Power storage device and method of manufacturing the same

Abstract

To provide a power storage device which maintains a capacitance maintaining rate and a high-rate characteristic and furthermore has such high safety that deposition of a metallic lithium is suppressed, and to provide a method of manufacturing the power storage device.SOLUTION: A power storage device comprises: a positive electrode having a positive electrode current collector foil and a positive electrode active material layer; a negative electrode having a negative electrode current collector foil and a negative electrode active material layer; and a separator which is interposed between the positive electrode and the negative electrode. The negative electrode active material layer includes: an active material; and a binder, and is constructed by lamination structure formed by a n-layer (n is two or more natural numbers) from a first layer which is approached to the negative electrode current collector foil to a nth layer which is approached to the separator. Here, when a solid weight ratio of all of the n-th layer is 100wt%, a binder content ratio (A) of the n-th layer is 5.0wt% or more and 35.0wt% or less, and when the solid weight ratio of all of the first layer is 100wt%, a binder content ratio (B) of the first layer satisfies the following equation of (1): (1)1.10≤A / B≤62.50. A mean resistance value of the negative electrode active material layer is 15 Ω / cm2 or more and 50 Ω / cm2 or less.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to a power storage device and a method for manufacturing the same.

Background Art

[0002] Power storage devices such as lithium-ion secondary batteries, nickel-metal hydride batteries, other secondary batteries, or capacitors are preferably used as power sources for portable devices such as personal computers and mobile terminals, or as high-output power sources for vehicles such as vehicle drive power sources for battery electric vehicles (BEVs) due to their light weight and high energy density. Electrodes included in such power storage devices include a current collector foil and a composite layer formed on the surface of the current collector foil. The composite layer is mainly composed of an active material. Typically, the composite layer also contains a conductive assistant and a binder. The binder contributes to maintaining the electrode structure, such as binding the active material to the current collector foil and binding the active materials to each other.

[0003] As power storage devices expand and develop in applications, it is required that they can be charged (or discharged) with a large current in a short time. In response to such needs, techniques for improving the capacity retention rate of power storage devices by reducing the electrical resistance of electrodes are known. For example, Japanese Patent Application Laid-Open No. 2018-174096 discloses an electrode having an active material layer in which the contents of the conductive assistant and the binder on the surface side are higher than the contents of the conductive assistant and the binder on the current collector base material side. The power storage element suppresses a decrease in input / output performance after repeated charge and discharge by reducing the contact resistance between the metal foil and the positive electrode active material. Japanese Patent Application Laid-Open No. 2022-100812 discloses an electrode having an active material layer in which the binder content on the side closest to the current collector is lower than the binder content on the side closest to the surface when the active material layer is divided into six equal parts. This document discloses a technique for reducing electrical resistance and maintaining cycle characteristics by further containing carbon nanotubes as a conductive assistant in the positive electrode active material layer.

Prior Art Documents

Patent Documents

[0004] Patent Document 1 Japanese Patent Application Laid-Open No. 2018-174096 Patent Document 2 Japanese Patent Application Laid-Open No. 2022-100812 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] By the way, in a power storage device, a metal (for example, metallic lithium or the like) serving as a charge carrier may be deposited on the surface of the negative electrode composite material layer (that is, the interface between the negative electrode composite material layer and the separator) during charging. In such a case, since the charge carriers (for example, lithium ions or the like) in the composite material layer decrease, the capacity retention rate after cycling decreases.

[0006] The present disclosure aims to provide a highly safe power storage device that suppresses the precipitation of a metal serving as a charge carrier while maintaining the capacity retention rate. MEANS FOR SOLVING THE PROBLEMS

[0007] The power storage device disclosed herein includes a positive electrode having a positive electrode current collector foil and a positive electrode composite material layer, a negative electrode having a negative electrode current collector foil and a negative electrode composite material layer, and a separator interposed between the positive electrode and the negative electrode. The negative electrode composite material layer contains an active material and a binder, and the negative electrode composite material layer is configured by a laminated structure including n layers (n is a natural number of 2 or more) from a first layer close to the current collector foil to an nth layer close to the separator. Here, when the total solid weight of the nth layer is 100 wt%, the binder content rate (A) of the nth layer is 8.0 wt% or more and 25.0 wt% or less, and the binder content rate (B) of the first layer when the total solid weight of the first layer is 100 wt% satisfies the formula (1) 1.10 ≦ A / B ≦ 62.50. Further, the average resistance value from the first layer close to the current collector foil to the nth layer close to the separator is 15 Ω / cm 2 or more and 50 Ω / cm 2 or less.

[0008] In a power storage device where the resistance value of the negative electrode is too low (the conductivity of charge carriers is too high), a large amount of charge carriers rapidly move from the positive electrode to the negative electrode during charging. As a result, the precipitation of the metal serving as the charge carrier is promoted on the surface of the composite material layer. On the other hand, as a result of the inventors' intensive studies, it has been found that by increasing the binder content rate (A) of the surface layer of the negative electrode composite material layer (specifically, 5.0 wt% to 35.0 wt%), the precipitation of the metal serving as the charge carrier can be suppressed. That is, according to such a power storage device, the resistance value of the surface layer of the negative electrode composite material layer preferably increases, and the conductivity of charge carriers (for example, lithium ions, etc.) on the surface layer side of the negative electrode composite material layer decreases. As a result, when charge carriers move from the positive electrode to the negative electrode during charging, the charge carriers can be uniformly occluded in the entire negative electrode composite material layer. As a result, the precipitation of the metal (for example, metallic lithium, etc.) serving as the charge carrier can be suppressed, and the capacity retention rate after cycling can be improved. Also, by making the binder content rates different between the surface layer and the bottom layer of the negative electrode composite material layer, the average resistance value of the negative electrode can be suppressed to 50 Ω / cm 2 or less. Specifically, the binder content rate (A) of the surface layer of the negative electrode composite material layer and the binder content rate (B) of the layer adjacent to the current collector foil are made to satisfy 1.10 ≦ A / B ≦ 62.50. By doing so, even if the vicinity of the surface layer has a high resistance, the conductivity in the vicinity of the current collector foil is improved, and while improving the capacity retention rate of the power storage device, it is also possible to suppress a decrease in battery performance.

