JP2022075345A - Positive electrode for power storage element and power storage element - Google Patents

Abstract

To provide a positive electrode for a power storage element having high output in a low temperature environment at the initial stage of the power storage element and can suppress the decrease in the output in the low temperature environment after high temperature storage, and the power storage element having high output in the low temperature environment at the initial stage and suppresses the decrease in output in the low temperature environment after high temperature storage.SOLUTION: A positive electrode for a power storage element includes a positive electrode base material, a positive electrode active material layer including a positive electrode active material, and an undercoat layer arranged between the positive electrode base material and the positive electrode active material layer, and the undercoat layer contains graphite, a half-value width of a (002) plane in XRD of the graphite is 0.15° or more and 0.24° or less, and the positive electrode active material contains a compound represented by the following formula (1). A density of the positive electrode active material layer is 1.80 g/cm3 or more and 2.10 g/cm3 or less. LiFexMn(1-x)PO4(0≤x≤1) (1).SELECTED DRAWING: None

Description

The present invention relates to a positive electrode for a power storage element and a power storage element.

Non-aqueous electrolyte secondary batteries typified by lithium ion secondary batteries are widely used in personal computers, electronic devices such as communication terminals, automobiles, etc. due to their high energy density. The non-aqueous electrolyte secondary battery generally has a pair of electrodes electrically separated by a separator and a non-aqueous electrolyte interposed between the electrodes, and transfers ions between the two electrodes. It is configured to charge and discharge. In addition, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as power storage elements other than non-aqueous electrolyte secondary batteries.

In recent years, inexpensive and highly safe olivine-type lithium iron phosphate has been attracting attention as a positive electrode active material used in the power storage element. For example, a non-aqueous electrolyte power storage element including a carbon material having a plurality of pores forming a three-dimensional network structure and a positive electrode containing a positive electrode active material containing lithium metal oxide particles has been proposed (Patent Document 1). reference).

Japanese Unexamined Patent Publication No. 2017-92000

However, the power storage element provided with the positive electrode as in Patent Document 1 may have a low output in an initial low temperature environment such as 0 ° C. or lower, and in addition, in the low temperature environment after high temperature storage such as 50 ° C. or higher. There is a risk that the output will decrease.

An object of the present invention is a positive electrode for a power storage element, which has a high output in an initial low temperature environment of the power storage element and can suppress a decrease in output in a low temperature environment after high temperature storage, and an initial low temperature environment. It is an object of the present invention to provide a power storage element having a high output in a high temperature and suppressing a decrease in the output in a low temperature environment after storage at a high temperature.

The positive electrode for a power storage element according to one aspect of the present invention includes a positive electrode base material, a positive electrode active material layer containing a positive electrode active material, and an undercoat layer arranged between the positive electrode base material and the positive electrode active material layer. The undercoat layer contains graphite, and the half-value width of the (002) plane in X-ray diffraction (hereinafter, also referred to as XRD) of the graphite is 0.15 ° or more and 0.24 ° or less, and the positive electrode active material. Contains the compound represented by the following formula 1, and the density of the positive electrode active material layer is 1.80 g / cm ³ or more and 2.10 g / cm ³ or less.
LiFe _x Mn _(1-x) PO ₄ (0 ≦ x ≦ 1) ・ ・ ・ 1

The power storage element according to another aspect of the present invention includes a positive electrode for the power storage element.

The positive electrode for a power storage element according to one aspect of the present invention can increase the output of the power storage element in a low temperature environment at the initial stage and suppress the decrease in the output in a low temperature environment after storage at a high temperature.

The power storage element according to another aspect of the present invention has a high output in an initial low temperature environment, and a decrease in output in a low temperature environment after high temperature storage is suppressed.

FIG. 1 is a perspective perspective view showing an embodiment of a non-aqueous electrolyte power storage device. FIG. 2 is a schematic view showing an embodiment of a power storage device in which a plurality of non-aqueous electrolyte power storage elements are assembled.

First, the outline of the positive electrode for a power storage element and the power storage element disclosed by the present specification will be described.

The positive electrode for a power storage element according to one aspect of the present invention includes a positive electrode base material, a positive electrode active material layer containing a positive electrode active material, and an undercoat layer arranged between the positive electrode base material and the positive electrode active material layer. The undercoat layer contains graphite, the half-value width of the (002) plane of the graphite is 0.15 ° or more and 0.24 ° or less, and the positive electrode active material is represented by the following formula 1. It contains a compound and the density of the positive electrode active material layer is 1.80 g / cm ³ or more and 2.10 g / cm ³ or less.
LiFe _x Mn _(1-x) PO ₄ (0 ≦ x ≦ 1) ・ ・ ・ 1

According to this positive electrode for a power storage element, it is possible to increase the output of the power storage element in a low temperature environment at the initial stage and suppress a decrease in the output in a low temperature environment after storage at a high temperature. The reason for this is not always clear, but it can be inferred as follows, for example. That is, the charge / discharge capacity can be increased by containing the compound of the above formula 1 as the positive electrode active material, that is, iron, manganese, or a combination thereof as the transition metal. Further, since the undercoat layer contains graphite, the contact resistance between the positive electrode base material and the positive electrode active material layer can be reduced as compared with the case where the positive electrode base material and the positive electrode active material are in direct contact with each other. Moreover, among carbon materials, graphite is less likely to swell during high-temperature storage than amorphous carbon, etc., and therefore, the inclusion of graphite in the undercoat layer causes expansion of the undercoat layer during high-temperature storage. It is possible to suppress an increase in resistance, and thereby it is possible to reduce a decrease in the output of the power storage element in a low temperature environment after the high temperature storage. In addition, since graphite having a half width of the (002) plane in XRD of 0.15 ° or more and 0.24 ° or less has appropriate crystallinity, the undercoat layer containing this graphite has excellent adhesion. And it has excellent conductivity. Therefore, it is possible to suppress the decrease in the output of the power storage element in the initial low temperature environment due to the resistance in the undercoat layer and the decrease in the output in the low temperature environment after the high temperature storage. In addition, the density of the positive electrode active material layer is 1.80 g / cm ³ or more and 2.10 g / cm ³ or less, so that the contact between the positive electrode active material layer and the undercoat layer is sufficient. Therefore, it is possible to suppress an increase in resistance at this interface and improve the diffusivity of lithium ions in the pores of the positive electrode active material layer. Therefore, it is presumed that the positive electrode for the power storage element can increase the output in the low temperature environment at the initial stage of the power storage element and suppress the decrease in the output in the low temperature environment after the high temperature storage.

The power storage element according to another aspect of the present invention includes a positive electrode for the power storage element.

