Positive electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

Abstract

Disclosed is a positive electrode for a nonaqueous electrolyte secondary battery, the positive electrode comprising: a positive electrode core body; and a positive electrode mixture layer that is disposed on at least a surface of the positive electrode core body. The positive electrode mixture layer contains at least a positive electrode active material and a positive electrode conductive agent. The positive electrode conductive agent contains porous carbon and carbon nanotubes. Particles of the porous carbon have a three-dimensional structure that has a plurality of through holes. The through holes are each a pore that continuously leads from a surface of a particle to another surface of the particle. The mode diameter of the pore size distribution of the porous carbon is in the range of 0.5 nm to less than 100 nm.

Description

Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-030595, filed on February 29, 2024, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

[0003] Patent Document 1 proposes "an electrode for a lithium ion secondary battery in which an electrolyte retention material made of a carbon material having through-holes is disposed in the gaps between electrode active materials."

[0004] Patent Document 2 proposes "a lithium secondary battery having a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a separator disposed between the positive electrode and the negative electrode, wherein at least one of the positive electrode and the negative electrode contains mesoporous carbon and conductive fine particles having an average particle size smaller than the average particle size of the mesoporous carbon."

[0005] Patent Document 3 states that "(A) the specific surface area value by the BET method is at least 600 m ２ / g and a composite of a metal oxide is used as the positive electrode active material, and (B) an aprotic organic solvent solution of a compound capable of generating ions that can be doped into the positive electrode active material by electrolysis is used as the electrolyte, and the activated carbon has interconnected pores with an average pore size of 10 μm or less.

[0006] JP 2006-147405 A JP 2009-026676 A JP 63-314766 A

[0007] As the capacity of non-aqueous electrolyte secondary batteries increases, the density of electrodes and the thickness of electrode mixture layers are increasing. Therefore, the development of positive electrodes with excellent electronic conductivity and high non-aqueous electrolyte retention is desired. Suppressing the depletion of non-aqueous electrolyte in the positive electrode and reducing reaction resistance are important challenges.

[0008] One aspect of the present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode core; and a positive electrode mixture layer disposed on at least a surface of the positive electrode core; the positive electrode mixture layer contains at least a positive electrode active material and a positive electrode conductive agent; the positive electrode conductive agent contains porous carbon and carbon nanotubes; particles of the porous carbon have a three-dimensional structure having a plurality of through holes; the through holes are voids that continuously connect from a surface of the particle to another surface; and the mode diameter of the pore size distribution of the porous carbon is 0.5 nm or more and less than 100 nm.

[0009] Another aspect of the present disclosure relates to a nonaqueous electrolyte secondary battery including the positive electrode for the nonaqueous electrolyte secondary battery, a negative electrode, and a nonaqueous electrolyte, wherein the negative electrode has at least a negative electrode core, and a volume Vc of the negative electrode in a fully charged state is 125% or more of a volume Vdc of the negative electrode in a fully discharged state.

[0010] According to the present disclosure, in a non-aqueous electrolyte secondary battery, it is possible to increase the electronic conductivity of the positive electrode and also increase the liquid retention of the non-aqueous electrolyte in the positive electrode.

[0011] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0012] 1 is a longitudinal sectional view schematically illustrating a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure.

[0013] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0014] The present disclosure encompasses any combination of two or more claims arbitrarily selected from the appended claims, i.e., any combination of two or more claims arbitrarily selected from the appended claims may be combined unless a technical contradiction arises.

[0015] [Positive Electrode] The present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery. Hereinafter, the positive electrode according to the present disclosure will also be referred to as a "positive electrode (P)." Non-aqueous electrolyte secondary batteries include lithium ion secondary batteries that use a liquid non-aqueous electrolyte (electrolytic solution) and solid-state batteries that contain a gel electrolyte. A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. A separator is usually disposed between the positive electrode and the negative electrode.

[0016] The positive electrode (P) has a positive electrode core (positive electrode current collector) and a positive electrode mixture layer disposed on at least the surface of the positive electrode core. The positive electrode mixture layer is composed of a positive electrode mixture and has a layer or film shape. The positive electrode mixture contains at least a positive electrode active material and a positive electrode conductive agent, and usually further contains a binder. The positive electrode conductive agent contains porous carbon and carbon nanotubes.

[0017] The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the coating. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode core. The dispersion medium is not particularly limited, but examples thereof include water, alcohol, and N-methyl-2-pyrrolidone (NMP).

[0018] The thickness of the positive electrode mixture layer is not particularly limited, and may be, for example, 10 μm to 200 μm, or 30 μm to 100 μm. A single positive electrode mixture layer may be formed by a plurality of layers having different compositions.

