Positive electrode for lithium ion secondary battery, lithium ion secondary battery and calculation method

Abstract

To provide a positive electrode for a lithium ion secondary battery, excellent in cycle life characteristics, a lithium ion secondary battery and a calculation method.SOLUTION: There is provided a positive electrode for a lithium ion secondary battery 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, Gv<0.710 is satisfied.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a lithium ion secondary battery, 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 appropriately dispersed. An electrode in which the positive electrode materials are uniformly dispersed has many conductive paths connecting the active materials within the positive electrode. Therefore, even if some of the conductive paths are lost due to repeated charging and discharging, many conductive paths remain, which extends the cycle life. [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] Generally, a positive electrode has improved cycle life characteristics when the positive electrode material is uniformly dispersed, since it has more conductive paths. On the other hand, if the dispersion is insufficient, the conductive paths decrease, resulting in a decrease in cycle life characteristics. However, quantifying the state of dispersion and the existence of conductive paths has not been sufficiently performed to date.

[0006] The present invention has been made in view of the above, and an object of the present invention is to provide a positive electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a calculation method that are excellent in cycle life characteristics. [Means for solving the problem]

[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 is, in a first aspect, a positive electrode for a lithium ion secondary battery, 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, and is characterized in that, in an SEM backscattered electron image of the surface of the positive electrode obtained with a scanning electron microscope (SEM) at a magnification of 5000x, an acceleration voltage of 15 kV, and an irradiation current of PC30, the distance between the center of gravity of the shape formed by the outline of an active material region and the center of gravity of the shape formed by the outline of the nearest other active material region, where Gv is the standard deviation of the distance for all centers of gravity, satisfies Gv<0.710 in μm terms.

[0008] Although SEM backscattered electron images can be substituted with secondary electron images, when emphasis is placed on differences in composition, SEM backscattered electron images, which are relatively more closely related to composition, are preferable.

[0009] The Gv indicates the variation in the distribution of each active material. A small standard deviation Gv means that the variation is small and the material is uniformly distributed. Conversely, a large standard deviation Gv means that the area is highly variable and the material is aggregated or unevenly distributed. In this way, the standard deviation Gv can be used to quantify the degree of dispersion of the electrode material.

[0010] Furthermore, as a second aspect, in addition to the features of the first aspect, the positive electrode for a lithium ion secondary battery 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.

[0011] Furthermore, as a third aspect, in addition to the first or second aspect, the positive electrode for a lithium ion secondary battery 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.

[0012] Furthermore, as a fourth aspect, in addition to any one of the first to third 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.

[0013] In addition, as a fifth aspect, a lithium ion secondary battery according to the present invention is characterized by comprising: the positive electrode for lithium ion secondary batteries according to any one of the first to fourth aspects; a negative electrode capable of absorbing and desorbing lithium ions; and a nonaqueous electrolyte solution.

[0014] Furthermore, as a sixth aspect, the calculation method according to the present invention is a calculation method for calculating parameters of a positive electrode for a lithium-ion secondary battery, 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 the positive electrode composite layer is subjected to a rolling process, the calculation method comprising the steps of: acquiring 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 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.

[0015] In addition, as a seventh aspect, the calculation method according to the present invention is characterized in that, in addition to the sixth aspect, it further comprises the step of calculating a standard deviation of the value.

[0016] Furthermore, as an eighth aspect, in addition to the sixth or seventh 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.

[0017] Furthermore, as a ninth aspect, the calculation method according to the present invention is characterized in that, in addition to any one of the sixth to eighth aspects, 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.

[0018] Furthermore, as a tenth aspect, in addition to any one of the sixth to ninth 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]

[0019] According to the present invention, a positive electrode for a lithium ion secondary battery having excellent cycle life characteristics can be obtained. [Brief explanation of the drawings]

[0020] [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

[0021] 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.

[0022] (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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] (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.

[0028] The positive electrode layer 41 contains a positive electrode active material, a conductive agent, and a binder.

[0029] <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 Mn 1 / 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 1.5Examples 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.

[0030] 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).

[0031] <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.

[0032] <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.

[0033] <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).

[0034] [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.

[0035] <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.

[0036] [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.

[0037] 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.

[0038] <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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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 processing targets in step S114 and thereafter. The reason for excluding the outline in this way is to distinguish between the outlines of the active material and the outlines of the non-active material. After the capture process, the control unit proceeds to step S112.