As described above, since the power storage element includes the positive electrode for the power storage element, as described above, the output in the low temperature environment at the initial stage is high, and the decrease in the output in the low temperature environment after the high temperature storage is suppressed. There is.

The positive electrode for a power storage element, the configuration of the power storage element, the configuration of the power storage device, the method of manufacturing the power storage element, and other embodiments according to the embodiment of the present invention will be described in detail. The name of each component (each component) used in each embodiment may be different from the name of each component (each component) used in the background technique.

<Positive electrode for power storage element>
The positive electrode for a power storage element (hereinafter, also simply referred to as a positive electrode) is an undercoat layer arranged between a positive electrode base material, a positive electrode active material layer containing a positive electrode active material, and the positive electrode base material and the positive electrode active material layer. The undercoat layer contains graphite, the half-value width of the (002) plane of the graphite is 0.15 ° or more and 0.24 ° or less, and the positive electrode active material is represented by the following formula 1. The density of the positive electrode active material layer is 1.80 g / cm ³ or more and 2.10 g / cm ³ or less.

(Positive electrode base material)
The positive electrode substrate has conductivity. Whether or not it has "conductivity" is determined with a volume resistivity of ¹⁰⁷ Ω · cm measured in accordance with JIS-H-0505 (1975) as a threshold value. As the material of the positive electrode base material, metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof are used. Among these, aluminum or an aluminum alloy is preferable from the viewpoint of potential resistance, high conductivity, and cost. Examples of the positive electrode base material include foils, thin-film deposition films, meshes, porous materials, and the like, and foils are preferable from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferable as the positive electrode base material. Examples of aluminum or aluminum alloy include A1085, A3003, and A1N30 specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

The average thickness of the positive electrode substrate is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and 40 μm or less, further preferably 8 μm or more and 30 μm or less, and particularly preferably 10 μm or more and 25 μm or less. By setting the average thickness of the positive electrode base material in the above range, it is possible to increase the energy density per volume of the secondary battery while increasing the strength of the positive electrode base material. The "average thickness of the base material" refers to a value obtained by dividing the punched mass when punching a base material having a predetermined area by the true density of the base material and the punched area, and the same applies to the negative electrode base material.

(Undercoat layer)
The undercoat layer is a layer arranged between the positive electrode base material and the positive electrode active material layer. The undercoat layer contains graphite, and the half width of the (002) plane of the graphite in XRD is 0.15 ° or more and 0.24 ° or less. When the undercoat layer contains graphite, the contact resistance between the positive electrode base material and the positive electrode active material layer can be reduced as compared with the case where the positive electrode base material and the positive electrode active material are in direct contact with each other. In addition, since graphite is less likely to swell during high-temperature storage than amorphous carbon such as acetylene black, the inclusion of graphite in the undercoat layer causes resistance due to expansion of the undercoat layer during high-temperature storage. The increase can be suppressed, and thus the decrease in the output of the power storage element in the low temperature environment after the high temperature storage can be suppressed. In addition to graphite, the undercoat layer may contain, for example, a binder and a conductive agent other than the above-mentioned graphite. When the undercoat layer contains the above-mentioned conductive agent, the contact resistance between the positive electrode base material and the positive electrode active material layer can be further reduced.

The half width of the (002) plane in XRD of graphite is measured as the half width of the peak corresponding to the (002) plane by the powder X-ray diffraction method using CuKα rays. The lower limit of the half width of the (200) plane of graphite is 0.15 °, preferably 0.18 °. When the half width of the surface (200) is equal to or greater than the lower limit, the adhesion between the graphite-containing undercoat layer and the positive electrode active material layer is improved, and the increase in resistance in the undercoat layer is reduced. be able to. In addition, the crystallinity of the graphite is not too low and is suitable for improving the conductivity between the undercoat layer and the positive electrode active material layer. The upper limit of the half width of the (200) plane of graphite is 0.24 °, preferably 0.21 °. When the half-value width of the (200) plane of graphite is not more than the upper limit, the crystallinity of the graphite is not too high, and the conductivity between the undercoat layer and the positive electrode active material layer is improved. It will be moderate. Therefore, when the half-value width of the (200) plane of graphite is within the above range, the output in the low temperature environment at the initial stage of the power storage element can be increased, and the output in the low temperature environment after high temperature storage can be further reduced. Can be reduced.

The full width at half maximum of the (002) plane of graphite is measured using an X-ray diffractometer (Rigaku, model name: MiniFlex II). Specifically, it is carried out according to the following conditions and procedures. The radiation source is CuKα ray, and the acceleration voltage and current are 30 kV and 15 mA, respectively. The sampling width is 0.01 deg, the scanning time is 14 minutes (scan speed is 5.0), the divergent slit width is 0.625 deg, the light receiving slit width is open, and the scattering slit is 8.0 mm. The obtained X-ray diffraction data is determined by the half-value width output by automatic analysis using the software "PDXL" attached to the X-ray diffraction device. When analyzing the X-ray diffraction data, the peak derived from Kα2 is not removed.

For the measurement of the half width of the above-mentioned (002) plane of graphite, if the graphite powder is before the formation of the undercoat layer, it is used as it is for the measurement. After the formation of the undercoat layer, the undercoat layer is peeled off from the positive electrode substrate and collected. This is used as it is for measurement.
When a measurement sample is collected from the positive electrode taken out by disassembling the power storage element, the power storage element is energized with a constant current for 1 hour in a 25 ° C environment before disassembling the power storage element. With a current value (0.1C) that is one tenth of the current value that has the same amount of electricity as the nominal capacity of the above, constant current discharge is performed up to a voltage that is the lower limit of the specified voltage. The power storage element is disassembled, the positive electrode is taken out, a battery with a metallic lithium electrode as the counter electrode is assembled, the current value is 10 mA per 1 g of the positive electrode mixture (positive electrode active material layer), and the voltage between the terminals of the positive electrode is 2.0 V (vs. Perform constant current discharge until Li / Li ⁺ ), and adjust to a completely discharged state. Re-disassemble and take out the positive electrode. The removed positive electrode was thoroughly washed with dimethyl carbonate to thoroughly wash the non-aqueous electrolyte adhering to the positive electrode, dried at room temperature for 24 hours, then the positive electrode active material layer was removed, and the exposed undercoat layer was peeled off from the positive electrode substrate. And collect. This is used as it is for measurement.
The work from dismantling to re-disassembling the power storage element, and the cleaning and drying work of the positive electrode are performed in an argon atmosphere having a dew point of −60 ° C. or lower.

