Power storage device and method for manufacturing power storage device

Abstract

An energy storage device (10) comprises: a positive electrode (21) having a positive electrode active material layer (21b); a negative electrode (22) having a negative electrode active material layer (22b); a separator (23) disposed between the positive electrode (21) and the negative electrode (22); and a liquid electrolyte disposed between the positive electrode (21) and the negative electrode (22), wherein the negative electrode active material layer (22b) has a unit area weight of 200 g / m². 2 The above describes the structure. The negative electrode active material layer (22b) comprises graphite particles, carbon fibers, and a negative electrode binder. The liquid electrolyte comprises lithium difluorophosphate and vinylene carbonate.

Description

Technical Field

[0001] This disclosure relates to energy storage devices and methods for manufacturing energy storage devices. Background Technology

[0002] Patent Document 1 discloses a flat energy storage device constructed by stacking multiple separately manufactured energy storage units in series. The energy storage unit comprises: a positive electrode, which is formed by forming a positive active material layer on the central portion of one side of a foil-shaped positive current collector; a negative electrode, which is formed by forming a negative active material layer on the central portion of one side of a foil-shaped negative current collector, arranged such that the negative active material layer faces the positive active material layer of the positive electrode; and a separator disposed between the positive and negative electrodes.

[0003] Furthermore, the aforementioned energy storage unit includes a sealing portion disposed between the positive and negative electrodes and located on the outer periphery of the positive and negative active material layers. The sealing portion maintains the distance between the positive and negative current collectors to prevent short circuits between the current collectors, and liquid-tightly seals the space between the positive and negative current collectors, forming a sealed space between the positive and negative current collectors to contain the liquid electrolyte. Existing technical documents Patent documents

[0004] Patent Document 1: Japanese Patent Application Publication No. 2017-16825 Summary of the Invention The problem the invention aims to solve

[0005] One method to increase the energy density of an energy storage unit is to increase the weight per unit area of ​​the active material layer. However, if the weight per unit area of ​​the negative electrode active material layer is increased to a certain extent, it will lead to a significant increase in the amount of gas generated from the electrode. Solution for solving the problem

[0006] In one aspect of this disclosure, an energy storage device is provided, comprising: a positive electrode having a positive active material layer; a negative electrode having a negative active material layer; a separator disposed between the positive and negative electrodes; and a liquid electrolyte disposed between the positive and negative electrodes. The negative active material layer has a unit area weight of 200 g / m². 2 The above describes the structure. The negative electrode active material layer comprises graphite particles, carbon fibers, and a negative electrode binder. The liquid electrolyte comprises lithium difluorophosphate and vinylene carbonate.

[0007] In the above-mentioned energy storage device, preferably, the carbon fiber comprises a fiber bundle formed by multiple single-layer carbon nanotubes being bundled together, the fiber bundle being connected across multiple graphite particles.

[0008] In the above-mentioned energy storage device, preferably, the average particle size (D50) of the graphite particles is 3 μm or more and 30 μm or less, and the fiber length of the monolayer carbon nanotube is 5 μm or more and 50 μm or less.

[0009] In the above-mentioned energy storage device, preferably, the liquid electrolyte comprises a cyclic sulfonate ester. In the above-mentioned energy storage device, preferably, the positive electrode active material layer comprises a positive electrode active material and a positive electrode binder, wherein the positive electrode binder is an aqueous binder.

[0010] In the above-mentioned energy storage device, preferably, a sealing part is provided between the positive electrode and the negative electrode to form a sealed space for containing the liquid electrolyte. In the aforementioned energy storage device, preferably, the graphite particle content in the negative electrode active material layer is 97% by mass or more, the average particle size (D50) of the graphite particles is 10 μm or more and 20 μm or less, and the specific surface area of ​​the negative electrode active material layer is 0.5 m². 2 / g or more and 1.5m 2 The content of lithium difluorophosphate per unit surface area of ​​the negative electrode active material layer in the liquid electrolyte is less than 3 mg / m³. 2 The content of vinylene carbonate per unit surface area of ​​the negative electrode active material layer in the liquid electrolyte is 3 mg / m³. 2 above.

[0011] In the aforementioned energy storage device, preferably, the liquid electrolyte comprises a cyclic sulfonate ester, the graphite particles in the negative electrode active material layer have a content of 97% by mass or more, the average particle size (D50) of the graphite particles is 10 μm or more and 20 μm or less, and the specific surface area of ​​the negative electrode active material layer is 0.5 m². 2 / g or more and 1.5m 2 The content of the cyclic sulfonate per unit surface area of ​​the negative electrode active material layer in the liquid electrolyte is less than / g, and is 0.6mg / m². 2 above.

[0012] In one aspect of this disclosure, a method for manufacturing the aforementioned energy storage device is provided. The method includes: an assembly step of assembling a battery assembly comprising the positive electrode, the negative electrode, the separator, and the liquid electrolyte; and an initial charging step of the battery assembly, the initial charging step comprising: a first charging step of charging to a first voltage of 2.0V or higher and 3.0V or lower; and a second charging step of charging from the first voltage to a predetermined target voltage, wherein the charging rate of the second charging step is higher than the charging rate of the first charging step.

[0013] In the above-mentioned method for manufacturing an energy storage device, preferably, the assembly process includes an electrode forming process that forms the negative electrode active material layer by coating the surface of the negative electrode current collector with a negative electrode composite material and drying the negative electrode composite material, wherein the negative electrode composite material comprises graphite particles, carbon fibers, a negative electrode binder and a dispersion medium, and the solid component concentration is less than 65% by mass. Invention Effects

[0014] According to the present invention, the amount of gas generated from the electrodes can be reduced. Attached Figure Description

[0015] Figure 1 This is a cross-sectional view of the energy storage device. Figure 2 This is a schematic diagram showing the state of graphite particles and carbon fibers in the negative electrode active material layer. Figure 3 This is an explanatory diagram of the activation process. Figure 4 (a) to (d) are schematic diagrams illustrating the formation process of the SEI film relative to graphite particles. Figure 5 This is a coordinate graph showing the results of a preservation test used to determine the amount of gas produced. Detailed Implementation

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Figure 1 The energy storage device 10 shown is, for example, an energy storage module used in the batteries of various vehicles such as forklifts, hybrid vehicles, and electric vehicles. The energy storage device 10 is, for example, a lithium-ion secondary battery. The energy storage device 10 may also be a double-layer capacitor. In this embodiment, the case where the energy storage device 10 is a lithium-ion secondary battery is illustrated.

[0017] <Brief Structure of Energy Storage Device> like Figure 1 As shown, the energy storage device 10 is configured as a unit stack 30 (stack) consisting of multiple energy storage cells 20 stacked in a stacking direction. Hereinafter, the stacking direction of the multiple energy storage cells 20 will be simply referred to as the stacking direction. Each energy storage cell 20 includes a positive electrode 21, a negative electrode 22, a separator 23, and a sealing portion 24.

[0018] The positive electrode 21 has a positive current collector 21a and a positive active material layer 21b disposed on the first surface 21a1 of the positive current collector 21a. In a top view (hereinafter referred to as top view) viewed from the stacking direction, the positive electrode active material layer 21b is formed at the center of the first surface 21a1 of the positive electrode current collector 21a. The peripheral portion of the first surface 21a1 of the positive electrode current collector 21a in the top view becomes the uncoated positive electrode portion 21c, where the positive electrode active material layer 21b is not provided. The uncoated positive electrode portion 21c is arranged to surround the positive electrode active material layer 21b in the top view.

[0019] The negative electrode 22 includes a negative electrode current collector 22a and a negative electrode active material layer 22b disposed on a first surface 22a1 of the negative electrode current collector 22a. In plan view, the negative electrode active material layer 22b is formed at the center of the first surface 22a1 of the negative electrode current collector 22a. In plan view, the peripheral portion of the first surface 22a1 of the negative electrode current collector 22a becomes the uncoated negative electrode portion 22c, where the negative electrode active material layer 22b is not disposed. The uncoated negative electrode portion 22c is arranged to surround the negative electrode active material layer 22b in plan view.