[0019] The positive electrode core is made of a sheet-like conductive material. A non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used as the positive electrode core. The positive electrode mixture layer is supported on one or both surfaces of the positive electrode core. The material of the positive electrode core is not particularly limited, but stainless steel, aluminum, aluminum alloy, titanium, etc. may be used.

[0020] 1m on one side of the positive electrode core ２ The mass of the positive electrode mixture layer formed per side may be, for example, 200 g or more, and preferably 250 g or more. By setting the mass to 250 g or more, it is possible to increase the capacity of the lithium-ion battery. The mass can be increased by thickening the positive electrode mixture layer or increasing the density of the positive electrode mixture layer. There is no particular limitation on the thickness of the positive electrode mixture layer, but it may be, for example, in the range of 50 μm to 250 μm per side of the core body. In the positive electrode according to the present disclosure, even if the positive electrode mixture layer is thickened, an increase in internal resistance is suppressed. The density of the positive electrode active material in the positive electrode mixture layer is, for example, 3.0 g / cm ３ ~3.7cm ３ It may be in the range of

[0021] (Porous Carbon (C)) The porous carbon used in the present disclosure has a structure that is superior in liquid retention for a non-aqueous electrolyte (electrolytic solution), and therefore has a structure different from that of carbon black and activated carbon. Hereinafter, the porous carbon used in the present disclosure will also be referred to as "porous carbon (C)." The porous carbon particles used in the present disclosure will also be referred to as "carbon particles (C)."

[0022] The carbon particles (C) have a three-dimensional structure having a plurality of through-holes. The through-holes are voids that are continuously connected from one surface of the carbon particles (C) to another surface. The plurality of through-holes serve to retain the electrolyte. It is believed that the electrolyte is absorbed into the through-holes by capillary action.

[0023] When carbon particles (C) are observed with an SEM, a plurality of pores (through holes) can be observed that connect from the surface corresponding to the SEM image surface to the back side. When a single carbon particle (C) is observed with a scanning electron microscope (SEM), a plurality of through holes are observed in a single carbon particle (C). For example, 2 to 1,000,000, preferably 3,000 to 1,000,000, more preferably 15,000 to 1,000,000, or even more preferably 5,000 to 300,000 through holes are observed in a single carbon particle (C). The number of through holes observed in a single carbon particle (C) may be calculated as the average number of through holes in 100 arbitrarily selected particles.

[0024] By binarizing and image processing an SEM image of the carbon particles (C), the area ratio of observable through holes to the area surrounded by the outline of the carbon particles (C) (hereinafter also referred to as "pore area ratio") can be calculated. The pore area ratio is, for example, 10% to 90%, and preferably 20% to 60%. The pore area ratio may be calculated as the average value of the pore area ratios of 100 arbitrarily selected particles.

[0025] The specific surface area of ​​porous carbon (C) is 100 m ２ / g or more. ２ By controlling the specific surface area to be 200 m / g or more, the contact area with the electrolyte increases, and the liquid retention ability of the porous carbon (C) is further improved. ２ / g or more, and ２ / g or more.

[0026] The specific surface area of ​​the porous carbon (C) is the BET specific surface area measured by the BET method. The BET specific surface area is measured by a gas adsorption method. Nitrogen gas is used as the gas. Details of the BET method may be in accordance with JIS Z8830:2013. Specifically, the amount of nitrogen adsorbed onto the porous carbon (C) at liquid nitrogen temperature is measured. The porous carbon (C) as a measurement sample is loaded into a sample tube of a measurement device (e.g., a Shimadzu Tristar II 3020 automatic specific surface area / pore distribution measurement device), and the sample tube is cooled to -196°C, and the pressure is reduced once. Nitrogen (purity 99.999%) is then adsorbed onto the measurement sample at a desired relative pressure, and the adsorption isotherm is measured. From the obtained adsorption isotherm, the BET specific surface area is determined by a multipoint method (e.g., three points) in the relative pressure range of 0.05 to 0.1.

[0027] In addition, the conductivity of the porous carbon (C) tends to decrease as the specific surface area increases. On the other hand, conductivity can be ensured by using carbon nanotubes in combination. In an electrode design that prioritizes liquid retention over conductivity, the specific surface area of ​​the porous carbon (C) is set to 100 m ２ It is preferable to control the value to be equal to or greater than 1 / g.

[0028] The porous carbon (C) has a pore size distribution suitable for retaining an electrolyte. The mode diameter of the pore size distribution of the carbon particles (C) is, for example, 0.5 nm or more and less than 100 nm, or may be 0.5 nm or more and less than 50 nm, or may be 0.5 nm or more and less than 10 nm.

[0029] The pore size distribution of the porous carbon (C) can be determined by analyzing the adsorption isotherm of the porous carbon measured when determining the BET specific surface area using the Barrett Joyner Hallenda (BJH) method and the Density Functional Theory (DTF) method. The QS-DTF method (Slitpore, Equilibrium Model) may be used as the DTF method.

