CN118974997A - Electricity storage element and electricity storage device - Google Patents

Abstract

An electric storage element according to one aspect of the present invention includes an electrode body in which a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer are laminated, and a sealable container for housing the electrode body, wherein the electrode body is in a state of being pressed in a lamination direction of the positive electrode and the negative electrode, a differential pore volume of the positive electrode active material layer having a pore diameter of 50nm is 0.0030cm ³/(g·nm) or more, and a difference between an open circuit voltage of 30% in SOC and an open circuit voltage of 70% in SOC is 0.2V or less.

Description

Electricity storage element and electricity storage device

Technical Field

The present invention relates to an electricity storage element and an electricity storage device.

Background Art

The non-aqueous electrolyte secondary battery represented by the lithium ion secondary battery is widely used in electronic equipment such as personal computers, communication terminals, and automobiles due to its high energy density. The above-mentioned non-aqueous electrolyte secondary battery generally has an electrode body including a pair of electrodes and a separator, a non-aqueous electrolyte, and a container for accommodating these electrode bodies and the non-aqueous electrolyte, and is configured to charge and discharge by giving and receiving charge transport ions between the two electrodes. In addition, as non-aqueous electrolyte storage elements other than non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and double electric layer capacitors have also been widely popularized.

In the past, carbon materials such as graphite were used as active materials contained in the negative electrode of non-aqueous electrolyte storage elements, and active materials have been developed to improve the charge and discharge characteristics of non-aqueous electrolyte storage elements. As an example of a non-aqueous electrolyte storage element, Patent Document 1 discloses a non-aqueous electrolyte secondary battery using lithium iron phosphate as a positive electrode active material and graphite as a negative electrode active material.

Prior art literature

Patent Literature

Patent Document 1: International Publication No. 2015/170561

Summary of the invention

In a non-aqueous electrolyte secondary battery using lithium iron phosphate as the positive active material and graphite as the negative active material, the change in voltage accompanying the change in the state of charge is excellent in a flat characteristic (voltage flatness) over a wide range of the state of charge. However, in a non-aqueous electrolyte secondary battery with voltage flatness, when the distance between the electrodes is uneven, etc., it is easy to produce deviations in the charge and discharge reactions in the electrode body. If the charge and discharge reactions in the electrode body are biased, lithium metal and the like are easily precipitated on the negative electrode surface during charging at the location where the charge and discharge reactions are concentrated, so there is a risk of a significant reduction in the capacity retention rate and the like accompanying the charge and discharge cycle.

The present invention has been made in view of the above-mentioned actual circumstances, and an object of the present invention is to provide an electric storage element and an electric storage device capable of suppressing a decrease in the capacity retention rate during charge and discharge cycles.

A storage element according to one aspect of the present invention comprises an electrode body formed by laminating a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and a sealable container for housing the electrode body, wherein the electrode body is in a state of being pressed in a laminating direction of the positive electrode and the negative electrode, the differential pore volume of the positive electrode active material layer at a pore diameter of 50 nm is 0.0030 cm ³ /(g·nm) or more, and the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% is 0.2 V or less.

A storage element according to another aspect of the present invention comprises an electrode body formed by laminating a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and a sealable container for accommodating the electrode body, wherein the electrode body is in a state of being pressed in the lamination direction of the positive electrode and the negative electrode, the differential pore volume of the positive electrode active material layer at a pore diameter of 50 nm is 0.0030 cm ³ /(g·nm) or more, the positive electrode active material comprises a compound represented by the following formula 1 or lithium manganate, and the negative electrode active material comprises graphite or lithium titanate.

LiFe _x Mn _(1－x) PO ₄ (0≤x≤1)···1

A power storage device according to another aspect of the present invention includes two or more power storage elements and includes one or more power storage elements according to one aspect of the present invention.

The electric storage element and the electric storage device according to one aspect of the present invention can suppress a decrease in the capacity retention rate during charge and discharge cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing an example of a discharge curve of an electric storage element for explaining voltage flatness.

FIG. 2 is a perspective view showing an embodiment of an electric storage element.

FIG. 3 is a schematic diagram showing an embodiment of an electric storage device including a plurality of electric storage elements assembled together.

FIG. 4 is a differential pore volume distribution curve of the positive electrode active material layer of Example 1 and Comparative Example 3.

DETAILED DESCRIPTION

One embodiment of the present invention provides the following aspects.

Item 1.

An electric storage element according to one embodiment of the present invention comprises an electrode body formed by stacking a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and a sealable container for housing the electrode body, the electrode body being in a state of being pressed in a stacking direction of the positive electrode and the negative electrode, the differential pore volume of the positive electrode active material layer having a pore diameter of 50 nm being not less than 0.0030 cm ³ /(g·nm), and the difference between an open circuit voltage at SOC 30% and an open circuit voltage at SOC 70% being not more than 0.2 V.

The electric storage device described in the above Item 1 can suppress a decrease in the capacity retention rate during charge and discharge cycles.

Item 2.

A storage element according to another embodiment of the present invention comprises an electrode body formed by laminating a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and a sealable container for accommodating the electrode body, the electrode body being in a state of being pressed in the lamination direction of the positive electrode and the negative electrode, the differential pore volume of the positive electrode active material layer with a pore diameter of 50 nm is 0.0030 cm ³ /(g·nm) or more, the positive electrode active material comprises a compound represented by the following formula 1 or lithium manganate, and the negative electrode active material comprises graphite or lithium titanate.

LiFe _x Mn _(1－x) PO ₄ (0≤x≤1)···1

The electric storage device described in the above Item 2 can suppress a decrease in the capacity retention rate during charge and discharge cycles.

Item 3.

In the electric storage device according to the above Item 1 or 2, the interior of the container may be in a negative pressure state.

According to the electric storage device described in the above Item 3, it is possible to further suppress a decrease in the capacity retention rate during charge and discharge cycles.

Item 4.

A power storage device according to another embodiment of the present invention includes two or more power storage elements and includes one or more power storage elements according to any one of Items 1 to 3 above.

According to the power storage device described in the above Item 4, it is possible to suppress a decrease in the capacity retention rate during charge and discharge cycles.

First, the outline of the power storage element and the power storage device disclosed in this specification will be described.

A storage element according to one aspect of the present invention comprises an electrode body formed by laminating a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and a sealable container for housing the electrode body, wherein the electrode body is in a state of being pressed in a laminating direction of the positive electrode and the negative electrode, the differential pore volume of the positive electrode active material layer at a pore diameter of 50 nm is 0.0030 cm ³ /(g·nm) or more, and the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% is 0.2 V or less.

This electric storage element can suppress a decrease in the capacity retention rate during charge and discharge cycles. The reason for this is presumably as follows.

