Positive electrode for lithium ion secondary battery and lithium ion secondary battery

Abstract

To provide a positive electrode for a lithium ion secondary battery, capable of achieving both good output characteristics and electrode adhesion strength and a lithium ion secondary battery.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, the positive electrode mixture layer containing lithium iron phosphate forming secondary particles as a positive electrode active material, a conductive agent, and a binder. When b represents an area including many conductive agents, binders and voids and w represents an area including few conductive agents, binders and voids in a SEM image, of a cross section of the positive electrode mixture layer, obtained by a scanning electron microscope (SEM), an area ratio B(=b / (b+w)) showing a ratio of the b in the SEM image satisfies 0.750≤B≤0.900.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] A positive electrode for a lithium ion secondary battery is produced by applying a positive electrode mixture paste containing a positive electrode active material, a conductive agent, a binder, and a solvent to a metal foil current collector and drying the paste.

[0003] Lithium-containing composite oxides such as lithium cobalt oxide, lithium manganese oxide, and lithium iron phosphate are used as positive electrode active materials, but because the active materials themselves do not have sufficient conductivity, conductive agents such as carbon black and graphite have been added to impart conductivity. The conductive agent serves to form conductive paths between active materials, and when fabricating electrodes, it is important to uniformly disperse this conductive agent to ensure as many conductive paths as possible.

[0004] There is a known technology for inspecting the performance of batteries based on the state of their electrodes (see, for example, Patent Document 1). This inspection method involves capturing images of the electrodes during transport and inspecting their appearance to check for defects in the electrode section without manual intervention. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 7-260704 Summary of the Invention [Problem to be solved by the invention]

[0006] However, although various studies have been conducted in the past on inspecting the state of electrodes and the performance of batteries based on the appearance of the electrodes, such as the inspection method described in Patent Document 1, no studies have been conducted on linking the appearance of the electrodes to battery performance, i.e., whether or not good output characteristics and electrode adhesion strength can be achieved at the same time, and using this information to advance development.

[0007] The present invention has been made in view of the above, and aims to provide a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery that can achieve both good output characteristics and good electrode adhesion strength. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems and achieve the object, a 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 lithium iron phosphate in the form of secondary particles as a positive electrode active material, a conductive agent, and a binder is provided on a current collector, and is characterized in that, in an SEM backscattered electron image of a cross section of the positive electrode composite layer obtained by a scanning electron microscope (SEM) at a magnification of 2500x, an acceleration voltage of 15 kV, and a working distance (WD) of 10 mm, when b denotes a region where the conductive agent, the binder, and voids are abundant, and w denotes a region where the conductive agent, the binder, and voids are few, an area ratio B (= b / (b + w)) indicating the ratio of b in the SEM backscattered electron image satisfies 0.750≦B≦0.900.

[0009] 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 90.0 mass% or more and 98.0 mass% or less, the mass proportion of the conductive agent is 1.0 mass% or more and 9.0 mass% or less, and the mass proportion of the binder is 1.0 mass% or more and 9.0 mass% or less.

[0010] 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 b represents a black region of a binarized image that has been binarized into white and black using a specific luminance signal of the SEM image as a threshold, and w represents a white region of the binarized image.

[0011] Furthermore, as a fourth aspect, in addition to the third aspect, the positive electrode for a lithium ion secondary battery according to the present invention is characterized in that the binarized image is binarized using an average value of a maximum value and a minimum value of a luminance signal of the image as a threshold value.

