Method for manufacturing a positive electrode, a positive electrode, and a lithium metal secondary battery

Abstract

The present invention provides a method for manufacturing a positive electrode and a positive electrode that maintains peel strength even with a small amount of binder, suppresses gas generation associated with charging and discharging of lithium metal secondary batteries, improves the capacity retention rate of lithium metal secondary batteries, and reduces volume resistivity. [Solution] The method for manufacturing a positive electrode includes the steps of: mixing a positive electrode active material and a binder solution to produce a coating solution; and applying the coating solution to a positive electrode current collector to form a positive electrode composite layer. The positive electrode active material is a single particle with a Ni content of 80 at% or more. The amount of water contained in the binder solution is 280 ppm by mass or less. The ratio of binder to solid content in the coating solution is 1% by mass or less.

Description

[Technical Field] 【0001】 This invention relates to a method for manufacturing a positive electrode, a positive electrode, and a lithium metal secondary battery. [Background technology] 【0002】 In recent years, research and development has been conducted on lithium metal rechargeable batteries that contribute to energy efficiency, in order to ensure that more people have access to affordable, reliable, sustainable, and advanced energy. 【0003】 Patent Document 1 describes a positive electrode comprising a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. Here, the positive electrode active material layer has an average particle size (D 50 The cathode material comprises a binder and a positive electrode material having a bimodal particle size distribution including large and small particles of different sizes. The large particles are lithium composite transition metal oxides with a nickel content of 80 atomic percent or more of the total transition metals. Furthermore, the small particles are lithium composite transition metal oxides containing nickel, cobalt, and aluminum, with a nickel content of 80 atomic percent to 85 atomic percent of the total transition metals, and a cobalt atom ratio (Co / Al) of 1.5 to 5. 【0004】 Furthermore, Patent Document 2 describes a positive electrode active material for a non-aqueous electrolyte secondary battery, specifically the 50% particle size D in the cumulative particle size distribution based on volume. 50 A lithium transition metal composite oxide single particle having a diameter of 1 μm or more and 21 μm or less is described. In this case, the positive electrode for a non-aqueous electrolyte secondary battery comprises a current collector and a positive electrode active material layer disposed on the current collector and containing a positive electrode active material for a non-aqueous electrolyte secondary battery and a binder. [Prior art documents] [Patent Documents] 【0005】 [Patent Document 1] Japanese Patent Publication No. 2022-508147 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-188445 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0006】 However, the large-particle-size particles and small-particle-size particles described in Patent Document 1 are lithium composite transition metal oxides in which the nickel content among all transition metals is 80 atomic% or more. Therefore, alkali remains as an impurity and is likely to absorb moisture. As a result, when manufacturing the positive electrode, the binder is likely to aggregate, and thus the peel strength of the positive electrode is likely to decrease. At this time, in order to maintain the peel strength of the positive electrode, if the content of the binder in the positive electrode active material layer is increased, the content of the positive electrode material in the positive electrode active material layer decreases, and the volume resistivity of the positive electrode increases. Also, the energy density of the positive electrode becomes low. 【0007】 In addition, the single particles of lithium transition metal composite oxide described in Patent Document 2 have a large specific surface area and thus are likely to adsorb moisture. Therefore, when the single particles of lithium transition metal composite oxide described in Patent Document 2 are applied to a lithium metal secondary battery, gas is likely to be generated during charge and discharge, and the capacity retention rate is likely to be low. 【0008】 An object of the present invention is to provide a method for manufacturing a positive electrode and a positive electrode that can maintain the peel strength even when the content of the binder is small, suppress the generation of gas during charge and discharge of a lithium metal secondary battery, improve the capacity retention rate of the lithium metal secondary battery, and reduce the volume resistivity. 