JP7601558B2 - Active material layer, electrode and lithium ion secondary battery - Google Patents

Description

The present invention relates to an active material layer, an electrode, and a lithium-ion secondary battery.

Lithium-ion secondary batteries are also widely used as a power source for mobile devices such as mobile phones and laptops, as well as hybrid cars. As these fields develop, there is a demand for higher performance from lithium-ion secondary batteries.

For example, Patent Document 1 discloses a lithium secondary battery in which a coating made of a mixture of two or more types is formed on the surface of the positive electrode. Patent Document 1 discloses that the coating suppresses decomposition of the electrolyte due to reaction between the active material and the electrolyte, improving cycle characteristics.

For example, Patent Document 2 describes a positive electrode in which the active material is coated with a specific metal, intermetallic compound, or oxide. It describes how the metal coating the active material absorbs oxygen, thereby improving the safety of the battery.

Japanese Patent Application Publication No. 8-236114 Japanese Patent Application Publication No. 11-16566

One of the indicators of battery performance is the discharge rate characteristic. The discharge rate is the speed required to reach the final voltage from a full charge. The discharge rate characteristic is the ratio of the discharge capacity when the discharge rate is increased to the discharge capacity when discharging at a specified discharge rate. A battery with excellent discharge rate characteristics is capable of rapid discharge.

This disclosure was made in consideration of the above problems, and aims to provide a lithium-ion secondary battery with excellent discharge rate characteristics.

To solve the above problems, the following measures are provided:

(1) The active material layer according to the first aspect has at least an active material and a binder, the active material comprising a first active material as a main component and a second active material as a subcomponent having a composition different from that of the first active material, the proportion of the first active material being 60% or more and less than 100% of the total active material, and the proportion of the second active material being greater than 0% and less than 5% of the total active material.

(2) In the active material layer according to the above aspect, the first active material may be a secondary particle formed by agglomeration of a plurality of primary particles, and the second active material may be a primary particle.

(3) In the active material layer according to the above aspect, the particle size of the first active material may be 0.6 to 1.5 times the particle size of the second active material.

(4) In the active material layer according to the above aspect, the reaction potential of the second active material may be higher than the reaction potential of the first active material.

(5) In the active material layer according to the above embodiment, the second active material may be expressed as Li _a Co _1-y M _y O ₂ ... (1), where a satisfies 0.8≦a≦1.2, y satisfies 0≦y<0.5, and M may be at least one of Ni, Mn, Fe, Mg, Zr, Al, Ti, Zn, Cu, Ca, and Na.

(6) The electrode according to the second aspect comprises a current collector and an active material layer according to the above aspect laminated on at least one surface of the current collector.

(7) The lithium ion secondary battery according to the third aspect includes the electrode according to the above aspect.

The lithium ion secondary battery according to the above aspect has excellent discharge rate characteristics.

1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment;

The following describes the embodiments in detail, with reference to the drawings as appropriate. The drawings used in the following description may show characteristic parts in an enlarged scale for the sake of clarity, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them. Appropriate modifications may be made without departing from the spirit of the invention.

"Lithium-ion secondary battery"
Fig. 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. The lithium ion secondary battery 100 shown in Fig. 1 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of connected terminals 60, 62. The non-aqueous electrolyte is contained within the exterior body 50.

(Power generation element)
The power generating element 40 includes a positive electrode 20 , a negative electrode 30 , and a separator 10 .

<Separator>
The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30, and prevents a short circuit between the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 may be, for example, a monolayer or laminate of a film made of a polyolefin such as polyethylene or polypropylene, or a stretched film of a mixture of the above resins, or a fibrous nonwoven fabric made of at least one material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene.

The separator 10 may be, for example, a solid electrolyte. The solid electrolyte may be, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. The polymer solid electrolyte may be, for example, a polyethylene oxide-based polymer having an alkali metal salt dissolved therein. Examples of _{oxide-based solid electrolytes include Li1.3Al0.3Ti1.7(PO4)3 (Nasicon type), Li1.07Al0.69Ti1.46(PO4)3 ( glass ceramics ) , Li0.34La0.51TiO2.94 ( perovskite type ) , Li7La3Zr2O12 ( garnet} type) _{, Li2.9PO3.3N0.46} ( _amorphous , _LIPON ), _{50Li4SiO4.50Li2BO3} ( _{glass ) , and 90Li3BO3.10Li2SO4} ( _glass ceramics _{) .} Examples of sulfide-based solid electrolytes include Li _3.25 Ge _0.25 P _0.75 S ₄ (crystal), Li ₁₀ GeP ₂ S ₁₂ (crystal, LGPS), Li ₆ PS ₅ Cl (crystal, argyrodite type), Li _9.54 Si _1.74 P _1.44 S _11.7 Cl _0.3 (crystal), Li _3.25 P _0.95 S ₄ (glass ceramics), Li ₇ P ₃ S ₁₁ (glass ceramics), 70 Li ₂ S · 30 P ₂ S ₅ (glass), 30 Li ₂ S · 26 B ₂ S ₃ · 44 LiI (glass), 50 Li ₂ S · 17 P ₂ S ₅ · 33 LiBH _{4 ( glass} ) _{, 63Li2S.36SiS2.Li3PO4 ( glass} ) _, and _{57Li2S.38SiS2.5Li4SiO4} ( _glass ).

