Positive electrode and lithium-ion secondary battery

Abstract

The positive electrode active material layer (14) includes multiple positive electrode active material particles (14P) and interparticle regions (14Z) formed between the multiple positive electrode active material particles (14P). The interparticle regions (14Z) have solid portions (14S) and multiple pores (14V) dispersed within the solid portions (14S). In a cross-sectional image of the positive electrode active material layer (14), the area distribution (D50) of the equivalent circular diameter of the pores (14V) is 0.82 μm or less, and the area distribution (D90) of the equivalent circular diameter of the pores (14V) is 1.37 μm or less. In a cross-sectional image of the positive electrode active material layer, when the interparticle regions (14Z) are divided into two-dimensional square lattices (GR) and the area ratio of the pores within each two-dimensional square lattice (GR) is obtained, the variation coefficient CV1 of the area ratio of the pores (14V) is 0.610 or less.

Description

Technical Field

[0001] This invention relates to positive electrodes and lithium-ion secondary batteries. Background Technology

[0002] Besides mobile devices such as mobile phones and laptops, lithium-ion secondary batteries are widely used as power sources for tools, hybrid vehicles, and more. In recent years, there has been a demand for lithium-ion secondary batteries with high energy density and excellent output characteristics. For example, Patent Document 1 describes a lithium-ion secondary battery capable of suppressing the increase in internal resistance during charging and discharging under high output conditions.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2007-109636 Summary of the Invention

[0006] The technical problem that the invention aims to solve

[0007] However, what is required is a lithium-ion secondary battery with superior capacity and fast discharge characteristics.

[0008] The present invention was made in view of the above-mentioned technical problems, and its purpose is to provide a positive electrode and a lithium-ion secondary battery that can take into account both high capacity and fast discharge characteristics.

[0009] Means for solving technical problems

[0010] [1] A positive electrode having a current collector and a layer of positive electrode active material in contact with at least one main surface of the current collector, wherein,

[0011] The aforementioned positive electrode active material layer comprises: a plurality of positive electrode active material particles and interparticle regions formed between the plurality of positive electrode active material particles.

[0012] The aforementioned interparticle region has a solid portion and multiple fine pores dispersed within the solid portion.

[0013] In the cross-sectional image of the aforementioned positive electrode active material layer, the D50 of the area reference distribution of the equivalent circular diameter of the aforementioned micropores is 0.82 μm or less, and the D90 of the area reference distribution of the equivalent circular diameter of the aforementioned micropores is 1.37 μm or less.

[0014] In the cross-sectional image of the above-mentioned positive electrode active material layer, when the interparticle region is divided into two-dimensional square grids and the area ratio of the pores in each two-dimensional square grid is obtained, the variation coefficient CV1 of the area ratio of the pores is less than 0.610.

[0015] The positive electrode described in [2][1], wherein the solid portion comprises a binder and a conductive additive.

[0016] The positive electrode described in [3][1] or [2], wherein the solid portion comprises additive particles composed of Co3O4 powder, LiCoO2 powder or a combination thereof.

[0017] The positive electrode described in [4][3], wherein, when the area ratio of the above-mentioned additive particles in each two-dimensional square lattice is obtained, the coefficient of variation CV2 of the area ratio of the above-mentioned additive particles is 0.601 or less.

[0018] [5] A lithium-ion secondary battery comprising: a positive electrode as described in any one of [1] to [4]; and a negative electrode.

[0019] The effects of the invention

[0020] We provide positive electrodes and lithium-ion secondary batteries that can balance high capacity and fast discharge characteristics. Attached Figure Description

[0021] Figure 1 This is a schematic cross-sectional view of a lithium-ion secondary battery, illustrating one example of this embodiment.

[0022] Figure 2 yes Figure 1 An enlarged schematic diagram of the cross-section of the positive electrode active material layer.

[0023] Symbol Explanation

[0024] 10……Positive electrode; 12……Positive electrode current collector; 14……Positive electrode active material layer; 14P……Positive electrode active material particles; 14S……Solid part; 14V……Fine pores; 14Z……Interparticle region; 20……Negative electrode; 22……Negative electrode current collector; 24……Negative electrode active material layer; 100……Lithium-ion secondary battery; GR……Two-dimensional cubic lattice. Detailed Implementation

[0025] (Lithium-ion secondary battery)

[0026] Figure 1 This is a schematic cross-sectional view of a lithium-ion secondary battery, illustrating one example of this embodiment. (e.g.) Figure 1 As shown, the lithium-ion secondary battery 100 mainly includes a laminate 30, a housing 50 that encloses the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30.