[0030] The volume average particle diameter of the porous carbon (C) may be less than 50 μm. By setting the volume average particle diameter to less than 50 μm, the dispersibility of the porous carbon (C) in the positive electrode mixture layer can be increased. By increasing the dispersibility of the porous carbon (C), it becomes easier to contribute to improving the conductivity of the positive electrode mixture layer. The volume average particle diameter may be less than 20 μm or less than 10 μm.

[0031] The volume average particle size of the porous carbon (C) is, for example, the particle size (median diameter: D50) at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method. For example, an "LA-750" manufactured by HORIBA, Ltd. can be used as the measuring device. It is preferable that the porous carbon (C) has a plurality of through-holes (for example, 100 or more) having a maximum diameter that is 1% or more of the volume average particle size (D50) of the porous carbon (C). For example, when the number of through-holes having a maximum diameter that is 1% or more of D50 is counted for 100 particles arbitrarily selected from the porous carbon (C), it is preferable that the 100 particles have, on average, 100 or more through-holes having a maximum diameter that is 1% or more of D50.

[0032] The porosity of the porous carbon (C) may be less than 50% by volume. By setting the porosity to less than 50% by volume, the strength of the porous carbon (C) is increased and collapse within the positive electrode mixture layer is suppressed. As a result, the liquid retention ability is maintained for a long period of time. The porosity may be 10% to 50% by volume, or 15% to 35% by volume.

[0033] The porosity of porous carbon is measured by mercury intrusion porosimetry. Details of the mercury intrusion porosimetry may be in accordance with JIS R1655:2003. Mercury has a contact angle of 90° or more with almost all solids, so even when the sample comes into contact with mercury, the mercury does not enter the pores. Therefore, the porosity can be calculated from the volume of the mercury when no pressure is applied (apparent volume) and the amount of mercury intrusion into the pores (pore volume) when external pressure is applied to the mercury.

[0034] The porous carbon (C), which is the measurement sample used to measure each physical property, may be prepared by disassembling a battery, removing the positive electrode, peeling off the positive electrode mixture layer, and separating it from the positive electrode mixture. For example, the positive electrode is removed from the battery, and the nonaqueous electrolyte and other components are removed by washing. The positive electrode mixture layer is then peeled off from the positive electrode core, and the positive electrode mixture is pulverized. Organic substances such as the positive electrode binder are dissolved and separated in an organic solvent such as NMP. The positive electrode active material is dissolved and separated in aqua regia. This separates only the carbonaceous material, carbon nanotubes, and porous carbon (C). The carbonaceous material is then classified to separate the porous carbon (C) from carbon nanotubes with a long average length. Alternatively, the porous carbon (C) may be obtained and prepared as a raw material for the positive electrode mixture layer. Furthermore, physical properties such as the content, specific surface area, total pore volume, average pore diameter, volume-average particle size, and porosity of the porous carbon (C) may be analyzed by morphological observation of a cross section parallel to the thickness direction of the positive electrode mixture layer.

[0035] The content of porous carbon (C) in the positive electrode mixture layer may be less than 10% by mass. Because porous carbon (C) is bulky, even a small amount can exhibit sufficient liquid retention. From the viewpoint of increasing the density of the positive electrode active material in the positive electrode mixture layer, the content of porous carbon (C) is preferably less than 10% by mass. The content of porous carbon (C) may be less than 5% by mass or less than 1% by mass. The content of porous carbon (C) may be 0.1% by mass or more.

[0036] (Carbon Nanotubes) The resistance of the positive electrode mixture layer can be significantly reduced by adding carbon nanotubes (hereinafter also referred to as "CNTs") to the positive electrode mixture. CNTs also have the effect of suppressing potential variations within the positive electrode.

[0037] It is believed that the CNTs form main conductive paths, and the porous carbon (C) forms fine conductive paths, resulting in conductive paths throughout the positive electrode mixture layer, improving current collection from the positive electrode mixture layer.

[0038] It is also possible to design the carbon particles (C) to be small, thereby increasing the number of point contacts with the positive electrode active material and improving the electronic conductivity of the positive electrode. However, when the carbon particles (C) are made small, the fine particles tend to aggregate, making it difficult to form long-distance conductive paths that connect the particles of the positive electrode active material. Therefore, it is effective to use CNTs in combination with porous carbon (C).

[0039] The CNTs may be either single-walled CNTs (SWCNTs) or multi-walled CNTs (MWCNTs). Examples of multi-walled CNTs include double-walled CNTs, triple-walled CNTs, and CNTs with four or more walls. The positive electrode mixture layer preferably contains single-walled CNTs and / or multi-walled CNTs. The multi-walled CNTs contained in the positive electrode mixture layer may be one type of multi-walled CNT or multiple types of multi-walled CNTs with different numbers of layers.