【Means for Solving the Problems】 【0009】 (1) The process includes the steps of: (1) mixing a positive electrode active material and a binder solution to produce a coating solution; and applying the coating solution to a positive electrode current collector to form a positive electrode composite layer, wherein the positive electrode active material is a single particle with a Ni content of 80 at% or more, the amount of water contained in the binder solution is 280 ppm by mass or less, the ratio of the binder to the solid content contained in the coating solution is 1% by mass or less, and the amount of water contained in the positive electrode composite layer is 200 μg / cm³. 3 The following is a method for manufacturing the positive electrode. 【0010】 (2) The positive electrode active material has a median diameter D based on volume. 50 A method for producing a positive electrode according to (1), comprising a first single particle having a size of 3 μm or more and 5.5 μm or less. 【0011】 (3) The positive electrode active material has a Ni content of 80 at% or more and a volume-based median diameter D 50 The method for producing a positive electrode according to (2), further comprising a second single particle having a size of 0.5 μm or more and 2.5 μm or less. 【0012】 (4) The method for manufacturing a positive electrode according to (3), wherein the positive electrode active material has a mass ratio of the second single particle to the first single particle of 1 / 9 or more and 3 / 2 or less. 【0013】 (5) A method for producing a positive electrode according to any one of (1) to (4), wherein the ratio of the binder to the solid content contained in the coating solution is 0.5% by mass or more and 0.8% by mass or less. 【0014】 (6) A method for manufacturing a positive electrode according to any one of (1) to (5), wherein when manufacturing the coating solution, a conductive additive is further mixed in such a way that the ratio of the conductive additive to the solid content in the coating solution is 2.5% by mass or less. 【0015】 (7) The method for producing a positive electrode according to (6), wherein the ratio of the conductive additive to the solid content contained in the coating liquid is 2.0% by mass or less. 【0016】 (8) A positive electrode current collector has a positive electrode composite material layer formed thereon. The positive electrode composite material layer contains a positive electrode active material and a binder, and is composed of single particles with a binder content of 1% by mass or less. The positive electrode active material has a Ni content of 80 at% or more, and the amount of moisture contained in the positive electrode composite material layer is 200 μg / cm 3 or less. This is the positive electrode. 【0017】 (9) The positive electrode composite material layer of the positive electrode according to (8) has no pores with a major diameter of 3 μm or more. 【0018】 (10) The positive electrode composite material layer further contains a conductive assistant, the content of the conductive assistant is 2.5% by mass or less, the density is 3.2 g / cm 3 or more and 3.6 g / cm 3 or less, and the volume resistivity is 5.0 Ω·cm or less. This is the positive electrode according to (8) or (9). 【0019】 (11) The positive electrode according to (10), wherein the content of the conductive assistant in the positive electrode composite material layer is 2.0% by mass or less. 【0020】 (12) The positive electrode active material includes first single particles with a volume-based median diameter D 50 of 3 μm or more and 5.5 μm or less, and second single particles with a volume-based median diameter D 50 of 0.5 μm or more and 2.5 μm or less. This is the positive electrode according to (11). 【0021】 (13) The positive electrode according to (12), wherein the mass ratio of the second single particles to the first single particles of the positive electrode active material is 1 / 9 or more and 3 / 2 or less. 【0022】 (14) A lithium metal secondary battery including the positive electrode according to any one of (8) to (13). 【Advantages of the Invention】 【0023】 According to the present invention, it is possible to provide a method for manufacturing a positive electrode and a positive electrode that can maintain peel strength even with a small amount of binder, suppress gas generation associated with charging and discharging of lithium metal secondary batteries, improve the capacity retention rate of lithium metal secondary batteries, and reduce volume resistivity. [Modes for carrying out the invention] 【0024】 Embodiments of the present invention will be described below. 