[0027] The laminate 30 has one or more positive electrodes 10 and one or more negative electrodes 20, with a separator 18 sandwiched between the positive electrodes 10 and the negative electrodes 20 and arranged opposite to each other. The positive electrode 10 has a positive electrode active material layer 14 disposed on one or two main surfaces of the plate-shaped (film-shaped) positive electrode current collector 12.

[0028] The negative electrode 20 is obtained by depositing a negative electrode active material layer 24 on one or both main surfaces of a plate-shaped (film-shaped) negative electrode current collector 22. The main surfaces of the positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with the main surface of the separator 18, respectively. Leads 60 and 62 are connected to the ends of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the ends of the leads 60 and 62 extend to the outside of the housing 50. First, the positive electrode 10 will be described in detail.

[0029] (Positive electrode 10)

[0030] (Positive current collector 12)

[0031] The positive current collector 12 can be any conductive plate material, such as a thin metal sheet (metal foil) made of aluminum, copper, nickel or their alloys.

[0032] (Positive electrode active material layer 14)

[0033] Figure 2 This is an enlarged cross-sectional view of the positive electrode active material layer 14. The positive electrode active material layer 14 has a plurality of positive electrode active material particles 14P and interparticle regions 14Z formed between the plurality of positive electrode active material particles 14P. The interparticle regions 14Z have solid portions 14S and a plurality of fine pores 14V dispersed within the solid portions 14S.

[0034] The solid portion 14S has an adhesive portion 14B that bonds the positive electrode active material particles 14P to each other and to the positive electrode current collector 12. The solid portion 14S may further include additive particles 14A. The adhesive portion 14B contains an adhesive and may also contain conductive additives.

[0035] (Positive electrode active material particles 14P)

[0036] As a positive electrode active material, it is suitable for reversibly performing lithium ion adsorption and release, lithium ion intercalation and deintercalation, or lithium ion and its equilibrium anion (e.g., ClO4). - There are no particular limitations on the doping and dedoping of lithium cobalt oxide (LiCoO2), and well-known active materials can be used. Examples include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese spinel (LiMn2O4), and materials derived from the general formula: LiNi x Co y Mn z Ma O2 (x+y+z+a=1, 0≤x≤1, 0≤y≤1, 0≤z≤1, 0≤a≤1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr) represents composite metal oxides, lithium vanadium compounds (LiV2O5), olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O) 12 LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1) and other composite metal oxides.

[0037] The particle size of the positive electrode active material particles is not particularly limited. In the D50 of the area reference distribution of the equivalent circle diameter of the positive electrode active material particles in the cross-sectional photograph of the positive electrode active material layer, it can be 0.1 to 30 μm.

[0038] (Adhesive section 14B)

[0039] The adhesive section 14B bonds the positive electrode active material particles 14P together and bonds the positive electrode active material particles 14P to the positive electrode current collector 12. The adhesive contained in the adhesive section can be any adhesive capable of the above-mentioned bonding; for example, fluoropolymers such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) can be used. Furthermore, in addition to the above, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, acrylic resin, polyimide resin, and polyamide-imide resin can also be used as the adhesive. Moreover, conductive polymers with electronic conductivity and conductive polymers with ionic conductivity can also be used as the adhesive. Examples of conductive polymers with electronic conductivity include polyacetylene. In this case, the adhesive also functions as conductive additive particles, so conductive additives may not be required. As conductive polymers with ionic conductivity, substances with ionic conductivity, such as lithium ions, can be used. Examples include substances formed by complexing monomers of polymer compounds (polyether polymers such as polyethylene oxide and polypropylene oxide, polyphosphazene, etc.) with lithium salts such as LiClO4, LiBF4, and LiPF6, or lithium-based alkali metal salts. As polymerization initiators used in the complexation, examples include photopolymerization initiators or thermal polymerization initiators suitable for the aforementioned monomers.