[0009] In a power storage device according to one aspect disclosed herein, the binder content rate (C) of any layer of the negative electrode composite material layer and the binder content rate (D) of the layer adjacent to the negative electrode current collector foil side in the thickness direction of the any layer satisfy the formula (2) 1.10 ≦ C / D ≦ 16.10. As a result, the difference in the conductivity of charge carriers between two adjacent layers becomes small, and the precipitation of the metal serving as the charge carrier between the layers can be suppressed.

[0010] In a power storage device according to one aspect disclosed herein, the negative electrode composite material layer has three or more layers. As a result, it becomes possible to more reliably suppress the precipitation of the metal serving as the charge carrier.

[0011] In one aspect of the power storage device disclosed herein, the binder content rate gradually increases from the first layer to the nth layer in the thickness direction of the negative electrode composite material layer. As a result, in the negative electrode composite material layer, a resistance value gradient can be formed from the vicinity of the negative electrode current collector foil to the vicinity of the separator. Such an effect is that the resistance value decreases as approaching the negative electrode current collector foil in the negative electrode composite material layer. As a result, during charging, charge carriers can easily diffuse and can be uniformly moved throughout the negative electrode composite material layer.

[0012] A method for manufacturing an electrode according to one aspect disclosed herein includes an electrode current collector foil and an electrode composite material layer, and is a method for manufacturing an electrode having a laminated structure composed of n layers (n is a natural number of 2 or more) from the first layer adjacent to the electrode current collector foil to the nth layer on the surface in the thickness direction of the electrode composite material layer. The method includes a step of preparing the composite materials of the first layer to the nth layer, and a step of sequentially laminating and drying the composite material layers from the first layer to the nth layer on the electrode current collector foil. Further, the drying after the second layer is performed so that the binder contained in the composite material composition does not move to other layers due to the convection of the solvent. Further, in the composite material layers from the first layer to the nth layer, the binder content rate increases as 1 to n increases.

[0013] In such a method for manufacturing an electrode, drying is performed while preventing the binder contained in the composite material layer from moving (migrating) to different layers due to the convection of the solvent along with excessive heating or evaporation of the solvent in the composite material layer having a laminated structure. As a result, a concentration gradient of the binder can be formed in the composite material layer from the vicinity of the current collector foil to the vicinity of the separator. As a result, it is possible to manufacture an electrode that suppresses the precipitation of a metal (for example, metallic lithium, etc.) serving as a charge carrier and improves the capacity retention rate after cycling. Further, the resistance value on the current collector foil side of the composite material layer can be decreased, and an electrode that suppresses the decrease in conductivity in the vicinity of the current collector foil can be manufactured.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Mode for Carrying Out the Invention

[0015] <Definition of Terms> Hereinafter, one embodiment of a power storage device in the present disclosure will be described in detail with reference to the drawings. Matters other than those specifically mentioned in this specification and necessary for implementation (for example, general configurations and manufacturing processes of power storage devices that do not characterize the present disclosure, etc.) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and common general technical knowledge in the relevant field. In the following drawings, members and parts having the same function are denoted by the same reference numerals for description. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships.

[0016] As used herein, the term "energy storage device" is a concept that includes devices in which a charge-discharge reaction occurs due to the movement of charge carriers between a pair of electrodes (a positive electrode and a negative electrode). That is, energy storage devices include batteries such as secondary batteries (e.g., lithium-ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries), and capacitors (physical batteries) such as lithium-ion capacitors and electric double-layer capacitors. Further, as used herein, the term "lithium-ion secondary battery" refers to an energy storage device that uses lithium ions as charge carriers and realizes repeated charge and discharge by the movement of charges associated with the lithium ions between the positive and negative electrodes.

[0017] When a numerical range is described in this specification as "A to B (where A and B are arbitrary numerical values)", it means "A or more and B or less", and also includes the meanings of "more than A and less than B", "more than A and B or less", and "A or more and less than B".

[0018] <Lithium-ion secondary battery> Hereinafter, a lithium-ion secondary battery, which is one of the embodiments in the present disclosure, will be described. FIG. 1 is a schematic longitudinal sectional view of a lithium-ion secondary battery 1 according to an embodiment. In the following description, the reference signs L, R, U, and D in the drawings represent the left, right, top, and bottom of the lithium-ion secondary battery 1. However, these are merely directions for convenience of explanation and do not limit the installation form of the energy storage device in any way.

[0019] <Battery case> As shown in FIG. 1, in the lithium-ion battery 1 according to the present embodiment, the shape of the battery case 10 is a rectangular parallelepiped shape and is a flat rectangle. However, the shape of the battery case 10 is not limited to this, and for example, it may be a cylindrical shape. The battery case 10 includes a battery case body 11 having an opening, and a sealing plate (lid body) that seals the opening. The battery case body 11 houses the electrode body 20 and an electrolytic solution (not shown). The battery case body 11 and the sealing plate 12 are welded and sealed by laser welding or the like. The material of the battery case 10 may be the same as that used in conventional power storage devices of this type, and there is no particular limitation. As an example, a lightweight and highly thermally conductive metal material such as aluminum is used for the material of the battery case 10. However, it is also possible to change the configuration of the battery case 10. For example, a flexible laminate film may be used as the battery case. In the present embodiment, the battery case body 11 is composed of a bottom wall, a pair of short side walls extending from the bottom wall and facing each other, and a long side wall. In the present embodiment, the sealing plate 12 of the battery case 10 is provided with a thin safety valve 13 set to release the internal pressure of the battery case when the internal pressure of the battery case rises above a predetermined level, and an injection port (not shown) for injecting the electrolytic solution.

[0020] An external positive electrode terminal 14 and an external negative electrode terminal 15 for external connection are provided outside the battery case. These electrode terminals are electrically connected to the electrode body 20 housed in the battery case via internal terminals 16 and 17. The external terminals 14 and 15 are made of metal. As the external positive electrode terminal 14, for example, aluminum or an aluminum alloy can be used. As the external negative electrode terminal 15, for example, copper or a copper alloy can be used.

[0021] The internal terminals 16 and 17 are made of metal. As the positive electrode internal terminal 16, for example, aluminum or an aluminum alloy can be used from the viewpoint of improving the bonding strength with the non-formed portion 31a of the positive electrode composite material layer. As the negative electrode internal terminal 17, for example, copper or a copper alloy can be used from the viewpoint of improving the bonding strength with the non-formed portion 41a of the negative electrode composite material layer.

