JP6989322B2 - Positive electrode for lithium-ion secondary battery - Google Patents

Description

The present invention relates to a positive electrode used in a non-aqueous electrolyte battery, particularly a lithium ion secondary battery.

Non-aqueous electrolyte batteries have been put into practical use as automobile batteries including hybrid automobiles and electric automobiles. A lithium ion secondary battery is used as such an in-vehicle power supply battery. Lithium-ion secondary batteries are required to have various characteristics such as output characteristics, energy density, capacity, life, and high temperature stability.

In particular, as for the positive electrode material as a lithium ion source, a material capable of stably inserting and removing lithium ions is constantly being searched for. In order to maintain the discharge characteristics, capacity, and safety of the battery, it is desired to develop a positive electrode material having a good balance between these performances.

It has been proposed to use an active material having a specific maximum particle size and a specific center particle size in order to form an electrode plate having good surface quality even with a thin coating and high film thickness uniformity, conductivity, and safety. (Patent Document 1).

Japanese Unexamined Patent Publication No. 2006-107779

In lithium-ion secondary batteries for hybrid automobiles that aim to charge and discharge with high current density (high rate), if the average particle size of the positive electrode active material is larger than the thickness of the positive electrode active material layer, lithium ions will be generated. It has been thought that it becomes easier to spread and the input / output characteristics are improved. This is because the interface between the positive electrode active material particles becomes a resistance when lithium ions diffuse, so by using a positive electrode active material with the largest possible average particle size, the interface between the positive electrode active material particles This is because it is considered convenient to reduce the number of particles. Based on this idea, Patent Document 1 proposes to use particles (particularly particles having a large particle size) in which the maximum particle size and the center particle size of the positive electrode active material are in a predetermined range. Here, when an attempt is made to charge and discharge the battery at a higher current density, the amount of lithium ions that permeates the entire separator arranged between the positive electrode and the negative electrode, not the diffusion rate of lithium ions in the positive electrode active material layer. Has been found to be rate-determining. At this time, the value of the positive electrode active material ([maximum particle size of positive electrode active material particles] / [average particle size of positive electrode active material particles]] having a larger maximum particle size with respect to the average particle size is large, that is, the particle size distribution is large. When the positive electrode active material) is used, the state of diffusion of lithium ions in the positive electrode active material layer becomes non-uniform, and therefore the amount of lithium ions reaching the separator varies depending on the position. Then, a large amount of lithium ions reach in a part of the separator, but lithium ions that exceed the performance of the separator cannot be transmitted, and in other parts, lithium ions do not reach much and lithium ions do not sufficiently permeate. Can happen. In this way, it was found that the entire separator does not diffuse a sufficient amount of lithium ions, and not only the performance of the separator cannot be utilized, but also the input / output characteristics of the battery deteriorate as a result.

Therefore, the present invention provides a positive electrode for a lithium ion secondary battery capable of making the amount of lithium ions permeating the entire separator as uniform as possible by setting the particle size distribution of the positive electrode active material particles in an appropriate range. With the goal. Further, it is an object of the present invention to provide a lithium ion battery which is excellent in input / output characteristics and can be charged / discharged by a high density current by using a positive electrode for a lithium ion secondary battery.

The positive electrode for a lithium ion secondary battery in the embodiment of the present invention is a positive electrode for a lithium ion secondary battery in which a positive electrode active material layer containing a positive electrode active material is provided on at least one surface of a positive electrode current collector, and is a positive electrode active material. The average particle size D ₅₀ , the maximum particle size D _max of the positive electrode active material, and the positive electrode active material layer thickness L are as follows.
[Number 1]
5 × D ₅₀ ≦ L (1)
D _max / D ₅₀ ≤ 3.5 (2)
The positive electrode for a lithium ion secondary battery, which satisfies both of the above.
Further, an embodiment of the present invention includes a positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery in which a negative electrode active material layer containing a negative electrode active material is provided on at least one surface of a negative electrode current collector, and a separator. , A lithium ion secondary battery containing a power generation element including an electrolytic solution inside the exterior body.

In the positive electrode for a lithium ion secondary battery of the present invention, lithium ions are uniformly diffused by utilizing the entire surface of the positive electrode active material layer. Therefore, lithium ions can be diffused to the maximum diffusion capacity of the separator even during high-density current charging / discharging. Further, the positive electrode for a lithium ion secondary battery of the present invention can provide a high-performance lithium ion secondary battery having excellent input / output characteristics and charge / discharge characteristics.

FIG. 1 is a diagram illustrating a structure of a positive electrode active material layer of a positive electrode for a conventional lithium ion secondary battery. FIG. 2 is a diagram illustrating the structure of the positive electrode active material layer of the positive electrode for the lithium ion secondary battery of the embodiment. FIG. 3 is a diagram illustrating a structure of a positive electrode active material layer of a positive electrode for a positive electrode for a lithium ion secondary battery of another conventional example. FIG. 4 is a schematic cross-sectional view showing a lithium ion secondary battery using the positive electrode for the lithium ion secondary battery according to the embodiment of the present invention.