[0020] The positive electrode 21 and the negative electrode 22 are arranged such that the positive electrode active material layer 21b and the negative electrode active material layer 22b are opposite to each other in the stacking direction. That is, the opposite direction of the positive electrode 21 and the negative electrode 22 is consistent with the stacking direction. The negative electrode active material layer 22b is formed to be one size larger than the positive electrode active material layer 21b, so that when viewed from above in the stacking direction, the entire formation region of the positive electrode active material layer 21b is located within the formation region of the negative electrode active material layer 22b.

[0021] Here, the positive current collector 21a has a second surface 21a2 located on the opposite side of the first surface 21a1, and the negative current collector 22a has a second surface 22a2 located on the opposite side of the first surface 22a1. The unit stack 30 has a structure in which multiple energy storage units 20 are stacked such that the second surface 21a2 of the positive current collector 21a is in contact with the second surface 22a2 of the negative current collector 22a. Thus, the multiple energy storage units 20 constituting the unit stack 30 are connected in series.

[0022] In the unit stack 30, a suspected bipolar electrode 25 is formed, which treats the positive current collector 21a and negative current collector 22a of two adjacent energy storage units 20 in the stacking direction as a single current collector. The suspected bipolar electrode 25 includes: a current collector, which is a structure formed by stacking the positive current collector 21a and the negative current collector 22a; a positive active material layer 21b, which is formed on one side of the current collector; and a negative active material layer 22b, which is formed on the other side of the current collector.

[0023] Alternatively, the positive current collector 21a and the negative current collector 22a can also form a bipolar current collector by joining the second surface 21a2 of the positive current collector 21a and the second surface 22a2 of the negative current collector 22a. In this case, the positive electrode 21 and the negative electrode 22 form a bipolar electrode 25 having a bipolar current collector formed by joining the positive current collector 21a and the negative current collector 22a. In other words, the bipolar electrode 25 is formed by joining the surface of the positive current collector 21a of the positive electrode 21 of one of the adjacent energy storage units 20, located opposite to the first surface 21a1, with the surface of the negative current collector 22a of the negative electrode 22 of the other energy storage unit 20, located opposite to the first surface 22a1.

[0024] The separator 23 is a component disposed between the positive electrode 21 and the negative electrode 22, which prevents short circuits caused by contact between the two electrodes by isolating the positive electrode 21 and the negative electrode 22 and allows charge carriers such as lithium ions to pass through.

[0025] The separator 23 is, for example, a porous sheet or nonwoven fabric containing a polymer that absorbs and retains the liquid electrolyte. Examples of materials constituting the separator 23 include polypropylene, polyethylene, polyolefins, and polyester. The separator 23 can have a single-layer or multi-layer structure. A multi-layer structure may include, for example, an adhesive layer and a ceramic layer as a heat-resistant layer.

[0026] The sealing portion 24 is disposed between the first surface 22a1 of the positive current collector 21a of the positive electrode 21 and the first surface 22a1 of the negative current collector 22a of the negative electrode 22, and at a position further outward than the positive active material layer 21b and the negative active material layer 22b, and is adhered to both the positive current collector 21a and the negative current collector 22a. The sealing portion 24 prevents short circuits between the current collectors by insulating the positive current collector 21a and the negative current collector 22a.

[0027] The sealing portion 24 is, for example, made of a polyolefin resin. Examples of polyolefin resins include polyethylene, polypropylene, modified polyethylene, modified polypropylene, isoprene, modified isoprene, polybutene, modified polybutene, and polybutadiene. Examples of modified polyethylene include acid-modified polyethylene and epoxy-modified polyethylene. Examples of modified polypropylene include acid-modified polypropylene and epoxy-modified polypropylene. Furthermore, two or more of these known polyolefin resins may be used in combination. Additionally, the polyolefin resin may be a thermoplastic resin or a thermosetting resin.

[0028] When viewed from above, the sealing portion 24 is formed as a frame extending along the periphery of the positive electrode current collector 21a and the negative electrode current collector 22a, and surrounding the periphery of the positive electrode active material layer 21b and the negative electrode active material layer 22b. The sealing portion 24 is disposed between the positive electrode uncoated portion 21c of the first surface 21a1 of the positive electrode current collector 21a and the negative electrode uncoated portion 22c of the first surface 22a1 of the negative electrode current collector 22a.

[0029] Inside the energy storage unit 20, a sealed space S is formed, surrounded by a frame-shaped sealing portion 24, a positive electrode 21, and a negative electrode 22. The sealed space S houses a separator 23 and a liquid electrolyte. Furthermore, the periphery of the separator 23 is embedded within the sealing portion 24.

[0030] The sealing portion 24, by sealing the enclosed space S between the positive electrode 21 and the negative electrode 22, can suppress the permeation of the liquid electrolyte contained in the enclosed space S to the outside. In addition, the sealing portion 24 can prevent moisture from entering the enclosed space S from the outside of the energy storage device 10. Moreover, the sealing portion 24 can, for example, prevent the leakage of gas generated from the positive electrode 21 or the negative electrode 22 due to charging and discharging reactions to the outside of the energy storage device 10.

[0031] Each energy storage cell 20 has a sealing portion 24 that extends outward from the edges of the positive current collector 21a and the negative current collector 22a. This outer peripheral portion, viewed from the stacking direction, protrudes in a direction orthogonal to the stacking direction from the edges of the positive current collector 21a and the negative current collector 22a. Adjacent energy storage cells 20 in the stacking direction are integrated by bonding the outer peripheral portions of their respective sealing portions 24 together. Known welding methods such as thermal welding, ultrasonic welding, or infrared welding can be used as methods for bonding adjacent sealing portions 24 together.

[0032] The energy storage device 10 includes a pair of current-carrying bodies, consisting of a positive electrode current-carrying plate 40 and a negative electrode current-carrying plate 50, arranged in the stacking direction of the unit stack 30 in such a way that they sandwich the unit stack 30. The positive electrode current-carrying plate 40 and the negative electrode current-carrying plate 50 are each made of a material with excellent conductivity.

[0033] The positive electrode energizing plate 40 is electrically connected to the second surface 21a2 of the positive current collector 21a, which is located on the outermost side of the positive electrode 21 in the stacking direction. The negative electrode energizing plate 50 is electrically connected to the second surface 22a2 of the negative current collector 22a, which is located on the outermost side of the negative electrode 22 in the stacking direction.

[0034] The energy storage device 10 is charged and discharged through terminals provided on the positive electrode current-carrying plate 40 and the negative electrode current-carrying plate 50, respectively. For example, the same material as the material constituting the positive electrode current-carrying plate 40 can be used to construct the positive electrode current-carrying member 21a. The positive electrode current-carrying plate 40 may also be constructed of a metal plate thicker than the positive electrode current-carrying member 21a used in the unit stack 30. For example, the same material as the material constituting the negative electrode current-carrying member 22a can be used to construct the negative electrode current-carrying plate 50. The negative electrode current-carrying plate 50 may also be constructed of a metal plate thicker than the negative electrode current-carrying member 22a used in the unit stack 30.

[0035] <Detailed information about the negative electrode> Next, we will explain the details of negative electrode 22. The negative electrode current collector 22a is a chemically inert conductor used to continuously allow current to flow through the negative electrode active material layer 22b during the discharge or charging of a lithium-ion secondary battery. An example of the negative electrode current collector 22a is a copper current collector whose first surface 22a1 is made of copper. The copper current collector can be a single element entirely composed of copper, or a composite having portions made of copper and portions made of materials other than copper. Examples of such single elements include copper foil, such as electrolytic copper foil. Examples of such composites include multilayer structures where the first surface 22a1 is a copper layer, and substrates containing the first surface 22a1 covered by a copper film.

[0036] Materials other than copper can include, for example, metallic materials, conductive resin materials, and conductive inorganic materials. Examples of metallic materials include, for example, aluminum, nickel, titanium, and stainless steel (e.g., SUS304, SUS316, SUS301, SUS304, etc. as specified in JIS G 4305:2015). Examples of conductive resin materials include, for example, conductive polymer materials or resins formed by adding conductive fillers to non-conductive polymer materials as needed. The negative electrode current collector 22a can be, for example, a foil, a sheet, or a film. The thickness of the negative electrode current collector 22a is, for example, 1 to 100 μm.

