JP2022145472A - 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 provide a cathode for a nonaqueous electrolyte secondary battery, small in the percentage of a component making no contribution to lithium ion conduction, and superior in discharge characteristic in a low-temperature region, a nonaqueous electrolyte secondary battery using the cathod, a battery module, and a battery system.

SOLUTION: A cathode 1 for a nonaqueous electrolyte secondary battery comprises: a cathode collector 11; and a cathode active material layer 12 present on the cathode collector 11. The cathode active material layer 12 has a thickness of 10 μm or more, and the cathode active material layer 12 contains a cathode active material. The cathode active material layer 12 has a surface reflectance of 5.5% or more in a wavelength region of 200-850 nm, which is equal to or smaller than a reflectance specific to a cathode active material included in the cathode active material layer 12.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2023,JPO&INPIT

Description

TECHNICAL FIELD The present invention relates to positive electrodes for non-aqueous electrolyte secondary batteries, non-aqueous electrolyte secondary batteries, battery modules, and battery systems 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.

Non-aqueous electrolyte secondary batteries are required to have battery characteristics at low temperatures. For example, in US Pat. No. 5,300,000, the steps of calculating and storing a battery CCA algorithm as a function of battery internal resistance (IR), measuring the IR of the battery in a test, and using the measured IR, determining a CCA value for a battery under test from the stored algorithm. By this method, Patent Document 1 attempts to improve CCA.

JP 2007-525354 A

However, the invention described in Patent Document 1 cannot satisfy the demand for further improving the battery characteristics of non-aqueous electrolyte secondary batteries at low temperatures.

The present invention has been made in view of the above circumstances, a positive electrode for a non-aqueous electrolyte secondary battery having a low proportion of components that do not contribute to lithium ion conduction and excellent discharge characteristics in a low temperature range, and a positive electrode using the same An object of the present invention is to provide a non-aqueous electrolyte secondary battery, a battery module, and a battery system.

The present invention has the following aspects.
[1] A positive electrode current collector and a positive electrode active material layer present on the positive electrode current collector, wherein the positive electrode active material layer has a thickness of 10 μm or more, and the positive electrode active material layer is a positive electrode active material layer. a non-aqueous substance containing a substance, wherein the reflectance of the surface of the positive electrode active material layer is 5.5% or more in a wavelength range of 200 nm to 850 nm and is equal to or less than the reflectance specific to the positive electrode active material contained in the positive electrode active material layer. Positive electrode for electrolyte secondary batteries.
[2] The positive electrode active material is 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). and the reflectance of the surface of the positive electrode active material layer is 9% or more in the wavelength range of 250 nm to 300 nm and is equal to or less than the reflectance specific to the positive electrode active material. A positive electrode for secondary batteries.
[3] The positive electrode for a nonaqueous electrolyte secondary battery according to [2], wherein the positive electrode active material is lithium iron phosphate represented by _LiFePO4 .
[4] Any one of [1] to [3], wherein the reflectance of the surface of the positive electrode active material layer is 8% or more in a wavelength range of 400 nm to 450 nm and is equal to or less than the reflectance specific to the positive electrode active material. positive electrode for non-aqueous electrolyte secondary batteries.
[5] The positive electrode for a nonaqueous electrolyte secondary battery according to any one of [1] to [4], wherein the positive electrode active material layer does not contain a conductive aid.
[6] The positive electrode current collector has a positive electrode current collector main body and a current collector coating layer present on the surface of the positive electrode current collector main body on the positive electrode active material layer side, [1] to [5] ] The positive electrode for non-aqueous electrolyte secondary batteries according to any one of the above.
[7] The positive electrode for a nonaqueous electrolyte secondary battery according to any one of [1] to [6], which has a potential of 7.5 V or more as measured by the following test method.
(Test method)
Four non-aqueous electrolyte secondary batteries having a rated capacity of 1 Ah are connected in series, and the potential is measured after discharging at a 1 C rate for 30 seconds from full charge.
[8] The positive electrode for the non-aqueous electrolyte secondary battery according to any one of [1] to [7], the negative electrode, and the non-aqueous electrolyte present between the positive electrode for the non-aqueous electrolyte secondary battery and the negative electrode and a non-aqueous electrolyte secondary battery.
[9] A battery module or battery system comprising a plurality of non-aqueous electrolyte secondary batteries according to [8].

