JP2024134235A - Positive electrode and lithium ion secondary battery - Google Patents

Abstract

To provide a positive electrode and a lithium ion secondary battery, each having capacity and quick discharge characteristic which can be made compatible with each other.SOLUTION: A positive electrode active material layer 14 comprises: a plurality of positive electrode active material particles 14P; and an inter-particle region 14Z which is formed between the plurality of positive electrode active material particles 14P. The inter-particle region 14Z includes: a solid portion 14S; and a plurality of fine holes 14V scattered in the solid portion 14S. In a cross-sectional image of the positive electrode active material layer 14, a D50 which denotes area-based distribution of an area circle equivalent diameter of each fine holes 14V is 0.82 μm or less, and a D90 which denotes area-based distribution of an area circle equivalent diameter of each of fine holes 14V is 1.37 μm or less. In the cross-sectional image of the positive electrode active material layer, the inter-particle region 14Z is divided into a two dimensional tetragonal lattice GR. When an area ratio of each fine hole in each secondary tetragonal lattice GR is acquired, a variation coefficient cv1 of each of the fine hole 14V is 0.610 or less.SELECTED DRAWING: Figure 2

Description

The present invention relates to a positive electrode and a lithium-ion secondary battery.

Lithium ion secondary batteries are widely used in mobile devices such as mobile phones and laptops, as well as power sources for tools, hybrid cars, and the like. 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 that can suppress an increase in the battery's internal resistance during charging and discharging under high output conditions.

JP 2007-109636 A

However, there is a demand for lithium-ion secondary batteries with superior capacity and rapid discharge characteristics.

The present invention was made in consideration of the above problems, and aims to provide a positive electrode and a lithium-ion secondary battery that can achieve both high capacity and rapid discharge characteristics.

[1] A positive electrode having a current collector and a positive electrode active material layer in contact with at least one main surface of the current collector,
the positive electrode active material layer includes a plurality of positive electrode active material particles and an interparticle region formed between the plurality of positive electrode active material particles,
the interparticle region has a solid portion and a plurality of pores dispersed within the solid portion;
In a cross-sectional image of the positive electrode active material layer, a D50 of an area-based distribution of the equivalent circle diameters of the pores is 0.82 μm or less, and a D90 of an area-based distribution of the equivalent circle diameters of the pores is 1.37 μm or less,
a positive electrode, in which, when the interparticle region is divided into a two-dimensional square lattice in a cross-sectional image of the positive electrode active material layer and the area ratio of the pores in each two-dimensional square lattice is obtained, a coefficient of variation CV1 of the area ratio of the pores is 0.610 or less.

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

[3] The positive electrode according to [1] or [2], wherein the solid portion contains additive particles made of Co ₃ O ₄ powder, LiCoO ₂ powder, or a combination thereof.

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

[5] A lithium ion secondary battery comprising a positive electrode according to any one of [1] to [4] and a negative electrode.

A positive electrode and lithium-ion secondary battery are provided that can achieve both high capacity and rapid discharge characteristics.

FIG. 1 is a schematic cross-sectional view showing a lithium-ion secondary battery according to an example of the present embodiment. 2 is an enlarged schematic view of a cross section of the positive electrode active material layer of FIG. 1.

(Lithium-ion secondary battery)
1 is a schematic cross-sectional view showing a lithium-ion secondary battery according to an example of the present embodiment. As shown in Fig. 1, the lithium-ion secondary battery 100 mainly includes a laminate 30, a case 50 that houses the laminate 30 in a sealed state, and a pair of leads 60, 62 connected to the laminate 30.

The laminate 30 includes one or more positive electrodes 10 and one or more negative electrodes 20, which are arranged facing each other with a separator 18 sandwiched between them. The positive electrode 10 includes a positive electrode active material layer 14 provided on one or both main surfaces of a plate-shaped (film-shaped) positive electrode current collector 12.

