WO2023195434A1

WO2023195434A1 - Power storage element and power storage device

Info

Publication number: WO2023195434A1
Application number: PCT/JP2023/013750
Authority: WO
Inventors: 大輔遠藤
Original assignee: 株式会社Ｇｓユアサ
Priority date: 2022-04-04
Filing date: 2023-04-03
Publication date: 2023-10-12

Abstract

A power storage element according to one aspect of the present invention comprises: 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. The electrode body is in a state of being pressed in a direction in which the positive electrode and the negative electrode are laminated. The differential pore volume of the positive electrode active material layer at a pore size of 50 nm is at least 0.0030 cm3/(g•nm). The difference between the open circuit voltage when the SOC is 30% and the open circuit voltage when the SOC is 70% is not more than 0.2 V.

Description

Energy storage elements and energy storage devices

The present invention relates to a power storage element and a power storage device.

Non-aqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, are widely used in electronic devices such as personal computers, communication terminals, automobiles, etc. due to their high energy density. The non-aqueous electrolyte secondary battery generally includes an electrode body having a pair of electrodes and a separator, a non-aqueous electrolyte, and a container housing the electrode body and the non-aqueous electrolyte. It is configured to charge and discharge by transferring charge transporting ions. In addition, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as nonaqueous electrolyte storage devices other than nonaqueous electrolyte secondary batteries.

Conventionally, carbon materials such as graphite have been used as the active material contained in the negative electrode of non-aqueous electrolyte energy storage elements, and active materials are being developed to improve the charging and discharging characteristics of non-aqueous electrolyte energy storage elements. It is being As an example of a nonaqueous electrolyte storage element, Patent Document 1 discloses a nonaqueous electrolyte secondary battery using lithium iron phosphate as a positive electrode active material and graphite or the like as a negative electrode active material.

International Publication No. 2015/170561

Non-aqueous electrolyte secondary batteries that use lithium iron phosphate as the positive electrode active material and graphite as the negative electrode active material have a characteristic (voltage flatness) that changes in voltage due to changes in the state of charge are flat over a wide range of the state of charge. ) is excellent. However, in a non-aqueous electrolyte secondary battery having voltage flatness, charging and discharging reactions are likely to occur unevenly within the electrode body, such as when the distance between the electrodes is uneven. If the charging and discharging reactions within the electrode body are biased, lithium metal and the like are likely to be deposited on the negative electrode surface during charging at locations where the charging and discharging reactions are concentrated, so there is a risk that the capacity retention rate and the like will be significantly reduced with charge and discharge cycles.

The present invention has been made based on the above circumstances, and an object of the present invention is to provide a power storage element and a power storage device that can suppress a decrease in capacity retention rate during charge/discharge cycles.

A power 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 accommodating the electrode body, The electrode body is in a state where it is pressed in the stacking direction of the positive electrode and the negative electrode, and the positive electrode active material layer has a differential pore volume of 0.0030 cm ³ /(g·nm) or more at a pore diameter of 50 nm. , the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC is 0.2 V or less.

A power storage element according to another 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 accommodating the electrode body. The electrode body is in a state of being pressed in the stacking direction of the positive electrode and the negative electrode, and the positive electrode active material layer has a differential pore volume of 0.0030 cm ³ /(g·nm) or more at a pore diameter of 50 nm. The above-mentioned positive electrode active material contains a compound represented by the following formula 1 or lithium manganate, and the above-mentioned negative electrode active material contains 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 power storage element and power storage device according to one aspect of the present invention can suppress a decrease in capacity retention rate during charge/discharge cycles.

FIG. 1 is a graph showing an example of a discharge curve of a power storage element for explaining voltage flatness. FIG. 2 is a transparent perspective view showing one embodiment of a power storage element. FIG. 3 is a schematic diagram showing an embodiment of a power storage device configured by collecting a plurality of power storage elements. FIG. 4 shows differential pore volume distribution curves of the positive electrode active material layers of Example 1 and Comparative Example 3.

One embodiment of the present invention provides the following aspects.

Item 1.
A power storage element according to an embodiment 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 accommodating the electrode body. , the electrode body is pressed in the lamination direction of the positive electrode and the negative electrode, and the positive electrode active material layer has a differential pore volume of 0.0030 cm ³ /(g·nm) or more at a pore diameter of 50 nm. Yes, the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC is 0.2V or less.

The electricity storage element according to item 1 above can suppress a decrease in capacity retention rate during charge/discharge cycles.

Item 2.
A power storage element according to another embodiment 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 accommodating the electrode body. , the electrode body is in a state of being pressed in the stacking direction of the positive electrode and the negative electrode, and the positive electrode active material layer has a differential pore volume of 0.0030 cm ³ /(g·nm) at a pore diameter of 50 nm. As above, the positive electrode active material contains a compound represented by the following formula 1 or lithium manganate, and the negative electrode active material contains graphite or lithium titanate.
LiFe _x Mn _(1-x) PO ₄ (0≦x≦1) ...1

The electricity storage element according to item 2 above can suppress a decrease in capacity retention rate during charge/discharge cycles.

Item 3.
In the electricity storage element according to item 1 or item 2 above, the inside of the container may be in a negative pressure state.

According to the electricity storage element described in item 3 above, it is possible to further suppress a decrease in capacity retention rate during charge/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 described in any one of Items 1 to 3 above.

According to the power storage device described in item 4 above, a decrease in capacity retention rate during charge/discharge cycles can be suppressed.

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

The power storage element can suppress a decrease in capacity retention rate during charge/discharge cycles. The reason for this is presumed to be as follows.