The lower limit of the average thickness of the undercoat layer is preferably 0.1 μm, more preferably 1 μm. When the average thickness of the undercoat layer is at least the above lower limit, the adhesion between the positive electrode base material and the positive electrode active material layer via the undercoat layer can be sufficiently increased. On the other hand, as the upper limit of the average thickness of the undercoat layer, 5 μm is preferable, and 3 μm is more preferable. When the average thickness of the undercoat layer is not more than the above upper limit, the increase in resistance due to the expansion of the undercoat layer during high temperature storage can be reduced. The average thickness of the undercoat layer is obtained by measuring the thickness at any 10 points and calculating the average value of the measurement results.

(Positive electrode active material layer)
The positive electrode active material layer contains a compound represented by the following formula 1 (hereinafter, also referred to as a first compound). Specifically, the positive electrode active material layer contains a positive electrode active material, and the positive electrode active material contains the first compound. The positive electrode active material layer contains optional components such as a conductive agent, a binder (binder), a thickener, and a filler, if necessary. The density of the positive electrode active material layer is 1.80 g / cm ³ or more and 2.10 g / cm ³ or less.
LiFe _x Mn _(1-x) PO ₄ (0 ≦ x ≦ 1) ・ ・ ・ 1

(Positive electrode active material)
The positive electrode active material contains the first compound. The first compound is represented by the above formula 1 and has an olivine type crystal structure. The compound having an olivine type crystal structure has a crystal structure that can be attributed to the space group Pnma. The crystal structure that can be attributed to the space group Pnma means that it has a peak that can be attributed to the space group Pnma in the X-ray diffraction pattern. Since the first compound is a polyanion salt in which the oxygen desorption reaction from the crystal lattice does not easily proceed, it is highly safe and inexpensive.

As the first compound, as represented by the above formula 1, lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ ), lithium iron manganese phosphate (LiFe _x Mn _1-x PO ₄ , 0 < It is composed of x <1) or a combination thereof. When the positive electrode active material contains iron, manganese, or a combination thereof as a transition metal, the charge / discharge capacity can be further increased.

As described above, the first compound is a phosphate compound containing iron, manganese or a combination thereof and lithium. The first compound may contain a transition metal element other than iron and manganese and a typical element such as aluminum as an unavoidable impurity. However, it is preferable that the first compound is substantially composed of iron, manganese or a combination thereof, lithium, phosphorus and oxygen.

In the above formula 1, the upper limit of x is 1, preferably 0.95. The lower limit of x is 0, preferably 0.25. When the value of x is not more than the above upper limit or more than the above lower limit, the first compound has more excellent lifetime characteristics. Note that x may be substantially 1.

The average particle size of the primary particles of the first compound is, for example, preferably 0.01 μm or more and 0.2 μm or less, and more preferably 0.02 μm or more and 0.1 μm or less. By setting the average particle size of the primary particles of the first compound in the above range, the diffusivity of lithium ions in the pores of the positive electrode active material layer is further improved. The average particle size of the primary particles is a value measured by observation with a scanning electron microscope.

The average particle size of the secondary particles of the first compound is, for example, preferably 3 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less. By setting the average particle of the secondary particles of the first compound in the above range, the production and handling are facilitated, and the diffusivity of lithium ions in the pores of the positive electrode active material layer is improved. The average particle size of the secondary particles is a value measured by particle size distribution measurement.

A crusher, a classifier, or the like is used to obtain the first compound with a predetermined particle size. Examples of the crushing method include a method using a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling airflow type jet mill, a sieve, or the like. At the time of pulverization, wet pulverization in which water or an organic solvent such as hexane coexists can also be used. As a classification method, a sieve, a wind power classifier, or the like is used as needed for both dry type and wet type.

At least a part of the surface of the first compound may be coated with carbon. As described above, the positive electrode active material may be the first compound in which at least a part of the surface thereof is coated with carbon. The electron conductivity is improved by covering at least a part of the surface of the first compound with carbon. The carbon content in the positive electrode active material is preferably 0.5% by mass or more and 5% by mass or less. By setting the carbon content in the above range, the electrical conductivity can be increased, and the electrode density and the capacity of the power storage element can be increased.

The positive electrode active material layer may further contain a positive electrode active material other than the first compound. As such another positive electrode active material, a known positive electrode active material for a lithium ion secondary battery can be appropriately selected. However, as the lower limit of the total content of the first compound in the total positive electrode active material contained in the positive electrode active material layer, 90% by mass is preferable, and 99% by mass is more preferable. The upper limit of the total content of the first compound in the total positive electrode active material may be 100% by mass. In this way, by using only the positive electrode active material having an olivine type crystal structure as the positive electrode active material, the output of the power storage element in the initial low temperature environment can be increased, and the output in the low temperature environment after high temperature storage can be increased. The effect of suppressing the decrease in output can be further enhanced.

As the positive electrode active material for the lithium ion secondary battery, a material capable of storing and releasing lithium ions is usually used. Examples of the positive electrode active material other than the first compound include a lithium transition metal composite oxide having an α-NaFeO type ₂ crystal structure, a lithium transition metal composite oxide having a spinel type crystal structure, a polyanion compound, a chalcogen compound, and sulfur. And so on. Examples of the lithium transition metal composite oxide having an α-NaFeO type ₂ crystal structure include Li [Li _x Ni _(1-x) ] O ₂ (0 ≦ x <0.5) and Li [Li _x Ni _γ Co ₍ 0 ≦ x <0.5). _1-x-γ) ] O ₂ (0 ≦ x <0.5, 0 <γ <1), Li [Li _x Co _(1-x) ] O ₂ (0 ≦ x <0.5), Li [ Li _x Ni _γ Mn _(1-x-γ) ] O ₂ (0 ≦ x <0.5, 0 <γ <1), Li [Li _x Ni _γ Mn _β Co _(1-x-γ-β) ] O ₂ (0≤x <0.5, 0 <γ, 0 <β, 0.5 <γ + β <1), Li [Li _x Ni _γ Co _β Al _(1-x-γ-β) ] O ₂ ( Examples thereof include 0 ≦ x <0.5, 0 <γ, 0 <β, 0.5 <γ + β <1). Examples of the lithium transition metal composite oxide having a spinel-type crystal structure include Li _x Mn ₂ O ₄ and Li _x Ni _γ Mn _(2-γ) O ₄ . Examples of the polyanionic compound include LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ MnSiO ₄ , Li ₂ CoPO ₄ F and the like. Examples of the chalcogen compound include titanium disulfide, molybdenum disulfide, molybdenum dioxide and the like. The atoms or polyanions in these materials may be partially substituted with atoms or anion species consisting of other elements. The surface of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more of them may be mixed and used.