Embodiments of the present invention will be described below. As used herein, the lithium ion secondary battery is a lithium ion secondary battery containing a power generation element including a positive electrode, a negative electrode, a separator, and an electrolytic solution inside the exterior body. Here, the positive electrode is a thin plate or a thin plate formed by applying, rolling and drying a mixture of a positive electrode active material, a binder and, if necessary, a conductive auxiliary agent to a positive electrode current collector such as a metal foil to form a positive electrode active material layer. It is a sheet-shaped battery member. The negative electrode is a thin plate-shaped or sheet-shaped battery member in which a mixture of a negative electrode active material, a binder, and a conductive auxiliary agent, if necessary, is applied to a negative electrode current collector to form a negative electrode active material layer. The separator is a film-shaped battery member for separating the positive electrode and the negative electrode and ensuring the conductivity of lithium ions between the negative electrode and the positive electrode. The electrolytic solution is an electrically conductive solution in which an ionic substance is dissolved in a solvent, and a non-aqueous electrolytic solution can be particularly used in this embodiment. A power generation element including a positive electrode, a negative electrode, a separator, and an electrolytic solution is a unit of a main component of a battery, and usually, a positive electrode and a negative electrode are laminated (laminated) via a separator, and this laminate is used. Is immersed in the electrolytic solution.

The lithium ion secondary battery comprises the power generation element inside the exterior body, and preferably the power generation element is sealed inside the exterior body. By being sealed, it means that the power generation element is wrapped with the exterior material described later so as not to be exposed to the outside air. The exterior body is either a housing in which the power generation element can be sealed inside, or a bag shape made of a flexible material. The lithium ion secondary battery may be in various forms such as a coin type battery, a laminated type battery, and a wound type battery.

In the lithium ion secondary battery of the embodiment, the positive electrode is a positive electrode active material layer formed by applying, rolling and drying a mixture of a positive electrode active material, a binder and a conductive auxiliary agent to a positive electrode current collector such as a metal foil. It is a thin plate-shaped or sheet-shaped battery member. Preferably, the positive electrode has a positive electrode active material layer obtained by applying or rolling a mixture of a positive electrode active material, a binder and a conductive auxiliary agent to a positive electrode current collector made of a metal foil such as an aluminum foil, and drying the mixture. .. The positive electrode active material layer contains at least two types of lithium-nickel-based composite oxides having different nickel content ratios and a lithium-manganese-based composite oxide. Here, the lithium-nickel composite oxide represented by the general formula _{Li x Ni y Me (1-} y) O 2 ( wherein Me, Al, Mn, Na, Fe , Co, Cr, Cu, Zn, Ca, At least one metal selected from the group consisting of K, Mg, and Pb, 1.0 ≦ x ≦ 1.2, and y is a positive number less than 1.0). It is a transition metal composite oxide containing lithium and nickel. For example, a lithium-nickel composite oxide in which Me is Co (cobalt) and Al (aluminum) and a lithium-nickel composite oxide in which Me is Co (cobalt) and Mn (manganese) are components of the positive electrode active material. Can be used as.

Examples of the lithium-manganese-based composite oxide include lithium manganate (LiMnO ₂ ) having a zigzag layered structure, spinel-type lithium manganate (LiMn ₂ O ₄ ), and the like. By using a lithium-manganese-based composite oxide in combination, a positive electrode can be produced at a lower cost. In particular, it is preferable to use a _{spinel-type lithium manganate (LiMn 2} O ₄ ), which is excellent in the stability of the crystal structure in an overcharged state. Mn (manganese) in LiMn ₂ O ₄ includes trivalent and tetravalent ones. Of these, manganese, which contributes to the redox of the battery reaction, is trivalent. By substituting a part of this trivalent manganese with another element, the crystal structure can be stabilized. Examples of the element that can be replaced with a part of trivalent manganese include Li, Mg, B, Al, V, Cr, Fe, Co, Ni, W and a combination of two or more thereof.

In all embodiments, the lithium-nickel composite oxide, lithium-nickel-cobalt-manganese composite oxide having the general formula _{Li x Ni y Co z Mn (} 1.0-y-z) layered crystal structure represented by _{O 2} It is preferable to include a substance as a positive electrode active material. Here, x in the general formula is 1.0 ≦ x ≦ 1.2, and y and z are positive numbers satisfying y + z <1.0. The lithium-nickel-based composite oxide having this general formula is, that is, a lithium-nickel-cobalt-manganese composite oxide (hereinafter, may be referred to as “NCM”). NCM is a lithium-nickel-based composite oxide preferably used for increasing the capacity of a battery. For example, in the general formula _{Li x Ni y Co z Mn (} 1.0-y-z) O 2, x = 1, y = 0.4, a composite oxide of z = 0.3 is referred to as "NCM433" A composite oxide having x = 1, y = 0.5, and z = 0.2 is referred to as "NCM523". Further, in the above general formula, the composite oxide of x = 1, y = 1/3, z = 1/3, 1-y-z = 1/3 is referred to as "NCM111".

The average particle size D ₅₀ of the positive electrode active material, the maximum particle size D _max of the positive electrode active material, and the positive electrode active material layer thickness L described above are the following formulas:
[Number 2]
5 × D ₅₀ ≦ L (1)
D _max / D ₅₀ ≤ 3.5 (2)
It is preferable to satisfy both. Here, the average particle size D ₅₀ can be measured by measuring the particle size of ordinary solid particles. Further, the maximum particle size D _max can be adjusted by sieving the particles using a sieve or a filter having a predetermined opening. The positive electrode active material layer thickness L is the thickness after applying the positive electrode active material mixture slurry to the positive electrode current collector, evaporating the solvent, and pressing. The production of the positive electrode active material layer will be described later.