[0022] <Electrolyte> As the electrolyte, a non-aqueous electrolyte obtained by dissolving a supporting salt in a suitable non-aqueous solvent can be used. Conventionally known non-aqueous electrolytes can be adopted without particular limitation. As an example of the non-aqueous solvent, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. can be used. Also, as an example of the supporting salt, a lithium salt (for example, LiBOB, LiPF6, etc.) can be used.

[0023] <Electrode body> In the present embodiment, the electrode body 20 is a laminated electrode body in which a rectangular positive electrode 30 and a rectangular negative electrode 40 are alternately stacked via a rectangular separator. FIG. 2 is an exploded perspective view schematically showing a cell unit constituting the laminated electrode body 20 which is one of the present embodiments. In the following description, the symbols LR, T, and UD in the drawings represent the width direction, the thickness direction, and the depth direction of the laminated electrode body 20. However, these are merely directions for convenience of explanation and do not limit the installation form of the lithium ion secondary battery 1 in any way. Note that the electrode body 20 is not limited to this and may be a wound electrode body in which a positive electrode and a negative electrode are wound via a separator. Also, the number of electrode bodies 20 is not particularly limited. A plurality of electrode bodies (for example, stacking a plurality of the above cell units, etc.) may be accommodated in the battery case 10.

[0024] The negative electrode 40 of the lithium ion battery 1 according to the present embodiment has a composite material layer 42 with a multilayer structure. FIG. 3 is a schematic cross-sectional view showing the laminated structure in the electrode body 20. The positive electrode 30 includes a positive electrode current collector foil 31 and a positive electrode composite material layer 32. Also, the negative electrode 40 includes a negative electrode current collector foil 41 and a negative electrode composite material layer 42. A separator 50 is interposed between the positive electrode 30 and the negative electrode 40.

[0025] <Negative electrode and negative electrode composite material layer> As shown in FIG. 2, the negative electrode 40 includes a rectangular negative electrode current collector foil 41 and a negative electrode composite material layer 42 formed on the surface of the negative electrode current collector foil 41. The negative electrode composite material layer 42 may be formed on one side or both sides (here, both sides) of the negative electrode current collector foil 41. Further, as shown in FIGS. 1 and 2, the negative electrode 40 may have a non-formed portion 41a of the negative electrode composite material layer where the negative electrode current collector foil 41 is exposed without the negative electrode composite material layer 42 being formed in a part. Here, the non-formed portion 41a of the negative electrode composite material layer is provided so as to protrude from one end of the laminated electrode body 20. The negative electrode internal terminal 17 can be joined to the non-formed portion 41a of the negative electrode composite material layer.

[0026] The negative electrode current collector foil 41 is rectangular. The shape and dimensions of the negative electrode current collector foil 41 are not particularly limited and may be appropriately determined according to the battery design. The material of the negative electrode current collector foil 41 may be a known material for a negative electrode current collector foil used in a power storage device and is not particularly limited. The material of the negative electrode current collector foil 41 is, for example, copper or a copper alloy. From the balance between the capacity density of the power storage device and the strength of the current collector, the lower limit of the film thickness of the negative electrode current collector foil 41 is preferably 3 μm or more, more preferably 5 μm or more, and most preferably 8 μm or more. The upper limit is preferably 40 μm or less, more preferably 25 μm or less, and most preferably 15 μm or less.

[0027] The negative electrode composite material layer 42 contains at least a negative electrode active material and a binder. As the negative electrode active material, a negative electrode active material used for a negative electrode of a general lithium-ion secondary battery can be used. For example, the negative electrode active material includes carbon materials such as soft carbon (easily graphitizable carbon), amorphous carbon materials, graphite, hard carbon (difficultly graphitizable carbon), carbon nanotubes, metal oxide materials such as silicon oxide, titanium oxide, vanadium oxide, lithium titanium composite oxides, metal nitride materials such as lithium nitride, lithium cobalt composite nitrides, and silicon compounds. Among these, from the viewpoint of improving the energy density, the negative electrode active material 45 is preferably graphite. These negative electrode active materials may be used alone or in combination of two or more. Further, as long as the effects of the technology according to the present disclosure are not significantly impaired, a conductive assistant, an inorganic filler, etc. may be added to each layer.

[0028] The negative electrode active material may be in particulate form. In that case, the average particle size of the negative electrode active material is not particularly limited. The average particle size of the negative electrode active material is typically 30 μm or less. If the average particle size is small, the electrode density increases, which can contribute to increasing the capacity of the power storage device. The upper limit of the average particle size of this negative electrode active material is preferably 25 μm or less, more preferably 20 μm or less. The average particle size of the active material can be determined by the 50 volume% particle size (D50 particle size) based on the laser diffraction / scattering method.

[0029] As the binder for the negative electrode composite material layer 42, binders used for the negative electrodes of general lithium-ion secondary batteries and lithium-ion capacitors can be used. For example, binders used in non-aqueous pastes include vinyl halide resins such as polyvinylidene fluoride (PVDF) and polyvinylidene chloride (PVDC), and polyalkylene oxides such as polyethylene oxide (PEO). When using an aqueous paste, a water-soluble polymer material or a water-dispersible polymer material can preferably be employed. For example, binders used in aqueous pastes include polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), etc. Since a relatively large amount of binder is used in the surface layer 44 of the negative electrode composite material layer 42 of the present disclosure, those with low cost and less environmental load are preferred. Therefore, the binder used for the negative electrode composite material layer 42 is preferably CMC or SBR. These binders may be used alone or in combination of two or more. Also, these binders may be of the same type from the first layer 42a to the negative electrode surface layer, or different types may be used.

[0030] Examples of the conductive auxiliary agent included in the negative electrode composite material layer 42 include metal powders such as silver, gold, or copper. Examples of the inorganic filler include aluminum hydroxide, silica, alumina, zirconia, titania, boehmite, or magnesia.