[0040] The CNT preferably has a thin and long shape. The average diameter Da of the CNT is, for example, preferably 10 nm or less, more preferably 7 nm or less, and even more preferably 2 nm or less. The average length La of the CNT is, for example, preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more. Here, the average length La of the CNT refers to the average length of CNTs in a bundle state formed by gathering two or more CNTs into a bundle. The aspect ratio represented by La / Da is, for example, 1000 or more, and preferably 10,000 or more. Note that the positive electrode mixture layer may contain CNTs in an isolated state without forming a bundle.

[0041] The average diameter Da of CNTs is determined by image analysis using a transmission electron microscope (TEM). The average diameter Da of CNTs can be measured by the following method. First, 100 CNTs are randomly selected, and the diameter (outer diameter) of each is measured at one arbitrary point. The average diameter Da is then determined by arithmetically averaging the measured diameters.

[0042] The average CNT length La is determined by image analysis using a scanning electron microscope (SEM) and is calculated by measuring the lengths of 100 randomly selected bundled CNTs (the lengths of the bundled CNTs when stretched linearly) and then calculating the arithmetic mean.

[0043] The positive electrode mixture layer may contain only a small amount of CNT. In combination with the presence of porous carbon (C), a small amount of CNT sufficiently improves the conductivity. The CNT content in the positive electrode mixture layer may be, for example, 1% by mass or less, or 0.5% by mass or less. The ratio of CNT to the total amount of porous carbon (C) and CNT is, for example, 0.5% by mass to 5% by mass.

[0044] The CNTs used as the measurement sample for measuring each physical property may be prepared by disassembling a battery, removing the positive electrode, peeling off the positive electrode mixture layer, and separating the CNTs from the positive electrode mixture. Alternatively, the CNTs may be obtained as the raw material for the positive electrode mixture layer.

[0045] (Positive Electrode Active Material) The positive electrode active material includes a material that electrochemically absorbs and releases lithium ions. The material that electrochemically absorbs and releases lithium ions may be a lithium transition metal composite oxide. Examples of the lithium transition metal composite oxide include layered compounds with a rock salt crystal structure, spinel compounds, and polyanion compounds. Among the lithium transition metal composite oxides, a composite oxide that contains at least Ni and in which the proportion of Ni relative to all metal elements other than Li is 50 mol % or more (hereinafter also referred to as "composite oxide (HN)") is preferred in terms of achieving high capacity.

[0046] In the composite oxide (HN), the proportion of Ni in the metal elements other than Li may be 50 atomic % or more, 80 atomic % or more, or 90 atomic % or more. In the composite oxide (HN), the proportion of Ni in the metal elements other than Li is less than 100 atomic %, may be 99 atomic % or less, or may be 98 atomic % or less.

[0047] A preferred example of the composite oxide (HN) is a rock salt type layered compound. For example, the composition of the composite oxide (HN) is represented by the formula (C): Li α Ni(1-x1-x2-x3-y)Coｘ１ Mn ｘ２ Al ｘ３ M ｙ O ２＋β Preferably, the lithium transition metal composite oxide is represented by the formula (C), where α satisfies 0.95≦α≦1.05, 0.5≦1−x1−x2−x3−y≦0.99, 0≦x1≦0.1, 0≦x2≦0.5, 0≦x3≦0.1, 0≦y≦0.1, and −0.05≦β≦0.05. M is at least one element selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y.

[0048] The composite oxide (HN) with a high Ni content tends to have an unstable crystal structure, and metals such as Ni may be eluted from the composite oxide (HN) due to side reactions. The eluted Ni forms a coating on the particle surface of the composite oxide (HN) that has a structure that prevents the absorption and release of Li ions, leading to an increase in internal resistance. On the other hand, when the positive electrode mixture layer contains porous carbon (C), the liquid retention of the positive electrode mixture layer is improved, thereby reducing the reaction resistance of the composite oxide (HN), suppressing side reactions, and suppressing an increase in internal resistance.

[0049] At least one element selected from the group consisting of B, N, P, Mg, Ca, Sr, Ba, Ti, W, Zr, and Al may be unevenly distributed in the surface layer of the lithium transition metal composite oxide. The element unevenly distributed in the surface layer has the effect of suppressing the formation of a high-resistance coating on the particle surfaces of the composite oxide (HN).

[0050] (Positive Electrode Binder) Examples of the binder for the positive electrode mixture include resin materials, such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; and polyimide resins such as polyimide and polyamideimide. These may be used alone or in combination of two or more.

[0051] [Negative Electrode] The negative electrode includes at least a negative electrode core (negative electrode current collector) and may include a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. The negative electrode mixture layer is composed of a negative electrode mixture and is in the form of a layer or film. The negative electrode mixture contains at least the negative electrode active material, and usually further contains a conductive agent and a binder.