<Positive electrode>
The positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is formed on at least one surface of the positive electrode current collector 22.

[Positive electrode current collector]
The positive electrode current collector 22 is, for example, a conductive plate material, such as a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, or the like.

[Positive electrode active material layer]
The positive electrode active material layer 24 includes, for example, a positive electrode active material, a conductive assistant, and a binder.

The positive electrode active material can reversibly absorb and release lithium ions, remove and insert lithium ions (intercalation), or dope and dedope lithium ions and counter anions.

The positive electrode active material is composed of at least two or more active materials. The positive electrode active material includes, for example, a first active material as a main component and a second active material as a secondary component. The abundance ratio of the second active material is extremely small compared to the first active material. The second active material is present in only a trace amount in the positive electrode active material layer 24.

The proportion of the first active material in the entire positive electrode active material is 60% or more and less than 100%, preferably 60% or more and less than 99%, more preferably 80% or more and less than 99%, and even more preferably 96% or more and less than 98%. The proportion of the second active material in the entire positive electrode active material is more than 0% and less than 5%, preferably 0.5% or more and less than 4.5%, and more preferably 1% or more and less than 4%. When the positive electrode active material is composed of the first active material and the second active material, the total proportion of the first active material and the second active material is 100%. When there are three or more types of positive electrode active materials, the total proportion of the first active material and the second active material is less than 100%.

The first active material has a different composition from the second active material. The reaction potential of the second active material is, for example, higher than the reaction potential of the first active material. The reaction potential is the electrical potential energy at which the lithium intercalation reaction occurs, and is the reaction potential relative to the equilibrium potential of Li. For example, the reaction potentials of spinel compounds (hereinafter referred to as LMO), manganese phosphate compounds (hereinafter referred to as LMP), lithium cobalt oxide (hereinafter referred to as LCO), nickel-cobalt-manganese ternary compounds (hereinafter referred to as NMC), nickel-cobalt-aluminum ternary compounds (hereinafter referred to as NCA), lithium iron phosphate (hereinafter referred to as LFP), sulfur-based compounds, and lithium titanium oxide (hereinafter referred to as LTO) satisfy the relationship of LMO>LMP>LCO>NMC≒NCA>LFP>sulfur-based compounds>LTO.

The second active material is, for example, a compound represented by Li _a Co _1-y M _y O ₂ ... (1), where a satisfies 0.8≦a≦1.2, y satisfies 0≦y<0.5, and M is at least one of Ni, Mn, Fe, Mg, Zr, Al, Ti, Zn, Cu, Ca, and Na. In the above formula (1), when y=0, it is LCO.

For example, the first active material may be a secondary particle, and the second active material may be a primary particle. A secondary particle is a particle formed by agglomeration of multiple primary particles. For example, when observing a cross section using a transmission electron microscope (TEM), if grain boundaries are visible within the particle, it is a secondary particle, and if no grain boundaries are visible, it is a primary particle.

The particle size of the first active material is, for example, 0.6 to 1.5 times the particle size of the second active material, and preferably 0.9 to 1.1 times. Here, the particle size is the average particle size of 10 or more particles. If the particles are irregular, the particle size is considered to be half the sum of the length of the major axis and the length of the minor axis. If secondary particles are formed, the particle size is the particle size of the secondary particles.

The binder binds the positive electrode active material in the positive electrode active material layer 24 together. Any known binder can be used. The binder is, for example, a fluororesin. Examples of the fluororesin include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc.

In addition to the above, the binder may be, for example, vinylidene fluoride-based fluororubber such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinylether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), or vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber).

