JP2016009543A - Positive electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a positive electrode for a lithium ion secondary battery, which is high in capacity and low in resistance; and a lithium ion secondary battery.SOLUTION: A positive electrode for a lithium ion secondary battery comprises: a collector; and a positive electrode active material layer bonded to the collector. The positive electrode active material layer includes: a positive electrode active material; a conductive assistant; and a binding agent. The conductive assistant includes: scale-like graphite and amorphous carbon. The content of the amorphous carbon is 1 to 5 times that of the scale-like graphite. The positive electrode active material layer has a porosity of 24-28%.

Description

  The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery.

  A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. Currently, it is mainly used as a power source for portable electronic devices, and further expected as a power source for electric vehicles that are expected to be widely used in the future.

  A lithium ion secondary battery has an active material capable of inserting and extracting lithium (Li) in a positive electrode and a negative electrode, respectively. And it operates by moving lithium ions in the electrolyte provided between the two electrodes. In lithium ion secondary batteries, lithium-containing metal composite oxides such as lithium cobalt composite oxide are mainly used as the active material for the positive electrode, and carbon materials having a multilayer structure are mainly used as the active material for the negative electrode. .

  Lithium ion secondary batteries are required to have higher capacities. For example, silicon-based materials such as silicon and silicon oxide having higher capacities than carbon materials have been studied as negative electrode active materials. The theoretical capacity of the silicon-based material is about 5 to 6 times the theoretical capacity of the carbon material. In order to increase the capacity of a lithium ion secondary battery using a silicon-based material as the negative electrode active material, the capacity of the positive electrode must be increased in accordance with the theoretical capacity of the negative electrode.

The capacity of the positive electrode can be controlled not only by the type of active material and the mixing ratio of the active materials, but also by the basis weight (g / cm ² ) of the positive electrode active material layer of the positive electrode. The basis weight of the positive electrode active material layer of the positive electrode means the mass of the positive electrode active material layer per unit area of the surface on which the positive electrode active material layer of the positive electrode current collector is formed. In general, when it is desired to greatly increase the capacity of the positive electrode, a method of increasing the weight of the positive electrode active material layer of the positive electrode and thereby increasing the content of the positive electrode active material in the positive electrode is taken. Moreover, the thickness of a positive electrode may increase with the increase in the fabric weight of the positive electrode active material layer of a positive electrode.

  The positive electrode active material generally has low conductivity. Increasing the content of the positive electrode active material in the positive electrode increases the resistance of the positive electrode. In general, in order to reduce the resistance of the electrode, a conductive assistant may be added to form a good conductive path in the electrode. A conductive path means a path through which electrons flow.

  Conventionally, various studies have been made to form a good conductive path in an electrode. For example, the types and blending amounts of conductive assistants are being studied.

  Patent Document 1 discloses a lithium secondary battery using a conductive agent including a carbonaceous material A having an average particle size of 100 nm or less and a carbonaceous material B having an average particle size of 1 μm or more. Patent Document 1 describes that the use of two types of carbonaceous materials having different average particle diameters improves the high-current discharge characteristics and cycle characteristics of the secondary battery. In Patent Document 1, the content of the carbonaceous material A having an average particle size of 100 nm or less is 25% by weight or more and 50% by weight or less with respect to the sum of the contents of the carbonaceous material A and the carbonaceous material B. Preferred is described.

  Patent Document 2 discloses a positive electrode for a lithium ion secondary battery containing scaly graphite and amorphous carbon as a conductive agent, and the content of scaly graphite in the positive electrode is 3% by mass or more and 7% by mass or less. It is described that the electrical conductivity of the positive electrode is improved and the capacity of the battery is improved by setting the mass ratio of flaky graphite to amorphous carbon to flaky graphite: amorphous carbon = 6: 4 to 7: 2. .

  Patent Document 3 includes a positive electrode mixture layer including a conductive agent composed of graphite powder and amorphous carbon powder, a positive electrode active material composed of a lithium transition metal composite oxide, and a binder. A nonaqueous electrolyte secondary battery is described in which the porosity of the agent layer is 26% or more and 39% or less, and amorphous carbon powder is contained in an amount of 10% by mass to 30% by mass in the conductive agent. Patent Document 3 describes that this nonaqueous electrolyte secondary battery can reduce electrode resistance and is excellent in load characteristics of the battery. If the content of the amorphous carbon powder in the conductive agent exceeds 30% by mass, even if the positive electrode mixture layer is compressed, the positive electrode mixture layer cannot be made sufficiently thin, and the electrode packing density becomes small. It is described that it is not preferable because the battery capacity is insufficient. Moreover, since the contact area with respect to a non-aqueous electrolyte is not fully ensured that a porosity is small, it describes that it is not preferable.

  However, in the inventions described in Patent Documents 1 to 3, it is unclear whether or not the increase in resistance of the positive electrode can be suppressed even when the content of the positive electrode active material in the positive electrode is large.

JP 2000-21407 JP 2003-331922 A Japanese Patent No. 3435731

  The present invention has been made in view of such circumstances, and even if the content of the positive electrode active material is increased to increase the capacity of the lithium ion secondary battery, the resistance at the positive electrode is increased. It aims at providing the positive electrode for lithium ion secondary batteries which can be suppressed, and a lithium ion secondary battery having the same.

  The positive electrode for a lithium ion secondary battery of the present invention that solves the above problems comprises a current collector and a positive electrode active material layer bound to the current collector. The positive electrode active material layer comprises a positive electrode active material, a conductive assistant. The conductive auxiliary agent is composed of scaly graphite and amorphous carbon, and the content of amorphous carbon is 1 to 5 times the content of scaly graphite. The porosity of the positive electrode active material layer is 24% or more and 28% or less.