Wherein formula (1), the thickness L of the positive electrode active material layer, which means that it is more than five times the length of the average particle diameter D ₅₀ of the positive electrode active material. The positive electrode active material particles of the embodiment have a relatively smaller average particle size than the conventionally used positive electrode active material particles. _{The formula (2) means that the ratio of the maximum particle size D max} and the average particle size D ₅₀ of the positive electrode active material particles is 3.5 or less. When the ratio of the maximum particle size D _max and the average particle size D ₅₀ of the positive electrode active material particles is 1, it _{means that the maximum particle size D max} and the average particle size D _{50 of} the positive electrode active material particles are equal. It means that there is no distribution of. The particle size distribution of the positive electrode active material of the embodiment is relatively smaller than the particle size distribution of the positive electrode active material that is often used conventionally. The positive electrode active material satisfying both the above formulas (1) and (2) means that the positive electrode active material is "particles having a small particle size with respect to the thickness of the positive electrode active material layer and having a uniform particle size". The positive electrode active material particles satisfying the formula (2) can be appropriately adjusted by changing the _{maximum particle size D max with a sieve having a predetermined opening.} You can also purchase commercial products. In order to satisfy the equation (1), first prepared positive electrode active material satisfying the formula (2), then to form a positive electrode active material layer to be 5 times or more the value of the average particle diameter D _50. The thickness L of the positive electrode active material layer can be adjusted by appropriately changing the amount of the positive electrode active material mixture arranged on the positive electrode current collector and the pressing pressure.

Next, the significance that the particle size of the positive electrode active material satisfies both the formulas (1) and (2) in the embodiment will be described with reference to the drawings. FIG. 1 is a drawing schematically illustrating the configuration of a positive electrode of a conventional example. In FIG. 1, 1: positive electrode for lithium ion secondary battery, 12: positive electrode current collector, 15: positive electrode active material layer, 17: separator, 151: positive electrode active material, and L: positive electrode active material layer thickness. Of the positive electrode active material layer 15, the portion not filled with the positive electrode active material 151 (the blank portion in FIG. 1) contains a conductive auxiliary agent, a binder and other electrode additives. Here, FIG. 1 shows a positive electrode active material having a wide particle size distribution such that _{the average particle size D 50} of the positive electrode active material 151 is large and the ratio of the maximum particle size D _max to the average particle size D _{50 is, for example, 5.} It shows the state of the positive electrode active material layer when particles are used. In FIG. 1, as in 1501, the zone in which the positive electrode active material particles having a length substantially equal to the positive electrode active material layer thickness L occupy the positive electrode active material layer has no interface between the positive electrode active material particles, and thus lithium. The diffusion of ions becomes easy. In FIG. 1, the arrows shown by 4 indicate the flow of lithium ions, and the number of arrows 4 indicates the magnitude of the diffusion of lithium ions. In FIG. 1, the zone 1501 in which four or five arrows 4 are described is a zone in which a large amount of lithium ions diffuse. On the other hand, a zone such as 1502 in which a positive electrode active material having a small particle size and a positive electrode active material having a large particle size are mixed and a plurality of positive electrode active material particles occupy the positive electrode active material layer is an interface between the positive electrode active materials. There are several. Therefore, in the zone 1502, the resistance to the diffusion of lithium ions becomes large, and it becomes difficult for lithium ions to flow. In FIG. 1, the zone 1502 in which two or one arrow 4 is described is a zone in which lithium ions are difficult to diffuse. Here, it is known that when the lithium ion secondary battery is charged and discharged by a high-density current, the diffusion of lithium ions in the separator 17 becomes rate-determining. That is, a zone in which a large amount of lithium ions are diffused, such as zone 1501, is affected by the diffusion rate-determining factor of the separator 17. Looking at the entire separator 17, as schematically shown in FIG. 1, the place where the lithium ion flows to the diffusion rate limiting limit of the separator 17 (zone 1501) and the place where the lithium ion flows to the diffusion rate limiting limit of the separator 17 are not affected by the diffusion rate limiting of the separator. There is a location (zone 1502) and the diffusion of lithium ions is not uniform across the separator 17.

On the other hand, FIG. 2 is a diagram schematically illustrating the positive electrode 1 according to the present embodiment. In FIG. 2, the particle size is fine with respect to the electrode active material layer thickness L, and the particle size distribution is narrow so that the ratio of the _{maximum particle size D max} to the average particle size D _{50 is 3.5 or less.} The state of the positive electrode active material layer when the positive electrode active material is used is shown. In FIG. 2, since a plurality of positive electrode active material particles occupy the positive electrode active material layer at any location of the positive electrode active material layer, the interface between the positive electrode active material particles naturally exists. However, as can be seen from the arrow 4 indicating the flow of lithium ions, the amount of diffusion of lithium ions becomes uniform throughout the separator 17. In this way, when the battery is charged and discharged at a high current density such that the diffusion of lithium ions in the separator is rate-determining by using the positive electrode active material in which _{the average particle size D 50} and the maximum particle size D _{max are adjusted.} Even so, lithium ions can be uniformly diffused over the entire separator.

FIG. 3 is a diagram schematically illustrating the positive electrode 1 according to another conventional example. FIG. 3 shows the case where the formula (2) is satisfied, but the average particle size D ₅₀ with respect to the thickness L of the positive electrode active material layer is large, that is, the positive electrode active material particles having a small particle size distribution are used, but the positive electrode active material layer is used. It shows the state when the particle size is large with respect to the thickness of. In FIG. 3, the zone in which the positive electrode active material particles having a length substantially equal to the thickness L of the positive electrode active material layer occupy the positive electrode active material layer, as in 1501, is lithium because there is no interface between the positive electrode active material particles. The diffusion of ions becomes easy. On the other hand, as in 1502, in the zone including the contact portion between the positive electrode active materials having a large particle size, the interface between the positive electrode active materials exists, so that the resistance to the diffusion of lithium ions becomes large and the lithium ions do not easily flow. Become. When a lithium ion secondary battery having a positive electrode in such a state is charged and discharged by a high-density current, a zone in which a large amount of lithium ions diffuses, such as zone 1501, is affected by the diffusion rate limiting of the separator 17. Looking at the entire separator 17, there are places where lithium ions flow to the diffusion rate-determining limit of the separator 17 (zone 1501) and places where the diffusion rate-determining of the separator does not affect (zone 1502), and diffusion of lithium ions. Is not uniform over the separator 17.

