JP2022145473A - Cathode for nonaqueous electrolyte secondary battery, nonaqueous electrolyte secondary battery using the same, battery module, and battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the high-rate cycle characteristic of a nonaqueous electrolyte secondary battery.

SOLUTION: A cathode for a nonaqueous electrolyte secondary battery has: a cathode collector 11; and a cathode active material layer 12 being present on the cathode collector 11. The cathode active material layer 12 has one or more cathode active material particles containing a cathode active material; a true density D of the cathode active material and a true density D1 of the cathode active material layer satisfy 0.96D≤D1<D. It is preferred that the cathode active material contains a compound represented by the general formula, LiFe_xM_(1-x)PO₄ (where 0≤x≤1, and M is Co, Ni, Mn, Al, Ti or Zr).

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2023,JPO&INPIT

Description

The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery, a battery module, and a battery system using the same.

A non-aqueous electrolyte secondary battery is generally composed of a positive electrode, a non-aqueous electrolyte, a negative electrode, and a separation membrane (separator) placed between the positive electrode and the negative electrode.
As a positive electrode for non-aqueous electrolyte secondary batteries, there is known one in which a composition comprising a positive electrode active material containing lithium ions, a conductive aid, and a binder is adhered to the surface of a metal foil (current collector). ing.
Examples of positive electrode active materials containing lithium ions include lithium transition metal composite oxides such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and lithium iron phosphate ( LiFePO ₄ ) and other lithium phosphate compounds have been put to practical use.

Patent Document 1 describes a positive electrode in which a positive electrode active material layer composed of a lithium phosphate compound, a binder, and a conductive aid is provided on an aluminum foil. In the positive electrode active material layer, the ratio of pores caused by the primary particles and the pores caused by the secondary particles of the lithium phosphate compound is set to a specific ratio, and the porosity is set to a specific range, thereby improving cycle identification. Examples are given.

Among lithium phosphate compounds, lithium iron phosphate has a high electrical resistance, so it is necessary to improve the performance by reducing the resistance.
Non-Patent Document 1 reports that the battery capacity is improved by coating the surface of the iron phosphate-based active material with carbon.

JP 2014-13748 A

I.Belharouak, C.Johnson, K.Amine, Synthesis and electrochemical analysis of vapor-deposited carbon-coated LiFePO4, Electrochemistry Communications, Volume 7, Issue 10, October 2005, Pages 983-988

However, these methods are not always satisfactory, and further improvement in battery characteristics is required.
The present invention provides a positive electrode for a non-aqueous electrolyte secondary battery that can improve the high rate cycle characteristics of the non-aqueous electrolyte secondary battery.

The present invention has the following aspects.
<1>
Having a positive electrode current collector and a positive electrode active material layer present on the positive electrode current collector,
The positive electrode active material layer has one or more positive electrode active material particles containing a positive electrode active material,
A positive electrode for a non-aqueous electrolyte secondary battery, wherein the true density D of the positive electrode active material and the true density D1 of the positive electrode active material layer satisfy the following formula (s).
0.96D≦D1<D (s)
<2>
The positive electrode active material is a compound represented by the general formula LiFe _x M _(1-x) PO ₄ (where 0≦x≦1 and M is Co, Ni, Mn, Al, Ti or Zr). The positive electrode for a non-aqueous electrolyte secondary battery according to <1>, comprising:
<3>
The positive electrode for a non-aqueous electrolyte secondary battery according to <2>, wherein the positive electrode active material is lithium iron phosphate represented by LiFePO ₄ .
<4>
The positive electrode for a nonaqueous electrolyte secondary battery according to <3>, wherein the true density D1 is 3.4 g/cm ³ or more and less than 3.6 g/cm ³ .
<5>
The positive electrode active material layer contains a conductive aid and a binder,
The content of the conductive aid is 1% by mass or less with respect to the total mass of the positive electrode active material layer,
The positive electrode for a nonaqueous electrolyte secondary battery according to any one of <1> to <4>, wherein the content of the binder is 1% by mass or less with respect to the total mass of the positive electrode active material layer.
<6>
The positive electrode for a non-aqueous electrolyte secondary battery according to any one of <1> to <5>, wherein the positive electrode active material layer does not contain a conductive aid.
<7>
Some or all of the positive electrode active material particles have a core portion of the positive electrode active material and a covering portion covering the core portion,
the covering includes a conductive material;
The positive electrode for a nonaqueous electrolyte secondary battery according to any one of <1> to <6>, wherein the content of the conductive material is 1.3% by mass or less with respect to the total mass of the positive electrode active material particles.
<8>
The positive electrode for a non-aqueous electrolyte secondary battery according to any one of <1> to <7>, which has a cycle capacity retention rate of 80% or more as determined by the following test method.
(Test method)
A non-aqueous electrolyte secondary battery with a rated capacity of 1 Ah, charging at 3C rate, 3.8 V, resting for 10 seconds, then discharging at 3 C rate, 2.0 V, resting for 10 seconds, repeating 1000 charge-discharge cycles. Measure the discharge capacity B when discharged at 0.2 C rate and 2.5 V, and divide the discharge capacity B by the discharge capacity A of the non-aqueous electrolyte secondary battery before being subjected to the charge / discharge cycle to obtain the cycle capacity maintenance rate ( %).
<9>
The positive electrode for the non-aqueous electrolyte secondary battery according to any one of <1> to <8>, the negative electrode, and the non-aqueous electrolyte present between the positive electrode for the non-aqueous electrolyte secondary battery and the negative electrode, A non-aqueous electrolyte secondary battery.
<10>
A battery module or battery system comprising a plurality of the non-aqueous electrolyte secondary batteries according to <9>.

According to the positive electrode for a non-aqueous electrolyte secondary battery of the present invention, the high rate cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

1 is a cross-sectional view schematically showing an example of a positive electrode for a non-aqueous electrolyte secondary battery according to the present invention; FIG. 1 is a cross-sectional view schematically showing an example of a non-aqueous electrolyte secondary battery according to the present invention; FIG.

In the present specification and claims, "-" indicating a numerical range means that the numerical values before and after it are included as lower and upper limits.
FIG. 1 is a schematic cross-sectional view showing one embodiment of a positive electrode for a non-aqueous electrolyte secondary battery of the present invention, and FIG. 2 is a schematic cross-sectional view showing one embodiment of a non-aqueous electrolyte secondary battery of the present invention. .
1 and 2 are schematic diagrams for explaining the configuration in an easy-to-understand manner, and the dimensional ratios and the like of each component may differ from the actual ones.