Here, the porosity is calculated by the following formula.
Porosity (%) = (apparent volume of positive electrode active material layer−volume of material of positive electrode active material layer) ÷ apparent volume of positive electrode active material layer × 100
Apparent volume of positive electrode active material layer = actual thickness of positive electrode active material layer × area of current collector surface on which positive electrode active material layer is formed Volume of positive electrode active material layer material = mass of positive electrode active material ÷ positive electrode active material True density of graphite + mass of flake graphite ÷ true density of flake graphite + mass of amorphous carbon ÷ true density of amorphous carbon + mass of binder ÷ true density of binder

The average particle diameter D ₅₀ of the positive electrode active material is 5 μm to ₁₅ μm, the average particle diameter D ₅₀ of the scaly graphite is 1 μm or more, and the average particle diameter D _{50 of the} positive electrode active material is an average particle of amorphous carbon it is preferable that the diameter _{D 50} is 10nm or more 100nm or less.

Note that the average particle diameter D ₅₀ refers to the particle size cumulative value of the volume distribution in the particle size distribution measurement by laser diffraction method is equivalent to 50%. That is, the average particle diameter D ₅₀ means the median size measured by volume.

  The content of the conductive additive is preferably more than 2% by mass and less than 7% by mass when the entire positive electrode active material layer is 100% by mass.

The basis weight of the positive electrode active material layer is preferably 17 mg / cm ² or more and 30 mg / cm ² or less.

The density of the positive electrode active material layer is preferably 2.9 g / cm ³ or more and 3.3 g / cm ³ or less. The density of the positive electrode active material layer means the mass of the positive electrode active material layer per apparent volume of the positive electrode active material layer.

  The lithium ion secondary battery of this invention has the said positive electrode for lithium ion secondary batteries, It is characterized by the above-mentioned.

  If it is set as the positive electrode for lithium ion secondary batteries of this invention, the favorable electroconductive path by scaly graphite and amorphous carbon can be formed between positive electrode active materials and between a positive electrode active material and a collector. For this reason, even if it increases content of the positive electrode active material in a positive electrode, the increase in the resistance of a positive electrode can be suppressed. In addition, since the porosity of the positive electrode active material layer is an appropriate amount, even if the content of the positive electrode active material in the positive electrode is increased and the thickness of the positive electrode active material layer is increased, lithium ions can move smoothly. Therefore, it is possible to suppress the deterioration of the capacity characteristics.

It is sectional drawing which represents typically the preferable one aspect | mode of the positive electrode for lithium ion secondary batteries of this invention. It is a graph which compares the value of the cell resistance of the laminate-type lithium ion secondary battery of Examples 1-4 and Comparative Examples 1-3. 4 is a graph comparing cell resistance and porosity of laminated lithium ion secondary batteries of Example 2, Example 5, Comparative Example 4 and Comparative Example 5. FIG.

<Positive electrode for lithium ion secondary battery>
The positive electrode for a lithium ion secondary battery of the present invention comprises a current collector and a positive electrode active material layer bound to the current collector.

  The current collector refers to a chemically inert electronic high conductor that keeps a current flowing through an electrode during discharge or charging of a lithium ion secondary battery. As the current collector, at least one selected from silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel, etc. Metal materials can be exemplified. The current collector may be covered with a known protective layer. What collected the surface of the electrical power collector by the well-known method may be used as an electrical power collector.

  The current collector can take the form of a foil, a sheet, a film, a linear shape, a rod shape, a mesh, or the like. Therefore, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, and a stainless steel foil can be suitably used as the current collector. When the current collector is in the form of foil, sheet or film, the thickness is preferably in the range of 1 μm to 100 μm.

  The positive electrode active material layer includes a positive electrode active material, a conductive additive, and a binder. The positive electrode active material layer further includes voids.

As the positive electrode active material, for example, of a layered compound _{Li a Ni b Co c Mn d} D e O f (0.2 ≦ a ≦ 1.2, b + c + d + e = 1,0 ≦ e <1, D is Li, Fe, At least one element selected from Cr, Cu, Zn, Ca, Mg, S, Si, Na, K, Al, Zr, Ti, P, Ga, Ge, V, Mo, Nb, W, La, 1.7 ≦ f ≦ 2.1) and Li ₂ MnO ₃ . Further, as the positive electrode active material, spinel such as LiMn ₂ O ₄ , solid solution composed of a mixture of spinel and layered compound, LiMPO ₄ , LiMVO ₄ or Li ₂ MSiO ₄ (wherein M is Co, Ni, Mn, Fe And a polyanionic compound selected from at least one of them. Furthermore, as the positive electrode active material, tavorite compound (the M a transition metal) LiMPO ₄ F, such as LiFePO ₄ F represented by, Limbo ₃ such LiFeBO ₃ (M is a transition metal) include borate-based compound represented by be able to. Any metal oxide used as the positive electrode active material may have the above composition formula as a basic composition, and a metal element contained in the basic composition may be substituted with another metal element. Further, as the positive electrode active material, a positive electrode active material that does not contain lithium ions contributing to charge / discharge, for example, sulfur alone (S), a compound in which sulfur and carbon are combined, a metal sulfide such as TiS ₂ , V ₂ O, etc. ₅ , oxides such as MnO ₂ , polyaniline and anthraquinone, compounds containing these aromatics in the chemical structure, conjugated materials such as conjugated diacetic acid organic materials, and other known materials can also be used. Further, a compound having a stable radical such as nitroxide, nitronyl nitroxide, galvinoxyl, phenoxyl, etc. may be adopted as the positive electrode active material. When using a positive electrode active material that does not contain lithium, it is necessary to add lithium ions to the positive electrode and / or the negative electrode in advance by a known method. Here, in order to add lithium ions, a compound containing lithium ions or lithium metal may be used.