Since the difference between the open circuit voltage at SOC30% and the open circuit voltage at SOC70% is less than 0.2V, the storage element has voltage flatness in which the voltage changes flatly with the change of the state of charge. Here, in order to illustrate the "voltage flatness", FIG. 1 shows an example of a discharge curve of the storage element with the horizontal axis being SOC (%) and the vertical axis being the battery voltage (V). The battery voltage (V) is the closed circuit voltage (CCV) of the storage element when the discharge current is set to 0.1C and constant current discharge is performed at 25°C. In FIG. 1, LFP/Gr is a lithium ion secondary battery using lithium iron phosphate (LFP) for the positive electrode and graphite (Gr) for the negative electrode, NCM/Gr is a lithium ion secondary battery using a composite oxide (NCM) containing Ni, Co and Mn for the positive electrode and graphite (Gr) for the negative electrode, and NCA/Gr is a lithium ion secondary battery using a composite oxide (NCA) containing Ni, Co and Al for the positive electrode and graphite (Gr) for the negative electrode. In Fig. 1 , the battery voltage (V) of NCM/Gr and NCA/Gr decreases monotonically from SOC 70% to SOC 30%, while the battery voltage (V) of LFP/Gr is substantially constant from SOC 70% to SOC 30%. That is, LFP/Gr of Fig. 1 has voltage flatness.

On the other hand, when the conventional storage element has the above-mentioned voltage flatness, when the distribution of charge transport ions such as lithium ions in the electrode body is biased, it is presumed that the force to eliminate the deviation through the difference in potential between the positive electrode active material layer and the negative electrode active material layer is difficult to work. In particular, when the thickness of the electrode body is large or the distance between the two electrodes is uneven, the diffusion of charge transport ions such as lithium ions tends to become uneven, and the deviation of the charge and discharge reaction is easily maintained in the electrode body. Therefore, there is the following risk: lithium metal and the like are easily precipitated locally on the negative electrode surface during charging, and the capacity retention rate decreases with the charge and discharge cycle.

In contrast, in the storage element of one aspect of the present invention, by pressing the electrode body in the stacking direction of the positive electrode and the negative electrode, the distance between the two electrodes can be easily uniformized, and since the differential pore volume of the positive electrode active material layer with a pore diameter of 50 nm is 0.0030 cm ³ / (g·nm) or more, the smooth and uniform movement of charge transport ions such as lithium ions is promoted in the positive electrode active material layer. Therefore, the charge and discharge reaction in the electrode body is not easily deviated, and as a result, the reduction in the capacity retention rate during the charge and discharge cycle can be suppressed.

A storage element according to another aspect of the present invention comprises an electrode body formed by laminating a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and a sealable container for accommodating the electrode body, wherein the electrode body is in a state of being pressed in the lamination direction of the positive electrode and the negative electrode, the differential pore volume of the positive electrode active material layer at a pore diameter of 50 nm is 0.0030 cm ³ /(g·nm) or more, the positive electrode active material comprises a compound represented by the following formula 1 or lithium manganate, and the negative electrode active material comprises graphite or lithium titanate.

LiFe _x Mn _(1－x) PO ₄ (0≤x≤1)···1

In this storage element, since the positive electrode active material includes the compound represented by the above formula 1 or lithium manganate, and the negative electrode active material includes graphite or lithium titanate, the voltage flatness is flat with the change of the voltage accompanying the change of the charging state. On the other hand, by pressing the electrode body in the stacking direction of the positive electrode and the negative electrode, it is easy to make the distance between the two electrodes uniform. In addition, since the differential pore volume of the pore diameter 50nm of the positive electrode active material layer is ^0.0030cm3 /(g·nm) or more, the smooth and uniform movement of charge transport ions such as lithium ions is promoted in the positive electrode active material layer. Therefore, the charge and discharge reaction in the electrode body is not easy to deviate, and as a result, the reduction of the capacity retention rate during the charge and discharge cycle can be suppressed.

The interior of the container may be in a negative pressure state. Thus, by making the interior of the container in a negative pressure state, the distance between the two electrodes can be more easily made uniform.

It should be noted that, in the present invention, "the stacking direction of the positive electrode and the negative electrode" refers to the direction in which the positive electrode and the negative electrode are stacked, and "the state of being pressed in the stacking direction" refers to the state in which pressure is applied in the direction in which the distance between the stacked layers of the electrode body becomes smaller. In addition, in the present invention, "negative pressure state" refers to a state in which the pressure inside the container that stores the electrode body is lower than the pressure outside the container. "SOC" is the abbreviation of State Of Charge, and the charging state of the storage element is expressed by the ratio of the remaining capacity at that time to the capacity of the fully charged state, and the fully charged state is marked as SOC100%. Here, "fully charged state" is a state in which the storage element is charged to the upper limit voltage using the recommended or specified charging conditions. Furthermore, "the difference between the open circuit voltage of SOC30% and the open circuit voltage of SOC70%" refers to the absolute value of the difference in open circuit voltage (OCV) measured after 30 minutes when the storage element is discharged at a constant current of 0.1C at 25°C and reaches SOC30% and SOC70%, respectively, without applying current.

In the present invention, "differential pore volume at a pore size of 50 nm" is a value obtained by the BJH method from the adsorption isotherm using the nitrogen adsorption method. Specifically, the differential pore volume is measured by the following method. 1.00 g of the powder of the sample to be measured (positive electrode active material layer) is placed in a sample tube for measurement and vacuum dried at 120°C for 12 hours to fully remove the moisture in the sample to be measured. Next, by a nitrogen adsorption method using liquid nitrogen, the isotherms on the adsorption side and the departure side are measured in the range of relative pressure P/P0 (P0 = about 770 mmHg) of 0 to 1. Then, the isotherm on the departure side is used and calculated by the BJH method to evaluate the differential pore volume distribution and calculate the differential pore volume at a pore size of 50 nm. The above-mentioned differential pore volume is measured using the gas adsorption amount measuring device "autosorb iQ" and the data analysis software "ASiQwin" manufactured by Quantachrome.

A power storage device according to another aspect of the present invention includes two or more power storage elements and one or more power storage elements according to the one aspect of the present invention. The power storage device includes the power storage element according to the one aspect of the present invention, and thus can suppress a decrease in capacity retention during charge and discharge cycles.

The structure of a storage element, the structure of a storage device, and a method for manufacturing a storage element according to an embodiment of the present invention are described in detail, as well as other embodiments. It should be noted that the names of the components (components) used in each embodiment are sometimes different from the names of the components (components) used in the background art.

＜Structure of energy storage element＞

The storage element of one embodiment of the present invention comprises: an electrode body having a positive electrode, a negative electrode and a separator, a nonaqueous electrolyte and a sealable container for accommodating the electrode body and the nonaqueous electrolyte. The electrode body has a structure in which a plurality of positive electrodes and a plurality of negative electrodes are stacked via a separator. As the structure of the electrode body, for example, a stacked type or a winding type in which a positive electrode and a negative electrode are stacked via a separator can be cited. The electrode body is in a state of being pressed in the stacking direction of the positive electrode and the negative electrode. The nonaqueous electrolyte exists in a state contained in the positive electrode, the negative electrode and the separator. As an example of a storage element, a nonaqueous electrolyte secondary battery (hereinafter also referred to as a "secondary battery") is described.