<Positive electrode for non-aqueous electrolyte secondary battery>
A positive electrode for a non-aqueous electrolyte secondary battery (also simply referred to as “positive electrode”) 1 of this embodiment has a positive electrode current collector 11 and a positive electrode active material layer 12 .
The positive electrode active material layer 12 exists on at least one surface of the positive electrode current collector 11 . A positive electrode active material layer 12 may be present on both surfaces of the positive electrode current collector 11 .
In the example of FIG. 1, the positive electrode current collector 11 has a positive electrode current collector main body 14 and a current collector coating layer 15 that covers the surface of the positive electrode current collector main body 14 on the positive electrode active material layer 12 side. Only the positive electrode current collector main body 14 may be used as the positive electrode current collector 11 .

[Positive electrode active material layer]
The positive electrode active material layer 12 has one or more positive electrode active material particles. The positive electrode active material layer 12 may further contain a binder. The positive electrode active material layer 12 may further contain a conductive aid.
The positive electrode active material particles contain a positive electrode active material. The positive electrode active material particles may be particles composed only of the positive electrode active material, or may have a core portion of the positive electrode active material and a coating portion (active material coating portion) covering the core portion (so-called coated particles ). At least part of the group of positive electrode active material particles contained in the positive electrode active material layer 12 is preferably coated particles.
The content of the positive electrode active material is preferably 80.0 to 99.9% by mass, more preferably 90.0 to 99.5% by mass, based on the total mass of the positive electrode active material layer 12 .

The positive electrode active material preferably contains a compound having an olivine crystal structure.
A compound having an olivine-type crystal structure is preferably a compound represented by the general formula LiFe _x M _(1-x) PO ₄ (hereinafter also referred to as “general formula (I)”). 0≦x≦1 in general formula (I). M is Co, Ni, Mn, Al, Ti or Zr. A small amount of Fe and M (Co, Ni, Mn, Al, Ti or Zr) can be substituted with other elements to the extent that the physical properties are not changed. Even if the compound represented by the general formula (I) contains a trace amount of metal impurities, the effects of the present invention are not impaired.
The compound represented by the general formula (I) is preferably lithium iron phosphate represented by LiFePO ₄ (hereinafter also simply referred to as “lithium iron phosphate”).

The positive electrode active material may contain a positive electrode active material other than the compound having an olivine crystal structure.
Another positive electrode active material is preferably a lithium transition metal composite oxide. For example, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium nickel cobalt oxide ( _{LiNixCoyAlzO2} , where _x + _y + _{z =} 1), lithium nickel cobalt manganate _{( LiNixCoyMnz O2} , where x+y+z= ₁ ), lithium manganate ₍ LiMn2O4), lithium cobalt manganate (LiMnCoO4), lithium manganese chromate ( _LiMnCrO4 ), lithium vanadium nickelate ( _LiNiVO4 ), nickel _- substituted manganates Lithium (for example, LiMn _1.5 Ni _0.5 O ₄ ), lithium vanadium cobaltate (LiCoVO ₄ ), non-stoichiometric compounds obtained by substituting a part of these compounds with metal elements, and the like. Examples of the metal element include one or more selected from the group consisting of Mn, Mg, Ni, Co, Cu, Zn and Ge.
1 type may be sufficient as another positive electrode active material, and 2 or more types may be sufficient as it.

The positive electrode active material particles of the present embodiment are preferably coated particles in which at least part of the surface of the positive electrode active material is coated with a conductive material. By using the coated particles as the positive electrode active material particles, the battery capacity and high rate cycle characteristics can be further enhanced.

The conductive material of the active material coating preferably contains carbon. The conductive material may consist of carbon only, or may be a conductive organic compound containing carbon and other elements than carbon. Nitrogen, hydrogen, oxygen and the like can be exemplified as other elements. In the conductive organic compound, the content of other elements is preferably 10 atomic % or less, more preferably 5 atomic % or less.
It is more preferable that the conductive material forming the active material coating portion consist of carbon only.

It is more preferable that the conductive material forming the active material coating portion consist of carbon only.
The content of the conductive material is preferably 0.1 to 3.0% by mass, more preferably 0.5 to 1.5% by mass, and 0.7 based on the total mass of the positive electrode active material having the active material coating. ~1.3% by mass is more preferred.

The coated particles are preferably coated particles having a core composed of a compound having an olivine-type crystal structure, more preferably coated particles having a core composed of a compound represented by general formula (I), and lithium iron phosphate as a core. Coated particles (hereinafter also referred to as “coated lithium iron phosphate”) are more preferable. These coated particles can improve battery capacity and cycle characteristics.
In addition, it is particularly preferable for the coated particles that the entire surface of the core is coated with a conductive material.

Coated particles can be produced by known methods. A method for producing coated particles will be described below using coated lithium iron phosphate as an example.
For example, lithium iron phosphate powder is prepared using the method described in Japanese Patent No. 5098146, and described in GS Yuasa Technical Report, June 2008, Vol. 5, No. 1, pp. 27-31. The method can be used to coat at least a portion of the surface of the lithium iron phosphate powder with carbon.
Specifically, first, iron oxalate dihydrate, ammonium dihydrogen phosphate, and lithium carbonate are weighed in a specific molar ratio, and these are pulverized and mixed under an inert atmosphere. Next, a lithium iron phosphate powder is produced by heat-treating the obtained mixture in a nitrogen atmosphere. Next, the lithium iron phosphate powder is placed in a rotary kiln and heat-treated while supplying methanol vapor using nitrogen as a carrier gas to obtain a lithium iron phosphate powder having at least a portion of the surface coated with carbon.
For example, the particle size of the lithium iron phosphate powder can be adjusted by adjusting the pulverization time in the pulverization step. The amount of carbon covering the lithium iron phosphate powder can be adjusted by the heating time and temperature in the step of heat-treating while supplying methanol vapor. It is desirable to remove the uncoated carbon particles by subsequent steps such as classification and washing.
Other positive electrode active materials may have the active material coating on at least part of the surface.

The content of the coated particles is preferably 50% by mass or more, more preferably 80% by mass or more, still more preferably 90% by mass or more, and may be 100% by mass with respect to the total mass of the positive electrode active material particles.