[0031] The negative electrode composite material layer 42 is composed of an n-layer (n is a natural number of 2 or more) laminated structure from the first layer close to the negative electrode current collector foil 41 to the nth layer close to the separator 50. In other words, the negative electrode composite material layer 42 of the present embodiment has a multilayer structure in which at least two or more composite material layers are laminated. Here, the layer close to the separator 50 in the negative electrode composite material layer 42 (that is, the nth layer 42n) is defined as the surface layer 44. Further, when the negative electrode composite material layer 42 is composed of a laminated structure of three or more layers, the layer provided between the first layer 42a close to the negative electrode current collector foil 41 and the surface layer 44 close to the separator 50 is defined as the intermediate layer 43. As shown in FIG. 3, the negative electrode composite material layer 42 of the present embodiment has the first layer 42a provided on the negative electrode current collector foil 41 and an n-layer composite material layer (intermediate layer 43 and surface layer 44) provided on the first layer 42a. In this specification, unless otherwise specified, the intermediate layer 43 refers to one or a plurality of layers.

[0032] The number of layers of the negative electrode composite material layer 42 is not particularly limited as long as the technology of the present disclosure is not significantly impaired. That is, the number of layers of the negative electrode composite material layer 42 is composed of a laminated structure of two or more layers. For example, as shown in FIG. 4, when n = 2, in one embodiment, the negative electrode composite material layer 42 has two layers. In this case, the surface layer 44 refers to the second layer 42b. As shown in FIG. 5, when n = 3, in one embodiment, the negative electrode composite material layer 42 has three layers. In this case, the negative electrode intermediate layer 43 refers to the second layer 42b. Further, the surface layer 44 refers to the third layer 42c. As shown in FIG. 6, when n = 4, in one embodiment, the negative electrode composite material layer 42 has four layers. In this case, the intermediate layer 43 refers to the second layer 42b and the third layer 42c. Further, the surface layer 44 refers to the fourth layer 42d.

[0033] The thickness of each layer from the first layer 42a to the surface layer 44 is not particularly limited. However, from the viewpoint of more reliably suppressing the precipitation of metallic lithium, it is preferable to provide a certain thickness for the surface layer 44. The upper limit value of the thickness of the surface layer 44 is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 60 μm or less from the viewpoint of maintaining battery performance. The lower limit value of the thickness of the surface layer 44 is preferably 10 μm or more, more preferably 20 μm or more, and even more preferably 30 μm or more from the viewpoint of suppressing the precipitation of metallic lithium. Also, from the viewpoint of maintaining the capacity of the lithium-ion battery, it is preferable to provide a certain thickness for each of the first layer 42a and the intermediate layer 43. The upper limit value of the thickness of the first layer 42a or the intermediate layer 43 is preferably 200 μm or less, more preferably 180 μm or less, and even more preferably 160 μm or less from the viewpoint of suppressing the precipitation of metallic lithium. The lower limit value of the thickness of the first layer 42a or the intermediate layer 43 is preferably 30 μm or more, more preferably 40 μm or more, and even more preferably 50 μm or more from the viewpoint of improving the capacity retention rate after cycling.

[0034] In addition, the thickness of each layer included in the negative electrode composite material layer 42 can be measured according to the following procedure. First, measurement by the TEM-EELS spectrum analysis method is carried out under the condition of full discharge, and the binder content at each of 8 measurement points along the thickness direction of the negative electrode composite material layer 42 is measured. Next, the change amount of the binder content between two adjacent measurement points is calculated. Then, when three or more measurement points where the change amount of the binder content rate exceeds 1.5% are continuous, the central measurement point of the continuous measurement points is regarded as the portion where the layer has switched. Based on this portion where the layer has switched, the thickness of each layer can be measured.

[0035] The content ratio (A) of the binder in the n-th layer 42n (i.e., the surface layer 44) typically satisfies 5.0 wt% or more and 35.0 wt% or less when the total weight of the solid content of the n-th layer 42n (i.e., the surface layer 44) is 100 wt%. By having a high binder content ratio (specifically, 5.0 wt% or more) in the n-th layer 42n (i.e., the surface layer 44), the precipitation of metallic lithium can be suppressed, and the capacity retention rate after cycling can be improved. Also, since the binder content ratio (A) of the n-th layer 42n (i.e., the surface layer 44) is 35.0 wt% or less, lithium ions can be occluded while maintaining the conductivity of the entire negative electrode. From the viewpoint of further suppressing the precipitation of metallic lithium, the lower limit value is preferably 8.0 wt% or more, more preferably 10.0 wt% or more, and most preferably 13.0 wt% or more. Also, by reducing the content ratio of the binder, it can contribute to ensuring the conductivity of lithium ions. Therefore, the upper limit value is preferably 30.0 wt% or less, more preferably 25.0 wt% or less, and even more preferably 20.0 wt% or less. Note that as the number of layers increases, the thickness of the composite layer also increases, and the mechanical strength of the composite layer becomes higher. On the other hand, a composite layer with a large thickness tends to lack flexibility and is likely to crack in the manufacturing process of the power storage device. In particular, stress concentrates near the surface of the composite layer due to winding or the like. The composite layer 42 of the present embodiment includes a binder with a high content ratio as it approaches the surface layer 44. For this reason, the binding property increases as it approaches the surface layer 44, and the occurrence of cracks in the manufacturing process of the power storage device can be reduced.

[0036] The content ratio (B) of the binder in the first layer of the negative electrode composite material layer adjacent to the negative electrode current collector foil disclosed herein is determined by the ratio to the content ratio (A) of the binder in the surface layer 44 (i.e., the nth layer 42n). That is, when the total solid weight of the first layer is 100 wt%, the formula (1) 1.10 ≦ A / B ≦ 62.50 is satisfied. Thereby, during charging, lithium ions can easily reach the vicinity of the current collector foil of the negative electrode composite material layer, and the battery performance can be maintained. Also, the upper limit value of A / B may be 100 or less. When A / B exceeds 100, the average resistance value of the entire negative electrode increases, and the high-rate characteristics deteriorate. From the viewpoint of maintaining battery performance, the upper limit value of A / B may be 45.00 or less, preferably 32.50 or less, more preferably 20.00 or less, and even more preferably 16.25 or less. From the viewpoint of suppressing the precipitation of metallic lithium, the lower limit value of A / B is preferably 3.25 or more, more preferably 4.50 or more, and even more preferably 6.50 or more.

[0037] The average resistance value of the negative electrode composite material layer 42 is 15 Ω / cm 2 or more and 50 Ω / cm 2 or less. Thereby, the effects of suppressing the precipitation of lithium ions and ensuring conductivity are achieved. From the viewpoint of further suppressing the precipitation of lithium ions, the lower limit value is preferably 20 Ω / cm 2 or more, more preferably 22 Ω / cm 2 or more, and most preferably 25 Ω / cm 2 or more. Also, from the viewpoint of maintaining the conductivity of lithium ions, the upper limit value of the average resistance value is preferably 48 Ω / cm 2 or less, more preferably 45 Ω / cm 2 or less, and even more preferably 43 Ω / cm 2 or less.