[0053] After that, the control unit increments N by 1 (step S112).

[0054] The control unit increases the number N by 1. MAX The control unit determines whether the number N is greater than or equal to N (step S113).MAX If it is determined that the number is equal to or less than N (step S113: No), the process proceeds to step S109, and the above-described process is repeated for the portion of the increased number N. MAX If it is determined that it is greater than the specified value (step S113: Yes), the process proceeds to step S114.

[0055] In step S114, the control unit calculates the center of gravity of the shape formed by the contours of each part present in the collection M (step S114).

[0056] The control unit calculates the center of gravity Gj closest to each center of gravity Gi, and incorporates the distance Gij, converted to μm, between the center of gravity Gi and the center of gravity Gj into the group L. After performing the same process for all centers of gravity Gi, the control unit calculates the standard deviation Gv of the group L (step S115). This standard deviation Gv indicates the standard deviation of the distances to all centers of gravity between the center of gravity of the shape formed by the outline of the active material region and the center of gravity of the shape formed by the outline of the nearest other active material region. In this way, the standard deviation Gv is calculated from the acquired SEM backscattered electron image. In this embodiment, a positive electrode having excellent cycle life characteristics can be obtained by satisfying the standard deviation Gv<0.710. If the standard deviation Gv is less than 0.710, the degree of dispersion of the material may be insufficient, reducing the number of conductive paths and resulting in a deterioration in cycle life characteristics.

[0057] [Method for controlling standard deviation Gv] The standard deviation Gv can be controlled by the slurry dispersion method, electrode drying conditions, etc. When controlling the standard deviation Gv by a dispersion method, for example, if a thin-film vortex mixer is used, the shear force applied to the positive electrode material is reduced, resulting in reduced dispersibility, and therefore the standard deviation Gv is likely to be large. When a thin-film vortex mixer is used, it is necessary to take into account the kneading time for dispersion and other factors to prevent the standard deviation Gv from becoming too large. When kneading is performed using a planetary mixer and then a solvent is added to reduce the solid content of the slurry (the ratio of the total weight of various positive electrode materials to the total weight of the slurry) and disperse the slurry, the shear force applied to the positive electrode material increases, improving dispersibility, and therefore the standard deviation Gv is likely to be small. When a planetary mixer is used, it is necessary to take into account the drying conditions described below and other factors to prevent the standard deviation Gv from becoming too small. Note that, in this invention, kneading refers to a process of kneading at a solid content ratio of at most 80% or more. The standard deviation Gv can also be controlled by adjusting the drying conditions. For example, a high drying temperature results in a uniform distribution of the conductive agent, resulting in a small standard deviation Gv. However, a low drying temperature results in a large standard deviation Gv due to uneven distribution of the conductive agent. To achieve a standard deviation Gv of Gv<0.710, for example, the slurry can be dispersed using a planetary mixer for one hour, the slurry can be applied by die coating, and the electrode can be dried at a maximum temperature of 120°C for seven minutes. Furthermore, a standard deviation Gv that is too small results in excessive dispersion, which is undesirable from the perspective of overall battery performance. Therefore, the lower limit of the standard deviation Gv is set to 0.06, which is approximately 1% of 6 μm, the lower limit of the desirable average particle diameter of the positive electrode active material.

[0058] (Negative electrode) The negative electrode 5 is composed of a negative electrode current collector 52 and a negative electrode layer 51 formed on one or both sides 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 of the negative electrode current collector 52 facing the separator 6. In contrast, the negative electrode layers 51 of the negative electrodes 5 located between the positive electrodes 4, excluding the negative electrode 5 located in the outermost layer, are formed on both sides of the negative electrode current collector 52.

[0059] 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.

[0060] <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.

[0061] <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.

[0062] <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.

[0063] 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.

[0064] (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.

[0065] (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.

[0066] 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.

[0067] 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.

[0068] In the present embodiment described above, by analyzing an SEM image, it is determined that the positive electrode is an active material region, and the standard deviation Gv of the center of gravity Gi of the outline of the region having a predetermined size satisfies Gv<0.710, whereby a positive electrode for a lithium ion secondary battery having excellent cycle life characteristics can be obtained.

[0069] In the above-described embodiment, an example has been described in which a contour is extracted from a binarized image and the standard deviation of the center of gravity of the contour is calculated. However, parameters other than the standard deviation can also be calculated based on the binarized image or the center of gravity.

