Lithium ion secondary battery positive electrode, lithium ion secondary battery and calculation method

Abstract

To provide a lithium ion secondary battery positive electrode, excellent in rate characteristics, a lithium ion secondary battery and a calculation method.SOLUTION: There is provided a lithium ion secondary battery positive electrode according to the present invention, in which a positive electrode mixture layer is provided on a current collector and rolled, the positive electrode mixture layer containing a positive electrode active material capable of storing and releasing lithium ions, a conductive agent, and a binder. When Gv represents a standard deviation of the center of gravity of a contour shape showing an active material area in a SEM reflection electron image, of a positive electrode surface, obtained by a scanning electron microscope (SEM) with an accelerating voltage of 15 kV and a working distance (WD) of 10 mm, 0.670<Gv is satisfied.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery positive electrode, a lithium ion secondary battery, and a calculation method. [Background technology]

[0002] Lithium-ion secondary batteries are used as power sources for conventional electronic devices in a wide range of applications, such as storage batteries for portable devices, electric vehicles, homes, drones, robots, and business facilities (see, for example, Patent Document 1). In particular, lithium-ion secondary batteries for drones are required to have high output density (high rate characteristics) due to their use.

[0003] To improve the rate characteristics of lithium-ion batteries, it is necessary to use a positive electrode in which the positive electrode materials (positive electrode active material, conductive agent, binder, etc.) are properly dispersed. Proper dispersion of the positive electrode materials reduces the reaction resistance of the positive electrode, improving the rate characteristics. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-11992 Summary of the Invention [Problem to be solved by the invention]

[0005] It has generally been thought that a homogeneously dispersed positive electrode material reduces reaction resistance. However, the inventors discovered that excessive dispersion of the positive electrode material increases reaction resistance and reduces rate characteristics. To achieve good rate characteristics, it is necessary to create a positive electrode in which the positive electrode material is not excessively dispersed but is aggregated to a certain extent.

[0006] The present invention has been made in view of the above, and an object thereof is to provide a lithium-ion secondary battery positive electrode having excellent rate characteristics, a lithium-ion secondary battery, and a calculation method.

Means for Solving the Problems

[0007] In order to solve the above-mentioned problems and achieve the object, the positive electrode for a lithium-ion secondary battery according to the present invention, from a first aspect, is a positive electrode composite layer containing a positive electrode active material capable of occluding and releasing lithium ions, a conductive agent, and a binder provided on a current collector, and is a lithium-ion secondary battery positive electrode subjected to a rolling treatment. In the SEM reflected electron image of the surface of the positive electrode obtained by a scanning electron microscope (SEM) with a magnification of 5000 times, an acceleration voltage of 15 kV, and an irradiation current P.C. 30, when the standard deviation of the distance between the centroid of the shape formed by the contour indicating the active material region and the centroid of the shape formed by the contour indicating the nearest other active material region is defined as Gv, in μm conversion, it satisfies 0.670 < Gv.

[0008] The SEM reflected electron image can be replaced with a secondary electron image, but when emphasizing the difference in composition, the SEM reflected electron image, which is relatively strongly related to the composition, is preferred.

[0009] The above Gv indicates the variation in the arrangement of the active materials. A small standard deviation Gv means that the variation is small and the materials are evenly dispersed. Conversely, a large standard deviation Gv means that the variation in the regions is large and the materials are aggregated or unevenly distributed. By using the standard deviation Gv in this way, the degree of dispersion of the electrode material can be quantified.

[0010] Further, the positive electrode for a lithium-ion secondary battery according to the present invention, from a second aspect, in addition to the first aspect, is characterized in that Gv satisfies 0.710 < Gv.

[0011] Furthermore, as a third aspect, in addition to the first or second aspect, the lithium ion secondary battery positive electrode according to the present invention is characterized in that, relative to the total mass of the positive electrode mixture layer, the mass proportion of the positive electrode active material is 94.0 mass% or more and 98.0 mass% or less, the mass proportion of the conductive agent is 1.0 mass% or more and 5.0 mass% or less, and the mass proportion of the binder is 1.0 mass% or more and 5.0 mass% or less.

[0012] Furthermore, as a fourth aspect, in addition to any one of the first to third aspects, the lithium ion secondary battery positive electrode according to the present invention is characterized in that the density of the positive electrode mixture layer is 3.3 g / cc or more and 3.7 g / cc or less.

[0013] Furthermore, as a fifth aspect, in addition to any one of the first to fourth aspects, the positive electrode for a lithium ion secondary battery according to the present invention is characterized in that the SEM backscattered electron image is obtained by scanning an area of ​​17.28 μm × 25.4 μm on the surface of the positive electrode.

[0014] In addition, as a sixth aspect, there is provided a lithium ion secondary battery according to the present invention, characterized in that it comprises the positive electrode for lithium ion secondary batteries according to any one of the first to fifth aspects, a negative electrode capable of absorbing and desorbing lithium ions, and a nonaqueous electrolyte solution.

[0015] Furthermore, as a seventh aspect, the calculation method according to the present invention is a calculation method for calculating parameters of a lithium-ion secondary battery positive electrode in which a positive electrode composite layer containing a positive electrode active material capable of absorbing and releasing lithium ions, a conductive agent, and a binder is provided on a current collector and rolled, the calculation method comprising the steps of: acquiring an image that is an SEM backscattered electron image or a secondary electron image of the surface of the positive electrode using a scanning electron microscope (SEM); binarizing the acquired image based on a brightness signal and a threshold value that is set based on the brightness signal of the image; extracting contours from the binarized image; and calculating a value obtained by measuring the distance between the center of gravity of the shape formed by the contour and the nearest center of gravity for all of the centers of gravity.

[0016] In addition, as an eighth aspect, the calculation method according to the present invention is characterized in that, in addition to the seventh aspect, it further comprises the step of calculating a standard deviation of the value.