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

(Positive electrode 10)
(Positive electrode current collector 12)
The positive electrode current collector 12 may be any conductive plate material, and may be, for example, a thin metal plate (metal foil) of aluminum, copper, nickel, or an alloy thereof.

(Positive electrode active material layer 14)
2 shows an enlarged cross-sectional view of the positive electrode active material layer 14. The positive electrode active material layer 14 includes 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 include a solid portion 14S and a plurality of pores 14V dispersed within the solid portion 14S.

The solid portion 14S has a binder portion 14B that binds the positive electrode active material particles 14P together and between the positive electrode active material particles 14P and the positive electrode current collector 12, and the solid portion 14S may further have additive particles 14A. The binder portion 14B contains a binder and may further contain a conductive assistant.

(Positive electrode active material particles 14P)
The positive electrode active material is a material that is capable of absorbing and releasing lithium ions, desorbing and inserting lithium ions (intercalation), or doping and dedoping lithium ions with a counter anion (e.g., ClO ₄ ⁻ ) of the lithium ions. There are no particular limitations on the active material as long as it is possible to cause the above-mentioned reaction to proceed reversibly, and any known active material can be used. For example, lithium cobalt oxide ( _LiCoO2 ), lithium nickel oxide ( _LiNiO2 ), lithium manganese spinel ( _LiMn2O4 ), and the general formula: LiNi _x Co _y Mn _z M _{a 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, and Cr. Composite metal oxides, lithium vanadium compounds (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (wherein M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr) or VO), lithium titanate (Li ₄ Ti ₅ O ₁₂ ), LiNi _x Co Examples of _the composite metal oxide include composite metal oxides such as _yAlzO2 (0.9<x ₊ y+z<1.1).

There is no particular limit to the particle size of the positive electrode active material particles, but the particle size can be 0.1 to 30 μm in terms of D50 of the area-based distribution of the diameter of the circle equivalent to the area of the positive electrode active material particles in a cross-sectional photograph of the positive electrode active material layer.

(Binder section 14B)
The binder portion 14B bonds the positive electrode active material particles 14P to each other and also bonds the positive electrode active material particles 14P to the positive electrode current collector 12. The binder contained in the binder portion may be any binder capable of the above-mentioned bonding, and may be, for example, a fluororesin such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). In addition to the above, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, acrylic resin, polyimide resin, polyamide-imide resin, etc. may be used as the binder. In addition, an electronically conductive conductive polymer or an ionically conductive conductive polymer may be used as the binder. For example, polyacetylene may be used as the electronically conductive conductive polymer. In this case, the binder also functions as a conductive additive particle, so that a conductive additive does not need to be added. As the ionically conductive conductive polymer, for example, one having ion conductivity such as lithium ion can be used, and examples thereof include a composite of a monomer of a polymer compound (polyether polymer compounds such as polyethylene oxide and polypropylene oxide, polyphosphazene, etc.) and a lithium salt or an alkali metal salt mainly composed of lithium, such as LiClO ₄ , LiBF ₄ , and LiPF _6. Examples of the polymerization initiator used for the composite include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-mentioned monomer.

The content of the binder in the positive electrode active material layer 14 is not particularly limited, but is preferably 1 to 10 mass % based on the sum of the masses of the active material, conductive additive, and binder. By setting the content of the active material and binder within the above range, it is possible to suppress the tendency in the obtained positive electrode active material layer 14 that the amount of binder is too small to form a strong active material layer. It is also possible to suppress the tendency that the amount of binder that does not contribute to the electric capacity increases, making it difficult to obtain a sufficient volumetric energy density.

(Conductive assistant)
The conductive assistant contained in the binder portion 14B is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and any known conductive assistant can be used. Examples of the conductive assistant include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO.

The amount of conductive additive contained in the positive electrode active material layer 14 is not particularly limited, but if added, it is preferably 0.5 to 5 mass % relative to the mass of the active material.