【0025】 [Manufacturing method for positive electrode] The method for manufacturing the positive electrode in this embodiment includes the steps of: mixing a positive electrode active material with a binder solution to produce a coating solution; and applying the coating solution to a positive electrode current collector to form a positive electrode composite layer, wherein the positive electrode active material is a single particle. 【0026】 The Ni content in the positive electrode active material is 80 at% or more, preferably 60 at% or more. Because the Ni content in the positive electrode active material is 80 at% or more, applying the positive electrode to a lithium metal secondary battery results in a higher initial capacity. For example, the Ni content in the positive electrode active material may be 90 at% or less. 【0027】 The positive electrode active material is, in general formula Li p Ni x Co y M 1 z O 2+α ...(1) (In the formula, p is between 1.0 and 1.3, x is between 0.3 and 0.95, y is between 0 and 0.4, z is between 0 and 0.5, the sum of x, y and z is 1, α is between -0.1 and 0.1, M 1 (This is Mn and / or Al.) It is preferable that the lithium transition metal composite oxide is represented by [formula]. 【0028】 The water content in the binder solution is 280 ppm by mass or less, preferably 200 ppm by mass or less. Because the water content in the binder solution is 280 ppm by mass or less, the adsorption of water onto the positive electrode active material is suppressed, reducing the water content in the positive electrode composite layer. As a result, the binder is less likely to aggregate during the manufacturing of the positive electrode, and the peel strength of the positive electrode is maintained even if the amount of binder added is reduced. Furthermore, when the positive electrode is applied to a lithium metal secondary battery, the generation of gas associated with charging and discharging is suppressed. This is presumed to be because the decomposition of the solvent in the electrolyte is suppressed. Moreover, when the positive electrode is applied to a lithium metal secondary battery, the capacity retention rate is improved. This is presumed to be because the formation of lithium hydroxide and lithium oxide due to the reaction between the lithium metal negative electrode and water is suppressed. The water content in the binder solution is, for example, 150 ppm or more. 【0029】 There are no particular limitations on the method for adjusting the water content in the binder solution to 280 ppm by mass or less, but one example is a method of preparing the binder solution under a low dew point environment (for example, an environment where the dew point is -40°C or lower). 【0030】 The binder is not particularly limited, but an example is polyvinylidene fluoride (PVDF). 【0031】 The solvent included in the binder solution is not particularly limited as long as it can dissolve the binder, but an example is N-methyl-2-pyrrolidone (NMP). 【0032】 The ratio of binder to solid content in the coating solution is 1% by mass or less, preferably 0.8% by mass or less. Because the ratio of binder to solid content in the coating solution is 1% by mass or less, the increase in the amount of water in the positive electrode composite layer due to the binder is suppressed, and the volume resistivity of the positive electrode is reduced. For example, the ratio of binder to solid content in the coating solution is 0.5% by mass or more. 【0033】 The moisture content in the positive electrode composite layer is 200 μg / cm³.3 The following is the case: 150 μg / cm³ 3 The following is preferable: The moisture content in the positive electrode composite layer is 200 μg / cm³. 3 Therefore, when the positive electrode of this embodiment is applied to a lithium metal secondary battery, gas generation during charging and discharging is suppressed. Furthermore, when the positive electrode of this embodiment is applied to a lithium metal secondary battery, the capacity retention rate is improved. The amount of moisture contained in the positive electrode composite layer is, for example, 100 μg / cm³. 3 That's all. 【0034】 The moisture content in the positive electrode composite layer is 200 μg / cm³. 3 The following are methods for adjustment, but are not limited to any particular, and include, for example, a method for manufacturing the coating solution under a low dew point environment (for example, an environment where the dew point is -40°C or lower), and a method for applying the coating solution to the positive electrode current collector under a low dew point environment (for example, an environment where the dew point is -40°C or lower). 【0035】 The positive electrode active material has a median diameter D based on volume. 