According to the present invention, a positive electrode for a non-aqueous electrolyte secondary battery having a low proportion of components that do not contribute to lithium ion conduction and excellent discharge characteristics in a low temperature range, a non-aqueous electrolyte secondary battery using the same, a battery module, and battery system can be provided.

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. FIG. 3 is a diagram showing the results of measuring the spectral reflectance of the surface of the positive electrode active material layer in Example 1 and Comparative Example.

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 contains a positive electrode active material. 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.
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% by mass to 99.9% by mass, more preferably 90.0% by mass 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 (1)”). 0≦x≦1 in the general formula (1). 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 (1) contains a trace amount of metal impurities, the effect of the present invention is not impaired.
The compound represented by the general formula (1) 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.

As the coated particles, coated particles having a core composed of a compound having an olivine-type crystal structure are preferable, and coated particles having a core composed of a compound represented by the general formula (1) are more preferable, and lithium iron phosphate is used 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 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, and even more preferably 90% by mass or more, relative to the total mass of the positive electrode active material particles. 100 mass % may be sufficient.

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 particles. 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. When 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 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.

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 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% 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 current collector main body 14 include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel.
The thickness of the current collector main body 14 is, for example, preferably 8 μm to 40 μm, more preferably 10 μm to 25 μm.
The thickness of current collector main body 14 and the thickness of 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 current collector main body 14 is coated with the current collector coating layer 15 is prepared by applying a slurry containing a conductive material, a binder, and a solvent by a known coating method such as gravure. It can be manufactured by a method of coating the surface of the current collector main body 14 using a solvent and drying it 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 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 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.

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 of the positive electrode 1 excluding the positive electrode current collector main body 14. ~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.

[Thickness of positive electrode active material layer]
In the positive electrode 1 of this embodiment, the thickness of the positive electrode active material layer 12 is preferably 10 μm or more. If the thickness of the positive electrode active material layer 12 is 10 μm or more, it can be formed sufficiently thicker than the current collector, so that the energy density of the battery can be increased. When the thickness of the positive electrode active material layer 12 is 10 μm or more, the surface of the positive electrode active material layer 12 is not affected by the reflectance of the positive electrode current collector 11 even if the positive electrode active material layer 12 is porous. can be measured.

[Reflectance of Surface of Positive Electrode Active Material Layer]
In the positive electrode 1 of the present embodiment, the reflectance of the surface of the positive electrode active material layer 12 is 5.5% or more in the wavelength range of 200 nm to 850 nm, and the reflectance specific to the positive electrode active material contained in the positive electrode active material layer 12 or less. be. The reflectance is preferably 5.8% or more, more preferably 6.0% or more. If the reflectance is 5.5% or more, the proportion of components that do not contribute to lithium ion conduction on the surface of the positive electrode active material layer 12 is reduced, so that hindrance to lithium ion conduction can be suppressed. can. As a result, the positive electrode 1 has excellent discharge characteristics in a low temperature range. In addition, when the proportion of components that do not contribute to the conduction of lithium ions on the surface of the positive electrode active material layer 12 decreases, the proportion of the positive electrode active material on the surface of the positive electrode active material layer 12 increases. The reflectance is equal to or less than the reflectance specific to the positive electrode active material contained in the layer 12 .
In the positive electrode 1 of the present embodiment, the blending amount of the positive electrode active material in the positive electrode active material layer 12 is increased in order to keep the reflectance of the surface of the positive electrode active material layer 12 within the above range.

The method for measuring the reflectance of the surface of the positive electrode active material layer 12 is evaluated by measuring the spectral reflectance of the surface of the positive electrode active material layer 12 .
For measuring the spectral reflectance, for example, a spectrophotometer U-4100 (with a 5° specular reflection accessory) manufactured by Hitachi, Ltd. is used. As a light source, a deuterium lamp in the ultraviolet region and a 50 W halogen lamp in the visible/near infrared region are used. As a detector, an ultraviolet/visible photomultiplier tube and a near-infrared cooled PbS are used. The measurement range is 200 nm-850 nm. Let the sample size be 15 mm×15 mm.
The spectral reflectance of the surface of the positive electrode active material layer 12 is measured while the positive electrode is fully discharged. Alternatively, the positive electrode is discharged to 2.0 V vs Li/Li ⁺ .
The reflectance of the conductive carbon material alone was obtained by adding 100 parts by mass of carbon black, 40 parts by mass of polyvinylidene fluoride as a binder, and N-methylpyrrolidone (NMP) as a solvent on the positive electrode current collector main body 14. is applied, and the reflectance of the obtained thin film is measured for evaluation. The amount of NMP used is the amount necessary to coat the slurry.
The surface reflection of the positive electrode active material layer 12 is affected by the surface roughness. In general, the electrode has a smooth surface by press treatment, and the effect of irregular reflection is low. Furthermore, in this measurement, the incident angle is 5°, and the incident angle is almost vertical, so that the material-specific reflectance can be stably measured.
In addition, when the thickness of the positive electrode active material layer 12 is less than several μm, reflected light from the positive electrode current collector main body 14 may be included. , the surface reflection of the positive electrode active material layer 12 reflects only the spectrum of the reflected light from the surface of the positive electrode active material layer 12 .