The content of the total positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and further preferably 80% by mass or more and 95% by mass or less. By setting the content of the total positive electrode active material in the above range, it is possible to achieve both high energy density and manufacturability of the positive electrode active material layer. The lower limit of the content of the first compound in the positive electrode active material layer is preferably 90% by mass, preferably 99% by mass.

(Conducting agent)
The conductive agent is not particularly limited as long as it is a conductive material. Examples of such a conductive agent include carbonaceous materials, metals, conductive ceramics and the like. Examples of the carbonaceous material include graphitized carbon, non-graphitized carbon, graphene-based carbon and the like. Examples of the non-graphitized carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNT), fullerenes and the like. Examples of the shape of the conductive agent include powder and fibrous. As the conductive agent, one of these materials may be used alone, or two or more of them may be mixed and used. Further, these materials may be combined and used. For example, a material in which carbon black and CNT are combined may be used. Among these, carbon black is preferable from the viewpoint of electron conductivity and coatability, and acetylene black is particularly preferable.

The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent in the above range, the energy density of the power storage element can be increased.

(Binder)
Examples of the binder include fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), thermoplastic resins such as polyethylene, polypropylene, polyacrylic, and polyimide; ethylene-propylene-diene rubber (EPDM), sulfone. Examples thereof include elastomers such as polyethyleneized EPDM, styrene butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

The binder content in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By setting the binder content within the above range, the active substance can be stably retained.

(Thickener)
Examples of the thickener include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has a functional group that reacts with lithium or the like, this functional group may be inactivated by methylation or the like in advance.

(Filler)
The filler is not particularly limited. Fillers include polyolefins such as polypropylene and polyethylene, silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, inorganic oxides such as aluminosilicate, magnesium hydroxide, calcium hydroxide, and hydroxide. Hydroxides such as aluminum, carbonates such as calcium carbonate, sparingly soluble ion crystals such as calcium fluoride, barium fluoride, barium sulfate, nitrides such as aluminum nitride and silicon nitride, talc, montmorillonite, boehmite, zeolite, etc. Examples thereof include mineral resource-derived substances such as apatite, kaolin, mulite, spinel, olivine, sericite, bentonite, and mica, or man-made products thereof.

The positive electrode active material layer includes typical non-metal elements such as B, N, P, F, Cl, Br, I, Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, Ba and the like. Typical metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, W and other transition metal elements are used as positive electrode active materials, conductive agents, binders, thickeners, fillers. It may be contained as a component other than.

The lower limit of the density of the positive electrode active material layer is 1.80 g / cm ³ , preferably 1.85 g / cm ³ . When the density of the positive electrode active material layer is at least the above lower limit, the contact between the positive electrode active material layer and the undercoat layer becomes sufficient, and an increase in resistance at this interface can be suppressed. On the other hand, the upper limit of the density of the positive electrode active material layer is 2.10 g / cm ³ , preferably 2.05 g / cm ³ . When the density of the positive electrode active material layer is not more than the above upper limit, the diffusivity of lithium ions in the pores of the positive electrode active material layer can be improved. The density of the positive electrode active material layer is measured by the following method.

For the density of the positive electrode active material layer, the mass per unit area of the positive electrode active material layer is measured, the average thickness of the positive electrode active material layer is measured, and the obtained mass per unit area is divided by the average thickness. Calculated by. Before assembling the power storage element, the density of the positive electrode active material layer is measured as it is. After assembling the power storage element, the density of the positive electrode active material layer of the positive electrode adjusted to the completely discharged state by the above method is measured. The average thickness of the positive electrode active material layer is obtained by measuring the thickness at any 10 points and calculating the average value of the measurement results.

<Manufacturing method of positive electrode for power storage element>
The positive electrode for a power storage element can be manufactured by forming an undercoat layer on a positive electrode base material and forming a positive electrode active material layer on the undercoat layer. The first compound contained in the positive electrode for a power storage element can be produced, for example, based on the following procedure.

(Formation of undercoat layer)
The undercoat layer is formed on the positive electrode substrate by mixing graphite, a conductive agent other than graphite and other components such as binder, and a dispersion medium as necessary, applying the mixture on the positive electrode substrate, and drying the mixture. Can be formed.

(Formation of positive electrode active material layer)
A first compound as a positive electrode active material,, if necessary, a positive electrode active material other than the above-mentioned first compound, a conductive agent, other components such as a binder, and a dispersion medium are mixed, and a mixture is formed on the positive electrode substrate. A positive electrode active material layer can be formed on the undercoat layer by applying it on the undercoat layer, drying it, and compressing it with a roll press or the like.

(Manufacturing of the first compound)
First, a mixed solution of FeSO ₄ and MnSO ₄ at an arbitrary ratio is added dropwise to the reaction vessel at a constant rate, and a NaOH aqueous solution, an NH ₃ aqueous solution, and an NH ₂ NH ₂ aqueous solution are added so that the pH between them remains constant. To prepare a Fe _x Mn _1-x (OH) ₂ precursor. Next, the prepared Fe _x Mn _(1-x) (OH) ₂ precursor is taken out from the reaction vessel and mixed with LiH ₂ PO ₄ and sucrose powder in a solid phase. Then, the obtained mixture is calcined in a nitrogen atmosphere at a calcining temperature of 550 ° C. or higher and 750 ° C. or lower to prepare a first compound having an olivine-type crystal structure and represented by the following formula 1. This first compound is coated with carbon. The first compound not coated with carbon can be produced by performing solid phase mixing and firing in the same manner as described above except that sucrose powder is not added.
LiFe _x Mn _(1-x) PO ₄ (0 ≦ x ≦ 1) ・ ・ ・ 1

<Adjustment of the density of the positive electrode active material layer>
The density of the positive electrode active material layer can be adjusted, for example, by changing the press pressure in the compression when forming the positive electrode active material layer described above, thereby changing the thickness of the positive electrode active material layer.

According to the positive electrode for the power storage element, it is possible to increase the output in the low temperature environment at the initial stage of the power storage element and suppress the decrease in the output in the low temperature environment after the high temperature storage.

<Structure of power storage element>
The power storage element according to an embodiment of the present invention includes an electrode body having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a container for accommodating the electrode body and the non-aqueous electrolyte. The electrode body is usually a laminated type in which a plurality of positive electrodes and a plurality of negative electrodes are laminated via a separator, or a wound type in which a positive electrode and a negative electrode are laminated in a laminated state via a separator. The non-aqueous electrolyte exists in the positive electrode, the negative electrode and the separator. As an example of the power storage element, a non-aqueous electrolyte secondary battery (hereinafter, also simply referred to as “secondary battery”) will be described.

[Positive electrode]
The positive electrode included in the power storage element is the above-mentioned positive electrode for the power storage element.