The conductive assistant enhances the conductivity between the positive electrode active materials in the positive electrode active material layer 24. Examples of the conductive assistant include carbon powders such as carbon black, carbon nanotubes, carbon materials, 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 conductive assistant is preferably a carbon material such as carbon black. If sufficient conductivity can be ensured with the active material alone, the positive electrode active material layer 24 does not need to contain a conductive assistant.

The positive electrode active material layer 24 may also contain a solid electrolyte or a gel electrolyte. The solid electrolyte may be, for example, the same as that which can be used in the separator.

<Negative Electrode>
The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[Negative electrode current collector]
The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 may be the same as the positive electrode current collector 22.

[Negative electrode active material layer]
The negative electrode active material layer 34 contains a negative electrode active material and may further contain a conductive material, a binder, and a solid electrolyte, as necessary.

The negative electrode active material may be any compound capable of absorbing and releasing ions, and may be any negative electrode active material used in known lithium ion secondary batteries. The negative electrode active material may be, for example, metallic lithium, lithium alloy, graphite (natural graphite, artificial graphite) capable of absorbing and releasing ions, carbon materials such as carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, and low-temperature fired carbon, semimetals or metals capable of combining with metals such as lithium, such as aluminum, silicon, tin, and germanium, amorphous compounds mainly composed of oxides such as SiO _x (0<x<2) and tin dioxide, and particles containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

The negative electrode active material layer 34 may contain, for example, silicon, tin, and germanium, as described above. Silicon, tin, and germanium may exist as single elements or as compounds. The compounds are, for example, alloys, oxides, and the like. As an example, when the negative electrode active material is silicon, the negative electrode 30 may be called a Si negative electrode. The negative electrode active material may be, for example, a mixture of a simple substance or compound of silicon, tin, or germanium and a carbon material. The carbon material is, for example, natural graphite. The negative electrode active material may also be, for example, a simple substance or compound of silicon, tin, or germanium, the surface of which is coated with carbon. The carbon material and the coated carbon increase the conductivity between the negative electrode active material and the conductive additive. When the negative electrode active material layer contains silicon, tin, or germanium, the capacity of the lithium ion secondary battery 100 increases.

The negative electrode active material layer 34 may contain, for example, lithium as described above. The lithium may be metallic lithium or a lithium alloy. The negative electrode active material layer 34 may be metallic lithium or a lithium alloy. The lithium alloy is, for example, an alloy of lithium and one or more elements selected from the group consisting of Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, and Al. As an example, when the negative electrode active material is metallic lithium, the negative electrode 30 may be called a Li negative electrode. The negative electrode active material layer 34 may be a sheet of lithium.

The negative electrode 30 may be made only of the negative electrode current collector 32 without having the negative electrode active material layer 34. When the lithium-ion secondary battery 100 is charged, metallic lithium is precipitated on the surface of the negative electrode current collector 32. Metallic lithium is elemental lithium with lithium ions precipitated therein, and metallic lithium functions as the negative electrode active material layer 34.

The conductive material and binder can be the same as those in the positive electrode 20. In addition to those listed for the positive electrode 20, the binder in the negative electrode 30 can be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc. The cellulose can be, for example, carboxymethyl cellulose (CMC).

(Terminal)
The terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. The terminals 60 and 62 are preferably protected with insulating tape to prevent short circuits.

(Exterior body)
The power generating element 40 and the non-aqueous electrolyte are sealed inside the exterior body 50. The exterior body 50 prevents the non-aqueous electrolyte from leaking to the outside and prevents moisture and the like from entering the lithium ion secondary battery 100 from the outside.

As shown in FIG. 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

The metal foil 52 may be, for example, aluminum foil. The resin layer 54 may be a polymer film such as polypropylene. The materials constituting the resin layer 54 may be different on the inside and outside. For example, the material on the outside may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the material for the polymer film on the inside may be polyethylene (PE), polypropylene (PP), etc.

(Non-aqueous electrolyte)
The non-aqueous electrolyte is sealed in the exterior body 50 and impregnates the power generating element 40 .
The non-aqueous electrolyte solution includes, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in the non-aqueous solvent.

The non-aqueous solvent contains, for example, a cyclic carbonate and a chain carbonate. The cyclic carbonate solvates the electrolyte. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, and butylene carbonate. It is preferable that the cyclic carbonate contains at least propylene carbonate. The chain carbonate reduces the viscosity of the cyclic carbonate. Examples of the chain carbonate include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. The non-aqueous solvent may also contain methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like.

The non-aqueous solvent may be, for example, a cyclic carbonate or a chain carbonate in which some of the hydrogen atoms are replaced with fluorine. For example, it may have fluoroethylene carbonate, difluoroethylene carbonate, etc.