[0012] In addition, as a fifth aspect, there is provided a positive electrode for a lithium ion secondary battery according to the present invention, characterized in that it comprises the positive electrode for a lithium ion secondary battery according to any one of the first to fourth aspects, a negative electrode capable of absorbing and desorbing lithium ions, and a non-aqueous electrolyte solution. [Effects of the Invention]

[0013] According to the present invention, by specifying the proportion of black area based on an SEM reflected image, it is possible to obtain a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery that achieve both good output characteristics and good electrode adhesion strength. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is an exploded perspective view illustrating the configuration of a lithium ion secondary battery including a positive electrode for a lithium ion secondary battery according to one embodiment of the present invention. [Figure 2] FIG. 2 is a flowchart showing a flow of calculating the black area ratio in a positive electrode for a lithium ion secondary battery according to one embodiment of the present invention. [Figure 3] FIG. 3 is a flowchart showing a flow of producing a positive electrode composite paste for a lithium ion secondary battery positive electrode according to one embodiment of the present invention. [Figure 4] FIG. 4 is a diagram showing an image obtained by performing binarization processing on the cross-sectional SEM image of the positive electrode composite material layer according to Example 1 of the present invention. [Figure 5] FIG. 5 is a diagram showing an image obtained by performing binarization processing on a cross-sectional SEM image of a positive electrode composite material layer according to Comparative Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] 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 shows an example of the configuration of a coin-type lithium ion secondary battery (coin cell) as an example of an embodiment, but the shape of the lithium ion secondary battery in the present invention is not particularly limited and may be flat, cylindrical, square, laminated, etc. Furthermore, the exterior 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.

[0016] (Embodiment) FIG. 1 is an exploded perspective view illustrating the configuration of a lithium ion secondary battery including a positive electrode for a lithium ion secondary battery according to one embodiment of the present invention.

[0017] The lithium ion secondary battery 10 includes a case 1, a leaf spring 2, a spacer 3, a positive electrode 4, a separator 5, a negative electrode 6, a gasket 7, and a cap 8. The lithium ion secondary battery 10 is a coin-type lithium ion secondary battery that includes a positive electrode for a lithium ion secondary battery that is configured using a positive electrode current collector and a positive electrode composite layer.

[0018] In the lithium ion secondary battery 10, the case 1 and the cap 8 are fixed together by caulking or the like, and the interior is filled with a non-aqueous electrolyte. The lithium ion secondary battery 10 is liquid-tightly sealed by the case 1, the gasket 7, and the cap 8. The spacer 3, the positive electrode 4, the separator 5, and the negative electrode 6 are biased toward the cap 8 by the leaf spring 2. This keeps the components in close contact with each other.

[0019] <Positive electrode> The positive electrode 4 is a positive electrode for a lithium secondary battery that includes at least a positive electrode current collector and a positive electrode mixture layer provided on one or both sides of the positive electrode current collector.

[0020] [Positive electrode current collector] Although there are no particular limitations on the material that constitutes the positive electrode current collector, it is preferable to use a metal, such as aluminum, nickel, stainless steel, titanium, or other alloys.

[0021] [Cathode active material] The positive electrode mixture layer is composed of a positive electrode active material, a conductive agent, and a binder. The positive electrode active material (also referred to as electrode material) can be lithium iron phosphate, an olivine-type compound that forms secondary particles. In this electrode material, it is preferable that 80% or more of the surface of the primary particles or the aggregates (secondary particles) formed by aggregation of the primary particles is covered with a carbonaceous coating containing carbon. The coverage of the carbonaceous coating on the surface of the electrode material of this embodiment can be measured using a transmission electron microscope (TEM, Energy Dispersive Spectrometer: EDS) or the like.

[0022] If the average primary particle diameter of the positive electrode active material is 50 nm or less, the specific surface area increases, improving ion diffusibility and electronic conductivity, but the amount of carbon required for carbonaceous coating increases.If the average primary particle diameter is 400 nm or more, the amount of carbon required for carbonaceous coating can be reduced, but ion diffusibility and electronic conductivity decrease.