[0052] The lithium metal secondary battery does not necessarily have to have a negative electrode material mixture layer, whereas the lithium ion secondary battery has a negative electrode material mixture layer.

[0053] The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode core and drying the applied film. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0054] The negative electrode core is made of a sheet-like conductive material. Examples of the negative electrode core include a non-porous conductive substrate (such as a metal foil) and a porous conductive substrate (such as a mesh, net, or punched sheet). The material for the negative electrode core is not particularly limited, but examples thereof include stainless steel, nickel, a nickel alloy, copper, and a copper alloy.

[0055] 1m on one side of the negative electrode core ２ The mass of the negative electrode mixture layer formed per surface may be, for example, 50 g or more, and preferably 100 g or more. By increasing the mass to 100 g or more, the coating amount of the positive electrode mixture layer can be increased from the viewpoint of maintaining an appropriate negative electrode / positive electrode capacity ratio, thereby enabling a high capacity lithium-ion battery. This mass can be increased by thickening the negative electrode mixture layer or increasing the density of the negative electrode active material in the negative electrode mixture layer. There are no particular limitations on the thickness of the negative electrode mixture layer, but it may be, for example, in the range of 60 μm to 240 μm per side of the core. By using the positive electrode (P) according to the present disclosure, an increase in internal resistance can be suppressed even when the negative electrode mixture layer is thickened. The negative electrode active material density of the negative electrode mixture layer is, for example, 1.0 g / cm. ３ ~2.0 g / cm ３ It may be in the range of

[0056] The volumes of the positive electrode and negative electrode increase and decrease with charge and discharge. The volume change of the negative electrode is particularly large. When the negative electrode expands during charge, the internal pressure of the electrode group increases, and the electrolyte is forced out of the positive electrode. Therefore, the liquid retention property of the porous carbon (C) works more effectively when the expansion rate of the negative electrode during charge is large.

[0057] A fully charged state refers to a state in which a battery is charged to a state of charge of, for example, 0.98×C or more (SOC = 98% or more), where C is the rated capacity of the battery. A fully discharged state refers to a state in which a battery is discharged to a state of charge of, for example, 0.05×C or less (SOC = 5% or less). Note that the amount of charge when a battery in a fully discharged state (SOC = 0%) is charged to a fully charged state (SOC = 100%) corresponds to the rated capacity. The voltage of a battery in a fully charged state corresponds to the end-of-charge voltage. The voltage of a battery in a fully discharged state corresponds to the end-of-charge voltage.

[0058] The volume Vc of the negative electrode in a fully charged battery may be 125% or more of the volume Vdc of the negative electrode in a fully discharged battery. Even when the expansion coefficient of the negative electrode is large, excellent charge-discharge cycle characteristics can be maintained by using the positive electrode (P).

[0059] (Negative electrode active material) The negative electrode active material includes a material that electrochemically absorbs and releases lithium ions. As the material that electrochemically absorbs and releases lithium ions, a carbonaceous material, a Si-containing material, or the like can be used. The negative electrode active material may be used alone or in combination of two or more. The negative electrode active material preferably includes a carbonaceous material and a Si-containing material.

[0060] (Carbonaceous Material) Examples of the carbonaceous material include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). One type of carbonaceous material may be used alone, or two or more types may be used in combination.

[0061] Among carbonaceous materials, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite particles may partially contain amorphous carbon, easily graphitized carbon, or hardly graphitized carbon.

[0062] Graphite is a carbonaceous material with a developed graphite crystal structure. The average interplanar spacing d002 of the (002) plane of graphite measured by X-ray diffraction may be, for example, 0.340 nm or less, or may be 0.3354 nm or more and 0.340 nm or less.

[0063] (Si-containing material) The Si-containing material may be, for example, a carbon composite particle containing a carbon phase and a Si phase dispersed within the carbon phase. The carbon phase may be, for example, amorphous carbon or crystalline carbon. The amorphous carbon may be, for example, hard carbon, soft carbon, or other. Amorphous carbon generally refers to a carbonaceous material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeds 0.340 nm. Examples of crystalline carbon include carbon having a graphite-type crystalline structure, such as graphite. Crystalline carbon such as graphite refers to a carbonaceous material in which d002 is 0.340 nm or less (for example, 0.3354 nm or more and 0.340 nm or less).

[0064] The content of the Si phase in the carbon composite particles is, for example, 30% by mass to 80% by mass, or may be 40% by mass to 70% by mass, in which case a higher initial capacity can be obtained.

[0065] The Si-containing material may be a composite particle in which a Si phase (fine Si phase) is dispersed in a lithium ion conductive phase (matrix). ２ It is preferable that the composite particles contain at least one selected from the group consisting of SiO phase and silicate phase. ２ phase and SiO ２ Examples of the composite particles include composite particles containing a silicate phase and a Si phase dispersed within the silicate phase (silicate composite particles).