[0017] Furthermore, in a ninth aspect, in addition to the seventh or eighth aspect, the calculation method according to the present invention is characterized in that the threshold value is a median value of luminance signals of the image.

[0018] Furthermore, as a tenth aspect, in addition to any one of the seventh to ninth aspects, the calculation method according to the present invention is characterized in that the number of pixels of the scale of the image is set, and the image is smoothed with an upper limit of the number of pixels equivalent to 0.1 μm.

[0019] Furthermore, as an eleventh aspect, in addition to any one of the seventh to tenth aspects, the calculation method according to the present invention is characterized in that, when the area of ​​the portion surrounded by the contour is smaller than a predetermined area, the portion surrounded by the contour is excluded from the contour for which the center of gravity is calculated. [Effects of the Invention]

[0020] According to the present invention, a positive electrode for a lithium ion secondary battery having excellent rate characteristics can be obtained. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a perspective view illustrating the configuration of a lithium ion secondary battery according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view taken along line AA shown in FIG. [Figure 3] FIG. 3 is a flowchart showing the flow of a method for calculating a standard deviation according to an embodiment of the present invention. [Figure 4] FIG. 4 is a diagram for explaining calculation of the center of gravity for the SEM image of the positive electrode composite material layer according to Example 1. As shown in FIG. [Figure 5] FIG. 5 is a diagram for explaining calculation of the center of gravity for an SEM image of the positive electrode composite layer according to Comparative Example 1. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following description. Furthermore, various modifications and improvements can be made to the present embodiments, and such modifications and improvements can also be included in the present invention. 1 and 2 show an example of the configuration of a laminated lithium ion secondary battery as an embodiment, but the shape of the lithium ion secondary battery of the present invention is not particularly limited and may be flat, cylindrical, prismatic, coin-shaped, etc. The exterior body of the lithium ion secondary battery is also not particularly limited and known materials such as laminate film, aluminum, aluminum alloy, and stainless steel can be used.

[0023] (Embodiment) Fig. 1 is a perspective view illustrating the configuration of a lithium ion secondary battery according to an embodiment of the present invention, and Fig. 2 is a cross-sectional view taken along line AA shown in Fig. 1.

[0024] The lithium ion secondary battery 1 is a laminated lithium ion battery having an exterior body 2 formed into a bag shape by stacking two laminate films with their heat-sealable resin layers facing each other and heat-sealing the outer periphery.

[0025] The exterior body 2 is formed into a bag shape by stacking two laminate films with their heat-sealable resin layers facing each other and heat-sealing the outer periphery. An electrode group 3 and a nonaqueous electrolyte are contained within the exterior body 2. The electrode group 3 is inserted through an opening in the exterior body 2, and the opening of the exterior body 2 is sealed by heat-sealing, thereby hermetically storing the electrode group 3 within the exterior body 2.

[0026] As the laminate film, a composite film in which a heat-sealable resin layer for heat sealing is provided on a metal layer is preferably used. The metal layer is not particularly limited as long as it prevents moisture from entering from the outside and improves the strength of the entire sheet. For example, aluminum foil, stainless steel foil, etc. can be used as the metal layer. The heat-sealable resin layer is not particularly limited, but polyethylene or polypropylene is preferably used from the viewpoint of the temperature range in which heat sealing is possible and the blocking property against the non-aqueous electrolyte. Here, to protect the metal layer, a protective layer may be provided on the surface opposite to the heat-sealable resin layer. The protective layer is not particularly limited, but nylon, PET, etc. are preferably used. Furthermore, to improve adhesion between the metal layer and the heat-sealable resin layer, an adhesive layer may be provided between them.

[0027] As shown in FIG. 2, the electrode group 3 has a structure in which a positive electrode 4, a negative electrode 5, and a separator 6 interposed between the positive electrode 4 and the negative electrode 5 are stacked as a set with the negative electrode 5 positioned as the outermost layer, and multiple such sets are stacked.

[0028] (positive electrode) The positive electrode 4 is composed of a positive electrode current collector 42 and a positive electrode layer 41 formed on one or both sides of the positive electrode current collector 42. The positive electrode 4 is in the form of a plate.

[0029] The positive electrode layer 41 contains a positive electrode active material, a conductive agent, and a binder.

[0030] <Cathode active material> The positive electrode active material is not particularly limited as long as it is a lithium-containing compound capable of absorbing and releasing lithium. The lithium-containing compound is, for example, a lithium-containing metal oxide or a lithium metal phosphate. Examples of the lithium-containing metal oxide include lithium cobalt composite oxides (e.g., LiCoO2), lithium nickel composite oxides (e.g., LiNiO2), lithium manganese composite oxides (e.g., LiMnO2, Li2MnO3, or LiMn2O4), and lithium nickel manganese composite oxides (e.g., Li(Co 1 / 3 Ni 1 / 3 Mn1 / 3 )O2, lithium cobalt nickel aluminum composite oxide (e.g., Li(Co 0.15 Ni 0.8 Al 0.05 )O2, lithium nickel manganese composite oxide (e.g., LiNi 0.5 Mn 1.5 Examples of lithium metal phosphate include lithium iron phosphorus composite oxides (e.g., LiFePO4). From the viewpoint of the discharge capacity and average voltage of the positive electrode active material, it is preferable to contain lithium cobalt oxide.

[0031] The average particle size of the positive electrode active material is preferably 6 μm or more and 9 μm or less. If it is less than 6 μm, the specific surface area of ​​the active material increases, resulting in a lack of conductive paths relative to the specific surface area, which may increase reaction resistance. If it is greater than 9 μm, the diffusibility of lithium ions within the active material particles may decrease, and the specific surface area of ​​the active material may decrease, reducing the reaction area and increasing reaction resistance. The average particle size is the median diameter value measured using the laser diffraction and scattering method described in JIS standard Z8825:2013. Measurements can be performed using a laser diffraction particle size distribution analyzer, SALD-2300 (Shimadzu Corporation).

