Power storage device and method for manufacturing power storage device

Abstract

A power storage device (10) is provided with: a positive electrode (21) that has a positive electrode active material layer (21b); a negative electrode (22) that has a negative electrode active material layer (22b); a separator (23) that is disposed between the positive electrode (21) and the negative electrode (22); and a liquid electrolyte that is disposed between the positive electrode (21) and the negative electrode (22). The basis weight of the negative electrode active material layer (22b) is 200 g / m2 or more. The negative electrode active material layer (22b) contains graphite particles, carbon fibers, and a negative electrode binder. The liquid electrolyte contains lithium difluorophosphate and vinylene carbonate.

Description

Electricity storage device and method for manufacturing the same

[0001] The present disclosure relates to an electricity storage device and a method for manufacturing an electricity storage device.

[0002] Patent Document 1 discloses a flat-type energy storage device constructed by stacking a plurality of individually manufactured energy storage cells in series. The energy storage cell includes a positive electrode having a positive electrode active material layer formed in the center of one side of a foil-shaped positive electrode current collector, a negative electrode having a negative electrode active material layer formed in the center of one side of a foil-shaped negative electrode current collector, the negative electrode active material layer being disposed so as to face the positive electrode active material layer of the positive electrode, and a separator disposed between the positive electrode and the negative electrode.

[0003] The energy storage cell further includes a seal portion disposed between the positive electrode and the negative electrode and on the outer circumferential side of the positive electrode active material layer and the negative electrode active material layer. The seal portion maintains a gap between the positive electrode current collector and the negative electrode current collector to prevent short circuits between the current collectors, and also liquid-tightly seals the gap between the positive electrode current collector and the negative electrode current collector, thereby forming an enclosed space for accommodating a liquid electrolyte between the positive electrode current collector and the negative electrode current collector.

[0004] JP 2017-16825 A

[0005] One method for increasing the energy density of a storage cell is to increase the basis weight of the active material layer. However, if the basis weight of the negative electrode active material layer is increased beyond a certain level, a problem occurs in that the amount of gas generated from the electrode increases significantly.

[0006] In one aspect of the present disclosure, there is provided an electricity storage device including 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. ２ The negative electrode active material layer includes graphite particles, carbon fibers, and a negative electrode binder, and the liquid electrolyte includes lithium difluorophosphate and vinylene carbonate.

[0007] In the above-described electricity storage device, it is preferable that the carbon fiber includes a fiber bundle formed by bundling a plurality of single-walled carbon nanotubes, and the fiber bundle is in contact with a plurality of the graphite particles across the plurality of graphite particles.

[0008] In the electricity storage device, it is preferable that the graphite particles have an average particle diameter (D50) of 3 μm or more and 30 μm or less, and the single-walled carbon nanotubes have a fiber length of 5 μm or more and 50 μm or less.

[0009] In the above-described power storage device, the liquid electrolyte preferably contains a cyclic sulfonic acid ester. In the above-described power storage device, the positive electrode active material layer preferably contains a positive electrode active material and a positive electrode binder, and the positive electrode binder is preferably an aqueous binder.

[0010] The electricity storage device preferably includes a seal portion that forms a sealed space between the positive electrode and the negative electrode to accommodate the liquid electrolyte. In the electricity storage device, the content of the graphite particles in the negative electrode active material layer is 97 mass % or more, the average particle diameter (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 m ２ / g or more 1.5m ２ / g or less, and the content of the lithium difluorophosphate per surface area of ​​the negative electrode active material layer in the liquid electrolyte is 3 mg / m ２ or more, and the content of the vinylene carbonate per surface area of ​​the negative electrode active material layer in the liquid electrolyte is 3 mg / m ２ It is preferable that this is equal to or greater than this.

[0011] In the electricity storage device, the liquid electrolyte contains a cyclic sulfonic acid ester, the content of the graphite particles in the negative electrode active material layer is 97 mass% or more, the average particle diameter (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 m ２ / g or more 1.5m ２ / g or less, and the content of the cyclic sulfonate ester per surface area of ​​the negative electrode active material layer in the liquid electrolyte is 0.6 mg / m ２ It is preferable that this is equal to or greater than this.

[0012] In one aspect of the present disclosure, there is provided a method for manufacturing the above-described power storage device, the method including assembling a battery assembly including the positive electrode, the negative electrode, the separator, and the liquid electrolyte, and initially charging the battery assembly, the initial charging step including a first charging step of charging the battery assembly to a first voltage of 2.0 V or more and 3.0 V or less, and a second charging step of charging from the first voltage to a predetermined target voltage, the second charging step having a charging rate higher than the charging rate of the first charging step.

[0013] In the method for manufacturing the electricity storage device, the assembly step preferably includes an electrode formation step of applying a negative electrode composite to a surface of a negative electrode current collector and drying the applied negative electrode composite to form the negative electrode active material layer, and the negative electrode composite preferably includes graphite particles, carbon fibers, a negative electrode binder, and a dispersion medium, and has a solid content concentration of 65 mass% or less.

[0014] According to the present invention, gas generated from the electrodes can be reduced.

[0015] FIG. 1 is a cross-sectional view of an electricity storage device. FIG. 2 is a schematic diagram showing the state of graphite particles and carbon fibers in a negative electrode active material layer. FIG. 3 is an explanatory diagram of an activation process. (a) to (d) of FIG. 4 are schematic diagrams showing the process of forming an SEI film on graphite particles. FIG. 5 is a graph showing the results of a storage test for measuring the amount of gas generated.

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

[0017] 1, the energy storage device 10 includes a cell stack 30 (laminated body) in which a plurality of energy storage cells 20 are stacked in a stacking direction. Hereinafter, the stacking direction of the plurality of 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 seal portion 24.

[0018] The positive electrode 21 includes a positive electrode current collector 21a and a positive electrode active material layer 21b provided on a first surface 21a1 of the positive electrode current collector 21a. In a plan view seen in the stacking direction (hereinafter simply referred to as a plan view), the positive electrode active material layer 21b is formed in the center of the first surface 21a1 of the positive electrode current collector 21a. In a plan view, the peripheral portion of the first surface 21a1 of the positive electrode current collector 21a is a positive electrode uncoated portion 21c where the positive electrode active material layer 21b is not provided. The positive electrode uncoated portion 21c is arranged to surround the periphery of the positive electrode active material layer 21b in a plan view.