The content of the compound having an olivine crystal structure is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, relative to the total mass of the positive electrode active material. 100 mass % may be sufficient.
When coated lithium iron phosphate is used, the content of coated lithium iron phosphate is preferably 50% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more, relative to the total mass of the positive electrode active material particles. preferable. 100 mass % may be sufficient.

The content of the positive electrode active material particles is preferably 90% by mass or more, more preferably 95% by mass or more, still more preferably over 99% by mass, and 99.5% by mass or more with respect to the total mass of the positive electrode active material layer 12. is particularly preferred, and may be 100% by mass. If the content of the positive electrode active material particles is at least the above lower limit, the battery capacity and cycle characteristics are improved.

The group of positive electrode active material particles (that is, the powder of positive electrode active material particles) preferably has an average particle size of, for example, 0.1 to 20.0 μm, more preferably 0.2 to 10.0 μm. When two or more kinds of positive electrode active material particles are used, each average particle size should be within the above range.
The average particle size of a group of positive electrode active material particles in the present specification is a volume-based median size measured using a particle size distribution analyzer based on a laser diffraction/scattering method.

The binder contained in the positive electrode active material layer 12 is an organic substance, and examples thereof include polyacrylic acid, lithium polyacrylate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber, polyvinyl alcohol, Polyvinyl acetal, polyethylene oxide, polyethylene glycol, carboxymethyl cellulose, polyacrylonitrile, polyimide and the like. One type of binder may be used, or two or more types may be used in combination.
The content of the binder in the positive electrode active material layer 12 is, for example, preferably 1% by mass or less, more preferably 0.5% by mass or less, relative to the total mass of the positive electrode active material layer 12 . If the content of the binder is equal to or less than the above upper limit, the proportion of the material that does not contribute to the conduction of lithium ions in the positive electrode active material layer 12 is reduced, the true density of the positive electrode active material layer 12 is increased, and furthermore, By reducing the ratio of the binder covering the surface of the positive electrode 1 and increasing the conductivity of lithium, the high rate cycle characteristics can be further improved.
When the positive electrode active material layer 12 contains a binder, the lower limit of the content of the binder is preferably 0.1% by mass or more with respect to the total mass of the positive electrode active material layer 12 .

Examples of conductive aids contained in the positive electrode active material layer 12 include carbon materials such as graphite, graphene, hard carbon, ketjen black, acetylene black, and carbon nanotubes (CNT). One type of conductive aid may be used, or two or more types may be used in combination.
The content of the conductive aid in the positive electrode active material layer 12 is, for example, preferably 1 part by mass or less, more preferably 0.5% by mass or less, and 0.2% by mass with respect to the total mass of the positive electrode active material layer 12. The following are more preferable, and 0% by mass (that is, containing no conductive aid) is particularly preferable. If the content of the conductive aid is equal to or less than the above upper limit, the ratio of the material that does not contribute to the conduction of lithium ions in the positive electrode active material layer 12 is reduced, the true density of the positive electrode active material layer 12 is increased, and the rate is increased. Cycle characteristics can be further improved.
When the positive electrode active material layer 12 is blended with the conductive aid, the lower limit of the conductive aid is appropriately determined according to the type of the conductive aid. % by mass.
It should be noted that the fact that the positive electrode active material layer 12 "does not contain a conductive aid" means that it does not substantially contain any conductive aid, and does not exclude substances contained to such an extent that the effect of the present invention is not affected. For example, if the content of the conductive aid is 0.1% by mass or less with respect to the total mass of the positive electrode active material layer 12, it can be determined that it is not substantially contained.

[Positive collector]
Examples of the material forming the positive electrode current collector main body 14 include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel.
The thickness of the positive electrode current collector main body 14 is, for example, preferably 8 to 40 μm, more preferably 10 to 25 μm.
The thickness of the positive electrode current collector main body 14 and the thickness of the positive electrode current collector 11 can be measured using a micrometer. An example of the measuring instrument is Mitutoyo's product name "MDH-25M".

[Current collector coating layer]
Current collector coating layer 15 includes a conductive material.
The conductive material in the current collector coating layer 15 preferably contains carbon, and more preferably a conductive material consisting only of carbon.
The current collector coating layer 15 is preferably a coating layer containing carbon particles such as carbon black and a binder. The binder for the current collector coating layer 15 can be exemplified by the same binder as the binder for the positive electrode active material layer 12 .
The positive electrode current collector 11 in which the surface of the positive electrode current collector main body 14 is coated with the current collector coating layer 15 is coated with a slurry containing a conductive material, a binder, and a solvent by a known coating method such as a gravure method. can be produced by coating the surface of the positive electrode current collector main body 14 using

The thickness of the current collector coating layer 15 is preferably 0.1 to 4.0 μm.
The thickness of the current collector coating layer can be measured by measuring the thickness of the coating layer in a cross-sectional electron microscope (SEM, TEM) image of the current collector coating layer. The thickness of the current collector coating layer may not be uniform. A current collector coating layer having a thickness of 0.1 μm or more is present on at least a portion of the surface of the positive electrode current collector main body 14, and the maximum thickness of the current collector coating layer is preferably 4.0 μm or less. .

[Manufacturing method of positive electrode]
The positive electrode 1 of the present embodiment is produced by, for example, applying a positive electrode manufacturing composition containing a positive electrode active material, a binder, and a solvent onto a positive electrode current collector 11, drying the solvent, and removing the solvent to obtain a positive electrode active material. It can be manufactured by the method of forming layer 12 . The composition for positive electrode production may contain a conductive aid.
The thickness of the positive electrode active material layer 12 can be adjusted by a method in which a laminate in which the positive electrode active material layer 12 is formed on the positive electrode current collector 11 is sandwiched between two flat jigs and is evenly pressed in the thickness direction. . For example, a method of applying pressure using a roll press can be used.

A non-aqueous solvent is preferable as the solvent for the positive electrode-manufacturing composition. Examples thereof include alcohols such as methanol, ethanol, 1-propanol and 2-propanol; linear or cyclic amides such as N-methylpyrrolidone and N,N-dimethylformamide; and ketones such as acetone. One solvent may be used, or two or more solvents may be used in combination.

The positive electrode active material layer 12 may contain a dispersant. Dispersants include, for example, polyvinylpyrrolidone (PVP) and one-shot varnish (manufactured by Toyocolor Co., Ltd.).