(Additive Particles 14A)
The solid portion 14S may contain additive particles 14A consisting of Co ₃ O ₄ powder, LiCoO ₂ powder, or a combination thereof. The average particle size of the additive particles 14A is not particularly limited, but may be 5 to 150 nm in the D50 of the area-based distribution of the area-equivalent circle diameter of the additive particles 14A in a cross-sectional photograph of the positive electrode active material layer. These additive particles function as a decomposition catalyst for lithium carbonate nanoparticles contained in the positive electrode active material layer before the first charge and discharge when manufacturing a battery. In addition, after the first charge and discharge, the electrostatic characteristics of the positive electrode active material layer are improved, the migration potential of lithium ions is increased, and the capacity and/or rapid discharge characteristics are further improved.

(hole 14V)
As described above, the inter-particle region 14Z formed between the plurality of positive electrode active material particles 14P has the solid portion 14S and the plurality of pores 14V dispersed within the solid portion 14S, and has a sponge-like shape.

In the cross-sectional image of the positive electrode active material layer, the area-based distribution D50 of the area-equivalent circle diameter of pores 14V is 0.82 μm or less. The D50 may be 0.70 μm or less, or 0.60 μm or less. D50 is the particle size at which the number ratio of particles smaller than this value is 50%.

In the cross-sectional image of the positive electrode active material layer, the area-based distribution D90 of the area-equivalent circle diameter of pores 14V is 1.37 μm or less. The D90 may be 1.30 μm or less, or 1.20 μm or less. D90 is the particle size at which 90% of the particles are smaller than this size.

When obtaining these particle size distributions, it is preferable to set the number of pores 14V to, for example, about 200 to 700.

Furthermore, in this embodiment, as shown in FIG. 2, when the interparticle region 14Z is divided into a two-dimensional square lattice GR in a cross-sectional image of the positive electrode active material layer, and the area ratio of the pores 14V in each two-dimensional square lattice GR is obtained, the coefficient of variation CV1 of the area ratio of the pores 14V is 0.610 or less. The coefficient of variation is the standard deviation/average value.

When the interparticle region 14Z contains additive particles 14A, when the interparticle region 14Z is divided into a two-dimensional square lattice GR and the area ratio of the additive particles 14A in each two-dimensional square lattice GR is obtained, it is preferable that the coefficient of variation CV2 of the area ratio of the additive particles is 0.601 or less.

There is no particular limitation on the direction of the cross-sectional image of the positive electrode active material layer, and the cross-section may be perpendicular to the main surface of the current collector or parallel to the main surface of the current collector, but a cross-section perpendicular to the main surface of the current collector is preferred. Each region in the cross-sectional image can be identified by the brightness of the image, the surface condition, element mapping, etc.

The size of the two-dimensional square lattice GR is preferably 0.5 to 4.0 μm on a side. Furthermore, the lattices for which the area ratio of the pores and additive particles is measured are limited to lattices in which the area ratio of the positive electrode active material particles 14P is 50% or less. Furthermore, closed pores in the positive electrode active material particles 14P are not included in the interparticle regions 14Z. The number of two-dimensional square lattices for which the area ratio is obtained is preferably 90 to 360.

(Action)
In such a positive electrode, the diameter of the pores 14V is smaller than that of the conventional one, and the small diameter pores 14V are spatially uniformly dispersed. Therefore, it is considered that the electrolyte is easily permeated into the positive electrode active material layer 14, the electrolyte and salt can be quickly contacted with the positive electrode active material, and the insertion and desorption of Li ions is efficiently performed. Specifically, the capacity can be increased by increasing the amount of Li ions moving between the positive electrode and the negative electrode. In addition, the rapid discharge characteristic can be improved by increasing the movement speed of Li ions between the positive electrode and the negative electrode. Therefore, a lithium ion secondary battery that is particularly effective for tools, electric vehicles, etc. can be realized.

In addition, when the solid portion of the positive electrode active material layer contains additive particles, the electrostatic properties are improved and the ion migration potential is increased, further improving the capacity and/or rapid discharge characteristics.

