JP2023029333A - Positive electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery, battery module and battery system that employ the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for a non-aqueous electrolyte secondary battery which can improve the heat resistance of the non-aqueous electrolyte secondary battery.

SOLUTION: A positive electrode 1 for a non-aqueous electrolyte secondary battery includes a collector 11, and a positive electrode active material layer 12 that is present on the collector 11 and contains positive electrode active material particles. In a spreading resistance value distribution of the positive electrode active material layer 12, the sum of frequencies of resistance values 4.0 to 6.0 (logΩ) is 0.0 to 5.0% based on the sum of frequencies of resistance values 4.0 to 12.5 (logΩ) being 100%.

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.

In Patent Document 1, as an index for evaluating the characteristics of the positive electrode active material, attention is paid to the spreading resistance value of the secondary particles of the positive electrode active material, and the positive electrode active material having different spreading resistance values is obtained by changing the composition and manufacturing conditions of the positive electrode active material. An example of how the material was produced is described.
Patent Document 2 describes an example in which the output characteristics and energy density of a secondary battery are improved by using flaky graphene as a conductive aid. In the examples using flaky graphene, when the cross section of the positive electrode is spread and mapped based on the resistance value, the aspect ratio of the portion below a specific resistance value is higher than that in the comparative example using the powdery conductive additive. It has been shown that the

WO2017/208894 WO2018/168059

From the viewpoint of expanding the use of non-aqueous electrolyte secondary batteries, there is a demand for heat resistance that can maintain good battery characteristics even in high-temperature environments.
The present invention provides a positive electrode for a non-aqueous electrolyte secondary battery that can improve the heat resistance of the non-aqueous electrolyte secondary battery.

The present invention has the following aspects.
[1] A current collector and a positive electrode active material layer containing positive electrode active material particles present on the current collector, wherein the spreading resistance value distribution of the positive electrode active material layer has a resistance value of 4.0 to 12. A positive electrode for a non-aqueous electrolyte secondary battery, wherein the total frequency of resistance values 4.0 to 6.0 (log Ω) is 0.0 to 5.0% when the total frequency of .5 (log Ω) is 100%. .
[2] In the spreading resistance value distribution, the average frequency of resistance values 6.0 to 9.0 (log Ω) is higher than the average frequency of resistance values 4.0 to 6.0 (log Ω), Positive electrode for water electrolyte secondary battery.
[3] The positive electrode for a non-aqueous electrolyte secondary battery of [1] or [2], wherein the positive electrode active material layer contains a conductive aid.
[4] The positive electrode for a non-aqueous electrolyte secondary battery according to [3], wherein an active material coating containing a conductive material is present on at least part of the surface of the positive electrode active material particles.
[5] The non-conductive material of [1] or [2], wherein the positive electrode active material layer does not contain a conductive aid, and an active material coating portion containing a conductive material is present on at least part of the surface of the positive electrode active material particles. Positive electrode for water electrolyte secondary battery.
[6] The positive electrode active material layer contains conductive carbon, and the content of the conductive carbon is 0.5% by mass or more and less than 3.0% by mass with respect to the total mass of the positive electrode active material layer, [ 3] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [5].
[7] The positive electrode active material particles have the general formula LiFe _x M _(1-x) PO ₄ (where 0≦x≦1 and M is Co, Ni, Mn, Al, Ti or Zr). A positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [6], containing the represented compound.
[8] The non-aqueous electrolyte according to any one of [1] to [7], wherein a current collector coating layer containing a conductive material is present on at least part of the surface of the current collector on the positive electrode active material layer side. A positive electrode for secondary batteries.
[9] A non-aqueous electrolyte comprising a positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [8], a negative electrode, and a non-aqueous electrolyte present between the positive electrode and the negative electrode for a non-aqueous electrolyte secondary battery. Water electrolyte secondary battery.
[10] A battery module or battery system comprising a plurality of non-aqueous electrolyte secondary batteries according to [9].

ADVANTAGE OF THE INVENTION According to this invention, the positive electrode for nonaqueous electrolyte secondary batteries which can improve the heat resistance of a nonaqueous electrolyte secondary battery is obtained.