[0066] SiO ２ The phase is an amorphous phase containing 95% or more by mass of silicon dioxide. ２ Composite particles with dispersed Si phases are SiO ｘwhere x is, for example, 0.5≦x<2, and may be 0.8≦x≦1.6. ｘ is, for example, obtained by heat treating silicon monoxide to produce SiO ２ It is obtained by separating the Si phase into a Si phase and a fine Si phase.

[0067] The silicate phase preferably contains at least one of an alkali metal element (a Group 1 element other than hydrogen in the long periodic table) and a Group 2 element in the long periodic table. The alkali metal element includes lithium (Li), potassium (K), sodium (Na), etc. The Group 2 element includes magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The lithium silicate phase has the formula: Li ２ｙ SiO ２＋ｙ The silicate composite particles may have a composition expressed as (0<y<2), where y may be 1 / 2 or 1. The silicate composite particles in which the Si phase is dispersed within the silicate phase can be obtained, for example, by pulverizing a mixture of silicate and raw silicon while stirring it in a ball mill or the like to form fine particles, and then heat-treating the mixture in an inert atmosphere.

[0068] The content of the Si phase dispersed in the silicate phase may be 30% by mass or more and 95% by mass or less, or 35% by mass or more and 75% by mass or less, based on the entire silicate composite particles.

[0069] When the negative electrode mixture layer contains a Si-containing material, the content of the Si-containing material in the negative electrode mixture layer is, for example, 3% by mass or more, and may be 5% by mass or more. The content of the Si-containing material is, for example, 15% by mass or less, and may be 10% by mass or less. A negative electrode mixture layer containing a Si-containing material at such a content exhibits large expansion and contraction, and is therefore desirably used in combination with a positive electrode (P).

[0070] (Negative electrode binder) Examples of binders for the negative electrode mixture include resin materials, such as fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aramid resins), polyimide resins (e.g., polyimide, polyamideimide), acrylic resins (e.g., polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer, or salts thereof), vinyl resins (e.g., polyvinyl acetate), and rubber-like materials (e.g., styrene-butadiene copolymer rubber (SBR)). One type of binder may be used alone, or two or more types may be used in combination.

[0071] (Thickener) The negative electrode mixture may contain a thickener that thickens the negative electrode slurry. Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include CMC and its modified products, methyl cellulose, etc. Modified CMC also includes salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts. One type of thickener may be used alone, or two or more types may be used in combination.

[0072] (Negative electrode conductive agent) Examples of the conductive agent for the negative electrode mixture include carbon blacks such as acetylene black; conductive fibers such as carbon fiber and metal fiber; carbon fluoride; metal powders such as aluminum; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; organic conductive materials such as phenylene derivatives, etc. These may be used alone or in combination of two or more.

[0073] [Separator] A separator is usually interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator can be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0074] An example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0075] The structure of the secondary battery will be described below with reference to Fig. 1. Fig. 1 is a longitudinal cross-sectional view of a cylindrical nonaqueous electrolyte secondary battery 10 that is an example of this embodiment. However, the present disclosure is not limited to the following configuration.

[0076] The secondary battery 10 includes an electrode group 18, an electrolyte (not shown), and a cylindrical battery can 22 with a bottom that accommodates these. A sealing body 11 is crimped to the opening of the battery can 22 via a gasket 21, thereby sealing the battery. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a extending from a positive electrode plate 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a extending from a negative electrode plate 16 is connected to the inner bottom surface of the battery can 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode plate 15 and a negative electrode plate 16 with a separator 17 interposed therebetween.