When at least one of the conductive material covering the positive electrode active material and the conductive aid contains carbon, the content of conductive carbon is 0.5 with respect to the mass of the remainder after removing the positive electrode current collector main body 14 from the positive electrode 1. ~3.5% by mass is preferable, and 1.5 to 3.0% by mass is more preferable.
When the positive electrode 1 is composed of the positive electrode current collector main body 14 and the positive electrode active material layer 12 , the mass of the positive electrode 1 excluding the positive electrode current collector main body 14 is the mass of the positive electrode active material layer 12 .
When the positive electrode 1 is composed of the positive electrode current collector main body 14, the current collector coating layer 15, and the positive electrode active material layer 12, the mass of the remainder after removing the positive electrode current collector main body 14 from the positive electrode 1 is the current collector coating layer 15. and the total mass of the positive electrode active material layer 12 .
When the content of the conductive carbon is within the above range with respect to the total mass of the positive electrode active material layer 12, the battery capacity can be further improved, and a non-aqueous electrolyte secondary battery having better cycle characteristics can be realized. .
The content of conductive carbon with respect to the mass of the remainder of the positive electrode 1 excluding the positive electrode current collector main body 14 is obtained by removing the entire amount of the layer existing on the positive electrode current collector main body 14 and vacuum-drying it in a 120 ° C. environment. (Powder) can be measured by the following <<Method for measuring conductive carbon content>>.
The content of conductive carbon measured by the following «Method for measuring conductive carbon content» includes carbon in the active material coating portion, carbon in the conductive aid, and carbon in the current collector coating layer 15. . Carbon in the binder is not included.

As a method for obtaining the measurement object, for example, the following method can be used.
First, the positive electrode 1 is punched out to an arbitrary size, immersed in a solvent (for example, N-methylpyrrolidone) and stirred to completely peel off the layer (powder) present on the positive electrode current collector body 14 . After confirming that no powder adheres to the positive electrode current collector main body 14, the positive electrode current collector main body 14 is removed from the solvent to obtain a suspension (slurry) containing the removed powder and the solvent. The suspension thus obtained is dried at 120° C. to completely volatilize the solvent to obtain the target measurement object (powder).

<<Method for measuring conductive carbon content>>
[Measurement method A]
The object to be measured is uniformly mixed, a sample (mass w1) is weighed, and thermogravimetric suggestive heat (TG-DTA) measurement is performed in the following steps A1 and A2 to obtain a TG curve. From the obtained TG curve, the following first weight reduction amount M1 (unit: mass %) and second weight reduction amount M2 (unit: mass %) are determined. Subtract M1 from M2 to obtain the content of conductive carbon (unit: % by mass).
Step A1: In an argon stream of 300 mL/min, the temperature is raised from 30° C. to 600° C. at a rate of temperature increase of 10° C./min and held at 600° C. for 10 minutes. A first weight reduction amount M1 is obtained.
M1=(w1-w2)/w1×100 (a1)
Step A2: Immediately after step A1, the temperature is lowered from 600° C. at a rate of 10° C./min, held at 200° C. for 10 minutes, and then the measurement gas is completely replaced from argon to oxygen with an oxygen flow of 100 mL/min. Inside, the temperature is increased from 200 ° C. to 1000 ° C. at a temperature increase rate of 10 ° C./min, and the mass w3 when held at 1000 ° C. for 10 minutes is calculated by the following formula (a2) to obtain the second weight reduction amount M2 ( Unit: % by mass).
M2=(w1-w3)/w1×100 (a2)

[Measurement method B]
The object to be measured is uniformly mixed and 0.0001 mg of the sample is precisely weighed, the sample is burned under the following combustion conditions, the carbon dioxide generated is quantified by a CHN elemental analyzer, and the total carbon content M3 ( Unit: % by mass). In addition, the first weight reduction amount M1 is obtained by the procedure of step A1 of the measuring method A described above. Subtract M1 from M3 to obtain the conductive carbon content (unit: % by mass).
[Combustion conditions]
Combustion furnace: 1150°C
Reduction furnace: 850°C
Helium flow rate: 200 mL/min Oxygen flow rate: 25-30 mL/min

[Measurement method C]
The total carbon content M3 (unit: % by mass) contained in the sample is measured in the same manner as in the measurement method B above. Also, the binder-derived carbon content M4 (unit: % by mass) is determined by the following method. Subtract M4 from M3 to obtain the conductive carbon content (unit: % by mass).
When the binder is polyvinylidene fluoride (PVDF: monomer (CH ₂ CF ₂ ) molecular weight 64), the content of fluoride ions (F ⁻ ) measured by combustion ion chromatography using a tubular combustion method ( Unit: % by mass), the fluorine atomic weight (19) of the monomer constituting PVDF, and the atomic weight (12) of carbon constituting PVDF by the following formula.
PVDF content (unit: mass%) = fluoride ion content (unit: mass%) x 64/38
PVDF-derived carbon content M4 (unit: mass%) = fluoride ion content (unit: mass%) x 12/19
The fact that the binder is polyvinylidene fluoride is confirmed by Fourier transform infrared spectrum (FT-IR) measurement of the sample or the liquid obtained by extracting the sample with N-N dimethylformamide (DMF) solvent, and the C-F bond derived It can be confirmed by a method for confirming absorption. It can also be confirmed by ¹⁹ F-NMR measurement.
If the binder is identified to be other than PVDF, the content of the binder (unit: mass %) corresponding to the molecular weight and the content of carbon (unit: mass %) of carbon content M4 can be calculated.
These techniques are described in the following publications.
Toray Research Center The TRC News No. 117 (Sep. 2013) pp. 34-37, [searched on February 10, 2021], Internet <https://www.toray-research.co.jp/technical-info/trcnews/pdf/TRC117(34- 37).pdf>.
Tosoh Analysis Center Technical Report No. T1019 2017.09.20, [retrieved on February 10, 2021], Internet <http://www.tosoh-arc.co.jp/techrepo/files/tarc00522/T1719N.pdf>.