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. It is a mapping image showing the measurement results of the spreading resistance value distribution. It is a mapping image showing the measurement results of the spreading resistance value distribution. 7 is a graph showing measurement results of spreading resistance value distribution.

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 (also simply referred to as “positive electrode”) 1 for a non-aqueous electrolyte secondary battery of this embodiment has a current collector (hereinafter referred to as “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 current collector coating layer 15 on the surface of the positive electrode active material layer 12 side. That is, 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 contains positive electrode active material particles.
The positive electrode active material layer 12 preferably further contains a binder.
The positive electrode active material layer 12 may further contain a conductive aid. As used herein, the term “conductive aid” refers to a conductive material having a shape such as a granular or fibrous shape, which is mixed with the positive electrode active material particles in forming the positive electrode active material layer. It refers to a conductive material present in the positive electrode active material layer in a form of connection.
The positive electrode active material layer 12 may further contain a dispersant.
The content of the positive electrode active material particles is preferably 80.0 to 99.9% by mass, more preferably 90 to 99.5% by mass, based on the total mass of the positive electrode active material layer 12 .

The thickness of the positive electrode active material layer (when the positive electrode active material layer is present on both sides of the positive electrode current collector, the total thickness of both surfaces) is preferably 30 to 500 μm, more preferably 40 to 400 μm, and 50 to 500 μm. 300 μm is particularly preferred. When the thickness of the positive electrode active material layer is at least the lower limit of the above range, the energy density of the battery incorporating the positive electrode tends to be high. Peeling can be suppressed during charging and discharging.

[Positive electrode active material particles]
It is preferable that an active material coating containing a conductive material is present on at least part of the surface of the positive electrode active material particles. From the viewpoint of better battery capacity and cycle characteristics, it is more preferable that the entire surface of the positive electrode active material particles is coated with a conductive material.
Here, "at least part of the surface of the positive electrode active material particle" means that the active material coating portion accounts for 50% or more, preferably 70% or more, and more preferably 90% of the total area of the outer surface of the positive electrode active material particle. It means covering the above. Note that this ratio (%) is the average value for all the positive electrode active material particles present in the positive electrode active material layer, and as long as this average value is equal to or higher than the above lower limit, the positive electrode active material without the active material coating portion It does not exclude the presence of minute amounts of particles of matter. When the positive electrode active material particles that do not have the active material coating part are present in the positive electrode active material layer, the amount thereof is preferably 30% by mass with respect to the total amount of the positive electrode active material particles present in the positive electrode active material layer. or less, more preferably 20% by mass or less, and particularly preferably 10% by mass or less.

The conductive material of the active material coating preferably contains carbon (conductive carbon). A conductive material consisting only of carbon may be used, or a conductive organic compound containing carbon and an element other than carbon may be used. 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.
The content of the conductive material is preferably 0.1 to 4.0% by mass, more preferably 0.5 to 3.0% by mass, and 0.1 to 4.0% by mass, more preferably 0.5 to 3.0% by mass, based on the total mass of the positive electrode active material particles having active material coating portions. 7 to 2.5% by mass is more preferable. If the amount is too large, the conductive material may peel off from the surface of the positive electrode active material particles and remain as independent conductive auxiliary particles, which is not preferable.

The positive electrode active material particles preferably contain a compound having an olivine crystal structure.
The 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”).
As the positive electrode active material particles, lithium iron phosphate particles having active material coating portions containing a conductive material on at least part of their surfaces (hereinafter also referred to as “coated lithium iron phosphate particles”) are more preferable. From the viewpoint of better battery capacity and cycle characteristics, it is more preferable that the lithium iron phosphate particles are entirely coated with a conductive material.
Coated lithium iron phosphate particles can be produced by known methods.
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 lithium iron phosphate particles having at least a portion of the surface coated with carbon.
For example, the particle size of the lithium iron phosphate particles can be adjusted by the pulverization time in the pulverization step. The amount of carbon covering the lithium iron phosphate particles can be adjusted by the heating time and temperature in the step of heat-treating while supplying methanol vapor. It is desirable to remove uncoated carbon particles by subsequent steps such as classification and washing.