50 It is preferable to include a first single particle whose diameter is 3 μm or more and 5.5 μm or less. This increases the packing density of the positive electrode composite layer. Furthermore, the positive electrode active material has a median diameter D based on volume. 50 Preferably, the material further contains a second single particle having a size of 0.5 μm or more and 2.5 μm or less. This further increases the packing density of the positive electrode composite layer. 【0036】 The mass ratio of the second single particle to the first single particle of the positive electrode active material is preferably 1 / 9 or more and 3 / 2 or less, and more preferably 3 / 7 or more and 2 / 3 or less. When the mass ratio of the second single particle to the first single particle of the positive electrode active material is 1 / 9 or more and 3 / 2 or less, the packing density of the positive electrode composite layer becomes even higher. 【0037】 The specific surface area of ​​the positive electrode active material is 0.5 m². 2 / g or more 1.0m 2 It is preferable that the concentration is less than or equal to / g. The specific surface area of ​​the positive electrode active material is 0.5 m². 2If the density is 1 / g or higher, the load required to achieve the desired density of the positive electrode composite layer becomes smaller, making the positive electrode active material less prone to cracking, thus improving the capacity retention rate of the lithium metal secondary battery. On the other hand, if the specific surface area of ​​the positive electrode active material is 1.0 m² or higher, 2 When the value is less than / g, the positive electrode active material is less likely to aggregate, thus improving the uniformity of the thickness of the positive electrode composite layer. 【0038】 A conductive additive may be further mixed in when manufacturing the coating solution. The conductive additive is not particularly limited as long as it has electronic conductivity, but examples include carbon black and carbon nanotubes. 【0039】 The ratio of conductive additive to solid content in the coating solution is preferably 2.5% by mass or less, and more preferably 2.0% by mass or less. When the ratio of conductive additive to solid content in the coating solution is 2.5% by mass or less, the peel strength of the positive electrode is increased. For example, the ratio of conductive additive to solid content in the coating solution is 1% by mass or more. 【0040】 The positive electrode current collector is not particularly limited, but an example is aluminum foil. 【0041】 [Positive electrode] In this embodiment, the positive electrode has a positive electrode composite layer formed on the positive electrode current collector, and the positive electrode composite layer contains a positive electrode active material and a binder. The positive electrode in this embodiment is manufactured by the positive electrode manufacturing method of this embodiment. 【0042】 The binder content in the positive electrode composite layer is 1% by mass or less, and preferably 0.8% by mass or less. Because the binder content in the positive electrode composite layer is 1% by mass or less, the increase in the amount of moisture contained in the positive electrode composite layer due to the binder is suppressed, and the volume resistivity of the positive electrode is reduced. For example, the binder content in the positive electrode composite layer is 0.5% by mass or more. 【0043】 The moisture content in the positive electrode composite layer is 200 μg / cm³. 3 The following is the case: 150 μg / cm³ 3The following is preferable: The moisture content in the positive electrode composite layer is 200 μg / cm³. 3 Therefore, when the positive electrode of this embodiment is applied to a lithium metal secondary battery, gas generation during charging and discharging is suppressed. Furthermore, when the positive electrode of this embodiment is applied to a lithium metal secondary battery, the capacity retention rate is improved. The amount of moisture contained in the positive electrode composite layer is, for example, 100 μg / cm³. 3 That's all. 【0044】 It is preferable that the positive electrode composite layer does not contain voids with a major axis of 3 μm or more. However, voids can be observed in electron microscope images of the cross-section of the positive electrode composite layer. 【0045】 In this case, if secondary particles are used as the positive electrode active material, cracks are more likely to occur, and voids with a major axis of 3 μm or more are more likely to exist in the positive electrode composite layer. 