[0032] <Conductive agent> The conductive agent is not particularly limited, and known or commercially available ones can be used. Examples of the conductive agent include carbon black such as acetylene black and ketjen black, carbon nanotubes, carbon fiber, activated carbon, graphite, and other conductive carbons. The conductive agent can be used alone or as a mixture of two or more of these.

[0033] <Binder> The binder is not particularly limited, and known or commercially available binders can be used. Examples include fluorine-based resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene-butadiene rubber (SBR), acrylic resin, polyvinyl alcohol (PVA), and carboxymethyl cellulose (CMC). The binder can be used as a single substance, or as a mixture or copolymer of two or more substances.

[0034] <Dispersant> A dispersant is not necessarily required, but may be added if it is desired to disperse the conductive agent more effectively. A typical dispersant is polyvinylpyrrolidone (PVP).

[0035] [Electrode composition] In the electrode composition, the higher the positive electrode active material ratio, the higher the energy density; the higher the conductive agent ratio, the better the electronic conductivity; and the higher the binder ratio, the higher the peel strength of the electrode coating surface. Considering these balances, the electrode composition preferably has a positive electrode active material mass ratio of 94.0% to 98.0% by mass, a conductive agent mass ratio of 1.0% to 5.0% by mass, and a binder mass ratio of 1.0% to 5.0% by mass, relative to the total mass of the positive electrode plate composite layer. In this case, if the positive electrode active material mass ratio is less than 94.0% by mass, the energy density cannot be increased. On the other hand, if the positive electrode active material mass ratio is greater than 98.0% by mass, the energy density increases, but the proportion of conductive agent or binder relative to the positive electrode active material decreases, resulting in a decrease in electronic conductivity or peel strength. If the mass ratio of the conductive agent is less than 1.0 mass%, the electronic conductivity decreases, whereas if the mass ratio of the conductive agent is more than 5.0 mass%, the electronic conductivity improves, but the ratio of the binder to the conductive agent becomes insufficient, resulting in a decrease in peel strength. If the mass ratio of the binder is 1.0 mass % or less, the peel strength decreases, whereas if the mass ratio of the binder is more than 5.0 mass %, the binder becomes a resistive component, resulting in a decrease in electronic conductivity.

[0036] <Positive electrode current collector> The positive electrode current collector 42 is not particularly limited, and known or commercially available ones can be used. Examples of the positive electrode current collector 42 include rolled foils made of aluminum, aluminum alloys, copper, nickel, or stainless steel, and porous metals such as porous aluminum. Among these positive electrode current collectors, aluminum or aluminum alloys are preferred because they have high electrical conductivity, excellent corrosion resistance in the electrolyte, and are lightweight metals.

[0037] [Electrode manufacturing method] The positive electrode can be produced, for example, by the following method. First, the positive electrode active material, conductive agent, binder, and dispersant described above are dispersed in a solvent to prepare a positive electrode slurry. A thickener may be further added to the positive electrode slurry. Next, the positive electrode slurry is applied to one or both surfaces of a positive electrode current collector, and then dried to form a positive electrode layer. The positive electrode can be produced by rolling the positive electrode slurry into a plate-shaped positive electrode.

[0038] Rolling is preferably performed so that the electrode density is 3.3 g / cc or more and 3.7 g / cc or less. If the density is less than 3.3 g / cc, the volumetric energy density may decrease and it may be difficult to form a conductive path. If the density is more than 3.7 g / cc, large shear stress is applied to the positive electrode composite layer and the composite layer-current collector foil interface during rolling, which may destroy the bonding between the materials in the positive electrode composite layer and the bonding between the composite layer and the current collector foil interface, reducing the electrode peel strength.

[0039] <Solvent> The solvent used in preparing the positive electrode slurry is not particularly limited, and known or commercially available solvents can be used. Examples of the solvent include N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and water. When polyvinylidene fluoride (PVDF) is used as the binder, N-methyl-2-pyrrolidone (NMP) is preferably used as the solvent. When styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), or carboxymethyl cellulose (CMC) is used as the binder, water is preferably used as the solvent. When polyvinylpyrrolidone (PVP) is used as the dispersant, polyvinylpyrrolidone (PVP) is preferably used.

[0040] The positive electrode 4 also has a positive electrode lead 43, which is a positive electrode current collector 42 extending from the positive electrode layer 41. The positive electrode lead 43 extends, for example, from the right side surface in FIG. 2. The positive electrode leads 43 are bundled at their tip ends within the exterior body 2 and joined to each other by ultrasonic welding, resistance welding, or the like. One end of the positive electrode terminal 7 is joined to the joint of the positive electrode lead 43, and the other end extends to the outside through the sealing portion of the exterior body 2.

[0041] In this embodiment, the surface of the positive electrode prepared by the above method was photographed with a scanning electron microscope (SEM), and the obtained SEM backscattered electron image was analyzed to obtain the center of gravity Gi of each active material particle region. After obtaining Gi, the center of gravity Gj closest to each center of gravity Gi was calculated, and the standard deviation Gv of the distance between the centers of gravity Gi and Gj, converted into μm, was calculated. Details of the image analysis method are as follows.

[0042] FIG. 3 is a flowchart showing the flow of a method for calculating standard deviation according to one embodiment of the present invention. This calculation method is executed using a computer. This computer is configured using one or more pieces of hardware, such as a central processing unit (CPU), a graphics processing unit (GPU), a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), or a complex programmable logic device (CPLD), as well as memory in which various programs are pre-installed. There are no particular restrictions on the language used to perform the processing, and any language, such as Python, Java, or VBA, can be selected. The calculation process will be described below as being executed by a control unit included in the computer.