The positive electrode active material particles may contain at least one other positive electrode active material particle containing 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 aluminate _{( LiNixCoyAlzO2} , where x ₊ y+z=1), lithium nickel cobalt manganate (LiNixCoyMn _zO2 , where x+y+z=1), lithium _manganate ( _LiMn2O4 ), lithium cobalt manganate ( _LiMnCoO4 ), lithium manganese chromate ( _{LiMnCrO4 )} , lithium vanadium nickelate ( _LiNiVO4 ), nickel-substituted manganese Lithium oxide (eg, 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.
The active material coating portion may be present on at least part of the surface of the other positive electrode active material particles.

The content of the compound having an olivine-type crystal structure is preferably 50% by mass or more, preferably 80% by mass, based on the total mass of the positive electrode active material particles (including the mass of the active material coating when the active material coating is present). The above is more preferable, and 90% by mass or more is even more preferable. 100 mass % may be sufficient.
When the coated lithium iron phosphate particles are used, the content of the coated lithium iron phosphate particles is preferably 50% by mass or more, more preferably 80% by mass or more, and 90% by mass or more with respect to the total mass of the positive electrode active material particles. is more preferred. 100 mass % may be sufficient.

The thickness of the active material coating portion of the positive electrode active material particles is preferably 1 to 100 nm.
The thickness of the active material coating portion of the positive electrode active material particles can be measured by measuring the thickness of the active material coating portion in a transmission electron microscope (TEM) image of the positive electrode active material particles. The thickness of the active material coating portion present on the surface of the positive electrode active material particles may not be uniform. It is preferable that an active material coating portion having a thickness of 1 nm or more exists on at least part of the surface of the positive electrode active material particles, and the maximum thickness of the active material coating portion is 100 nm or less.

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

[Binder]
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, and polyvinylidene. Acetal, polyethylene oxide, polyethylene glycol, carboxymethyl cellulose, polyacrylonitrile, polyimide and the like can be mentioned. 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 4.0% by mass or less, more preferably 2.0% 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 ratio of substances that do not contribute to the conduction of lithium ions in the positive electrode active material layer 12 is reduced, and the battery 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, more preferably 0.5% by mass, based on the total mass of the positive electrode active material layer 12. The above is more preferable.

[Conductive agent]
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 is, for example, preferably 4 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 1 part by mass or less with respect to 100 parts by mass of the total mass of the positive electrode active material. , it is particularly preferable not to contain a conductive additive, and a state in which independent conductive additive particles (for example, independent carbon particles) do not exist is desirable.
When the positive electrode active material layer contains the conductive support agent, the lower limit of the content of the conductive support agent is appropriately determined according to the type of the conductive support agent. More than 1% by mass.
In addition, the fact that the positive electrode active material layer "does not contain a conductive aid" means that it does not substantially contain a conductive aid, and does not exclude substances contained to such an extent that the effects of the present invention are 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, it can be determined that it is not substantially contained.

[Dispersant]
The dispersant contained in the positive electrode active material layer 12 is an organic substance, and examples thereof include polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl formal (PVF), and the like. One dispersant may be used, or two or more dispersants may be used in combination.

[Positive electrode current collector body]
The positive electrode current collector main body 14 is made of a metal material. Examples of metal materials 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]
It is preferable that the current collector coating layer 15 is present on at least part of the surface of the positive electrode current collector main body 14 . Current collector coating layer 15 includes a conductive material.
Here, "at least part of the surface" means 10% to 100%, preferably 30% to 100%, more preferably 50% to 100% of the surface area of the positive electrode current collector body.
The conductive material in the current collector coating layer 15 preferably contains carbon (conductive carbon). A conductive material consisting only of carbon is more preferable.
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 applied to the surface of the positive electrode current collector body 14 using and dried to remove the solvent.

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 transmission electron microscope (TEM) image or a scanning electron microscope (SEM) image of the cross section 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. .