[0023] Furthermore, if the average secondary particle diameter of the positive electrode active material is less than 0.3 μm, it is difficult to obtain sufficient adhesion strength between the electrode current collector and the electrode material, and if a binder resin is included, the amount of binder resin added will be excessive, which may result in a loss of battery capacity. On the other hand, if the average secondary particle diameter exceeds 50 μm, when an electrode for a lithium-ion secondary battery is formed, a distribution of density of the electrode material is likely to occur within the electrode. This results in different current densities in microscopic regions of the electrode, which in turn results in different rates of deterioration of the electrode material in microscopic regions of the electrode.

[0024] Taking these factors into consideration, the present invention specifies an average primary particle size greater than 50 nm and less than 400 nm. The average secondary particle size is 5 μm or greater and 30 μm or less. The average secondary particle size is the median diameter measured by the laser diffraction / scattering method described in JIS standard Z8825:2013. A laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation) can be used for the measurement.

[0025] [Conductive agent] The conductive agent is not particularly limited, and known or commercially available conductive agents can be used. For example, carbon black such as acetylene black or ketjen black, activated carbon, graphite, carbon fiber, carbon nanofiber, and carbon nanotubes can be used. The carbon nanotubes can be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) such as double-walled carbon nanotubes (DWCNTs).

[0026] [Binder] The type of binder is not particularly limited, but examples that can be used include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene-butadiene rubber (SBR), and acrylic resin.

[0027] [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).

[0028] [Electrode composition] In the electrode composition of the positive electrode 4, a higher ratio of the positive electrode active material increases the energy density, a higher ratio of the conductive agent improves the electronic conductivity, and a higher ratio of the binder increases the peel strength of the electrode coating surface. Considering these balances, the electrode composition preferably has a positive electrode active material mass ratio of 90.0 mass% to 98.0 mass%, a conductive agent mass ratio of 1.0 mass% to 9.0 mass%, and a binder mass ratio of 1.0 mass% to 9.0 mass%, relative to the total mass of the positive electrode plate composite layer. In this case, if the mass ratio of the positive electrode active material is less than 90.0 mass%, the energy density cannot be increased, whereas if the mass ratio of the positive electrode active material is more than 98.0 mass%, the energy density increases, but the ratio of the conductive agent or binder to the positive electrode active material decreases, resulting in a decrease in electronic conductivity or peel strength. Furthermore, if the mass ratio of the conductive agent is less than 1.0 mass%, the electronic conductivity decreases. On the other hand, if the mass ratio of the conductive agent is more than 9.0 mass%, the electronic conductivity improves, but the ratio of the binder to the conductive agent becomes insufficient, resulting in a decrease in peel strength. Furthermore, 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 9.0 mass %, the binder becomes a resistive component, resulting in a decrease in electronic conductivity.

[0029] [Coating process] The positive electrode composite layer is applied to the current collector as a positive electrode composite paste. There are no particular limitations on the method for applying this positive electrode composite paste to the current collector, and any general coating method can be appropriately selected and used. A known coating method selected from gravure coating, gravure reverse coating, roll coating, Mayer bar coating, blade coating, knife coating, air knife coating, commart coating, slot die coating, slide die coating, dip coating, etc. can be used. In this embodiment, the current collector foil is coated and dried by table coating. For example, the temperature inside the oven is set to 100°C, and the current collector foil is dried with hot air for 3 minutes. For example, aluminum foil with a thickness of 20 μm is used as the current collector foil used for coating. Here, the coating amount on one side is 6 mg / cm. 2 In the case of below 13mg / cm, the output characteristics improve but the capacity per unit area decreases. 2 In this case, the output characteristics will decrease, but the capacity per unit area will increase. Therefore, the coating amount should be 6 mg / cm on one side. 2 Larger than 13 mg / cm 2 In this embodiment, for example, the coating amount on one side is 11 mg / cm 2 A single-sided coated electrode is produced so that The coating amount of the positive electrode was determined by cutting out a positive electrode to a certain area, peeling the positive electrode composite layer from the current collector foil, measuring the mass, and dividing the obtained mass by the area. Similarly, the electrode density was determined by cutting out a positive electrode to a certain area, measuring the thickness of the positive electrode composite layer after removing the current collector foil, peeling the positive electrode composite layer from the current collector foil, measuring the mass, and dividing the obtained mass by the volume obtained by multiplying the thickness and the area.