[0038] A power storage device including an electrode having a composite material layer 42 with a multilayer laminated structure in which the binder content increases from the current collector foil 41 side toward the separator 50 side as described above can suppress the precipitation of metallic lithium without reducing the capacity retention rate. Generally, in order to improve the capacity retention rate and high-rate characteristics after cycling of a power storage device, the resistance of the electrode is suppressed so that lithium ions can move easily. However, if the resistance of the electrode is made too small, metallic lithium is likely to precipitate during charging. The inventors have found that since the resistance value of the surface layer of the composite material layer is excessively low, lithium ions saturate near the interface between the separator and the composite material layer before being occluded in the entire composite material layer and precipitate as a metal. On the other hand, the power storage device according to the present embodiment includes the composite material layer 42 having a laminated structure. In the composite material layer 42 having the laminated structure, the binder content of the surface layer 44 close to the separator 50 is set as high as 5.0 wt% to 35.0 wt%. Thereby, the resistance value near the interface between the separator 50 and the composite material layer 42 increases, and the precipitation of metallic lithium is suppressed. Further, the binder content of the layer close to the current collector foil 41 is suppressed within a certain range (here, 1.10 ≦ A / B ≦ 62.5) based on the surface layer 44. Thereby, the resistance value on the current collector foil 41 side decreases, the conductivity between the current collector foil 41 and the composite material layer 42 can be suitably maintained, and a decrease in the capacity retention rate after cycling can be suppressed. By reducing the binder content on the current collector foil 41 side, the average resistance of the entire electrode is reduced (specifically, 50 Ω / cm 2 ), so that the battery performance can be maintained. As will be described later, the laminated structure in the composite material layer 42 according to the present embodiment can also be applied to the positive electrode 30. However, as described herein, the composite material layer 42 having the laminated structure exhibits a higher effect in the negative electrode 40 that occludes lithium ions during charging.

[0039] By adjusting the increase amount of the binder content in each layer, while suppressing the reaction of lithium ions, lithium ions can be suitably distributed throughout the entire negative electrode composite material layer 42. When the solid content weight of the entire arbitrary layer of the negative electrode composite material layer 42 is 100 wt%, the binder content (C) of the above arbitrary layer and the solid content weight of the layer adjacent to the negative electrode current collector foil side 41 in the lamination direction (thickness direction T) with respect to the above arbitrary layer When it is 100 wt%, it is preferable that the binder content (D) satisfies the formula (2) 1.10 ≦ C / D ≦ 16.10. Thereby, the gradient of the binder content between the layers becomes gentle. Such an effect reduces the difference in the conductivity of charge carriers between two adjacent layers, and can suppress the precipitation of metallic lithium between the layers. From the viewpoint of further suppressing the precipitation of metallic lithium and improving the capacity retention rate of the power storage device, the lower limit value of C / D may be 1.10 or more, preferably 1.25 or more, more preferably 1.80 or more, and even more preferably 2.10 or more. Further, by setting the upper limit value, when moving to the current collector foil side, charge carriers can be suitably occluded, which can contribute to maintaining the high rate characteristics and the capacity retention rate of the power storage device. The upper limit value of C / D may be 16.10 or less, preferably 15.00 or less, more preferably 10.50 or less, and even more preferably 9.00 or less.

[0040] The negative electrode composite material layer 42 of the power storage device disclosed herein may include an intermediate layer 43 located between the first layer 42a and the surface layer 44 in the lamination direction (thickness direction T) of the negative electrode composite material layer 42. In the negative electrode composite material layer 42 of the present embodiment, each layer has a different binder content from the first layer 42a close to the negative electrode current collector foil 41 to the nth layer 42n close to the separator 50. Further, the binder content in each layer gradually increases from the vicinity of the negative electrode current collector foil 41 toward the vicinity of the opposing separator 50 in the lamination direction (thickness direction T) of the negative electrode composite material layer 42. The thickness and binder content of each layer can be measured according to the procedure of the TEM-EELS spectrum analysis scan described above.

[0041] The negative electrode composite material layer 42 of the power storage device disclosed herein is configured, for example, in a laminated structure of three or more layers. As the number of layers of the negative electrode composite material layer 42 increases, the metal serving as a charge carrier diffuses more easily and can be uniformly moved throughout the negative electrode composite material layer. Therefore, the precipitation of lithium ions can be further suppressed.

[0042] In the negative electrode composite material layer 42 of the power storage device disclosed herein, it is sufficient that the binder content rate in each layer gradually increases from the vicinity of the negative electrode current collector foil 41 toward the vicinity of the opposing separator 50 in the stacking direction (thickness direction T) of the negative electrode composite material layer 42. In other words, in the negative electrode composite material layer 42, the binder content rate of each layer may change so as to increase from the first layer 42a to the nth layer 42n (i.e., the surface layer 44). Thereby, the resistance value becomes smaller as it approaches the negative electrode current collector foil. That is, a gradient of the binder content rate of each layer can be formed. As a result, during charging, the metal serving as a charge carrier diffuses more easily and can be uniformly moved throughout the negative electrode composite material layer.

[0043] Further, in the negative electrode composite material layer 42 from the first layer 42a to the nth layer 42n, it is preferable that the above C / D decreases as 1 to n increases. For example, the increase amount of the binder content rate from the second layer 42b to the third layer 42c is smaller than the increase amount of the binder content rate from the first layer 42a to the second layer 42b. Thereby, the resistance value gradually increases as it approaches the separator 50 from the negative electrode current collector foil 41. When lithium ions move between layers, the precipitation of metallic lithium due to a rapid change in the resistance value can be reduced.

[0044] The negative electrode composite material layer 42 may have voids as long as the effects of the technology according to the present disclosure are not significantly impaired. When the negative electrode composite material layer 42 has voids, the porosity of the negative electrode composite material layer 42 can be 5% or more and 35% or less. If the porosity is high, the composite material layer is likely to crack and the resistance also increases. Therefore, the porosity of the negative electrode composite material layer 42 is preferably 30% or less. Further, from the viewpoint of increasing the capacity of the power storage device, the porosity of the negative electrode composite material layer 42 is more preferably 20% or less, and most preferably 10% or less. Note that the porosity may be the same from the first layer to the negative electrode surface layer, or may be different.