[0040] The content of the binder in the positive electrode active material layer 14 is not particularly limited, but is preferably 1 to 10% by mass, based on the sum of the masses of the active material, conductive additive, and binder. By setting the contents of the active material and binder within the above range, the tendency for the amount of binder in the obtained positive electrode active material layer 14 to be too small, thus failing to form a firm active material layer, can be suppressed. In addition, the tendency for the amount of binder, which does not contribute to the capacitance, to increase, making it difficult to obtain a sufficient volumetric energy density, can also be suppressed.

[0041] (Conductive additive)

[0042] The conductive additive contained in the binder section 14B is not particularly limited as long as it ensures good conductivity of the positive electrode active material layer 14, and known conductive additives can be used. Examples include carbon materials such as graphite and carbon black, metal powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal powders, and conductive oxides such as ITO.

[0043] The content of conductive additives in the positive electrode active material layer 14 is not particularly limited, but when added, it is preferably 0.5 to 5% by mass relative to the active material.

[0044] (Additive Granules 14A)

[0045] The solid portion 14S may contain additive particles 14A composed of Co3O4 powder, LiCoO2 powder, or a combination thereof. The average particle size of the additive particles 14A is not particularly limited, but can be from 5 nm to 150 nm in the D50 of the distribution of the equivalent circle diameter of the additive particles 14A in a cross-sectional photograph of the positive electrode active material layer. These additive particles act as a catalyst for the decomposition of lithium carbonate nanoparticles contained in the positive electrode active material layer before the first charge and discharge during battery manufacturing. Furthermore, after the first charge and discharge, they improve the electrostatic properties of the positive electrode active material layer, resulting in a higher lithium-ion mobility potential and better capacity and / or fast discharge characteristics.

[0046] (14V fine aperture)

[0047] As described above, the interparticle region 14Z formed between multiple positive electrode active material particles 14P has a solid portion 14S and multiple fine pores 14V dispersed within the solid portion 14S, and is sponge-like.

[0048] In the cross-sectional image of the positive electrode active material layer, the area distribution of the equivalent circular diameter of the fine pores (14V) has a D50 of less than 0.82 μm. This D50 can be less than 0.70 μm or less than 0.60 μm. D50 is the particle size at which the ratio of particles smaller than this is 50%.

[0049] In the cross-sectional image of the positive electrode active material layer, the area distribution of the equivalent circular diameter of the fine pores (14V) has a D90 of less than 1.37 μm. This D90 can be less than 1.30 μm or less than 1.20 μm. D90 is the particle size at which the ratio of particles smaller than this value is 90%.

[0050] When obtaining these particle size distributions, the number of fine pores 14V is preferably around 200 to 700.

[0051] Furthermore, in this embodiment, as Figure 2 As shown, in the cross-sectional image of the positive electrode active material layer, when the interparticle region 14Z is divided into two-dimensional square lattices GR, and the area ratio of the micropores 14V within each two-dimensional square lattice GR is obtained, the coefficient of variation CV1 of the area ratio of the micropores 14V is less than 0.610. The coefficient of variation refers to the standard deviation / mean value.

[0052] When the interparticle region 14Z contains additive particles 14A, and when the interparticle region 14Z is divided into two-dimensional square lattices GR and the area ratio of additive particles 14A in each two-dimensional square lattice GR is obtained, the variation coefficient CV2 of the area ratio of additive particles is preferably 0.601 or less.

[0053] The orientation of the cross-sectional image of the positive electrode active material layer is not particularly limited; it can be a cross-section perpendicular to the main surface of the current collector or a cross-section parallel to the main surface of the current collector, preferably a cross-section perpendicular to the main surface of the current collector. The regions in the cross-sectional image can be determined by the image brightness, surface condition, element mapping, etc.

[0054] The dimensions of the two-dimensional square lattice GR are preferably set to have one side of 0.5 to 4.0 μm. Furthermore, the lattice used to measure the area ratio of pores and additive particles is limited to a lattice where the area ratio of the positive electrode active material particles 14P is 50% or less. Further, the closed pores within the positive electrode active material particles 14P are not included in the interparticle region 14Z. The number of two-dimensional square lattices used to obtain the area ratio is preferably set to 90 to 360.