[Spreading resistance value distribution of positive electrode active material layer]
In the present embodiment, in the spreading resistance value distribution of the positive electrode active material layer, when the total frequency of resistance values 4.0 to 12.5 (log Ω) is 100%, the resistance value is 4.0 to 6.0 (log Ω). is 0.0 to 5.0%, preferably 0.0 to 4.0%, more preferably 0.0 to 3.0%, even more preferably 0.0 to 2.0%.
The spreading resistance value distribution of the positive electrode active material layer in this specification is measured using a scanning spreading resistance microscope (SSRM) using a cross section of the positive electrode active material layer as a measurement object, and the following <<spreading resistance value distribution Measurement method>>.

<<How to measure the spreading resistance value distribution>>
The SSRM applies a bias voltage to an object to be measured, scans the surface with a conductive probe, and two-dimensionally measures the distribution of resistance values (spreading resistance values) immediately below the probe.
The spreading resistance value distribution was measured using an SSRM under the conditions of a DC bias voltage of +2.0 V, a scan size of 60 μm×60 μm, and the number of measurement points (number of data points) of 1024×1024. A frequency distribution graph (spreading resistance value distribution) is obtained.
The frequency on the vertical axis is defined as the total frequency (number of measurement points) at which the resistance value is 4.0 log Ω (1 × 10 ⁴ Ω) or more and 12.5 log Ω (1 × 10 ^12.5 Ω) or less. relative frequency (unit: %, also simply referred to as “frequency”).

If the total frequency of the resistance values of 4.0 to 6.0 (log Ω) in the spreading resistance value distribution is 5.0% or less, the effect of improving the heat resistance of the non-aqueous electrolyte secondary battery is excellent.
If there is a portion with a low resistance of 4.0 to 6.0 (log Ω) in the cross section of the positive electrode active material layer, that portion becomes an active point when the non-aqueous electrolyte secondary battery is exposed to high temperatures. It is considered that a side reaction between the positive electrode and the electrolytic solution is likely to occur.
The frequency sum of resistance values of 4.0 to 6.0 (log Ω) can be reduced, for example, by reducing independent conductivity aid particles (eg, independent carbon particles).

In the spread resistance value distribution, the average frequency A of resistance values 4.0 to 6.0 (log Ω) is the frequency (% ). Specifically, the average frequency A is calculated by dividing the sum of the resistance values of each measurement point existing in the resistance value range of 4.0 to 6.0 (logΩ) by the number of measurement points.
In the spread resistance value distribution, the average frequency B of resistance values 6.0 to 9.0 (log Ω) is the frequency (% ). Specifically, the average frequency B is calculated by dividing the sum of the resistance values of each measurement point existing in the resistance value range of 6.0 to 9.0 (logΩ) by the number of measurement points.
In this embodiment, it is preferable that the average frequency B is larger than the average frequency A (A<B). When A<B, the effect of improving the heat resistance of the non-aqueous electrolyte secondary battery is excellent. In addition, it is easy to obtain a sufficient output of the non-aqueous electrolyte secondary battery.
The average frequency B is, for example, preferably 0.05 to 0.5%, more preferably 0.1 to 0.4%, even more preferably 0.15 to 0.35%. The average frequency B can be increased, for example, by having more conductive material present as the active material coating.
When A<B, the difference BA is preferably more than 0%, more preferably 0.05% or more, and even more preferably 0.20% or more.

[Conductive carbon content]
In this embodiment, the positive electrode active material layer 12 preferably contains conductive carbon. Examples of embodiments in which the positive electrode active material layer contains conductive carbon include the following embodiments 1 to 3.
Aspect 1: A mode in which the positive electrode active material layer contains a conductive aid, and the conductive aid contains conductive carbon.
Aspect 2: The positive electrode active material layer contains a conductive aid, and an active material coating portion containing a conductive material is present on at least part of the surface of the positive electrode active material particles, and the conductive material of the active material coating portion and the conductive material Embodiments in which one or both of the auxiliaries comprise conductive carbon.
Aspect 3: The positive electrode active material layer does not contain a conductive aid, an active material coating portion containing a conductive material is present on at least part of the surface of the positive electrode active material particles, and the conductive material of the active material coating portion is conductive. Embodiments containing carbon.
Aspect 3 is more preferable in terms of the effect of improving the heat resistance of the non-aqueous electrolyte secondary battery.