A method for measuring the reflectance specific to the positive electrode active material contained in the positive electrode active material layer 12 is as follows.
A slurry containing only the positive electrode active material is applied onto the main body of the positive electrode current collector, and the reflectance of the thin film obtained by drying is measured to evaluate the reflectance specific to the positive electrode active material.
The reflectance specific to the positive electrode active material is measured in the same manner as the reflectance of the surface of the positive electrode active material layer 12 is measured.

In the positive electrode 1 of the present embodiment, the reflectance of the surface of the positive electrode active material layer 12 is 9% or more in the wavelength range of 250 nm to 300 nm, and the reflectance specific to the positive electrode active material contained in the positive electrode active material layer 12 or less. is preferred. When the reflectance is 9% or more, the proportion of the components that do not contribute to the conduction of lithium ions is smaller, so that the hindrance to the conduction of lithium ions can be further suppressed. In addition, when the proportion of components that do not contribute to the conduction of lithium ions on the surface of the positive electrode active material layer 12 decreases, the proportion of the positive electrode active material on the surface of the positive electrode active material layer 12 increases. The reflectance is equal to or less than the reflectance specific to the positive electrode active material contained in the layer 12 .

In the positive electrode 1 of the present embodiment, the reflectance of the surface of the positive electrode active material layer 12 is 8% or more in the wavelength range of 400 nm to 450 nm, and the reflectance specific to the positive electrode active material contained in the positive electrode active material layer 12 or less. is preferred. The reflectance is preferably equal to or lower than the reflectance specific to the positive electrode active material contained in the positive electrode active material layer 12 . When the reflectance is 8% or more, the ratio of the components that do not contribute to the conduction of lithium ions is smaller, so that the hindrance to the conduction of lithium ions can be further suppressed. In addition, when the proportion of components that do not contribute to the conduction of lithium ions on the surface of the positive electrode active material layer 12 decreases, the proportion of the positive electrode active material on the surface of the positive electrode active material layer 12 increases. It becomes equal to or less than the reflectance specific to the positive electrode active material contained in the layer 12 .

In the positive electrode 1 of this embodiment, the potential measured by the following test method is 7.5 V or higher.
(Test method)
Four non-aqueous electrolyte secondary batteries having a rated capacity of 1 Ah are connected in series, and the potential is measured after discharging at a 1 C rate for 30 seconds from full charge.
When the positive electrode 1 has a potential of 7.5 V or higher as measured by the above test method, the cold cranking characteristics for use in a lead-acid battery can be satisfied.

According to the positive electrode 1 of the present embodiment, the reflectance of the surface of the positive electrode active material layer 12 is 5.5% or more in the wavelength range of 200 nm to 850 nm, and the reflectance specific to the positive electrode active material contained in the positive electrode active material layer 12. Therefore, the proportion of components that do not contribute to the conduction of lithium ions on the surface of the positive electrode active material layer 12 is small, and discharge characteristics in a low temperature range are excellent. That is, the positive electrode 1 of the present embodiment has good cold cranking characteristics in a low temperature range, suppresses an increase in resistance particularly after starting use, and can be used stably for a long period of time.
Conventionally, a conductive material such as carbon, a conductive aid, a binder, and other additives that cover the surface of the positive electrode active material usually segregate on the surface of the positive electrode active material layer that constitutes the positive electrode. Segregation of the conductive material and the binder on the surface of the positive electrode active material layer increases interfacial resistance and adversely affects battery characteristics. In particular, in the discharge characteristics in the low temperature range, if the ion conduction of the positive electrode active material existing near the surface of the positive electrode active material layer, which is closest to the negative electrode side, is hindered, the characteristics are adversely affected, resulting in a decrease in potential at the beginning of discharge (due to DC resistance IR drop) is a problem. In the positive electrode 1 of the present embodiment, the reflectance of the surface of the positive electrode active material layer 12 is 5.5% or more in the wavelength range of 200 nm to 850 nm, and the reflectance specific to the positive electrode active material contained in the positive electrode active material layer 12 or less. By doing so, the ratio of components that do not contribute to the conduction of lithium ions on the surface of the positive electrode active material layer 12 is reduced, thereby solving the above problem.