[0043] First, the control unit acquires a backscattered electron image (step S101). This backscattered electron image is an SEM backscattered electron image captured by an SEM. As an example, this SEM backscattered electron image is a 5000x magnification SEM backscattered electron image obtained by scanning a 17.28 μm × 25.4 μm area on the positive electrode surface with an SEM. The scanning range is not limited to 17.28 μm × 25.4 μm. In this case, an auto-contrast / brightness adjustment function (ACB) is used, if available. Regarding the luminance signal, as will be described later, if the maximum value of the image luminance signal is 240 or less, or the median value of the image luminance signal is 120 or more, there is a possibility of a condition such as "whiteout" or "blackout." This may result in an inaccurate standard deviation, as will be described later. Therefore, a new image should be captured and used. In order to avoid the above-mentioned "bloated highlights" and "crushed shadows," in the case of the SEM used in the present invention, scanning under conditions of an acceleration voltage of 15 kV, a working distance (WD) of 10 mm, and a probe current (PC) of 30 is effective, if not completely avoidable.

[0044] Thereafter, the control unit checks the number of pixels of the scale in the backscattered electron image (step S102). The control unit acquires the magnification of the SEM backscattered electron image and determines how many pixels the scale indicating the length standard is made up of.

[0045] The control unit then smooths the image (step S103). Based on the number of pixels of the scale, the control unit performs smoothing with an upper limit of the number of pixels equivalent to 0.1 μm if the magnification is 5000 times. This smoothing evens out roughness in the image. The upper limit of the number of pixels in the smoothing can be changed depending on the imaging conditions such as the magnification.

[0046] After the smoothing process, the control unit binarizes the image (step S104). The control unit first calculates the luminance signal for each pixel by adding together three values: a value obtained by weighting the red luminance by 0.299, a value obtained by weighting the green luminance by 0.587, and a value obtained by weighting the blue luminance by 0.114. The control unit then calculates the median of the luminance signals for the image and sets the color of each pixel position to either white or black using this median as a threshold. By changing the color to the set color, the control unit converts the image into an image with only two colors: black and white.

[0047] Then, the control unit extracts contours from the binarized image (step S105). The control unit extracts the boundary between white and black and depicts it as a contour.

[0048] The reason for using the median value of the image luminance signal as the threshold is that the luminance signal of an image may vary from image to image or depending on the SEM imaging environment. If the threshold is set as the absolute value of the luminance signal, if the luminance signal of the entire image varies even within the same field of view, the contour may change depending on the luminance signal of the image, making it difficult to accurately grasp the boundary of the active material. Furthermore, if a value other than the median is set as the threshold, even if the dispersion state of the electrode surface differs between fields of view, there may be no difference in Gv, and the dispersion state may not be accurately quantified. Therefore, when binarizing, it is preferable to use the median value as the threshold to distinguish colors.

[0049] After the contour extraction, the control unit assigns a number N to the portion (area) surrounded by the contour (step S106). MAX Numbers up to are assigned to each.

[0050] Then, the control unit calculates the area of ​​each part (step S107). Furthermore, the control unit sets the number N to N=1 (step S108). Note that step S107 and step S108 may be performed first, or may be performed simultaneously.

[0051] The control unit determines that the area of ​​the portion numbered N is 0.5 squared ((0.5) 2 )μm 2 The control unit determines whether the area is equal to or greater than (0.5) (step S109). 2 μm 2 If it is determined that the area is less than (0.5) (step S109: No), the process proceeds to step S112. 2 μm 2 If it is determined that the number is equal to or greater than the number (step S109: Yes), the process proceeds to step S110.

[0052] In step S110, the control unit determines whether the portion with number N is a white region or a black region. For example, if white dots occupy 20% or more of the dots in the region, it is determined to be a white region. In this embodiment, the white region is a region corresponding to the active material, and the black region is a region corresponding to the conductive agent, binder, or voids between the active material particles. If the control unit determines that it is a white region (step S110: Yes), it proceeds to step S112. On the other hand, if the control unit determines that it is a black region (step S110: No), it proceeds to step S111. Note that this step is not limited to this, and in embodiments other than this embodiment, processing in which the white region and the black region are reversed may also be possible.

[0053] In step S111, the control unit incorporates the number N into the group M. As a result, the number N (outline) that is not in the group M is excluded from the targets of the processing after step S114. The reason for excluding the outline in this way is to distinguish the outline between the active material and the non-active part. After the incorporation process, the control unit proceeds to step S112.

[0054] After that, the control unit increases N by 1 (step S112).

[0055] For the number N increased by 1, the control unit MAX determines whether it is greater than N (step S113). When the control unit determines that the number N is MAX less than or equal to N (step S113: No), it proceeds to step S109 and repeats the above-described processing for the part of the increased number N. In contrast, when the control unit determines that the number N is MAX greater than N (step S113: Yes), it proceeds to step S114.

[0056] s In step S114, the control unit calculates the centroid of the shape formed by the outline of each part existing in the group M (step S114).

[0057] The control unit calculates the centroid Gj closest to each centroid Gi and incorporates the distance Gij in μm conversion between the centroid Gi and the centroid Gj into the group L. After performing the same operation for all centroids Gi, the standard deviation Gv of the group L is calculated (step S115). In this way, the standard deviation Gv is calculated from the acquired SEM backscattered electron image. In this embodiment, when the standard deviation Gv satisfies 0.670 < Gv, a positive electrode with excellent rate characteristics can be obtained. Here, if the standard deviation Gv is 0.670 or less, the degree of dispersion of the material is insufficient, the conductive path decreases, and the rate characteristics may deteriorate. More preferably, the standard deviation Gv satisfies 0.710 < Gv. Thereby, a further improvement in rate characteristics is possible.