[0055] (effect)

[0056] In this positive electrode, compared to existing technologies, the diameter of the pores 14V is smaller, and the small-diameter pores 14V are spatially uniformly dispersed. Therefore, it is believed that the electrolyte can easily penetrate into the positive electrode active material layer 14, enabling rapid contact between the electrolyte and salt and the positive electrode active material, and efficient insertion and removal of Li ions. Specifically, by increasing the amount of Li ions moving between the positive and negative electrodes, the capacity can be increased. In addition, by increasing the movement speed of Li ions between the positive and negative electrodes, the fast discharge characteristics can be improved. Therefore, it is particularly possible to realize efficient lithium-ion secondary batteries for tools, electric vehicles, etc.

[0057] Furthermore, if the solid portion of the positive electrode active material layer contains additive particles, the electrostatic properties are improved, the ion mobility potential becomes higher, and the capacity and / or fast discharge characteristics become better.

[0058] Furthermore, by uniformly dispersing the additive particles in space, the electrostatic properties are further improved, thus optimizing the dielectric material balance, resulting in a particularly high ion mobility potential and further enhancing the aforementioned properties.

[0059] (The manufacturing method of the positive electrode)

[0060] The following is an example of a method for manufacturing such a positive electrode.

[0061] (Preparation of lithium carbonate nanoparticles or composite particles containing them)

[0062] First, lithium carbonate powder with a large particle size (e.g., a few μm) is dissolved in water. The lower the lithium carbonate concentration in the aqueous solution, the smaller the particle size of the subsequently precipitated lithium carbonate nanoparticles. Next, when a poor solvent 1 is added to the aqueous solution, lithium carbonate nanoparticles, for example, particles with a particle size D50 of 1–1000 nm based on a volumetric particle size distribution using laser diffraction, precipitate, resulting in a slurry (precipitation step). The amount of poor solvent 1 added is preferably 70 wt% or more relative to water. The more poor solvent 1 is added, the smaller the particle size of the precipitated lithium carbonate nanoparticles. Poor solvent 1 can be appropriately selected from solvents that are soluble in water but do not dissolve lithium carbonate. Furthermore, poor solvent 1 preferably has a higher boiling point than water. This is because, in the next step of removing lithium carbonate nanoparticles from the slurry, if poor solvent 1 evaporates before water during heat drying, the precipitated lithium carbonate will re-dissolve in water. An example of poor solvent 1 is N-methyl-2-pyrrolidone (NMP).

[0063] To separate the precipitated lithium carbonate nanoparticles from the slurry and obtain them as a dried powder, the slurry can be heated and dried. However, during drying, the lithium carbonate nanoparticles may undergo liquid cross-linking and agglomerate. Therefore, by further adding a poor solvent 2 to the slurry containing precipitated lithium carbonate and poor solvent 1, the nanoparticles are slightly agglomerated in the liquid. After the agglomerated nanoparticles settle, the supernatant is removed, and the same steps are repeated by adding poor solvent 2. This allows for the easy acquisition of lithium carbonate nanoparticles dispersed in poor solvent 2, for example, with a particle size D50 of 1–1000 nm. The poor solvent 2 is not limited to any solvent that is soluble in water and NMP but does not dissolve lithium carbonate; acetone, ethanol, etc., are preferred.

[0064] The D50 of the area criterion distribution of the equivalent circle diameter of the obtained lithium carbonate nanoparticles based on SEM observation is preferably less than 500 nm, and more preferably less than 200 nm.

[0065] Furthermore, when adding additive particles to the positive electrode active material layer, before adding undesirable solvent 1, additive nanoparticles, such as particles with a particle size D50 of 1 to 1000 nm, are added to the lithium carbonate aqueous solution and dispersed. Then, undesirable solvent 1 is added, thereby obtaining composite particles of lithium carbonate nanoparticles and additive nanoparticles.

[0066] Next, a coating comprising positive electrode active material, binder, solvent, conductive additive, and lithium carbonate nanoparticles and / or the above composite particles is applied to the positive electrode current collector, and the solvent in the coating applied to the positive electrode current collector is removed.

[0067] As a solvent, for example, water, N-methyl-2-pyrrolidone, N,N-dimethylformamide, etc. can be used.