In one embodiment of the present invention, the upper limit of the difference between the open circuit voltage of SOC30% and the open circuit voltage of SOC70% is 0.2V, more preferably 0.15V, and further preferably 0.10V. By making the difference between the open circuit voltage of SOC30% and the open circuit voltage of SOC70% below the above upper limit, the range of the charging state in which the change of the voltage accompanying the change of the charging state becomes flat can be relatively expanded (the voltage flatness can be improved). On the other hand, the lower limit of the difference between the open circuit voltage of SOC30% and the open circuit voltage of SOC70% is not particularly limited, for example, it can be 0.01V or 0.05V. The difference between the open circuit voltage of SOC30% and the open circuit voltage of SOC70% can be above any of the above lower limits and below any of the above upper limits.

(positive electrode)

The positive electrode includes a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

The positive electrode substrate has conductivity. Whether it has "conductivity" is determined based on the volume resistivity of 10 ⁷ Ω·cm measured in accordance with JIS-H-0505 (1975) as a threshold value. As the material of the positive electrode substrate, metals such as aluminum, titanium, tantalum, stainless steel, or their alloys can be used. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity and cost. As the positive electrode substrate, foil, vapor-deposited film, mesh, porous material, etc. can be cited. From the viewpoint of cost, foil is preferred. Therefore, as the positive electrode substrate, aluminum foil or aluminum alloy foil is preferred. As aluminum or aluminum alloy, A1085, A3003, A1N30, etc. specified in JIS-H-4000 (2014) or JIS-H-4160 (2006) can be exemplified.

The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, further preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate to the above range, the strength of the positive electrode substrate can be increased, and the energy density per unit volume of the secondary battery can be increased.

The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer reduces the contact resistance between the positive electrode substrate and the positive electrode active material layer by including a conductive agent such as carbon particles. The composition of the intermediate layer is not particularly limited, and for example, includes a binder and a conductive agent.

In one embodiment of the present invention, the positive electrode active material includes a compound represented by the following formula 1 or lithium manganate.

LiFe _x Mn _(1－x) PO ₄ (0≤x≤1)···1

The positive electrode active material may include both the compound represented by the above formula 1 and lithium manganate. In addition, the positive electrode active material may include only one compound represented by the above formula 1, or may include two or more compounds represented by the above formula 1 having different molar ratios (x) of Fe relative to the sum of the molar numbers of Fe and Mn. The compound represented by the above formula 1 or the lithium manganate may be partially substituted by atoms or anionic substances composed of other elements, or may be coated with other materials.

The lower limit of the total content of the compound represented by the above formula 1 and lithium manganate relative to all positive electrode active materials is preferably 70% by mass, more preferably 80% by mass, and further preferably 90% by mass. By making the above total content above the above lower limit, it is easy to reduce the difference between the open circuit voltage of SOC30% and the open circuit voltage of SOC70%. On the other hand, the total content of the compound represented by the above formula 1 and lithium manganate relative to all positive electrode active materials can be 100% by mass.

The positive electrode active material preferably contains lithium iron phosphate (LiFePO ₄ ) as the compound represented by the above formula 1. The lower limit of the content of lithium iron phosphate relative to all positive electrode active materials is preferably 70% by mass, more preferably 80% by mass, and further preferably 90% by mass. By making the content of lithium iron phosphate relative to all positive electrode active materials greater than the above lower limit, it is easy to further reduce the difference between the open circuit voltage of SOC30% and the open circuit voltage of SOC70%. On the other hand, the content of lithium iron phosphate relative to all positive electrode active materials can be 100% by mass.

The positive electrode active material is usually a particle (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm to 20 μm. By making the average particle size of the positive electrode active material above the above lower limit, the manufacture or handling of the positive electrode active material becomes easy. By making the average particle size of the positive electrode active material below the above upper limit, the electron conductivity of the positive electrode active material layer is improved. It should be noted that when a composite of a positive electrode active material and other materials is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. "Average particle size" refers to the particle size distribution measured by a laser diffraction/scattering method based on a dilution solution after diluting the particles in a solvent in accordance with JIS-Z-8825 (2013), and the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50%.

In order to obtain a powder with a predetermined particle size, a pulverizer, a classifier, etc. can be used. As a pulverization method, for example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a reverse jet mill, a rotating airflow jet mill, or a sieve can be cited. During pulverization, wet pulverization in which an organic solvent such as water or hexane coexists can also be used. As a classification method, a sieve, a wind classifier, etc. can be used together with a dry method or a wet method as needed.

The content of the positive electrode active material in the positive electrode active material layer is preferably 50% to 99% by mass, more preferably 70% to 98% by mass, and further preferably 80% to 95% by mass. By making the content of the positive electrode active material within the above range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

The conductive agent is not particularly limited as long as it is a material with conductivity. As such a conductive agent, for example, carbonaceous materials, metals, conductive ceramics, etc. can be mentioned. As carbonaceous materials, graphite, non-graphite carbon, graphene-based carbon, etc. can be mentioned. As non-graphite carbon, carbon nanofibers, pitch-based carbon fibers, carbon black, etc. can be mentioned. As carbon black, furnace black, acetylene black, Ketjen black, etc. can be mentioned. As graphene-based carbon, graphene, carbon nanotubes (CNT), fullerene, etc. can be mentioned. As the shape of the conductive agent, powder, fiber, etc. can be mentioned. As the conductive agent, one of these materials can be used alone, or two or more can be mixed and used. In addition, these materials can also be used in a composite. For example, a material after carbon black and CNT composite can be used. Among these, from the viewpoint of electronic conductivity and coating properties, carbon black is preferred, and acetylene black is preferred.

The content of the conductive agent in the positive electrode active material layer is preferably 1% to 10% by mass, and more preferably 3% to 9% by mass. When the content of the conductive agent is within the above range, the energy density of the secondary battery can be increased.

Examples of adhesives include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic acid, polyimide, etc.; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, etc.; polysaccharide polymers, etc.

The content of the binder in the positive electrode active material layer is preferably 1% by mass to 10% by mass, and more preferably 3% by mass to 9% by mass. When the content of the binder is within the above range, the active material can be stably retained.

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, the functional group may be inactivated in advance by methylation or the like.

The filler is not particularly limited. As the filler, there can be mentioned polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, aluminum oxide, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, insoluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, mica, and other materials derived from mineral resources or their artificial products, etc.