Since the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC is 0.2 V or less, the electricity storage element has voltage flatness in which the change in voltage due to the change in the state of charge is flat. Here, in order to explain "voltage flatness", FIG. 1 shows an example of a discharge curve of a power storage element with the horizontal axis as SOC (%) and the vertical axis as battery voltage (V). The battery voltage (V) is the closed circuit voltage (CCV) of the storage element when constant current discharge is performed at 25° C. with a discharge current of 0.1C. In Figure 1, LFP/Gr is a lithium ion secondary battery using lithium iron phosphate (LFP) as a positive electrode and graphite (Gr) as a negative electrode, and NCM/Gr is a composite oxide containing Ni, Co, and Mn. It is a lithium ion secondary battery that uses a composite oxide (NCA) containing Ni, Co and Al as a positive electrode and graphite (Gr) as a negative electrode. This is a lithium ion secondary battery that uses a lithium ion battery as a negative electrode. In Figure 1, the battery voltage (V) of NCM/Gr and NCA/Gr monotonically decreases from 70% SOC to 30% SOC, whereas the battery voltage (V) of LFP/Gr almost decreases from 70% SOC to 30% SOC. constant. That is, the LFP/Gr in FIG. 1 has voltage flatness.

On the other hand, when a conventional energy storage element has the above-mentioned voltage flatness, when the distribution of charge transport ions such as lithium ions is uneven in the electrode body, the potential in the positive electrode active material layer and the negative electrode active material layer changes. It is inferred that the difference makes it difficult for the force to eliminate this bias 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 transporting ions such as lithium ions tends to become uneven, and the charging and discharging reactions within the electrode body tend to be uneven. Easy to maintain. For this reason, lithium metal or the like tends to be locally deposited on the surface of the negative electrode during charging, and there is a risk that the capacity retention rate will decrease with charge/discharge cycles.

In contrast, in the electricity storage element according to one aspect of the present invention, the distance between the two electrodes is easily equalized by pressing the electrode body in the stacking direction of the positive electrode and the negative electrode, and the pore diameter of the positive electrode active material layer is 50 nm. Since the differential pore volume is 0.0030 cm ³ /(g·nm) or more, smooth and uniform movement of charge transport ions such as lithium ions is promoted within the positive electrode active material layer. Therefore, it is presumed that the charging and discharging reactions within the electrode body are less likely to be biased, and as a result, it is possible to suppress a decrease in the capacity retention rate during charging and discharging cycles.

In the electricity storage element, since the positive electrode active material contains the compound represented by the above formula 1 or lithium manganate, and the negative electrode active material contains graphite or lithium titanate, the change in voltage due to the change in the state of charge is flat. It has voltage flatness. On the other hand, by pressing the electrode body in the direction in which the positive electrode and negative electrode are stacked, the distance between the two electrodes is easily equalized, and 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, smooth and uniform movement of charge transport ions such as lithium ions is promoted within the positive electrode active material layer. Therefore, the charging and discharging reactions within the electrode body are less likely to be biased, and as a result, it is possible to suppress a decrease in the capacity retention rate during charging and discharging cycles.

The interior of the container may be under negative pressure. In this way, by having the inside of the container in a negative pressure state, the distance between both electrodes can be more easily equalized.

In the present invention, "the lamination direction of the positive electrode and the negative electrode" means the direction in which the positive electrode and the negative electrode are laminated, and the "state of being pressed in the lamination direction" means the direction in which the positive electrode and the negative electrode are laminated, and the term "state of being pressed in the lamination direction" means the direction in which the positive electrode and the negative electrode are laminated. This means that pressure is applied in the direction where the distance between Furthermore, in the present invention, a "negative pressure state" means a state in which the pressure inside the container housing the electrode body is lower than the pressure outside the container. "SOC" is an abbreviation for State of Charge, and represents the state of charge of a power storage element as a ratio of the remaining capacity at that time to the capacity in a fully charged state, and a fully charged state is expressed as SOC 100%. Here, the "fully charged state" refers to a state in which the electricity storage element is charged to the upper limit voltage using recommended or designated charging conditions. Furthermore, the "difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70%" is calculated by subjecting the storage element to constant current discharge at 25° C. with a discharge current of 0.1 C to reach SOC 30% and SOC 70%. It refers to the absolute value of the difference in open circuit voltage (OCV) measured after 30 minutes without applying any current.

In the present invention, the "differential pore volume at a pore diameter of 50 nm" is a value determined by the BJH method from an adsorption isotherm using the nitrogen gas adsorption method. Specifically, the differential pore volume is measured by the following method. 1.00 g of 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 sufficiently remove water in the sample to be measured. Next, by a nitrogen gas adsorption method using liquid nitrogen, isotherms on the adsorption side and the desorption side are measured while the relative pressure P/P0 (P0 = about 770 mmHg) is in the range of 0 to 1. Then, the differential pore volume distribution is evaluated by calculation by the BJH method using the isotherm on the desorption side, and the differential pore volume at a pore diameter of 50 nm is determined. The above-described differential pore volume is measured using a gas adsorption amount measuring device "autosorb iQ" manufactured by Quantachrome and data analysis software "ASiQwin".

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 the above-described one aspect of the present invention. Since the power storage device includes the power storage element according to the above-described one aspect of the present invention, it is possible to suppress a decrease in capacity retention rate during charge/discharge cycles.

A configuration of a power storage element, a configuration of a power storage device, a method for manufacturing a power storage element, and other embodiments of the present invention will be described in detail. Note that 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 art.

<Configuration of power storage element>
A 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 sealable container for accommodating the electrode body and the non-aqueous electrolyte. The electrode body has a structure in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with a separator in between. Examples of the structure of the electrode body include a laminated type, or a wound type in which a positive electrode and a negative electrode are laminated with a separator interposed in between and wound. The electrode body is in a state where it is pressed in the stacking direction of the positive electrode and the negative electrode. The non-aqueous electrolyte exists in the positive electrode, negative electrode, and separator. As an example of a power storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") will be described.

In one embodiment of the present invention, the upper limit of the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC is 0.2V, more preferably 0.15V, and even more preferably 0.10V. By keeping the difference between the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% below the above upper limit, it is possible to relatively widen the range of charge states in which the change in voltage due to change in charge state is flat (voltage flatness). ). On the other hand, the lower limit of the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC is not particularly limited, but may be, for example, 0.01V or 0.05V. The difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC may be greater than or equal to any of the above lower limits and below any of the above upper limits.

