (CC
CV, 1C, cut-off current 0.05C), discharge is 3V (CC, 1C), each 10
0 cycle charging/discharging was performed. The pause time of the cycle test for sample 01 was 1 minute. Note that 1C here is a current value per weight of positive electrode active material, which is 160 mAh/g.

FIG. 11 shows the cycle test results for sample 01. A cycle test was conducted with a rest time of 1 minute, and the initial discharge capacity was 211.6 mAh/g, and the discharge capacity after 100 cycles was 186.9 mAh/g. As shown in FIG. 11, the capacity retention rate of the 100th charge/discharge cycle could be suppressed to 88.3%, which was good. Note that sample 01 had good charge-discharge characteristics with a wide plateau.

100a: Positive electrode active material 100b: Positive electrode active material 100c: Positive electrode active material 130: Binder 131a, 131b: Carbon fiber

Claims (10)

A positive electrode active material layer comprising a positive electrode active material, a conductive auxiliary agent, and a binder,
The positive electrode active material includes lithium, titanium, cobalt, magnesium, oxygen, and fluorine,
The conductive auxiliary agent includes carbon nanotubes with a specific surface area of less than 60 m ² /g,
The positive electrode active material layer has an electrode density of 4.0 g/cm ³ or more after pressure molding. 2. The positive electrode active material layer according to claim 1, wherein the press pressure of the pressure molding is 1050 kN/m or more. A positive electrode active material layer comprising a positive electrode active material, a conductive auxiliary agent, and a binder,
The positive electrode active material includes lithium, titanium, cobalt, magnesium, oxygen, and fluorine,
The conductive auxiliary agent includes carbon nanotubes with a specific surface area of less than 60 m ² /g,
The positive electrode active material layer has an electrode density of 4.3 g/cm ³ or more after pressure molding. 4. The positive electrode active material layer according to claim 3, wherein the press pressure of the pressure molding is 1450 kN/m or more. In any one of claims 1 to 4,
The positive electrode active material is 96% by weight,
The conductivity auxiliary agent is 2% by weight,
The binder is 2% by weight of the positive electrode active material layer. 6. The positive electrode active material layer according to claim 1, wherein the conductive auxiliary agent further contains carbon black having a specific surface area of 60 m ² /g or more. A secondary battery having a positive electrode current collector, a positive electrode including a positive electrode active material layer, and a negative electrode,
The positive electrode active material layer includes a positive electrode active material, a conductive aid, and a binder,
The positive electrode active material layer contains lithium, titanium, cobalt, magnesium, oxygen, and fluorine,
The conductive auxiliary agent is a secondary battery containing carbon nanotubes having a specific surface area of less than 60 m ² /g. In claim 7, the electrode density of the positive electrode active material layer after pressure molding is 4.3 g/cm ³ or more,
The press pressure of the pressure molding is a pressure of 1450 kN/m or more. In claim 7 or claim 8,
The positive electrode active material is 96% by weight,
The conductivity auxiliary agent is 2% by weight,
A secondary battery in which the binder is 2% by weight. The secondary battery according to any one of claims 7 to 9, wherein the conductive auxiliary agent further contains carbon black having a specific surface area of 60 m ² /g or more.

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