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

[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 face each other in the stacking direction. That is, the facing direction of the positive electrode 21 and the negative electrode 22 coincides with the stacking direction. The negative electrode active material layer 22b is formed to be slightly larger than the positive electrode active material layer 21b, and in a plan view seen from 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 electrode current collector 21a has a second surface 21a2 located opposite to the first surface 21a1, and the negative electrode current collector 22a has a second surface 22a2 located opposite to the first surface 22a1. The cell stack 30 has a structure in which a plurality of power storage cells 20 are stacked such that the second surface 21a2 of the positive electrode current collector 21a and the second surface 22a2 of the negative electrode current collector 22a are in contact with each other. In this way, the plurality of power storage cells 20 constituting the cell stack 30 are connected in series.

[0022] In the cell stack 30, a pseudo bipolar electrode 25 is formed in which the mutually contacting positive electrode current collectors 21 a and negative electrode current collectors 22 a of two storage cells 20 adjacent to each other in the stacking direction are regarded as a single current collector. The pseudo bipolar electrode 25 includes a current collector having a structure in which the positive electrode current collector 21 a and the negative electrode current collector 22 a are stacked, a positive electrode active material layer 21 b formed on one surface of the current collector, and a negative electrode active material layer 22 b formed on the other surface.

[0023] Alternatively, the positive electrode current collector 21a and the negative electrode current collector 22a may form a bipolar current collector in which the second surface 21a2 of the positive electrode current collector 21a and the second surface 22a2 of the negative electrode current collector 22a are joined together. In this case, the positive electrode 21 and the negative electrode 22 form a bipolar electrode 25 including a single bipolar current collector in which the positive electrode current collector 21a and the negative electrode current collector 22a are joined together. In other words, the bipolar electrode 25 is formed by joining a surface of the positive electrode current collector 21a of the positive electrode 21 constituting one of the adjacent energy storage cells 20 opposite to the first surface 21a1 and a surface of the negative electrode current collector 22a of the negative electrode 22 constituting the other of the adjacent energy storage cells 20 opposite to the first surface 22a1.

[0024] The separator 23 is disposed between the positive electrode 21 and the negative electrode 22, and is a component that separates the positive electrode 21 and the negative electrode 22 to prevent short circuits due to contact between the two electrodes, while allowing 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 that form the separator 23 include polypropylene, polyethylene, polyolefin, and polyester. The separator 23 may have a single-layer structure or a multi-layer structure. The multi-layer structure may include, for example, an adhesive layer, a ceramic layer as a heat-resistant layer, and the like.

[0026] The seal portion 24 is disposed between the first surface 22a1 of the positive electrode current collector 21a of the positive electrode 21 and the first surface 22a1 of the negative electrode current collector 22a of the negative electrode 22, and is disposed on the outer circumferential side of the positive electrode active material layer 21b and the negative electrode active material layer 22b, and is adhered to both the positive electrode current collector 21a and the negative electrode current collector 22a. The seal portion 24 insulates the positive electrode current collector 21a from the negative electrode current collector 22a, thereby preventing a short circuit between the current collectors.

[0027] The seal portion 24 is made of, for example, a polyolefin-based resin. Examples of polyolefin-based 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. Two or more of these known polyolefin-based resins may be used in combination. The polyolefin-based resin may be a thermoplastic resin or a thermosetting resin.

[0028] The sealing portion 24 extends along the peripheral edges of the positive electrode current collector 21 a and the negative electrode current collector 22 a in a plan view, and is formed in a frame shape surrounding the peripheries of the positive electrode active material layer 21 b and the negative electrode active material layer 22 b. The sealing portion 24 is disposed between the positive electrode uncoated portion 21 c on the first surface 21 a 1 of the positive electrode current collector 21 a and the negative electrode uncoated portion 22 c on the first surface 22 a 1 of the negative electrode current collector 22 a.

[0029] An enclosed space S is formed inside the energy storage cell 20 and is surrounded by a frame-shaped seal portion 24, the positive electrode 21, and the negative electrode 22. A separator 23 and a liquid electrolyte are housed in the enclosed space S. The peripheral portion of the separator 23 is embedded in the seal portion 24.

[0030] The seal portion 24 can prevent the liquid electrolyte contained in the sealed space S from permeating to the outside by sealing the sealed space S between the positive electrode 21 and the negative electrode 22. The seal portion 24 can also prevent moisture from entering the sealed space S from the outside of the energy storage device 10. Furthermore, the seal portion 24 can prevent gas generated from the positive electrode 21 or the negative electrode 22 due to, for example, a charge / discharge reaction, from leaking to the outside of the energy storage device 10.

[0031] The seal portion 24 of each storage cell 20 has an outer peripheral portion that extends outward beyond the respective edges of the positive electrode current collector 21 a and the negative electrode current collector 22 a. When viewed from the stacking direction, the outer peripheral portion protrudes beyond the respective edges of the positive electrode current collector 21 a and the negative electrode current collector 22 a in a direction perpendicular to the stacking direction. Adjacent storage cells 20 in the stacking direction are integrated by bonding the outer peripheral portions of their seal portions 24 together. Examples of methods for bonding adjacent seal portions 24 together include known welding methods such as heat welding, ultrasonic welding, and infrared welding.

[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, which are arranged to sandwich the cell stack 30 in the stacking direction of the cell 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 current-carrying plate 40 is electrically connected to the second surface 21a2 of the positive electrode current collector 21a of the positive electrode 21 arranged outermost at one end in the stacking direction. The negative electrode current-carrying plate 50 is electrically connected to the second surface 22a2 of the negative electrode current collector 22a of the negative electrode 22 arranged outermost at the other end 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. The material constituting the positive electrode current-carrying plate 40 may be, for example, the same material as the material constituting the positive electrode current collector 21a. The positive electrode current-carrying plate 40 may be formed of a metal plate that is thicker than the positive electrode current collector 21a used in the cell stack 30. The material constituting the negative electrode current-carrying plate 50 may be, for example, the same material as the material constituting the negative electrode current collector 22a. The negative electrode current-carrying plate 50 may be formed of a metal plate that is thicker than the negative electrode current collector 22a used in the cell stack 30.

[0035] <Details of the Negative Electrode> Next, the details of the negative electrode 22 will be described. The negative electrode current collector 22a is a chemically inactive electrical conductor that continues to pass current through the negative electrode active material layer 22b during discharge or charge of the lithium-ion secondary battery. An example of the negative electrode current collector 22a is a copper current collector whose surface, which becomes the first surface 22a1, is made of copper. The copper current collector may be a simple substance made entirely of copper, or a composite having a portion made of copper and a portion made of a material other than copper. Examples of the simple substance include copper foil such as electrolytic copper foil. Examples of the composite include a multilayer structure in which the layer making up the first surface 22a1 is a copper layer, and a substrate whose surface, including the first surface 22a1, is coated with a copper film.