<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. The negative electrode 3 and the separator 2 are used one more than the number of the positive electrodes 1, and 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, the negative electrode current collector 31 is coated with the composition, and the negative electrode active material is dried to remove the solvent. 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% by mass, more preferably 85.0% to 98.0% by mass, relative to the total mass of the negative electrode active material layer 32. is more preferred.

[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, γ-butyrolactone, sulfolane, dimethylsulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrohydrafuran, 2-methyl Polar solvents such as tetrahydrofuran, 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, a battery module configured by connecting a plurality of non-aqueous electrolyte secondary batteries in series or in parallel, a battery pack comprising a plurality of electrically connected battery modules and a battery control system, and a plurality of electrically connected It can be used for a storage battery system or the like that includes individual battery modules and a battery control system.

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

<Measurement method>
[Method for measuring spectral reflectance]
(Measurement condition)
Measuring device: Spectrophotometer U-4100 manufactured by Hitachi, Ltd. (with 5° specular reflection accessory)
Light source: UV deuterium lamp, visible/near infrared 50W halogen lamp Detector: UV/visible photomultiplier tube, near infrared cooled PbS
Measurement range: 200-850nm
Sample size: 15mm x 15mm

(Measurement of spectral reflectance)
The spectral reflectance of the positive electrode active material layer is measured while the positive electrode is fully discharged. Alternatively, the positive electrode is discharged to 2.0 V vs Li/Li ⁺ .
The reflectance of the conductive carbon material alone is obtained by adding 100 parts by mass of carbon black, 40 parts by mass of polyvinylidene fluoride as a binder, and a solvent on an aluminum foil (thickness of 15 μm) that is the main body of the positive electrode current collector. A slurry mixed with N-methylpyrrolidone (NMP) was applied, dried, and the reflectance of the obtained thin film was measured for evaluation. The amount of NMP used was the amount necessary for coating the slurry.
The surface reflection of the positive electrode active material layer is affected by the surface roughness. In general, the electrode has a smooth surface by press treatment, and the effect of irregular reflection is low. Furthermore, in this measurement, the incident angle is 5°, and the incident angle is almost vertical, so that the material-specific reflectance can be stably measured.
In addition, when the thickness of the positive electrode active material layer is less than several μm, the reflected light of the positive electrode current collector body may be included. Therefore, the surface reflection of the positive electrode active material layer reflects only the spectrum of light reflected from the surface of the positive electrode active material layer.

<Production example: 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 into a 42 mm square electrode shape to obtain a negative electrode.

[Example 1]
As the positive electrode active material, carbon-coated lithium iron phosphate (hereinafter also referred to as “carbon-coated active material”; average particle size: 1.0 μm) was used. The coating amount of carbon was set to 1.0% by mass.
No conductive aid was used.
First, 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.
Next, a positive electrode active material layer was formed by the following method.
100 parts by mass of the carbon-coated active material, 0.5 parts by mass of polyvinylidene fluoride as a binder, and NMP as a solvent 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.
The composition for manufacturing a positive electrode was coated on both sides of the positive electrode current collector, pre-dried, and then vacuum-dried at 120° C. to form a positive electrode active material layer having a thickness of 70 μm.
The resulting laminate was pressed under a load of 10 kN to obtain a positive electrode sheet.
The obtained positive electrode sheet was punched into a 40 mm square electrode shape to obtain a positive electrode.