[Negative electrode]
The negative electrode has a negative electrode base material and a negative electrode active material layer arranged directly on the negative electrode base material or via an intermediate layer. The configuration of the intermediate layer is not particularly limited, and can be selected from, for example, the configurations exemplified by the positive electrode for the power storage element.

(Negative electrode base material)
The negative electrode substrate has conductivity. As the material of the negative electrode base material, metals such as copper, nickel, stainless steel, nickel-plated steel and aluminum, alloys thereof, carbonaceous materials and the like are used. Among these, copper or a copper alloy is preferable. Examples of the negative electrode base material include foils, thin-film deposition films, meshes, porous materials, and the like, and foils are preferable from the viewpoint of cost. Therefore, a copper foil or a copper alloy foil is preferable as the negative electrode base material. Examples of the copper foil include rolled copper foil, electrolytic copper foil and the like.

The average thickness of the negative electrode substrate is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 30 μm or less, further preferably 4 μm or more and 25 μm or less, and particularly preferably 5 μm or more and 20 μm or less. By setting the average thickness of the negative electrode base material in the above range, it is possible to increase the energy density per volume of the secondary battery while increasing the strength of the negative electrode base material.

(Negative electrode active material layer)
The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, if necessary. Optional components such as a conductive agent, a binder, a thickener, and a filler can be selected from the materials exemplified for the positive electrode.

The negative electrode active material layer is a typical non-metal element such as B, N, P, F, Cl, Br, I, Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, Ba, etc. Typical metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, W and other transition metal elements are used as negative electrode active materials, conductive agents, binders, etc. It may be contained as a component other than the thickener and the filler.

The negative electrode active material can be appropriately selected from known negative electrode active materials. As the negative electrode active material for a lithium ion secondary battery, a material capable of storing and releasing lithium ions is usually used. Examples of the negative electrode active material include metal Li; metal or semi-metal such as Si and Sn; metal oxide or semi-metal oxide such as Si oxide, Ti oxide and Sn oxide; Li ₄ Ti ₅ O ₁₂ ; Titanium-containing oxides such as LiTIO _{2 and} TiNb _{2O 7} ; polyphosphate compounds; silicon carbide; carbon materials such as graphite (graphitite) and non-graphitizable carbon (graphitizable carbon or non-graphitizable carbon). Be done. Among these materials, graphite and non-graphitic carbon are preferable. In the negative electrode active material layer, one of these materials may be used alone, or two or more thereof may be mixed and used.

“Graphite” refers to a carbon material having an average lattice spacing (d ₀₀₂ ) of (002) planes determined by X-ray diffraction method before charging / discharging or in a discharged state of 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferable from the viewpoint that a material having stable physical properties can be obtained.

"Non-graphitic carbon" refers to a carbon material having an average lattice spacing (d ₀₀₂ ) of the (002) plane determined by the X-ray diffraction method before charging / discharging or in a discharged state of 0.34 nm or more and 0.42 nm or less. .. Examples of non-graphitizable carbon include non-graphitizable carbon and easily graphitizable carbon. Examples of the non-planar carbon include a resin-derived material, a petroleum pitch or a petroleum pitch-derived material, a petroleum coke or a petroleum coke-derived material, a plant-derived material, an alcohol-derived material, and the like.

Here, the "discharged state" means a state in which the carbon material, which is the negative electrode active material, is discharged so as to sufficiently release lithium ions that can be occluded and discharged by charging and discharging. For example, in a unipolar battery in which a negative electrode containing a carbon material as a negative electrode active material is used as a working electrode and metallic Li is used as a counter electrode, the open circuit voltage is 0.7 V or more.

The “non-graphitizable carbon” refers to a carbon material having d ₀₀₂ of 0.36 nm or more and 0.42 nm or less.

The “graphitizable carbon” refers to a carbon material having d ₀₀₂ of 0.34 nm or more and less than 0.36 nm.

The negative electrode active material is usually particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide or a polyphosphate compound, the average particle size thereof may be 1 μm or more and 100 μm or less. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide or the like, the average particle size thereof may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to the above lower limit or more, the production or handling of the negative electrode active material becomes easy. By setting the average particle size of the negative electrode active material to the above upper limit or less, the electron conductivity of the active material layer is improved. A crusher, a classifier, or the like is used to obtain a powder having a predetermined particle size. The pulverization method and the powder grade method can be selected from, for example, the methods exemplified for the positive electrode. When the negative electrode active material is a metal such as metal Li, the negative electrode active material may be in the form of a foil.

The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material within the above range, it is possible to achieve both high energy density and manufacturability of the negative electrode active material layer.

[Separator]
The separator can be appropriately selected from known separators. As the separator, for example, a separator composed of only a base material layer, a separator having a heat-resistant layer containing heat-resistant particles and a binder formed on one surface or both surfaces of the base material layer can be used. Examples of the shape of the base material layer of the separator include woven fabrics, non-woven fabrics, and porous resin films. Among these shapes, a porous resin film is preferable from the viewpoint of strength, and a non-woven fabric is preferable from the viewpoint of liquid retention of a non-aqueous electrolyte. As the material of the base material layer of the separator, polyolefins such as polyethylene and polypropylene are preferable from the viewpoint of shutdown function, and polyimide and aramid are preferable from the viewpoint of oxidative decomposition resistance. As the base material layer of the separator, a material in which these resins are combined may be used.

The heat-resistant particles contained in the heat-resistant layer preferably have a mass reduction of 5% or less when the temperature is raised from room temperature to 500 ° C. in an air atmosphere of 1 atm, and the mass reduction when the temperature is raised from room temperature to 800 ° C. Is more preferably 5% or less. Inorganic compounds can be mentioned as materials whose mass reduction is less than or equal to a predetermined value. Examples of the inorganic compound include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, barium titanate, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide and aluminosilicate; aluminum nitride and silicon nitride. Nitrides such as; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ion crystals such as calcium fluoride and barium fluoride; covalent crystals such as silicon and diamond; talc, montmorillonite, boehmite, etc. Examples thereof include substances derived from mineral resources such as zeolite, apatite, kaolin, mulite, spinel, olivine, sericite, bentonite, and mica, or man-made products thereof. As the inorganic compound, a simple substance or a complex of these substances may be used alone, or two or more kinds thereof may be mixed and used. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferable from the viewpoint of safety of the power storage element.

The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and preferably 20% by volume or more from the viewpoint of discharge performance. Here, the "porosity" is a volume-based value and means a measured value with a mercury porosity meter.