[0037] The negative electrode active material layer 22b contains graphite particles, carbon fibers, and a negative electrode binder. (Graphite particles) Graphite particles, acting as charge carriers such as lithium ions, are contained in the negative electrode active material layer 22b. Examples of graphite constituting the graphite particles include natural graphite, artificial graphite, hard carbon (difficult-to-graphitize carbon), and soft carbon (easily-graphitize carbon). Examples of artificial graphite include highly oriented graphite and mesophase carbon microspheres.

[0038] The average particle size (D50) of the graphite particles is, for example, 3 μm or more, preferably 5 μm or more, and more preferably 10 μm or more. The average particle size (D50) of the graphite particles is, for example, 30 μm or less, preferably 25 μm or less, and more preferably 20 μm or less. The average particle size (D50) of the graphite particles can be determined, for example, using a laser diffraction particle size analyzer.

[0039] The content of graphite particles in the negative electrode active material layer 22b is, for example, 93% by mass or more, preferably 95% by mass or more, and more preferably 97% by mass or more. The content of graphite particles in the negative electrode active material layer 22b is, for example, 99% by mass or less, preferably 98% by mass or less, and more preferably 97.5% by mass or less.

[0040] like Figure 4 As shown in (c) and (d), graphite particles P can also have an SEI (Solid Electrolyte Interface) film produced by the decomposition of liquid electrolytes. Figure 4 The example graphite particle P shown in (c) and (d) has an SEI film M. The SEI film M comprises a first layer M1 and a second layer M2. The first layer M1 of the SEI film M is formed on the surface of the graphite particle P, covering a portion or all of the surface of the graphite particle P. The first layer M1 is composed of decomposition products of lithium difluorophosphate contained in the liquid electrolyte. The second layer M2 is formed on the surface of the first layer M1, covering a portion or all of the surface of the first layer M1. The second layer M2 is composed of decomposition products of vinylene carbonate contained in the liquid electrolyte. The SEI film M functions as a protective film that inhibits contact between the graphite particle P and the liquid electrolyte. Furthermore, the detailed formation mechanism of the SEI film M will be described later.

[0041] (Carbon fiber) Carbon fibers are included in the negative electrode active material layer 22b as a conductive additive to improve electrical conductivity. Carbon fibers can be, for example, carbon nanotubes. Carbon nanotubes can be broadly classified into two types: single-layer carbon nanotubes and multilayer carbon nanotubes. A single-layer carbon nanotube is a cylindrical shape formed by seamlessly winding a single graphene sheet. A multilayer carbon nanotube is a composite formed by multiple single-layer carbon nanotubes of different diameters contained within a single single-layer carbon nanotube. Preferably, the carbon fibers included in the negative electrode active material layer 22b are single-layer carbon nanotubes.

[0042] There are no particular limitations on the fiber length and fiber diameter of carbon fibers. For example, the fiber length of carbon fibers is 5 μm or more and 1000 μm or less. The fiber diameter of carbon fibers is, for example, 1 nm or more and 20 nm or less. The fiber length and fiber diameter of carbon fibers can be measured, for example, using electron microscopy such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

[0043] The carbon fiber content in the negative electrode active material layer 22b is, for example, 0.005% by mass or more and 0.05% by mass or less. Reference Figure 2 This is an example illustrating the preferred state of graphite particles and carbon fibers in the negative electrode active material layer 22b. Figure 2 The diagrams of the negative electrode binder and other components described later are omitted.

[0044] like Figure 2 As shown, the negative electrode active material layer 22b contains graphite particles P and fiber bundles F. Fiber bundle F is a bundle of multiple monolayer carbon nanotubes bound together. The number of monolayer carbon nanotubes constituting one fiber bundle F is, for example, a few to several dozen. Alternatively, the negative electrode active material layer 22b may also contain individual monolayer carbon nanotubes that do not form fiber bundles F.

[0045] exist Figure 2 In the negative electrode active material layer 22b shown, monolayer carbon nanotubes, which are carbon fibers, are dispersed in units of fiber bundles F and are configured as a mesh (e.g., spider web). The fiber bundles F are connected across multiple graphite particles P. In this specification, the state in which fiber bundles F are connected across multiple graphite particles (hereinafter sometimes referred to as a specific dispersion state) refers to the state in which more than 50% of the fiber bundles F, based on the number of bundles, are connected across multiple graphite particles.

[0046] Furthermore, regarding a fiber bundle F, if at least one fiber constituting fiber bundle F is connected to graphite particle A, then fiber bundle F is considered to be connected to graphite particle A. Additionally, regarding a fiber bundle F, if at least one fiber constituting fiber bundle F is connected to graphite particle A and at least one fiber constituting fiber bundle F is connected to graphite particle B, then fiber bundle F is considered to be connected across graphite particles A and B. In this case, the fiber connected to graphite particle A and the fiber connected to graphite particle B can be the same or different. Specific dispersion states can be confirmed, for example, using electron microscopy such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

[0047] From the viewpoint of forming a specific dispersion state, the fiber length of the monolayer carbon nanotubes is preferably, for example, 5 μm or more and 50 μm or less. More preferably, the fiber length is 10 μm or more, and even more preferably 15 μm or more. More preferably, the fiber length is 40 μm or less, and even more preferably 30 μm or less. The ratio of the fiber length to the average particle size of the graphite particles P (fiber length of the monolayer carbon nanotube / average particle size of the graphite particles) is preferably greater than 1, for example. This ratio is, for example, 20 or less.

[0048] From the viewpoint of forming a specific dispersion state, the content of monolayer carbon nanotubes in the negative electrode active material layer 22b is preferably 0.005% by mass or more and 0.05% by mass or less. This content is more preferably 0.007% by mass or more, and even more preferably 0.009% by mass or more. This content is more preferably 0.03% by mass or less, and even more preferably 0.02% by mass or less. The ratio of this content to the content of graphite particles P (content of monolayer carbon nanotubes / content of graphite particles) is preferably 0.0001 or more and 0.0005 or less.

[0049] (Negative electrode binder) Examples of negative electrode binders include fluorinated resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber; thermoplastic resins such as polypropylene and polyethylene; imide resins such as polyimide and polyamide-imide; resins containing alkoxysilyl groups; acrylic resins such as poly(meth)acrylic acid; styrene-butadiene rubber; carboxymethyl cellulose; alginates such as sodium alginate and ammonium alginate; water-soluble cellulose ester crosslinks; and starch-acrylic acid graft polymers. Negative electrode binders can be used alone or in combination. Examples of solvents or dispersion media used to dissolve or disperse negative electrode binders include water and N-methyl-2-pyrrolidone.

[0050] (Other ingredients) The negative electrode active material layer 22b may also include other components besides graphite particles, carbon fibers, and negative electrode binders, as needed. Examples of such other components include negative electrode active materials other than graphite particles, conductive additives other than carbon fibers, electrolytes (polymer matrix, ion-conducting polymer, liquid electrolyte, etc.), and electrolyte support salts (lithium salts) for improving ion conductivity.

[0051] Other anode active materials besides graphite particles include elements that can alloy with lithium, such as silicon and tin. Furthermore, any element, alloy, or compound capable of absorbing and releasing charge carriers such as lithium ions can be used as an anode active material without particular limitations.

[0052] Examples of conductive additives other than carbon fiber include acetylene black, carbon black, and graphite. The types and proportions of other components contained in the negative electrode active material layer 22b are not particularly limited, and can be appropriately referenced to previously known views on lithium-ion secondary batteries.

[0053] (Weight and thickness per unit area of ​​negative electrode active material layer 22b) The weight per unit area of ​​the negative electrode active material layer 22b is 200 g / m². 2 The preferred value is 250g / m³. 2 The above, more preferably 300g / m2 The above. The weight per unit area of ​​the negative electrode active material layer 22b is, for example, 500 g / m². 2 The preferred value is 450g / m³. 2 The following is more preferably 400g / m 2 the following.

[0054] The thickness of the negative electrode active material layer 22b is, for example, 0.2 mm or more, preferably 0.25 mm or more, and more preferably 0.3 mm or more. The thickness of the negative electrode active material layer 22b is, for example, 0.4 mm or less.