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

The positive electrode base material has electrical conductivity. Whether or not it has "conductivity" is determined by using a volume resistivity of 10 ⁷ Ω·cm as a threshold value, which is measured in accordance with JIS-H-0505 (1975). As the material of the positive electrode base material, metals such as aluminum, titanium, tantalum, stainless steel, or alloys thereof are used. Among these, aluminum or aluminum alloy is preferred from the viewpoint of potential resistance, high conductivity, and cost. Examples of the positive electrode base material include foil, vapor deposited film, mesh, porous material, etc., and foil is 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, A1N30, etc. specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

The average thickness of the positive electrode base material is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and 40 μm or less, even more 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 within 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 intermediate layer is a layer disposed between the positive electrode base material and the positive electrode active material layer. The intermediate layer reduces contact resistance between the positive electrode base material and the positive electrode active material layer by containing a conductive agent such as carbon particles. The structure of the intermediate layer is not particularly limited, and includes, for example, 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 contain both the compound represented by Formula 1 above and lithium manganate. Further, the positive electrode active material may contain only one type of compound represented by the above formula 1, or two or more types of the above formula 1 having different molar ratios (x) of Fe to the sum of moles of Fe and Mn. It may contain the compound represented by. The compound represented by the above formula 1 or lithium manganate may be partially substituted with atoms or anion species of other elements, or may be coated with other materials.

The lower limit of the total content of the compound represented by Formula 1 above and lithium manganate for all positive electrode active materials is preferably 70% by mass, more preferably 80% by mass, and even more preferably 90% by mass. When the total content is equal to or greater than the lower limit, it is easy to reduce the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC. On the other hand, the total content of the compound represented by Formula 1 and lithium manganate in all positive electrode active materials may be 100% by mass.

The positive electrode active material preferably contains lithium iron phosphate (LiFePO ₄ ) as a compound represented by Formula 1 above. The lower limit of the content of lithium iron phosphate for all positive electrode active materials is preferably 70% by mass, more preferably 80% by mass, and even more preferably 90% by mass. When the content of lithium iron phosphate in all positive electrode active materials is equal to or higher than the above lower limit, it is easier to reduce the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC. On the other hand, the content of lithium iron phosphate in all positive electrode active materials may be 100% by mass.

The positive electrode active material is usually particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to be equal to or larger than the above lower limit, manufacturing or handling of the positive electrode active material becomes easier. By setting the average particle size of the positive electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the positive electrode active material layer is improved. In addition, when using a composite of a positive electrode active material and another material, let the average particle diameter of the composite be the average particle diameter of the positive electrode active material. "Average particle size" is based on the particle size distribution measured by laser diffraction/scattering method on a diluted solution of particles diluted with a solvent, in accordance with JIS-Z-8825 (2013). -2 (2001), meaning the value at which the volume-based cumulative distribution calculated in accordance with 2001 is 50%.

A pulverizer, classifier, etc. are used to obtain powder with a predetermined particle size. Examples of the pulverization method include methods 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 jet mill, a sieve, and the like. At the time of pulverization, wet pulverization in which water or an organic solvent such as hexane is present can also be used. As for the classification method, a sieve, a wind classifier, etc. may be used, both dry and wet, as necessary.

The content of the 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 even more preferably 80% by mass or more and 95% by mass or less. By setting the content of the positive electrode active material within the above range, it is possible to achieve both high energy density and manufacturability of the positive electrode active material layer.

The conductive agent is not particularly limited as long as it is a material that has conductivity. Examples of such conductive agents include carbonaceous materials, metals, conductive ceramics, and the like. Examples of the carbonaceous material include graphite, non-graphitic carbon, graphene-based carbon, and the like. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, carbon black, and the like. Examples of carbon black include furnace black, acetylene black, Ketjen black, and the like. Examples of graphene-based carbon include graphene, carbon nanotubes (CNT), and fullerene. Examples of the shape of the conductive agent include powder, fiber, and the like. As the conductive agent, one type of these materials may be used alone, or two or more types may be used in combination. Further, these materials may be used in combination. For example, a composite material of carbon black and CNT may be used. Among these, carbon black is preferred from the viewpoint of electronic conductivity and coatability, and acetylene black is particularly preferred.

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, more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the secondary battery can be increased.

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

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

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

The filler is not particularly limited. Fillers include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate, magnesium hydroxide, calcium hydroxide, and hydroxide. Hydroxides such as aluminum, carbonates such as calcium carbonate, poorly soluble ionic crystals such as calcium fluoride, barium fluoride, barium sulfate, nitrides such as aluminum nitride and silicon nitride, talc, montmorillonite, boehmite, zeolite, Examples include substances derived from mineral resources such as apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof.

The positive electrode active material layer is made of typical nonmetallic elements 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, Nb, W, and other transition metal elements are used as positive electrode active materials, conductive agents, binders, thickeners, and fillers. It may be contained as a component other than the above.

The lower limit of the differential pore volume at a pore diameter of 50 nm in the positive electrode active material layer is 0.0030 cm ³ /(g·nm), preferably 0.0040 cm ³ /(g·nm), and 0.0050 cm ³ /(g・nm) is more preferable. When the differential pore volume is equal to or greater than the lower limit, the nonaqueous electrolyte easily permeates into the positive electrode active material layer, so that the movement of charge transport ions such as lithium ions in the positive electrode active material layer is likely to be uniform. On the other hand, the upper limit of the differential pore volume is preferably 0.0100 cm ³ /(g·nm), more preferably 0.0080 cm ³ /(g·nm), from the viewpoint of ease of production. Since the differential pore volume is difficult to change due to charging and discharging, for example, the positive electrode active material layer is obtained by disassembling and removing a power storage device manufactured based on the positive electrode active material after charging and discharging the power storage device. It can be the value measured against. Note that the differential pore volume of the positive electrode active material layer at a pore diameter of 50 nm can be adjusted, for example, by changing the manufacturing conditions when producing the precursor of the positive electrode active material, or by changing the firing temperature during producing the active material. .