[0030] [Pressing process] The coated electrode was rolled using a press. The coating density of the electrode can be adjusted by adjusting the gap and press pressure of the press rolls of the press. If the electrode is rolled so that the coating density is 1.8 g / cc or less, the capacity per volume decreases. If the electrode is rolled so that the coating density is 2.2 g / cc or more, strong stress is applied to the current collector foil via the composite layer, causing electrode distortion and wrinkles. Taking these factors into consideration, the density set during rolling in the present invention was set to 2.0 g / cc. The press can be a mechanical press, a hydraulic press, or the like.

[0031] [solvent] The type of solvent is not particularly limited, but solvents such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide can be used. When polyvinylidene fluoride is used as the binder, it is preferable to use N-methyl-2-pyrrolidone as the solvent.

[0032] <Negative electrode> The negative electrode 6 includes at least a negative electrode current collector and a negative electrode mixture layer provided on one or both sides of the negative electrode current collector, and is capable of absorbing and releasing lithium ions.

[0033] <Nonaqueous electrolyte> The non-aqueous electrolyte contains a non-aqueous solvent in which a lithium salt is dissolved. The lithium salt may be, for example, one or a mixture of two or more selected from the group consisting of LiBF4, LiPF6, Li(FSO2)2N, and Li(CF3SO2)2N.

[0034] The nonaqueous solvent is not particularly limited, but examples thereof include one or a mixture of two or more solvents selected from the group consisting of dimethyl carbonate (DEC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl 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, EC is preferred because it can form a good coating on the negative electrode active material.

[0035] The nonaqueous electrolyte preferably further contains additives other than the lithium salts described above for the purpose of forming a high-quality coating on the surface of the negative electrode active material through reductive decomposition during charge and discharge. The additives are not particularly limited, but examples include vinylene carbonate, fluoroethylene carbonate, 1,3,2-dioxathiolane-2,2-dioxide (MMDS), 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide, tris(trimethylsilyl) phosphite, 1-propene-1,3-sultone, and Li2PO2F2. These additives may be used alone or in combination.

[0036] [Separator] The separator may be, for example, a porous sheet made of polymer or fiber, or a nonwoven fabric separator. The separator may also be one in which a ceramic layer, which is a heat-resistant insulating layer, is laminated on a porous substrate.

[0037] In this embodiment, a cross section of the positive electrode prepared by the above method was photographed with a scanning electron microscope (SEM), and the obtained SEM backscattered electron image (hereinafter sometimes simply referred to as "SEM image") was analyzed to calculate the black area ratio B in the image. At this time, the positive electrode 4 was processed using a cross-section polisher (manufactured by JEOL Ltd.) so that the cross section of the positive electrode composite layer was flat, and the processed portion was photographed with an SEM. The cross section of the positive electrode composite layer refers to a cross section cut in the thickness direction of the layer.

[0038] 2 is a flowchart showing the flow of calculating the black area ratio in a positive electrode for a lithium-ion secondary battery 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 a 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.

[0039] 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. While a secondary electron image can be used instead of an SEM backscattered electron image, if emphasis is placed on differences in composition, an SEM backscattered electron image, which is relatively strongly related to composition, is preferable. As an example, this SEM backscattered electron image is an SEM backscattered electron image at 2500x magnification obtained by scanning a 51.2 μm × 34.8 μm area on the positive electrode surface with an SEM. The scanning range is not limited to 51.2 μm × 34.8 μm. In this case, if an auto-contrast / brightness adjustment function (ACB) is available, it should be used. 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 known as "blown-out highlights" or "crushed blacks," which may result in inaccurate distribution, as will be described later. Therefore, a new image should be captured and used. To avoid the above-mentioned "bloated highlights" and "crushed blacks," in the case of the SEM used in the present invention, scanning under the conditions of an acceleration voltage of 15 kV, a working distance (WD) of 10 mm, and a probe current (PC) of 30 is effective, although not completely avoidable.