Furthermore, the uniform spatial distribution of the additive particles further improves the electrostatic properties, optimizing the dielectric material balance and resulting in a particularly high ion migration potential, further enhancing the above properties.

(Method of manufacturing positive electrode)
An example of a method for producing such a positive electrode is shown below.

(Preparation of lithium carbonate nanoparticles or composite particles containing the same)
First, lithium carbonate powder with a large particle size (for example, several μm) is dissolved in water. The lower the lithium carbonate concentration of the lithium carbonate aqueous solution, the smaller the particle size of the lithium carbonate nanoparticles that are precipitated later. Next, when poor solvent 1 is added to the lithium carbonate aqueous solution, lithium carbonate nanoparticles, for example, particles with a particle size D50 of 1 to 1000 nm in the volume-based particle size distribution by laser diffraction, are precipitated to obtain a slurry (precipitation process). The amount of poor solvent 1 added is preferably 70 wt % or more relative to water. The larger the amount of poor solvent 1 added, the smaller the particle size of the precipitated lithium carbonate nanoparticles. The poor solvent 1 can be appropriately selected from those that are soluble in water and insoluble in lithium carbonate. In addition, it is preferable that the poor solvent 1 has a boiling point higher than that of water. This is because in the next process of extracting lithium carbonate nanoparticles from the slurry, if the poor solvent 1 evaporates before water when performing thermal drying, the precipitated lithium carbonate will be re-dissolved in water. An example of poor solvent 1 is N-methyl-2-pyrrolidone (NMP).

The slurry can be heated and dried to separate the precipitated lithium carbonate nanoparticles from the slurry and obtain them as a dry powder, but there is a possibility that the lithium carbonate nanoparticles may form liquid bridges and aggregate during drying. Therefore, by further adding poor solvent 2 to the slurry containing lithium carbonate from which the nanoparticles have precipitated and poor solvent 1, the nanoparticles are lightly aggregated in the liquid, the aggregated nanoparticles are allowed to settle, the supernatant liquid is removed, and poor solvent 2 is further added. By repeating the same procedure, lithium carbonate nanoparticles dispersed in poor solvent 2, for example, with a particle size D50 of 1 to 1000 nm, can be easily obtained. There are no limitations on the poor solvent 2 as long as it dissolves in water and NMP and does not dissolve lithium carbonate, but acetone, ethanol, etc. are preferable.

The resulting lithium carbonate nanoparticles preferably have a D50 of 500 nm or less, more preferably 200 nm or less, in the area-based distribution of the area-equivalent circle diameter as observed by SEM.

Furthermore, when additive particles are added to the positive electrode active material layer, additive nanoparticles, for example, particles with a particle size D50 of 1 to 1000 nm, are added and dispersed in the lithium carbonate aqueous solution before adding poor solvent 1, and then poor solvent 1 is added to obtain composite particles of lithium carbonate nanoparticles and additive nanoparticles.

Next, a paint containing a positive electrode active material, a binder, a solvent, a conductive assistant, and lithium carbonate nanoparticles and/or the above composite particles is applied onto the positive electrode current collector, and the solvent in the paint applied onto the positive electrode current collector is removed.

Examples of solvents that can be used include water, N-methyl-2-pyrrolidone, and N,N-dimethylformamide.

There are no particular limitations on the coating method, and any method that is normally used to fabricate a positive electrode can be used. Examples include the slit die coating method and the doctor blade method.

The method for removing the solvent in the paint applied to the positive electrode collector 12 is not particularly limited, and the positive electrode collector 12 to which the paint has been applied may be dried, for example, in an atmosphere of 80°C to 150°C.

The positive electrode on which the positive electrode active material layer 14 has been formed in this manner may then be pressed, for example, by a roll press device, if necessary. The linear pressure of the roll press may be, for example, 10 to 50 kgf/cm.