[0036] Examples of materials other than copper include metal materials, conductive resin materials, and conductive inorganic materials. Examples of metal materials include aluminum, nickel, titanium, and stainless steel (e.g., SUS304, SUS316, SUS301, SUS304, etc., as defined in JIS G 4305:2015). Examples of conductive resin materials include resins obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material as needed. The negative electrode current collector 22a may be in the form of, 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) The graphite particles are contained in the negative electrode active material layer 22b as a negative electrode active material capable of absorbing and releasing charge carriers such as lithium ions. Examples of graphite constituting the graphite particles include natural graphite, artificial graphite, hard carbon (non-graphitizable carbon), and soft carbon (easily graphitizable carbon). Examples of artificial graphite include highly oriented graphite and mesocarbon microbeads.

[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 measured, for example, using a laser diffraction particle size analyzer.

[0039] The graphite particle content 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 graphite particle content 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] As shown in (c) and (d) of Figure 4, the graphite particles P may have an SEI (Solid Electrolyte Interface) film formed by decomposition of the liquid electrolyte. The graphite particles P shown in (c) and (d) have an SEI film M. The SEI film M includes 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 particles P and covers a part or the entire surface of the graphite particles P. The first layer M1 is composed of a decomposition product of lithium difluorophosphate contained in the liquid electrolyte. The second layer M2 is formed on the surface of the first layer M1 and covers a part or the entire surface of the first layer M1. The second layer M2 is composed of a decomposition product of vinylene carbonate contained in the liquid electrolyte. The SEI film M functions as a protective film that suppresses contact between the graphite particles P and the liquid electrolyte. The mechanism by which the SEI film M is formed will be described in detail later.

[0041] (Carbon Fiber) Carbon fiber is contained in the negative electrode active material layer 22b as a conductive additive for improving electrical conductivity. The carbon fiber is, for example, a carbon nanotube. Carbon nanotubes can be broadly divided into two types: single-walled carbon nanotubes and multi-walled carbon nanotubes. Single-walled carbon nanotubes are cylindrical bodies formed by seamlessly rolling up a single graphene sheet. Multi-walled carbon nanotubes are composites in which multiple single-walled carbon nanotubes with different diameters are contained within a single single-walled carbon nanotube. The carbon fiber contained in the negative electrode active material layer 22b is preferably a single-walled carbon nanotube.

[0042] The fiber length and fiber diameter of the carbon fiber are not particularly limited. The fiber length of the carbon fiber is, for example, 5 μm or more and 1000 μm or less. The fiber diameter of the carbon fiber is, for example, 1 nm or more and 20 nm or less. The fiber length and fiber diameter of the carbon fiber can be measured using, for example, an electron microscope such as a scanning electron microscope (SEM) or a transmission electron microscope (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. An example of a preferred state of the graphite particles and carbon fibers in the negative electrode active material layer 22b will be described with reference to Fig. 2. In Fig. 2, the negative electrode binder and other components described later are not shown.

[0044] As shown in Fig. 2, the negative electrode active material layer 22b includes graphite particles P and fiber bundles F. The fiber bundles F are formed by bundling a plurality of single-walled carbon nanotubes. The number of single-walled carbon nanotubes constituting one fiber bundle F is, for example, several to several tens of single-walled carbon nanotubes. The negative electrode active material layer 22b may also include a single single-walled carbon nanotube that is not part of the fiber bundle F.

[0045] 2 , single-walled carbon nanotubes, which are carbon fibers, are dispersed in fiber bundles F and are arranged in a mesh-like pattern (e.g., spider web-like) as a whole. The fiber bundles F are in contact with and straddle a plurality of graphite particles P. In this specification, the state in which the fiber bundles F are in contact with and straddle a plurality of graphite particles (hereinafter, this state may be referred to as a specific dispersion state) means a state in which 50% or more of the fiber bundles F, based on the number of bundles, of the plurality of fiber bundles F are in contact with and straddle a plurality of graphite particles.

[0046] Note that, with respect to one fiber bundle F, if at least one fiber constituting the fiber bundle F is in contact with graphite particle A, the fiber bundle F is considered to be in contact with graphite particle A. Furthermore, with respect to one fiber bundle F, if at least one fiber constituting the fiber bundle F is in contact with graphite particle A and at least one fiber constituting the fiber bundle F is in contact with graphite particle B, the fiber bundle F is considered to be in contact with both graphite particle A and graphite particle B. In this case, the fiber in contact with graphite particle A and the fiber in contact with graphite particle B may be the same or different. The specific dispersion state can be confirmed using, for example, an electron microscope such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0047] From the viewpoint of forming a specific dispersion state, the fiber length of the single-walled carbon nanotubes is preferably, for example, 5 μm or more and 50 μm or less. The fiber length is more preferably 10 μm or more, and even more preferably 15 μm or more. The fiber length is more preferably 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 single-walled carbon nanotubes / average particle size of graphite particles) is preferably, for example, greater than 1. The ratio is, for example, 20 or less.

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

[0049] (Negative Electrode Binder) Examples of negative electrode binders include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, 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 crosslinked bodies, and starch-acrylic acid graft polymers. The negative electrode binders can be used alone or in combination. Examples of solvents or dispersion media used to dissolve or disperse the negative electrode binder include water and N-methyl-2-pyrrolidone.

[0050] (Other Components) The negative electrode active material layer 22b may further contain other components in addition to the graphite particles, carbon fibers, and negative electrode binder, as necessary. Examples of the other components include a negative electrode active material other than graphite particles, a conductive additive other than carbon fibers, an electrolyte (a polymer matrix, an ion-conductive polymer, a liquid electrolyte, etc.), and an electrolyte supporting salt (lithium salt) for increasing ion conductivity.

[0051] Examples of negative electrode active materials other than graphite particles include elements that can be alloyed with lithium, such as silicon and tin. The negative electrode active material is not particularly limited as long as it is a simple substance, alloy, or compound that can absorb and release charge carriers such as lithium ions.

[0052] Examples of conductive additives other than carbon fiber include acetylene black, carbon black, and graphite. The types and blending ratios of other components contained in the negative electrode active material layer 22b are not particularly limited, and publicly known knowledge about lithium ion secondary batteries can be used as appropriate.

[0053] (Weight and Thickness of Negative Electrode Active Material Layer 22b) The weight of the negative electrode active material layer 22b is 200 g / m ２ or more, and preferably 250 g / m ２ More preferably, 300 g / m ２ The weight of the negative electrode active material layer 22b is, for example, 500 g / m ２ and preferably 450 g / m ２ More preferably, 400 g / m or less. ２ The following is the result.