The content of the conductive carbon is preferably 0.5% by mass or more and less than 3.0% by mass, more preferably 1.0 to 2.8% by mass, and 1.3 to 1.3% by mass, based on the total mass of the positive electrode active material layer. 2.5% by mass is more preferred.
When the content of conductive carbon in the positive electrode active material layer is at least the lower limit of the above range, good conductive path formation and low resistance characteristics are excellent. A positive electrode active material layer having few active sites can be formed.

The content of conductive carbon with respect to the total mass of the positive electrode active material layer is obtained by peeling the positive electrode active material layer from the positive electrode and vacuum-drying it in a 120 ° C environment. Quantity measurement method>>.
For example, an object to be measured can be obtained by removing powder from the outermost surface of the positive electrode active material layer with a depth of several μm using a spatula or the like and vacuum-drying it in a 120° C. environment.
The content of conductive carbon measured by <<Method for Measuring Content of Conductive Carbon>> below includes carbon in the active material coating portion and carbon in the conductive aid. Carbon in the binder is not included. Carbon in the dispersant is not included.

<<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 obtained by measuring the sample or the liquid obtained by extracting the sample with an N,N-dimethylformamide (DMF) solvent by Fourier transform infrared spectrum (FT-IR), can be confirmed by a method for confirming the absorption of 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.
When a dispersant is included, the content of conductive carbon (unit: mass %) can be obtained by subtracting M4 from M3 and further subtracting the amount of carbon derived from the dispersant.
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, [searched 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. Here, "near the particle surface" means a region up to about 100 nm deep from the particle surface, and "inside the particle" means a region inside the vicinity of the particle surface.
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.
As yet another method, a scanning spread resistance microscope (SSRM) is used to observe the cross section of the positive electrode active material layer. It can be determined that it is the conductive carbon present in the active material coating portion. It can be determined that a portion that exists independently and has a low resistance other than such particles is the conductive aid.
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.

[Volume Density of Positive Electrode Active Material Layer]
In the present embodiment, the volume density of the positive electrode active material layer 12 is preferably 2.20-2.70 g/cm ³ , more preferably 2.25-2.50 g/cm ³ .
The volume density of the positive electrode active material layer 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 the positive electrode 1 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)

<Manufacturing method of positive electrode>
The method for manufacturing the positive electrode 1 of the present embodiment includes a composition preparation step of preparing a positive electrode manufacturing composition containing a positive electrode active material, and a coating step of coating the positive electrode manufacturing composition on the positive electrode current collector 11. have.
For example, the positive electrode 1 can be manufactured by applying a positive electrode manufacturing composition containing a positive electrode active material and a solvent onto the positive electrode current collector 11 and drying it to remove the solvent to form the positive electrode active material layer 12 . The composition for positive electrode production may contain a conductive aid. The composition for manufacturing a positive electrode may contain a binder. The manufacturing composition may contain a dispersant.
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 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 type of solvent may be used, or two or more types may be used in combination.

<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 sides 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 laminated so that the negative electrode 3 is the outermost layer.

[Negative electrode]
The negative electrode active material layer 32 contains a negative electrode active material. Furthermore, a binding material may be included. Further, it may 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, lithium titanate (LTO), silicon, and silicon monoxide. Examples of carbon materials include 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.

The material of the negative electrode current collector 31 can be exemplified by the same materials as those of the positive electrode current collector 11 described above.
Binders in the negative electrode production composition include polyacrylic acid (PAA), lithium polyacrylate (PAALi), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-propylene hexafluoride copolymer (PVDF-HFP ), styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene glycol (PEG), carboxymethylcellulose (CMC), polyacrylonitrile (PAN), polyimide (PI), and the like. One type of binder may be used, or two or more types may be used in combination.
Examples of the solvent in the negative electrode-producing composition include water and organic solvents. Examples of organic solvents include alcohols such as methanol, ethanol, 1-propanol and 2-propanol; linear or cyclic amides such as N-methylpyrrolidone (NMP) and N,N-dimethylformamide (DMF); and ketones such as acetone. I can give an example. The solvent 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.