In the differential pore volume distribution curve evaluated by the method described above, the positive electrode active material layer has at least one peak in the pore diameter range of 25 nm or more and 75 nm or less, and the maximum peak is in the pore diameter range of 25 nm or more and 75 nm or less. It is preferable to have it within the range. In addition, the positive electrode active material layer should have at least one peak in the pore diameter range of 45 nm or more and 55 nm or less, and have the largest peak in the pore diameter range of 45 nm or more and 55 nm or less, in the differential pore volume distribution curve. is more preferable. In this way, the positive electrode active material layer has at least one peak in the above range in the differential pore volume distribution curve, and has the maximum peak in the above range, so that the positive electrode active material layer of the non-aqueous electrolyte penetration and movement of charge transport ions such as lithium ions in the positive electrode active material layer are likely to be uniform.

From the viewpoint of increasing energy density, the lower limit of the mass per unit area of the positive electrode active material layer is preferably 1 mg/cm ² , more preferably 5 mg/cm ² , and even more preferably 10 mg/cm ² . On the other hand, the upper limit of the mass per unit area of the positive electrode active material layer is preferably 30 mg/cm ² , more preferably 25 mg/cm ² , and even more preferably 20 mg/cm ² . When the mass per unit area of the positive electrode active material layer is below the above upper limit, charge transport ions such as lithium ions are likely to be uniformly diffused. Note that "mass per unit area of the positive electrode active material layer" refers to the solid content equivalent mass of the positive electrode active material layer, which is the mass of the positive electrode active material layer disposed directly or via an intermediate layer on the surface of the positive electrode base material. Means the value divided by the area of the region. When the positive electrode active material layer is arranged on both sides of the positive electrode base material, the value is determined from the mass and area per unit area of the positive electrode active material layer on one surface of the positive electrode base material.

(Negative electrode)
The negative electrode includes a negative electrode base material and a negative electrode active material layer disposed on the negative electrode base material directly 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 for the positive electrode.

The negative electrode base material has electrical conductivity. As the material of the negative electrode base material, metals such as copper, nickel, stainless steel, nickel-plated steel, aluminum, alloys thereof, carbonaceous materials, etc. are used. Among these, copper or copper alloy is preferred. Examples of the negative electrode base material include foil, vapor deposited film, mesh, porous material, etc. Foil is preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferable as the negative electrode base material. Examples of copper foil include rolled copper foil, electrolytic copper foil, and the like.

The average thickness of the negative electrode base material is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 30 μm or less, even more 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 within 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.

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, as 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 made of typical nonmetallic elements 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 of It may be contained as a component other than the adhesive and filler.

In one embodiment of the invention, the negative electrode active material includes graphite or lithium titanate. Here, "graphite" refers to a carbon material whose average lattice spacing (d ₀₀₂ ) of the (002) plane is 0.33 nm or more and less than 0.34 nm, as determined by X-ray diffraction before charging and discharging or in a discharge state. means. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties. Moreover, the "discharged state" of the negative electrode means a state in which the negative electrode active material is discharged such that charge transport ions such as lithium ions that can be intercalated and released during charging and discharging are sufficiently released. For example, 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 for all negative electrode active materials is preferably 70% by mass, more preferably 80% by mass, and even more preferably 90% by mass. When the content of graphite or lithium titanate is at least the above lower limit, it is easy to reduce the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC. On the other hand, the content of graphite or lithium titanate in all negative electrode active materials may be 100% by mass.

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. The average particle size of the negative electrode active material may be 1 μm or more and 100 μm or less. By setting the average particle size of the negative electrode active material to be equal to or larger than the above lower limit, manufacturing or handling of the negative electrode active material becomes easier. By setting the average particle size of the negative electrode active material to be less than or equal to the above upper limit, the electronic conductivity of the active material layer is improved. A pulverizer, classifier, etc. are used to obtain powder with a predetermined particle size. The pulverization method and classification method can be selected from, for example, the methods exemplified for the positive electrode.

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, 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.

From the viewpoint of increasing energy density, the lower limit of the mass per unit area of the negative electrode active material layer is preferably 1 mg/cm ² , more preferably 3 mg/cm ² , and even more preferably 5 mg/cm ² . On the other hand, the upper limit of the mass per unit area of the negative electrode active material layer is preferably 20 mg/cm ² , more preferably 15 mg/cm ² , and even more preferably 12 mg/cm ² . When the mass per unit area of the negative electrode active material layer is below the above upper limit, charge transport ions such as lithium ions are likely to be uniformly diffused. Note that "mass per unit area of the negative electrode active material layer" refers to the solid content equivalent mass of the negative electrode active material layer, which is the mass of the negative electrode active material layer disposed directly or through an intermediate layer on the surface of the negative electrode base material. Means the value divided by the area of the region. When the negative electrode active material layers are arranged on both sides of the negative electrode base material, the value is determined from the mass and area per unit area of the negative electrode active material layer on one surface of the negative electrode base material.

(Separator)
The separator can be appropriately selected from known separators. As the separator, for example, a separator consisting of only a base material layer, a separator in which a heat resistant layer containing heat resistant particles and a binder is formed on one or both surfaces of the base material layer, etc. can be used. Examples of the shape of the base material layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, a porous resin film is preferred from the viewpoint of strength, and a nonwoven fabric is preferred from the viewpoint of liquid retention of the nonaqueous electrolyte. As the material for the base layer of the separator, polyolefins such as polyethylene and polypropylene are preferred from the viewpoint of shutdown function, and polyimide, aramid, etc. are preferred from the viewpoint of oxidative decomposition resistance. A composite material of these resins may be used as the base material layer of the separator.