[0045] The resistance value of the surface layer 44 is not particularly limited as long as the effects of the technology according to the present disclosure are not significantly impaired. From the viewpoint of maintaining the conductivity of lithium ions, the upper limit value of the resistance value of the surface layer 44 is 150 Ω / cm 2 or less, preferably 140 Ω / cm 2 or less, more preferably 130 Ω / cm 2 or less, still more preferably 120 Ω / cm 2 or less, and even more preferably. Further, from the viewpoint of suppressing the precipitation of lithium ions, the lower limit value of the resistance value of the surface layer 44 is 10 Ω / cm 2 or more, preferably 15 Ω / cm 2 or more, more preferably 20 Ω / cm 2 or more, still more preferably 25 Ω / cm 2 or more, and most preferably.

[0046] The resistance value of the first layer 42a is not particularly limited as long as the effects of the technology according to the present disclosure are not significantly impaired. From the viewpoint of ensuring conductivity with the negative electrode current collector foil 41, the upper limit value of the resistance value of the first layer 42a is 30 Ω / cm 2 or less, preferably 28 Ω / cm 2 or less, more preferably 27 Ω / cm 2 or less, still more preferably 25 Ω / cm 2 or less, and even more preferably. Further, the lower limit value of the resistance value of the first layer 42a is 2 Ω / cm 2 or more, preferably 3 Ω / cm 2 or more, preferably 4 Ω / cm 2 or more, preferably 5 Ω / cm 2 or more.

[0047] <Positive electrode and positive electrode composite material layer> As shown in FIG. 2, the positive electrode 30 includes a rectangular positive electrode current collector foil 31 and a positive electrode composite material layer 32 formed on the surface of the positive electrode current collector foil 31. Note that the positive electrode composite material layer 32 may be formed on one side or both sides (here, both sides) of the positive electrode current collector foil 31. Further, as shown in FIGS. 1 and 2, the positive electrode 30 may have a non-formed portion 31a of the positive electrode composite material layer where the positive electrode current collector foil 31 is exposed without the positive electrode composite material layer 32 being formed. The non-formed portion 31a of the positive electrode composite material layer is provided so as to protrude from one end of the laminated electrode body 20. The positive electrode internal terminal 16 can be joined to the non-formed portion 31a of the positive electrode composite material layer.

[0048] For example, the material of the positive electrode current collector foil 31 is aluminum or an aluminum alloy. As the positive electrode active material of the positive electrode composite material layer 32, a positive electrode active material used for the positive electrode of a general lithium-ion secondary battery can be used. Specifically, the positive electrode active material is a lithium composite metal oxide having a layered rock salt structure, a spinel structure, or an olivine structure. Examples of the lithium composite metal oxide include LiCoO2, LiNiO2, LiFeO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), LiNi 0.5 Mn 1.5 O4, LiNi 0.8 Co 0.15 A l0.05 O2 (NCA), LiCrMO4, LiMn2O4, LiFePO4 (LFP), and the like. Note that these positive electrode active materials may be used alone or in combination of two or more. Among these, from the viewpoint of improving the cycle characteristics of the power storage device, the positive electrode active material is preferably NCM.

[0049] Note that the average resistance value of the positive electrode composite material layer 32 is not particularly limited as long as the technology of the present disclosure is not significantly impaired. If the average resistance value of the positive electrode composite material layer 32 is low, excellent output characteristics can be exhibited. Therefore, the upper limit value of the resistance value of the positive electrode composite material layer 32 is preferably 25 Ω / cm 2 or less, more preferably 20 Ω / cm 2The following is more preferable, 15 Ω / cm 2 The following is even more preferable. Also, the lower limit value may be 1 Ω / cm 2 or more, and may be 2 Ω / cm 2 or more, and may be 3 Ω / cm 2 or more.

[0050] <Separator> The separator 50 disclosed herein is a porous sheet having insulating properties, but its shape and dimensions are not particularly limited and may be appropriately determined according to the battery design. Typically, since the separator 50 insulates the positive electrode and the negative electrode, the dimensions of the separator 50 are larger than those of the positive electrode 30 and the negative electrode 40. Also, as the material of the separator 50, a known separator used in a power storage device may be used and is not particularly limited. For example, as the separator 50, polyolefins such as polyethylene or polypropylene, polyesters, celluloses, or resins such as polyamides are preferably used. Further, the surface of the separator 50 may be provided with a heat-resistant layer as long as the effects of the technology according to the present disclosure are not significantly impaired. Regarding the film thickness of the separator 50, considering the balance between the capacity density of the power storage device and the strength of the current collector, the lower limit value is preferably 5 μm or more, more preferably 8 μm or more, and most preferably 10 μm or more. Also, the upper limit value of the film thickness of the separator 50 is preferably 40 μm or less, more preferably 30 μm or less, and most preferably 20 μm or less.

[0051] <Resistance ratio of positive and negative electrodes> The resistance ratio of the positive and negative electrodes in the power storage device disclosed herein is not particularly limited. Assuming that the average resistance value of the positive electrode 30 is α, it is preferable that the resistance value (X) of the negative electrode surface layer 44 satisfies the formula (3): 1.0 ≦ X / α ≦ 20.0. In a power storage device including an electrode body that satisfies the above formula (3), the resistance value of the positive electrode composite layer 31 is smaller than the resistance value of the surface layer 44 of the negative electrode composite layer 42 close to the separator 50. As a result, during charging, lithium ions can move preferably from the positive electrode to the negative electrode, so that the high rate characteristics can be improved. From the viewpoint of improving the capacity retention rate, the lower limit value of X / α is preferably 2.0 or more, more preferably 4.0 or more, and still more preferably 6.0 or more. Also, from the viewpoint of maintaining battery performance, the upper limit value of the resistance ratio of the positive and negative electrodes is preferably 18.0 or less, more preferably 15.0 or less, and still more preferably 10.0 or less.