<<Method for analyzing conductive carbon>>
The conductive carbon that constitutes the active material coating portion of the positive electrode active material and the conductive carbon that is the conductive aid can be distinguished by the following analysis method.
For example, the particles in the positive electrode active material layer are analyzed by transmission electron microscope electron energy loss spectroscopy (TEM-EELS), and particles having a carbon-derived peak near 290 eV only in the vicinity of the particle surface are positive electrode active materials, Particles in which carbon-derived peaks are present even inside the particles can be determined to be conductive aids.
As another method, the particles in the positive electrode active material layer are subjected to mapping analysis by Raman spectroscopy. Particles that are positive electrode active materials and in which only the G-band and D-band are observed can be determined as conductive aids. A small amount of carbon considered as an impurity, a small amount of carbon unintentionally peeled off from the surface of the positive electrode active material during production, and the like are not determined to be conductive aids.
Using these methods, it is possible to confirm whether or not the positive electrode active material layer contains a conductive aid made of a carbon material.

[Pore Specific Surface Area and Central Pore Diameter of Positive Electrode Active Material Layer]
The pore specific surface area of the positive electrode active material layer 12 of the present embodiment is preferably 5.0 to 10.0 m ² /g, more preferably 6.0 to 9.5 m ² /g, and more preferably 7.0 to 9.0 m 2 /g. ² /g is more preferred.
The center pore diameter of the positive electrode active material layer 12 of the present embodiment is preferably 0.06 to 0.150 μm, more preferably 0.06 to 0.130 μm, even more preferably 0.08 to 0.120 μm.
In this specification, the pore specific surface area and central pore diameter of the positive electrode active material layer 12 are values measured by a mercury porosimetry method. The central pore diameter is calculated as the median diameter (D50, unit: μm) in the pore diameter range of 0.003 to 1.000 μm in the pore diameter distribution.

When the pore specific surface area and central pore diameter of the positive electrode active material layer 12 are within the above ranges, the effect of improving the high rate cycle characteristics of the non-aqueous electrolyte secondary battery is excellent.
If the pore surface area is equal to or less than the upper limit of the above range, the reaction surface area is small, so current is locally concentrated in the fine powder of the positive electrode active material, the conductive aid, etc. during high-rate charge-discharge cycles, and the connection between the positive electrode 1 and the electrolyte solution occurs. The number of places where the side reaction is high is reduced, and deterioration is easily suppressed.
When the central pore diameter is at least the lower limit of the above range, there are few places where the fine powder of the positive electrode active material, the conductive agent, etc. aggregate, so that uneven reaction hardly occurs during high-rate charge-discharge cycles, and the positive electrode 1 and the electrolyte solution do not have a secondary reaction. The number of places where the reaction is high is reduced, and deterioration is easily suppressed.
The pore specific surface area and central pore diameter of the positive electrode active material layer 12 can be adjusted by, for example, the content of the positive electrode active material, the particle size of the positive electrode active material, the thickness of the positive electrode active material layer 12, and the like. When the positive electrode active material layer 12 contains a conductive aid, it can also be adjusted by the content of the conductive aid and the particle size of the conductive aid. It is also affected by the amount of fine powder contained in the positive electrode active material and the state of dispersion when preparing the composition for manufacturing a positive electrode.
For example, when the particle size of the conductive additive is smaller than that of the positive electrode active material, the pore surface area can be reduced and the central pore size can be increased by reducing the content of the conductive additive.

[Density of positive electrode active material layer]
In the present embodiment, the true density D1 of the positive electrode active material layer 12 and the true density D of the positive electrode active material particles satisfy the following formula (s).

0.96D≦D1<D (s)

That is, the ratio of the true density D1 to the true density D (D1/D ratio) is 0.96 or more and less than 1.
The D1/D ratio is preferably 0.97-0.99, more preferably 0.98-0.99. If the D1/D ratio is equal to or higher than the above lower limit, the proportion of the substance that contributes to lithium ion conduction in the positive electrode active material layer 12 is increased, and uniform charge-discharge reactions are smoothly performed, thereby improving high-rate cycle characteristics. . If the D1/D ratio is equal to or less than the above upper limit, the effect of blending other components (for example, a binder, a conductive aid, etc.) can be exhibited.
Note that the D1/D ratio can be adjusted by the content of the positive electrode active material in the positive electrode active material layer 12 .

The true density D of the positive electrode active material can be measured by the so-called Archimedes method.
When the positive electrode active material particles are made of the positive electrode active material, the positive electrode active material particles are used as a sample for measuring the true density D as they are.
When the positive electrode active material particles are coated particles, the coating layer is removed, the core portion is taken out, and this core portion (that is, the positive electrode active material) is used as a sample for measuring the true density D.
As a method for removing the coating layer of the coated particles, when the core portion is a layer of conductive carbon, a method of baking the coated particles can be mentioned. The firing conditions for the coated particles are, for example, 800 to 900° C. for 3 hours or more.
As a method for extracting the positive electrode active material in the positive electrode active material layer 12, there is a method of washing the positive electrode active material layer 12 with a solvent such as N-methylpyrrolidone (NMP) and then baking the positive electrode active material layer 12 in the presence of oxygen. mentioned. According to this method, the positive electrode active material can be collected by removing the organic material such as the binder, the conductive aid, and the like. The firing conditions for the positive electrode active material layer 12 are, for example, 800 to 900° C. and 3 hours or more.
Examples of the method for measuring the true density D include a He gas replacement method. In the He gas replacement method, for example, Micromeritics, dry density meter, Accupic II 1340 (low volume expansion method, for example, 10 cm ³ type, sample amount 0.2 cm ³ or more) can be used.
As an example of the measurement results of the true density D, 4 g of LiFePO ₄ obtained by firing the positive electrode active material layer 12 was sampled in a 10 cm ³ cell, and the measurements were repeated five times or more to obtain an average value. A value of 55 g/cm ³ was obtained. Similarly, the true density of LiCoO ₂ was measured to be 5.0 g/cm ³ .
Also, if the positive electrode active material can be identified from the charge/discharge potential, battery capacity, and composition analysis by ICP, the true density inherent to the crystal material can be obtained from literature values or similarly prepared measurements of the same material. The literature value for LiFePO ₄ is 3.6 g/cm ³ , which roughly agrees with the above test results.