The average particle diameter _{D 50} of the positive electrode active material is preferably 2 .mu.m to 20 .mu.m. When the average particle diameter D ₅₀ of the positive electrode active material is too small, the difference in particle size between the positive electrode active material and the amorphous carbon is decreased, hardly conductive path by amorphous carbon formed. When the average particle diameter D ₅₀ of the positive electrode active material is too large resistance of the positive electrode increases. The average particle diameter D50 of the positive electrode active material is particularly preferably ₅ μm to 15 μm.

  The binder has a function of connecting the positive electrode active material and the conductive additive to the current collector. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorine-containing resins such as fluorine rubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, poly ( Examples thereof include acrylic resins such as (meth) acrylic acid, alkoxysilyl group-containing resins, styrene / butadiene rubber (SBR), and carboxymethylcellulose (CMC). These binders can be used alone or in combination of two or more.

  The blending ratio of the binder in the positive electrode active material layer is preferably a mass ratio of positive electrode active material: binder = 1: 0.001 to 1: 0.3. The positive electrode active material: binder is more preferably 1: 0.005 to 1: 0.2, and further preferably 1: 0.01 to 1: 0.15. This is because if the amount of the binder is too small, the moldability of the electrode may be lowered, and if the amount of the binder is too large, the energy density of the electrode is lowered.

  A conductive support agent consists of flaky graphite and amorphous carbon.

Scale-like graphite is flaky graphite, and both natural and artificial ones can be used. The electrical resistivity of scaly graphite is very low, about 0.0014 Ω · cm. The average particle diameter D ₅₀ of the flake graphite is preferably greater than the average particle diameter D ₅₀ of the amorphous carbon. A piece of flake graphite having a very low electrical resistivity and a large average particle diameter D ₅₀ is disposed between the positive electrode active materials and between the positive electrode active material and the current collector, thereby forming one piece of flake shape. Even with graphite, a long-distance conductive path can be easily formed in the positive electrode active material layer.

It is preferable that the average particle diameter _{D 50} of the flake graphite is 1μm or more 20μm or less. In particular, the average particle diameter D ₅₀ of the flake graphite is preferably 1 μm or more and the average particle diameter D _{50 of the} positive electrode active material. When the average particle diameter D ₅₀ of the flake graphite is too small, long-distance conductive path is hard to be formed. Scaly average particle diameter D ₅₀ of the graphite is not easily mixed with the larger average particle diameter D ₅₀ of the positive electrode active material and flaky graphite and the positive electrode active material.

Also, if the same amount of flake graphite in the positive electrode active material layer, the number of flake graphite contained as the positive electrode active material layer Average particle diameter D ₅₀ of the flake graphite is small increases. An increase in the number of flake graphite per unit mass is advantageous because it is advantageous for forming a good conductive path. When the average particle diameter D ₅₀ of the scaly graphite too large, becomes small number of scale-like graphite per unit mass, it is difficult to form good electrically conductive paths in the positive electrode active material layer.

Amorphous carbon has an average particle diameter D ₅₀ smaller than the flake graphite. Amorphous carbon enters the gap formed by the scaly graphite, the positive electrode active material, and the current collector, and forms a conductive path in cooperation with the scaly graphite.

  As the amorphous carbon, furnace black, ketjen black (registered trademark) (KB), channel black, acetylene black (AB), or the like can be used. In some cases, thermal black may be used.

The amorphous carbon may preferably have an average particle size D ₅₀ of the nano-order of less than 1nm and not more than 1 [mu] m. However amorphous carbon of an average particle diameter _{D 50} 1 nm to 10 nm is likely to aggregate easily coarsened. Therefore, the average particle diameter _{D 50} of the amorphous carbon is preferably 10 nm to 100 nm.

The average particle diameter D ₅₀ of the amorphous carbon is too large, difficult conductive path by amorphous carbon formed. Amorphous carbon enters the gap formed by the positive electrode active material, the scaly graphite, and the current collector in the positive electrode to form a conductive path. For that reason the average particle diameter D ₅₀ of the amorphous carbon is too large, the difference in particle size between the positive electrode active material and flaky graphite is hardly becomes to amorphous carbon fine gaps in the positive electrode penetrates small non It becomes difficult to form a conductive path by crystalline carbon.

  The content of amorphous carbon is 1 to 5 times the content of scaly graphite. The content of amorphous carbon is more preferably 1 to 3 times the content of scaly graphite.

  Scalable graphite and amorphous carbon combine with each other to form a good conductive path. In the present invention, the amorphous carbon content is equal to or higher than the scaly graphite content. If the amorphous carbon content is less than the scaly graphite content, it is difficult to form a good conductive path, and the increase in resistance of the positive electrode cannot be effectively suppressed.

  In order to increase the capacity of the lithium ion secondary battery, it is preferable that the content of the entire conductive additive in the positive electrode active material layer is small because the amount of the positive electrode active material in the positive electrode active material layer can be increased accordingly.

  Here, since scaly graphite is a flake, the distance for conducting electricity is longer than that of granular graphite having the same mass. That is, scaly graphite can form a long-distance conductive path with less mass than granular graphite. Therefore, scaly graphite is advantageous for forming a long-distance conductive path in the positive electrode active material layer.

  It is preferable that the content of the entire conductive additive including the scaly graphite and the amorphous carbon is more than 2% by mass and less than 7% by mass when the entire positive electrode active material layer is 100% by mass. By making content of a conductive support agent into the said range, electroconductivity can be ensured, content of the positive electrode active material in a positive electrode active material layer can be increased relatively, and it can be set as a high capacity | capacitance positive electrode.