As described with reference to FIGS. 1, 2 and 3, in the present embodiment, the positive electrode active material is selected so as to satisfy both the formula (1) and the formula (2), and the thickness of the positive electrode active material layer is determined. It can be seen that the adjustment, that is, the formation of the positive electrode active material layer as schematically shown in FIG. 2, is very important for improving the performance of the lithium ion secondary battery.

The average particle diameter D ₅₀ of the positive electrode active material is greater than 2 microns, it is preferably 5 micrometers or less. If the average particle size D ₅₀ is too small, the number of contacts between the positive electrode active material particles increases, so that the amount of binder (described later) required for connecting them increases. However, if a large amount of binder is present in the positive electrode active material layer, the proportion of the binder covering the surface of the positive electrode active material increases, the lithium ion path between the positive electrode active material particles is interrupted, and the diffusion resistance of lithium ions may increase. .. Further, since the binder absorbs the electrolytic solution and swells, if the amount of the binder is too large, the swelling amount also becomes large, the conductive path of electrons is interrupted, and the battery resistance may increase after storage for a certain period of time. Be thus considering storage characteristics of the battery, the average particle diameter D ₅₀ of the positive electrode active material is greater than 2 microns, it is preferably 5 micrometers or less. A preferred range of the positive electrode active material layer thickness L derived from a suitable range of the average particle diameter D ₅₀ of the positive electrode active material is at least 10 micrometers or more. In this way, by considering the balance between the particle size of the positive electrode active material and the thickness of the positive electrode active material layer, it is possible to maintain a high output battery without impairing the input / output characteristics even during charging and discharging at a high current density. It will be possible.

The positive electrode active material layer preferably further contains a conductive auxiliary agent. The conductive auxiliary agent assists electrical conduction in the positive electrode active material layer and improves the electrical properties of the lithium ion secondary battery. Examples of the conductive auxiliary agent include carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and ketjen black, and carbon materials such as activated carbon, graphite, mesoporous carbon, fullerenes, and carbon nanotubes. In addition, an electrode additive generally used for electrode formation, such as a thickener, a dispersant, and a stabilizer, can be appropriately used for the positive electrode active material layer.

The positive electrode active material layer preferably further contains a binder. The binder plays a role of adhering the positive electrode active material particles to each other or the positive electrode active material layer and the metal foil. As the binder, fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF), conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole, and styrene butadiene rubber ( Use synthetic rubber such as SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), acrylonitrile butadiene rubber (NBR), or polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin. be able to.

In the embodiment, the positive electrode active material layer is formed by mixing the above-mentioned positive electrode active material, binder, and conductive auxiliary agent with a solvent (N-methylpyrrolidone (NMP), water, etc.) at an appropriate ratio to form a slurry. It can be formed by applying or rolling to a positive electrode current collector made of a metal foil (aluminum foil or the like), heating to evaporate the solvent, and finally pressing.

The negative electrode used together with the positive electrode for the lithium ion secondary battery of the embodiment and constitutes the lithium ion secondary battery is a negative electrode collection such as a metal foil containing a mixture of a negative electrode active material, a binder, and a conductive auxiliary agent if necessary. It is a thin plate-shaped or sheet-shaped battery member having a negative electrode active material layer formed by coating, rolling, and drying on an electric body.

When using a negative electrode in which a mixture of a negative electrode active material, a binder, and a conductive auxiliary agent, if necessary, is applied to a negative electrode current collector or rolled and dried to form a negative electrode active material layer, a carbon material is used as the negative electrode active material. It is preferable to use it. Here, the carbon material contains graphite. In particular, when graphite is contained in the negative electrode active material layer, there is an advantage that the output of the battery can be improved even when the remaining capacity (SOC) of the battery is low. Graphite is a carbon material of hexagonal hexagonal plate crystals, and is sometimes referred to as graphite, graphite, or the like. Graphite is preferably in the form of particles.

Graphite includes natural graphite and artificial graphite. Natural graphite can be obtained in large quantities at low cost, has a stable structure, and has excellent durability. Artificial graphite is artificially produced graphite, which has high purity (it contains almost no impurities such as allotropes) and therefore has low electrical resistance. As the carbon material in the embodiment, both natural graphite and artificial graphite can be suitably used.

Amorphous carbon can also be used as the carbon material in the embodiment. Amorphous carbon is a carbon material that is generally amorphous and has a structure in which microcrystals are randomly networked, which may have a structure partially similar to graphite. .. Examples of the amorphous carbon include carbon black, coke, activated carbon, carbon fiber, hard carbon, soft carbon, mesoporous carbon and the like. When natural graphite particles having a coating with amorphous carbon or artificial graphite having a coating with amorphous carbon are used as the carbon material of the negative electrode active material, the decomposition of the electrolytic solution is suppressed and the durability of the negative electrode is improved. In addition, natural graphite having a coating with amorphous carbon or artificial graphite having a coating with amorphous carbon can also be used.