[0068] There are no particular restrictions on the coating method; methods commonly used in the production of positive electrodes can be used. Examples include slot die coating and blade coating.

[0069] There is no particular limitation on the method for removing the solvent from the coating on the positive current collector 12. The positive current collector 12 coated with the coating can be dried in an atmosphere at, for example, 80°C to 150°C.

[0070] Furthermore, the positive electrode with the positive active material layer 14 formed in this way can then be pressed as needed, for example, by a rolling device. The linear pressure of the rolling can be set to, for example, 10 to 50 kgf / cm.

[0071] Then, when the positive electrode active material layer 14 containing lithium carbonate nanoparticles is exposed to the initial charging process, the lithium carbonate decomposes, generating lithium ions that move towards the negative electrode. That is, lithium carbonate functions as a pre-doping material for lithium. From the viewpoint of ensuring sufficient decomposition of lithium carbonate, the initial charging voltage is preferably 4.3V or higher. During the initial charging, due to the decomposition of lithium carbonate and the movement of lithium ions, the location of the lithium carbonate nanoparticles becomes a pore 14V. In the above method, since lithium carbonate nanoparticles are used as a pre-doping agent, fine pores 14V can be spatially uniformly arranged relative to the positive electrode after the first charge and discharge. When additive particles are used in conjunction, if lithium carbonate nanoparticles are precipitated in the presence of additive particles, it is easy to reduce the diameter of the pores 14V and to distribute them spatially uniformly; furthermore, it is also easy to distribute the additive particles 14A spatially uniformly.

[0072] (negative electrode)

[0073] (Negative current collector)

[0074] The negative current collector 22 can be any conductive sheet material, such as a thin sheet (metal foil) of copper, nickel, stainless steel, or their alloys. From the viewpoint of conductivity, copper foil is preferred. The copper foil can be rolled copper foil or other types of copper foil. The thickness of the negative current collector is not limited, and can be, for example, 5 μm to 20 μm.

[0075] (Negative electrode active material layer)

[0076] The negative electrode active material layer 24 is mainly composed of negative electrode active material, binder and conductive additives in the required amount.

[0077] (Negative electrode active material)

[0078] Examples of anode active materials include carbon materials such as graphite, non-graphitized carbon, easily graphitized carbon, and low-temperature sintered carbon, which can adsorb / release (intercalation / deintercalation, or doping / dedoping) lithium ions; metals and alloys such as Al, Si, and Sn, which can combine with lithium; and SiO2. x (0<x<2), TiO2, SnO2 and other oxide-based crystalline / amorphous compounds, lithium titanate (Li4Ti5O) 12 Particles such as )

[0079] (Negative electrode adhesive and negative electrode conductive additive)

[0080] The adhesive and conductive additive can be the same materials used in the positive electrode 10 described above. Furthermore, the content of the adhesive and conductive additive should be appropriately adjusted, taking into account the volume change of the negative electrode active material and its adhesion to the foil; the same content as in the positive electrode 10 described above is acceptable. When added, the amount of adhesive added is preferably 2 to 20% by mass relative to the active material. The amount of conductive additive added is preferably 0.5 to 5% by mass relative to the active material. Additionally, depending on the type of thermosetting adhesive, heat treatment at any temperature above 200°C is required.

[0081] (Method for manufacturing the negative electrode)

[0082] The manufacturing method of the negative electrode is well known. For example, a coating containing a negative electrode active material, a binder, a solvent and a conductive additive is applied to the negative electrode current collector, and the solvent in the coating applied to the current collector is removed.

[0083] (Diaphragm)

[0084] There are no particular restrictions on the diaphragm as long as it is stable to the electrolyte and has excellent liquid retention. Porous sheets of polyolefins such as polyethylene and polypropylene, or non-woven fabrics can usually be listed.