After that, when the positive electrode active material layer 14 containing the lithium carbonate nanoparticles is exposed to the initial charging process, the lithium carbonate decomposes, lithium ions are generated, and the lithium carbonate migrates to the negative electrode. That is, the lithium carbonate functions as a pre-dope material for lithium. From the viewpoint of sufficiently decomposing the lithium carbonate, it is preferable that the initial charging voltage is 4.3 V or more. After the initial charging, the locations where the lithium carbonate nanoparticles were present become pores 14V due to the decomposition of lithium carbonate and the migration of lithium ions. In the above method, since the lithium carbonate nanoparticles are used as a pre-dope agent, it is possible to arrange the fine pores 14V spatially uniformly in the positive electrode that has undergone the initial charging and discharging. When additive particles are used in combination, if the lithium carbonate nanoparticles are precipitated in the presence of the additive particles, the diameter of the pores 14V is reduced and it is easy to distribute them spatially uniformly, and further, it is easy to distribute the additive particles 14A spatially uniformly.

(Negative electrode)
(Negative electrode current collector)
The negative electrode current collector 22 may be any conductive plate material, and may be, for example, a metal thin plate (metal foil) of copper, nickel, stainless steel, or an alloy thereof. Copper foil is preferable from the viewpoint of electrical conductivity. The copper foil may be a rolled copper foil or other copper foil. There is no limitation on the thickness of the negative electrode current collector, but it may be, for example, 5 to 20 μm.

(Negative Electrode Active Material Layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material, a binder, and a necessary amount of a conductive assistant.

(Negative Electrode Active Material)
Examples of the negative electrode active material include carbon materials such as graphite, difficult-to-graphitize carbon, easy-to-graphitize carbon, and low-temperature fired carbon that can absorb and release lithium ions (intercalate/deintercalate, or dope/dedop), metals and alloys that can combine with lithium such as Al, Si, and Sn, crystalline and amorphous compounds mainly composed of oxides such as SiO _x (0<x<2), TiO ₂ , and SnO _2, and particles containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

(Negative Electrode Binder and Negative Electrode Conductive Assistant)
The binder and conductive assistant may be the same as the materials used in the positive electrode 10 described above. In addition, the content of the binder and conductive assistant may be appropriately adjusted when the magnitude of the volume change of the negative electrode active material and the adhesion to the foil must be taken into consideration, and the same content as the content in the positive electrode 10 described above may be adopted. When added, the amount of the binder added is preferably 2 to 20 mass% with respect to the mass of the active material. The amount of the conductive assistant added is preferably 0.5 to 5 mass% with respect to the mass of the active material. In addition, depending on the type, such as a thermosetting binder, heat treatment at any temperature of 200°C or higher is required.

(Method of Manufacturing Negative Electrode)
Methods for producing a negative electrode are known, and include, for example, applying a coating material containing a negative electrode active material, a binder, a solvent, and a conductive assistant onto a negative electrode current collector, and then removing the solvent from the coating material applied to the current collector.

(Separator)
There are no particular limitations on the separator as long as it is stable to the electrolyte and has excellent electrolyte retention, but typical examples include porous sheets of polyolefins such as polyethylene and polypropylene, or nonwoven fabrics.

(Electrolytes)
The electrolyte is contained in the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. The electrolyte is not particularly limited, and for example, in this embodiment, an electrolyte solution containing a lithium salt (aqueous electrolyte solution, an electrolyte solution using an organic solvent) can be used. However, since the electrolyte aqueous solution has a low electrochemical decomposition voltage, the withstand voltage during charging is limited to a low level, so an electrolyte solution using an organic solvent (non-aqueous electrolyte solution) is preferable. As the electrolyte solution, a solution in which a lithium salt is dissolved in a non-aqueous solvent (organic solvent) is preferably used. The lithium salt is not particularly limited, and lithium salts used as electrolytes for lithium ion secondary batteries can be used. For example, as the lithium salt, inorganic acid anion salts such as LiPF ₆ and LiBF ₄ , organic acid anion salts such as LiCF ₃ SO ₃ and (CF ₃ SO ₂ ) ₂ NLi, etc. can be used.