Like the true density D, the true density D1 of the positive electrode active material layer 12 can be obtained by the He gas replacement method. First, the true density D2 of the positive electrode 1 is measured while including the positive electrode current collector 11 . Next, considering the thickness and mass of the positive electrode current collector 11, the true density D1 is calculated from the value of the true density D2. When calculating the true density D1, the thickness and mass of the positive electrode current collector 11 are measured and subtracted from the volume and mass of the positive electrode 1 to obtain the true density D1 of only the positive electrode active material layer 12. can. The true density D1 may be obtained by peeling the positive electrode active material layer 12 from the positive electrode current collector 11 and measuring the true density of the peeled positive electrode active material layer 12 . However, according to this method, an error occurs if deposits remain on the positive electrode current collector 11 . Therefore, as a method for obtaining the true density D1, a method of measuring the true density D2 of the positive electrode 1 in a state including the positive electrode current collector 11 and calculating the true density D1 from the true density D2 is preferable.

In this embodiment, the volume density of the positive electrode active material layer 12 is not particularly limited, but is preferably 2.05 to 2.80 g/cm ³ and more preferably 2.15 to 2.50 g/cm ³ .
The volume density of the positive electrode active material layer 12 can be measured, for example, by the following measuring method.
The thicknesses of the positive electrode 1 and the positive electrode current collector 11 are each measured with a microgauge, and the thickness of the positive electrode active material layer 12 is calculated from the difference between them. The thickness of the positive electrode 1 and the positive electrode current collector 11 is the average value of the values measured at five or more arbitrary points. As the thickness of the positive electrode current collector 11, the thickness of the positive electrode current collector exposed portion 13, which will be described later, may be used.
The mass of a measurement sample obtained by punching out a positive electrode to have a predetermined area is measured, and the mass of the positive electrode active material layer 12 is calculated by subtracting the pre-measured mass of the positive electrode current collector 11 .
The volume density of the positive electrode active material layer 12 is calculated based on the following formula (1).
Volume density (unit: g/cm ³ )=mass of positive electrode active material layer (unit: g)/[(thickness of positive electrode active material layer (unit: cm)×area of measurement sample (unit: cm ² )]...・(1)

When the volume density of the positive electrode active material layer 12 is within the above range, the volume energy density of the battery can be further improved, and a non-aqueous electrolyte secondary battery having superior cycle characteristics can be realized.
The volume density of the positive electrode active material layer 12 can be adjusted by, for example, the content of the positive electrode active material, the particle size of the positive electrode active material, the thickness of the positive electrode active material layer 12, and the like. When the positive electrode active material layer 12 contains a conductive aid, it can also be adjusted by the type (specific surface area, specific gravity) of the conductive aid, the content of the conductive aid, and the particle size of the conductive aid.

[Cycle capacity retention rate]
In the positive electrode 1 of the present embodiment, the cycle capacity retention rate obtained by the following test method is preferably 80% or more, more preferably 85% or more, still more preferably 90% or more, and may be 100%. If the cycle capacity retention rate is equal to or higher than the above lower limit, high rate cycle characteristics can be further enhanced.
(Test method)
A non-aqueous electrolyte secondary battery with a rated capacity of 1 Ah, charging at 3C rate, 3.8 V, resting for 10 seconds, then discharging at 3 C rate, 2.0 V, resting for 10 seconds, repeating 1000 charge-discharge cycles. Measure the discharge capacity B when discharged at 0.2 C rate and 2.5 V, and divide the discharge capacity B by the discharge capacity A of the non-aqueous electrolyte secondary battery before being subjected to the charge / discharge cycle to obtain the cycle capacity maintenance rate ( %).

<Non-aqueous electrolyte secondary battery>
A non-aqueous electrolyte secondary battery 10 of the present embodiment shown in FIG. 2 includes the positive electrode 1 for non-aqueous electrolyte secondary batteries of the present embodiment, a negative electrode 3, and a non-aqueous electrolyte. Further, a separator 2 may be provided. Reference numeral 5 in the figure is an exterior body.
In this embodiment, the positive electrode 1 has a plate-like positive electrode current collector 11 and positive electrode active material layers 12 provided on both surfaces thereof. The positive electrode active material layer 12 exists on part of the surface of the positive electrode current collector 11 . An edge portion of the surface of the positive electrode current collector 11 is a positive electrode current collector exposed portion 13 where the positive electrode active material layer 12 does not exist. A terminal tab (not shown) is electrically connected to an arbitrary portion of the positive electrode current collector exposed portion 13 .
The negative electrode 3 has a plate-like negative electrode current collector 31 and negative electrode active material layers 32 provided on both sides thereof. The negative electrode active material layer 32 exists on part of the surface of the negative electrode current collector 31 . An edge portion of the surface of the negative electrode current collector 31 is a negative electrode current collector exposed portion 33 where the negative electrode active material layer 32 does not exist. A terminal tab (not shown) is electrically connected to an arbitrary portion of the negative electrode current collector exposed portion 33 .
The shapes of the positive electrode 1, the negative electrode 3, and the separator 2 are not particularly limited. For example, it may be rectangular in plan view.
The non-aqueous electrolyte secondary battery 10 of the present embodiment is produced, for example, by fabricating an electrode laminate in which the positive electrode 1 and the negative electrode 3 are alternately laminated with the separator 2 interposed therebetween, and the electrode laminate is packaged in an outer package such as an aluminum laminate bag ( It can be manufactured by a method of enclosing in a housing 5, injecting a non-aqueous electrolyte (not shown), and sealing.
FIG. 2 typically shows a structure in which negative electrodes/separators/positive electrodes/separators/negative electrodes are laminated in this order, but the number of electrodes can be changed as appropriate. One or more positive electrodes 1 are sufficient, and any number of positive electrodes 1 can be used according to the battery capacity to be obtained. One more negative electrode 3 and separator 2 than the number of positive electrodes 1 are used, and they are arranged so that the outermost layer is the negative electrode 3 .

[Negative electrode]
The negative electrode active material layer 32 contains a negative electrode active material. The negative electrode active material layer 32 may further contain a binder. The negative electrode active material layer 32 may further contain a conductive aid. The shape of the negative electrode active material is preferably particulate.
For the negative electrode 3, for example, a negative electrode manufacturing composition containing a negative electrode active material, a binder, and a solvent is prepared, coated on the negative electrode current collector 31, and dried to remove the solvent to obtain the negative electrode active material. It can be manufactured by any method that forms layer 32 . The negative electrode production composition may contain a conductive aid.