It is preferable that the ratio of cyclic carbonate to chain carbonate in the non-aqueous solvent is 1:9 to 1:1 by volume.

The electrolyte is, for example, a lithium salt. The electrolyte is, for example, _LiPF6 , LiClO4, _LiBF4 , _LiCF3SO3 , _LiCF3CF2SO3 , LiC _{( CF3SO2} ) _{3 , LiN( CF3SO2 ) 2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2 ) , LiN ( CF3CF2CO)2 , LiBOB , etc. The lithium salt} may be used _alone or in combination of two or more. From the viewpoint of the degree of ionization, _it is preferable that the electrolyte contains _LiPF6 .

The non-aqueous solvent may, for example, have a room temperature molten salt. A room temperature molten salt is a salt that is liquid even at temperatures below 100°C and is obtained by combining a cation and an anion. Since a room temperature molten salt is a liquid composed only of ions, it has strong electrostatic interactions and is characterized by being non-volatile and non-flammable.

Examples of the cationic component of the room-temperature molten salt include nitrogen-based cations containing nitrogen, phosphorus-based cations containing phosphorus, sulfur-based cations containing sulfur, etc. These cationic components may be contained alone or in combination of two or more kinds.
Examples of the nitrogen-based cation include linear or cyclic ammonium cations such as imidazolium cation, pyrrolidinium cation, piperidinium cation, pyridinium cation, and azoniaspiro cation.
The phosphorus-based cation includes linear or cyclic phosphonium cations.
Examples of sulfur-based cations include linear or cyclic sulfonium cations.
As the cationic component, the nitrogen-based cation N-methyl-N-propyl-pyrrolidinium (P13) is particularly preferred because it has high lithium ion conductivity and wide redox resistance when a lithium imide salt is dissolved therein.

The anion components of the room temperature molten salt include AlCl ₄ ^- , NO ₂ ^- , NO ₃ ^- , I ^- , BF ₄ ^- , PF ₆ ^- , AsF ₆ ^- , SbF ₆ ^- , NbF ₆ ^- , TaF ₆ ^- , F(HF) _2.3 ^- , p-CH ₃ PhSO ₃ ^- , CH ₃ CO ₂ ^- , CF ₃ CO ₂ ^- , CH ₃ SO ₃ ^- , CF ₃ SO ₃ ^- , (CF ₃ SO ₂ ) ₃ C ^- , C ₃ F ₇ CO ₂ , C ₄ F ₉ SO ₃ ^- , (FSO ₂ ) ₂ N ^- (bis(fluorosulfonyl)imide) (FSI), (CF ₃ SO ₂ ) _2N- ⁽ bis( _{trifluoromethanesulfonyl} )imide) (TFSI), (C2F5SO2 ^{)2N-(bis(pentafluoroethanesulfonyl)imide), (CF3SO2)(CF3CO)N-} _{( ( trifluoromethanesulfonyl ) (} ^{trifluoromethanecarbonyl} )imide), (CN) _2N- ( _dicyanoimide ⁾ , etc. These anion components may be used alone or in combination of two or more.

"Method of manufacturing lithium-ion secondary batteries"
First, the positive electrode 20 is prepared. The positive electrode 20 is prepared by mixing a positive electrode active material, a binder, and a solvent to prepare a paste-like positive electrode slurry. At least two types of positive electrode active material, a first active material and a second active material, are prepared. The amount of the second active material mixed with respect to the first active material is very small. There is no particular restriction on the method of mixing these components constituting the positive electrode slurry, and there is also no particular restriction on the mixing order. Next, the positive electrode slurry is applied to the positive electrode current collector 22. There is no particular restriction on the application method. For example, a slit die coating method or a doctor blade method can be used.

Then, the solvent in the positive electrode slurry applied onto the positive electrode current collector 22 is removed. There are no particular limitations on the method of removal. For example, the positive electrode current collector 22 to which the positive electrode slurry has been applied is dried in an atmosphere at 80°C to 150°C. The resulting coating is then pressed to densify the positive electrode active material layer 24, thereby obtaining the positive electrode 20. For example, a roll press, a hydrostatic press, or the like can be used as the pressing means.

Next, the negative electrode 30 is prepared. The negative electrode 30 can be prepared in the same manner as the positive electrode 20. The negative electrode 30 is prepared by mixing a negative electrode active material, a binder, and a solvent to prepare a paste-like negative electrode slurry. The negative electrode slurry is applied to the negative electrode current collector 32 and dried to obtain the negative electrode 30. If the negative electrode active material is metallic lithium, lithium foil may be attached to the negative electrode current collector 32.