[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, and 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 the liquid electrolyte to penetrate. The surface area of ​​the negative electrode active material layer 22b is, for example, 250 mm ２ Over 1350mm ２ The following is the result.

[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. ２ 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 negative electrode current collector 22a where 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 m ２ / g or more 1.5m ２The total mass of one negative electrode active material layer 22b, i.e., the total mass of the negative electrode active material layers 22b included in one power storage cell 20, is, for example, 20 g or more and 1500 g or less, and preferably 500 g or more and 900 g or less. The area of ​​the range in which the negative electrode active material layers 22b are formed on the negative electrode current collector 22a is, for example, 1000 cm ２ More than 30000cm ２ is preferably 10,000 cm or less. ２ More than 30000cm ２ The following is the result.

[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 a conventionally known method such as roll coating can be used. A coating may be provided on the surface of the negative electrode current collector 22a. An example of the coating is a heat-resistant layer provided to improve the thermal stability of the negative electrode 22.

[0059] <Details of the Positive Electrode> Next, the details of the positive electrode 21 will be described. The positive electrode current collector 21a is a chemically inactive electrical conductor that continues to pass current through the positive electrode active material layer 21b during discharge or charge of the lithium-ion secondary battery. An example of the positive electrode current collector 21a is an aluminum current collector whose surface, which becomes the first surface 21a1, is made of aluminum. The aluminum current collector may be a simple substance made entirely of aluminum, or a composite having a portion made of aluminum and a portion made of a material other than aluminum. Examples of the simple substance include aluminum foil such as rolled aluminum foil. Examples of the composite include a multilayer structure in which the layer making up the first surface 21a1 is an aluminum layer, and a substrate whose surface, including the first surface 21a1, is coated with an aluminum film.

[0060] Examples of materials other than aluminum include metal materials, conductive resin materials, and conductive inorganic materials. Examples of metal materials include copper, nickel, titanium, and stainless steel (e.g., SUS304, SUS316, SUS301, SUS304, etc., as defined in JIS G 4305:2015). Examples of conductive resin materials include resins obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material as needed. The positive electrode current collector 21a may be in the form of, for example, a foil, a sheet, or a film. The thickness of the positive electrode 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 that can be used as a positive electrode active material for a lithium ion secondary battery, such as a lithium composite metal oxide having a layered rock salt structure, a metal oxide having a spinel structure, or a polyanion-based compound, may be used. Two or more positive electrode active materials may also be used in combination. In this embodiment, the positive electrode active material layer 21b contains an olivine-type lithium iron phosphate (LiFePO ), which is a polyanion-based compound. ４ ) is included.

[0062] An example of the positive electrode active material layer 21b includes a positive electrode binder. Examples of the positive electrode binder include those exemplified above as negative electrode binders. The positive electrode binders may be used alone or in combination. Examples of the solvent or dispersion medium used to dissolve or disperse the positive electrode binder include water and N-methyl-2-pyrrolidone.

[0063] An example of a positive electrode binder is an aqueous binder. The aqueous binder is a binder that is soluble or dispersible in an aqueous solvent and is used by mixing the aqueous binder with the positive electrode active material in a dispersed or dissolved state in the aqueous solvent. The aqueous binder is not particularly limited, and any conventionally known material can be used as the aqueous binder contained in the positive electrode active material layer of a lithium ion secondary battery. Examples of aqueous binders include those exemplified above as negative electrode binders.

[0064] The positive electrode active material layer 21b may further contain, as necessary, a conductive additive for increasing electrical conductivity, an electrolyte (such as a polymer matrix, an ion-conductive polymer, or a liquid electrolyte), an electrolyte supporting salt (lithium salt) for increasing ionic conductivity, etc. The conductive additive is added to increase the conductivity of the positive electrode 21. Examples of the conductive additive include acetylene black, carbon black, graphite, and carbon nanotubes. The types and blending ratios of the components contained in the positive electrode active material layer 21b are not particularly limited, and conventionally known knowledge about lithium ion secondary batteries may be referenced as appropriate.

[0065] The basis weight and thickness of the positive electrode active material layer 21b are not particularly limited, and conventionally known knowledge about lithium ion secondary batteries can be referred to as appropriate. The basis weight of the positive electrode active material layer 21b is, for example, 500 g / m ２ 800g / m or more 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 a conventionally known method such as roll coating can be used. A coating may be provided on the surface of the positive electrode current collector 21a. An example of the coating is a heat-resistant layer provided to improve the thermal stability of the positive electrode 21.

[0067] <Liquid Electrolyte> The liquid electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent, and further contains a specific additive.

[0068] Examples of the electrolyte salt include LiClO ４ , LiAsF ６ , LiPF ６ , LiBF ４ , LiCF ３ SO ３ , LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ Examples of known lithium salts include:

[0069] Examples of non-aqueous solvents include known solvents such as cyclic carbonates, cyclic esters, chain carbonates, chain esters, and ethers. Two or more non-aqueous solvents may be used in combination. Examples of non-aqueous solvents include esters such as cyclic esters and chain esters. Esters are more likely to generate gas due to self-discharge than other solvents such as carbonates.

[0070] Examples of the non-aqueous electrolyte include a solution in which the lithium salt is dissolved in the non-aqueous solvent at a concentration of about 0.5 mol / L to 2.5 mol / L, and a specific additive is added to the solution.

[0071] The liquid electrolyte contains lithium difluorophosphate (LiPO ), which is one of the specific additives. ２ F ２ The content of lithium difluorophosphate in the liquid electrolyte is set according to the size of the surface area of ​​the negative electrode active material layer 22b that is disposed in the same sealed space S as the liquid electrolyte. More specifically, the content of lithium difluorophosphate in the liquid electrolyte per surface area of ​​the negative electrode active material layer 22b is, for example, 3 mg / m ２ or more, preferably 4 mg / m ２ More preferably, it is 5 mg / m ２ The content of lithium difluorophosphate per surface area of ​​the negative electrode active material layer 22b is, for example, 20 mg / m ２ or less, preferably 15 mg / m ２ More preferably, it is 9 mg / m or less. ２ The following is the result.

[0072] In each sealed space S, the content of lithium difluorophosphate in the liquid electrolyte is adjusted to fall within the above range. When multiple negative electrode active material layers 22b are disposed in one sealed space S, the content of lithium difluorophosphate is adjusted to fall within the above range based on the total surface area of ​​the multiple negative electrode active material layers 22b. Hereinafter, the "surface area of ​​the negative electrode active material layer 22b disposed in the same sealed space S as the sealed space S in which the liquid electrolyte is accommodated" may be referred to 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, from 0.3% by mass to 2.0% by mass, and preferably from 1.0% by mass to 2.0% by mass.