[0055] (Surface area of ​​negative electrode active material layer 22b) The negative electrode active material layer 22b has a porous shape that allows it to be wetted by a liquid electrolyte. The surface area of ​​the negative electrode active material layer 22b is, for example, 250 mm². 2 Above and 1350mm 2 the following.

[0056] The surface area of ​​the negative electrode active material layer 22b can be calculated as the product of the specific surface area of ​​the negative electrode active material layer 22b and the total mass of the negative electrode active material layer 22b. In this specification, the specific surface area of ​​the negative electrode active material layer 22b refers to the BET surface area per unit mass of the negative electrode active material layer 22b, as measured by N2 adsorption using the BET method. The total mass of the negative electrode active material layer 22b can be calculated as the product of the weight per unit area of ​​the negative electrode active material layer 22b and the area of ​​the region in the negative electrode current collector 22a in which the negative electrode active material layer 22b is formed.

[0057] The specific surface area of ​​the negative electrode active material layer 22b is, for example, 0.5 m². 2 / g or more and 1.5m 2 / g or less. The total mass of one negative electrode active material layer 22b, that is, the total mass of the negative electrode active material layer 22b contained in one energy storage unit 20, is, for example, 20g or more and 1500g or less, preferably 500g or more and 900g or less. The area of ​​the region in the negative electrode current collector 22a in which the negative electrode active material layer 22b is formed is, for example, 1000cm². 2 Above and 30000cm 2 The preferred value is 10000cm. 2 Above and 30000cm 2 the following.

[0058] The method for forming the negative electrode active material layer 22b on the surface of the negative electrode current collector 22a is not particularly limited, and conventionally known methods such as roller coating can be used. Alternatively, a coating film can be provided on the surface of the negative electrode current collector 22a. For example, a heat-resistant layer provided to improve the thermal stability of the negative electrode 22 can be cited as an example of such a coating film.

[0059] <Detailed information about the positive electrode> Next, we will explain the details of positive electrode 21. The positive electrode current collector 21a is a chemically inert conductor used to continuously allow current to flow through the positive electrode active material layer 21b during the discharge or charging of a lithium-ion secondary battery. An example of the positive electrode current collector 21a is an aluminum current collector whose first surface 21a1 is made of aluminum. The aluminum current collector can be a single material entirely composed of aluminum, or a composite having portions made of aluminum and portions made of materials other than aluminum. Examples of such a single material include rolled aluminum foil. Examples of such composites include a multilayer structure whose first surface 21a1 is an aluminum layer, and a substrate containing the first surface 21a1 covered by an aluminum film.

[0060] Materials other than aluminum can include, for example, metallic materials, conductive resin materials, and conductive inorganic materials. Examples of metallic materials include, for example, copper, nickel, titanium, and stainless steel (e.g., SUS304, SUS316, SUS301, SUS304, etc. as specified in JIS G 4305:2015). Examples of conductive resin materials include, for example, conductive polymer materials or resins formed by adding conductive fillers to non-conductive polymer materials as needed. The positive current collector 21a can be, for example, a foil, a sheet, or a film. The thickness of the positive current collector 21a is, for example, 1 to 100 μm.

[0061] The positive electrode active material layer 21b contains a positive electrode active material capable of absorbing and releasing charge carriers such as lithium ions. As the positive electrode active material, any material suitable for use as a positive electrode active material in lithium-ion secondary batteries, such as lithium composite metal oxides with a layered rock salt structure, metal oxides with a spinel structure, or polyanionic compounds, can be used. Alternatively, two or more positive electrode active materials can be used in combination. In this embodiment, the positive electrode active material layer 21b contains olivine-type lithium iron phosphate (LiFePO4) as a polyanionic compound.

[0062] An example of the positive electrode active material layer 21b includes a positive electrode binder. Examples of positive electrode binders include the substances exemplified above as negative electrode binders. The positive electrode binder can be used alone or in combination. Examples of solvents or dispersion media used for dissolving or dispersing the positive electrode binder include water and N-methyl-2-pyrrolidone.

[0063] An example of a positive electrode binder is an aqueous binder. An aqueous binder is a binder that can be dissolved or dispersed in an aqueous solvent, and is used by mixing it with the positive electrode active material in a dispersed or dissolved state in an aqueous solvent. There are no particular limitations on the aqueous binder; conventionally known materials can be used as aqueous binders incorporated into the positive electrode active material layer of a lithium-ion secondary battery. Examples of aqueous binders, such as the substances exemplified above as negative electrode binders, can be cited as examples.

[0064] The positive electrode active material layer 21b may, as needed, include conductive additives to improve electrical conductivity, electrolytes (polymer matrix, ion-conducting polymer, liquid electrolyte, etc.), and electrolyte support salts (lithium salts) to improve ion conductivity. The conductive additives are added to improve the conductivity of the positive electrode 21. Examples of conductive additives include acetylene black, carbon black, graphite, and carbon nanotubes. The types and mixing ratios of the components included in the positive electrode active material layer 21b are not particularly limited and can be appropriately referenced to previously known insights regarding lithium-ion secondary batteries.

[0065] The area weight and thickness of the positive electrode active material layer 21b are not particularly limited, and can be appropriately referenced to previously known insights regarding lithium-ion secondary batteries. For example, the area weight of the positive electrode active material layer 21b is 500 g / m². 2 Above and 800g / m 2 The thickness of the positive electrode active material layer 21b is, for example, 0.3 mm or more and 0.4 mm or less.

[0066] The method for forming the positive electrode active material layer 21b on the surface of the positive electrode current collector 21a is not particularly limited, and conventionally known methods such as roll coating can be used. Alternatively, a coating film can be provided on the surface of the positive electrode current collector 21a. For example, a heat-resistant layer provided to improve the thermal stability of the positive electrode 21 can be cited as an example of such a coating film.

[0067] <Liquid Electrolytes> Liquid electrolytes contain a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent, and further contain specific additives.

[0068] Examples of known lithium salts that can be cited as electrolyte salts include LiClO4, LiAsF6, LiPF6, LiBF4, LiCF3SO3, LiN(FSO2)2, and LiN(CF3SO2)2.

[0069] Examples of non-aqueous solvents include, for example, cyclic carbonates, cyclic esters, linear carbonates, linear esters, and ethers. Two or more non-aqueous solvents can also be used in combination. An example of a non-aqueous solvent is esters, such as cyclic esters and linear esters. Compared to other solvents like carbonates, esters are more prone to gas generation due to self-discharge.

[0070] Examples of non-aqueous electrolytes include those obtained by adding specific additives to a solution in which the lithium salt is dissolved at a concentration of about 0.5 mol / L to 2.5 mol / L in the non-aqueous solvent.

[0071] The liquid electrolyte contains lithium difluorophosphate (LiPO2F2) as one of the aforementioned specific additives. The content of lithium difluorophosphate in the liquid electrolyte is determined based on the surface area of ​​the negative electrode active material layer 22b disposed within the same enclosed space S as the liquid electrolyte. Specifically, the content of lithium difluorophosphate per unit surface area of ​​the negative electrode active material layer 22b in the liquid electrolyte is, for example, 3 mg / m³. 2 The above is preferably 4 mg / m³. 2 The above, more preferably 5 mg / m² 2 That's all. Additionally, the lithium difluorophosphate content per unit surface area of ​​the negative electrode active material layer 22b is, for example, 20 mg / m². 2 The preferred concentration is 15 mg / m³. 2 The following is more preferably 9 mg / m³ 2 the following.

[0072] In each enclosed space S, the content of lithium difluorophosphate in the liquid electrolyte is adjusted to reach the aforementioned range. Furthermore, when multiple negative electrode active material layers 22b are disposed within one enclosed space S, the content of lithium difluorophosphate is adjusted to reach the aforementioned range based on the total surface area of ​​the multiple negative electrode active material layers 22b. Additionally, hereafter, "the surface area of ​​the negative electrode active material layer 22b disposed within the same enclosed space S as the enclosed space containing the liquid electrolyte" is sometimes described as "the specific surface area of ​​the negative electrode active material layer 22b". The concentration of lithium difluorophosphate in the liquid electrolyte is, for example, 0.3% by mass or more and 2.0% by mass or less, preferably 1.0% by mass or more and 2.0% by mass or less.