[0040] The control unit then smooths the image (step S102). The control unit performs smoothing based on the number of pixels of the scale, with the number of pixels equivalent to 0.1 μm as the upper limit. This smoothing evens out the roughness of the image. At this time, the control unit acquires the magnification of the SEM backscattered electron image and determines how many pixels the scale, which indicates the standard of length, is made up of, thereby setting the number of pixels of the scale.

[0041] The control unit then binarizes the image (step S103). For example, the control unit first calculates the luminance signal for each pixel by summing 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. If the luminance signal has a decimal point, it may be rounded to 0 or an integer. Any rounding method, such as rounding up, rounding down, or rounding, may be selected. The control unit then calculates the average value of the maximum and minimum luminance signals of the image, and sets the color of each pixel position to either white or black using this average value as a threshold. If the median value of the luminance signal has a decimal point, it may be rounded to 0 or an integer. Any rounding method, such as rounding up, rounding down, or rounding, may be selected. The control unit converts the image into an image with only two colors, black and white, by changing the color to the set color. In this embodiment, the area where a large amount of active material is present is white, and the area where a large amount of conductive agent, binder, or voids is present is black. However, what is important is that the area where a large amount of conductive agent, binder, or voids is present is made into a molecule, and it is also possible to reverse the white and black colors as appropriate.

[0042] The reason for using the average value of the image luminance signal as the threshold here is that the luminance signal of an image may differ from image to image or depending on the SEM imaging environment, and if the threshold is set as the absolute value of the luminance signal, when 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. Therefore, when binarizing, it is preferable to separate the colors using the average value as the threshold.

[0043] Then, the control unit calculates the black area ratio B (0≦B≦1) in the binarized image (step S104). The black area ratio B is calculated using the following formula (1). B = b / (b + w) (1) Here, b is the proportion of black in the binarized image, and w is the proportion of white in the binarized image. b and w may be expressed by the proportion of the image occupied, the area occupied by each color, or the number of pixels. The maximum possible value of b / b+w is 1 unless there are pixel defects or the like. The maximum possible value of b+w is the total area of ​​the binarized image in the case of area, or the total number of pixels in the case of number of pixels.

[0044] In this embodiment, the black area ratio B is adjusted to satisfy the relationship 0.750≦B≦0.900. Here, the black area ratio B indicates the degree to which the secondary particle structure is maintained. The white region represented by w corresponds to a region with little conductive agent, binder, and voids, i.e., a region corresponding to the active material, and the black region represented by b corresponds to a region with a large amount of conductive agent, binder, or voids. When the secondary particle structure of the positive electrode active material is maintained, the secondary particle portion contains little conductive agent, binder, or voids, so the region of w increases (b decreases) and the black area ratio B decreases. In contrast, when the secondary particle structure is not maintained, the conductive agent, binder, or voids are uniformly dispersed, so the region of w decreases (b increases) and the black area ratio B increases.

[0045] The black area ratio B increases as the maximum value of the ratio of solids consisting of active material, conductive agent, binder, etc. to solvent during kneading (intermediate solids ratio) increases, and as the kneading time increases.

[0046] [Relationship between black area ratio B, output characteristics (10C discharge capacity / 0.2C discharge capacity (%)), and electrode peel strength] When the black area ratio B is less than 0.750, the output characteristics are high but the peel strength is low. This is because the secondary particle structure is maintained, the number of conductive paths is increased, and the output characteristics are high, but the degree of dispersion of the binder is insufficient. When the black area ratio B is 0.750≦B≦0.900, the output characteristics and peel strength are both high. This is because the secondary particle structure is maintained and the binder is appropriately dispersed, making it possible to achieve both good output characteristics and high peel strength. When the black area ratio B is greater than 0.900, the peel strength is very high, but the output characteristics are low. This is because the binder is well dispersed, so the peel strength is high, but the secondary particle structure is destroyed, preventing the formation of a conductive path and resulting in a decrease in output.