[0052] <Manufacturing method of laminated composite layer> One embodiment of the manufacturing method of the electrode disclosed herein includes a step of preparing a composite material from the first layer to the nth layer, and a step of sequentially laminating and drying the composite material layers from the first layer to the nth layer (n is a natural number of 2 or more) on the electrode current collector foil. In the manufacturing method of the electrode of the present disclosure, the drying after the second layer is performed so that the binder contained in the composite material composition does not move to other layers due to the convection of the solvent. Also, in the composite material layers from the first layer to the nth layer, the content rate of the binder increases as 1 to n increases. Hereinafter, taking the negative electrode 40 as an example, an example of the manufacturing method of the electrode disclosed herein will be described in detail.

[0053] <First step S10> The method for manufacturing the electrode of the present embodiment first has a step of preparing a negative electrode composite material for the first layer 42a to the nth layer 42n (i.e., the surface layer 44) shown in FIG. 3 (hereinafter also referred to as the "composite material preparation step"). In the composite material preparation step of the present embodiment, a negative electrode composite material from the first layer 42a to the nth layer 42n in which at least an active material and a binder are dispersed in an appropriate solvent (such as ion-exchanged water or an organic solvent) is prepared. Further, the negative electrode composite material from the first layer 42a to the nth layer 42n at this time is prepared such that the content rate of the binder increases as 1 to n increases. Here, the content rate of the binder is determined from the weight of the binder when the total solid content weight is 100 wt% for each layer. Note that the negative electrode composite material is a paste-like (slurry-like, ink-like) composition. For kneading the negative electrode composite material, for example, conventionally known stirring and mixing apparatuses such as a ball mill, a roll mill, a mixer, a disperser, and a kneader can be appropriately used. The kneading time may be the time until the negative electrode active material and the binder are evenly dispersed. Although it may vary depending on the apparatus configuration and kneading conditions, typically it is 10 minutes to 3 hours, preferably 10 minutes to 30 minutes.

[0054] In addition, the paste-like composition prepared in the composite material preparation step may contain optional components other than those described above as long as the effects of the present disclosure are not significantly impaired. Such optional components include a conductive auxiliary agent and an inorganic filler.

[0055] <Second Step S20> Here, a negative electrode composite material layer from the first layer 42a to the surface layer 44 is sequentially laminated and dried on the negative electrode current collector foil. First, the negative electrode composite material for the first layer 42a prepared in the composite material preparation step is applied onto the negative electrode current collector foil 41 (hereinafter also referred to as the "first application step"). For applying the negative electrode composite material, for example, conventionally known coating apparatuses such as a slit coater, a die coater, a comma coater, a gravure coater, and a dip coater can be appropriately used. Note that the negative electrode composite material may be applied to both sides of the negative electrode current collector foil 41 or only to one side. Further, a non-formed portion 41a of the negative electrode composite material layer may be provided by not applying the negative electrode composite material to a part of the negative electrode current collector foil 41.

[0056] The negative electrode composite material applied on the negative electrode current collector foil 41 in the first coating step is dried to form the first layer 42a (hereinafter also referred to as the "first drying step"). In the first drying step, the drying means is not particularly limited. For example, conventionally known drying devices such as a hot air drying device, a vacuum drying device, dry air, and infrared rays may be appropriately used.

[0057] Next, as shown in FIGS. 3 to 5, a negative electrode composite material for the second layer 42b is applied to the surface of the first layer 42a (hereinafter also referred to as the "second coating step"). Note that the second coating step is also a step of applying a negative electrode composite material for the nth layer 42n to the surface of the (n - 1)th layer 42n - 1. In the second coating step, the negative electrode composite material can be applied in the same manner as in the first coating step. The coating speed of each layer is not particularly limited as long as the technology of the present disclosure is not significantly impaired. The coating speed of the negative electrode composite material after the second layer is preferably 0.5 times or more and 0.8 times or less the coating speed of the first layer 42a. Since there are irregularities on the surface of each layer, by reducing the coating speed, unevenness in the layer thickness can be less likely to occur.

[0058] When drying the negative electrode composite material after the second coating step (that is, after the second layer and subsequent layers), it is performed so that the binder contained in the composite material does not move to other layers due to the convection of the solvent. Since the composite material before drying has fluidity, in a conventionally known drying method, due to excessive heating and evaporation of the solvent, uneven distribution (migration) occurs in which the binder contained in the composite material layer moves to the surface side of the composite material layer due to the convection of the solvent. In the method for manufacturing an electrode of the present embodiment, it is performed by drying for 5 minutes or more and 15 minutes or less (for example, in a dry atmosphere, under normal pressure, at a temperature of 150°C). Thereby, in the multi-layered electrode composite material layer, the binders of each layer are not homogenized, and the binder content different for each layer can be maintained. Note that as the drying means, while blowing dry air in a drying furnace, it is typically at a temperature below the melting point of the binder (for example, 100°C to 150°C) and is dried within, for example, 15 minutes. The drying wind speed is preferably about 1 m / second to 15 m / second.

[0059] In the drying of the second layer and subsequent layers, the binder can be prevented from becoming uniform by drying for 5 minutes or more and 15 minutes or less. The reason for this is that since the composite material of the second layer and subsequent layers before drying has fluidity, it is considered that the movement of the binder into the voids of the lower layer is suppressed by drying in a short time (specifically, 15 minutes or less).

[0060] In the method for manufacturing an electrode according to an embodiment disclosed herein, the above-described second coating step and second drying step are repeatedly performed until the number of arbitrary layers n (n is a natural number of 2 or more) of the composite material layer is reached. For example, when n = 3, as shown in FIG. 4, a third layer 42c is applied to the surface of the second layer 42b applied by the second drying step. Then, the applied third layer 42c is dried under the same drying conditions as the second layer 42b (that is, the second drying step). In this case, in the composite material preparation step, it is preferable to adjust the binder content in advance so that the binder content of the third layer 42c is higher than the binder content of the second layer 42b. From the viewpoint of improving the manufacturing efficiency by reducing the number of steps, the number of layers n of the composite material layer is preferably 2 or less, 3 or less, 4 or less, or 5 or less.

[0061] After drying, pressing or the like may be performed as necessary for density adjustment. The pressing process can be performed, for example, by a roll press, a flat press, or the like. By performing pressing, an anchor effect is exerted between the layers of the composite material layer, so that the strength of the electrode body can be increased. Note that the number of pressing processes is not particularly limited. From the viewpoint of improving the manufacturing efficiency by reducing the number of steps, coating and drying may be repeated until the number of arbitrary layers n of the target composite material layer is reached, and finally pressed only once. Alternatively, pressing may be performed after coating and drying the composite material of each layer. By performing pressing for each layer, the flow of the binder into the voids of each layer can be eliminated, and the occurrence of binder migration can be suppressed.