The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, I, typical metal elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, Ba, and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

The lower limit of the differential pore volume of the positive electrode active material layer with a pore size of 50 nm is 0.0030 cm ³ / (g·nm), preferably 0.0040 cm ³ / (g·nm), and more preferably 0.0050 cm ³ / (g·nm). By making the differential pore volume above the lower limit, the non-aqueous electrolyte easily penetrates into the positive electrode active material layer, and thus it is easy to make the movement of charge transport ions such as lithium ions in the positive electrode active material layer uniform. On the other hand, from the viewpoint of ease of manufacture, the upper limit of the differential pore volume is preferably 0.0100 cm ³ / (g·nm), and more preferably 0.0080 cm ³ / (g·nm). Since the differential pore volume is difficult to change mainly due to charge and discharge, it can be, for example, a value obtained by measuring the positive electrode active material layer after disassembling the storage element made based on the positive electrode active material and taking it out. The differential pore volume of the positive electrode active material layer at a pore diameter of 50 nm can be adjusted by, for example, changing the production conditions when preparing the precursor of the positive electrode active material or changing the firing temperature when preparing the active material.

The positive electrode active material layer preferably has at least one peak in the range of pore diameter 25nm to 75nm in the differential pore volume distribution curve evaluated by the above method, and has the largest peak in the range of pore diameter 25nm to 75nm. In addition, the positive electrode active material layer more preferably has at least one peak in the range of pore diameter 45nm to 55nm in the above differential pore volume distribution curve, and has the largest peak in the range of pore diameter 45nm to 55nm. In this way, the positive electrode active material layer has at least one peak in the differential pore volume distribution curve in the above range, and has the largest peak in the above range, thereby making it easy to uniformize the penetration of non-aqueous electrolyte into the positive electrode active material layer and the movement of charge transport ions such as lithium ions in the positive electrode active material layer.

As the lower limit of the mass per unit area of the positive electrode active material layer, from the viewpoint of improving the energy density, it is preferably 1 mg/ ^cm2 , more preferably 5 mg/ ^cm2 , and further preferably 10 mg/ ^cm2 . On the other hand, as the upper limit of the mass per unit area of the positive electrode active material layer, it is preferably 30 mg/ ^cm2 , more preferably 25 mg/ ^cm2 , and further preferably 20 mg/ ^cm2 . If the mass per unit area of the positive electrode active material layer is below the above upper limit, it is easy to make the charge transport ions such as lithium ions diffuse uniformly. It should be noted that "the mass per unit area of the positive electrode active material layer" refers to the value obtained by dividing the mass converted by the solid content of the positive electrode active material layer by the area of the region where the positive electrode active material layer is directly or via the intermediate layer on the surface of the positive electrode substrate. In the case where the positive electrode active material layer is arranged on both sides of the positive electrode substrate, it is a value obtained by the mass per unit area and the area of the positive electrode active material layer on one side of the positive electrode substrate.

(negative electrode)

The negative electrode comprises a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The structure of the intermediate layer is not particularly limited, and can be selected from the structures exemplified in the above-mentioned positive electrode.

The negative electrode substrate has conductivity. As the material of the negative electrode substrate, metals such as copper, nickel, stainless steel, nickel-plated steel, aluminum, or their alloys, carbonaceous materials, etc. can be used. Among these, copper or copper alloys are preferred. As the negative electrode substrate, foil, vapor-deposited film, net, porous material, etc. can be cited. From the perspective of cost, foil is preferred. Therefore, as the negative electrode substrate, copper foil or copper alloy foil is preferred. As examples of copper foil, rolled copper foil, electrolytic copper foil, etc. can be cited.

The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, further preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate to the above range, the strength of the negative electrode substrate can be increased, and the energy density per unit volume of the secondary battery can be increased.

The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains any component such as a conductive agent, a binder, a thickener, and a filler as needed. The conductive agent, the binder, the thickener, the filler, and any component can be selected from the materials exemplified in the positive electrode.

The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, I, typical metal elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, Ba, and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

In one embodiment of the present invention, the negative electrode active material comprises graphite or lithium titanate. Here, "graphite" refers to a carbon material having an average lattice plane spacing (d ₀₀₂ ) of 0.33 nm or more and less than 0.34 nm on the (002) plane determined by X-ray diffraction method before charge and discharge or in a discharge state. As graphite, natural graphite and artificial graphite can be cited. From the viewpoint of a material with stable physical properties, artificial graphite is preferred. In addition, the "discharge state" of the negative electrode refers to a state in which the charge transport ions such as lithium ions that can be absorbed and released accompanying charge and discharge are fully released from the negative electrode active material. For example, the following state: in a monopolar battery using a negative electrode containing graphite as a negative electrode active material as a working electrode and metal Li as a counter electrode, the open circuit voltage is 0.7 V or more.

The lower limit of the content of graphite or lithium titanate relative to all negative electrode active materials is preferably 70% by mass, more preferably 80% by mass, and further preferably 90% by mass. When the content of graphite or lithium titanate is above the above lower limit, the difference between the open circuit voltage of SOC30% and the open circuit voltage of SOC70% can be easily reduced. On the other hand, the content of graphite or lithium titanate relative to all negative electrode active materials can be 100% by mass.

The negative electrode active material is usually a particle (powder). The average particle size of the negative electrode active material can be, for example, 1 nm to 100 μm. The average particle size of the negative electrode active material can also be 1 μm to 100 μm. By making the average particle size of the negative electrode active material above the above lower limit, the manufacture or handling of the negative electrode active material becomes easy. By making the average particle size of the negative electrode active material below the above upper limit, the electronic conductivity of the active material layer is improved. In order to obtain a powder with a specified particle size, a pulverizer, a classifier, etc. can be used. The pulverization method and the classification method can be selected from the methods exemplified in the above-mentioned positive electrode, for example.

The content of the negative electrode active material in the negative electrode active material layer is preferably 60% to 99% by mass, more preferably 90% to 98% by mass. When the content of the negative electrode active material is within the above range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

As the lower limit of the mass per unit area of the negative electrode active material layer, from the viewpoint of improving the energy density, it is preferably 1 mg/cm ² , more preferably 3 mg/cm ² , and further preferably 5 mg/cm ^2. On the other hand, as the upper limit of the mass per unit area of the negative electrode active material layer, it is preferably 20 mg/cm ² , more preferably 15 mg/cm ² , and further preferably 12 mg/cm ^2. If the mass per unit area of the negative electrode active material layer is below the above upper limit, it is easy to make the charge transport ions such as lithium ions diffuse uniformly. It should be noted that "the mass per unit area of the negative electrode active material layer" refers to the value obtained by dividing the mass converted from the solid content of the negative electrode active material layer by the area of the region where the negative electrode active material layer is arranged directly or via an intermediate layer on the surface of the negative electrode substrate. In the case where the negative electrode active material layer is arranged on both sides of the negative electrode substrate, it is a value obtained by the mass per unit area and the area of the negative electrode active material layer on one side of the negative electrode substrate.

(Isolator)

The separator can be appropriately selected from known separators. As the separator, for example, a separator consisting only of a substrate layer, a separator in which a heat-resistant layer containing heat-resistant particles and an adhesive is formed on one or both surfaces of the substrate layer, etc. can be used. As the shape of the substrate layer of the separator, for example, woven fabrics, non-woven fabrics, porous resin films, etc. can be cited. Among these shapes, porous resin films are preferred from the viewpoint of strength, and non-woven fabrics are preferred from the viewpoint of liquid retention of non-aqueous electrolytes. As the material of the substrate layer of the separator, from the viewpoint of the cutting function, for example, polyolefins such as polyethylene and polypropylene are preferred, and from the viewpoint of resistance to oxidative decomposition, for example, polyimide, aromatic polyamide, etc. are preferred. As the substrate layer of the separator, a material composited with these resins can also be used.