[0047] In the present embodiment described above, the black area ratio B, calculated using b, which indicates the ratio of black in a binarized image, and w, which indicates the ratio of white in the binarized image, is set to satisfy the condition 0.750≦B≦0.900. According to the present embodiment, since the black area ratio B satisfies the above condition, a positive electrode for a lithium ion secondary battery that achieves both good output characteristics and electrode adhesion strength can be obtained. By fabricating a lithium ion secondary battery using this positive electrode for a lithium ion secondary battery, a lithium ion secondary battery with good output characteristics can be obtained.

[0048] In the above-described embodiment, an example in which smoothing is performed in calculating the black area ratio has been described, but smoothing need not be performed if, for example, there is no effect on the calculation of the black area ratio. Also, in the above-described embodiment, an example in which a binarized image is used in calculating the black area ratio has been described, but instead of generating a binarized image, a threshold may be set for the luminance signal, and the black area ratio B may be calculated by setting the area below this threshold as b and the area equal to or greater than the threshold as w. [Example]

[0049] EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to the embodiments described in the Examples.

[0050] Example 1 [Electrode materials and solvents] The electrode constituent materials used were a positive electrode active material (lithium iron phosphate), a conductive agent (carbon black (Li400) manufactured by Denka Co., Ltd.), and a binder (PVDF (KF polymer #9300) manufactured by Kureha Corporation). The material ratio was positive electrode active material / conductive agent / binder = 93 / 3 / 4, and in the present invention, 131.13 g of positive electrode active material, 4.23 g of conductive agent, and 5.64 g of binder were used. N-methyl-2-pyrrolidone (NMP: manufactured by Mitsubishi Chemical Corporation) was used as the solvent for preparing the positive electrode composite paste. The positive electrode composite paste was prepared according to the following procedures <Mixing 1> to <Mixing 3> (see FIG. 3).

[0051] [Kneading] <Mixing 1> 4.23 g of carbon black as a conductive agent, 5.64 g of PVDF as a binder, and 125 g of NMP as a solvent were mixed at 2000 rpm for 2 minutes using a Mixer (manufactured by Thinky Corporation) to obtain a mixture. <Mixing 2> To the mixture prepared in the kneading 1, 131.13 g of lithium iron phosphate as a positive electrode active material was added, and the mixture was mixed at 2000 rpm for 6 minutes using a Thinky Mixer (manufactured by Thinky Corporation) to obtain a mixture. <Mixing 3> A solvent was added so that the viscosity value, measured at 25°C and 60 rpm using a Brookfield viscometer (manufactured by Toki Sangyo Co., Ltd.: BMII rotor No. 4), was 1000 mPa s or more and 7000 mPa s or less, and the mixture was kneaded using a Mixer (manufactured by Thinky Corporation) to obtain a positive electrode composite paste.

[0052] [Electrode fabrication] The positive electrode mixture paste was applied to one side of the aluminum foil at a coating amount of 11 mg / cm. 2 The mixture was then dried with hot air at 100° C. for 10 minutes, and then rolled using a press to give a coating density of 2.0 g / cc, thereby producing a positive electrode.

[0053] [Battery assembly] Charge / discharge evaluations were performed using a 2032-type coin-type lithium secondary battery (hereinafter abbreviated as coin-type cell) having the structure described below.

[0054] A 300 μm thick lithium metal foil was attached to the surface of a 100 μm thick stainless steel plate negative electrode current collector to form a negative electrode layer, thereby producing a negative electrode. A non-aqueous electrolyte solution was prepared by dissolving LiPF6 as a lithium salt at a ratio of 1.3 mol / L in a mixed solvent prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:5:3. A coin-type cell was produced using the obtained positive electrode, negative electrode, electrolyte, and a separator made of a microporous polyolefin film in an argon atmosphere with a dew point of −50° C. or lower.