【0046】 The density of the positive electrode composite layer is 3.2 g / cm³. 3 More than 3.6g / cm 3 Preferably, it is 3.3 g / cm³. 3 More than 3.5g / cm 3 It is even more preferable that the following conditions are met: The density of the positive electrode composite layer is 3.2 g / cm³. 3 More than 3.6g / cm 3 The following conditions suppress the occurrence of cracks in the positive electrode active material, and as a result, voids with a major axis of 3 μm or more are less likely to exist in the positive electrode composite layer. 【0047】 The positive electrode composite layer may further contain a conductive additive. This lowers the volume resistivity of the positive electrode. In this case, the volume resistivity of the positive electrode is preferably 5.0 Ω·cm or less. 【0048】 The content of the conductive additive in the positive electrode composite layer is preferably 2.5% by mass or less, and more preferably 2.0% by mass or less. When the content of the conductive additive in the positive electrode composite layer is 2.5% by mass or less, the peel strength of the positive electrode is increased. For example, the content of the conductive additive in the positive electrode composite layer is 1% by mass or more. 【0049】 [Lithium metal rechargeable battery] The lithium metal secondary battery of this embodiment comprises a positive electrode of this embodiment, a negative electrode, and a separator impregnated with an electrolyte. 【0050】 The negative electrode has a lithium metal layer formed on the negative electrode current collector. The negative electrode current collector is not particularly limited, but an example is copper foil. 【0051】 The electrolyte is an electrolyte dissolved in a non-aqueous solvent. 【0052】 The electrolyte is not particularly limited, but an example is lithium bis(fluorosulfonyl)imide (LiFSI). 【0053】 The non-aqueous solvent is not particularly limited, but examples include ether solvents such as 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). 【0054】 The electrolyte may contain lithium difluoro(oxalate) borate (LiDFOB). The LiDFOB content in the electrolyte is not particularly limited, but for example, it is 0.2% by mass or more and 1.0% by mass or less. 【0055】 The separator is not particularly limited, but examples include porous resin sheets. Examples of resins that make up the porous resin sheet include polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. 【0056】 The lithium metal secondary battery of this embodiment may further include an outer covering material (for example, a laminate film) that surrounds the positive electrode, the negative electrode, and the separator impregnated with electrolyte. 【0057】 Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and the above embodiments may be modified as appropriate within the scope of the spirit of the present invention. [Examples] 【0058】 The following describes embodiments of the present invention, but the present invention is not limited to these embodiments. Note that "parts" refers to parts by mass. 【0059】 (Amount of water contained in the binding agent solution) The amount of water contained in the binder solution was measured using a moisture meter AQ-2200 (manufactured by HIRANUMA). At the same time, the water contained in the binder solution was evaporated using a moisture vaporizer set to 150°C. 【0060】 (Moisture content in the positive electrode composite layer) The amount of moisture contained in the positive electrode composite layer was measured using a moisture measuring device AQ-2200 (manufactured by HIRANUMA). At the same time, the moisture contained in the positive electrode composite layer was evaporated using a moisture vaporization device set to 150°C. 【0061】 (Density of the positive electrode composite layer) Using a GR-202 analytical balance with built-in weights (manufactured by A&D Company, Limited) and a GT2-P12K pencil-type high-precision sensor head (manufactured by Keyence), the basis weight and thickness of the cathode composite layer were measured, and the density of the cathode composite layer was calculated. 【0062】 (Cracks in the positive electrode composite layer) The cross-sectional image of the cathode composite layer was observed using an electron microscope JSM-IT800 (manufactured by JEOL). The criteria for determining cracks in the cathode composite layer are as follows. A: When there are no voids with a major axis of 3 μm or larger. B: If there are voids with a major axis of 3 μm or more 【0063】 (Volume resistivity of the positive electrode) The volume resistivity of the positive electrode was measured using the RM2610 electrode resistance measurement system (manufactured by HIOKI E.C.). 【0064】 (Peel strength of the positive electrode) Using a digital force gauge ZTS-5N (manufactured by IMADA) and a load measuring stand MX2-500N-L (manufactured by IMADA), the load when the positive electrode was peeled at 180°C was measured and determined as the peel strength of the positive electrode. 【0065】 (Example 1) The positive electrode was manufactured in an environment with a dew point of -50°C as follows. 