As the separator, a polymer gel composed of a polymer and a non-aqueous electrolyte may be used. Examples of the polymer include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, polyvinylidene fluoride and the like. The use of polymer gel has the effect of suppressing liquid leakage. As the separator, a polymer gel may be used in combination with a porous resin film or a non-woven fabric as described above.

[Non-water electrolyte]
As the non-aqueous electrolyte, a known non-aqueous electrolyte can be appropriately selected. A non-aqueous electrolyte solution may be used as the non-aqueous electrolyte. The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

As the non-aqueous solvent, a known non-aqueous solvent can be appropriately selected. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphoric acid esters, sulfonic acid esters, ethers, amides, nitriles and the like. As the non-aqueous solvent, a solvent in which some of the hydrogen atoms contained in these compounds are replaced with halogen may be used.

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), and difluoroethylene carbonate. (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate and the like can be mentioned. Among these, EC is preferable.

Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diphenyl carbonate, trifluoroethylmethyl carbonate, bis (trifluoroethyl) carbonate and the like. Among these, EMC is preferable.

As the non-aqueous solvent, it is preferable to use cyclic carbonate or chain carbonate, and it is more preferable to use cyclic carbonate and chain carbonate in combination. By using the cyclic carbonate, the dissociation of the electrolyte salt can be promoted and the ionic conductivity of the non-aqueous electrolyte solution can be improved. By using the chain carbonate, the viscosity of the non-aqueous electrolytic solution can be kept low. When the cyclic carbonate and the chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate: chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of the electrolyte salt include lithium salt, sodium salt, potassium salt, magnesium salt, onium salt and the like. Of these, lithium salts are preferred.

Examples of the lithium salt include inorganic lithium salts such as LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ F) ₂ , lithium bis (oxalate) borate (LiBOB), and lithium difluorooxalate borate (LiFOB). , Lithium oxalate salts such as lithium bis (oxalate) difluorophosphate (LiFOP), LiSO ₃ CF ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiN (SO ₂ CF ₃ ) Examples thereof include lithium salts having a halogenated hydrocarbon group such as (SO ₂ C ₄ F ₉ ), LiC (SO ₂ CF ₃ ) ₃ , and LiC (SO ₂ C ₂ F ₅ ) ₃ . Among these, an inorganic lithium salt is preferable, and LiPF ₆ is more preferable.

The content of the electrolyte salt in the non-aqueous electrolyte solution is preferably 0.1 mol / dm ³ or more and 2.5 mol / dm ³ or less at 20 ° C. and 1 atm, and 0.3 mol / dm ³ or more and 2.0 mol / dm. It is more preferably ³ or less, more preferably 0.5 mol / dm ³ or more and 1.7 mol / dm ³ or less, and particularly preferably 0.7 mol / dm ³ or more and 1.5 mol / dm ³ or less. By setting the content of the electrolyte salt in the above range, the ionic conductivity of the non-aqueous electrolyte solution can be increased.

The non-aqueous electrolyte solution may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additive include halogenated carbonate esters such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); lithium bis (oxalate) borate (LiBOB), lithium difluorooxalate borate (LiFOB), and lithium bis (oxalate). ) Difluorophosphate (LiFOP) and other oxalates; Lithiumbis (fluorosulfonyl) imide (LiFSI) and other imide salts; Biphenyl, alkylbiphenyl, terphenyl, partially hydride of terphenyl, cyclohexylbenzene, t-butylbenzene , T-Amilbenzene, diphenyl ether, dibenzofuran and other aromatic compounds; partial halides of the aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene; 2,4-difluoroanisole, 2 , 5-Difluoroanisole, 2,6-difluoroanisole, 3,5-difluoroanisole and other halogenated anisole compounds; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, anhydrous. Citraconic acid, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic acid anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sulton, propensulton, butane sulton, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, sulfate. Ethylene, sulfolane, dimethylsulfone, diethylsulfone, dimethylsulfoxide, diethylsulfoxide, tetramethylenesulfoxide, diphenylsulfide, 4,4'-bis (2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxy Methyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisol, diphenyldisulfide, dipyridinium disulfide, 1,3-propensulton, 1,3-propanesulton, 1,4-butanesulton, 1,4- Examples thereof include buten sulton, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate and the like. These additives may be used alone or in combination of two or more.

The content of the additive contained in the non-aqueous electrolytic solution is preferably 0.01% by mass or more and 10% by mass or less, and is 0.1% by mass or more and 7% by mass or less with respect to the total mass of the non-aqueous electrolytic solution. It is more preferable to have it, more preferably 0.2% by mass or more and 5% by mass or less, and particularly preferably 0.3% by mass or more and 3% by mass or less. By setting the content of the additive in the above range, it is possible to improve the capacity maintenance performance or the cycle performance after high temperature storage, and further improve the safety.

As the non-aqueous electrolyte, a solid electrolyte may be used, or a non-aqueous electrolyte solution and a solid electrolyte may be used in combination.

The solid electrolyte can be selected from any material having ionic conductivity such as lithium, sodium and calcium and being solid at room temperature (for example, 15 ° C to 25 ° C). Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, polymer solid electrolytes and the like.

Examples of the lithium ion secondary battery include Li ₂ SP ₂ S ₅ , LiI-Li ₂ SP ₂ S ₅ , Li ₁₀ Ge-P ₂ S ₁₂ , and the like as the sulfide solid electrolyte. ..

<Specific configuration of power storage element>
The shape of the power storage element of the present embodiment is not particularly limited, and examples thereof include a cylindrical battery, a square battery, a flat battery, a coin battery, and a button battery.
FIG. 1 shows a power storage element 1 as an example of a square battery. The figure is a perspective view of the inside of the container. The electrode body 2 having the positive electrode and the negative electrode wound around the separator is housed in the square container 3. The positive electrode is electrically connected to the positive electrode terminal 4 via the positive electrode lead 41. The negative electrode is electrically connected to the negative electrode terminal 5 via the negative electrode lead 51. A non-aqueous electrolyte is injected into the container 3.

<Configuration of power storage device>
The power storage element of the present embodiment is a power source for automobiles such as an electric vehicle (EV), a hybrid vehicle (HEV), and a plug-in hybrid vehicle (PHEV), a power source for electronic devices such as a personal computer and a communication terminal, or a power source for power storage. For example, it can be mounted as a power storage unit (battery module) composed of a plurality of power storage elements 1 assembled together. In this case, the technique of the present invention may be applied to at least one power storage element included in the power storage device.
FIG. 2 shows an example of a power storage device 30 in which a power storage unit 20 in which two or more electrically connected power storage elements 1 are assembled is further assembled. The power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1, a bus bar (not shown) that electrically connects two or more power storage units 20 and the like. The power storage unit 20 or the power storage device 30 may include a state monitoring device (not shown) for monitoring the state of one or more power storage elements.