Examples of the organic solvent include aprotic high-dielectric constant solvents such as ethylene carbonate and propylene carbonate, and aprotic low-viscosity solvents such as acetates and propionates such as dimethyl carbonate and ethyl methyl carbonate. It is desirable to use these aprotic high-dielectric constant solvents and aprotic low-viscosity solvents in a suitable mixing ratio. Furthermore, ionic liquids using imidazolium, ammonium, and pyridinium cations can be used. The counter anion is not particularly limited, but examples include BF ₄ ^- , PF ₆ ^- , (CF ₃ SO ₂ ) ₂ N ^-, and the like. The ionic liquid can be used by mixing with the organic solvent described above.

From the viewpoint of electrical conductivity, the concentration of the lithium salt in the electrolyte is preferably 0.5 to 2.0 M. The conductivity of this electrolyte at a temperature of 25°C is preferably 0.01 S/m or more, and is adjusted according to the type or concentration of the electrolyte salt.

When the electrolyte is a solid electrolyte or a gel electrolyte, it is possible to include silicone gel, poly(vinylidene fluoride), etc. as a polymer material.

Furthermore, various additives may be added to the electrolyte of this embodiment as necessary. Examples of additives include vinylene carbonate, methylvinylene carbonate, etc., for improving cycle life, biphenyl, alkylbiphenyl, etc., for preventing overcharging, and various carbonate compounds, various carboxylic acid anhydrides, and various nitrogen- and sulfur-containing compounds for deoxidization and dehydration.

(case)
The case 50 seals the laminate 30 and the electrolyte therein. The case 50 is not particularly limited as long as it can prevent leakage of the electrolyte to the outside and intrusion of moisture from the outside into the lithium ion secondary battery 100. For example, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated on both sides with a polymer film 54 can be used as the case 50. 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 preferable as the material of the outer polymer film 54, and a polyethylene (PE) or polypropylene (PP) is preferable as the material of the inner polymer film 54.

(Lead)
The leads 60 and 62 are made of a conductive material such as aluminum.

Then, by using a known method, the leads 60, 62 are welded to the positive electrode collector 12 and the negative electrode collector 22, respectively, and the separator 18 is sandwiched between the positive electrode active material layer 14 of the positive electrode 10 and the negative electrode active material layer 24 of the negative electrode 20. The separator 18 is then inserted into the case 50 together with the electrolyte, and the entrance of the case 50 is sealed.

Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. For example, the lithium ion secondary battery is not limited to the shape shown in FIG. 1, but may be a coin type in which electrodes punched into a coin shape and a separator are laminated, or a cylinder type in which an electrode sheet and a separator are wound in a spiral shape.

[Example 1]
(Preparation of lithium carbonate particles)
600 mL of NMP was added to 200 mL of a 0.4 wt % aqueous lithium carbonate solution to precipitate lithium carbonate nanoparticles. The D50 of the area-based distribution of the area-equivalent circle diameter of the lithium carbonate nanoparticles observed with a SEM was 200 nm.

(Preparation of Positive Electrode)
A slurry was prepared by dispersing heated and dried lithium carbonate nanoparticles, lithium-containing nickel-cobalt-manganese composite oxide particles as a positive electrode active material, acetylene black as a conductive assistant, and polyvinylidene fluoride as a binder in a solvent, N-methyl-2-pyrrolidone (NMP). The slurry was prepared so that the weight ratio of the nickel-cobalt-manganese composite oxide particles, acetylene black, and polyvinylidene fluoride in the slurry was 97:1.5:1.5. This slurry was applied onto an aluminum foil as a current collector, dried, and then rolled to produce a positive electrode on which the positive electrode active material layer of Example 1 was formed.