[0070] Furthermore, in the above-described embodiment, an example was described in which smoothing, selection of a portion surrounded by a contour, and selection of pixels are performed in calculating the standard deviation. However, for example, smoothing need not be performed if it does not affect the calculation of the standard deviation, and if there is no constraint on the time required to calculate the standard deviation, the center of gravity may be calculated using all contours without selecting a portion.

[0071] Furthermore, in the above-described embodiment, an example in which a backscattered electron image is acquired at 5000 magnifications has been described, but a backscattered electron image captured at other magnifications can also be used. [Example]

[0072] The present invention will be described in more detail below by way of examples, but the present invention is not limited to the following examples in any way.

[0073] Example 1 <Preparation of positive electrode> The 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 method involved kneading the mixture in a planetary mixer for 1 hour, and then adding solvent to achieve a viscosity of 5000 cp. Next, the positive electrode slurry was applied to both sides of aluminum foil (12 μm thick) as the positive electrode current collector, with a coating weight of 15.4 mg / cm. 2 The mixture was then dried for 7 minutes at a maximum temperature of 120° C. Thereafter, the mixture was pressed until the electrode density reached 3.5 g / cc to prepare a positive electrode.

[0074] [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.

[0075] <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.

[0076] <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.

[0077] <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.

[0078] <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).

[0079] <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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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 disperse the material for 1 hour using a thin film rotary mixer (the amount of solvent was adjusted so that the slurry viscosity became 5000 cp).

[0084] Example 3 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 disperse the material using a thin film vortex mixer for 10 hours (the amount of solvent was adjusted so that the slurry viscosity became 5000 cp) and the drying method was to dry the material at a maximum temperature of 95°C for 3 minutes, and then further dry the material at a maximum temperature of 130°C for 1 minute.

[0085] Example 4 The positive electrode slurry is composed of 97% by weight of lithium iron cobaltate (LiCoO2), 1.63% by weight of carbon black as a conductive agent, 1.25% by weight of polyvinylidene fluoride (PVDF) as a binder, and 0.12% by weight of polyvinylpyrrolidone (PVP) as a dispersant. The slurry was dispersed by kneading it in a planetary mixer for 1 hour, adding a solvent to make the viscosity 5000cp, and dispersing it. The slurry coating amount was 15.3 mg / cm. 2 A prototype battery was obtained in the same manner as in Example 1, except that the coating method was comma coating, the maximum drying temperature was 120°C, and the drying time was 7 minutes.

[0086] (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 dispersion for 1 hour using a thin film vortex mixer, and the drying method was drying at a maximum temperature of 95°C for 3 minutes, and then further drying at a maximum temperature of 130°C for 1 minute.

[0087] (Comparative 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 disperse the material using a thin film vortex mixer for 0.75 hours, and the drying method was to dry the material at a maximum temperature of 95°C for 3 minutes, and then further dry it at a maximum temperature of 130°C for 1 minute.

[0088] (Comparative Example 3) A prototype battery was obtained in the same manner as in Example 1, except that the dispersion method for producing the positive electrode was dispersion for 0.5 hours using a thin film vortex mixer, and the drying method was drying at a maximum temperature of 95°C for 3 minutes, followed by further drying at a maximum temperature of 130°C for 1 minute.

[0089] Comparative Example 4 The positive electrode slurry was composed of 95.0 wt% lithium iron cobaltate (LiCoO2), 3.6 wt% carbon black as a conductive agent, 1.2 wt% polyvinylidene fluoride (PVDF) as a binder, and 0.2 wt% polyvinylpyrrolidone (PVP) as a dispersant. The slurry was dispersed using a thin film gyratory mixer for 10 hours, and the slurry coating amount was 15.6 mg / cm. 2 A prototype battery was obtained in the same manner as in Example 1, except that the coating method was comma coating, the drying method was a maximum temperature of 130° C., and the drying time was 7 minutes.