[0058] 〔Method for Controlling Standard Deviation Gv〕 The standard deviation Gv can be controlled by the dispersion method of the slurry, the electrode drying conditions, etc. When controlling the standard deviation Gv by the dispersion method, for example, in the case of the dispersion method using a thin-film swirling mixer, the shearing force applied to the positive electrode material becomes small and the dispersibility decreases, so the standard deviation Gv tends to increase. When using a thin-film swirling mixer, it is necessary to consider the kneading time for dispersion, etc., so that the standard deviation Gv does not become too large. If a planetary mixer is used for solid kneading and then a solvent is added to reduce the solid content ratio of the slurry (the ratio of the total weight of various positive electrode materials to the total weight of the slurry) for dispersion, the shearing force applied to the positive electrode material becomes large and the dispersibility increases, so the standard deviation Gv tends to decrease. When using a planetary mixer, it is necessary to consider the drying conditions described later, etc., so that the standard deviation Gv does not become too small. In addition, the solid kneading in the present invention means a step of kneading with a solid content ratio of 80% or more at maximum. Also, the standard deviation Gv can be controlled by adjusting the drying conditions. For example, when the drying temperature is high, the conductive agent spreads evenly and the standard deviation Gv becomes small, but when the drying temperature is low, the conductive agent is unevenly distributed and spreads, so the standard deviation Gv becomes large. In order to make the standard deviation Gv satisfy 0.670 < Gv, as an example, a thin-film swirling mixer can be used for 1 hour to disperse the slurry, the slurry is coated by die coating, and the electrode drying can be carried out at a maximum drying temperature of 95 °C for 3 minutes and then at a maximum temperature of 130 °C for 1 minute. Also, when the standard deviation Gv is too large, it becomes excessive uneven distribution, which is not desirable from the perspective of overall battery performance. Therefore, 4.5, which is half of 9 μm, the upper limit of the preferable value of the average particle diameter of the positive electrode active material, is the upper limit of the standard deviation Gv.

[0059] (Negative Electrode) The negative electrode 5 is composed of a negative electrode current collector 52 and a negative electrode layer 51 formed on both sides or one side of the negative electrode current collector 52. Here, the negative electrode layer 51 of the negative electrode 5 located in the outermost layer is formed on the surface facing the separator 6 of the negative electrode current collector 52. On the other hand, the negative electrode layer 51 of the negative electrode 5 located between the positive electrodes 4, excluding the negative electrode 5 located in the outermost layer, is formed on both sides of the negative electrode current collector 52.

[0060] The negative electrode layer 51 contains a negative electrode active material, a conductive agent, and a binder. The negative electrode active material is not particularly limited as long as it is a material capable of absorbing and releasing lithium. Examples of the negative electrode active material include cokes such as pyrolytic carbons, pitch coke, needle coke, and petroleum coke; natural graphite; artificial graphite; mesocarbon microbeads (MCMB); hard carbon; soft carbon; glassy carbon; sintered organic polymer compounds (e.g., phenolic resin, furan resin, etc., sintered and carbonized); carbon fibers; carbon black; activated carbon; metal materials such as Al, Si, and Sn; and alloy materials. Preferred negative electrode active materials are cokes and carbons that have stable charge / discharge characteristics and easily form a solid electrolyte interphase (SEI) coating on their surfaces. Graphite such as natural graphite or artificial graphite is more preferred.

[0061] <Conductive agent> The conductive agent is not particularly limited, and known or commercially available conductive agents can be used. For example, the conductive agent can be the same as that used in the positive electrode described above. Note that a configuration without a conductive agent is also possible.

[0062] <Binder> The binder is not particularly limited, and known or commercially available binders can be used. Examples include fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), fluororubbers, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), core-shell binders, polyvinyl alcohol, and polyimide resins such as polyimide and polyamideimide. The binder can be used as a single substance, or as a mixture or copolymer of two or more substances.

[0063] <Negative electrode current collector> The negative electrode current collector 52 is not particularly limited, and a known or commercially available one can be used. For example, rolled foil made of copper, a copper alloy, aluminum, or stainless steel, or a porous metal such as porous aluminum can be used as the negative electrode current collector 52. The negative electrode current collector is preferably made of copper or a copper alloy.

[0064] The negative electrode 5 also has a negative electrode lead 53, for example, in which the negative electrode current collector 52 extends from the negative electrode layer 51. The negative electrode lead 53 extends, for example, from the left side surface in FIG. 2. The negative electrode leads 53 are bundled at their tip ends within the exterior body 2 and joined to each other. One end of the negative electrode terminal 8 is joined to the joint of the negative electrode lead 53, and the other end extends to the outside through the sealing portion of the exterior body 2.

[0065] (Non-aqueous electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent in which a lithium salt is dissolved. Examples of lithium salts include one or a mixture of two or more selected from the group consisting of LiBF, LiPF, Li(FSO)N, Li(CFSO)N, and LiPOF. The nonaqueous solvent is not particularly limited, but examples include one or a mixture of two or more selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl propionate, ethyl propionate, propyl propionate, methyl acetate, methyl formate, methyl butyrate, dioxolane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethoxyethane, γ-butyrolactone, acetonitrile, and benzonitrile. DMC, DEC, DPC, EMC, EC, and PC are particularly preferred. Among these, it is preferable to include EC, which is capable of forming a good coating on the negative electrode active material.

[0066] (separator) The separator 6 is not particularly limited, and known or commercially available separators can be used. The separator 6 can be, for example, a microporous membrane or nonwoven fabric made of a polyolefin resin such as a polyethylene resin or a polypropylene resin, a polyimide resin, or a polytetrafluoroethylene (PTFE) resin. The microporous membrane or nonwoven fabric may have a single-layer or multilayer structure. The separator 6 may be a microporous membrane as is, or an inorganic ceramic layer may be formed on one or both sides of the microporous membrane to improve heat resistance and nonaqueous electrolyte replenishment.

[0067] Such lithium-ion secondary batteries are preferably subjected to pretreatments such as initial charging and gas evacuation before use. For example, initial charging involves injecting a nonaqueous electrolyte solution and then performing constant current charging at a minute current value down to 10% of the rated capacity. After such initial charging, gas generated within the exterior body during initial charging is vented. Gas evacuation is performed, for example, by placing the exterior body in a decompression chamber with a portion of the sealed portion open. Gas evacuation can be accelerated, for example, by applying pressure to the exterior body from the outside during gas evacuation.