The heat-resistant particles contained in the heat-resistant layer preferably have a mass loss 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 loss when the temperature is raised from room temperature to 800°C. is more preferably 5% or less. Inorganic compounds are examples of materials whose mass loss is less than a predetermined value. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride. carbonates such as calcium carbonate; sulfates such as barium sulfate; poorly soluble ionic crystals such as calcium fluoride, barium fluoride, barium titanate; covalent crystals such as silicon and diamond; talc, montmorillonite, boehmite, Examples include substances derived from mineral resources such as zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. As the inorganic compound, these substances may be used alone or in combination, or two or more types may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferable from the viewpoint of safety of the electricity 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, "porosity" is a value based on volume, and means a value measured with a mercury porosimeter.

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

(Nonaqueous electrolyte)
The non-aqueous electrolyte can be appropriately selected from known non-aqueous electrolytes. A non-aqueous electrolyte may be used as the non-aqueous electrolyte. The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

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

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

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

As the nonaqueous solvent, it is preferable to use a cyclic carbonate or a chain carbonate, and it is more preferable to use a cyclic carbonate and a chain carbonate together. By using a cyclic carbonate, it is possible to promote the dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. By using chain carbonate, the viscosity of the non-aqueous electrolyte can be kept low. When a cyclic carbonate and a chain carbonate are used together, 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 electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, onium salts, and the like. Among these, lithium salts are preferred.

Examples of lithium salts include inorganic lithium salts such as LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ , LiClO ₄ , and 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 include lithium salts having halogenated hydrocarbon groups 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, inorganic lithium salts are preferred, and LiPF ₆ is more preferred.

The content of the electrolyte salt in the nonaqueous electrolyte is preferably 0.1 mol/dm ³ or more and 2.5 mol/dm 3 or less, and 0.3 mol/dm ³ ^or more and 2.0 mol/dm at 20° C. and 1 atmosphere. It is more preferably ³ or less, even more preferably 0.5 mol/dm ³ or more and 1.7 mol/dm ³ or less, 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 within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and electrolyte salt. Examples of additives include halogenated carbonate esters such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate). ) Oxalates such as difluorophosphate (LiFOP); Imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene , t-amylbenzene, diphenyl ether, dibenzofuran and other aromatic compounds; 2-fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene and other aromatic compounds such as partial halides; 2,4-difluoroanisole, 2 , 5-difluoroanisole, 2,6-difluoroanisole, 3,5-difluoroanisole and other halogenated anisole compounds; vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, anhydride Citraconic acid, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethylsulfone, diethyl Sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo- 1,3,2-Dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propenesultone, 1,3-propanesultone, 1,4-butanesultone, 1,4-butenesultone, perfluorooctane, boric acid Examples include tristrimethylsilyl, 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 nonaqueous electrolyte is preferably 0.01% by mass or more and 10% by mass or less, and 0.1% by mass or more and 7% by mass or less based on the mass of the entire nonaqueous electrolyte. It is more preferable if it is present, more preferably from 0.2% by mass to 5% by mass, and particularly preferably from 0.3% by mass to 3% by mass. By setting the content of the additive within the above range, capacity retention performance or cycle performance after high-temperature storage can be improved, and safety can be further improved.

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

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

Examples of the sulfide solid electrolyte in the case of a 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.

The shape of the power storage element of this embodiment is not particularly limited, and examples include a cylindrical battery, a square battery, a flat battery, a coin battery, a button battery, and the like.
FIG. 2 shows a power storage element 1 (non-aqueous electrolyte secondary battery) as an example of a square battery. Note that this figure is a perspective view of the inside of the container. An electrode body 2 having a positive electrode and a negative electrode wound together with a separator in between is housed in a rectangular container 3. Further, the container 3 is sealed with the non-aqueous electrolyte added thereto. As the container 3, a known metal container, resin container, or the like that is commonly used as a container for a power storage element can be used. The container 3 is preferably a thin and flexible container, such as a container made of a composite film in which a metal layer and a resin film layer are laminated, from the viewpoint of making it easier to press the electrode body 2 by the method described later.

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 a negative electrode lead 51. In FIG. 2, the axial direction of the winding of the electrode body 2 in the power storage element 1 is the X direction, the thickness direction of the power storage element 1 is the Y direction, and the direction perpendicular to the axis direction (X direction) and the thickness direction (Y direction) is shown. The direction perpendicular to is indicated as the Z direction. Note that the Z direction is parallel to the flat portion (that is, the surface of the flat portion) of the electrode body 2 in the power storage element 1, and coincides with the winding direction of the electrode body 2 on the flat portion. Here, the thickness direction of power storage element 1 coincides with the thickness direction of electrode body 2. The thickness direction of the electrode body 2 corresponds to the lamination direction of the positive electrode, negative electrode, and separator, and also corresponds to the direction perpendicular to the surfaces of the positive electrode, negative electrode, and separator.

As described above, the electrode body 2 of the power storage element 1 is in a state where it is pressed in the stacking direction (Y direction) of the positive electrode and the negative electrode. However, a part of the electrode body 2 (for example, a pair of curved surfaces at both ends of the flat part of the electrode body 2) may not be pressed. In this way, by being in a state where the electrode body 2 is pressed in the stacking direction of the positive electrode and the negative electrode (pressed state), the distance between both electrodes can be easily equalized, and as a result, lithium ions, etc. Smooth and uniform movement of charge transport ions is promoted.

As a method for bringing the electrode body 2 into a pressed state, a method of bringing the inside of the container 3 into a negative pressure state (closing the container 3 while maintaining the pressure inside the container 3 at a negative pressure) is preferable. In this way, by bringing the inside of the container 3 into a negative pressure state, the container 3 is deformed (concave in the direction in which the positive electrode and the negative electrode are stacked) and presses the electrode body 2 in the direction in which the positive electrode and the negative electrode are stacked. As a result, the distance between the positive electrode and the negative electrode in the electrode body 2 can be easily equalized.

In the state before the container 3 is sealed, the upper limit of the ratio of the minimum width in the stacking direction (minimum width in the Y direction) of the internal space of the container 3 to the thickness in the stacking direction (thickness in the Y direction) of the electrode body 2 is preferably 1.20, more preferably 1.10. When the ratio is less than or equal to the upper limit, the container 3 tends to press the electrode body 2 in the stacking direction due to deformation of the container 3. Note that the lower limit of the ratio is not particularly limited, but may be, for example, 1.05.