[0085] (electrolytes)

[0086] The electrolyte is contained within the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. There is no particular limitation on the electrolyte; for example, in this embodiment, an electrolyte containing a lithium salt (an aqueous electrolyte solution, or an electrolyte solution using an organic solvent) can be used. However, aqueous electrolyte solutions have low electrochemical decomposition voltages, which limits their withstand voltage during charging; therefore, an electrolyte using an organic solvent (a non-aqueous electrolyte solution) is preferred. As the electrolyte, an electrolyte prepared by dissolving a lithium salt in a non-aqueous solvent (organic solvent) is preferred. There is no particular limitation on the lithium salt; lithium salts used as electrolytes in lithium-ion secondary batteries can be used. For example, inorganic acid anions such as LiPF6 and LiBF4, and organic acid anions such as LiCF3SO3 and (CF3SO2)2NLi can be used as lithium salts.

[0087] Furthermore, examples of organic solvents include aprotic high-dielectric-constant solvents such as ethylene carbonate and propylene carbonate, and aprotic low-viscosity solvents such as acetates or propionates such as dimethyl carbonate and methyl ethyl carbonate. It is preferable to use these aprotic high-dielectric-constant solvents and aprotic low-viscosity solvents in appropriate mixing ratios. Furthermore, ionic liquids utilizing imidazolium, ammonium, and pyridinium-type cations can be used. The anti-anion is not particularly limited; BF4 is an example. - PF6 -(CF3SO2)2N - Etc. Ionic liquids can be mixed with the above-mentioned organic solvents.

[0088] From the viewpoint of conductivity, the concentration of lithium salt in the electrolyte is preferably 0.5–2.0 M. The conductivity of the electrolyte at 25°C is preferably 0.01 S / m or higher, and can be adjusted by the type or concentration of the electrolyte salt.

[0089] When the electrolyte is a solid electrolyte or a gel electrolyte, it can be used as a polymer material containing organosilicon gel, poly(vinylidene fluoride), etc.

[0090] Furthermore, various additives can be added to the electrolyte of this embodiment as needed. Examples of additives include vinylene carbonate and methyl vinylene carbonate for improving cycle life, biphenyl and alkyl biphenyl for preventing overcharging, and various carbonate compounds, carboxylic anhydrides, and nitrogen- and sulfur-containing compounds for deoxygenation and dehydration.

[0091] (case)

[0092] The housing 50 seals the laminate 30 and the electrolyte inside. The housing 50 is not particularly limited; it can be any object capable of preventing electrolyte leakage to the outside and the intrusion of moisture or other substances into the lithium-ion secondary battery 100. For example, the housing 50 could be as follows: Figure 1 As shown, a metal laminate film is obtained by coating a metal foil 52 with a polymer film 54 on both sides. For example, aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the polymer film 54. For example, a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide, is preferred as the material of the outer polymer film 54, while polyethylene (PE) or polypropylene (PP) is preferred as the material of the inner polymer film 54.

[0093] (lead)

[0094] Leads 60 and 62 are made of conductive materials such as aluminum.

[0095] Then, using known methods, leads 60 and 62 are soldered to the positive current collector 12 and the negative current collector 22 respectively, with the separator 18 sandwiched between the positive active material layer 14 of the positive electrode 10 and the negative active material layer 24 of the negative electrode 20. They are then inserted into the housing 50 together with the electrolyte, and the inlet of the housing 50 is sealed.

[0096] The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments. For example, lithium-ion secondary batteries are not limited to... Figure 1The shape shown can also be a coin-shaped electrode and a diaphragm stacked together, or a cylindrical shape formed by winding electrode sheets and diaphragms into a spiral.

[0097] Example

[0098] [Example 1]

[0099] (Production of lithium carbonate granules)

[0100] 600 mL of NMP was added to 200 mL of a 0.4 wt% lithium carbonate aqueous solution to precipitate lithium carbonate nanoparticles. SEM observation showed that the D50 of the area-equivalent circle diameter distribution of the lithium carbonate nanoparticles was 200 nm.

[0101] (The production of the positive electrode)

[0102] A slurry was prepared by dispersing heated and dried lithium carbonate nanoparticles, lithium-containing nickel-cobalt-manganese composite oxide particles (as the positive electrode active material), acetylene black (as a conductive additive), and polyvinylidene fluoride (PVDF) (as a binder) in N-methyl-2-pyrrolidone (NMP) as a solvent. The slurry was prepared with a weight ratio of nickel-cobalt-manganese composite oxide particles, acetylene black, and PVDF of 97:1.5:1.5. This slurry was coated onto an aluminum foil (as a current collector), dried, and then calendered to fabricate a positive electrode with the positive electrode active material layer of Example 1.