The binder contained in the negative electrode active material layer plays a role of adhering the particles of the carbon material which is the negative electrode active material to each other and the negative electrode active material layer and the metal foil. For example, when PVDF is used as a binder, N-methylpyrrolidone (NMP) can be used as a solvent instead of water, so that the generation of gas due to residual water can be prevented. In particular, it is preferable that the binder content is 4 to 7% by weight based on the weight of the entire negative electrode active material layer. When the binder content is within the above range, the binding force of the negative electrode material can be secured and the resistance of the negative electrode can be kept low. As binders, in addition to PVDF, fluororesins such as polytetrafluoroethylene (PTFE) and polyvinyl fluoride (PVF), conductive polymers such as polyaniline, polythiophene, polyacetylene and polypyrrole, and styrene butadiene rubber (SBR). ), Synthetic rubber such as butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), acrylonitrile butadiene rubber (NBR), or water-soluble such as polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin. Sex binders can also be used.

The negative electrode active material layer may contain a conductive auxiliary agent. The conductive aid assists electrical conduction in the negative electrode active material layer and improves the electrical properties of the lithium ion secondary battery. Examples of the conductive auxiliary agent include carbon fibers such as carbon nanofibers, carbon black such as acetylene black and ketjen black, and carbon materials such as activated carbon, mesoporous carbon, fullerene, and carbon nanotubes. In addition, an electrode additive generally used for electrode formation, such as a thickener, a dispersant, and a stabilizer, can be appropriately used for the negative electrode active material layer.

In the negative electrode active material layer, a carbon material, a binder, and a conductive auxiliary agent, which are negative electrode active materials, are mixed with a solvent (N-methylpyrrolidone (NMP), water, etc.) at an appropriate ratio to form a slurry, which is then mixed with a metal. It can be formed by applying or rolling to a negative electrode current collector made of a foil (copper foil or the like) and heating to evaporate the solvent.

The separator used together with the positive electrode for a lithium ion secondary battery of the embodiment and constituting the lithium ion secondary battery is a film for separating the positive electrode and the negative electrode and ensuring the conductivity of lithium ions between the negative electrode and the positive electrode. It is a battery member in the shape of a battery. The separator is composed of an olefin resin layer. The olefin-based resin layer is a layer composed of a polyolefin obtained by polymerizing or copolymerizing an α-olefin such as ethylene, propylene, butene, pentene, or hexene. In the embodiment, a structure having pores that are closed when the battery temperature rises, that is, a layer made of porous or microporous polyolefin is preferable. Since the olefin resin layer has such a structure, even if the battery temperature rises, the separator can be blocked (shut down) and the ion flow can be cut off. In order to exert the shutdown effect, it is very preferable to use a porous polyethylene film. The separator may optionally have a heat resistant fine particle layer. At this time, the heat-resistant fine particle layer provided to prevent abnormal heat generation of the battery is composed of inorganic fine particles having a heat-resistant temperature of 150 ° C. or higher and stable in an electrochemical reaction. Examples of such inorganic fine particles include inorganic oxides such as silica, alumina (α-alumina, β-alumina, θ-alumina), iron oxide, titanium oxide, barium titanate, and zirconium oxide; boehmite, zeolite, apatite, and kaolin. Minerals such as spinel, mica, and mullite can be mentioned. As described above, a ceramic separator having a heat-resistant resin layer can also be used.

The electrolytic solution used together with the positive electrode for a lithium ion secondary battery of the embodiment and constituting the lithium ion secondary battery is an electrically conductive solution in which an ionic substance is dissolved in a solvent. In particular, a non-aqueous electrolytic solution can be used. A power generation element including a positive electrode, a negative electrode, a separator, and an electrolytic solution is a unit of a main component of a battery. Normally, a positive electrode and a negative electrode are laminated via a separator, and the laminate is immersed in the electrolytic solution. Has been done.

The electrolytic solution is a non-aqueous electrolytic solution, which is dimethyl carbonate (hereinafter referred to as “DMC”), diethyl carbonate (hereinafter referred to as “DEC”), ethyl methyl carbonate (hereinafter referred to as “EMC”), and di. Chain carbonates such as -n-propyl carbonate, di-t-propyl carbonate, di-n-butyl carbonate, di-isobutyl carbonate, or di-t-butyl carbonate, propylene carbonate (PC), ethylene carbonate (hereinafter "" It is preferably a mixture containing a cyclic carbonate such as "EC"). The electrolytic solution is such a carbonate mixture in which lithium salts such as lithium _{hexafluorophosphate (LiPF 6} ), lithium borofluoride (LiBF ₄ ), and lithium perchlorate (LiClO _{4) are dissolved.}

The electrolytic solution preferably contains PC and / or EC, which are cyclic carbonates, and DMC and / or EMC, which are chain carbonates, in an appropriate combination. PC is a solvent having a low freezing point and is used for improving the output of a battery at a low temperature. However, it is known that PC has a little low compatibility with graphite used as a negative electrode. EC is a solvent having high polarity and high dielectric constant, and is used as a constituent component of an electrolytic solution for a lithium ion secondary battery. However, since EC has a high melting point (freezing point) and is solid at room temperature, even if it is used as a mixed solvent, it may solidify and precipitate at a low temperature. DMC is a solvent having a large diffusion coefficient and a low viscosity. However, since DMC has a high melting point (freezing point), the electrolytic solution may solidify at a low temperature. Like DMC, EMC is a solvent with a large diffusion coefficient and low viscosity. As described above, the constituent components of the electrolytic solution have different characteristics, and it is important to consider the balance between them in order to improve the output of the battery at low temperature, for example. By adjusting the content ratio of the cyclic carbonate and the chain carbonate, it is possible to obtain an electrolytic solution having a low viscosity at room temperature and not losing its performance even at a low temperature.