  Furthermore, in order to obtain a high-capacity positive electrode, the content of the conductive additive is preferably 5% by mass or less when the entire positive electrode active material layer is 100% by mass.

  The positive electrode active material layer further includes voids. The gap is between the positive electrode active materials, between the positive electrode active material and scaly graphite, between the positive electrode active material and amorphous carbon, between the positive electrode active material and the binder, and between the positive electrode active material and the current collector. Between flaky graphite, between flaky graphite, between flaky graphite and amorphous carbon, between flaky graphite and binder, between flaky graphite and current collector, between amorphous carbon And between the amorphous carbon and the binder, between the amorphous carbon and the current collector, between the binders, and between the binder and the current collector. Through these voids, the electrolyte can move favorably in the positive electrode active material layer. The porosity of the positive electrode active material layer is 24% or more and 28% or less. If the porosity of the positive electrode active material layer is too low, lithium ions will not move smoothly and the resistance of the lithium ion secondary battery may be increased. When the porosity of the positive electrode active material layer is too high, it becomes difficult for the conductive assistants to come into contact with each other, and it becomes difficult to form a conductive path, which may increase the resistance.

The basis weight of the positive electrode active material layer is preferably 17 mg / cm ² or more and 30 mg / cm ² or less. The basis weight of the positive electrode active material layer is more preferably 17 mg / cm ² or more and 20 mg / cm ² or less. The basis weight of the positive electrode active material layer is determined by dividing the mass of the entire positive electrode active material layer by the area of the surface of the current collector on which the positive electrode active material layer is formed. If the basis weight is too low, the battery capacity may be low. If the basis weight is too high, desired output characteristics may not be obtained.

The density of the positive electrode active material layer is preferably 2.7 g / cm ³ or more and 3.5 g / cm ³ or less. The density of the positive electrode active material layer is more preferably 2.9 g / cm ³ or more and 3.3 g / cm ³ or less. The density of the positive electrode active material layer is determined by dividing the mass of the entire positive electrode active material layer by the apparent volume of the positive electrode active material layer. If the density of the positive electrode active material layer is too low, the battery capacity may be reduced. If the density of the positive electrode active material layer is too high, desired output characteristics may not be obtained.

  The thickness of the positive electrode active material layer is preferably 50 μm or more and 100 μm or less. The thickness of the positive electrode active material layer can be calculated by measuring the thickness of the positive electrode and subtracting the thickness of the current collector used from the measured value. From the viewpoint of increasing the capacity of the positive electrode, the thickness of the positive electrode active material layer is preferably 50 μm or more. When the positive electrode active material layer is too thin, the ratio of the current collector in the positive electrode increases, so that the energy density of the battery is reduced regardless of whether the lithium ion secondary battery is a wound type or a stacked type. If the thickness of the positive electrode active material layer is too thick, it becomes difficult for the positive electrode active material layer to uniformly absorb and release lithium ions into the positive electrode active material in the depth direction.

  Here, FIG. 1 is a sectional view schematically showing a preferred embodiment of the positive electrode for a lithium ion secondary battery of the present invention. As shown in FIG. 1, the positive electrode for a lithium ion secondary battery includes a current collector 1 and a positive electrode active material layer 2 formed on the surface of the current collector 1. The positive electrode active material layer 2 includes a positive electrode active material 3, scaly graphite 41, amorphous carbon 42, and a binder 5. A large number of voids 6 exist in the positive electrode active material layer 2. The gap 6 is formed between the positive electrode active materials 3, between the positive electrode active material 3 and the flaky graphite 41, between the positive electrode active material 3 and the amorphous carbon 42, and between the positive electrode active material 3 and the binder 5. Between the positive electrode active material 3 and the current collector 1, between the flaky graphites 41, between the flaky graphite 41 and the amorphous carbon 42, between the flaky graphite 41 and the binder 5, and between the flaky graphite Between the graphite graphite 41 and the current collector 1, between the amorphous carbons 42, between the amorphous carbon 42 and the binder 5, between the amorphous carbon 42 and the current collector 1, It is formed between the adhesives 5 and between the binder 5 and the current collector 1. Through these voids 6, the electrolytic solution can move well in the positive electrode active material layer 2.

In a preferred embodiment, the positive electrode active material 3 is in a powder form, and the flaky graphite 41 is a flake having an average particle diameter D ₅₀ that is equal to or smaller than the average particle diameter D ₅₀ of the positive electrode active material 3. The scaly graphite 41 is disposed in a gap formed between the current collector 1 and each positive electrode active material 3, and a long-distance conductive path can be formed in the positive electrode active material layer 2 even with one scaly graphite 41. Preferred amorphous carbon 42 in one embodiment is a powder form, the average particle diameter D ₅₀ of amorphous carbon 42, the average particle diameter D ₅₀ and the average particle diameter D ₅₀ of the flake graphite 41 of the positive electrode active material 3 Smaller than that. Therefore, the amorphous carbon 42 enters a gap formed by the current collector 1, each positive electrode active material 3, and each scaly graphite 41. The amorphous carbon 42 can electrically connect the flaky graphites 41 by entering the gaps. In this way, the amorphous carbon 42 cooperates with the flaky graphite 41 to form a good conductive path in the positive electrode active material layer.

  The content of the amorphous carbon 42 is 1 to 5 times the content of the scaly graphite 41. In the positive electrode active material layer 2, a conductive path is formed by the flaky graphite 41 and the amorphous carbon 42 between the positive electrode active material 3 and the current collector 1, and good conductivity is ensured. Therefore, an increase in resistance in the positive electrode for a lithium ion secondary battery of the present invention is suppressed.