As shown in the examples below, the present invention can improve the heat resistance of non-aqueous electrolyte secondary batteries. For example, in a 1C output test before and after storage at 80°C for 20 days, an output retention rate of 50% or more, preferably 60% or more can be achieved.
Therefore, it becomes easy to use the non-aqueous electrolyte secondary battery even in a high-temperature environment where it has been difficult to use the non-aqueous electrolyte secondary battery. For example, it is possible to provide a non-aqueous electrolyte secondary battery as a substitute for a lead-acid battery used in the engine room of a vehicle.

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

<Measurement method>
[Spreading resistance distribution]
Using a cross section parallel to the thickness direction of the positive electrode active material layer as a measurement target, the spreading resistance value distribution was measured using SSRM under the following conditions.
(Apparatus used) Bruker, product name: NanoScopeV Dimension Icon, Glovebox.
(Preparation of Sample) A test piece cut out from the positive electrode sheet was embedded in an epoxy resin, cut by broad ion beam processing to prepare a cross section, and introduced into a measuring apparatus under an inert atmosphere.
(Measurement condition)
Scan mode: Simultaneous measurement of contact mode and spreading resistance.
Tip: Diamond coated silicon cantilever (DDESP 10).
Measurement environment: room temperature, high-purity Ar gas atmosphere (H ₂ O=0.1 ppm, O ₂ =0.1 ppm).
Applied voltage: DC bias voltage = +2.0V.
Scan size: 60 μm×60 μm.
Number of measurement points (number of data points): 1024×1024.

[Method for measuring volume density]
Using a microgauge, the thickness of the positive electrode sheet and the thickness of the exposed portion of the positive electrode current collector were measured. Each was measured at arbitrary five points and an average value was obtained. The thickness of the positive electrode active material layer was calculated by subtracting the thickness of the positive electrode current collector exposed portion from the thickness of the positive electrode sheet.
Five measurement samples were prepared by punching a positive electrode sheet into a circle with a diameter of 16 mm.
The mass of each measurement sample was weighed with a precision balance, and the mass of the positive electrode active material layer in the measurement sample was calculated by subtracting the previously measured mass of the positive electrode current collector from the measurement result. The volume density of the positive electrode active material layer was calculated based on the above formula (1) from the average value of each measured value.

<Evaluation method>
[Evaluation of heat resistance: measurement of output retention rate]
(1) Regarding the non-aqueous electrolyte secondary battery (initial state), the initial output power (unit: Wh) was measured by the following method.
A non-aqueous electrolyte secondary battery (cell) was produced so as to have a rated capacity of 1.5 Ah. The resulting cell was charged at a constant current of 0.2 C rate (i.e., 300 mA) at a constant current at a final voltage of 3.6 V under an environment of 25 ° C., and then charged at a constant voltage of 1/10 of the charging current. was set as the final current (that is, 30 mA).
Then, in a 25° C. environment, discharge was performed at a constant current of 1.0 C rate (ie, 1500 mA) with a final voltage of 3.0 V. The discharge power at this time was defined as the power that can be output in the initial state (hereinafter also referred to as "initial output") E1.
(2) Then, in an environment of 25° C., the cell is charged at a constant current at a rate of 0.2 C (that is, 300 mA) at a final voltage of 3.6 V, and then at a constant voltage of 1/10 of the charging current. was set as the final current (ie, 30 mA) and adjusted to the fully charged state.
(3) The non-aqueous electrolyte secondary battery subjected to the measurement in (1) and the adjustment to full charge in (2) was stored in an atmosphere of 80° C. for 20 days.
(4) After completing the storage in (3) above, the power (unit: Wh) that can be output after storage was measured by the following method.
First, in a 25° C. environment, discharge was performed at a constant current of 0.2 C rate (that is, 300 mA) and a final voltage of 2.5 V.
Then, in an environment of 25 ° C., after charging at a constant current of 0.2 C rate (that is, 300 mA) and a final voltage of 3.6 V, 1/10 of the charging current at a constant voltage is charged to the final current. (that is, 30 mA).
Then, in a 25° C. environment, discharge was performed at a constant current of 1.0 C rate (ie, 1500 mA) with a final voltage of 3.0 V. The discharge power at this time was defined as the power that can be output in the state after storage (hereinafter also referred to as "post-storage output") E2.
(5) The ratio of the post-storage output E2 obtained in (4) above to the initial output E1 obtained in (1) above was determined by the following formula and defined as an output retention rate (unit: %).
Output maintenance rate = (E2/E1) x 100