For the heat-resistant particles contained in the heat-resistant layer, the mass reduction when the temperature is increased from room temperature to 500°C is preferably 5% or less in an air atmosphere of 1 atmosphere, and the mass reduction when the temperature is increased from room temperature to 800°C is more preferably 5% or less. As materials whose mass reduction is less than the prescribed value, inorganic compounds can be cited. As inorganic compounds, for example, oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminum silicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; insoluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalent crystals such as silicon and diamond; talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, mica, and other mineral resources or their artificial products, etc. As inorganic compounds, monomers or composites of these substances can be used alone, or two or more can be mixed and used. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of safety of the electric storage element.

The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, the "porosity" is a value based on volume, and refers to a value measured using a mercury porosimeter.

As a separator, a polymer gel composed of a polymer and a non-aqueous electrolyte can be used. As polymers, for example, polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinyl pyrrolidone, polyvinylidene fluoride, etc. can be cited. If a polymer gel is used, it has the effect of suppressing leakage. As a separator, a porous resin film or nonwoven fabric as described above can also be used together with a polymer gel.

(Non-aqueous electrolyte)

The non-aqueous electrolyte may be appropriately selected from known non-aqueous electrolytes. A non-aqueous electrolyte solution may also be used. The non-aqueous electrolyte solution includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

As nonaqueous solvent, it can be suitably selected from known nonaqueous solvents. As nonaqueous solvent, cyclic carbonate, linear carbonate, carboxylate, phosphoric acid ester, sulfonic acid ester, ether, amides, nitrile etc. can be enumerated. As nonaqueous solvent, it is also possible to use a solvent whose part of the hydrogen atom contained by these compounds is replaced by halogen.

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), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate. Among these, EC is preferred.

Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

As the non-aqueous solvent, preferably a cyclic carbonate or a chain carbonate is used, and more preferably a cyclic carbonate and a chain carbonate are used in combination. By using a cyclic carbonate, the dissociation of the electrolyte salt can be promoted and the ion conductivity of the non-aqueous electrolyte can be improved. By using a chain carbonate, the viscosity of the non-aqueous electrolyte can be suppressed to a low level. In the case of using a cyclic carbonate and a chain carbonate in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate: chain carbonate) is preferably in the range of 5:95 to 50:50, for example.

The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of the electrolyte salt include lithium salts, sodium salts, potassium salts, magnesium salts, Salts, etc. Among these, lithium salts are preferred.

Examples of the lithium salt include inorganic lithium salts such as _LiPF6 , _{LiPO2F2 , LiBF4} , _LiClO4 , and LiN( _SO2F ) ₂ ; lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalatoborate (LiFOB), and lithium bis _{( oxalate} )difluorophosphate (LiFOP) _{; and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2 , LiN ( SO2C2F5 ) 2 , LiN} ( _SO2CF3 )( _SO2C4F9 ) _, LiC( _SO2CF3 ) ₃ , _and LiC( _SO2C2F5 ) ₃ . _Among these, inorganic lithium salts are preferred _, and _LiPF6 is more preferred.

The content of the electrolyte salt in the non-aqueous electrolyte is preferably 0.1 mol/dm ³ to 2.5 mol/dm ³ , more preferably 0.3 mol/dm ³ to 2.0 mol/dm ³ , further preferably 0.5 mol/dm ³ to 1.7 mol/dm ³ , and particularly preferably 0.7 mol/dm ³ to 1.5 mol/dm ³ at 20° C. and 1 atmosphere. When the content of the electrolyte salt is within the above range, the ion conductivity of the non-aqueous electrolyte can be improved.

In addition to the non-aqueous solvent and the electrolyte salt, the non-aqueous electrolyte may also contain additives. Examples of additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partial hydride of terphenyl, cyclohexylbenzene, tert-butylbenzene, tert-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, 3,5-difluoro Anisole and other halogenated anisole compounds; vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; vinyl sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, vinyl sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxa-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxa-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, bipyridine disulfide, 1,3-propylene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, tetrakis(trimethylsilyl)titanate, monofluorolithium phosphate, difluorolithium phosphate, etc. These additives may be used alone or in combination of two or more.

The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% to 10% by mass, more preferably 0.1% to 7% by mass, further preferably 0.2% to 5% by mass, and particularly preferably 0.3% to 3% by mass, relative to the mass of the entire non-aqueous electrolyte. By making the content of the additive within the above range, the capacity retention performance or cycle performance after high temperature storage can be improved, or the safety can be further improved.

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

The solid electrolyte can be selected from any material that has ion conductivity and is solid at room temperature (e.g., 15° C. to 25° C.), such as lithium, sodium, and calcium. Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and polymer solid electrolytes.

In the case of a lithium ion secondary battery, examples of the sulfide solid electrolyte include Li ₂ S—P ₂ S ₅ , LiI—Li ₂ S—P ₂ S ₅ , and Li ₁₀ Ge—P ₂ S ₁₂ .

The shape of the electric storage element of the present embodiment is not particularly limited, and examples thereof include cylindrical batteries, square batteries, flat batteries, coin batteries, and button batteries.

FIG. 2 shows an electric storage element 1 (non-aqueous electrolyte secondary battery) as an example of a square battery. It should be noted that the figure is a perspective view of the inside of the container. An electrode body 2 having a positive electrode and a negative electrode wound with a separator clamped therebetween is housed in a square container 3. In addition, the container 3 is sealed in a state in which a non-aqueous electrolyte is added. As the container 3, a well-known metal container, resin container, etc., which are generally used as containers for electric storage elements, can be used. As the container 3, from the viewpoint of making it easy to put the electrode body 2 in a pressed state by the method described later, a thin and flexible container such as a composite film container laminated with a metal layer and a resin film layer is preferred.

The positive electrode is electrically connected to the positive terminal 4 via the positive electrode lead 41. The negative electrode is electrically connected to the negative terminal 5 via the negative electrode lead 51. In FIG2 , the axial direction of the winding of the electrode body 2 in the energy storage element 1 is represented as the X direction, the thickness direction of the energy storage element 1 is represented as the Y direction, and the direction perpendicular to the above-mentioned axial direction (X direction) and perpendicular to the above-mentioned thickness direction (Y direction) is represented as the Z direction. It should be noted that the Z direction is parallel to the flat portion (i.e., the surface of the flat portion) of the electrode body 2 in the energy storage element 1, and is consistent with the winding direction of the electrode body 2 in the above-mentioned flat portion. Here, the thickness direction of the energy storage element 1 is consistent with the thickness direction of the electrode body 2. The thickness direction of the electrode body 2 is equivalent to the stacking direction of the positive electrode, the negative electrode, and the separator, and is also equivalent to the direction perpendicular to the surfaces of these positive electrodes, negative electrodes, and separators.