The spectral reflectance of the surface of the positive electrode active material layer of the obtained positive electrode was measured. The results are shown in FIG. From the results shown in FIG. 3, the reflectance is 6.5% or more in the wavelength range of 200 nm to 850 nm, and the reflection spectrum specific to lithium iron phosphate contained in the positive electrode active material layer is reflected in the wavelength range of 250 nm to 300 nm. , the reflectance was 11%. In addition, the reflectance was 8.9% or more in the wavelength region of 400 nm to 450 nm, reflecting the small amount of carbon black. The reflectance tended to decrease as the wavelength became longer, and the reflectance at 850 nm was 6.6%.
When the amount of reflected light of only the conductive aid (carbon black) was measured, the reflectance was 3% to 5% in the wavelength range of 200 nm to 300 nm, and the reflectance was 3% or less in the wavelength range of 300 nm to 850 nm. It was found that carbon black has a much higher light absorption than the positive electrode active material layer in this example. Since the amount of carbon black added was small, reflection specific to the lithium iron phosphate particles was observed on the surface of the positive electrode active material layer, and it is considered that a state in which the amount of material that does not contribute to lithium conduction was small was achieved.

A non-aqueous electrolyte secondary battery having the configuration shown in FIG. 2 was manufactured by the following method.
Ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed so that the volume ratio of EC:PC:DEC was 30: ₅ :65. A non-aqueous electrolytic solution was prepared by dissolving so as to obtain 1 liter.
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 and the positive electrode were laminated, and then the negative electrode was laminated on the separator.
A terminal tab was electrically connected to each of the positive electrode current collector exposed portion and the negative electrode current collector exposed portion of the electrode laminate, and the electrode laminate was wrapped with an aluminum laminate film so that the terminal tab protruded outside. It was sandwiched and sealed by laminating three sides.
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 capacity of the obtained non-aqueous electrolyte secondary battery was 1.0 Ah.

After the obtained non-aqueous electrolyte secondary battery was fully charged (100% SOC) at 25 ° C., it was discharged at -30 ° C. from the fully charged state at 1 C, and 30 seconds after the start of discharge (equivalent to 8 mAh) was observed.
As a result, the potential of the non-aqueous electrolyte secondary battery of this example was 2.52V. A battery module obtained by connecting four cells (non-aqueous electrolyte secondary batteries) in series had a potential of 10.08 V, which satisfied the cold cranking characteristics for a lead-acid battery.

Further, the non-aqueous electrolyte secondary battery of this example was subjected to 1000 charge/discharge cycles at 40° C., one cycle of 1C charge/1C discharge. After that, the non-aqueous electrolyte secondary battery was fully charged (100% SOC) at 25° C., and then discharged at 1 C from the fully charged state at −30° C. The potential after 30 seconds from the start of discharge was 2.0. It was 45V. The electric potential of the battery obtained by connecting four cells (non-aqueous electrolyte secondary batteries) in series was 9.8 V, which satisfied the cold cranking characteristics for lead-acid battery applications.
These results are shown in Table 1.

[Example 2]
A positive electrode of Example 2 was produced in the same manner as in Example 1, except that 1.0% by mass of carbon black as a conductive aid was added.
The spectral reflectance of the surface of the positive electrode active material layer of the obtained positive electrode was measured. As a result, the reflectance was 6.2% or more in the wavelength range of 200 nm to 850 nm, and in the wavelength range of 250 nm to 300 nm, the reflectance was was 9.5%. Moreover, the reflectance was 8.9% or more in the wavelength range of 400 nm to 450 nm. The reason why the reflectance is lower than that of Example 1 is that a small amount of carbon black is added.

A non-aqueous electrolyte secondary battery (laminate cell) was manufactured using the obtained positive electrode. The capacity of the obtained non-aqueous electrolyte secondary battery was 1.0 Ah.
After the obtained non-aqueous electrolyte secondary battery was fully charged (100% SOC) at 25 ° C., it was discharged at -30 ° C. from the fully charged state at 1 C, and 30 seconds after the start of discharge (equivalent to 8 mAh) When the potential of was observed, the potential was 2.5V. The reason why the potential was slightly lower than in Example 1 is considered to be that the small amount of carbon black contained slightly hindered the lithium conduction path.

Further, the non-aqueous electrolyte secondary battery of this example was subjected to 1000 charge/discharge cycles at 40° C., one cycle of 1C charge/1C discharge. After that, the non-aqueous electrolyte secondary battery was fully charged (100% SOC) at 25° C., and then discharged at 1 C from the fully charged state at −30° C. The potential after 30 seconds from the start of discharge was 2.0. It was 35V. A battery module obtained by connecting four cells (non-aqueous electrolyte secondary batteries) in series had a potential of 9.4 V, which satisfied the cold cranking characteristics for a lead-acid battery.
These results are shown in Table 1.
As in Example 1, the longer the wavelength, the lower the reflectance, and the reflectance at 850 nm was 6.2%.