【0066】 Using Hibiscus Disperser Mix 3D-2 (Primix), 19.15 g of polyvinylidene fluoride KF polymer #9700 (Kureha Corporation) was dissolved in 300.00 g of N-methyl-2-pyrrolidone (Nippon Refine Corporation) as a solvent to obtain a 6% by mass solution of the binder. The water content in the binder solution was 264 ppm by mass. 【0067】 Using a Primix 2P-03 kneader and a Primix 56-30 thin-film swirling constrained mixer, the positive electrode active material had a Ni content of 84 at%, and the volume-based median diameter D 50 A coating solution was obtained by mixing lithium nickel manganese cobalt oxide single particles having a diameter of 5 μm, a dispersion of conductive additive 1 containing 12% by mass of acetylene black and 1% by mass of carbon nanotubes, and a binder solution, such that the ratio of the binder and conductive additive 1 to the solid content in the coating solution was 0.8% by mass and 3.0% by mass, respectively. 【0068】 As a positive electrode current collector, it has a thickness of 12 μm and an electrical conductivity of 3.37 × 10⁻⁶. 5 A coating solution was applied to aluminum foil with a viscosity of S / cm to form a 65 μm thick cathode composite layer, thereby obtaining the cathode. The water content in the cathode composite layer was 157 μg / cm³. 3 Therefore, the density of the positive electrode composite layer is 3.3 / cm³. 3 That was the case. 【0069】 (Example 2) The positive electrode active material has a Ni content of 84 at%, and the volume-based median diameter D 50 The lithium nickel manganese cobalt oxide single particles are 5 μm in size, with a content of 70 mass and 84 at% Ni, and the median diameter D is based on volume. 50 A positive electrode was obtained in the same manner as in Example 1, except that a mixture of 30% by mass of lithium nickel manganese cobalt oxide single particles, each having a diameter of 2 μm, was used, and the mixture was prepared so that the ratio of conductive additive 1 to the solid content in the coating solution was 1.2% by mass. At this time, the water content in the binder solution was 278 ppm by mass. The water content in the positive electrode composite layer was 156 μg / cm³. 3 Therefore, the density of the positive electrode composite layer is 3.5 / cm³. 3 That was the case. 【0070】 (Example 3) A positive electrode was obtained in the same manner as in Example 2, except that instead of conductive additive 1, a dispersion containing 3% by mass of acetylene black and 6% by mass of carbon nanotubes was used as the dispersion of conductive additive 2, and the mixture was prepared so that the ratio of conductive additive 2 to the solid content in the coating solution was 2.2% by mass. At this time, the water content in the binder solution was 279 ppm by mass. The water content in the positive electrode composite layer was 179 μg / cm³. 3 Therefore, the density of the positive electrode composite layer is 3.4 / cm³. 3 That was the case. 【0071】 (Example 4) The positive electrode active material has a Ni content of 84 at%, and the volume-based median diameter D 50 The lithium nickel manganese cobalt oxide single particles are 5 μm in size, with a content of 60 mass and 84 at% Ni, and the median diameter D is based on volume. 50 A positive electrode was obtained in the same manner as in Example 3, except that a mixture of 40% by mass of lithium nickel manganese cobalt oxide single particles with a diameter of 2 μm was used. At this time, the water content in the binder solution was 277 ppm by mass. The water content in the positive electrode composite layer was 188 μg / cm³. 3 Therefore, the density of the positive electrode composite layer is 3.4 / cm³.3 That was the case. 【0072】 (Example 5) A positive electrode was obtained in the same manner as in Example 3, except that the ratio of the binder and conductive additive 2 to the solid content in the coating solution was 0.7% by mass and 1.8% by mass, respectively. At this time, the water content in the binder solution was 278 ppm by mass. The water content in the positive electrode composite layer was 172 μg / cm³. 3 Therefore, the density of the positive electrode composite layer is 3.4 / cm³. 3 That was the case. 【0073】 (Example 6) A positive electrode was obtained in the same manner as in Example 3, except that the ratio of the binder and conductive additive 2 to the solid content in the coating solution was 0.6% by mass and 1.9% by mass, respectively. At this time, the water content in the binder solution was 277 ppm by mass. The water content in the positive electrode composite layer was 171 μg / cm³. 3 Therefore, the density of the positive electrode composite layer is 3.4 / cm³. 3 That was the case. 