[0073] The liquid electrolyte contains vinylene carbonate (1,3-dioxol-2-one), which is one of the specific additives. The content of vinylene carbonate in the liquid electrolyte is set according to the size of the surface area of ​​the negative electrode active material layer 22b, which is disposed in the same sealed space S as the liquid electrolyte. More specifically, the content of vinylene carbonate in the liquid electrolyte per surface area of ​​the negative electrode active material layer 22b is, for example, 3 mg / m ２ or more, preferably 4 mg / m ２ More preferably, it is 5 mg / m ２ The content of vinylene carbonate per surface area of ​​the negative electrode active material layer 22b is, for example, 20 mg / m ２ or less, preferably 15 mg / m ２ More preferably, it is 9 mg / m or less. ２ The following is the result.

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

[0075] The liquid electrolyte preferably contains a cyclic sulfonate ester, which is one of the specific additives. The cyclic sulfonate ester is an optional specific additive. Therefore, the liquid electrolyte may not contain a cyclic sulfonate ester.

[0076] Examples of the cyclic sulfonate ester include propane sultone and propene sultone. The content of the cyclic sulfonate ester in the liquid electrolyte is set according to the size of the surface area of ​​the negative electrode active material layer 22b that is disposed in the same sealed space S as the liquid electrolyte. More specifically, the content of the cyclic sulfonate ester in the liquid electrolyte per surface area of ​​the negative electrode active material layer 22b is, for example, 0.6 mg / m ２ or more, preferably 0.8 mg / m ２ More preferably, it is 1 mg / m ２ The content of the cyclic sulfonic acid ester per surface area of ​​the negative electrode active material layer 22b is, for example, 20 mg / m ２ or less, preferably 15 mg / m ２ More preferably, it is 9 mg / m or less. ２ The following is the result.

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

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

[0079] (Assembly Process) The assembly process includes an electrode forming process, a storage cell forming process, and a cell stack forming process.

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

[0081] The positive electrode active material layer 21b is formed by applying a positive electrode mixture, which will become the positive electrode active material layer 21b when solidified, to one surface of the positive electrode current collector 21a to a predetermined thickness, and then performing a solidification treatment appropriate for the positive electrode mixture. An example of the positive electrode mixture includes a positive electrode active material, an aqueous positive electrode binder, and an aqueous solvent such as water. The solid content of the positive electrode mixture 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 depositing a negative electrode composite, which will solidify to become the negative electrode active material layer 22b, to one surface of the negative electrode current collector 22a to a predetermined thickness, followed by a solidification treatment appropriate for the negative electrode composite. The negative electrode composite includes, for example, a negative electrode active material, graphite particles, carbon fiber, a negative electrode binder, and a dispersion medium such as water. The solid content of the negative electrode composite is, for example, 50% by mass or more and 70% by mass or less. The solid content of the negative electrode composite is preferably 65% ​​by mass or less, and more preferably 60% by mass or less. By reducing the solid content of the negative electrode composite, the resulting negative electrode active material layer 22b is more likely to have the dispersion state shown in FIG. 2, i.e., a specific dispersion state in which the fiber bundles F are dispersed.

[0083] In the energy storage cell formation process, first, 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 face each other in the stacking direction with the separator 23 sandwiched therebetween. In addition, a sealing material that will become the sealing portion 24 is arranged between the positive electrode 21 and the negative electrode 22 and on the outer periphery of the positive electrode current collector 21a and the negative electrode current collector 22a. As the sealing material, for example, a resin sheet made of an acid-modified polyolefin resin cut into the same shape as the sealing portion 24 in a planar view is used.

[0084] Thereafter, the positive electrode 21, the negative electrode 22, and the separator 23 are welded to the sealing material to form a single assembly in which the positive electrode 21, the negative electrode 22, the separator 23, and the sealing portion 24 are integrated. Examples of methods for bonding the sealing material include known welding methods such as heat welding, ultrasonic welding, and infrared welding.

[0085] Next, a liquid electrolyte is injected into the sealed space S inside the assembly unit through an injection port provided in a part of the seal portion 24, and then the injection port is sealed. In this way, an assembly unit that constitutes one storage cell 20 is formed.

[0086] In the cell stack forming process, first, a plurality of assembly units are stacked on top of each other so that the second surface 21a2 of the positive electrode current collector 21a faces the second surface 22a2 of the negative electrode current collector 22a. Then, the plurality of assembly units are integrated by bonding the outer circumferential portions 24a of the seal portions 24 of adjacent assembly units in the stacking direction.

[0087] Next, a positive electrode current-carrying plate 40 is overlapped and fixed in an electrically connected state to the second surface 21a2 of the positive electrode current collector 21a of the positive electrode 21 arranged outermost at one end in the stacking direction. Similarly, a negative electrode current-carrying plate 50 is overlapped and fixed in an electrically connected state to the second surface 22a2 of the negative electrode current collector 22a of the negative electrode 22 arranged outermost at the other end in the stacking direction. A battery assembly is formed through the above steps.

[0088] 3, the activation process includes an initial charging process and an aging process. The battery assembly is subjected to the activation process to obtain the power storage device 10.

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

[0090] The initial charging process includes a first charging process for charging up to a first voltage and a second charging process for charging from the first voltage to a target voltage. The first charging process and the second charging process have different charging rates. The charging rate of the first charging process is lower than the charging rate of the second charging process.

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

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

[0093] As described above, the decomposition voltage of lithium difluorophosphate is lower than that of vinylene carbonate. Therefore, by performing charging in the first charging step at a low charge rate, lithium difluorophosphate and vinylene carbonate decompose in the following order: lithium difluorophosphate, vinylene carbonate. The decomposition of vinylene carbonate may start after all of the lithium difluorophosphate has decomposed, or may start after the decomposition of lithium difluorophosphate has started but before all of the lithium difluorophosphate has decomposed.

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

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

[0096] The charging rate in the second charging step is higher than the charging rate in the first charging step, for example, more than 0.01 C, preferably 0.1 C or more, or 0.5 C or less.

[0097] By making the charge rate in the second charging step higher than the charge rate in the first charging step, the time required for the initial charging step can be shortened. Furthermore, by setting the first voltage to a low value within a range equal to or higher than the voltage at which lithium difluorophosphate and vinylene carbonate decompose, the time required for the initial charging step can be shortened. In other words, in this case, the period during which the battery is charged at a low charge rate in the first charging step is relatively short, and the period during which the battery is charged at a high charge rate in the second charging step is relatively long. As a result, the time required for the initial charging step can be shortened.