The electrolytic solution may also contain a cyclic carbonate compound as an additive. Examples of the cyclic carbonate used as an additive include vinylene carbonate (hereinafter referred to as “VC”). Further, a cyclic carbonate compound having a halogen may be used as an additive. These cyclic carbonates are also compounds that form protective films for the positive and negative electrodes during the charging and discharging process of the battery. In particular, it is a compound capable of preventing an attack on a positive electrode active material containing a lithium-nickel-based composite oxide by a sulfur-containing compound such as the above-mentioned disulfonic acid compound or disulfonic acid ester compound. Examples of the cyclic carbonate compound having a halogen include fluoroethylene carbonate (hereinafter referred to as “FEC”), difluoroethylene carbonate, trifluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, trichlorethylene carbonate and the like. Fluoroethylene carbonate, which is a cyclic carbonate compound having a halogen and an unsaturated bond, is particularly preferably used.

Further, the electrolytic solution may further contain a disulfonic acid compound as an additive. The disulfonic acid compound is a compound having two sulfo groups in one molecule, and includes a disulfonate compound in which a sulfo group forms a salt together with a metal ion, or a disulfonic acid ester compound in which a sulfo group forms an ester. .. One or two of the sulfo groups of the disulfonic acid compound may form a salt with a metal ion or may be in an anionic state. Examples of disulfonic acid compounds are methanedisulfonic acid, 1,2-ethanedisulfonic acid, 1,3-propanedisulfonic acid, 1,4-butanedisulfonic acid, benzenedisulfonic acid, naphthalenedisulfonic acid, biphenyldisulfonic acid, and these. Examples thereof include salts (lithium methanedisulfonate, lithium 1,3-ethanedisulfonate, etc.) and anions thereof (anion of methanedisulfonic acid, anion of 1,3-ethanedisulfonate, etc.). Examples of the disulfonic acid compound include disulfonic acid ester compounds, which include methanedisulfonic acid, 1,2-ethanedisulfonic acid, 1,3-propanedisulfonic acid, 1,4-butanedisulfonic acid, benzenedisulfonic acid, and naphthalenedisulfonic acid. Alternatively, chain disulfonic acid esters such as alkyl diesters or aryl diesters of biphenyl disulfonic acid; and cyclic disulfonic acid esters such as methylene methane disulfonic acid ester, ethylene methane disulfonic acid ester and propylene methane disulfonic acid ester are preferably used. Methylenemethane disulfonic acid ester (hereinafter referred to as "MMDS") is particularly preferably used.

Used together with the positive electrode for a lithium ion secondary battery of the embodiment, the exterior body constituting the lithium ion secondary battery includes a coating layer such as a nylon layer and a polyethylene terephthalate layer, a metal base material, an acid-modified polypropylene layer, and a polypropylene layer. It may be in the shape of a bag composed of a laminated body in which and is laminated. Here, the metal base material constituting the laminate used as the material of the exterior body is a base material preferably used as an exterior film of a battery, preferably a metal foil, for example, aluminum, nickel, iron, copper, stainless steel, tin. It is a foil of. The exterior body has a function of sealing the non-aqueous electrolytic solution inside the exterior body. An exterior body is formed by bending the laminate and heat-sealing the three sides other than the bent portion, or by stacking the two laminates and heat-sealing the four sides, and inside this, a positive electrode, a negative electrode, and a separator are formed. And the power generation element composed of the electrolytic solution is sealed.

The "acid-modified polypropylene" in the acid-modified polypropylene layer constituting the laminate means polypropylene in which an acid is introduced by a graft reaction, but in the present specification, it is a propylene / ethylene copolymer or a propylene / ethylene / butene copolymer. , A copolymer in which propylene is introduced as a copolymerization component, such as a polypropylene / butene copolymer, in which an acid is introduced is also referred to as "acid-modified polypropylene". Examples of the acid introduced by the graft reaction include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, itaconic acid, and itaconic anhydride. Maleic anhydride-modified polypropylene introduced with maleic anhydride, maleic anhydride-modified propylene / ethylene copolymer, maleic anhydride-modified propylene / ethylene / butene copolymer, and maleic anhydride-modified polypropylene / butene copolymer are representative. It is a typical "acid-modified polypropylene". The acid-modified polypropylene has a function of adhering a metal base material and a polypropylene layer described later.

In the present specification, "polypropylene" in the polypropylene layer constituting the laminate means a propylene homopolymer, a propylene / ethylene copolymer, a propylene / ethylene / butene copolymer, a polypropylene / butene copolymer, etc. All copolymers of propylene and other olefins such as propylene / 4-methylpentene-1 copolymer and propylene / hexene copolymer are included, and a mixture thereof may be used. The polypropylene layer serves to give flexibility to the laminate. The polypropylene layer preferably contains a lubricant. The lubricant provides ease of molding when forming the polypropylene layer. As a lubricant, it is preferable to use a higher fatty acid amide containing 18 or more carbon atoms. Examples of higher fatty acid amides containing 18 or more carbon atoms are oleic acid amides, behenic acid amides, erucic acid amides and the like.