Examples of negative electrode active materials and conductive aids include carbon materials such as graphite, graphene, hard carbon, ketjen black, acetylene black, and carbon nanotubes (CNT). Each of the negative electrode active material and the conductive aid may be used alone or in combination of two or more.

As the material of the negative electrode current collector 31 and the binder and solvent in the composition for manufacturing the negative electrode, the same materials as the material of the positive electrode current collector 11 and the binder and solvent in the composition for manufacturing the positive electrode are used. I can give an example. The binder and the solvent in the negative electrode-producing composition may be used alone or in combination of two or more.

The total content of the negative electrode active material and the conductive aid is preferably 80.0 to 99.9 mass %, more preferably 85.0 to 98.0 mass %, relative to the total mass of the negative electrode active material layer 32 .

[Separator]
A separator 2 is arranged between the negative electrode 3 and the positive electrode 1 to prevent short circuit or the like. The separator 2 may hold a non-aqueous electrolyte, which will be described later.
The separator 2 is not particularly limited, and can be exemplified by porous polymer membranes, non-woven fabrics, glass fibers, and the like.
An insulating layer may be provided on one or both surfaces of the separator 2 . The insulating layer is preferably a layer having a porous structure in which insulating fine particles are bound with an insulating layer binder.

The separator 2 may contain various plasticizers, antioxidants and flame retardants.
Antioxidants include phenolic antioxidants such as hindered phenolic antioxidants, monophenolic antioxidants, bisphenolic antioxidants, and polyphenolic antioxidants; hindered amine antioxidants; phosphorus antioxidants. benzotriazole-based antioxidants; benzophenone-based antioxidants; triazine-based antioxidants; salicylic acid ester-based antioxidants, and the like. Phenolic antioxidants and phosphorus antioxidants are preferred.

[Non-aqueous electrolyte]
A non-aqueous electrolyte fills between the positive electrode 1 and the negative electrode 3 . For example, known nonaqueous electrolytes can be used in lithium ion secondary batteries, electric double layer capacitors and the like.
As the non-aqueous electrolyte, a non-aqueous electrolytic solution obtained by dissolving an electrolyte salt in an organic solvent is preferable.

The organic solvent preferably has resistance to high voltage. For example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, Polar solvents such as tetrahydrafuran, 2-methyltetrahydrofuran, dioxolane, methyl acetate, or mixtures of two or more of these polar solvents are included.

The electrolyte salt is not particularly limited, and examples thereof include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₆ , LiCF ₃ CO ₂ , LiPF ₆ SO ₃ , LiN(SO ₂ F) ₂ and LiN(SO ₂ CF ₃ ). ₂ , Li( _SO2CF2CF3 ) ₂ , LiN( _COCF3 ) _{2 ,} LiN _{( COCF2CF3} ) ₂ , or a mixture of _two or more of these salts.

The non-aqueous electrolyte secondary battery of the present embodiment can be used as a lithium ion secondary battery for various uses such as industrial use, consumer use, automobile use, and residential use.
The mode of use of the non-aqueous electrolyte secondary battery of this embodiment is not particularly limited. For example, it can be used for a battery module configured by connecting a plurality of non-aqueous electrolyte secondary batteries in series or in parallel, a battery system including a plurality of electrically connected battery modules and a battery control system, and the like.
Examples of battery systems include battery packs, stationary storage battery systems, automotive power storage battery systems, automotive auxiliary equipment storage battery systems, and emergency power supply storage battery systems.

EXAMPLES The present invention will be described in more detail below using examples, but the present invention is not limited to these examples.

<Raw materials used>
[Positive electrode active material particles]
LiFePO ₄ coated particles: core (LiFePO ₄ ) true density 3.55 g/cm ³ . Conductive carbon coating. The amount of conductive carbon in the table is a ratio with respect to 100% by mass of the positive electrode active material particles.
· LiCoO ₂ (no covering portion): True density 5.00 g/cm ³ .
[Conductive agent]
- Carbon black: True density 2.30 g/cm ³ .
[Binder]
• Polyvinylidene fluoride (PVDF): True density 1.20 g/cm ³ .
[Dispersant]
• Polyvinylpyrrolidone (PVP): True density 1.78 g/cm ³ .

[Positive collector]
- A positive electrode current collector having a current collector coating layer (thickness: 2 µm) on both sides of an aluminum foil (thickness: 15 µm). The current collector coating layer contains carbon black (100 parts by mass) and a binder (40 parts by mass).
(Production method)
A positive electrode current collector was produced by coating both the front and back surfaces of a positive electrode current collector body with a current collector coating layer by the following method. An aluminum foil (thickness: 15 μm) was used as the main body of the positive electrode current collector.
A slurry was obtained by mixing 100 parts by mass of carbon black, 40 parts by mass of polyvinylidene fluoride as a binder, and N-methylpyrrolidone (NMP) as a solvent. The amount of NMP used was the amount necessary for coating the slurry.
The obtained slurry is applied to both sides of the positive electrode current collector body by gravure so that the thickness of the coating film after drying (both sides total) is 2 μm, dried to remove the solvent, and the positive electrode current collector is coated. body. The current collector coating layers 15 on both sides were formed so that the coating amount and thickness were uniform.

<Measurement method>
[Cycle capacity retention rate]
The cycle capacity retention rate was evaluated according to the following procedures (1) to (7).
(1) A non-aqueous electrolyte secondary battery (cell) was produced so as to have a rated capacity of 1 Ah, and cycle evaluation was performed at room temperature (25°C).
(2) The resulting cell was charged at a constant current rate of 0.2C (i.e., 200mA) with a final voltage of 3.6V, and then terminated with 1/10 of the charging current at a constant voltage. Charging was done as a current (ie 20 mA).
(3) Discharge for capacity confirmation was performed at a constant current of 0.2C rate and a final voltage of 2.5V. The discharge capacity at this time was taken as the reference capacity, and the reference capacity was taken as the current value of the 1C rate (that is, 1,000 mA).
(4) After charging the cell at a constant current at a 3C rate (that is, 3000 mA) with a final voltage of 3.8 V, resting for 10 seconds, and discharging from this state at a 3 C rate with a final voltage of 2.0 V , rested for 10 seconds.
(5) The cycle test of (4) was repeated 1,000 times.
(6) After performing the same charging as in (2), the same capacity confirmation as in (3) was performed.
(7) By dividing the discharge capacity in capacity confirmation measured in (6) by the reference capacity before the cycle test and making it a percentage, the cycle capacity retention rate after 1,000 cycles (1,000 cycle capacity retention rate, unit: %).