[0077] (Additional Notes) The above description discloses the following technologies: (Technology 1) A positive electrode for a non-aqueous electrolyte secondary battery, comprising a positive electrode core and a positive electrode mixture layer disposed on at least the surface of the positive electrode core, wherein the positive electrode mixture layer contains at least a positive electrode active material and a positive electrode conductive agent, and the positive electrode conductive agent contains porous carbon and carbon nanotubes, wherein the porous carbon particles have a three-dimensional structure having a plurality of through-holes, and the through-holes are pores that are continuously connected from one surface of the particle to another surface, and wherein the mode diameter of the pore size distribution of the porous carbon is 0.5 nm or more and less than 100 nm. (Technology 2) A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode core; and a positive electrode mixture layer disposed on at least the surface of the positive electrode core, wherein the positive electrode mixture layer contains at least a positive electrode active material and a positive electrode conductive agent, and the positive electrode conductive agent contains porous carbon and carbon nanotubes, wherein the porous carbon particles have a three-dimensional structure having a plurality of through-holes, and the through-holes are pores that are continuously connected from one surface of the particle to another surface, and the mode diameter of the pore size distribution of the porous carbon is 0.5 nm or more and less than 100 nm. ２ / g or more. (Technology 3) The positive electrode for a non-aqueous electrolyte secondary battery according to Technology 1 or 2, wherein the volume average particle size of the porous carbon is less than 50 μm. (Technology 4) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 3, wherein the porosity of the porous carbon is less than 50 volume %. (Technology 5) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 4, wherein the content of the porous carbon in the positive electrode mixture layer is less than 10 mass %. (Technology 6) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 5, wherein the average diameter of the carbon nanotubes is 10 nm or less. (Technology 7) The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 6, wherein the positive electrode active material contains a lithium transition metal composite oxide, and the lithium transition metal composite oxide contains at least Ni, and the proportion of Ni to all metal elements other than Li is 50 mol % or more. (Technology 8) The lithium transition metal composite oxide has the formula: Li α Ni(1-x1-x2-x3-y)Co ｘ１ Mn ｘ２ Al ｘ３ M ｙ O ２＋βThe positive electrode for a non-aqueous electrolyte secondary battery according to Technical Field 7, wherein the following conditions are satisfied: 0.95≦α≦1.05, 0.5≦1−x1−x2−x3−y≦0.99, 0≦x1≦0.1, 0≦x2≦0.5, 0≦x3≦0.1, 0≦y≦0.1, −0.05≦β≦0.05, and M is at least one element selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y. (Technology 9) The positive electrode for a non-aqueous electrolyte secondary battery according to Technical Field 8, wherein at least one element selected from the group consisting of B, N, P, Mg, Ca, Sr, Ba, Ti, W, Zr, and Al is unevenly distributed in a surface layer portion of the lithium transition metal composite oxide. (Technology 10) A nonaqueous electrolyte secondary battery comprising: the positive electrode for a nonaqueous electrolyte secondary battery according to any one of Techniques 1 to 9, a negative electrode, and a nonaqueous electrolyte, wherein the negative electrode has at least a negative electrode core, and a volume Vc of the negative electrode in a fully charged state is 125% or more of a volume Vdc of the negative electrode in a fully discharged state. (Technology 11) The nonaqueous electrolyte secondary battery according to Technique 10, wherein the negative electrode has the negative electrode core and a negative electrode mixture layer disposed on a surface of the negative electrode core, the negative electrode mixture layer contains a Si-containing material, and a content of the Si-containing material in the negative electrode mixture layer is 3% or more.

[0078] [Examples] The present disclosure will be specifically described below based on examples and comparative examples relating to lithium ion secondary batteries, but the present disclosure is not limited to the following examples.

[0079] Examples 1 to 9, Comparative Examples 1 to 3 Lithium ion secondary batteries (batteries A1 to A9 of Examples 1 to 9, batteries B1 to B3 of Comparative Examples 1 to 3) were fabricated and evaluated according to the following procedure.

[0080] (1) Preparation of Negative Electrode 92 parts by mass of graphite powder and 8 parts by mass of SiO powder as negative electrode active materials, 0.8 parts by mass of carboxymethyl cellulose (CMC), 1.2 parts by mass of styrene butadiene rubber (SBR), and an appropriate amount of water were mixed to obtain a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of copper foil so as to obtain an appropriate positive / negative electrode capacity ratio, and the coating was dried and then rolled to form a negative electrode mixture on each side of the copper foil. The active material density of the negative electrode mixture layer was 1.6 g / cm. ３ It was decided.

[0081] (2) Preparation of the positive electrode: Lithium-containing composite oxide (LiNi ０．８８ Co ０．０７ Mn ０．０５ O ２ ) was mixed with polyvinylidene fluoride, porous carbon (C) having the physical properties shown in Table 1, and CNT at the content ratio in the positive electrode mixture shown in Table 1, and mixed with an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, the coating was dried, and then rolled to form a positive electrode mixture layer (density 3.6 g / cm) with a thickness of 95 μm on each side of the aluminum foil. ３ ) was formed to obtain a positive electrode.

[0082] In addition, in the battery B1 of Comparative Example 1, no CNT was used.

[0083] In the battery B2 of Comparative Example 2, the porous carbon (C) was not used.

[0084] In the battery B3 of Comparative Example 3, acetylene black (AB) was used instead of CNT.

[0085] The number of through-holes in the porous carbon (C) was calculated by image processing of a cross-sectional SEM image of the porous carbon, and was reported as a relative value when the number of through-holes in the porous carbon of Example 1 was set to 100.

[0086] (3) Preparation of Electrolyte Solution Lithium hexafluorophosphate (LiPF) was added to a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC=4:1:15. ６ ) was added in an amount of 1.3 mol / L to prepare an electrolyte solution.