[0103] (Making the negative electrode)

[0104] [Evaluation of the fabrication of the negative electrode cell]

[0105] A slurry was prepared by dispersing a mixture of silicon particles, polyimide (PI) as a binder, and acetylene black in N-methyl-2-pyrrolidone (NMP) as a solvent. The slurry was prepared with a weight ratio of silicon particles, acetylene black, and polyimide of 80:10:10. The slurry was coated onto a copper foil serving as the negative electrode current collector, dried, calendered, and the negative electrode was fabricated.

[0106] (Battery manufacturing)

[0107] Next, the fabricated negative electrode and positive electrode were punched to achieve a battery capacity of 1000mAh. A separator made of polyethylene microporous membrane was then sandwiched between the fabricated negative electrode and positive electrode to obtain a laminate (power generation element). The laminate was placed in an aluminum laminate package, and a solution of fluoroethylene carbonate (FEC): vinylene carbonate (VC): ethyl methyl carbonate (EMC) mixed in a volume ratio of 1:1:8 and in which LiPF6 was dissolved at a concentration of 1.3 mol / L was injected into the aluminum laminate package as an electrolyte. The package was then vacuum sealed to produce the lithium-ion secondary battery of Example 1.

[0108] (Initial charge and discharge)

[0109] The lithium-ion secondary battery was initially charged in a constant temperature bath at 25°C under conditions of an upper limit voltage of 4.48V and a charging rate of 0.1C (the current value at which charging ends after 10 hours during constant current charging). Then, the lithium-ion secondary battery was initially discharged in a constant temperature bath at 25°C under conditions of a lower limit voltage of 2.5V and a discharge rate of 0.1C.

[0110] (Evaluation of battery characteristics)

[0111] Next, the discharge capacity and discharge rate characteristics (fast discharge characteristics) of the lithium-ion secondary battery are determined. The charging capacity is measured in a constant temperature bath at 25°C with the charging rate set to 0.1C (the current value at which charging ends after 10 hours of constant current charging). Then, the initial discharge capacity is measured in a constant temperature bath at 25°C with the discharge rate set to 0.2C, and the discharge capacity is determined.

[0112] Regarding the 4C discharge rate characteristics, after charging at a 0.1C charging rate (the current value at which the discharge ends after 10 hours of constant current discharge at 25℃), the discharge capacity at a 0.2C discharge rate was determined in a constant temperature bath at 25℃. After charging again at a 0.1C charging rate (the current value at which the discharge ends after 10 hours of constant current discharge at 25℃), the discharge capacity at a 4C discharge rate was determined in a constant temperature bath at 25℃. The 4C discharge rate characteristics were determined by the ratio of the 4C discharge capacity to the 0.2C discharge capacity.

[0113] A SEM image of the cross-section of the positive electrode active material layer after initial charge and discharge, perpendicular to the main surface of the current collector, was obtained. The image size is 1357 × 967 pixels, and the image size is 12.7 × 9.0 μm. Solid portions 14S and multiple micropores 14V dispersed within the solid portions 14S were observed in the interparticle region 14Z formed between multiple positive electrode active material particles 14P. The equivalent circular diameter of each micropore was calculated, and the area distribution of the micropore diameter was obtained, yielding D50 and D90. The number of micropores used in the calculation was 261.

[0114] Furthermore, the obtained SEM image was segmented into a two-dimensional square lattice with sides of 1 μm. Taking lattices where the area ratio of the positive electrode active material particles 14P was less than 50% as the target, the pore area ratio of each was obtained, and the variation coefficient CV1 was calculated. The number of lattices was 95.

[0115] [Example 2]

[0116] (Preparation of composite particles)

[0117] 0.4 g of cobalt oxide (Co3O4) particles with a particle size D50 of 30 nm were added as an additive to 200 ml of a 0.4 wt% lithium carbonate aqueous solution and dispersed. 200 ml of NMP was added to the dispersion, precipitating composite particles of lithium carbonate nanoparticles with cobalt oxide particles on their surface. The size of the lithium carbonate particles constituting the composite particles was 1000 nm in D50. The remaining steps were the same as in Example 1 to obtain and evaluate a lithium-ion secondary battery.