[0068] When the initial charge is performed at such a small current value, the SEI film is likely to form at 1.2 to 0.8 V vs. Li / Li. + Since the negative electrode potential is maintained at this state for a long period of time, a denser and stronger SEI film can be formed on the surface of the negative electrode active material. Furthermore, during initial charging, there is a high possibility that the nonaqueous electrolyte may not be sufficiently impregnated into the constituent materials of the lithium-ion secondary battery, increasing the risk of lithium deposition due to overvoltage. Therefore, by performing the initial charging at such a small current value, it is possible to suppress the occurrence of overvoltage and thereby suppress lithium deposition. Furthermore, by performing the initial charging at a small current value, it is possible to promote gas generation during the initial charging, and the gas can be efficiently discharged through a portion of the sealing portion of the exterior body that is opened. By sufficiently discharging the gas, it is possible to suppress expansion of the exterior body due to gas generation when the lithium-ion secondary battery is used.

[0069] In the embodiment described above, by analyzing the SEM image, when the standard deviation Gv of the centroid Gi of the contour of the region having a predetermined size in the active material region of the positive electrode satisfies 0.670 < Gv, a lithium-ion secondary battery positive electrode excellent in rate characteristics can be obtained.

[0070] In the above-described embodiment, an example of extracting the contour from the binarized image and calculating the standard deviation of the centroid of the contour has been described. In addition, based on the binarized image and the centroid, parameters other than the standard deviation can be calculated. [[ID=​​​​​​​​​​​​​​​​​​​​​A positive electrode slurry was prepared by dispersing 96 wt% lithium iron cobaltate (LiCoO2) as the positive electrode active material, 2.6 wt% carbon black as the conductive agent, 1.2 wt% polyvinylidene fluoride (PVDF) as the binder, and 0.2 wt% polyvinylpyrrolidone (PVP) as the dispersant in N-methyl-2-pyrrolidone (NMP) as the solvent. The dispersion was performed using a thin-film gyratory mixer for 1 hour. The amount of solvent was adjusted to achieve a slurry viscosity of 5000 cp. Next, the positive electrode slurry was coated onto both sides of a 12 μm thick aluminum foil positive electrode current collector using a die coater to a coating weight of 15.4 mg / cm2, and the electrode was then dried. The drying method involved drying at a maximum temperature of 95°C for 3 minutes, followed by a further drying at a maximum temperature of 130°C for 1 minute. Thereafter, the mixture was pressed until the electrode density reached 3.5 g / cc to prepare a positive electrode.

[0075] [Standard deviation calculation] The surface of the obtained positive electrode was photographed using an SEM. The SEM backscattered electron image was obtained by scanning an area of ​​17.28 μm × 25.4 μm on the positive electrode surface at a magnification of 5000x, an acceleration voltage of 15 kV, and an irradiation current of PC30. Using this SEM backscattered electron image (SEM image), the standard deviation was calculated according to the flowchart shown in Figure 3.

[0076] <Preparation of negative electrode> The negative electrode slurry was prepared by dispersing 98% by weight of graphite as the negative electrode active material and 1.0% by weight each of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders in ion-exchanged water as the solvent. Next, the negative electrode slurry was applied to both sides of a copper foil (6 μm thick) as the negative electrode current collector in a coating amount of 8.3 mg / cm. 2 The mixture was coated using a coater so that the thickness became 1.6 g / cc, and then dried at 80 to 110° C. Thereafter, the mixture was pressed until the electrode density became 1.6 g / cc to prepare a negative electrode.

[0077] <separator> The separator used was a 12 μm thick microporous membrane made of polyethylene (PE) resin, with a 2 μm thick coating layer made of AlOOH (boehmite) on one side thereof.

[0078] <Exterior body> The exterior body consisted of a rectangular first laminate film with a thickness of 151 μm, a recess, and a flat edge around the recess, and a flat, rectangular second laminate film with a thickness of 151 μm. Each of these laminate films had a laminated structure consisting of a heat-sealable resin layer (inner layer), a metal layer, and a protective layer (outer layer). The heat-sealable resin layer was an 80 μm thick polyolefin resin film. The metal layer was an aluminum foil with a thickness of 40 μm. The protective layer was a 25 μm thick polyamide film.

[0079] <Nonaqueous electrolyte> The nonaqueous electrolyte was a 3 / 1 / 4 / 2 volume mixture of ethylene carbonate (EC), diethyl carbonate (DEC), ethyl propionate (EP), and propyl propionate (PP), dissolved in 1.3 mol / L lithium hexafluorophosphate (LiPF6). The additives included 2.25 wt% vinylene carbonate (VC), 1.25 wt% adiponitrile (ADN), 1.25 wt% succinonitrile, and 0.15 wt% lithium tetrafluoroborate (LiBF4) based on the total weight of the electrolyte (EC + DEC + EP + PP + LiPF6).

[0080] <Prototype battery assembly> Using the non-aqueous electrolyte, a prototype battery according to Example 1 was assembled as follows: The prototype battery had the same structure as the lithium-ion secondary battery shown in Figures 1 and 2, except that the manufacturing process for the positive electrode was changed. The positive electrodes, negative electrodes, and separators were each cut to a specified area. Next, as shown in Figure 2, eight positive electrodes and nine negative electrodes were alternately stacked with separators in between to obtain an electrode group. The positive electrode leads and negative electrode leads were bundled at their tips and joined together by ultrasonic welding. An aluminum tab was ultrasonically welded to the joint of the positive electrode lead as a positive electrode terminal. A copper tab was ultrasonically welded to the joint of the negative electrode lead as a negative electrode terminal.