[0087] (4) Fabrication of a Non-Aqueous Electrolyte Secondary Battery An Al positive electrode lead was attached to the positive electrode obtained above, and a Ni negative electrode lead was attached to the negative electrode obtained above. The positive electrode and negative electrode were spirally wound with a polyethylene thin film (separator) in between in an inert gas atmosphere to fabricate a wound electrode assembly. The electrode assembly was housed in a cylindrical outer can, and the non-aqueous electrolyte was injected. The outer can was then sealed to fabricate a cylindrical 2170 battery with the rated capacity (approximately 5 Ah) shown in Table 2.

[0088]

[0089] Evaluation Resistance (DCIR) The battery was charged at a constant current of 1.5 A from a discharged state to 50% SOC (State of Charge), and then left in an open circuit state for 2 hours. The DCIR of the battery was measured at 25°C. The voltage drop (ΔV) when a current of 2.5 A was applied for 10 seconds was divided by the current value to obtain the DCIR. The results are shown in Table 2.

[0090] [Charge-Discharge Cycle Test] A charge-discharge cycle test was conducted in an environment of 25°C. Charging and discharging were performed under the following conditions. A 20-minute break was allowed between charging and discharging. 200 charge-discharge cycles were repeated, and the capacity retention rate was calculated using the formula "Capacity retention rate (%) = (discharge capacity at 200th cycle ÷ discharge capacity at 1st cycle) × 100". The results are shown in Table 2.

[0091] (Charging) The battery was charged at a constant current of 5 A until the voltage reached 4.2 V, and then was charged at a constant voltage of 4.2 V until the current reached 0.1 A.

[0092] (Discharge) Discharge was carried out at a constant current of 5 A until the voltage reached 2.5 V.

[0093] The evaluation results of batteries A1 to A9 and B1 to B3 are shown in Table 2. In Table 2, the DCIR, initial capacity, and capacity retention rate are each shown as relative values, with the result for battery B1 of Comparative Example 1 set at 100.

[0094]

[0095] It can be seen from Tables 1 and 2 that only when a positive electrode containing porous carbon (C) and CNT is used, low resistance and excellent charge-discharge cycle characteristics can be obtained.

[0096] A nonaqueous electrolyte secondary battery including a positive electrode according to the present disclosure is useful for a variety of applications, such as electronic devices such as mobile phones, smartphones, and tablet terminals, hybrid vehicles, plug-in hybrid vehicles, and electric vehicles.

[0097] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0098] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 15: Positive electrode plate, 15a: Positive electrode lead, 16: Negative electrode plate, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove portion, 23: First insulating plate, 24: Second insulating plate

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode core; and a positive electrode mixture layer disposed on at least a surface of the positive electrode core; the positive electrode mixture layer contains at least a positive electrode active material and a positive electrode conductive agent; the positive electrode conductive agent contains porous carbon and carbon nanotubes; the porous carbon particles have a three-dimensional structure having a plurality of through holes, the through holes being voids that continuously connect from one surface of the particle to another surface; and the mode diameter of the pore size distribution of the porous carbon is 0.5 nm or more and less than 100 nm.

2. The specific surface area of ​​the porous carbon is 100 m ２ 2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode has a SiO2 content of 0.1% or more.

3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the volume average particle size of said porous carbon is less than 50 μm.

4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the porosity of the porous carbon is less than 50% by volume.

5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of said porous carbon in said positive electrode mixture layer is less than 10 mass %.

6. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the carbon nanotubes have an average diameter of 10 nm or less.

7. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material comprises a lithium transition metal composite oxide, and the lithium transition metal composite oxide contains at least Ni, with the proportion of Ni relative to all metal elements other than Li being 50 mol % or more.

8. The lithium transition metal composite oxide has the formula: Li α Ni(1-x1-x2-x3-y)Co ｘ１ Mn ｘ２ Al ｘ３ M ｙ O ２＋β 8. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 7, wherein the following conditions are satisfied: 0.95≦α≦1.05, 0.5≦1−x1−x2−x3−y≦0.99, 0≦x1≦0.1, 0≦x2≦0.5, 0≦x3≦0.1, 0≦y≦0.1, and −0.05≦β≦0.05; and M is at least one element selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y.

9. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 8, wherein at least one element selected from the group consisting of B, N, P, Mg, Ca, Sr, Ba, Ti, W, Zr, and Al is unevenly distributed in a surface layer portion of the lithium transition metal composite oxide.

10. A non-aqueous electrolyte secondary battery comprising the positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode has at least a negative electrode core, and the volume Vc of the negative electrode in a fully charged state is 125% or more of the volume Vdc of the negative electrode in a fully discharged state.

11. The nonaqueous electrolyte secondary battery according to claim 10, wherein the negative electrode has the negative electrode core and a negative electrode mixture layer disposed on a surface of the negative electrode core, the negative electrode mixture layer contains a Si-containing material, and the content of the Si-containing material in the negative electrode mixture layer is 3% or more.

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