<Production Example 1: Production of Negative Electrode>
100 parts by mass of artificial graphite as a negative electrode active material, 1.5 parts by mass of styrene-butadiene rubber as a binder, 1.5 parts by mass of carboxymethyl cellulose Na as a thickener, and water as a solvent are mixed, A composition for manufacturing a negative electrode having a solid content of 50% by mass was obtained.
The obtained composition for manufacturing a negative electrode was coated on both sides of a copper foil (thickness: 8 μm), dried in vacuum at 100° C., and then pressed under a load of 2 kN to obtain a negative electrode sheet. The obtained negative electrode sheet was punched out to obtain a negative electrode.

<Production Example 2: Production of Nonaqueous Electrolyte Secondary Battery>
A non-aqueous electrolyte secondary battery having the configuration shown in FIG. 2 was manufactured by the following method.
Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed so that the volume ratio of EC:DEC was ₃ :7. An aqueous electrolyte was prepared.
The positive electrode of each example and the negative electrode obtained in Production Example 1 were alternately laminated via a separator to prepare an electrode laminate having the negative electrode as the outermost layer. A polyolefin film (thickness: 15 μm) was used as the separator.
In the process of producing the electrode laminate, first, the separator 2 and the positive electrode 1 were laminated, and then the negative electrode 3 was laminated on the separator 2 .
A terminal tab is electrically connected to each of the positive electrode current collector exposed portion 13 and the negative electrode current collector exposed portion 33 of the electrode laminate, and the electrodes are stacked with an aluminum laminate film so that the terminal tab protrudes to the outside. The body was sandwiched, and three sides were laminated and sealed.
Subsequently, a non-aqueous electrolyte was injected from one side that was left unsealed, and vacuum-sealed to manufacture a non-aqueous electrolyte secondary battery (laminate cell).
The cycle capacity retention rate was measured using the obtained non-aqueous electrolyte secondary battery.

<Examples 1-3, Comparative Examples 1-2>
According to the composition shown in Table 1, the positive electrode active material particles, the conductive aid, the binder, and the dispersing agent were dispersed in a solvent (NMP) to prepare a composition for manufacturing a positive electrode. The amount of the solvent used was the amount necessary for coating the composition for manufacturing a positive electrode. The amounts of the positive electrode active material particles, the conductive aid, the binder, and the dispersant in the table are the total amount other than the solvent (i.e., the total amount of the positive electrode active material particles, the conductive aid, the binder, and the dispersant). ) is a ratio to 100% by mass. In the table, the content of each composition represents % by mass, and "-" indicates that it is not blended. Vacuum drying was performed to form a positive electrode active material layer. The coating amount of the positive electrode manufacturing composition was set to 31 mg/cm ² . The resulting laminate was pressure-pressed with a load of 10 kN to form a positive electrode sheet. Then, the positive electrode sheet was punched out to obtain a positive electrode.
The true density D1 was obtained for the positive electrode of each example.
A non-aqueous electrolyte secondary battery was produced using the positive electrode of each example, and the cycle capacity retention rate was determined for this non-aqueous electrolyte secondary battery. The results are shown in the table.

As shown in Table 1, Examples 1 to 3 to which the present invention was applied had a cycle capacity retention rate of 82% or more.
In Comparative Examples 1 and 2, in which the D1/D ratio was less than 96%, the cycle capacity retention rate was 68% or less.
From these results, it was confirmed that the high rate cycle characteristics can be improved by applying the present invention.

REFERENCE SIGNS LIST 1 positive electrode 2 separator 3 negative electrode 5 exterior body 10 secondary battery 11 positive electrode current collector 12 positive electrode active material layer 13 positive electrode current collector exposed portion 14 positive electrode current collector body 15 current collector coating layer 31 negative electrode current collector 32 negative electrode active Material layer 33 Negative electrode current collector exposed portion

Claims (10)

Having a positive electrode current collector and a positive electrode active material layer present on the positive electrode current collector,
The positive electrode active material layer has one or more positive electrode active material particles containing a positive electrode active material,
A positive electrode for a non-aqueous electrolyte secondary battery, wherein the true density D of the positive electrode active material and the true density D1 of the positive electrode active material layer satisfy the following formula (s).
0.96D≦D1<D (s) The positive electrode active material is a compound represented by the general formula LiFe _x M _(1-x) PO ₄ (where 0≦x≦1 and M is Co, Ni, Mn, Al, Ti or Zr). The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, comprising: 2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein said positive electrode active material is lithium iron phosphate represented by _LiFePO4 . The positive electrode for a non-aqueous electrolyte secondary battery according to claim 3, wherein said true density D1 is 3.4 g/ ^cm3 or more and less than 3.6 g/ ^cm3 . The positive electrode active material layer contains a conductive aid and a binder,
The content of the conductive aid is 1% by mass or less with respect to the total mass of the positive electrode active material layer,
5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of said binder is 1% by mass or less with respect to the total mass of said positive electrode active material layer. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the positive electrode active material layer does not contain a conductive aid. Some or all of the positive electrode active material particles have a core portion of the positive electrode active material and a covering portion covering the core portion,
the covering includes a conductive material;
The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the content of said conductive material is 1.3% by mass or less with respect to the total mass of said positive electrode active material particles. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 7, wherein a cycle capacity retention rate determined by the following test method is 80% or more.
(Test method)
A non-aqueous electrolyte secondary battery with a rated capacity of 1 Ah, charging at 3C rate, 3.8 V, resting for 10 seconds, then discharging at 3 C rate, 2.0 V, resting for 10 seconds, repeating 1000 charge-discharge cycles. Measure the discharge capacity B when discharged at 0.2 C rate and 2.5 V, and divide the discharge capacity B by the discharge capacity A of the non-aqueous electrolyte secondary battery before being subjected to the charge / discharge cycle to obtain the cycle capacity maintenance rate ( %). The positive electrode for the non-aqueous electrolyte secondary battery according to any one of claims 1 to 8, the negative electrode, and the non-aqueous electrolyte present between the positive electrode for the non-aqueous electrolyte secondary battery and the negative electrode, A non-aqueous electrolyte secondary battery. A battery module or battery system comprising a plurality of the non-aqueous electrolyte secondary batteries according to claim 9 .

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