Next, the prepared positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to prepare the power generating element 40. If the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around the axis.

Finally, the power generating element 40 is enclosed in the exterior body 50. The non-aqueous electrolyte is injected into the exterior body 50. After the non-aqueous electrolyte is injected, the pressure is reduced, heating, etc. is performed, so that the non-aqueous electrolyte is impregnated into the power generating element 40. The exterior body 50 is sealed by applying heat, etc., to obtain the lithium ion secondary battery 100.

The lithium ion secondary battery 100 according to the first embodiment has improved discharge rate characteristics due to the addition of a small amount of the second active material, which is a secondary component, to the first active material, which is a primary component. The reason for this is unclear, but it is believed that the inclusion of a small amount of the second active material, which has a different reaction potential, in the positive electrode active material layer 24 creates a Li ion concentration gradient in the active material layer. It is believed that the charge/discharge reaction is promoted by promoting the movement of Li ions in accordance with the Li ion concentration gradient, thereby improving the discharge rate characteristics. Furthermore, when primary particles are included in the active material, the discharge rate tends to improve. This tendency is particularly noticeable when the second active material is primary particles.

The above describes the embodiments of the present invention in detail with reference to the drawings, but each configuration and their combinations in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications of configurations are possible without departing from the spirit of the present invention.

"Example 1"
(Preparation of Positive Electrode)
A positive electrode active material, a conductive material, and a binder were mixed to prepare a positive electrode mixture. The conductive material was carbon black, and the binder was polyvinylidene fluoride (PVDF). The mass ratio of the positive electrode active material, the conductive material, and the binder was 90:5:5. The positive electrode active material used LiNi _0.33 Mn _0.33 Co _0.33 O ₂ (referred to as NMC333 in Table 1) as the first active material, and LiCoO ₂ (referred to as LCO in Table 1) as the second active material. The abundance ratio of the first active material to the second active material was 99.5% for the first active material and 0.5% for the second active material. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry. Then, the positive electrode slurry was applied to one side of an aluminum foil having a thickness of 15 μm so that the weight per unit area after drying was about 10.0 mg/cm ^2. After application, the positive electrode slurry was applied to the other side of the aluminum foil so that the weight per unit area after drying was about 10.0 mg/cm ^2. After application, the positive electrode slurry was applied to both sides ...

(Preparation of negative electrode)
The negative electrode active material, the conductive material, and the binder were mixed to prepare a negative electrode mixture. The negative electrode active material was graphite, the binder was styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) was added as a thickener. The mass ratio of the negative electrode active material, the binder, and the thickener was 95:3:2. The negative electrode mixture was dispersed in distilled water to prepare a negative electrode slurry. Then, the negative electrode slurry was applied to one side of a copper foil having a thickness of 10 μm so that the weight per unit area after drying was about 6.0 mg/cm ^2. After application, the negative electrode slurry was dried at 100° C. to remove the solvent, and a negative electrode active material layer was formed on one side of the copper foil. After drying, the negative electrode slurry was applied to the other side of the copper foil so that the weight per unit area after drying was about 6.0 mg/cm ^2. After application, the negative electrode slurry was dried at 100° C. to remove the solvent, and a negative electrode active material layer was formed on both sides of the copper foil.

(Cell Preparation)
The negative and positive electrodes thus prepared were punched out into a predetermined shape, and were alternately laminated with 25 μm thick polypropylene separators in between, to prepare a laminate of nine negative electrodes and eight positive electrodes. The laminate prepared in the same manner was cut as a sample, and the morphology of the first active material and the second active material was confirmed by TEM. The first active material was a secondary particle, and the second active material was a primary particle.

The laminate was inserted into an exterior body of an aluminum laminate film and heat-sealed except for one peripheral portion to form an opening. A non-aqueous electrolyte was injected into the exterior body. The non-aqueous electrolyte was prepared by dissolving 1.5 MOL/L of lithium hexafluorophosphate (LiPF ₆ ) in a solvent in which equal amounts of ethylene carbonate (EC) and dimethyl carbonate (DEC) were mixed. The remaining portion was then heat-sealed while reducing the pressure using a vacuum sealer, to produce a lithium ion secondary battery according to Example 1.