[0098] The aging process is a process in which the battery assembly in a high-voltage state after the initial charging process is held in a high-temperature environment for a predetermined time. The temperature in the aging process is, for example, 55° C. or higher and 70° C. or lower. The time for the aging process is, for example, 20 hours or higher and 50 hours or lower.

[0099] In the aging step, the state of the SEI film M formed on the surface of the graphite particles P changes from the state shown in Fig. 4(c) to the state shown in Fig. 4(d). More specifically, the SEI film M shown in Fig. 4(c) includes a portion where the second layer M2 made of a vinylene carbonate decomposition product is not formed, i.e., a portion where the first layer M1 or the graphite particles P is exposed. The SEI film M shown in Fig. 4(d) has no portion where the first layer M1 or the graphite particles P is exposed, or only a small portion.

[0100] Immediately after the initial charging step, the SEI film has a high proportion of the state shown in FIG. 4(c) and a low proportion of the state shown in FIG. 4(d). By performing the aging step, the state of the SEI film M changes from the state shown in FIG. 4(c) to the state shown in FIG. 4(d). As a result, the SEI film M in the state shown in FIG. 4(c) decreases, and the SEI film M in the state shown in FIG. 4(d) increases. The SEI film M in the state shown in FIG. 4(d) is superior as a protective film that suppresses contact between the graphite particles P and the liquid electrolyte because it covers the entire surface or a wide area of ​​the graphite particles P.

[0101] Next, the operation of this embodiment will be described. In the energy storage device 10, the negative electrode active material layer 22b has a weight per unit area of ​​200 g / m ２ In the conventional energy storage device, the weight of the negative electrode active material layer is adjusted to 200 g / m or more. ２ Increasing the weight of the negative electrode active material layer to this level results in a significant increase in the amount of gas generated from the electrode. Specifically, when the weight of the negative electrode active material layer is large, the amount of electrons released from the negative electrode due to self-discharge increases. The electrons released from the negative electrode react with the liquid electrolyte at the negative electrode to generate gas. Furthermore, the electrons released from the negative electrode reach the positive electrode through the liquid electrolyte and react with water contained in the positive electrode active material layer of the positive electrode to generate gas at the positive electrode. Therefore, when the weight of the negative electrode active material layer is increased, the amount of electrons released from the negative electrode due to self-discharge increases, resulting in a significant increase in the amount of gas generated from the electrode.

[0102] Here, in the energy storage device 10 of this embodiment, 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 contained in the liquid electrolyte. As a result, as shown in (c) and (d) of Figure 4, an SEI film M composed of decomposition products of lithium difluorophosphate and vinylene carbonate is formed on the surface of the graphite particles P. The SEI film M derived from lithium difluorophosphate and vinylene carbonate functions as a protective film that suppresses contact between the graphite particles P and the liquid electrolyte. By suppressing contact between the graphite particles P and the liquid electrolyte, self-discharge on the surface of the graphite particles P is suppressed. As a result, gas generation from the electrode due to electrons released due to self-discharge in the negative electrode active material layer 22b is reduced.

[0103] The SEI film M, which is made of decomposition products of lithium difluorophosphate and vinylene carbonate, has a first layer M1 made of decomposition products of lithium difluorophosphate and a second layer M2 made of decomposition products of vinylene carbonate. It is believed that the second layer M2 functions as a barrier to prevent contact with the liquid electrolyte, and the first layer M1 functions as an adhesive layer to firmly hold the second layer M2 around the graphite particles P.

[0104] Next, the effects of this embodiment will be described. (1) The energy storage device 10 includes a positive electrode 21 having a positive electrode active material layer 21 b, a negative electrode 22 having a negative electrode active material layer 22 b, 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, and the negative electrode active material layer 22 b has a basis weight of 200 g / m ２ The negative electrode active material layer 22b contains graphite particles, carbon fibers, and a negative electrode binder, and the liquid electrolyte contains lithium difluorophosphate and vinylene carbonate.

[0105] According to the above configuration, an SEI film M is formed on the graphite particles P, which functions as a protective film that suppresses contact between the graphite particles P and the liquid electrolyte. By suppressing contact between the graphite particles P and the liquid electrolyte, it is possible to suppress self-discharge on the surfaces of the graphite particles P. As a result, it is possible to reduce gas generation from the electrodes due to electrons emitted from the graphite particles P.

[0106] (2) The carbon fiber includes fiber bundles formed by bundling a plurality of single-walled carbon nanotubes, and the fiber bundles are in contact with a plurality of graphite particles across the fiber bundles. According to the above configuration, a decrease in the electronic conductivity of the anode active material layer 22b, which is caused by increasing the basis weight of the anode active material layer 22b, can be suppressed with a small amount of carbon fiber (conductive additive). In particular, the amount of conductive additive can be reduced compared to when a particulate conductive additive such as carbon black is used. Furthermore, when carbon black is used as the conductive additive, the carbon black may react with the liquid electrolyte during charging, causing the liquid electrolyte to decompose and generate gas. The use of carbon fiber also suppresses gas generation caused by charging and discharging the carbon black.

[0107] (3) The liquid electrolyte contains a cyclic sulfonic acid ester. According to the above configuration, an SEI film M is formed that is even more effective as a protective film that suppresses contact between the graphite particles P and the liquid electrolyte. This improves the effect of suppressing self-discharge on the surface of the graphite particles P. This further reduces gas generation from the electrode due to electrons emitted from the graphite particles P.

[0108] (4) The positive electrode active material layer 21b includes a positive electrode active material and a positive electrode binder. The positive electrode binder is an aqueous binder. The positive electrode active material layer 21b including the aqueous binder is formed using a positive electrode mixture including water, and therefore is likely to contain water such as crystal water therein. Therefore, the positive electrode active material layer 21b including the aqueous binder can be said to have a configuration that is likely to generate gas. By applying the configuration of (1) above to an electricity storage device that is likely to generate gas, a significant effect of reducing gas generation can be obtained.

[0109] (5) The non-aqueous solvent contained in the liquid electrolyte is an ester. Esters are more likely to generate gas due to self-discharge than other solvents such as carbonates. By applying the configuration of (1) above to an electricity storage device that is prone to gas generation, a significant effect of reducing gas generation can be achieved.

[0110] (6) A seal portion 24 is provided between the positive electrode 21 and the negative electrode 22 to form a sealed space S that contains a 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 swell and deform. By reducing gas generation, deformation of the electricity storage device 10 due to gas generation can be suppressed. Furthermore, peeling of the seal portion 24 due to an increase in the internal pressure of the sealed space S can be suppressed.