As described above, the electrode body 2 of the storage element 1 is in a state of being pressed in the stacking direction (Y direction) of the positive electrode and the negative electrode. However, a portion of the electrode body 2 (for example, a pair of curved surface portions at both ends of the flat portion of the electrode body 2, etc.) may not be pressed. In this way, by putting the electrode body 2 in a state of being pressed in the stacking direction of the positive electrode and the negative electrode (pressed state), it is easy to make the distance between the two electrodes uniform, and as a result, the smooth and uniform movement of charge transport ions such as lithium ions in the electrode body 2 is promoted.

As a method for pressing the electrode body 2, it is preferable to make the inside of the container 3 a negative pressure state (sealing the container 3 while maintaining the pressure inside the container 3 at a negative pressure). In this way, by making the inside of the container 3 a negative pressure state, the container 3 is deformed (depressed in the stacking direction of the positive electrode and the negative electrode), and the electrode body 2 is pressed in the stacking direction of the positive electrode and the negative electrode. As a result, the distance between the positive electrode and the negative electrode in the electrode body 2 can be easily made uniform.

In the state before the container 3 is sealed, the upper limit of the ratio of the minimum width in the stacking direction (the minimum width in the Y direction) of the internal space of the container 3 to the thickness in the stacking direction (the thickness in the Y direction) of the electrode body 2 is preferably 1.20, and more preferably 1.10. If the above ratio is below the above upper limit, the container 3 is easily pressed in the stacking direction by the deformation of the container 3. It should be noted that the lower limit of the above ratio is not particularly limited, and may be 1.05, for example.

It is preferred that a gas soluble in a non-aqueous electrolyte is contained in the container 3. The gas soluble in a non-aqueous electrolyte is contained in the container 3, and the gas dissolves in the non-aqueous electrolyte, thereby making it easy to make the inside of the container 3 a negative pressure state. Here, the gas soluble in a non-aqueous electrolyte refers to a gas having a solubility of 1 cm ³ or more in 1 cm ³ of a non-aqueous solvent at 1 atmosphere and 25° C. As the above-mentioned gas, when a cyclic carbonate or a chain carbonate is used as the non-aqueous solvent of the non-aqueous electrolyte, carbon dioxide is preferred.

The volume of the gas soluble in the non-aqueous electrolyte contained in the container 3 (under 1 atmosphere, 25° C.) can be determined by the volume of the non-aqueous electrolyte contained in the container 3 and the solubility of the gas soluble in the non-aqueous electrolyte in the non-aqueous electrolyte. For example, when a cyclic carbonate or a chain carbonate is used as a non-aqueous solvent for the non-aqueous electrolyte and carbon dioxide is used as a gas soluble in the non-aqueous electrolyte, from the viewpoint of improving the negative pressure effect, the lower limit of the volume of carbon dioxide relative to the volume of the non-aqueous electrolyte contained in the container 3 is preferably 10%, and more preferably 20%. On the other hand, from the viewpoint of shortening the time from sealing the container 3 to dissolving the carbon dioxide, the upper limit of the volume of carbon dioxide relative to the volume of the above-mentioned non-aqueous electrolyte is preferably 400%, more preferably 200%, and further preferably 100%.

From the viewpoint of reducing the pressure in the container 3, the lower limit of the volume of the gas soluble in the non-aqueous electrolyte contained in the container 3 (at 1 atmosphere, 25°C) relative to the volume of the remaining space in the container 3 is preferably 40%, more preferably 70%, and further preferably 95%. On the other hand, the upper limit of the above volume relative to the volume of the remaining space in the container 3 can be 100%. Here, the "volume of the remaining space in the container 3" refers to the volume obtained by subtracting the volume of the structure such as the electrode body 2 and the non-aqueous electrolyte from the internal volume of the container 3. In addition, the volume of the electrode body 2 refers to the apparent volume of the constituent elements of the electrode body (active material layer, separator, etc.), and does not include the voids between the active material layers and in the separator.

＜Configuration of power storage device＞

The storage element of this embodiment can be installed as a storage unit (battery module) composed of a plurality of storage elements in a vehicle power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), a power source for electronic equipment such as a personal computer, a communication terminal, or a power source for storing electricity. In this case, the technology of the present invention can be applied to at least one storage element contained in the storage unit.

A power storage device according to another embodiment of the present invention has two or more power storage elements and has one or more power storage elements of the above-mentioned one embodiment of the present invention (hereinafter referred to as the "second embodiment"). As for at least one power storage element contained in the power storage device of the second embodiment, as long as the technology of one embodiment of the present invention is applied, it can have one power storage element of the above-mentioned one embodiment of the present invention, and one or more power storage elements other than the above-mentioned one embodiment of the present invention, or it can have two or more power storage elements of the above-mentioned one embodiment of the present invention. FIG. 3 shows an example of a power storage device 30 formed by further integrating a power storage unit 20 formed by integrating two or more electrically connected power storage elements 1. The power storage device 30 of the second embodiment can include a bus (not shown) that electrically connects the two or more power storage elements 1, a bus (not shown) that electrically connects the two or more power storage units 20, and the like. The power storage unit 20 or the power storage device 30 can include a state monitoring device (not shown) that monitors the state of one or more power storage elements.

<Method for manufacturing energy storage device>

The manufacturing method of the storage element of the present embodiment can be appropriately selected from known methods. The manufacturing method, for example, comprises: preparing an electrode body, preparing a non-aqueous electrolyte, and storing the electrode body and the non-aqueous electrolyte in a container. Preparing the electrode body comprises: preparing a positive electrode and a negative electrode, and stacking or winding the positive electrode and the negative electrode through an isolating member to form an electrode body. The manufacturing method of the storage element of the present embodiment further comprises pressing the electrode body in the stacking direction of the positive electrode and the negative electrode. As for pressing the electrode body in the stacking direction of the positive electrode and the negative electrode, it is preferred to contain a gas soluble in the non-aqueous electrolyte in a container.

The non-aqueous electrolyte may be contained in the container by any known method. For example, when a non-aqueous electrolyte is used as the non-aqueous electrolyte, the non-aqueous electrolyte may be injected from an injection port formed in the container and then the injection port may be sealed.

<Other embodiments>

It should be noted that the storage element of the present invention is not limited to the above-mentioned embodiment, and various changes can be made within the scope of the gist of the present invention. For example, the structure of another embodiment can be added to the structure of one embodiment, and a part of the structure of one embodiment can be replaced with the structure of another embodiment or a known technology. Furthermore, a part of the structure of one embodiment can be deleted. In addition, a known technology can be added to the structure of one embodiment.

In the above embodiment, the case where the storage element is used as a rechargeable non-aqueous electrolyte secondary battery (e.g., a lithium ion secondary battery) is described, but the type, shape, size, capacity, etc. of the storage element are arbitrary. The present invention can be applied to various secondary batteries, double-layer capacitors, or capacitors such as lithium ion capacitors.