【0074】 (Comparative Example 1) A positive electrode was obtained in the same manner as in Example 2, except that the dew point of the environment was changed to -10°C and the ratio of conductive additive 1 to the solid content in the coating solution was mixed to 2.2% by mass. At this time, the water content in the binder solution was 289 ppm by mass. The water content in the positive electrode composite layer was 862 μg / cm³. 3 Therefore, the density of the positive electrode composite layer is 3.5 / cm³. 3 That was the case. 【0075】 (Comparative Example 2) A positive electrode was obtained in the same manner as in Comparative Example 1, except that the dew point of the environment was changed to -20°C. At this time, the water content in the binder solution was 260 ppm by mass. The water content in the positive electrode composite layer was 331 μg / cm³. 3 Therefore, the density of the positive electrode composite layer is 3.4 / cm³. 3 That was the case. 【0076】 (Cell creation) An electrolyte was obtained by mixing lithium bis(fluorosulfonyl)imide (LiFSI) as an electrolyte equivalent to 2.1 mol / L, a mixture of 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) in a molar ratio of 3:2 as solvents, and lithium difluoro(oxalate) borate (LiDFOB) as an additive equivalent to 0.5 mass%. 【0077】 As a negative electrode current collector, it has a thickness of 10 μm and an electrical conductivity of 6.5 × 10⁻⁶. 6 A clad material was prepared by bonding copper foil with a density of S / cm to lithium foil with a thickness of 20μm. The clad material was punched out to a size of 34mm x 44mm to obtain the negative electrode. 【0078】 A positive electrode, a polyethylene porous film coated with Al2O3 as a separator, and a negative electrode were laminated together and housed in a laminate case. After that, an electrolyte solution was poured into the inside of the laminate case, and the laminate case was sealed to obtain a cell (lithium metal secondary battery cell). 【0079】 (Initial cell capacity) Using a charge / discharge evaluation device TOSCAT-3000S (manufactured by Toyo System) and a LightSpec constant temperature incubator LU-124 (manufactured by ESPEC), the cells were charged to 4.30V with a constant current of 16.83mA in an environment of 25°C, then charged at a constant voltage of 4.30V for 1.5 hours, and left for 10 minutes. Next, the cells were discharged to 2.65V with a constant current of 16.83mA and left for 30 minutes. The above procedure was repeated twice to charge and discharge the cells, and the initial capacity of the cells was measured. The current value that allows for complete discharge in 1 hour was defined as 1C for the obtained discharge capacity. 【0080】 (Cell capacity retention rate) Using a charge / discharge evaluation device TOSCAT-3000S (manufactured by Toyo System) and a LightSpec constant temperature incubator LU-124 (manufactured by ESPEC), the cells were charged to 4.3V with a constant current of 0.33C in an environment of 25°C, left for 10 minutes, then discharged to 3.00V with a constant current of 0.33C, and left for 30 minutes. This operation was repeated 48 times to charge and discharge the cells. Next, the operation at (initial cell capacity) was repeated twice to charge and discharge the cells, and the capacity of the cells after charging and discharging was measured. Next, the formula... (Cell capacity after charging and discharging) / (Cell initial capacity) × 100 The cell capacity retention rate was calculated based on this. 【0081】 (Amount of gas generated after charging and discharging) Under the same conditions as for (cell capacity retention rate), the cell was charged and discharged twice (50 times), and then the amount of gas generated was measured using an electronic hydrometer MDS-3000 (manufactured by Alpha-Mirage). 【0082】 Table 1 shows the characteristics and evaluation results of the positive electrode. 【0083】 [Table 1] 【0084】 Table 1 shows that the positive electrodes of Examples 1-6 maintain peel strength even with a low binder content, generate less gas after charging and discharging the cell, and have a high cell capacity retention rate. In contrast, the positive electrode of Comparative Example 1 has a water content of 289 ppm by mass in the binder solution and a water content of 862 μg / cm³ in the positive electrode composite layer. 3 Therefore, when the binder content is low, the peel strength is not maintained, the amount of gas generated after charging and discharging the cell is high, and the cell's capacity retention rate is low. In addition, the positive electrode of Comparative Example 2 has a moisture content of 331 μg / cm³ in the positive electrode composite layer. 3 Therefore, if the binder content is low, the peel strength is not maintained, the amount of gas generated after charging and discharging the cell increases, and the cell's capacity retention rate is low.