[0081] An electrode group with welded positive and negative terminals was placed in the recess of a first laminate film, and a flat second laminate film was placed over the flat edges around the recess of the first laminate film so that the positive and negative terminals partially extended outward. Three of the four overlapping sides of the first and second laminate films were heat-sealed to each other, and the electrode group was housed in an exterior housing made of the first and second laminate films. Next, 7 g of nonaqueous electrolyte was injected into the remaining side of the exterior housing. This side was then heat-sealed under a reduced pressure of 1 hPa to 100 hPa to obtain a prototype battery after the injection of the nonaqueous electrolyte. The theoretical capacity of the prototype battery was 2.55 Ah.

[0082] After the non-aqueous electrolyte was poured into the prototype battery, the battery was left standing for 12 hours to allow the non-aqueous electrolyte to permeate the constituent materials of the electrode group.

[0083] Next, the positive and negative terminals of the prototype battery were connected to a power source, and the prototype battery was initially charged at a current value of 0.25 C to 10% of its rated capacity. The prototype battery was then placed in a decompression chamber with a portion of the sealed portion of the exterior body open, allowing gas generated within the battery to be vented. The sealed portion of the exterior body was then resealed, and the prototype battery was then charged a second time at a current value of 0.5 C to 10-100% of its rated capacity. The prototype battery was then left standing in a high-temperature environment for 3 hours, after which it was subjected to constant-current charging at a current value of 0.5 C to 4.4 V, and constant-voltage charging to 0.02 C. The rated capacity was then measured at a current value of 0.2 C, and the battery was charged to a state of charge (SOC) of 10% of its rated capacity, yielding a prototype battery according to Example 1.

[0084] Example 2 A prototype battery was obtained in the same manner as in Example 1, except that the dispersion method for producing the positive electrode was to carry out kneading with a planetary mixer for 1 hour and then add a solvent to disperse the mixture to a viscosity of 5000 cp.

[0085] Example 3 A prototype battery was obtained in the same manner as in Example 1, except that the coating method for producing the positive electrode was comma coating and the drying method was a maximum temperature of 120° C. and a drying time of 7 minutes.

[0086] Example 4 A prototype battery was obtained in the same manner as in Example 1, except that the positive electrode slurry was composed of 97 wt% lithium iron cobaltate (LiCoO), 1.63 wt% carbon black as a conductive agent, 1.25 wt% polyvinylidene fluoride (PVDF) as a binder, and 0.12 wt% polyvinylpyrrolidone (PVP) as a dispersant, the slurry was dispersed using a thin film gyratory mixer for 1 hour, the slurry coating amount was 15.3 mg / cm, the coating method was comma coating, and the drying method was a maximum temperature of 120°C and a drying time of 7 minutes.

[0087] (Comparative Example 1) A prototype battery was obtained in the same manner as in Example 1, except that the dispersion method for producing the positive electrode was to knead the mixture with a planetary mixer for 1 hour and then add a solvent to achieve a viscosity of 5000 cp, the coating method was comma coating, and the drying method was at a maximum temperature of 120°C and for a drying time of 7 minutes.

[0088] (Comparative Example 2) A prototype battery was obtained in the same manner as in Example 1, except that the dispersion method for preparing the positive electrode was 10 hours using a thin film vortex mixer, the coating method was comma coating, and the drying method was a maximum temperature of 130°C and a drying time of 7 minutes.

[0089] (Comparative Example 3) A prototype battery was obtained in the same manner as in Example 1, except that the dispersion method for preparing the positive electrode was 10 hours using a thin film vortex mixer, the coating method was comma coating, and the drying method was a maximum temperature of 120°C and a drying time of 7 minutes.

[0090] Comparative Example 4 A prototype battery was obtained in the same manner as in Example 1, except that the positive electrode slurry was composed of 93.7 wt% lithium iron cobaltate (LiCoO), 4.0 wt% carbon black as a conductive agent, 2.0 wt% polyvinylidene fluoride (PVDF) as a binder, and 0.3 wt% polyvinylpyrrolidone (PVP) as a dispersant, the slurry was dispersed using a thin film gyratory mixer for 10 hours, the slurry coating amount was 15.8 mg / cm, the coating method was comma coating, the drying temperature was a maximum temperature of 130°C, and the drying time was 7 minutes.

[0091] <Evaluation 1: 0℃, 7C discharge load test> A 7C discharge load test was conducted in a 0°C environment for each of the prototype batteries of Examples 1 to 4 and Comparative Examples 1 to 4. First, before the 7C discharge load test, the 0.2C discharge capacity of each prototype battery was measured under the following charge / discharge condition 1. Then, each prototype battery was subjected to a 7C discharge load test under the following charge / discharge condition 2. From the discharge capacity measurement results before and after the 7C discharge load test, the 7C discharge capacity retention rate was calculated using the following formula (I). The results are shown in Table 1 below. Discharge capacity maintenance rate / %=(7C discharge capacity / 0.2C discharge capacity)×100...(I)

[0092] (Charge / Discharge Condition 1 (Measurement of Discharge Capacity)) Environmental temperature: 0℃ Charge: 1.0C up to 4.4V (Continue charging until the charging current drops to 0.1C) Discharge: 0.2C to 2.5V Pause time between charging and discharging: 10 minutes (Charge / discharge condition 2 (7C discharge load test)) Environmental temperature: 0℃ Charge: 1.0C up to 4.4V (Continue charging until the charging current drops to 0.1C) Discharge: 7.0C to 2.5V Charging and discharging pause time: 10 minutes

[0093] The characteristics of the prototype batteries according to Examples 1 to 4 and Comparative Examples 1 to 4 are shown in Table 1 below.