[0073] The liquid electrolyte contains vinylene carbonate (1,3-dioxane-2-one), which is one of the aforementioned specific additives. The content of vinylene carbonate in the liquid electrolyte is determined based on the surface area of ​​the negative electrode active material layer 22b disposed within the same sealed space S as the liquid electrolyte. Specifically, the content of vinylene carbonate per unit surface area of ​​the negative electrode active material layer 22b in the liquid electrolyte is, for example, 3 mg / m³. 2 The above is preferably 4 mg / m³. 2 The above, more preferably 5 mg / m² 2 That's all. Additionally, the content of vinylene carbonate per unit surface area of ​​the negative electrode active material layer 22b is, for example, 20 mg / m². 2 The preferred concentration is 15 mg / m³.2 The following is more preferably 9 mg / m³ 2 the following.

[0074] In the liquid electrolyte, the ratio of lithium difluorophosphate content per unit surface area to vinylene carbonate content per unit surface area of ​​the negative electrode active material layer 22b (lithium difluorophosphate / vinylene carbonate) is, for example, 0.5 or more and 2.0 or less. The concentration of vinylene carbonate in the liquid electrolyte is, for example, 0.3% by mass or more and 2.0% by mass or less, preferably 1.0% by mass or more and 2.0% by mass or less.

[0075] Preferably, the liquid electrolyte contains a cyclic sulfonate as one of the aforementioned specific additives. The cyclic sulfonate is any of the aforementioned specific additives that can be added. Therefore, the liquid electrolyte may also be composed without containing a cyclic sulfonate.

[0076] Examples of cyclic sulfonates include propane sulfonyl lactone and propenyl sulfonyl lactone. The content of cyclic sulfonates in the liquid electrolyte is determined based on the surface area of ​​the negative electrode active material layer 22b disposed in the same enclosed space S as the liquid electrolyte. Specifically, the content of cyclic sulfonates per unit surface area of ​​the negative electrode active material layer 22b in the liquid electrolyte is, for example, 0.6 mg / m³. 2 The above is preferably 0.8 mg / m³. 2 The above, more preferably 1 mg / m² 2 That's all. Additionally, the content of cyclic sulfonate per unit surface area of ​​the negative electrode active material layer 22b is, for example, 20 mg / m². 2 The preferred concentration is 15 mg / m³. 2 The following is more preferably 9 mg / m³ 2 the following.

[0077] In the liquid electrolyte, the ratio of the content of cyclic sulfonates per unit surface area of ​​the negative electrode active material layer 22b to the content of vinylene carbonate per unit surface area of ​​the negative electrode active material layer 22b (cyclic sulfonates / vinylene carbonate) is, for example, 0.1 or more and 2.0 or less, preferably 0.5 or more and 2.0 or less. The concentration of cyclic sulfonates in the liquid electrolyte is, for example, 0.1% by mass or more and 2.0% by mass or less, preferably 1.0% by mass or more and 2.0% by mass or less.

[0078] <Manufacturing Method of Energy Storage Device> Next, the manufacturing method of the energy storage device 10 of this embodiment will be described. The method for manufacturing the energy storage device 10 includes an assembly step of assembling a battery assembly and an activation step of activating the assembled battery assembly. In this specification, the battery assembly refers to the energy storage device 10 before the activation step.

[0079] (Assembly process) The assembly process includes electrode formation, energy storage unit formation, and unit stack formation.

[0080] In the electrode forming process, a positive electrode is formed by forming a positive electrode active material layer 21b on the surface of the positive electrode current collector 21a, and a negative electrode is formed by forming a negative electrode active material layer 22b on the surface of the negative electrode current collector 22a.

[0081] The positive electrode active material layer 21b is formed by adhering the positive electrode composite material, which has been cured to become the positive electrode active material layer 21b, to one side of the positive electrode current collector 21a to a predetermined thickness, and then performing a curing treatment corresponding to the positive electrode composite material. An example of the positive electrode composite material includes a positive electrode active material, an aqueous positive electrode binder, and an aqueous solvent such as water. The proportion of solid components in the positive electrode composite material is, for example, 70% by mass or more and 80% by mass or less.

[0082] The negative electrode active material layer 22b is formed by attaching a negative electrode composite material, which has been cured to become the negative electrode active material layer 22b, to one side of the negative electrode current collector 22a to a predetermined thickness, followed by a curing treatment corresponding to the negative electrode composite material. The negative electrode composite material includes, for example, a negative electrode active material, graphite particles, carbon fibers, a negative electrode binder, and a dispersion medium such as water. The proportion of solid components in the negative electrode composite material is, for example, 50% by mass or more and 70% by mass or less. Furthermore, the proportion of solid components in the negative electrode composite material is preferably 65% ​​by mass or less, more preferably 60% by mass or less. By reducing the proportion of solid components in the negative electrode composite material, it is easier to obtain [a specific type of material] in the formed negative electrode active material layer 22b. Figure 2 The dispersion state shown is a specific dispersion state in which fiber bundles F are dispersed.

[0083] In the energy storage cell forming process, firstly, the positive electrode 21 and the negative electrode 22 are arranged such that the separator 23 is sandwiched between the positive electrode active material layer 21b and the negative electrode active material layer 22b, and they are opposite each other in the stacking direction. Furthermore, a sealing member, forming a sealing portion 24, is disposed between the positive electrode 21 and the negative electrode 22, and on the outer periphery side of the positive electrode current collector 21a and the negative electrode current collector 22a. As the sealing member, a resin sheet made of acid-modified polyolefin resin is cut into a shape identical to the top view shape of the sealing portion 24.

[0084] Then, the positive electrode 21, negative electrode 22, and separator 23 are bonded to the sealing element by welding, thereby forming an assembly unit in which the positive electrode 21, negative electrode 22, separator 23, and sealing element 24 are integrated. Known welding methods such as thermal welding, ultrasonic welding, and infrared welding can be listed as methods for bonding the sealing element.

[0085] Next, liquid electrolyte is injected into the sealed space S inside the assembly unit through an injection port provided in a part of the sealing part 24, and then the injection port is closed. Thus, an assembly unit constituting one energy storage unit 20 is formed.

[0086] In the unit stack forming process, firstly, multiple assembly units are stacked such that the second surface 21a2 of the positive current collector 21a faces the second surface 22a2 of the negative current collector 22a. Then, the multiple assembly units are integrated by bonding the outer peripheral portions 24a of the sealing portions 24 in adjacent assembly units in the stacking direction to each other.

[0087] Next, the positive electrode current-carrying plate 40 is overlapped onto the second surface 21a2 of the positive electrode current collector 21a, which is located on the outermost side of the positive electrode 21 in the stacking direction, and fixed to the second surface 21a2 in an electrically connected state. Similarly, the negative electrode current-carrying plate 50 is overlapped onto the second surface 22a2 of the negative electrode current collector 22a, which is located on the outermost side of the negative electrode 22 in the stacking direction, and fixed to the second surface 22a2 in an electrically connected state. Through the above-described processes, a battery assembly is formed.

[0088] (Activation process) like Figure 3 As shown, the activation process includes an initial charging process and an aging process. By performing the activation process on the battery assembly, the energy storage device 10 is obtained.

[0089] The initial charging process is a process of using a charger to initially charge the battery assembly so that the voltage of the battery assembly reaches a target voltage that is set to be above the rated voltage and below the threshold voltage. The rated voltage is set as the upper limit of the voltage at which the energy storage device 10 can be used stably. The target voltage is the voltage set as the state of charge (SOC) of 100%. The first target voltage (voltage per cell) is, for example, 3.5V or higher and 4.0V or lower. An example of the first target voltage is 3.75V.

[0090] The initial charging process includes: a first charging step, charging to a first voltage; and a second charging step, charging from the first voltage to a target voltage. The charging rates of the first and second charging steps are different. The charging rate of the first charging step is lower than that of the second charging step.

[0091] The charging rate of the first charging step is, for example, 0.01C or less, preferably 0.0075C or less. The charging rate of the first charging step is, for example, 0.001C or more, preferably 0.0025C or more.

[0092] The first voltage is the voltage at which both lithium difluorophosphate and vinylene carbonate decompose. The decomposition voltage of lithium difluorophosphate is, for example, 1.0V or higher. The decomposition voltage of vinylene carbonate is, for example, 2.0V or higher. Therefore, the first voltage is, for example, 2.0V or higher, preferably 2.1V or higher, more preferably 2.2V or higher. The first voltage is, for example, 3.0V or lower, preferably 2.8V or lower, more preferably 2.7V or lower.