  A positive electrode is prepared by preparing a composition for forming a positive electrode active material layer containing the positive electrode active material, a conductive additive, and a binder, and further adding a suitable solvent to the composition to form a paste. After coating on the surface of the film, it can be dried and, if necessary, compressed to increase the electrode density.

  As a method for applying the composition for forming a positive electrode active material layer, conventionally known methods such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, and a curtain coating method may be used.

  As the solvent for adjusting the viscosity, for example, N-methyl-2-pyrrolidone (NMP), methanol, methyl isobutyl ketone (MIBK), water and the like can be used.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention includes the positive electrode of the present invention. In the lithium ion secondary battery of the present invention, known materials can be used for the negative electrode, the separator and the electrolytic solution.

  The negative electrode has a current collector and a negative electrode active material layer bound to the surface of the current collector. The negative electrode active material layer includes a negative electrode active material and a binder, and optionally includes a negative electrode conductive additive. The current collector and the binder are the same as those described for the positive electrode.

As the negative electrode active material, a material capable of inserting and extracting lithium ions can be used. Accordingly, the negative electrode active material is not particularly limited as long as it is a simple substance, an alloy, or a compound that can occlude and release lithium ions. Examples of the negative electrode active material include Li, group 14 elements such as carbon, silicon, germanium, and tin, group 13 elements such as aluminum and indium, group 12 elements such as zinc and cadmium, group 15 elements such as antimony and bismuth, An alkaline earth metal such as magnesium or calcium, or a group 11 element such as silver or gold may be used alone. When silicon or the like is used for the negative electrode active material, a silicon atom reacts with a plurality of lithiums, so that it becomes a high-capacity active material. However, there is a problem that volume expansion and contraction due to insertion and extraction of lithium becomes significant. May occur. In order to reduce the fear, it is also preferable to employ an alloy or compound in which another element such as a transition metal is combined with a simple substance such as silicon as the negative electrode active material. Specific examples of the alloy or compound include tin-based materials such as Ag-Sn alloy, Cu-Sn alloy, Co-Sn alloy, carbon-based materials such as various graphites, SiO _x (disproportionated to silicon simple substance and silicon dioxide). Examples thereof include silicon-based materials such as 0.5 ≦ x ≦ 1.5), silicon alone, or a composite of a silicon-based material and a carbon-based material.

  Examples of the carbon-based material include non-graphitizable carbon, artificial graphite, natural graphite, cokes, graphites, glassy carbons, organic polymer compound fired bodies, carbon fibers, activated carbon, and carbon blacks. Here, the organic polymer compound fired body refers to a material obtained by firing and carbonizing a polymer material such as phenols and furans at an appropriate temperature.

In addition, as the negative electrode active material, oxides such as Nb ₂ O ₅ , TiO ₂ , Li ₄ Ti ₅ O ₁₂ , WO ₂ , MoO ₂ , Fe ₂ O ₃ , or Li _3-x M _x N (M = A nitride represented by (Co, Ni, Cu) may be employed. One or more of these materials can be used as the negative electrode active material.

The negative electrode active material is preferably in powder form. If the anode active material is in powder form, it is preferable that the average particle size _{D 50} of the negative electrode active material is 0.1μm or more 30μm or less, and more preferably 1μm or more 20μm or less. When the average particle diameter D ₅₀ of the negative electrode active material is too small, the specific surface area of the powder of the negative electrode active material is increased, it increases the contact area of the powder of the anode active material and the electrolyte solution, proceed decomposition of the electrolyte solution As a result, the cycle characteristics may be deteriorated. If the average particle diameter D ₅₀ of the negative electrode active material is too large, if the conductivity using a low negative electrode active material, conductive entire electrode becomes uneven, charging and discharging characteristics may deteriorate.

In particular, SiO _x (0.5 ≦ x ≦ 1.5) having a high capacity theoretical capacity is preferably used as the negative electrode active material.

  Examples of the conductive aid for the negative electrode include carbon black, graphite, acetylene black (AB), ketjen black (registered trademark) (KB), and vapor grown carbon fiber (VGCF), which are carbonaceous fine particles. These conductive additives for negative electrodes can be used alone or in combination of two or more. The amount of the conductive additive for the negative electrode is not particularly limited. For example, the conductive additive for the negative electrode is about 1 part by mass to 30 parts by mass with respect to 100 parts by mass of the active material contained in the negative electrode. be able to.

  The separator separates the positive electrode and the negative electrode and allows lithium ions to pass through while preventing a short circuit of current due to contact between the two electrodes. As the separator, for example, a porous film made of a synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramics can be used.

  The electrolytic solution includes a solvent and an electrolyte dissolved in the solvent.

  As the solvent, for example, cyclic esters, chain esters, and ethers can be used. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone. Examples of chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, ethyl methyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. As ethers, for example, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane can be used.

Moreover, as an electrolyte dissolved in the electrolytic solution, for example, a lithium salt such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ can be used.

As an electrolytic solution, for example, a lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 is} added to a solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, dimethyl carbonate, and the like from 0.5 mol / l to 1.7 mol / l. A solution dissolved at a certain concentration can be used.

  The lithium ion secondary battery can be mounted on a vehicle. Since the lithium ion secondary battery has a high capacity and a low resistance, a vehicle equipped with the lithium ion secondary battery has high performance in terms of output and life.