Preferably, the container 3 contains a gas soluble in the non-aqueous electrolyte. A gas soluble in the non-aqueous electrolyte is stored inside the container 3, and by dissolving this gas in the non-aqueous electrolyte, the inside of the container 3 can be easily brought into a negative pressure state. Here, a gas soluble in a non-aqueous electrolyte means a gas having a solubility of 1 cm ³ or more in 1 cm ³ of a non-aqueous solvent at 25° C. under 1 atmosphere. The gas is preferably carbon dioxide when a cyclic carbonate or a chain carbonate is used as the nonaqueous solvent of the nonaqueous electrolyte.

The volume of the gas soluble in the non-aqueous electrolyte accommodated inside the container 3 (at 1 atm, 25°C) is the volume of the non-aqueous electrolyte accommodated inside the container 3 and the volume of the gas soluble in the non-aqueous electrolyte. It can be determined by the solubility in non-aqueous electrolytes. For example, when a cyclic carbonate or a chain carbonate is used as the nonaqueous solvent of the nonaqueous electrolyte and carbon dioxide is used as the gas soluble in the nonaqueous electrolyte, the amount of carbon dioxide relative to the volume of the nonaqueous electrolyte accommodated inside the container 3 is The lower limit of the volume is preferably 10%, more preferably 20%, from the viewpoint of enhancing the effect of negative pressure. On the other hand, the upper limit of the volume of carbon dioxide gas relative to the volume of the non-aqueous electrolyte is preferably 400%, more preferably 200%, and 100% from the viewpoint of shortening the time until carbon dioxide gas is dissolved after sealing the container 3. % is more preferred.

From the viewpoint of reducing the pressure inside the container 3, the lower limit of the volume of the gas soluble in the non-aqueous electrolyte contained inside the container 3 (at 1 atmosphere, 25° C.) with respect to the volume of the surplus space inside the container 3 is 40°C. % is preferred, 70% is more preferred, and even more preferably 95%. On the other hand, the upper limit of the volume of the surplus space in the container 3 may be 100%. Here, "the volume of the surplus space inside the container 3" means the volume obtained by subtracting the volume of structures such as the electrode body 2 and the nonaqueous electrolyte from the internal volume of the container 3. Further, the volume of the electrode body 2 means the apparent volume of the constituent elements of the electrode body (active material layer, separator, etc.), and does not include voids existing between active material layers or within the separator.

<Configuration of power storage device>
The power storage element of this embodiment can be used as a power source for automobiles such as an electric vehicle (EV), a hybrid vehicle (HEV), or a plug-in hybrid vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power source for power storage. etc., it can be mounted as a power storage unit (battery module) configured by collecting a plurality of power storage elements. In this case, the technology of the present invention may be applied to at least one power storage element included in the power storage unit.
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 the embodiment of the present invention (hereinafter referred to as "second embodiment"). . It is sufficient that the technology according to one embodiment of the present invention is applied to at least one power storage element included in the power storage device according to the second embodiment, and the above-described power storage element according to one embodiment of the present invention may be applied. The battery may include one or more power storage elements that are not related to the embodiment of the present invention, or may include two or more power storage elements that are not related to the embodiment of the present invention. FIG. 3 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 according to the second embodiment includes 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, etc. may be provided. The power storage unit 20 or the power storage device 30 may include a state monitoring device (not shown) that monitors the state of one or more power storage elements.

<Method for manufacturing electricity storage element>
The method for manufacturing the electricity storage element of this embodiment can be appropriately selected from known methods. 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 a positive electrode and a negative electrode, and forming the electrode body by laminating or winding the positive electrode and the negative electrode with a separator in between. The method for manufacturing a power storage element of the present embodiment further includes pressing the electrode body in the stacking direction of the positive electrode and the negative electrode. Pressing the electrode body in the stacking direction of the positive electrode and the negative electrode preferably includes storing a gas soluble in the non-aqueous electrolyte in a container.

Storing the non-aqueous electrolyte in a container can be appropriately selected from known methods. For example, when a nonaqueous electrolyte is used as the nonaqueous electrolyte, the injection port may be sealed after the nonaqueous electrolyte is injected through an injection port formed in the container.

<Other embodiments>
Note that the power storage element of the present invention is not limited to the above embodiments, and various changes 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 known technique. Additionally, some of the configurations of certain embodiments may be deleted. Furthermore, well-known techniques can be added to the configuration of a certain embodiment.

In the above embodiment, a case has been described in which the electricity storage element is used as a chargeable/dischargeable non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery), but the type, shape, size, capacity, etc. of the electricity 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.

Although the above embodiment describes an electrode body in which a positive electrode and a negative electrode are stacked with a separator in between, the electrode body does not need to include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other with a non-conductive layer formed on the active material layer of the positive electrode or the negative electrode.

In the above embodiment, a case has been described in which the positive electrode active material contains the compound represented by the above formula 1 or lithium manganate, and the negative electrode active material contains graphite or lithium titanate, but the positive electrode active material and the negative electrode active material The configuration is not limited to the above embodiment. For example, if a positive electrode active material is selected other than the compound represented by formula 1 or lithium manganate, and a negative electrode active material is selected other than graphite or lithium titanate, and the open circuit voltage at SOC 30% and the open circuit voltage at SOC 70% are selected, A configuration may be adopted in which the difference from the circuit voltage is 0.2 V or less.

In the above embodiment, the method of bringing the inside of the container into a negative pressure state is explained as a method of putting the electrode body in a pressed state. method may be adopted. For example, the electrode body can be brought into a pressed state by restraining the electricity storage element in a state where it is pressed in the stacking direction (Y direction) of the positive electrode and the negative electrode using a conventionally known restraining member or the like. The restraining member is not particularly limited as long as it can press (pressurize) the electricity storage element. Examples of the mode of pressing the electrode body using a restraining member or the like include pressing the power storage element with the restraint member so that the pressing force applied to the power storage element becomes a constant value (constant pressure restraint). The area and shape of the region pressed by the restraining member or the like on the outer surface of the power storage element (pressing region) are not particularly limited, and may have the effect of suppressing a decrease in capacity retention during charge/discharge cycles and the characteristics of the power storage element. It can be set appropriately in consideration. The container is preferably thin and flexible. Further, it is preferable to arrange a known buffer member between the restraining member and the container, since the pressure applied to the electrode body can be maintained at a constant value from the beginning.