(Battery evaluation: discharge rate characteristics)
The discharge rate characteristics of the lithium ion battery thus produced were evaluated. The discharge rate characteristics were determined as the ratio (%) of the discharge capacity at a discharge rate of 3C (current value at which discharge is completed in 20 minutes when constant current discharge is performed at 25°C) to the discharge capacity at a discharge rate of 1C (current value at which discharge is completed in 1 hour when constant current discharge is performed at 25°C) being 100%.

As an evaluation condition for the discharge rate characteristics, the fabricated cell was initially charged and discharged at a current density of 0.15 mA/cm ² , and the actual capacity of the fabricated cell was measured. Based on the obtained actual capacity, the current densities for discharge rates of 1C and 3C were determined.

After the initial charge and discharge, the battery was charged at a constant current of 0.5 C up to 4.3 V, and then charged at a constant voltage until the current density reached 0.02 C. After a 5-minute rest period, the battery was discharged at a constant current of 1 C up to 2.8 V, and the discharge capacity at 1 C was measured, followed by a 5-minute rest period.

Then, constant current charging was performed at 0.5 C up to 4.3 V, and constant voltage charging was performed until the current density reached 0.02 C. After a 5-minute rest period, constant current discharging was performed at 3 C up to 2.8 V, and the discharge capacity at 3 C was measured.

"Examples 2 to 5, Comparative Examples 1 and 2"
Examples 2 to 5 and Comparative Examples 1 and 2 differ from Example 1 in that the ratio of the first active material to the second active material was changed. Comparative Example 1 consisted of only the first active material.
In Comparative Example 1, the first active material was 100.0% and the second active material was 0.0%.
In Example 2, the first active material was 99.0% and the second active material was 1.0%.
In Example 3, the first active material was 97.0% and the second active material was 3.0%.
In Example 4, the first active material was 96.0% and the second active material was 4.0%.
In Example 5, the first active material was 96.5% and the second active material was 4.5%.
In Comparative Example 2, the first active material was 95.0% and the second active material was 5.0%.
The discharge rate characteristics were measured for Examples 2 to 5 and Comparative Examples 1 and 2. Table 1 shows the percentages of the rate characteristics of Examples 2 to 5 and Comparative Example 2 when the rate characteristic of Comparative Example 1 is taken as 100%. In other words, the rate characteristics of Examples 2 to 5 and Comparative Example 2 shown in Table 1 are relative values to the rate characteristic of Comparative Example 1.

"Examples 6 to 10, Comparative Examples 3 and 4"
Examples 6 to 10 and Comparative Examples 3 and 4 differ from Examples 1 to 5 and Comparative Examples 1 and 2 in that LiNi _0.6 Mn _0.2 Co _0.2 O ₂ (referred to as NMC622 in Table 1) was used as the first active material.
The discharge rate characteristics were measured for Examples 6 to 10 and Comparative Examples 3 and 4. Table 1 shows the percentages of the rate characteristics of Examples 6 to 10 and Comparative Example 4 when the rate characteristic of Comparative Example 3 is taken as 100%. In other words, the rate characteristics of Examples 6 to 10 and Comparative Example 4 shown in Table 1 are relative values to the rate characteristic of Comparative Example 3.

"Examples 11 to 15, Comparative Examples 5 and 6"
Examples 11 to 15 and Comparative Examples 5 and 6 differ from Examples 1 to 5 and Comparative Examples 1 and 2 in that LiNi _0.85 Co _0.10 Al _0.05 O ₂ (referred to as NCA in Table 1) was used as the first active material.
The discharge rate characteristics were measured for Examples 11 to 15 and Comparative Examples 5 and 6. Table 1 shows the percentages of the rate characteristics of Examples 11 to 15 and Comparative Example 6 when the rate characteristic of Comparative Example 5 is taken as 100%. In other words, the rate characteristics of Examples 11 to 15 and Comparative Example 6 shown in Table 1 are relative values to the rate characteristic of Comparative Example 5.

"Examples 16 to 20, Comparative Examples 7 and 8"
Examples 16 to 20 and Comparative Examples 7 and 8 differ from Examples 1 to 5 and Comparative Examples 1 and 2 in that LiFePO ₄ (referred to as LFP in Table 2) was used as the first active material.
The discharge rate characteristics were measured for Examples 16 to 20 and Comparative Examples 7 and 8. Table 2 shows the percentages of the rate characteristics of Examples 16 to 20 and Comparative Example 8 when the rate characteristic of Comparative Example 7 is taken as 100%. That is, the rate characteristics of Examples 16 to 20 and Comparative Example 8 shown in Table 2 are relative values to the rate characteristic of Comparative Example 7.