<Manufacturing method of power storage element>
The method for manufacturing the power storage element of the present embodiment can be appropriately selected from known methods except that the positive electrode for the power storage element is used as the positive electrode. The manufacturing method includes, for example, preparing an electrode body, preparing a non-aqueous electrolyte, and accommodating the electrode body and the non-aqueous electrolyte in a container. Preparing the electrode body includes preparing the positive electrode body and the negative electrode body, and forming the electrode body by laminating or winding the positive electrode body and the negative electrode body via the separator.

The storage of the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte solution, the non-aqueous electrolyte solution may be injected from the injection port formed in the container, and then the injection port may be sealed.

<Other embodiments>
The power storage element of the present invention is not limited to the above embodiment, and various modifications may be made without departing from the gist of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and a part of the configuration of one embodiment can be replaced with the configuration of another embodiment or a well-known technique. In addition, some of the configurations of certain embodiments can be deleted. Further, a well-known technique can be added to the configuration of a certain embodiment.

In the above embodiment, the case where the power storage element is used as a non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery) capable of charging and discharging has been described, but the type, shape, size, capacity, etc. of the power storage element are arbitrary. .. The present invention can also be applied to capacitors such as various secondary batteries, electric double layer capacitors and lithium ion capacitors.

In the above embodiment, the electrode body in which the positive electrode and the negative electrode are laminated via the separator has been described, but the electrode body may not include the separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state where a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode.

Hereinafter, the present invention will be described in more detail by way of examples. The present invention is not limited to the following examples.

[Preparation of positive electrode active material]
(1) Preparation of LiFePO ₄ First, a 1 mol / dm ³ FeSO ₄ aqueous solution was dropped into a 2 dm ³ reaction vessel containing 750 cm ³ ion-exchanged water at a constant rate, and the pH during that period was a constant value of 10.0. A 4 mol / dm ³ NaOH aqueous solution, a 0.5 mol / dm ³ NH ₃ aqueous solution, and a 0.5 mol / dm ³ NH ₂ NH ₂ aqueous solution were added dropwise so as to maintain ± 0.1, and Fe (OH) ₂ was added. A precursor was prepared. The temperature of the reaction vessel was set to 50 ° C. (± 2 ° C.). Next, the prepared Fe (OH) ₂ precursor was taken out from the reaction vessel and mixed with LiH ₂ PO ₄ and sucrose powder in a solid phase. Then, the obtained mixture was calcined in a nitrogen atmosphere at a calcining temperature of 650 ° C. to prepare LiFePO ₄ , a positive electrode active material having an olivine-type crystal structure.
(2) Preparation of LiFe _0.25 Mn _0.75 PO ₄ First, adjust the molar ratio of FeSO ₄ and MnSO ₄ to 1: 3 in a 2 dm ³ reaction vessel containing 750 cm ³ ion-exchanged water. While dropping the 1 mol / dm ³ aqueous solution at a constant rate, a 4 mol / dm ³ NaOH aqueous solution and a 0.5 mol / dm ³ NH ₃ so that the pH during that period is maintained at a constant value of 10.0 ± 0.1. An aqueous solution and a 0.5 mol / dm ³ NH ₂ NH ₂ aqueous solution were added dropwise to prepare a Fe _0.25 Mn _0.75 (OH) ₂ precursor. The temperature of the reaction vessel was set to 50 ° C. (± 2 ° C.). Next, the prepared Fe _0.25 Mn _0.75 (OH) ₂ precursor was taken out from the reaction vessel and mixed with LiH ₂ PO ₄ and sucrose powder in a solid phase. Then, the obtained mixture was fired in a nitrogen atmosphere at a firing temperature of 650 ° C. to prepare a positive electrode active material LiFe _0.25 Mn _0.75 PO ₄ having an olivine type crystal structure.

Table 1 shows the values of x in the following formula 1 of the positive electrode active materials of Examples 1 to 8 and Comparative Examples 1 to 11.
LiFe _x Mn _(1-x) PO ₄ (0 ≦ x ≦ 1) ・ ・ ・ 1

[Preparation of positive electrode]
<Examples 1 to 7 and Comparative Examples 1 to 5>
(Formation of undercoat layer)
As graphite, graphite having a half width at half maximum of the (002) plane in XRD as shown in Table 1 was used. The above graphite, PVDF as a binder, and a dispersion medium were mixed. At that time, the solid content mass ratio of graphite: binder was set to 95: 5, and an appropriate amount of a dispersion medium was added to the mixture to adjust the viscosity to prepare a paste for an undercoat layer. Next, the paste for the undercoat layer is applied to both sides of the aluminum foil which is the positive electrode base material, leaving an uncoated part (undercoat layer non-forming part), and dried at 120 ° C. to obtain a positive electrode base material. An undercoat layer having an average thickness of 2 to 3 μm was formed on the layer.

(Formation of positive electrode active material layer)
LiFePO ₄ was used as the positive electrode active material. The positive electrode active material, N-methylpyrrolidone (NMP) as a dispersion medium, acetylene black as a conductive agent, and PVDF as a binder were used. The positive electrode active material, the conductive agent, the binder and the dispersion medium were mixed. At that time, the solid content mass ratio of the positive electrode active material: conductive agent: binder was set to 90: 5: 5, and an appropriate amount of a dispersion medium was added to the mixture to adjust the viscosity, and a positive electrode active material mixture paste was prepared. Next, the positive electrode active material mixture paste was applied onto the undercoat layer formed on the positive electrode base material, dried at 120 ° C., and roll-pressed so as to obtain the densities shown in Table 1. A positive electrode active material layer was formed on the undercoat layer. The amount of the positive electrode active material mixture paste applied was 10 mg / cm ² in terms of solid content. In this way, positive electrodes of Example 1 to Example 7 and Comparative Example 1 to Comparative Example 5 were obtained. The density of the positive electrode active material layer in the obtained positive electrode was measured based on the above method. The results are shown in Table 1.

<Comparative Example 6 to Comparative Example 9>
Comparison from Examples 1 to 7 and Comparative Example 1 except that amorphous carbon as shown in Table 1 is used instead of graphite and roll-pressed so that the density of the positive electrode active material layer as shown in Table 1 is obtained. The positive electrodes of Comparative Examples 6 to 9 were prepared in the same manner as in Example 5. The density of the positive electrode active material layer in the obtained positive electrode was measured based on the above method. The results are shown in Table 1.