The pressure applied to the electrode body by the restraining member etc. is not particularly limited and can be set as appropriate. For example, the lower limit of the pressure applied to the electrode body is preferably 0.5 MPa, more preferably 0.7 MPa, even more preferably 0.9 MPa, and even more preferably 1.0 MPa. When the pressure is equal to or higher than the lower limit, the effect of suppressing a decrease in capacity retention during charge/discharge cycles is more significantly exhibited. On the other hand, the upper limit of the pressure is preferably 3.7 MPa, more preferably 3.5 MPa, even more preferably 3.3 MPa, and even more preferably 3.0 MPa. When the pressure is below the upper limit, it is possible to prevent the pores of the separator from being clogged due to excessive pressure being applied. The pressure applied to the electrode body is the pressure in the discharge state. The method of measuring the pressure applied to the electrode body is to separate the energy storage element from the restraining member etc. in the discharge state, and press the energy storage element using an autograph until it has the same thickness as when it was being pressed by the restraining member etc. The pressure applied to the electrode body may be determined by dividing the load at that time by the area of the contact surface between the autograph and the electrode body. Note that, although a load is normally applied by the container to a pair of opposing surfaces of the electricity storage element, the area of only one of the pair of surfaces is defined as the area of the surface to which the load is applied.

Hereinafter, the present invention will be explained in more detail with reference to Examples. The invention is not limited to the following examples.

[Example 1]
(Preparation of positive electrode)
Lithium iron phosphate (LiFePO ₄ ) was used as the positive electrode active material. Using N-methylpyrrolidone (NMP) as a dispersion medium, the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder are mixed in a mass ratio of 90:5:5 in terms of solid content. A positive electrode mixture paste was prepared containing: The positive electrode mixture paste was applied to both sides of an aluminum foil serving as a positive electrode base material, dried, and then pressed. Thereby, a positive electrode was obtained in which positive electrode active material layers were laminated on both sides of the positive electrode base material. At this time, the mass per unit area of the positive electrode active material layer laminated on one side of the positive electrode base material was 15 mg/cm ² in terms of solid content. Further, the differential pore volume of the positive electrode active material layer in the obtained positive electrode at a pore diameter of 50 nm was 0.0039 cm ³ /(g·nm). Note that the differential pore volume of the positive electrode active material layer at a pore diameter of 50 nm is a value determined by the method described above. In addition, in this example, the above differential pore volume is a value measured at the time of producing the positive electrode, but the above differential pore volume should be considered to be the same value even after charging and discharging a storage element produced based on this positive electrode. I can do it. The differential pore volume distribution curve of the positive electrode active material layer of Example 1 is shown in FIG.

(Preparation 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. Note that the mass ratio of Gr, SBR, and CMC was 96:2:2 (in terms of solid content). A negative electrode mixture paste was applied to both sides of a copper foil serving as a negative electrode base material and dried. Thereafter, roll pressing was performed to obtain a negative electrode. At this time, the mass per unit area of the negative electrode active material layer laminated on one side of the negative electrode base material was 8 mg/cm ² in terms of solid matter.

(Preparation of non-aqueous electrolyte)
A solution was prepared by dissolving LiPF ₆ as an electrolyte salt at a concentration of 1.0 mol/dm ³ in a non-aqueous solvent in which EC and EMC were mixed at a volume ratio of 30:70. The above solution was obtained as a non-aqueous electrolyte.

(Preparation of electricity storage element)
A microporous polyolefin membrane was used as a separator. An electrode body was produced by laminating the above positive electrode and the above negative electrode with the separator interposed therebetween. This electrode body was housed in a container made of a composite film (total thickness: approximately 150 μm) in which an aluminum layer and a resin film layer were laminated, and the nonaqueous electrolyte was poured into the container. 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 was 1.10 or less. After that, carbonic acid was added so that the volume was approximately 50% of the volume of the injected non-aqueous electrolyte and approximately 80% of the volume of the surplus space in the container, calculated under 1 atm and 25°C. Gas was filled in and sealed by thermal welding to obtain the electricity storage element of Example 1. Note that the inside of the container after the electricity storage element of Example 1 was sealed was in a negative pressure state due to the carbon dioxide gas sealed in the container being dissolved in the non-aqueous electrolyte. Further, since the container was thin and made of a flexible composite film, the container was deformed due to the negative pressure inside the container. Furthermore, since the minimum width of the internal space of the container in the stacking direction is sufficiently small relative to the thickness of the electrode body, deformation of the container causes the electrode body to be pressed in the stacking direction of the positive electrode and the negative electrode. became.

[Examples 2 and 3 and Comparative Examples 1 to 9]
Each of the energy storage elements of Examples 2 and 3 and Comparative Examples 1 to 3 was prepared in the same manner as in Example 1 except that the differential pore volume at a pore diameter of 50 nm in the positive electrode active material layer was changed as shown in Table 1. Obtained.
Comparative Example 4 was carried out in the same manner as in Example 1, except that carbon dioxide gas was not sealed in the container and the differential pore volume at a pore diameter of 50 nm in the positive electrode active material layer was changed as shown in Table 1. Nine electricity storage elements were obtained from the following. However, in Comparative Example 3, the differential pore volume at all pore diameters was evaluated, and in Examples 2, 3, and Comparative Examples 1, 2, and 4 to 9, only the differential pore volume at a pore diameter of 50 nm was evaluated. The differential pore volume distribution curve of the positive electrode active material layer of Comparative Example 3 is shown in FIG. Moreover, since carbon dioxide gas was not sealed in the container, the pressure inside the container of the electricity storage elements of Comparative Examples 4 to 9 was equal to atmospheric pressure (1 atm).