  The vehicle may be a vehicle that uses electric energy from a lithium ion secondary battery for all or a part of its power source, such as an electric vehicle, a hybrid vehicle, a plug-in hybrid vehicle, a hybrid railway vehicle, and an electric forklift. , Electric wheelchairs, electric assist bicycles, electric motorcycles. When a lithium ion secondary battery is mounted on a vehicle, a plurality of lithium ion secondary batteries may be connected in series to form an assembled battery. Examples of devices equipped with lithium ion secondary batteries include various home appliances driven by batteries such as personal computers and portable communication devices, office devices, and industrial devices in addition to vehicles.

  As mentioned above, although embodiment of the positive electrode for lithium ion secondary batteries and lithium ion secondary battery of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be described in more detail with reference to examples.

<Production of laminated lithium-ion secondary battery>
[Production of positive electrode]
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ having an average particle diameter D ₅₀ of 10 μm as a positive electrode active material, and flaky graphite having an average particle diameter D ₅₀ of 5 μm and an average particle diameter D _{50 as} flaky graphite. Of 10-μm scale graphite, acetylene black (AB) having an average particle diameter D ₅₀ of 50 nm as amorphous carbon, and polyvinylidene fluoride (PVDF) as a binder were prepared. Here, the electrical resistivity of the flake graphite was 0.0014 Ω · cm, and the electrical resistivity of AB was 0.14 Ω · cm. True density of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ is 4.69 g / cm ^3, the true density of the average particle diameter _{D 50} of 5μm flake graphite is at 2.23 g / cm ³ , true density of the average particle diameter _{D 50} of 10μm flake graphite is 2.23 g / cm ^3, the true density of the AB is 1.31 g / cm ^3, the true density of PVDF is 1.71 g / cm ³ Met.

Example 1
94% by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , 0.5% by mass of the average particle diameter D ₅₀ of scaly graphite having 5 μm, 2.5% by mass of AB and 3% by mass of PVDF Were mixed to prepare a composition for forming a positive electrode active material layer. This positive electrode active material layer forming composition was dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a slurry.

An aluminum foil having a thickness of 20 μm was prepared as a current collector. The slurry was applied to the current collector so as to form a film using a doctor blade. The aluminum foil coated with the slurry was dried at 80 ° C. for 20 minutes, and NMP was removed by volatilization to form a positive electrode active material layer on the surface of the aluminum foil. Thereafter, the aluminum foil and the positive electrode active material layer on the aluminum foil were firmly bonded to each other by a roll press. Here, the basis weight of the positive electrode active material layer was 18.4 mg / cm ² . The joined product was heated with a vacuum dryer at 120 ° C. for 6 hours, cut into a predetermined shape (rectangular shape of 25 mm × 30 mm), and used as the positive electrode of Example 1. The thickness of the positive electrode active material layer of the positive electrode of Example 1 was about 59 μm.

The density and porosity of the positive electrode of Example 1 were calculated. The density of the positive electrode active material layer of Example 1 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Example 1 was 27.0%.

[Production of negative electrode]
The negative electrode was produced as follows.

98 weight% of the average particle diameter _{D 50} of the graphite powder of 20 [mu] m, as a binder, 1 wt% styrene - mixing a butadiene rubber (SBR) and 0.7 wt% carboxymethyl cellulose (CMC), the The mixture was dispersed in an appropriate amount of distilled water to prepare a slurry. This slurry was applied to a copper foil having a thickness of 10 μm, which is a negative electrode current collector, using a doctor blade so as to form a film, and the current collector coated with the slurry was dried and pressed. The basis weight of the negative electrode active material layer of the negative electrode was 11.1 mg / cm ² . The joined product was heated with a vacuum dryer at 80 ° C. for 6 hours, cut into a predetermined shape (rectangular shape of 25 mm × 30 mm), and used as a negative electrode. The thickness of the negative electrode active material layer of this negative electrode was about 84 μm.

[Production of laminated lithium-ion secondary battery]
Using the positive electrode and the negative electrode in Example 1, a laminate type lithium ion secondary battery was produced. Specifically, a rectangular sheet (27 × 32 mm, thickness 25 μm) made of polypropylene resin as a separator was sandwiched between the positive electrode and the negative electrode to form an electrode plate group. The electrode plate group was covered with a set of two laminated films, and the three sides were sealed, and then an electrolyte solution was injected into the bag-like laminated film. As an electrolytic solution, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and fluoroethylene carbonate (FEC) are EC: EMC: DMC: FEC = 26: 30: 40: 4 (volume ratio). A solution obtained by dissolving LiPF ₆ in a mixed solvent so as to be 1 mol / l was used. Thereafter, the remaining one side was sealed to obtain a laminate type lithium ion secondary battery in which the four sides were hermetically sealed and the electrode plate group and the electrolyte were sealed. Note that the positive electrode and the negative electrode each have a tab that can be electrically connected to the outside, and a part of the tab extends to the outside of the laminated lithium ion secondary battery. The laminated lithium ion secondary battery of Example 1 was produced through the above steps.

(Example 2)
As except that the average particle diameter D ₅₀ was used a mixture of 1.0 wt% and AB2.0 wt% flake graphite 5μm conductive additive, laminated lithium of Example 2 in the same manner as in Example 1 An ion secondary battery was produced. The basis weight of the positive electrode active material layer of the positive electrode of Example 2 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Example 2 was about 59 μm. The density of the positive electrode active material layer of Example 2 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Example 2 was 27.2%.

(Example 3)
As except that the average particle diameter D ₅₀ was used a mixture of 1.5 wt% and AB1.5 wt% flake graphite 5μm conductive additive, laminated lithium of Example 3 in the same manner as in Example 1 An ion secondary battery was produced. The basis weight of the positive electrode active material layer of the positive electrode of Example 3 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Example 3 was about 59 μm. The density of the positive electrode active material layer of Example 3 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Example 3 was 27.4%.