Table 1

[0094] The standard deviation Gv in Table 1 above is the value obtained by averaging the values for 5 fields for each level of the electrode. Also, the discharge capacity retention rate is the value obtained by averaging the values for 3 cells for each level of the cell. In addition, in each level such as this example, when obtaining the standard deviation Gv, for the image acquired at a magnification of 5,000 times, smoothing is performed with the upper limit being the number of pixels corresponding to 0.1 μm, and contours with an area less than 0.5 squared ((0.5) 2 ) μm 2 are excluded, and a region where white points occupy 20% or more of the points within the region is determined as the white region.

[0095] Regarding the calculation of the standard deviation in Example 1 and Comparative Example 1, it will be described with reference to FIGS. 4 and 5. FIG. 4 is a diagram for explaining the calculation of the centroid with respect to the SEM image of the positive electrode composite layer according to Example 1. FIG. 5 is a diagram for explaining the calculation of the centroid with respect to the SEM image of the positive electrode composite layer according to Comparative Example 1. In FIGS. 4 and 5, according to the flowchart of FIG. 3, the SEM image is binarized with the median value of the luminance signal, and an image in which centroids (● shown in the figure) are generated in each white region is shown. The standard deviation Gv is calculated by obtaining the distance (shown as a straight line in the figure) between each centroid and the closest centroid.

[0096] When the discharge capacity retention rate is 45% or more, it is appropriate as the application of the lithium-ion battery for drones. However, referring to the results shown in Table 1, it can be seen that the prototype batteries according to Examples 1 to 4 that satisfy 0.670 < Gv can maintain the discharge capacity retention rate at 45% or more compared to Comparative Examples 1 to 4 that do not satisfy 0.670 < Gv. On the other hand, Comparative Examples 1 to 4 had a discharge capacity retention rate of less than 45%. Therefore, it can be understood that Examples 1 to 4 are appropriate batteries for the application of lithium-ion batteries for drones. Here, the reason why Examples 1 to 4 have a high discharge capacity retention rate is thought to be that conductive paths between the positive electrode active material particles are appropriately formed, and at the same time, they become paths through which lithium ions in the positive electrode can easily move, thereby reducing reaction resistance.In contrast, in Comparative Examples 1 to 4, the positive electrode material is over-dispersed, which causes the conductive paths to become finer, and at the same time, it becomes paths through which lithium ions in the positive electrode cannot easily move, thereby increasing reaction resistance.This is thought to be the reason why the capacity retention rate is low.

[0097] As described above, according to this embodiment, a positive electrode plate for a lithium ion secondary battery having good rate characteristics can be provided.

[0098] Although the embodiments of the present invention have been specifically described, the present invention is not limited to these embodiments and examples, and various modifications based on the technical concept of the present invention are possible. [Explanation of symbols]

[0099] 1. Lithium-ion secondary battery 2. Exterior body 3 electrode groups 4 Positive electrode 5 negative electrode 6 Separator 7 Positive terminal 8 Negative terminal 41 Positive electrode layer 42 Positive electrode current collector 51 negative electrode layer 52 Negative electrode current collector

Claims

1. A lithium ion secondary battery positive electrode in which a positive electrode mixture layer including a positive electrode active material capable of absorbing and releasing lithium ions, a conductive agent, and a binder is provided on a current collector and subjected to a rolling treatment, In an SEM backscattered electron image of the surface of the positive electrode obtained using a scanning electron microscope (SEM) with an acceleration voltage of 15 kV and a working distance (WD) of 10 mm, when the standard deviation of the distances between the center of gravity of the shape formed by the outline of an active material region and the nearest center of gravity of the shape formed by the outline of another active material region with respect to all the centers of gravity is Gv, the standard deviation of the distances satisfies 0.670<Gv in terms of μm. A positive electrode for a lithium ion secondary battery.

2. The Gv satisfies 0.710<Gv, 2. The positive electrode for a lithium ion secondary battery according to claim 1 .

3. With respect to the total mass of the positive electrode mixture layer, the mass ratio of the positive electrode active material is 94.0 mass% or more and 98.0 mass% or less, the mass ratio of the conductive agent is 1.0 mass% or more and 5.0 mass% or less, and the mass ratio of the binder is 1.0 mass% or more and 5.0 mass% or less.

2. The positive electrode for a lithium ion secondary battery according to claim 1 .

4. The density of the positive electrode mixture layer is 3.3 g / cc or more and 3.7 g / cc or less.

2. The positive electrode for a lithium ion secondary battery according to claim 1 .

5. The SEM backscattered electron image was obtained by scanning an area of ​​17.28 μm × 25.4 μm on the surface of the positive electrode.

2. The positive electrode for a lithium ion secondary battery according to claim 1 .

6. The positive electrode for a lithium ion secondary battery according to claim 1 or 2, a negative electrode capable of absorbing and releasing lithium ions; a nonaqueous electrolyte; A lithium ion secondary battery comprising:

7. A method for calculating parameters of a lithium ion secondary battery positive electrode in which a positive electrode mixture layer including a positive electrode active material capable of absorbing and releasing lithium ions, a conductive agent, and a binder is provided on a current collector and the positive electrode mixture layer is subjected to a rolling treatment, the method comprising: Obtaining an image, which is an SEM backscattered electron image or a secondary electron image, of the surface of the positive electrode using a scanning electron microscope (SEM); binarizing the acquired image based on a luminance signal and a threshold value set based on the luminance signal of the image; Extracting a contour from the binarized image; Calculating the distance between the center of gravity of the shape of the outline and the nearest center of gravity for all centers of gravity; A calculation method characterized by:

8. The method of claim 7, further comprising calculating the standard deviation of the values.

9. the threshold is the median value of the luminance signal of the image; 8. The calculation method according to claim 7, wherein:

10. Set the number of pixels of the scale of the image; smoothing the image up to a maximum number of pixels equivalent to 0.1 μm; 8. The calculation method according to claim 7, wherein:

11. If the area of ​​the portion surrounded by the contour is smaller than a predetermined area, the portion surrounded by the contour is excluded from the contours for which the center of gravity is calculated.

8. The calculation method according to claim 7, wherein:

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