"Examples 21 to 25, Comparative Examples 9 and 10"
Examples 21 to 25 and Comparative Examples 9 and 10 differ from Examples 1 to 5 and Comparative Examples 1 and 2 in that LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (referred to as NMC811 in Table 2) was used as the first active material.
The discharge rate characteristics were measured for Examples 21 to 25 and Comparative Examples 9 and 10. Table 2 shows the percentages of the rate characteristics of Examples 21 to 25 and Comparative Example 10 when the rate characteristic of Comparative Example 9 is taken as 100%. That is, the rate characteristics of Examples 21 to 25 and Comparative Example 10 shown in Table 2 are relative values to the rate characteristic of Comparative Example 9.

"Examples 26 to 30, Comparative Examples 11 and 12"
Examples 26 to 30 and Comparative Examples 11 and 12 differ from Examples 1 to 5 and Comparative Examples 1 and 2 in that the first active material is LiCoO ₂ (referred to as LCO in Table 3) and the second active material is LiMn ₂ O ₄ (referred to as LMO in Table 3) having a spinel structure.
The discharge rate characteristics were measured for Examples 26 to 30 and Comparative Examples 11 and 12. Table 3 shows the ratios of the rate characteristics of Examples 26 to 30 and Comparative Example 12 when the rate characteristic of Comparative Example 11 is taken as 100%. That is, the rate characteristics of Examples 26 to 30 and Comparative Example 12 shown in Table 3 are relative values to the rate characteristic of Comparative Example 11.

"Examples 31 to 35, Comparative Examples 13 and 14"
Examples 31 to 35 and Comparative Examples 13 and 14 differ from Examples 26 to 30 and Comparative Examples 11 and 12 in that the second active material was NMC333.
The discharge rate characteristics were measured for Examples 31 to 35 and Comparative Examples 13 and 14. Table 3 shows the ratios of the rate characteristics of Examples 31 to 35 and Comparative Example 14 when the rate characteristic of Comparative Example 13 is taken as 100%. That is, the rate characteristics of Examples 31 to 35 and Comparative Example 14 shown in Table 3 are relative values to the rate characteristic of Comparative Example 13.

"Examples 36 to 40, Comparative Examples 15 and 16"
Examples 36 to 40 and Comparative Examples 15 and 16 differ from Examples 26 to 30 and Comparative Examples 11 and 12 in that the second active material was NMC622.
The discharge rate characteristics were measured for Examples 36 to 40 and Comparative Examples 15 and 16. Table 3 shows the ratios of the rate characteristics of Examples 36 to 40 and Comparative Example 16 when the rate characteristic of Comparative Example 15 is taken as 100%. That is, the rate characteristics of Examples 36 to 40 and Comparative Example 16 shown in Table 3 are relative values to the rate characteristic of Comparative Example 15.

"Examples 41 to 45, Comparative Examples 17 and 18"
Examples 41 to 45 and Comparative Examples 17 and 18 differ from Examples 26 to 30 and Comparative Examples 11 and 12 in that the second active material was LiFePO ₄ (referred to as LFP in Table 1).
The discharge rate characteristics were measured for Examples 41 to 45 and Comparative Examples 17 and 18. Table 4 shows the ratios of the rate characteristics of Examples 41 to 45 and Comparative Example 18 when the rate characteristic of Comparative Example 17 is taken as 100%. That is, the rate characteristics of Examples 41 to 45 and Comparative Example 18 shown in Table 4 are relative values to the rate characteristic of Comparative Example 17.

"Examples 46 to 50, Comparative Examples 19 and 20"
Examples 46 to 50 and Comparative Examples 19 and 20 differ from Examples 1 to 5 and Comparative Examples 1 and 2 in that three types of active materials were used. The first active material was LCO, the second active material was NMC333, and the third active material was NMC622. The abundance ratio of the first active material was fixed at 62%, and the abundance ratio of the second active material and the third active material was changed.

In Comparative Example 19, the first active material was 62.0%, the second active material was 0.0%, and the third active material was 38.0%.
In Example 46, the first active material was 62.0%, the second active material was 0.5%, and the third active material was 37.5%.
In Example 47, the first active material was 62.0%, the second active material was 1.0%, and the third active material was 37.0%.
In Example 48, the first active material was 62.0%, the second active material was 3.0%, and the third active material was 35.0%.
In Example 49, the first active material was 62.0%, the second active material was 4.0%, and the third active material was 34.0%.
In Example 50, the first active material was 62.0%, the second active material was 4.5%, and the third active material was 33.5%.
In Comparative Example 20, the first active material was 62.0%, the second active material was 5.0%, and the third active material was 33.0%.
The discharge rate characteristics were measured for Examples 46 to 50 and Comparative Examples 19 and 20. Table 4 shows the ratios of the rate characteristics of Examples 46 to 50 and Comparative Example 20 when the rate characteristic of Comparative Example 19 is taken as 100%. That is, the rate characteristics of Examples 46 to 50 and Comparative Example 20 shown in Table 4 are relative values to the rate characteristic of Comparative Example 19.