[0118] Additionally, the area ratio of the additive particles within each two-dimensional cubic lattice is obtained. The number of lattices is set to the same number as in the case of voids.

[0119] [Example 3]

[0120] The amount of NMP added was set to 600 ml, and the size of the lithium carbonate particles constituting the composite particles was set to 200 nm in terms of D50. Otherwise, the lithium-ion secondary battery was obtained and evaluated according to the same steps as in Example 2.

[0121] [Example 4]

[0122] The amount of NMP added was set to 800 ml, and the size of the lithium carbonate particles constituting the composite particles was set to 100 nm in terms of D50. Otherwise, the lithium-ion secondary battery was obtained and evaluated following the same steps as in Example 2.

[0123] [Example 5]

[0124] The amount of NMP added was set to 1000 ml, and the size of the lithium carbonate particles constituting the composite particles was set to 80 nm in terms of D50. Otherwise, the lithium-ion secondary battery was obtained by following the same steps as in Example 2.

[0125] [Example 6]

[0126] The amount of NMP added was 600 ml, the size of the lithium carbonate particles constituting the composite particles was 200 nm in terms of D50, and lithium cobalt oxide particles with a particle size of 30 nm were used instead of cobalt oxide particles. Otherwise, the lithium-ion secondary battery was obtained by following the same steps as in Example 2.

[0127] [Example 7]

[0128] The amount of NMP added was 800 ml, and the size of the lithium carbonate particles constituting the composite particles was 100 nm in terms of D50. Otherwise, the lithium-ion secondary battery was obtained by following the same steps as in Example 6.

[0129] [Example 8]

[0130] The amount of NMP added was 1000 ml, and the size of the lithium carbonate particles constituting the composite particles was 80 nm in terms of D50. Otherwise, the lithium-ion secondary battery was obtained by following the same steps as in Example 6.

[0131] [Comparative Example 1]

[0132] In the fabrication of the composite particles, the lithium carbonate precipitation process in the presence of additive particles, as in the example, is not used. Instead, a composite particle with cobalt oxide particles of 30 nm in diameter is loaded onto the surface of lithium carbonate particles with a particle size of 2000 nm in D50 by electrophoresis is used. Otherwise, the lithium-ion secondary battery is obtained by the same steps as in Example 2.

[0133] [Comparative Example 2]

[0134] In the fabrication of the composite particles, the precipitation process as in the example was not used. Instead, lithium carbonate raw material powder was mixed with cobalt oxide particles with a particle size of 30 nm, and composite particles with a particle size of 1000 nm (D50) and cobalt oxide particles on the surface were obtained by mechanochemical method. Otherwise, the lithium-ion secondary battery was obtained using the same steps as in Example 2. The conditions and results are shown in Table 1.

[0135]

Claims

1. A positive electrode, wherein, The positive electrode has a current collector and a layer of positive electrode active material in contact with at least one main surface of the current collector. The positive electrode active material layer has multiple positive electrode active material particles and interparticle regions formed between the multiple positive electrode active material particles. The interparticle region has a solid portion and multiple fine pores dispersed within the solid portion. In the cross-sectional image of the positive electrode active material layer, the D50 of the area reference distribution of the equivalent circular diameter of the pores is less than 0.82 μm, and the D90 of the area reference distribution of the equivalent circular diameter of the pores is less than 1.37 μm. In the cross-sectional image of the positive electrode active material layer, when the interparticle region is divided into two-dimensional square grids and the area ratio of the pores in each two-dimensional square grid is obtained, the variation coefficient CV1 of the area ratio of the pores is less than 0.

610.

2. The positive electrode as described in claim 1, wherein, The solid portion contains an adhesive and a conductive additive.

3. The positive electrode as described in claim 1, wherein, The solid portion comprises additive particles consisting of Co3O4 powder, LiCoO2 powder, or a combination thereof.

4. The positive electrode as described in claim 3, wherein, When the area ratio of the additive particles in each two-dimensional square is obtained, the coefficient of variation (CV2) of the area ratio of the additive particles is less than 0.

601.

5. A lithium-ion secondary battery, wherein, have: The positive electrode according to any one of claims 1 to 4; and negative electrode.

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