(Preparation of negative electrode)
[Preparation of negative electrode cell for evaluation]
A mixture of silicon-containing particles, polyimide (PI) as a binder, and acetylene black was dispersed in N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a slurry. The slurry was prepared so that the weight ratio of silicon particles, acetylene black, and polyimide in the slurry was 80:10:10. This slurry was applied onto a copper foil as a negative electrode current collector, dried, and then rolled to prepare a negative electrode.

(Battery Construction)
Next, the negative electrode and the negative electrode were punched out so that the capacity of the battery was 1000mAh, and then a separator made of a polyethylene microporous film was sandwiched between the negative electrode and the positive electrode to obtain a laminate (power generation element). This laminate was placed in an aluminum laminator pack, and an electrolyte solution was mixed in the aluminum laminate pack so that fluoroethylene carbonate (FEC): vinylene carbonate (VC): ethyl methyl carbonate (EMC) = 1: 1: 8 in volume ratio, and _LiPF6 was dissolved in the electrolyte solution to a concentration of 1.3 mol / L, and then vacuum sealed to prepare the lithium ion secondary battery of Example 1.

(Initial charge/discharge)
The lithium ion secondary battery was initially charged in a thermostatic chamber at 25° C. under the conditions of an upper limit voltage of 4.48 V and a charge rate of 0.1 C (a current value at which charging is completed in 10 hours when constant current charging is performed). Thereafter, the lithium ion secondary battery was initially discharged in a thermostatic chamber at 25° C. under the conditions of a lower limit voltage of 2.5 V and a discharge rate of 0.1 C.

(Evaluation of Battery Characteristics)
Next, the discharge capacity and discharge rate characteristics (rapid discharge characteristics) of the lithium ion secondary battery were determined. The charge capacity was measured in a thermostatic chamber at 25° C. when the charge rate was 0.1 C (the current value at which charging is completed in 10 hours when constant current charging is performed), and then the initial discharge capacity was measured in a thermostatic chamber at 25° C. when the discharge rate was 0.2 C, to determine the discharge capacity.

The 4C discharge rate characteristics were calculated by charging at a 0.1C charge rate (current value at which discharge ends in 10 hours when constant current discharge is performed at 25°C), then measuring the discharge capacity at a 0.2C discharge rate in a thermostatic chamber at 25°C, then charging again at a 0.1C charge rate (current value at which discharge ends in 10 hours when constant current discharge is performed at 25°C), then measuring the discharge capacity at a 4C discharge rate in a thermostatic chamber at 25°C, and calculating the ratio of the 4C discharge capacity to the 0.2C discharge capacity.

An SEM image of a cross section perpendicular to the main surface of the current collector of the positive electrode active material layer after initial charging and discharging was obtained. The image size was 1357 x 967 pixels, and the image dimension was 12.7 x 9.0 μm. A solid portion 14S and a plurality of pores 14V dispersed within the solid portion 14S were observed in the interparticle region 14Z formed between a plurality of positive electrode active material particles 14P. The area circle equivalent diameter of each pore was calculated to obtain the area-based distribution of pore diameters, and D50 and D90 were obtained. The number of pores used in the calculation was 261.

The obtained SEM image was divided into a two-dimensional square lattice with sides of 1 μm. For lattices in which the area ratio of the positive electrode active material particles 14P was 50% or less, the pore area ratio for each was obtained, and the coefficient of variation CV1 was obtained. The number of lattices was 95.

[Example 2]
(Preparation of Composite Particles)
0.4 g of cobalt oxide (Co ₃ O ₄ ) particles having a particle size D50 of 30 nm were added as an additive and dispersed in 200 ml of a lithium carbonate aqueous solution having a concentration of 0.4 wt %. 200 ml of NMP was added to this dispersion to precipitate composite particles that are lithium carbonate nanoparticles carrying cobalt oxide particles on their surfaces. The size of the lithium carbonate particles constituting the composite particles was 1000 nm as D50. The other procedures were the same as in Example 1, and a lithium ion secondary battery was obtained and evaluated.