[Comparative example]
A positive electrode of a comparative example was produced in the same manner as in Example 1, except that 5.0% by mass of carbon black as a conductive aid was added.
The spectral reflectance of the surface of the positive electrode active material layer of the obtained positive electrode was measured. The results are shown in FIG. As a result, the reflectance was 5.0% in the wavelength range of 200 nm to 850 nm, and the reflectance was 5.0% in the wavelength range of 250 nm to 300 nm, reflecting the reflection spectrum specific to lithium iron phosphate contained in the positive electrode active material layer. It was 6.2%. In addition, the reflectance was 5.0% or more in the wavelength range of 400 nm to 450 nm. The reason why the reflectance is lower than that of Examples 1 and 2 is that the amount of carbon black added is large. Due to the influence of carbon black, the reflectance has a minimum value at a wavelength of 410 nm. rice field.

A non-aqueous electrolyte secondary battery (laminate cell) was manufactured using the obtained positive electrode. The capacity of the obtained non-aqueous electrolyte secondary battery was 1.0 Ah.
After the obtained non-aqueous electrolyte secondary battery was fully charged (100% SOC) at 25 ° C., it was discharged at -30 ° C. from the fully charged state at 1 C, and 30 seconds after the start of discharge (equivalent to 8 mAh) When the potential of was observed, the potential was 2.4V. The reason why the potential was even lower than in Example 2 is considered to be that the large amount of carbon black added hindered the lithium conduction path to such an extent that the battery characteristics were greatly affected.

Further, the non-aqueous electrolyte secondary battery of this example was subjected to 1000 charge/discharge cycles at 40° C., one cycle of 1C charge/1C discharge. After that, the non-aqueous electrolyte secondary battery was fully charged (100% SOC) at 25° C., and then discharged at 1 C from the fully charged state at −30° C. The potential after 30 seconds from the start of discharge was 1.0. It was 8V. A battery module obtained by connecting four cells (non-aqueous electrolyte secondary batteries) in series had a potential of 7.2 V, which did not satisfy the cold cranking characteristics for use in a lead-acid battery. The increase in resistance and the decrease in potential after cycling are thought to be due to side reactions caused by carbon black. It was found that the deterioration was significant.
These results are shown in Table 1.

1 positive electrode 2 separator 3 negative electrode 5 outer package 10 secondary battery 11 positive electrode current collector 12 positive electrode active material layer 13 positive electrode current collector exposed portion 14 current collector main 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 (9)

Having a positive electrode current collector and a positive electrode active material layer present on the positive electrode current collector,
The thickness of the positive electrode active material layer is 10 μm or more,
The positive electrode active material layer contains a positive electrode active material,
The non-aqueous electrolyte secondary battery, wherein the reflectance of the surface of the positive electrode active material layer is 5.5% or more in a wavelength range of 200 nm to 850 nm and is equal to or less than the reflectance specific to the positive electrode active material contained in the positive electrode active material layer. positive electrode. 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). and
2. The positive electrode for a non-aqueous electrolyte secondary battery in accordance with claim 1, wherein the surface reflectance of said positive electrode active material layer is 9% or more in a wavelength range of 250 nm to 300 nm and equal to or less than the reflectance specific to said positive electrode active material. 3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 2, wherein said positive electrode active material is lithium iron phosphate represented by _LiFePO4 . The nonaqueous according to any one of claims 1 to 3, wherein the reflectance of the surface of the positive electrode active material layer is 8% or more in the wavelength range of 400 nm to 450 nm, and the reflectance specific to the positive electrode active material or less. Positive electrode for electrolyte secondary batteries. 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. 6. Any one of claims 1 to 5, wherein the positive electrode current collector has a positive electrode current collector main body and a current collector coating layer present on the surface of the positive electrode current collector main body on the positive electrode active material layer side. The positive electrode for a non-aqueous electrolyte secondary battery according to Item 1. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the potential measured by the following test method is 7.5 V or higher.
(Test method)
Four non-aqueous electrolyte secondary batteries having a rated capacity of 1 Ah are connected in series, and the potential is measured after discharging at a 1 C rate for 30 seconds from full charge. A positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 7, a negative electrode, and a non-aqueous electrolyte present between the positive electrode for a non-aqueous electrolyte secondary battery and the negative electrode , non-aqueous electrolyte secondary battery. A battery module or battery system comprising a plurality of non-aqueous electrolyte secondary batteries according to claim 8 .

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