Example 4
A laminated lithium ion secondary battery of Example 4 was obtained in the same manner as in Example 2 except that flaky graphite having an average particle diameter D ₅₀ of 10 μm was used instead of the flaky graphite having an average particle diameter D ₅₀ of 5 μm. Produced. The basis weight of the positive electrode active material layer of the positive electrode of Example 4 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Example 4 was about 59 μm. The density of the positive electrode active material layer of Example 4 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Example 4 was 27.2%.

(Example 5)
92% by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , 1.7% by mass of an average particle diameter D ₅₀ of 5 μm of flaky graphite, 3.3% by mass of AB and 3% by mass of PVDF A laminate type lithium ion secondary battery of Example 5 was produced in the same manner as in Example 1 except that was used as a mixture. The basis weight of the positive electrode active material layer of the positive electrode of Example 5 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Example 5 was about 59 μm. The density of the positive electrode active material layer of Example 5 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Example 5 was 25.3%.

(Comparative Example 1)
A laminated lithium ion secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 except that AB 3.0% by mass was used as the conductive additive. The basis weight of the positive electrode active material layer of the positive electrode of Comparative Example 1 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Comparative Example 1 was about 59 μm. The density of the positive electrode active material layer of Comparative Example 1 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Comparative Example 1 was 26.9%.

(Comparative Example 2)
As except that the average particle diameter D ₅₀ was used a mixture of 2.0 wt% and AB1.0 wt% flake graphite 5μm conductive additive, laminated lithium in Comparative Example 2 in the same manner as in Example 1 An ion secondary battery was produced. The basis weight of the positive electrode active material layer of the positive electrode of Comparative Example 2 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Comparative Example 2 was about 59 μm. The density of the positive electrode active material layer of Comparative Example 2 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Comparative Example 2 was 27.5%.

(Comparative Example 3)
Laminated lithium ion with an average particle diameter D ₅₀ except that flaky graphite average particle size D ₅₀ in place of 5μm was used flake graphite of 10μm Comparative Example 2 in analogy to comparative example 3 as a conductive additive A secondary battery was produced. The basis weight of the positive electrode active material layer of the positive electrode of Comparative Example 3 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Comparative Example 3 was about 59 μm. The density of the positive electrode active material layer of Comparative Example 3 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Comparative Example 3 was 27.5%.

(Comparative Example 4)
95% by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , 0.7% by mass of an average particle diameter D ₅₀ of 5 μm of flaky graphite, 1.3% by mass of AB and 3% by mass of PVDF A laminate type lithium ion secondary battery of Comparative Example 4 was produced in the same manner as in Example 1 except that was used as a mixture. The basis weight of the positive electrode active material layer of the positive electrode of Comparative Example 4 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Comparative Example 4 was about 59 μm. The density of the positive electrode active material layer of Comparative Example 4 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Comparative Example 4 was 28.2%.

(Comparative Example 5)
90% by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , 2.4% by mass of average particle diameter D ₅₀ of scaly graphite having 5 μm, 4.6% by mass of AB and 3% by mass of PVDF A laminate type lithium ion secondary battery of Comparative Example 5 was produced in the same manner as in Example 1 except that was used as a mixture. The basis weight of the positive electrode active material layer of the positive electrode of Comparative Example 5 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Comparative Example 5 was about 59 μm. The density of the positive electrode active material layer of Comparative Example 5 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Comparative Example 5 was 23.4%.

(Comparative Example 6)
93% by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , 3.0% by mass of an average particle diameter D ₅₀ of 10 μm of flaky graphite, 1.0% by mass of AB and 3% by mass of PVDF A laminate type lithium ion secondary battery of Comparative Example 6 was produced in the same manner as in Example 1 except that was used as a mixture. The basis weight of the positive electrode active material layer of the positive electrode of Comparative Example 6 was 18.4 mg / cm ² , and the thickness of the positive electrode active material layer of the positive electrode of Comparative Example 6 was about 59 μm. The density of the positive electrode active material layer of Comparative Example 6 was 3.1 g / cm ³ , and the porosity of the positive electrode active material layer of Comparative Example 6 was 26.8%.

<Cell resistance evaluation>
The cell resistances of the laminated lithium ion secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 3 were measured. The cell resistance (Ω) was measured at a voltage of 20% SOC (State of charge) at 2.5 C rate and 10 seconds discharge. Furthermore, the cell resistance of the laminated lithium ion secondary batteries of Example 2, Example 5, Comparative Example 4, Comparative Example 5 and Comparative Example 6 is 2.5 C at a voltage of 10% SOC (State of charge). The rate was measured at 10 seconds discharge.

  A smaller cell resistance value indicates a lower positive electrode resistance. Further, since the cell resistance is measured at a 2.5 C rate, the measured value of the cell resistance is also an index indicating high rate characteristics. In each example and each comparative example, six batteries each having the same configuration were prepared, the resistance of each battery was measured, and the average value was calculated.

  The result of the average value of the cell resistance of Examples 1-4 and Comparative Examples 1-3 is shown in FIG. FIG. 3 shows a graph comparing the cell resistance and the porosity of Example 2, Example 5, Comparative Example 4 and Comparative Example 5.

  Table 1 summarizes the compounding ratio, porosity, and cell resistance of the positive electrode active material layer components of each example and comparative example.