[0093] As described above, the decomposition voltage of lithium difluorophosphate is lower than that of vinylene carbonate. Therefore, by charging the first charging step at a low charging rate, lithium difluorophosphate and vinylene carbonate decompose in the order of lithium difluorophosphate, followed by vinylene carbonate. The decomposition of vinylene carbonate can begin after all lithium difluorophosphate has decomposed, or it can begin after the decomposition of lithium difluorophosphate has begun but before all lithium difluorophosphate has decomposed.

[0094] like Figure 4 As shown in (a) and (b), during the first charging step, when lithium difluorophosphate C1 in the liquid electrolyte decomposes, its decomposition products adhere to the graphite particles P in the negative electrode active material layer 22b. Thus, a first layer M1 composed of the decomposition products of lithium difluorophosphate C1 is formed on the surface of the graphite particles P. By performing the first charging step at a low charging rate, the first layer M1 can be formed more reliably.

[0095] Next, as Figure 4 As shown in (b) and (c), during the first charging step, when vinylene carbonate C2 in the liquid electrolyte decomposes, its decomposition products adhere to the graphite particles P in the negative electrode active material layer 22b. Consequently, a second layer M2 composed of the decomposition products of vinylene carbonate C2 is formed on the surface of the first layer M1 formed on the surface of the graphite particles P. As a result, an SEI film M having the first layer M1 and the second layer M2 is formed for the graphite particles P. By charging the first charging step at a low charging rate, the formation of the second layer M2 occurs after the formation of the first layer M1. As a result, the second layer M2 can be formed more reliably.

[0096] The charging rate of the second charging step is higher than that of the first charging step. The charging rate of the second charging step is, for example, greater than 0.01C, preferably 0.1C or higher. The charging rate of the second charging step is, for example, 0.5C or lower.

[0097] By using a higher charging rate in the second charging step than in the first charging step, the time required for the initial charging step can be shortened. Furthermore, by setting the first voltage to a lower value within the range above the decomposition voltage of lithium difluorophosphate and vinylene carbonate, the time required for the initial charging step can be shortened. In other words, in this case, the period of charging at the lower charging rate of the first charging step is relatively shorter, and the period of charging at the higher charging rate of the second charging step is relatively longer. As a result, the time required for the initial charging step can be shortened.

[0098] The aging process involves maintaining the battery assembly in a high-voltage state after the initial charging process in a high-temperature environment for a specified period of time. The aging process temperature is, for example, above 55°C and below 70°C. The aging process time is, for example, above 20 hours and below 50 hours.

[0099] During the aging process, the state of the SEI film M formed on the surface of the graphite particles P changes from... Figure 4 The state change shown in (c) is as follows: Figure 4 The state shown in (d). In detail, Figure 4 The SEI film M shown in (c) includes a portion where the second layer M2, which is composed of decomposition products of vinylene carbonate, is not formed, i.e., the portion where the first layer M1 or graphite particles P are exposed. Figure 4 The SEI film M shown in (d) has no exposed portion of the first layer M1 or graphite particles P, or the portion is small.

[0100] In the SEI film immediately after the initial charging process, Figure 4 The proportion of states shown in (c) is higher. Figure 4 The proportion of states shown in (d) is small. Through an aging process, the state of the SEI film M changes from... Figure 4 The state change shown in (c) is as follows: Figure 4 The state shown in (d) is as follows. The result is that... Figure 4 The SEI film M in state (c) is reduced. Figure 4 The state shown in (d) indicates an increase in the M of the SEI film. Figure 4 The SEI film M in state (d) covers the entire or large area of ​​the surface of the graphite particles P, and is therefore superior as a protective film to inhibit the contact between the graphite particles P and the liquid electrolyte.

[0101] Next, the function of this implementation method will be explained. The weight per unit area of ​​the negative electrode active material layer 22b of the energy storage device 10 is adjusted to 200 g / m². 2 The above. In existing energy storage devices, when the weight per unit area of ​​the negative electrode active material layer is increased to 200 g / m²... 2In the above case, a significant increase in the amount of gas generated from the electrodes occurs. Specifically, when the weight per unit area of ​​the negative electrode active material layer is large, the amount of electrons emitted from the negative electrode due to self-discharge increases. These emitted electrons react with the liquid electrolyte in the negative electrode to generate gas. Additionally, electrons emitted from the negative electrode travel through the liquid electrolyte to the positive electrode and react with the water contained in the positive electrode active material layer, thereby generating gas in the positive electrode. Therefore, as the weight per unit area of ​​the negative electrode active material layer increases, the increased amount of electrons emitted from the negative electrode due to self-discharge results in a significant increase in the amount of gas generated from the electrodes.

[0102] In this embodiment of the energy storage device 10, graphite particles are used as the negative electrode active material constituting the negative electrode active material layer 22b, and lithium difluorophosphate and vinylene carbonate are included in the liquid electrolyte. Thus, as... Figure 4 As shown in (c) and (d), an SEI film M composed of the decomposition products of lithium difluorophosphate and vinylene carbonate is formed on the surface of graphite particles P. The SEI film M derived from lithium difluorophosphate and vinylene carbonate functions as a protective film to inhibit the contact between graphite particles P and the liquid electrolyte. Since the contact between graphite particles P and the liquid electrolyte is suppressed, self-discharge on the surface of graphite particles P is suppressed. As a result, the generation of gas from the electrode caused by electrons released through self-discharge in the negative electrode active material layer 22b is reduced.

[0103] Furthermore, the SEI film M, composed of the decomposition products of lithium difluorophosphate and vinylene carbonate, has a first layer M1 composed of the decomposition products of lithium difluorophosphate and a second layer M2 composed of the decomposition products of vinylene carbonate. It is conceivable that the second layer M2 functions as a barrier to inhibit contact with the liquid electrolyte, while the first layer M1 functions as an adhesive layer that firmly holds the second layer M2 around the graphite particles P.

[0104] Next, the effects of this implementation method will be explained. (1) The energy storage device 10 comprises: a positive electrode 21 having a positive electrode active material layer 21b; a negative electrode 22 having a negative electrode active material layer 22b; a separator 23 disposed between the positive electrode 21 and the negative electrode 22; and a liquid electrolyte disposed between the positive electrode 21 and the negative electrode 22, wherein the weight per unit area of ​​the negative electrode active material layer 22b is 200 g / m². 2 The above describes the structure. The negative electrode active material layer 22b comprises graphite particles, carbon fibers, and a negative electrode binder. The liquid electrolyte comprises lithium difluorophosphate and vinylene carbonate.

[0105] Based on the above configuration, for the graphite particles P, an SEI film M is formed that functions as a protective film to suppress contact between the graphite particles P and the liquid electrolyte. By suppressing the contact between the graphite particles P and the liquid electrolyte, self-discharge on the surface of the graphite particles P can be suppressed. As a result, the generation of gas from the electrode caused by electrons emitted from the graphite particles P can be reduced.

[0106] (2) Carbon fiber consists of a bundle of fibers formed by multiple single-layer carbon nanotubes, which are connected across multiple graphite particles. Based on the above configuration, a small amount of carbon fiber (conductive additive) can suppress the decrease in the electronic conductivity of the negative electrode active material layer 22b caused by increasing the unit area weight of the negative electrode active material layer 22b. In particular, compared with the use of particulate conductive additives such as carbon black, the amount of conductive additive can be reduced. Furthermore, when carbon black is used as a conductive additive, sometimes the carbon black reacts with the liquid electrolyte during charging, and the liquid electrolyte decomposes to generate gas. By using carbon fiber, the generation of gas caused by charging and discharging in carbon black can also be suppressed.

[0107] (3) The liquid electrolyte contains cyclic sulfonates. Based on the above configuration, a superior SEI film M is formed as a protective film to suppress the contact between graphite particles P and the liquid electrolyte. Therefore, the effect of suppressing self-discharge on the surface of graphite particles P is improved. Consequently, the generation of gas from the electrodes caused by electrons emitted from graphite particles P can be further reduced.