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

[0112] According to the above configuration, charging is performed at a relatively low charge rate in the first charging step. This makes it possible to more reliably form an SEI film having a first layer M1 made of a decomposition product of lithium difluorophosphate and a second layer M2 made of a decomposition product of vinylene carbonate on the surface of the graphite particles P constituting the negative electrode current collector 22a. Therefore, the effect of reducing the amount of gas generation based on the function of the SEI film M, which suppresses contact between the graphite particles P and the liquid electrolyte, is more significantly achieved. Furthermore, charging is performed at a relatively high charge rate in the second charging step. This makes it possible to shorten the time required for the initial charging step.

[0113] (8) The assembly process includes an electrode formation process in which a negative electrode mixture is applied to the surface of the negative electrode current collector 22a and dried to form the negative electrode active material layer 22b. The negative electrode mixture includes graphite particles, carbon fibers, a negative electrode binder, and a dispersion medium, and has a solid content concentration of 65 mass% or less. This configuration makes it easy to form a state in which the fiber bundles F of single-walled carbon nanotubes are in contact with a plurality of graphite particles P across the fiber bundles F.

[0114] This embodiment can be modified as follows. This embodiment and the following modifications can be combined with each other as long as there is no technical contradiction. The planar shapes of the positive electrode current collector 21a and the positive electrode active material layer 21b are not particularly limited. They may be polygonal, such as rectangular, or may be circular or elliptical. The same applies to the negative electrode current collector 22a and the negative electrode active material layer 22b.

[0115] The planar shape of the seal portion 24 is not particularly limited and may be a polygonal shape such as a rectangle, or may be a circle or an ellipse. A conductive layer that adheres closely to the positive electrode current collector 21a may be disposed between the positive electrode current-carrying plate 40 and the positive electrode current collector 21a to improve the conductive contact between the two components. Examples of the conductive layer include a layer containing carbon such as acetylene black or graphite, or a layer with a lower hardness than the positive electrode current collector 21a, such as a plating layer containing Au. A similar conductive layer may also be disposed between the negative electrode current-carrying plate 50 and the negative electrode current collector 22a.

[0116] The number of storage cells 20 constituting the energy storage device 10 is not particularly limited. The number of storage cells 20 constituting the energy storage device 10 may be one. The positive electrode active material layer 21b or the negative electrode active material layer 22b may be provided on the second surface 21a2 of the positive electrode current collector 21a. Furthermore, the positive electrode active material layer 21b or the 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 include a restraining member that restrains the cell stack 30. The restraining member applies a restraining load to the region where the energy storage cells 20 face each other in the stacking direction of the cell stack 30, particularly to the region where the area where the positive electrode active material layer 21 b is provided overlaps with the area where the negative electrode active material layer 22 b is provided in a plan view.

[0118] The specific configuration of the restraining member is not particularly limited as long as it is capable of applying a restraining load to the cell stack 30. For example, the restraining member may be configured to include plate-shaped restraining plates disposed at both ends of the cell stack 30 in the stacking direction so as to sandwich the cell stack 30, and fastening members made of bolts and nuts that fasten the restraining plates together. In the case of a restraining member configured as described above, the fastening members urge the restraining plates in directions that bring them closer to each other, thereby applying a restraining load to the cell stack 30 in the stacking direction.

[0119] In the activation process, a charge / discharge process may be performed as necessary. The charge / discharge process is performed, for example, before or after the aging process. The charge / discharge process is a process in which multiple cycles of charge / discharge are performed, with one cycle consisting of discharge and charge. For example, in the case of a charge / discharge process performed before the aging process, multiple cycles of charge / discharge are performed, with one cycle consisting of discharge to 0% SOC and charge to 15% SOC. In the case of a charge / discharge process performed after the aging process, multiple cycles of charge / discharge are performed, with one cycle consisting of discharge to 0% SOC and charge to 100% SOC.

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

[0121] The charge rate in the first charging step may be constant or may vary. For example, the charge rate may increase stepwise, or there may be a period during which the charge rate remains constant. Similarly, the charge rate in the first charging step may be constant or may vary.

[0122] The charging rate in the first charging step may be more than 0.01 C. Even in this case, the amount of graphite particles P having the SEI film M is reduced, but the graphite particles P themselves are formed.

[0123] Examples that further specify the above-described embodiment will be described below. (Example 1) A positive electrode composite was applied to one surface of an aluminum foil having a thickness of 35 μm. The applied positive electrode composite was dried and pressed to produce a positive electrode sheet in which a positive electrode active material layer was formed on the surface of a positive electrode current collector. The basis weight of the positive electrode active material layer was 750 g / m ２ The positive electrode mixture was LiFePO ４ A slurry containing single-walled carbon nanotubes, carboxymethyl cellulose sodium salt, and styrene-butadiene rubber in a solid mass ratio of 98.55:0.05:0.4:1.0, with water as the solvent, and having a solid concentration of 75% by mass, was used.

[0124] The negative electrode composite was applied to one side of a copper foil having a thickness of 12 μm. The applied negative electrode composite was dried and pressed to prepare a negative electrode sheet in which a negative electrode active material layer was formed on the surface of a negative electrode current collector. The weight of the negative electrode active material layer was 378 g / m ２ The negative electrode mixture used was a slurry containing graphite, single-walled carbon nanotubes, carboxymethyl cellulose sodium salt, styrene-butadiene rubber, and a thickener in a solid content mass ratio of 97.17:0.01:0.4:2.3:0.12, with water as the solvent, and having a solid content concentration of 56% by mass.

[0125] A separator was sandwiched between a positive electrode made from a positive electrode sheet cut to 25 mm × 30 mm and a negative electrode made from a negative electrode sheet cut to 26 mm × 31 mm, and the resulting electrode assembly was housed in a laminate exterior material. A liquid electrolyte was injected into the exterior material, and the exterior material was sealed to obtain the electricity storage device of Example 1.

[0126] The liquid electrolyte was a mixture of ethylene carbonate and methyl propionate in a volume ratio of 15:85, with LiN(FSO ２ ) ２was dissolved to a concentration of 1.4 M, and the additives shown in Table 1 were added. In Example 1, a liquid electrolyte containing lithium difluorophosphate and vinylene carbonate as additives was used. In Table 1, the numerical value in the column for each additive indicates the content of the additive per surface area of ​​the negative electrode active material layer 22b, and the numerical value in parentheses below it indicates the concentration of the additive in the liquid electrolyte.