<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 current collector having current collector coating layer]
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 resulting slurry is applied to both the front and back surfaces of a 15 μm thick aluminum foil (positive electrode current collector body) by a gravure method so that the thickness of the current collector coating layer after drying (both sides total) is 2 μm. and dried to remove the solvent to obtain a positive electrode current collector. The current collector coating layers on both sides were formed so that the coating amount and thickness were uniform.

<Examples 1 to 5>
Examples 1 to 3 are working examples, and examples 4 and 5 are comparative examples.
As the positive electrode active material particles, lithium iron phosphate particles (hereinafter also referred to as “carbon-coated active material”) having the following two kinds of active material coating portions were used.
Carbon-coated active material (1): average particle diameter of 1 μm, carbon content of 1.5% by mass.
Carbon-coated active material (2): average particle size of 10 μm, carbon content of 2.5% by mass.
Both of the carbon-coated active materials (1) and (2) had a thickness of the active material coating within a range of 1 to 100 nm.
Carbon black (CB) or carbon nanotube (CNT) was used as a conductive aid. CB and CNT have impurities below the limit of quantification, and can be regarded as having a carbon content of 100% by mass.
Polyvinylidene fluoride (PVDF) was used as a binder.
N-methylpyrrolidone (NMP) was used as solvent.
As the positive electrode current collector, the aluminum foil having the current collector coating layer obtained in Production Example 2 was used.

A positive electrode active material layer was formed by the following method.
The positive electrode active material particles, the conductive aid, the binder and the solvent (NMP) having the compositions shown in Table 1 were mixed in a mixer to obtain 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. In addition, the compounding amounts of the positive electrode active material particles, the conductive aid, and the binder in the table are based on the total amount other than the solvent (that is, the total amount of the positive electrode active material particles, the conductive aid, and the binder) being 100% by mass. It is the ratio when
The obtained composition for manufacturing a positive electrode was applied on both sides of a positive electrode current collector, and after preliminary drying, vacuum drying was performed in an environment of 120° C. to form a positive electrode active material layer. The resulting laminate was pressure-pressed to obtain a positive electrode sheet. Table 1 shows the coating amount (both sides total), the thickness of the positive electrode active material layer (both sides total), and the volume density. The positive electrode active material layers on both sides were formed so that the coating amount and thickness were uniform.
The obtained positive electrode sheet was punched out to obtain a positive electrode.

For the obtained positive electrode sheet, the spreading resistance value distribution of the cross section of the positive electrode active material layer was measured by the method described above, and the values of each item shown in Table 2 were obtained. Also, the conductive carbon content relative to the total mass of the positive electrode active material layer was determined. Table 2 shows the results.
3 is a mapping image showing the measurement results of the spreading resistance value distribution of Example 1, and FIG. 4 is a mapping image of Example 4. As shown in FIG. FIG. 5 is a graph showing the spreading resistance value distribution of Examples 1 and 3, the horizontal axis represents the spreading resistance value (unit: log Ω), and the vertical axis represents the total frequency of resistance values 4.0 to 12.5 (log Ω). represents the relative frequency (unit: %) when is 100%.
Based on the carbon content and compounding amount of the carbon-coated active material and the carbon content and compounding amount of the conductive aid, the content of conductive carbon with respect to the total mass of the positive electrode active material layer was calculated. It is also possible to confirm using the method described in the above <<Method for measuring conductive carbon content>>.