[0108] (4) The positive electrode active material layer 21b contains positive electrode active material and positive electrode binder. The positive electrode binder is an aqueous binder. Since the positive electrode active material layer 21b containing an aqueous binder is formed using a positive electrode composite material containing water, it is easy to contain water such as water of crystallization within it. Therefore, it can be said that the positive electrode active material layer 21b containing an aqueous binder is a structure that is prone to gas generation. For an energy storage device with a structure that is prone to gas generation, by applying the structure described above (1), the effect of significantly reducing gas generation can be achieved.

[0109] (5) The non-aqueous solvent contained in liquid electrolytes is ester. Compared to other solvents such as esters and carbonates, esters are more prone to generating gas due to self-discharge. For energy storage devices with a structure that easily generates gas, the structure described in (1) above can significantly reduce gas generation.

[0110] (6) A sealing part 24 is provided between the positive electrode 21 and the negative electrode 22 to form a closed space S for containing liquid electrolyte. In the above configuration, when gas is generated from the electrodes, the internal pressure of the sealed space S increases, which may cause the sealed space S to deform by expansion. By reducing gas generation, deformation of the energy storage device 10 caused by gas generation can be suppressed. In addition, peeling of the seal 24 caused by the increase in internal pressure of the sealed space S can be suppressed.

[0111] (7) The manufacturing method of the energy storage device 10 includes: an assembly step, assembling a battery assembly; and an initial charging step, performing an initial charge on the battery assembly. The initial charging step includes: a first charging step, charging to a first voltage; and a second charging step, charging from the first voltage to a target voltage. The charging rate of the first charging step is lower than the charging rate of the second charging step. The first voltage is the voltage at which both lithium difluorophosphate and vinylene carbonate contained in the liquid electrolyte decompose.

[0112] Based on the above configuration, in the first charging step, charging is performed at a relatively low charging rate. This allows for a more reliable formation of an SEI film on the surface of the graphite particles P constituting the negative electrode current collector 22a, comprising a first layer M1 composed of lithium difluorophosphate decomposition products and a second layer M2 composed of vinylene carbonate decomposition products. Therefore, a more significant reduction in gas generation based on the function of the SEI film M in suppressing contact between the graphite particles P and the liquid electrolyte can be achieved. Furthermore, in the second charging step, charging is performed at a relatively high charging rate. This shortens the time required for the initial charging step.

[0113] (8) The assembly process includes an electrode forming process that forms a negative electrode active material layer 22b by coating a negative electrode composite material onto the surface of the negative electrode current collector 22a and drying the negative electrode composite material. The negative electrode composite material includes graphite particles, carbon fibers, a negative electrode binder, and a dispersion medium, with a solid component concentration of 65% by mass or less. Based on the above configuration, it is easy to form a state in which a single layer of carbon nanotube fiber bundle F spans multiple graphite particles P.

[0114] Furthermore, this embodiment can be implemented with the following modifications. This embodiment and the following modifications can be combined with each other to implement them within the scope of technical non-inconsistency. The top-view shape of the positive current collector 21a and the positive active material layer 21b is not particularly limited. It can be rectangular or polygonal, or circular or elliptical. The same applies to the negative current collector 22a and the negative active material layer 22b.

[0115] The top view shape of the sealing part 24 is not particularly limited; it can be a rectangle or other polygonal shape, or it can be a circle or an ellipse. To ensure good conductive contact between the positive electrode current-carrying plate 40 and the positive electrode current collector 21a, a conductive layer that is in close contact with the positive electrode current collector 21a can be disposed between the two components. Examples of such conductive layers include carbon-containing layers such as acetylene black or graphite, and plating layers containing Au, which have a lower hardness than the positive electrode current collector 21a. Similarly, the same conductive layer can be disposed between the negative electrode current-carrying plate 50 and the negative electrode current collector 22a.

[0116] The number of energy storage units 20 constituting the energy storage device 10 is not particularly limited. The number of energy storage units 20 constituting the energy storage device 10 may also be one. Alternatively, a positive electrode active material layer 21b or a negative electrode active material layer 22b may be provided on the second surface 21a2 of the positive electrode current collector 21a. Alternatively, a positive electrode active material layer 21b or a negative electrode active material layer 22b may be provided on the second surface 22a2 of the negative electrode current collector 22a.

[0117] The energy storage device 10 may also be an energy storage device having a constraint member of the constraint unit stack 30. The constraint member applies a constraint load to the area where the energy storage units 20 are opposite to each other in the stacking direction of the unit stack 30, especially the area where the positive electrode active material layer 21b is provided and the area where the negative electrode active material layer 22b is provided when viewed from above.

[0118] Any configuration capable of applying a constraint load to the unit stack 30 is acceptable, and the specific configuration of the constraint member is not particularly limited. Examples of constraint members include plate-shaped constraint plates positioned at both ends of the unit stack 30 in the stacking direction to clamp the unit stack 30, and fastening members consisting of bolts and nuts that fasten the constraint plates to each other. In the case of the constraint member configured as described above, the constraint plates are forced together in a direction that brings them closer to each other by the fastening members, thereby applying a constraint load in the stacking direction to the unit stack 30.

[0119] In the activation process, a charge-discharge process may also be performed as needed. This charge-discharge process can be performed, for example, before or after the aging process. The charge-discharge process involves performing multiple cycles of charge-discharge, with each cycle consisting of discharging and charging. For example, in the case of a charge-discharge process performed before the aging process, multiple cycles of charge-discharge are performed, with each cycle consisting of discharging up to 0% SOC and charging up to 15% SOC. In the case of a charge-discharge process performed after the aging process, multiple cycles of charge-discharge are performed, with each cycle consisting of discharging up to 0% SOC and charging up to 100% SOC.

[0120] The charging rate of the second charging step can also be the same as that of the first charging step. In this case, the charging rate of the second charging step is, for example, 0.01C or less, preferably 0.0075C or less. The charging rate of the second charging step is, for example, 0.001C or more, preferably 0.0025C or more.

[0121] The charging rate in the first charging step can be constant or variable. For example, the charging rate can increase in stages, or it can be maintained within a constant range. Similarly, the charging rate in the first charging step can also be constant or variable.

[0122] Alternatively, the charging rate of the first charging step can be set to exceed 0.01C. Even in this case, although the amount generated will decrease, graphite particles P with SEI film M will still be formed. Example

[0123] The following describes embodiments that further embellish the above-described implementation methods. (Example 1) A positive electrode composite material was coated on one side of an aluminum foil with a thickness of 35 μm. By drying and pressing the coated positive electrode composite material, a positive electrode sheet with a positive electrode active material layer formed on the surface of the positive electrode current collector was fabricated. The area weight of the positive electrode active material layer was 750 g / m². 2 As the positive electrode composite material, a slurry containing LiFePO4, monolayer carbon nanotubes, sodium carboxymethyl cellulose, and styrene-butadiene rubber in a solid component mass ratio of 98.55:0.05:0.4:1.0 and water as the solvent was used, with a solid component concentration of 75% by mass.

[0124] A negative electrode composite material was coated on one side of a 12 μm thick copper foil. By drying and pressing the coated negative electrode composite material, a negative electrode sheet with a negative electrode active material layer formed on the surface of the negative electrode current collector was fabricated. The area weight of the negative electrode active material layer was 378 g / m². 2 As the negative electrode composite material, a slurry containing graphite, monolayer carbon nanotubes, sodium carboxymethyl cellulose, styrene-butadiene rubber, and thickener, with a solid component mass ratio of 97.17:0.01:0.4:2.3:0.12 and water as the solvent, was used, with a solid component concentration of 56% by mass.

[0125] An electrode body, with a separator sandwiched between a positive electrode (cut to 25mm x 30mm) and a negative electrode (cut to 26mm x 31mm), is housed within a laminate serving as an outer packaging. By injecting liquid electrolyte into the outer packaging and sealing it, the energy storage device of Example 1 is obtained.

[0126] The liquid electrolyte used was prepared by dissolving LiN(FSO2)2 at a concentration of 1.4 M in a mixed solvent of ethylene carbonate and methyl propionate at a volume ratio of 15:85, and adding the additives shown in Table 1. In Example 1, a liquid electrolyte with lithium difluorophosphate and vinylene carbonate as additives was used. In Table 1, the values ​​in each additive column indicate the content of additive per unit surface area of ​​the negative electrode active material layer 22b, and the values ​​in parentheses below indicate the concentration of the additive in the liquid electrolyte.