[0055] [Mixture resistivity] The composite resistivity was measured using an electrode resistance measuring device manufactured by HIOKI Co., Ltd. In this specification, a custom-made product was used, but RM3544 is generally expected to be suitable.

[0056] [Peel test] The peel test was conducted in accordance with JIS standard JIS-K6854-1 (test method for peel adhesion strength, 90-degree peel), and the electrode strength was evaluated by measuring the peel strength at a sample width of 20 mm and a pulling speed of 50 mm / min.

[0057] [Charge / discharge evaluation] The discharge load characteristics were evaluated using the coin cell prepared above. The discharge load characteristic test was performed with charging [0.2 ItA-cc / cv-4.2V, 3h cut] and discharging [0.2 or 10.0 ItA-cc, 2.0V cut].

[0058] [Black area ratio calculation] The cross section of the positive electrode composite layer was photographed with an SEM, and the SEM reflected image was binarized according to the flowchart shown in FIG. 2 to calculate the black area ratio B (see formula (1) above).

[0059] Example 2 In Example 2, a coin-shaped cell was produced in the same manner as in Example 1, except that the kneading time in the kneading 2 step was set to 3 minutes.

[0060] Example 3 In Example 3, a coin-shaped cell was produced in the same manner as in Example 1, except that the amount of NMP added in the kneading 1 was 111 g.

[0061] Example 4 In Example 4, a coin-shaped cell was produced in the same manner as in Example 1, except that the amount of NMP added in the kneading 1 was 111 g and the kneading time in the kneading 2 was 3 minutes.

[0062] (Comparative Example 1) In Comparative Example 1, a coin-shaped cell was produced in the same manner as in Example 1, except that the amount of NMP added in the kneading 1 was 159 g.

[0063] (Comparative Example 2) In Comparative Example 2, a coin-shaped cell was produced in the same manner as in Example 1, except that the amount of NMP added in the kneading 1 was 159 g and the kneading time in the kneading 2 was 3 minutes.

[0064] (Comparative Example 3) In Comparative Example 3, a coin-shaped cell was produced in the same manner as in Example 1, except that the amount of NMP added in the kneading 1 was 86 g.

[0065] Comparative Example 4 In Comparative Example 4, a coin-shaped cell was produced in the same manner as in Example 1, except that the amount of NMP added in the kneading 1 was 86 g and the kneading time in the kneading 2 was 3 minutes.

[0066] Table 1 shows the intermediate solid content values ​​at the time of kneading 2, the intermediate solid content values ​​at the time of kneading 3, and the kneading time of kneading 2 for Examples 1 to 4 and Comparative Examples 1 to 4. Table 2 also shows the black area ratio B, composite resistivity, electrode peel strength, and charge / discharge characteristics for Examples 1 to 4 and Comparative Examples 1 to 4, respectively.

[0067] [Table 1]

[0068] [Table 2]

[0069] Here, as an example, binarized images in Example 1 and Comparative Example 1 will be described with reference to Fig. 4 and Fig. 5. Fig. 4 is a diagram showing an image obtained by performing binarization processing on a cross-sectional SEM image of a positive electrode composite layer according to Example 1 of the present invention. Fig. 5 is a diagram showing an image obtained by performing binarization processing on a cross-sectional SEM image of a positive electrode composite layer according to Comparative Example 1 of the present invention. As shown in Figs. 4 and 5, by performing binarization processing on the SEM image, it is possible to distinguish between regions with a large amount of conductive agent, binder, and voids and regions with few (active material regions).