[0127] (Examples 2 to 4) Power storage devices of Examples 2 to 4 were obtained in the same manner as Example 1, except that the additives added to the liquid electrolyte were different. As shown in Table 1, in Examples 2 and 3, a liquid electrolyte to which lithium difluorophosphate, vinylene carbonate, and propene sultone were added was used. The amount of propene sultone added differed between Examples 2 and 3. In Example 4, a liquid electrolyte to which lithium difluorophosphate, vinylene carbonate, and propane sultone were added was used.

[0128] (Comparative Examples 1 and 2) Power storage devices of Comparative Examples 1 and 2 were obtained in the same manner as in Example 1, except that the additives added to the liquid electrolyte were different. As shown in Table 1, in Comparative Example 1, a liquid electrolyte to which vinylene carbonate was added was used. In Comparative Example 2, a liquid electrolyte to which vinylene carbonate and lithium difluoro(oxalato)borate were added was used. The content of each additive in the liquid electrolyte is as shown in Table 1.

[0129]

[0130] The additives shown in Table 1 are as follows: LiPO ２ F ２: Lithium difluorophosphate VC : Vinylene carbonate PS : 1,3-propane sultone PRS : 1-propene 1,3-sultone DFOB : Lithium difluoro(oxalato)borate (Measurement of gas generation amount) Each of the power storage devices of the examples and comparative examples was charged to 2.7 V (first voltage) at a constant current of 0.005 C. Next, after charging to 3.75 V (target voltage) at a constant current of 0.05 C, the power storage device in a high voltage state was held at 65° C. for 35 hours. Thereafter, it was discharged to 3.0 V.

[0131] The volume of the electricity storage device after discharge was measured, and this measured value was designated as the initial volume. Thereafter, the electricity storage device after discharge was stored at 60°C for several days, and the volume of the electricity storage device was measured periodically during the storage period, and the increase in volume from the initial volume was calculated. The calculated increase in volume was designated as the amount of gas generated. The results are shown in the graph of FIG. 5. The volume of the electricity storage device was measured by Archimedes' method using an electronic hydrometer (MDS-300 manufactured by Alpha Mirage).

[0132] As described above, Comparative Example 1 is an example in which a liquid electrolyte containing only vinylene carbonate was used, and Comparative Example 2 is an example in which vinylene carbonate and the comparative additive DFOB were added. As shown in Figure 5, the amount of gas generated in Comparative Examples 1 and 2 increased with increasing storage days. This result indicates that gas continued to be generated in the devices of Comparative Examples 1 and 2.

[0133] Here, detailed data is omitted, but as a reference test, the weight of the negative electrode active material layer was reduced (100 g / m ２ When a similar test was carried out using an electricity storage device obtained in the same manner as in Comparative Example 1 except that the negative electrode active material layer was changed to a negative electrode active material layer having a mass per unit area of ​​1000 sq. m, almost no gas generation was confirmed. This result indicates that the gas generation in Comparative Examples 1 and 2 is due to the negative electrode active material layer having a mass per unit area of ​​1000 sq. m or more.

[0134] 5, in Example 1, which used a liquid electrolyte containing lithium difluorophosphate and vinylene carbonate, the amount of gas generated with increasing storage time was reduced to less than half compared to Comparative Examples 1 and 2. This result demonstrates that the simultaneous addition of lithium difluorophosphate and vinylene carbonate can reduce gas generation from the electrodes.

[0135] Examples 2 to 4 are examples in which a liquid electrolyte containing propane sultone or propene sultone in addition to lithium difluorophosphate and vinylene carbonate was used. As shown in Figure 5, no gas generation was observed in Examples 2 to 4. These results demonstrate that gas generation from the electrodes can be significantly reduced or eliminated by further adding a cyclic sulfonate ester such as propane sultone or propene sultone.

[0136] Although detailed data is omitted, as an additional test, a power storage device was obtained in the same manner as in Example 1, except that the single-walled carbon nanotubes (1% by mass) were replaced with particulate carbon black in an amount (3.8% by mass) that provided an equivalent capacity retention rate. When the amount of gas generated from this power storage device was measured, the amount of gas generated was greater than that of Example 1. This result shows that the effect of reducing the amount of gas generated in each example is an effect obtained when carbon fiber is used as the conductive additive.

[0137] F... fiber bundle P... graphite particles S... sealed space 10... power storage device 21... positive electrode 21b... positive electrode active material layer 22... negative electrode 22b... negative electrode active material layer 23... separator 24... seal portion

Claims

1. A power 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, wherein the basis weight of the negative electrode active material layer is 200 g / m ２ or more, wherein the negative electrode active material layer includes graphite particles, carbon fibers, and a negative electrode binder, and the liquid electrolyte includes lithium difluorophosphate and vinylene carbonate.

2. The carbon fiber includes a fiber bundle formed by bundling a plurality of single-walled carbon nanotubes, and the fiber bundle is in contact across a plurality of the graphite particles. The power storage device according to claim 1.

3. The average particle diameter (D50) of the graphite particles is 3 μm or more and 30 μm or less, and the fiber length of the single-walled carbon nanotubes is 5 μm or more and 50 μm or less. The power storage device according to claim 1 or claim 2.

4. The liquid electrolyte contains a cyclic sulfonic acid ester. The power storage device according to any one of claims 1 to 3.

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

6. The power storage device according to any one of claims 1 to 5, further comprising a seal portion that forms a sealed space for accommodating the liquid electrolyte between the positive electrode and the negative electrode.

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

8. The liquid electrolyte contains a cyclic sulfonic acid ester, the content of the graphite particles in the negative electrode active material layer is 97% by mass or more, the average particle diameter (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 m ２ / g or more and 1.5 m ２ / g or less. The content of the cyclic sulfonic acid ester per unit surface area of the negative electrode active material layer in the liquid electrolyte is 0.6 mg / m ２ or more. The power storage device according to any one of claims 1 to 7.

9. A method for manufacturing a power storage device according to any one of claims 1 to 8, including an assembling step of assembling a battery assembly including the positive electrode, the negative electrode, the separator, and the liquid electrolyte, and a first charging step for the battery assembly. The first charging step includes a first charging step of charging to a first voltage of 2.0 V or more and 3.0 V or less, and a second charging step of charging from the first voltage to a predetermined target voltage. The charging rate of the second charging step is higher than the charging rate of the first charging step. A method for manufacturing a power storage device characterized by this.

10. The assembling step includes an electrode forming step of forming the negative electrode active material layer by applying and drying a negative electrode composite material on the surface of a negative electrode current collector. The negative electrode composite material includes graphite particles, carbon fibers, a negative electrode binder, and a dispersion medium, and has a solid content concentration of 65% by mass or less. The method for manufacturing a power storage device according to claim 9.

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