In the above embodiment, the electrode body is described as a stacked positive electrode and a negative electrode with a separator interposed therebetween, but the electrode body may not have a separator. For example, the positive electrode and the negative electrode may be directly connected with a non-conductive layer formed on the active material layer of the positive electrode or the negative electrode.

In the above embodiment, the case where the positive electrode active material includes the compound represented by the above formula 1 or lithium manganate and the negative electrode active material includes graphite or lithium titanate is described, but the composition of the positive electrode active material and the negative electrode active material is not limited to the above embodiment. For example, it can be composed as follows: the compound represented by the above formula 1 or lithium manganate is selected as the positive electrode active material, graphite or lithium titanate is selected as the negative electrode active material, and the difference between the open circuit voltage of SOC30% and the open circuit voltage of SOC70% is 0.2V or less.

In the above embodiment, the method of making the inside of the container a negative pressure state is described as a method of making the electrode body a pressed state, but a method other than the method of making the inside of the container a negative pressure state can also be used as a method of making the electrode body a pressed state. For example, by using a previously known constraint component, etc. to constrain the storage element in a state of pressing in the stacking direction (Y direction) of the positive and negative electrodes, the electrode body can be made into a pressed state. As a constraint component, as long as it can press (pressurize) the storage element, there is no particular limitation. As a method of pressing the electrode body using a constraint component, etc., there can be cited a method of pressing the storage element using a constraint component in a manner that the pressing force applied to the storage element becomes a constant value (constant pressure constraint), etc. The area and shape of the region (pressing region) of the outer surface of the storage element pressed by the constraint component, etc. are not particularly limited, and can be appropriately set in consideration of the effect of suppressing the reduction of the capacity retention rate during the charge and discharge cycle and the characteristics of the storage element. As the above-mentioned container, it is preferably thin and flexible. In addition, from the viewpoint of being able to maintain the pressure applied to the electrode assembly at a constant value from the initial stage, it is preferable to arrange a known buffer member between the restraining member and the container.

The pressure applied to the electrode body by the restraining member is not particularly limited and can be appropriately set. For example, as the lower limit of the pressure applied to the electrode body, it is sometimes preferably 0.5MPa, more preferably 0.7MPa, further preferably 0.9MPa, and further preferably 1.0MPa. By the above-mentioned pressure being above the above-mentioned lower limit, the effect of suppressing the reduction of the capacity retention rate during the charge and discharge cycle is more significantly exerted. On the other hand, as the upper limit of the above-mentioned pressure, it is sometimes preferably 3.7MPa, more preferably 3.5MPa, further preferably 3.3MPa, and further preferably 3.0MPa. By the above-mentioned pressure being below the above-mentioned upper limit, the clogging of the pores of the separator caused by applying excessive pressure can be suppressed. The pressure applied to the electrode body is the pressure in the discharge state. As a method for measuring the pressure applied to the electrode body, the following value can be cited as the pressure applied to the electrode body: the storage element is separated from the restraining member, etc. in the discharge state, and the storage element is pressed with an automatic recorder until it becomes the same thickness as when pressed by the restraining member, etc., and the load at this time is divided by the area of the contact surface between the automatic recorder and the electrode body. In addition, although a load is usually applied to a pair of opposing surfaces of the electric storage element through the container, the area of only one surface of the pair of surfaces is set as the area of the surface to which the load is applied.

Example

The present invention is further described below with reference to examples. The present invention is not limited to the following examples.

[Example 1]

(Production of positive electrode)

As a positive electrode active material, lithium iron phosphate (LiFePO ₄ ) was used. A positive electrode mixture paste containing the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of 90:5:5 in terms of solid content was prepared using N-methylpyrrolidone (NMP) as a dispersion medium. The positive electrode mixture paste was applied to both sides of an aluminum foil as a positive electrode substrate, and pressed after drying. Thus, a positive electrode having positive electrode active material layers stacked on both sides of the positive electrode substrate was obtained. At this time, the mass per unit area of the positive electrode active material layer stacked on one side of the positive electrode substrate was 15 mg/cm ² in terms of solid content. In addition, the differential pore volume of the positive electrode active material layer with a pore diameter of 50 nm in the obtained positive electrode was 0.0039 cm ³ /(g·nm). It should be noted that the differential pore volume of the positive electrode active material layer with a pore diameter of 50 nm is a value obtained by the above method. In this example, the differential pore volume is a value measured when the positive electrode is made, but even after the storage element made based on the positive electrode is charged and discharged, the differential pore volume can be regarded as the same value. The differential pore volume distribution curve of the positive electrode active material layer of Example 1 is shown in FIG.

(Production of negative electrode)

A negative electrode mixture paste was prepared by mixing graphite (Gr) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. It should be noted that the mass ratio of Gr, SBR, and CMC was 96:2:2 (solid content conversion). The negative electrode mixture paste was applied to both sides of a copper foil as a negative electrode substrate and dried. Then, roll pressing was performed to obtain a negative electrode. At this time, the mass per unit area of the negative electrode active material layer stacked on one side of the negative electrode substrate was 8 mg/ ^cm2 in terms of solid content.

(Preparation of non-aqueous electrolyte)

LiPF ₆ as an electrolyte salt was dissolved at a concentration of 1.0 mol/dm ³ in a non-aqueous solvent obtained by mixing EC and EMC at a volume ratio of 30:70 to prepare a solution. The solution was obtained as a non-aqueous electrolyte.

(Production of energy storage elements)

As a separator, a polyolefin microporous film is used. The electrode body is prepared by stacking the positive electrode and the negative electrode via the separator. The electrode body is placed in a container made of a composite film (total thickness: about 150 μm) in which an aluminum layer and a resin film layer are stacked, and the non-aqueous electrolyte is injected into the interior. The ratio of the minimum width of the internal space of the container in the stacking direction to the thickness of the electrode body in the stacking direction is 1.10 or less. Then, converted to the conditions of 1 atmosphere and 25°C, carbon dioxide is sealed in a manner of about 50% of the volume of the injected non-aqueous electrolyte and about 80% of the volume of the remaining space in the container, and sealed by heat welding to obtain the storage element of Example 1. It should be noted that the carbon dioxide sealed in the container dissolves in the non-aqueous electrolyte, and the interior of the container after the storage element of Example 1 is sealed becomes a negative pressure state. In addition, since the container is thin and made of a flexible composite film, the interior of the container is in a negative pressure state, thereby deforming the container. Furthermore, since the minimum width of the internal space of the container in the stacking direction is sufficiently smaller than the thickness of the electrode body, the electrode body is pressed in the stacking direction of the positive and negative electrodes due to deformation of the container.

[Examples 2, 3 and Comparative Examples 1 to 9]

Except that the differential pore volume of the pore diameter of 50 nm of the positive electrode active material layer was changed as described in Table 1, the same procedure as in Example 1 was carried out to obtain each of the electric storage elements of Examples 2 and 3 and Comparative Examples 1 to 3.