"Examples 51 to 56"
Examples 51 to 53 differ from Example 3 in that the forms of the first and second active materials were changed.
Examples 54 to 56 differ from Example 8 in that the forms of the first and second active materials were changed.
The discharge rate characteristics of Examples 51 to 56 were measured. The results are shown in Table 5. The rate characteristics of Examples 51 to 53 in Table 5 are relative values to Comparative Example 1, and the rate characteristics of Examples 54 to 56 are relative values to Comparative Example 3. Table 5 also shows the results of Examples 3 and 8 for comparison.

"Examples 57 to 70"
Examples 57 to 63 differ from Example 3 in that the relationship between the particle sizes of the first and second active materials was changed.
Examples 64 to 70 differ from Example 8 in that the forms of the first and second active materials were changed.
The discharge rate characteristics of Examples 57 to 70 were measured. The results are shown in Table 6. The rate characteristics of Examples 57 to 63 are relative values to Comparative Example 1, and the rate characteristics of Examples 64 to 70 are relative values to Comparative Example 3. Table 6 also shows the results of Examples 3 and 8 for comparison.

REFERENCE SIGNS LIST 10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 30 Negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior body 52 Metal foil 54 Resin layers 60, 62 Terminal 100 Lithium ion secondary battery

Claims (7)

At least an active material and a binder;
The active material includes a first active material as a main component and a second active material as a subcomponent having a composition different from that of the first active material,
A ratio of the first active material is 60% or more and less than 100% of the total active material,
The ratio of the second active material is greater than 0% and less than 5% of the total active material,
The combination of the first active material and the second active material is
The active material layer, wherein the first active material is a nickel-cobalt-manganese ternary compound or a nickel-cobalt-aluminum ternary compound, and the second active material is lithium cobalt oxide. At least an active material and a binder;
The active material includes a first active material as a main component and a second active material as a subcomponent having a composition different from that of the first active material,
A ratio of the first active material is 60% or more and less than 100% of the total active material,
The ratio of the second active material is greater than 0% and less than 5% of the total active material,
The combination of the first active material and the second active material is
The first active material is a nickel-cobalt-manganese ternary compound or a nickel-cobalt-aluminum ternary compound, and the second active material is lithium cobalt oxide; or
The first active material is lithium cobalt oxide, and the second active material is any one selected from the group consisting of a nickel-cobalt-manganese ternary compound, a nickel-cobalt-aluminum ternary compound, and lithium iron phosphate;
the first active material is a secondary particle formed by agglomeration of a plurality of primary particles,
The active material layer, wherein the second active material is a primary particle. The active material layer according to claim 1 or 2, wherein the particle size of the first active material is 0.6 to 1.5 times the particle size of the second active material. At least an active material and a binder;
The active material includes a first active material as a main component and a second active material as a subcomponent having a composition different from that of the first active material,
A ratio of the first active material is 60% or more and less than 100% of the total active material,
The ratio of the second active material is greater than 0% and less than 5% of the total active material,
The combination of the first active material and the second active material is
The first active material is a nickel-cobalt-manganese ternary compound or a nickel-cobalt-aluminum ternary compound, and the second active material is lithium cobalt oxide; or
The first active material is lithium cobalt oxide, and the second active material is any one selected from the group consisting of a nickel-cobalt-manganese ternary compound, a nickel-cobalt-aluminum ternary compound, and lithium iron phosphate;
The reaction potential of the second active material is higher than the reaction potential of the first active material. The second active material is represented by Li _a Co _1-y M _y O ₂ ... (1),
a satisfies 0.8≦a≦1.2 and y satisfies 0≦y<0.5,
The active material layer according to any one of claims 1 to 4, wherein M is at least one of Ni, Mn, Fe, Mg, Zr, Al, Ti, Zn, Cu, Ca, and Na. A current collector;
An electrode comprising: the active material layer according to any one of claims 1 to 5 laminated on at least one surface of the current collector. A lithium ion secondary battery comprising the electrode of claim 6.

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