[0070] [About black area ratio B] Table 2 shows that the black area ratios B of Examples 1 to 4 satisfied the condition 0.750≦B≦0.900. In contrast, the black area ratios of Comparative Examples 1 and 2 were B<0.750, and the black area ratios of Comparative Examples 3 and 4 were B>0.900. The results shown in Tables 1 and 2 indicate that the lower the intermediate solid content ratio (Table 1) at the time of kneading 2, the smaller the black area ratio B, and the higher the intermediate solid content ratio, the larger the black area ratio B. This is because the higher the intermediate solid content ratio at the time of kneading 2, the greater the shear stress applied to the positive electrode composite paste during kneading, dispersing the conductive agent.

[0071] [About peel strength] Examples 1 to 4, where B is 0.750≦B≦0.900, have higher peel strengths than Comparative Examples 1 and 2, where B is <0.750 (see Table 2). This is presumably because the binder is dispersed without being unevenly distributed, resulting in higher peel strengths. On the other hand, Comparative Example 2, where B is >0.900, has a higher peel strength than Examples 1 to 4, presumably because the binder is dispersed.

[0072] [Output characteristics] In Examples 1 to 4, where B was 0.750≦B≦0.900, the 10 ItA discharge capacity / 0.2 ItA discharge capacity [%] was 73% or higher (see Table 2). This is presumably because the secondary particle structure of the positive electrode active material was maintained and many conductive paths were formed, resulting in improved output characteristics. On the other hand, in Comparative Examples 1 to 4, where B<0.750 and B>0.900, the 10 ItA discharge capacity / 0.2 ItA discharge capacity [%] was lower than in Examples 1 to 4 (see Table 2). In contrast, the composite resistivity was lower in Comparative Examples 1 and 2, where B<0.750, than in Examples 1 to 4, and higher in Comparative Examples 3 and 4, where B>0.900 (see Table 2).

[0073] [Conditions for achieving both output characteristics and peel strength] From the above results, it can be seen that Examples 1 to 4 maintain good output characteristics while maintaining peel strength to a degree that prevents the composite layer from slipping off. From these results, in order to achieve both output characteristics and peel strength, if the black area ratio B calculated by the above formula (1) satisfies 0.750≦B≦0.900, a positive electrode plate in which the secondary particle structure of the positive electrode active material is maintained and the conductive agent and binder are appropriately dispersed can be obtained. [Explanation of symbols]

[0074] 1 case 2 leaf springs 3 spacers 4 Positive electrode 5 Separator 6 negative electrode 7 Gasket 8 Cap 10 Lithium-ion secondary battery

Claims

1. A positive electrode for a lithium ion secondary battery, comprising: a positive electrode mixture layer provided on a current collector, the positive electrode mixture layer including lithium iron phosphate in the form of secondary particles as a positive electrode active material, a conductive agent, and a binder; In an SEM backscattered electron image of a cross section of the positive electrode composite layer obtained by a scanning electron microscope (SEM) at a magnification of 2500 times, an acceleration voltage of 15 kV, and a working distance (WD) of 10 mm, when a region containing a large amount of the conductive agent, the binder, and voids is designated as b and a region containing a small amount of the conductive agent, the binder, and voids is designated as w, an area ratio B (= b / (b + w)) indicating a ratio of b in the SEM backscattered electron image satisfies 0.750≦B≦0.

900. 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 90.0 mass% or more and 98.0 mass% or less, the mass ratio of the conductive agent is 1.0 mass% or more and 9.0 mass% or less, and the mass ratio of the binder is 1.0 mass% or more and 9.0 mass% or less.

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

3. The b represents a black region of a binarized image that has been binarized into white and black using a specific brightness signal of the SEM image as a threshold value, The w represents a white area of ​​the binarized image.

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

4. The binarized image is binarized using an average value of the maximum and minimum values ​​of the luminance signal of the image as a threshold value.

4. The positive electrode for a lithium ion secondary battery according to claim 3.

5. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, a negative electrode capable of absorbing and releasing lithium ions; a nonaqueous electrolyte; A lithium ion secondary battery comprising:

Images