In addition, carbon dioxide was not sealed in the container, and the differential pore volume of the pore diameter of 50 nm of the positive electrode active material layer was changed as described in Table 1. The storage elements of Comparative Examples 4 to 9 were obtained in the same manner as in Example 1. Among them, the differential pore volume of all pore diameters was evaluated in Comparative Example 3, and only the differential pore volume of the pore diameter of 50 nm was evaluated in Examples 2, 3, Comparative Examples 1, 2, and 4 to 9. The differential pore volume distribution curve of the positive electrode active material layer of Comparative Example 3 is shown in FIG4. In addition, since carbon dioxide was not sealed in the container, the pressure inside the container of the storage element of Comparative Examples 4 to 9 was equal to the atmospheric pressure (1 atmosphere).

(Initial charge and discharge)

For each of the obtained storage elements, initial charge and discharge were performed at 25°C according to the following procedures. The charging current was set to 0.1C and the charging end voltage was 3.6V for constant current and constant voltage charging. The end condition of charging was that the charging current reached 0.02C. Then, a rest time of 10 minutes was set. Then, the discharge current was set to 0.1C and the discharge end voltage was 2.0V for constant current discharge.

(Initial capacity confirmation test)

Next, an initial capacity confirmation test was performed on each power storage element at 25° C. according to the following procedure.

The charging current is set to 0.1C and the charging end voltage is 3.6V for constant current and constant voltage charging. The end condition of charging is that the charging current reaches 0.02C. Then, a rest period of 10 minutes is set. Then, the discharge current is set to 0.1C and the discharge end voltage is 2.0V for constant current discharge. The discharge capacity at this time is referred to as the "initial discharge capacity". In addition, for each storage element, the charging current is set to 0.1C and the charging end voltage is 3.6V for constant current and constant voltage charging. The end condition of charging is that the charging current reaches 0.02C. Then, a rest period of 10 minutes is set. Then, constant current discharge is performed with a discharge current of 0.1C to SOC70%, and then the voltage is measured after 30 minutes without applying current as the "open circuit voltage of SOC70%". Then, constant current discharge is performed with a discharge current of 0.1C to SOC30%, and then the voltage is measured after 30 minutes without applying current as the "open circuit voltage of SOC30%". In each storage element, the difference between the open circuit voltage at SOC30% and the open circuit voltage at SOC70% is less than 0.2 V. It should be noted that here, the state after the initial capacity confirmation test is marked as SOC0% as a fully discharged state, and the state after constant current and constant voltage charging with a charge end voltage of 3.6 V for the same amount of electricity as the initial discharge capacity is marked as SOC100% as a fully charged state.

(Charge and discharge cycle test)

After the initial capacity confirmation test, for each storage element, a charge and discharge cycle test was performed at 25°C according to the following instructions. The charging current was set to 1.0C and the charge end voltage was 3.6V for constant current and constant voltage charging. The end condition of charging was that the charging current reached 0.05C. Then, the discharge current was set to 1.0C and the discharge end voltage was 2.0V for constant current discharge. A rest time of 10 minutes was set after charging and after discharging. This charge and discharge was performed for 50 cycles.

After the charge-discharge cycle test, a capacity confirmation test after the charge-discharge cycle test is performed in the same manner as the above-mentioned "initial capacity confirmation test". The discharge capacity at this time is referred to as the "discharge capacity after the charge-discharge cycle test". The discharge capacity after the charge-discharge cycle test is divided by the initial discharge capacity to calculate the capacity retention rate (%). The results are shown in Table 1.

[Table 1

As shown in Table 1 above, when comparing Comparative Example 1 and Comparative Example 7, which have the same differential pore volume of 50 nm of the pore diameter of the positive electrode active material layer, the capacity retention rate of Comparative Example 1, in which the pressure in the container is a negative pressure state, is higher than that of Comparative Example 7, in which the pressure in the container is equal to atmospheric pressure. Similarly, when comparing Comparative Example 2 and Comparative Example 8, and Comparative Example 3 and Comparative Example 9, the capacity retention rate of Comparative Example 2 and Comparative Example 3, in which the pressure in the container is a negative pressure state, is higher than that of Comparative Example 8 and Comparative Example 9, in which the pressure in the container is equal to atmospheric pressure.

In addition, when the pressure in the container is equal to the atmospheric pressure in Comparative Examples 4 to 9, the differential pore volume of the positive electrode active material layer at a pore size of 50 nm is 0.0030 cm ³ / (g·nm) or more, and the capacity retention rate is higher in Comparative Examples 4 to 6 than in Comparative Examples 7 to 9 in which the differential pore volume of the positive electrode active material layer at a pore size of 50 nm is less than 0.0030 cm ³ / (g·nm). On the other hand, the differential pore volume of the positive electrode active material layer at a pore size of 50 nm is 0.0030 cm ³ / (g·nm) or more, and the pressure in the container is in a negative pressure state. The capacity retention rate is greatly improved compared to Comparative Examples 1 to 9. It is speculated that, for example, the fact that the electrode body is pressed due to the negative pressure state in the container and the fact that the positive electrode active material layer has appropriate pores work synergistically, significantly promoting the smooth and uniform movement of charge transport ions such as lithium ions in the electrode body, so that the charge and discharge reaction in the electrode body is not easily deviated, and as a result, the reduction in the capacity retention rate during the charge and discharge cycle can be suppressed.

Industrial Applicability

The present invention can be applied to power storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles.

Explanation of symbols

1 Storage element

2 Electrode body

3 Container

4 Positive terminal

41 positive lead

5 Negative terminal

51 negative lead

20 Storage Unit

30 Power storage device

Claims (4)

1. A power storage element is provided with:

An electrode body formed by stacking a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and

A sealable container for receiving the electrode body;

the electrode body is in a state of being pressed in a lamination direction of the positive electrode and the negative electrode,

The positive electrode active material layer has a differential pore volume of 0.0030cm ³/(g.nm) or more at a pore diameter of 50nm,

The difference between the open circuit voltage of 30% SOC and the open circuit voltage of 70% SOC is 0.2V or less.

2. A power storage element is provided with:

An electrode body formed by stacking a positive electrode having a positive electrode active material layer and a negative electrode having a negative electrode active material layer, and

A sealable container for receiving the electrode body;

the electrode body is in a state of being pressed in a lamination direction of the positive electrode and the negative electrode,

The positive electrode active material layer has a differential pore volume of 0.0030cm ³/(g.nm) or more at a pore diameter of 50nm,

The positive electrode active material contains a compound represented by the following formula 1 or lithium manganate,

The negative electrode active material contains graphite or lithium titanate,

LiFe_xMn_(1－x)PO₄(0≤x≤1)···1。

3. The power storage element according to claim 1 or 2, wherein an inside of the container is in a negative pressure state.

4. An electric storage device comprising two or more electric storage elements and one or more electric storage elements according to claim 1 or 2.