Here, a configuration example of a lithium ion secondary battery manufactured by using the positive electrode active material of the embodiment will be described with reference to the drawings. FIG. 4 shows an example of a cross-sectional view of a lithium ion secondary battery. The lithium ion secondary battery 10 includes a negative electrode current collector 11, a negative electrode active material layer 13, a separator 17, a positive electrode current collector 12, and a positive electrode active material layer 15 as main components. In FIG. 4, the negative electrode active material layers 13 are provided on both sides of the negative electrode current collector 11, and the positive electrode active material layers 15 are provided on both sides of the positive electrode current collector 12, but only on one side of each current collector. It is also possible to form an active material layer. The negative electrode current collector 11, the positive electrode current collector 12, the negative electrode active material layer 13, the positive electrode active material layer 15, and the separator 17 are constituent units of one battery, that is, a power generation element (single battery 19 in the figure). The separator 17 may be composed of a heat-resistant fine particle layer and an olefin-based resin film (neither of them is shown). A plurality of such cell cells 19 are stacked via the separator 17. The extending portions extending from the negative electrode current collectors 11 are collectively bonded onto the negative electrode leads 25, and the extending portions extending from the positive electrode current collectors 12 are collectively bonded onto the positive electrode leads 27. An aluminum plate is preferably used as the positive electrode lead, and a copper plate is preferably used as the negative electrode lead, and in some cases, it may have a partial coating with another metal (for example, nickel, tin, solder) or a polymer material. The positive electrode lead and the negative electrode lead are welded to the positive electrode and the negative electrode, respectively. The battery formed by stacking a plurality of cells in this way is packaged by the exterior body 29 in such a form that the welded negative electrode lead 25 and the positive electrode lead 27 are pulled out to the outside. In FIG. 4, a laminated body (laminate) is used as the exterior body 29. The electrolytic solution 31 is injected into the exterior body 29. The exterior body 29 has a shape in which two laminated bodies are superposed and the peripheral edge portion is heat-sealed. In FIG. 4, the negative electrode lead 25 and the positive electrode lead 27 are provided on opposite sides of the exterior body 29 (referred to as “both tab type”), but the negative electrode lead 25 and the positive electrode lead 27 are provided on the outer body 29. It is also possible to provide it on one side (that is, the negative electrode lead 25 and the positive electrode lead 27 are pulled out from one side of the exterior body 29, which is referred to as “single tab type”).

The lithium ion secondary battery using the positive electrode for the lithium ion secondary battery according to the embodiment has a high capacity and discharge because its performance can be maximized even if it is charged and discharged at a high current density. Excellent characteristics. In particular, the positive electrode for a lithium ion secondary battery can uniformly transmit lithium ions when the battery is charged and discharged, so that the efficiency is high and the performance of the battery can be improved. Such a lithium ion secondary battery is conveniently used as a vehicle-mounted battery or a stationary battery that requires charging and discharging by a high-density current.

<Manufacturing of positive electrode>
Lithium-nickel-cobalt-manganese composite oxide (NCM111) was prepared as the positive electrode active material. Each positive electrode active material had a maximum particle size D _max , an average particle size D ₅₀ , and a particle surface area (SSA) as shown in Table 1. A positive electrode active material, carbon black (CB) (TIMCAL, SC65) having a BET specific surface area of 62 m ² / g as a conductive auxiliary agent, and PVDF (Kureha, # 7200) as a binder resin are used as a positive electrode in terms of solid content mass ratio. The active material: CB: PVDF was mixed in the ratio shown in Table 1 and added to the solvent NMP. Further, oxalic anhydride (molecular weight 90) as an organic water scavenger was added to this mixture in an amount of 0.03 part by mass based on 100 parts by mass of the solid content excluding NMP from the mixture, and then a planetary dispersion mixing was performed. By carrying out for 30 minutes, these materials were uniformly dispersed to prepare a slurry. The obtained slurry was applied onto both sides of an aluminum foil having a thickness of 20 μm as a positive electrode current collector so that the weight after drying would be the value shown in Table 1 for the weight per side. Then, the electrode was heated at 125 ° C. for 10 minutes to evaporate the NMP to form a positive electrode active material layer. Further, the positive electrode active material layer was pressed so that the pore ratio was 30%. In this way, a positive electrode having a positive electrode active material layer having a thickness shown in Table 1 formed on one surface of the positive electrode current collector was produced. Table 1 shows the capacity per unit area of one side of the positive electrode at this time. Positive electrode size: 280 mm x 260 mm.

<Manufacturing of negative electrode>
Natural graphite powder was used as the negative electrode active material. This carbon material powder, styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) which are binder resins, and carbon black powder (CB) as a conductive auxiliary agent are used as graphite powder: SBR: CMC: CB = 95: 2: The mixture was uniformly mixed so as to have a ratio of 1: 2, and added to pure water as a solvent to prepare a slurry. The obtained slurry was applied on both sides of a rectangular copper foil having a thickness of 10 μm as a negative electrode current collector by a doctor blade method so that the ^{weight after drying would be 3.0 mg / cm 2 per side.} Then, it was dried at 100 ° C. and pressed so that the porosity of the negative electrode active material layer was 35%. In this way, a negative electrode having a negative electrode active material layer formed on one side of the negative electrode current collector so as to face the capacity per unit area of one side of the positive electrode (Table 1) with a capacity 1.2 times larger was produced. Negative electrode size: 300 mm x 280 mm.

<Separator>
A separator having a thickness of 16 μm and a size of 320 mm × 320 mm made of uniaxially stretched polypropylene was used. The air permeability of the separator is 80 seconds / 100 ml, and the porosity is 60%.