The area ratio of additive particles within each two-dimensional square lattice was also obtained. The number of lattices was the same as in the case of voids.

[Example 3]
A lithium ion secondary battery was obtained and evaluated in the same manner as in Example 2, except that the amount of NMP added was 600 ml and the size of the lithium carbonate particles constituting the composite particles was 200 nm as D50.

[Example 4]
A lithium ion secondary battery was obtained and evaluated in the same manner as in Example 2, except that the amount of NMP added was 800 ml and the size of the lithium carbonate particles constituting the composite particles was 100 nm as D50.

[Example 5]
A lithium ion secondary battery was obtained in the same manner as in Example 2, except that the amount of NMP added was 1000 ml and the size of the lithium carbonate particles constituting the composite particles was 80 nm as D50.

[Example 6]
A lithium ion secondary battery was obtained in the same manner as in Example 2, except that the amount of NMP added was 600 ml, the size of the lithium carbonate particles constituting the composite particles was 200 nm as D50, and lithium cobalt oxide particles having a particle size of 30 nm were used instead of the cobalt oxide particles.

[Example 7]
A lithium ion secondary battery was obtained in the same manner as in Example 6, except that the amount of NMP added was 800 ml and the size of the lithium carbonate particles constituting the composite particles was set to 100 nm as D50.

[Example 8]
A lithium ion secondary battery was obtained in the same manner as in Example 6, except that the amount of NMP added was 1000 ml and the size of the lithium carbonate particles constituting the composite particles was set to 80 nm as D50.

[Comparative Example 1]
A lithium ion secondary battery was obtained in the same manner as in Example 2, except that in the preparation of the composite particles, a precipitation step of lithium carbonate in the presence of additive particles as in the Examples was not used, and composite particles were used in which cobalt oxide particles having a particle size of 30 nm were supported by a migration method on the surfaces of lithium carbonate particles having a particle size D50 of 2000 nm.

[Comparative Example 2]
A lithium ion secondary battery was obtained in the same manner as in Example 2, except that in the preparation of the composite particles, a precipitation step as in the Example was not used, but instead, lithium carbonate raw material powder was mixed with cobalt oxide particles having a particle size of 30 nm, and composite particles having a particle size of 1000 nm as D50 of the lithium carbonate particles and having cobalt oxide particles supported on the surface were obtained by a mechanochemical method. The conditions and results are shown in Table 1.

10... positive electrode, 12... positive electrode current collector, 14... positive electrode active material layer, 14P... positive electrode active material particle, 14S... solid portion, 14V... pore, 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 square lattice.

Claims (5)

A positive electrode having a current collector and a positive electrode active material layer in contact with at least one main surface of the current collector,
the positive electrode active material layer includes a plurality of positive electrode active material particles and an interparticle region formed between the plurality of positive electrode active material particles,
the interparticle region has a solid portion and a plurality of pores dispersed within the solid portion;
In a cross-sectional image of the positive electrode active material layer, a D50 of an area-based distribution of the equivalent circle diameters of the pores is 0.82 μm or less, and a D90 of an area-based distribution of the equivalent circle diameters of the pores is 1.37 μm or less,
a positive electrode, in which, when the interparticle region is divided into a two-dimensional square lattice in a cross-sectional image of the positive electrode active material layer and the area ratio of the pores in each two-dimensional square lattice is obtained, a coefficient of variation CV1 of the area ratio of the pores is 0.610 or less. The positive electrode according to claim 1, wherein the solid portion includes a binder and a conductive additive. The positive electrode of claim 1 , wherein the solid portion includes additive particles of Co ₃ O ₄ powder, LiCoO ₂ powder, or a combination thereof. The positive electrode according to claim 3, wherein when the area ratio of the additive particles in each two-dimensional square lattice is obtained, the coefficient of variation CV2 of the area ratio of the additive particles is 0.601 or less. A lithium ion secondary battery comprising the positive electrode according to any one of claims 1 to 4 and a negative electrode.

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