(Initial charge/discharge)
Initial charging and discharging of each of the obtained power storage elements was performed at 25° C. in the following manner. Constant current and constant voltage charging was performed with a charging current of 0.1 C and a charging end voltage of 3.6 V. The charging termination condition was until the charging current reached 0.02C. Thereafter, a rest period of 10 minutes was provided. Thereafter, constant current discharge was performed with a discharge current of 0.1C and a discharge end voltage of 2.0V.

(Initial capacity confirmation test)
Next, an initial capacity confirmation test was conducted on each power storage element at 25° C. in the following manner.
Constant current and constant voltage charging was performed with a charging current of 0.1 C and a charging end voltage of 3.6 V. The charging termination condition was until the charging current reached 0.02C. Thereafter, a rest period of 10 minutes was provided. Thereafter, constant current discharge was performed with a discharge current of 0.1C and a discharge end voltage of 2.0V. The discharge capacity at this time was defined as the "initial discharge capacity." Furthermore, constant current and constant voltage charging was performed for each power storage element at a charging current of 0.1 C and a charging end voltage of 3.6 V. The charging termination condition was until the charging current reached 0.02C. Thereafter, a rest period of 10 minutes was provided. Thereafter, constant current discharge was performed with a discharge current of 0.1 C until SOC 70%, and the voltage after 30 minutes with no current applied was measured and defined as "open circuit voltage at SOC 70%". Thereafter, constant current discharge was performed at a discharge current of 0.1 C to SOC 30%, and the voltage after 30 minutes with no current applied was measured and defined as "open circuit voltage at SOC 30%". In each power storage element, the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC was 0.2 V or less. In addition, here, the state after performing the initial capacity confirmation test is expressed as a fully discharged state and SOC 0%, and the state in which the same amount of electricity as the initial discharge capacity is charged at a constant current and constant voltage with a charge end voltage of 3.6V is referred to as a fully discharged state. A fully charged state is expressed as SOC 100%.

(Charge/discharge cycle test)
After the above initial capacity confirmation test, a charge/discharge cycle test was conducted on each power storage element at 25° C. in the following manner. Constant current and constant voltage charging was performed with a charging current of 1.0 C and a charging end voltage of 3.6 V. The charging termination condition was until the charging current reached 0.05C. Thereafter, constant current discharge was performed with a discharge current of 1.0 C and a discharge end voltage of 2.0 V. A rest period of 10 minutes was provided after charging and discharging, respectively. This charging and discharging was performed for 50 cycles.
After the charge/discharge cycle test, a capacity confirmation test after the charge/discharge cycle test was conducted in the same manner as the "initial capacity confirmation test" described above. The discharge capacity at this time was defined as "discharge capacity after charge/discharge cycle test." The discharge capacity after the charge/discharge cycle test was divided by the initial discharge capacity to determine the capacity retention rate (%). The results are shown in Table 1.

As shown in Table 1 above, comparing Comparative Example 1 and Comparative Example 7 in which the differential pore volume at a pore diameter of 50 nm in the positive electrode active material layer is the same, Comparative Example 1 in which the pressure inside the container is in a negative pressure state is The capacity retention rate is higher than that of Comparative Example 7 in which the internal pressure is equal to atmospheric pressure. Similarly, comparing Comparative Example 2 and Comparative Example 8, and Comparative Example 3 and Comparative Example 9, Comparative Example 2 has a negative pressure inside the container, and Comparative Example 3 has a negative pressure inside the container. The capacity retention rate is higher than that of Example 8 and Comparative Example 9.
In addition, when comparing Comparative Examples 4 to 9 where the pressure inside the container is equal to atmospheric pressure, Comparative Examples 4 to 9 where the differential pore volume at a pore diameter of 50 nm in the positive electrode active material layer is 0.0030 cm ³ / (g · nm) or more. Sample No. 6 has a higher capacity retention rate than Comparative Examples 7 to 9 in which the differential pore volume of the positive electrode active material layer at a pore diameter of 50 nm is less than 0.0030 cm ³ /(g·nm). On the other hand, Examples 1 to 3, in which the positive electrode active material layer has a differential pore volume of 0.0030 cm ³ /(g·nm) or more at a pore diameter of 50 nm, and the pressure inside the container is in a negative pressure state, are different from Comparative Example 1. The capacity retention rate is greatly improved compared to 9. From these facts, for example, the electrode body is pressed due to the negative pressure inside the container, and the positive electrode active material layer has appropriate pores, which act synergistically to cause the inside of the electrode body to As the smooth and uniform movement of charge transport ions such as lithium ions was significantly promoted, the charge/discharge reaction within the electrode body was less likely to be biased, and as a result, the decline in capacity retention during charge/discharge cycles was suppressed. It is assumed that

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.

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

Claims

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 stacked;
and a sealable container for accommodating the electrode body,
The electrode body is in a state where it is pressed in the stacking direction of the positive electrode and the negative electrode,
The positive electrode active material layer has a differential pore volume of 0.0030 cm 3 /(g·nm) or more at a pore diameter of 50 nm,
A power storage element in which the difference between the open circuit voltage at 30% SOC and the open circuit voltage at 70% SOC is 0.2V or less.
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 stacked;
and a sealable container for accommodating the electrode body,
The electrode body is in a state where it is pressed in the stacking direction of the positive electrode and the negative electrode,
The positive electrode active material layer has a differential pore volume of 0.0030 cm 3 /(g·nm) or more at a pore diameter of 50 nm,
The positive electrode active material contains a compound represented by the following formula 1 or lithium manganate,
A power storage element in which the negative electrode active material includes graphite or lithium titanate.
LiFe x Mn (1-x) PO 4 (0≦x≦1) ...1
The electricity storage element according to claim 1 or 2, wherein the interior of the container is under negative pressure.
A power storage device comprising two or more power storage elements and one or more power storage elements according to claim 1 or claim 2.