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 obtained in this 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 heat resistance was evaluated by measuring the output retention rate before and after high-temperature storage by the above method. Table 2 shows the results.

As shown in the results of Table 2, Examples 1 to 3, in which the cross section of the positive electrode active material layer has a resistance value of 4.0 to 6.0 (log Ω) as small as 5.0% or less, are in a high temperature environment. Even when stored, the output retention rate was high and the heat resistance was good. It is conceivable that the increase in resistance was difficult to occur because there was almost no low-resistance portion in the positive electrode active material layer, which is likely to undergo a deterioration reaction at high temperatures.
Comparing Examples 1 and 3, Example 1, which does not contain a conductive aid, has a lower total frequency of resistance values of 4.0 to 6.0 (logΩ) and a higher output retention rate.
Example 2 is an example in which the content of conductive carbon is increased without increasing the amount of the conductive additive in Example 1. Compared to Example 1, the total frequency of resistance values 4.0 to 6.0 (log Ω) is less, the average frequency B of resistance values 6.0 to 9.0 (log Ω) is greater, and the output maintenance rate is further increased. Improved.

On the other hand, Examples 4 and 5, in which the total frequency of resistance values 4.0 to 6.0 (log Ω) exceeds 5.0%, have a remarkable output retention rate when stored in a high temperature environment compared to Examples 1 to 3. was low, and the heat resistance was poor. As can be seen from the mapping image of FIG. 4, the positive electrode active material layer of Example 4 is locally dotted with low-resistance portions. It is considered that such a low-resistance portion became an active point in a high-temperature environment, and a deterioration reaction occurred.

REFERENCE SIGNS LIST 1 positive electrode 2 separator 3 negative electrode 5 exterior body 10 secondary battery 11 current collector (positive electrode current collector)
REFERENCE SIGNS LIST 12 Positive electrode active material layer 13 Positive electrode current collector exposed portion 14 Positive electrode current collector main body 15 Current collector coating layer

Claims (10)

having a current collector and a positive electrode active material layer containing positive electrode active material particles present on the current collector;
In the spreading resistance value distribution of the positive electrode active material layer, when the total frequency of resistance values 4.0 to 12.5 (log Ω) is 100%, the total frequency of resistance values 4.0 to 6.0 (log Ω) is A positive electrode for a non-aqueous electrolyte secondary battery, which is 0.0 to 5.0%. 2. The nonaqueous according to claim 1, wherein in the spreading resistance value distribution, the average frequency of resistance values 6.0 to 9.0 (log Ω) is higher than the average frequency of resistance values 4.0 to 6.0 (log Ω). Positive electrode for electrolyte secondary batteries. 3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein said positive electrode active material layer contains a conductive aid. 4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 3, wherein an active material coating portion containing a conductive material is present on at least part of the surface of said positive electrode active material particles. 3. The non-aqueous electrolyte 2 according to claim 1, wherein the positive electrode active material layer does not contain a conductive aid, and an active material coating portion containing a conductive material is present on at least part of the surface of the positive electrode active material particles. Positive electrode for secondary batteries. Claims 3 to 3, wherein the positive electrode active material layer contains conductive carbon, and the content of the conductive carbon is 0.5% by mass or more and less than 3.0% by mass 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 5. The positive electrode active material particles are 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 any one of claims 1 to 6, comprising a compound. The non-aqueous electrolyte 2 according to any one of claims 1 to 7, wherein a current collector coating layer containing a conductive material is present on at least part of the surface of the current collector on the positive electrode active material layer side. Positive electrode for secondary batteries. A positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 8, a negative electrode, and a non-aqueous electrolyte present between the positive electrode and the negative electrode for the non-aqueous electrolyte secondary battery, non-aqueous Electrolyte secondary battery. A battery module or battery system comprising a plurality of non-aqueous electrolyte secondary batteries according to claim 9 .

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