[0127] (Examples 2-4) Except for the different additives added to the liquid electrolyte, the energy storage devices of Examples 2 to 4 were obtained in the same manner as in Example 1. As shown in Table 1, in Examples 2 and 3, liquid electrolytes containing lithium difluorophosphate, vinylene carbonate, and propane sulpholactone were used. The amount of propane sulpholactone added in Examples 2 and 3 was different. In Example 4, a liquid electrolyte containing lithium difluorophosphate, vinylene carbonate, and propane sulpholactone was used.

[0128] (Comparative Examples 1-2) Except for the difference in the additives added to the liquid electrolyte, the energy storage devices of Comparative Examples 1 and 2 were obtained in the same manner as in Example 1. As shown in Table 1, in Comparative Example 1, a liquid electrolyte containing vinylene carbonate was used. In Comparative Example 2, a liquid electrolyte containing vinylene carbonate and lithium difluoro(oxalate)borate was used. The content of each additive in the liquid electrolyte is shown in Table 1.

[0129] [Table 1]

[0130] The additives shown in Table 1 are as follows. LiPO2F2: Lithium difluorophosphate VC: Vinylene carbonate PS: 1,3-Propanesulfonyl lactone PRS: 1-Propylene-1,3-Sulfolactone DFOB: Lithium difluoro(oxalate)borate (Determination of gas production) Each energy storage device in the embodiments and comparative examples was charged to 2.7V (first voltage) with a constant current of 0.005C. Then, after charging to 3.75V (target voltage) with a constant current of 0.05C, the energy storage device in the high-voltage state was kept at 65°C for 35 hours. Then, it was discharged to 3.0V.

[0131] The volume of the energy storage device after discharge was measured, and this measured value was taken as the initial volume. Then, the discharged energy storage device was stored at 60°C for several days, and its volume was periodically measured during the storage period to calculate the volume increase from the initial volume. This calculated volume increase was taken as the gas production rate. Figure 5 The results are shown in the coordinate graph. The volume of the energy storage device was determined using an electronic hydrometer (MDS-300, manufactured by Alfa Mirage) via the Archimedes method.

[0132] As described above, Comparative Example 1 uses a liquid electrolyte containing only vinylene carbonate, and Comparative Example 2 contains both vinylene carbonate and DFOB as a comparative additive. Figure 5 As shown, in Comparative Example 1 and Comparative Example 2, the amount of gas produced increased with the increase of storage days. This result indicates that gas was continuously produced within the apparatus of Comparative Example 1 and Comparative Example 2.

[0133] Although detailed data is omitted here, as a reference experiment, the method used reduced the unit area weight of the negative electrode active material layer (set at 100g / m²). 2 The same energy storage device as Comparative Example 1 was subjected to the same test, and the result was that almost no gas generation was detected. This result indicates that the gas generation in Comparative Example 1 and Comparative Example 2 was caused by the negative electrode active material layer having a unit area weight that was too large.

[0134] like Figure 5 As shown, compared to Comparative Examples 1 and 2, in Example 1, which used a liquid electrolyte containing lithium difluorophosphate and vinylene carbonate, the amount of gas generated due to the increased storage days was reduced to less than half. This result demonstrates that by simultaneously adding lithium difluorophosphate and vinylene carbonate, the generation of gas from the electrodes can be reduced.

[0135] Examples 2-4 are examples of liquid electrolytes that not only contain lithium difluorophosphate and vinylene carbonate, but also propane sulpholactone or propene sulpholactone. For example... Figure 5 As shown, no gas generation was observed in Examples 2-4. This result indicates that by further adding cyclic sulfonates such as propane sulfonyl lactone and allyl sulfonyl lactone, gas generation from the electrode can be significantly reduced or even eliminated.

[0136] Furthermore, although detailed data is omitted, as an additional experiment, a storage device was obtained in the same manner as in Example 1, except that the monolayer carbon nanotubes (1% by mass) were replaced with particulate carbon black (3.8% by mass) in an amount that yielded the same capacity retention rate. Regarding this storage device, the amount of gas generated was measured, and the result showed that the amount of gas generated was increased compared to Example 1. This result indicates that the reduction in gas generation in each example was achieved by using carbon fiber as a conductive additive. Explanation of reference numerals in the attached figures

[0137] F…fiber bundle P… Graphite particles S…enclosed space 10…Electric Storage Devices 21… Positive electrode 21b… Positive electrode active material layer 22… Negative electrode 22b…Negative electrode active material layer 23…isolation materials 24…Sealing part.

Claims

1. An electricity storage device comprising: a positive electrode having a positive electrode active material layer; a negative electrode having a negative electrode active material layer; a separator disposed between the positive electrode and the negative electrode; and a liquid electrolyte disposed between the positive electrode and the negative electrode, the negative electrode active material layer having a weight per unit area of 200 g / m2 or less. 2 The electricity storage device is characterized by the following. The negative electrode active material layer comprises graphite particles, carbon fibers, and a negative electrode binder. The liquid electrolyte contains lithium difluorophosphate and vinylene carbonate.

2. The energy storage device according to claim 1, wherein, The carbon fiber comprises a fiber bundle consisting of multiple single-layer carbon nanotubes bound together, the fiber bundle spanning multiple graphite particles.

3. The energy storage device according to claim 1 or claim 2, wherein, The average particle size (D50) of the graphite particles is greater than 3 μm and less than 30 μm. The fiber length of the monolayer carbon nanotube is greater than 5 μm and less than 50 μm.

4. The energy storage device according to any one of claims 1 to 3, wherein, The liquid electrolyte contains cyclic sulfonates.

5. The energy storage device according to any one of claims 1 to 4, wherein, The positive electrode active material layer comprises a positive electrode active material and a positive electrode binder. The positive electrode binder is an aqueous binder.

6. The energy storage device according to any one of claims 1 to 5, wherein, A sealing portion is provided between the positive electrode and the negative electrode to form a sealed space for containing the liquid electrolyte.

7. The energy storage device according to any one of claims 1 to 6, wherein, The graphite particles in the negative electrode active material layer contain more than 97% by mass. The average particle size (D50) of the graphite particles is greater than 10 μm and less than 20 μm. The specific surface area of ​​the negative electrode active material layer is 0.5 m². 2 / g or more and 1.5m 2 / g or less The content of lithium difluorophosphate per unit surface area of ​​the negative electrode active material layer in the liquid electrolyte is 3 mg / m³. 2 above, The content of vinylene carbonate per unit surface area of ​​the negative electrode active material layer in the liquid electrolyte is 3 mg / m³. 2 above.

8. The energy storage device according to any one of claims 1 to 7, wherein, The liquid electrolyte contains cyclic sulfonate esters. The graphite particles in the negative electrode active material layer contain more than 97% by mass. The average particle size (D50) of the graphite particles is greater than 10 μm and less than 20 μm. The specific surface area of ​​the negative electrode active material layer is 0.5 m². 2 / g or more and 1.5m 2 / g or less The content of the cyclic sulfonate per unit surface area of ​​the negative electrode active material layer in the liquid electrolyte is 0.6 mg / m³. 2 above.

9. A method for manufacturing an energy storage device, which is the method for manufacturing an energy storage device according to any one of claims 1 to 8, characterized in that, The process includes: an assembly step, which assembles a battery assembly comprising the positive electrode, the negative electrode, the separator, and the liquid electrolyte; and an initial charging step for the battery assembly. The initial charging process includes: The first charging step involves charging to a first voltage of 2.0V or higher and 3.0V or lower; and The second charging step involves charging from the first voltage to a predetermined target voltage. The charging rate of the second charging step is higher than that of the first charging step.

10. The method for manufacturing an energy storage device according to claim 9, wherein, The assembly process includes an electrode forming process that forms the negative electrode active material layer by coating the surface of the negative electrode current collector with a negative electrode composite material and drying the negative electrode composite material. The negative electrode composite material comprises graphite particles, carbon fibers, a negative electrode binder, and a dispersion medium, with a solid component concentration of less than 65% by mass.

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