<Electrolytic solution>
_{Lithium hexafluorophosphate (LiPF 6} ) as an electrolyte salt was added to a mixed non-aqueous solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a ratio of 30:70 (volume ratio) at a concentration of 1.0 mol / L. Then, vinylene carbonate (VC) as an additive was dissolved so as to be 1.0% by weight. This non-aqueous mixed solvent was used as an electrolytic solution.

<Manufacturing of lithium-ion secondary battery>
An aluminum positive electrode lead terminal was ultrasonically welded to an uncoated portion for connecting the positive electrode terminal to the positive electrode produced as described above. Similarly, a nickel negative electrode lead terminal was ultrasonically welded to the uncoated portion for connecting the negative electrode terminal. The negative electrode / separator / positive electrode are arranged in this order so that the active material layers of the negative electrode and the positive electrode overlap each other with the separator separated from each other, and the outermost electrode is laminated so as to be the negative electrode. , 6 negative electrodes) were obtained. This electrode laminate was wrapped in two exterior bodies (aluminum laminate, thickness: 0.15 mm), and three sides were bonded by heat fusion except for one of the long sides. A laminated lithium-ion battery was produced by injecting 370 microliters of an electrolytic solution into the solution and impregnating it with vacuum, and then sealing the openings by heat fusion under reduced pressure. After the initial charge of this laminated lithium ion battery, it was aged at 45 ° C. for several days to obtain a laminated lithium ion secondary battery.

<First charge / discharge>
The first charge / discharge was performed using the laminated lithium ion secondary battery prepared as described above. For the first charge and discharge, first, constant current constant voltage (CC-CV) charging was performed at an atmospheric temperature of 25 ° C., a 10 mA current, and an upper limit voltage of 4.2 V, and then aging was performed at 45 ° C. for several days. After that, constant current discharge was performed at a current of 20 mA up to 2.5 V.

<Evaluation of lithium-ion secondary battery element: Initial output characteristics>
The created lithium-ion battery was charged to 3.7V. Discharge was started at a current amount of 44 mA / cm ² , and the battery voltage was measured after 10 seconds had elapsed. If the battery voltage is less than 3V 10 seconds after the start of discharging, or if the battery cannot be discharged for 10 seconds, it is evaluated as defective.

<Evaluation of lithium-ion secondary battery element: high temperature storage test>
As described above, the laminated lithium ion secondary battery subjected to the initial charge / discharge was stored at 45 ° C. at SOC 60%. After 16 weeks, the resistance of the battery was measured. It was calculated how much the resistance increased compared to the battery resistance before storage. However, this high temperature storage test has not been carried out for the comparative example evaluated as defective in the above "initial output characteristics".

A lithium ion secondary battery using a positive electrode in which the relationship between the average particle size D ₅₀ , the maximum particle size D _max , and the positive electrode active material layer thickness L satisfies the range of the present invention is charged and discharged by a high-density current. Also shows excellent initial output characteristics. The coefficient of increase in resistance measured in the high temperature storage test simply represents the coefficient of expansion of the power generation element. A battery with a large increase in resistance means that the battery (particularly the electrodes) has expanded and the distance between the electrodes has increased. It can be seen that the positive electrode of the present invention is unlikely to expand due to charge / discharge, especially in the specifications of Examples 2 to 6, but this is because lithium ions are inserted and removed almost uniformly from the entire positive electrode, and lithium ions are emitted throughout the battery. It is considered that this is because it is uniformly diffused.

Although the embodiments of the present invention have been described above, the above embodiments are merely examples of the embodiments of the present invention, and the technical scope of the present invention is limited to a specific embodiment or a specific configuration. No.

1 Positive electrode for lithium ion secondary battery 10 Lithium ion secondary battery 11 Negative electrode current collector 12 Positive electrode current collector 13 Negative electrode active material layer 15 Positive electrode active material layer 151 Positive electrode active material layer 151 Positive electrode active material 1501 Zone 1502 that is not affected by the diffusion rate control of the separator Zone affected by the diffusion rate control of the separator 17 Separator 4 Lithium ion flow 25 Negative electrode lead 27 Positive electrode lead 29 Exterior 31 Electrolyte

Claims (4)

A positive electrode for a lithium ion secondary battery in which a positive electrode active material layer containing a positive electrode active material is provided on at least one surface of a positive electrode current collector.
Positive electrode active material is, in the general formula _{Li x Ni y Co z Mn (} 1.0-y-z) O 2 ( wherein, is 1.0 ≦ x ≦ 1.2, y and z y + z <1. It is a lithium nickel-cobalt-manganese composite oxide having a layered crystal structure represented by (a positive number satisfying 0).
The average particle size D ₅₀ of the positive electrode active material, the maximum particle size D _max of the positive electrode active material, and the thickness L of the positive electrode active material layer are as follows.
[Number 1]
5 × D ₅₀ ≦ L (1)
D _max / D ₅₀ ≤ 3.5 (2)
The positive electrode for a lithium ion secondary battery that satisfies both of the above. The average particle diameter D ₅₀ of the positive electrode active material, 2 larger than micrometers, 5 micrometers or less, the positive electrode for a lithium ion secondary battery according to claim 1. The positive electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode active material layer contains 4 to 6% of a binder based on the mass of the positive electrode active material layer. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3.
A negative electrode for a lithium ion secondary battery in which a negative electrode active material layer containing a negative electrode active material is provided on at least one surface of the negative electrode current collector, and a negative electrode.
Separator and
With the electrolyte
A lithium-ion secondary battery that contains a power generation element including the inside of the exterior body.

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