[0090] <Evaluation 1: 1C charge / 3C discharge cycle test in a 45°C environment> A charge-discharge cycle test was conducted in a 45°C environment for each of the prototype batteries of Examples 1 to 4 and Comparative Examples 1 to 4. First, each prototype battery was left standing in a thermostatic chamber at 45°C for 1 to 4 hours, and then 350 charge-discharge cycles were performed, with one cycle consisting of charge and discharge according to the following charge-discharge condition 1. The discharge capacity retention rate was calculated from the discharge capacity at the first cycle and the discharge capacity at the 350th cycle. The calculation was performed according to the following formula (I). The results are shown in Table 1 below. Discharge capacity maintenance rate / % = (discharge capacity at 350th cycle / discharge capacity at 1st cycle) × 100 (I)

[0091] (Charge / discharge condition 1 (1C charge / 3C discharge cycle charge / discharge test)) Environmental temperature: 45℃ Charge: 1.0C up to 4.4V (Continue charging until the charging current drops to 0.1C) Discharge: 3.0C to 2.5V Pause time between charging and discharging: 10 minutes

[0092] 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]

[0093] The standard deviation Gv in Table 1 above is the average value of values ​​for five fields of view for each level of electrode. The cycle discharge capacity retention rate is the average value of values ​​for three cells for each level of cell. When calculating the standard deviation Gv for each level of this example, etc., the image acquired at a magnification of 5000 times is smoothed with an upper limit of the number of pixels equivalent to 0.1 μm, and the area is adjusted to the square of 0.5 ((0.5) 2 )μm 2 Contours smaller than this were excluded, and areas where white dots accounted for 20% or more of the points within the area were determined to be white areas.

[0094] Calculation of the standard deviation in Example 1 and Comparative Example 1 will be described with reference to FIGS. 4 and 5. FIG. 4 is a diagram for explaining calculation of the center of gravity for an SEM image of a positive electrode composite layer according to Example 1. FIG. 5 is a diagram for explaining calculation of the center of gravity for an SEM image of a positive electrode composite layer according to Comparative Example 1. FIGS. 4 and 5 show images in which the SEM image was binarized using the median value of the luminance signal according to the flowchart of FIG. 3, and a center of gravity (● shown in the diagram) was generated in each white region. The standard deviation Gv was calculated by determining the distance (shown by a straight line in the diagram) between each center of gravity and the nearest center of gravity.

[0095] A cycle discharge capacity retention rate of 70% or higher is appropriate for use as a lithium-ion battery for drones. Referring to the results shown in Table 1, it can be seen that the prototype batteries according to Examples 1 to 4, which satisfy Gv<0.710, can maintain a discharge capacity retention rate of 70% or higher, compared to Comparative Examples 1 to 4, which do not satisfy Gv<0.710. In contrast, Comparative Examples 1 to 4 had discharge capacity retention rates below 70%. Therefore, it can be seen that Examples 1 to 4 are appropriate for use as lithium-ion batteries for drones. The reason why Examples 1 to 4 have high cycle discharge capacity retention is thought to be that there are many conductive paths between the positive electrode active material particles, and even if some of the conductive paths are lost due to repeated charge and discharge, many of the conductive paths remain. In contrast, Comparative Examples 1 to 4 have few conductive paths, and therefore, when some of the conductive paths are lost, fewer conductive paths remain, which is thought to reduce the battery reaction within the active material and result in a decrease in cycle discharge capacity retention.

[0096] As described above, according to this embodiment, a positive electrode plate for a lithium ion secondary battery having good cycle life characteristics can be provided.

[0097] 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]

[0098] 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 positive electrode for a lithium ion secondary battery, comprising: a positive electrode mixture layer including a positive electrode active material capable of absorbing and releasing lithium ions, a conductive agent, and a binder, provided on a current collector; and a rolling treatment performed on the positive electrode mixture layer; 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, the distance between the center of gravity of the shape formed by the outline of an active material region and the center of gravity of the shape formed by the outline of another active material region that is closest to the active material region satisfies Gv<0.710 in terms of μm, where Gv is the standard deviation of the distance with respect to all the centers of gravity. A positive electrode for a lithium ion secondary battery.

2. 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 .

3. 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 .

4. 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 .

5. 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:

6. A method for calculating parameters of a positive electrode for a lithium ion secondary battery, the positive electrode being formed by providing a positive electrode mixture layer on a current collector and including a positive electrode active material capable of absorbing and releasing lithium ions, a conductive agent, and a binder, and then rolling the positive electrode mixture layer, 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:

7. The method of claim 6, further comprising the step of calculating the standard deviation of the values.

8. the threshold is the median value of the luminance signal of the image; 7. The calculation method according to claim 6,

9. 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; 7. The calculation method according to claim 6,

10. 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.

7. The calculation method according to claim 6,

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