<Example 8 and Comparative Example 10>
From Examples 1 to 7 and Comparative Example 1 except that LiFe _0.25 Mn _0.75 PO ₄ is used as the positive electrode active material and roll-pressed so that the density of the positive electrode active material layer as shown in Table 1 is obtained. The positive electrodes of Example 8 and Comparative Example 10 were produced in the same manner as in Comparative Example 5. The density of the positive electrode active material layer in the obtained positive electrode was measured based on the above method. The results are shown in Table 1.

<Comparative Example 11>
LiFe _0.25 Mn _0.75 PO ₄ is used as the positive electrode active material, and amorphous carbon as shown in Table 1 is used instead of graphite so that the density of the positive electrode active material layer as shown in Table 1 can be obtained. The positive electrodes of Comparative Example 11 were produced in the same manner as in Examples 1 to 7 and Comparative Examples 1 to 5 except for roll pressing. The density of the positive electrode active material layer in the obtained positive electrode was measured based on the above method. The results are shown in Table 1.

[Manufacturing of negative electrode]
Graphite was used as the negative electrode active material, SBR was used as the binder, and CMC was used as the thickener. Negative electrode active material, binder, thickener and water as a dispersion medium were mixed. At that time, the solid content mass ratio of the negative electrode active material: binder: thickener was set to 97: 2: 1. An appropriate amount of water was added to the obtained mixture to adjust the viscosity, and a negative electrode mixture paste was prepared. This negative electrode mixture paste was applied to both sides of the copper foil, leaving an uncoated portion (negative electrode active material layer non-forming portion), and dried to prepare a negative electrode active material layer. Then, a roll press was performed to prepare a negative electrode.

[Preparation of non-aqueous electrolyte]
A non-aqueous electrolyte was prepared by dissolving LiPF ₆ at a concentration of 1 mol / dm ³ in a mixed solvent in which EC and EMC were mixed at a volume ratio of 3: 7.

[Manufacturing of power storage element]
Next, each of the above positive electrodes was laminated with the above negative electrode via a separator composed of a polyethylene microporous membrane base material and a heat-resistant layer formed on the polyethylene microporous membrane base material to prepare electrode bodies. .. The heat-resistant layer was arranged on the surface facing the positive electrode. Each electrode body was housed in a square container made of aluminum, and a positive electrode terminal and a negative electrode terminal were attached. After injecting the non-aqueous electrolyte into the container, the container was sealed to obtain a power storage element of Examples and Comparative Examples.

[Capacity confirmation test]
Each of the above-mentioned power storage elements was charged with a constant current of 0.1 C to 3.6 V in an environment of 25 ° C., and then charged with a constant voltage of 3.6 V. The charging end condition was until the charging current reached 0.02C. After a 10-minute pause after charging, constant current discharge was performed at a discharge current of 0.1 C up to 2.0 V in an environment of 25 ° C. A 10-minute rest was provided after discharge. The above cycle was repeated twice, and the second discharge capacity was set to 0.1 C capacity.

[Output in low temperature environment at the initial stage and output in low temperature environment after high temperature storage]
Each of the above-mentioned power storage elements was charged with a constant current of 0.1 C to 3.6 V in an environment of 25 ° C., and then charged with a constant voltage of 3.6 V. The charging end condition was until the charging current reached 0.02C. After a rest period of 10 minutes after charging, constant current discharge was performed up to 2.0 V with a discharge current of 0.1 C, and "0.1 C discharge capacity at 25 ° C" was measured. Next, half the amount of electricity of this "0.1C discharge capacity at 25 ° C." was set to 50% SOC, and constant current charging was performed from a completely discharged state until the charging current reached 50% SOC. Then, after storing for 3 hours in an environment of 0 ° C., the battery was discharged with a discharge current of 0.1 C for 30 seconds, a rest period of 10 minutes was provided, and then supplementary charging was performed with a charging current of 0.1 C for 30 seconds. .. Similarly, the discharge currents are adjusted to 0.3C and 0.5C, each is discharged for 30 seconds, a rest period of 10 minutes is provided, and then supplementary charging is performed with a charging current of 0.1C until the SOC reaches 50%. rice field. The "initial 0 ° C. output (indicated as" 0 ° C. initial output "in Table 1)" was calculated from the current at each discharge and the battery voltage 10 seconds after the start of discharge.
Next, each of the above-mentioned power storage elements was stored at SOC 50% in an environment of 60 ° C. for 7 days. After storage, charge and discharge for 2 cycles in the same method as the above "capacity confirmation test" in a 25 ° C environment, and then perform an output test by the same method as the above "initial 0 ° C output". This was carried out, and "0 ° C output after storage at 60 ° C (shown in Table 1 as" 0 ° C output after storage at 60 ° C ")" was calculated. The results are shown in Table 1.

As shown in Table 1 above, the undercoat layer contains graphite having a half-value width of the (002) plane in XRD of 0.15 ° or more and 0.24 ° or less, and the positive electrode active material layer contains the first compound. In Examples 1 to 8 which are contained and the density of the positive electrode active material layer is 1.80 g / cm ³ or more and 2.10 g / cm ³ or less, the initial 0 ° C. output is 8 W or more and stored at 60 ° C. Later, the decrease in 0 ° C. output was less than 4 W. On the other hand, in Comparative Examples 1 to 11, the initial output at 0 ° C. was less than 8 W, or the decrease in 0 ° C. output after storage at 60 ° C. was 4 W or more. As described above, in Examples 1 to 8, the output in the low temperature environment at the initial stage is higher than that in Comparative Examples 1 to 11, and the decrease in the output in the low temperature environment after the high temperature storage is suppressed. You can see that there is.

As a result of the above, it was shown that the positive electrode can increase the output of the power storage element in the low temperature environment at the initial stage and suppress the decrease of the output in the low temperature environment after the high temperature storage.

The present invention can be applied to a power storage element used as a power source for personal computers, electronic devices such as communication terminals, automobiles and the like.

1 Power storage element 2 Electrode body 3 Container 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Power storage unit 30 Power storage device

Claims (2)

With the positive electrode base material,
A positive electrode active material layer containing a positive electrode active material and
It is provided with an undercoat layer arranged between the positive electrode base material and the positive electrode active material layer.
The undercoat layer contains graphite and
The half width of the (002) plane in the X-ray diffraction of graphite is 0.15 ° or more and 0.24 ° or less.
The positive electrode active material contains a compound represented by the following formula 1 and contains.
A positive electrode for a power storage element having a density of the positive electrode active material layer of 1.80 g / cm ³ or more and 2.10 g / cm ³ or less.
LiFe _x Mn _(1-x) PO ₄ (0 ≦ x ≦ 1) ・ ・ ・ 1 The power storage element including the positive electrode for the power storage element according to claim 1.

Images