Claims

[Claim 1] A process of mixing a positive electrode active material and a binder solution to produce a coating solution, The process includes applying the coating liquid to the positive electrode current collector to form a positive electrode composite layer, The positive electrode active material is a single particle having a Ni content of 80 at% or more. The amount of water contained in the binder solution is 280 ppm by mass or less. The ratio of the binder to the solid content in the coating solution is 1% by mass or less. The moisture content in the aforementioned cathode composite layer is 200 μg / cm³. ３ The following is a method for manufacturing the positive electrode. [Claim 2] The positive electrode active material has a median diameter D based on volume. ５０ A method for producing a positive electrode according to claim 1, comprising a first single particle having a size of 3 μm or more and 5.5 μm or less. [Claim 3] The positive electrode active material has a median diameter D based on volume. ５０ The method for producing a positive electrode according to claim 2, further comprising a second single particle having a size of 0.5 μm or more and 2.5 μm or less. [Claim 4] The method for manufacturing a positive electrode according to claim 3, wherein the positive electrode active material has a mass ratio of the second single particle to the first single particle of 1 / 9 or more and 3 / 2 or less. [Claim 5] A method for producing a positive electrode according to any one of claims 1 to 4, wherein the ratio of the binder to the solid content contained in the coating liquid is 0.5% by mass or more and 0.8% by mass or less. [Claim 6] When manufacturing the aforementioned coating solution, a conductive additive is further mixed in. A method for manufacturing a positive electrode according to any one of claims 1 to 4, wherein the ratio of the conductive additive to the solid content contained in the coating liquid is 2.5% by mass or less. [Claim 7] The method for producing a positive electrode according to claim 6, wherein the ratio of the conductive additive to the solid content contained in the coating liquid is 2.0% by mass or less. [Claim 8] A positive electrode composite material layer is formed on the positive electrode current collector. The positive electrode composite layer comprises a positive electrode active material and a binder, wherein the binder content is 1% by mass or less. The positive electrode active material is a single particle having a Ni content of 80 at% or more. The moisture content in the aforementioned cathode composite layer is 200 μg / cm³. ３ The positive electrode is as follows. [Claim 9] The positive electrode according to claim 8, wherein the positive electrode composite layer does not contain pores with a major axis of 3 μm or more. [Claim 10] The positive electrode composite layer further contains a conductive additive, the content of which is 2.5% by mass or less, and the density is 3.2 g / cm³. ３ 3.6g / cm or more ３ The following: The positive electrode according to claim 8 or 9, wherein the volume resistivity is 5.0 Ω·cm or less. [Claim 11] The positive electrode according to claim 10, wherein the positive electrode composite layer contains 2.0% by mass or less of the conductive additive. [Claim 12] The positive electrode active material has a median diameter D based on volume. ５０ A first single particle having a diameter of 3 μm or more and 5.5 μm or less, and a volume-based median diameter D ５０ The positive electrode according to claim 11, comprising a second single particle having a size of 0.5 μm or more and 2.5 μm or less. [Claim 13] The positive electrode according to claim 12, wherein the positive electrode active material has a mass ratio of the second single particle to the first single particle of 1 / 9 or more and 3 / 2 or less. [Claim 14] A lithium metal secondary battery comprising the positive electrode described in claim 8 or 9.

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