  As can be seen in FIG. 2 and Table 1, all of the average cell resistance values at 20% SOC of the laminated lithium ion secondary batteries of Examples 1 to 4 were lower than 5Ω. The cell resistance at 20% SOC of the laminated lithium ion secondary batteries of Comparative Examples 1 to 3 was higher than 5Ω. Here, in the laminate type lithium ion secondary battery of Comparative Example 1, scaly graphite is not added as a conductive additive for the positive electrode. Therefore, it was confirmed that it was difficult to suppress an increase in cell resistance unless flaky graphite was added to the positive electrode active material layer. Further, the laminated lithium ion secondary battery of Comparative Example 2 is added with 2.0% by mass of flake graphite and 1.0% by mass of AB. Here, the cell resistance of the laminate-type lithium ion secondary battery of Comparative Example 2 to which flaky graphite is added is the laminate of Comparative Example 1 to which 3.0% by mass of AB is added although flaky graphite is not added. Higher than the cell resistance of the lithium ion secondary battery. This indicates that even if scaly graphite is added, an increase in cell resistance is difficult to suppress if the proportion of amorphous carbon is low. The reason why the cell resistance of the laminate type lithium ion secondary battery of Comparative Example 2 is higher than the cell resistance of the laminate type lithium ion secondary battery of Comparative Example 1 is that the content of amorphous carbon is contained with respect to the content of flake graphite. It is presumed that the conductive path could not be formed well due to the small amount. Similarly, in the laminated lithium ion secondary battery of Comparative Example 6, the cell resistance at 10% SOC was 10% when the SOC of the laminated lithium ion secondary battery of Example 2, Example 5, Comparative Example 4 and Comparative Example 5 was 10%. It became higher than the cell resistance.

In addition, the laminated lithium ion secondary battery of Example 4 using flaky graphite having an average particle diameter D ₅₀ of 10 μm also used flaky graphite having an average particle diameter D ₅₀ of 5 μm of the same mass% as in Example 4. Similar to the laminate type lithium ion secondary battery of Example 2, the average value of the cell resistance at 20% SOC was lower than that of the laminate type lithium ion secondary batteries of Comparative Examples 1 to 3. The average particle diameter D ₅₀ of the laminated lithium ion secondary battery is also laminated lithium ion secondary Both cell also positive electrode active material of Example 2 of Example 4 was 10 [mu] m. Thus it was confirmed that it was possible to suppress an increase in the cell resistance of the laminated lithium-ion secondary batteries be used scaly graphite having an average particle diameter D ₅₀ and equivalent mean particle diameter D ₅₀ of the positive electrode active material.

Here, all of the laminated lithium ion secondary batteries of Examples 1 to 4 had a cell resistance lower than that of the laminated lithium ion secondary batteries of Comparative Examples 1 to 3. In each of the laminated lithium ion secondary batteries of Examples 1 to 4, the weight of the positive electrode active material layer of the positive electrode is 18.4 mg / cm ^2, which is a high weight. Therefore, it was confirmed that the increase in cell resistance of the positive electrode can be suppressed even if the positive electrode active material layer of the positive electrode is high in weight.

  Moreover, the thickness of the positive electrode active material layer of each of the laminated lithium ion secondary batteries of Examples 1 to 4 is 59 μm, and even if the thickness of the positive electrode active material layer is as thick as 59 μm, an increase in cell resistance of the positive electrode can be suppressed. It could be confirmed.

  Moreover, as seen in FIG. 3 and Table 1, the contents of Example 2, Example 5, Comparative Example 4 and Comparative Example 5 in which the content of amorphous carbon was increased relative to the content of scaly graphite. In the laminated lithium ion secondary battery, the average value of the cell resistance when the SOC of the laminated lithium ion secondary battery of Example 2 and Example 5 in which the porosity of the positive electrode active material layer is 24% or more and 28% or less is 10%. All were lower than 2.5Ω. The cell resistance at 10% SOC of the laminate type lithium ion secondary battery of Comparative Example 4 in which the porosity of the positive electrode active material layer was 28.2% and Comparative Example 5 in which the porosity of the positive electrode active material layer was 23.4% was 2 It was higher than 5Ω. When the porosity of the positive electrode active material layer is 24% or more and 28% or less, the movement of lithium ions is smooth even when the content of the positive electrode active material in the positive electrode is increased and the thickness of the positive electrode active material layer is increased. It was found that an increase in cell resistance was suppressed because the conductive path was formed satisfactorily.

  1: current collector, 2: positive electrode active material layer, 3: positive electrode active material, 41: flaky graphite, 42: amorphous carbon, 5: binder, 6: void.

Claims (6)

A current collector,
A positive electrode active material layer bound to the current collector,
The positive electrode active material layer has a positive electrode active material, a conductive auxiliary agent, and a binder,
The conductive auxiliary agent consists of scaly graphite and amorphous carbon,
The content of the amorphous carbon is 1 to 5 times the content of the scaly graphite,
A positive electrode for a lithium ion secondary battery, wherein the positive electrode active material layer has a porosity of 24% to 28%. The average particle diameter _{D 50} of the positive electrode active material is 5Myuemu～15myuemu,
The average particle diameter D ₅₀ of該鱗flake graphite 1μm or more and not more than the average particle diameter D ₅₀ of the positive electrode active material,
2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the average particle diameter D ₅₀ of the amorphous carbon is 10 nm or more and 100 nm or less.   3. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the content of the conductive auxiliary is more than 2% by mass and less than 7% by mass when the entire positive electrode active material layer is 100% by mass. 4. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the weight of the positive electrode active material layer is 17 mg / cm ² or more and 30 mg / cm ² or less. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the density of the positive electrode active material layer is 2.9 g / cm ³ or more and 3.3 g / cm ³ or less.   The lithium ion secondary battery which has a positive electrode for lithium ion secondary batteries as described in any one of Claims 1-5.

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