[0062] In the method for manufacturing an electrode as described above, a composite material layer having a multilayer structure can be formed without heating the composite material. That is, by the convection of the solvent, it is possible to suppress the uneven distribution of the binder contained in the composite material. Thereby, a concentration gradient of the binder can be created from the vicinity of the current collector foil to the vicinity of the separator. Note that the method for manufacturing the electrode of the present embodiment can also be applied to the positive electrode 30 accordingly.

[0063] As described above, the preferred embodiments of the present disclosure have been described based on the drawings. However, such descriptions are not limiting matters, and of course, various modifications are possible. For example, the configuration of the negative electrode composite material layer 42 in the negative electrode 40 described above can be applied to the positive electrode composite material layer 32. However, in that case, various materials are changed to those that can generally be used for the positive electrode of the power storage device. Even when the technology of the present embodiment is applied to the positive electrode, since the resistance value at the interface between the separator and the positive electrode composite material layer becomes high, the rapid movement of lithium ions from the positive electrode to the negative electrode during charging is suppressed.

Examples

[0064] Hereinafter, examples and comparative examples will be shown to specifically describe the power storage device of the present disclosure. Here, when n = 3, that is, a negative electrode composite material layer having a three-layer laminated structure will be described as an example. Note that the present disclosure is not intended to be limited to what is shown in such examples.

[0065] <Example 1: Production of positive electrode sheet> As the positive electrode active material, 97.5 wt% of NCM powder, 1.5 wt% of PVDF as a binder, and 1.0 wt% of CNT as a conductive assistant were mixed to prepare a paste for forming a positive electrode composite layer. Note that N-methyl-2-pyrrolidone (NMP) was used as the solvent. Next, an aluminum foil with a thickness of 13 μm was prepared. The paste for forming the positive electrode composite layer was applied to both sides of the aluminum foil. At this time, the thickness of the aluminum foil and the positive electrode composite layers on both of its sides (i.e., the average film thickness of the positive electrode) was adjusted to be 140 μm and coated. After the application of the positive electrode composite layer, it was air-dried and then heat-dried at 120 °C. Then, leaving the conductive part, it was cut to be 45 cm in length and 35 cm in width to produce a positive electrode sheet.

[0066] <Example 1: Preparation of Composite Material for Negative Electrode Sheet> As the negative electrode active material, 96.6 wt% of graphite particles and 0.4 wt% of SBR as a binder were mixed to prepare a paste for forming the first layer. As the negative electrode active material, 95.6 wt% of graphite particles and 4.4 wt% of SBR as a binder were mixed to prepare a paste for forming the second layer. As the negative electrode active material, 92.0 wt% of graphite particles and 8.0 wt% of SBR as a binder were mixed to prepare a paste for forming the third layer. Note that ion-exchanged water was used as the solvent.

[0067] <Example 1: Coating and Drying of the First Layer> Next, a copper foil with a thickness of 8 μm was prepared. The paste for forming the first layer was applied to both sides of the copper foil. Also, at this time, the thickness of the first layer on one side was adjusted to be 60 μm and coated. After the coating of the first layer, it was air-dried and then heat-dried at 120 °C.

[0068] <Example 1: Coating and Drying of the Second to Third Layers> After drying the first layer, the paste for forming the second layer was applied to the surface of the first layer. At this time, the thickness of the second layer on one side was adjusted to 60 μm and applied. After applying the second layer, it was dried in a dry atmosphere at normal pressure at a temperature of 150 °C for 5 minutes or more and 15 minutes or less. Further, after drying the second layer, the paste for forming the third layer was applied to the surface of the second layer. At this time, the thickness of the third layer on one side was adjusted to 60 μm and applied. In addition, a comma coater (registered trademark) was used for the application of each layer. After applying the third layer, it was dried in a dry atmosphere at normal pressure at a temperature of 150 °C for 5 minutes or more and 15 minutes or less, and then pressed. Thereafter, leaving the conductive part, it was cut to a size of 50 cm in length and 40 cm in width to produce a negative electrode sheet.

[0069] <Fabrication of laminated three - electrode cell> In the measurement of the resistance value and capacitance retention rate shown below, a laminated three - electrode cell composed of a positive electrode sheet, a reference electrode (RE), a negative electrode sheet, a separator, and a non - aqueous electrolyte was used. The reference electrode was fabricated using a platinum wire and included a conductive part and a coating part coated with LFP. As the separator, two porous polyolefin sheets made of polyethylene with a size of 53 cm in length, 43 cm in width, and 16 μm in thickness were prepared. The above - mentioned positive electrode sheet, separator, reference electrode, separator, and negative electrode sheet were stacked in this order and sealed with an aluminum / PP laminate film so that each conductive part was exposed to fabricate a laminated three - electrode cell. In addition, as the non - aqueous electrolyte, a solution in which a supporting salt (LiPF6) was dissolved in an organic mixed solvent (EC:EMC:DMC = 3:3:4) at a concentration of about 1.15 mol / L was used. Terminals were connected to the reference electrode and the negative electrode of the laminated three - electrode cell, and preliminary charging was performed at 0.6C until the state of charge (SOC) reached 50%.

[0070] <Measurement of resistance value> At 25 °C, an alternating current impedance measurement method (frequency range: 10000 Hz to 10 mHz, applied voltage: 30 mV) was performed to measure the resistance value. The resistance values from 1000 Hz to 50 mHz were taken as the positive electrode resistance value / negative electrode resistance value, and each resistance value (Ω / cm 2 ) was calculated by dividing by the area (cm 2 ) of the above positive and negative electrode sheets.

[0071] <Fabrication of a laminated bipolar cell> In the measurement of the capacity retention rate shown below, a laminated bipolar cell composed of a positive electrode sheet, a negative electrode sheet, a separator, and a non-aqueous electrolyte solution was used. As the separator, one porous polyolefin sheet made of polyethylene with a length of 53 cm, a width of 43 cm, and a thickness of 16 μm was prepared. The above positive electrode sheet, separator, and negative electrode sheet were stacked in this order, and sealed with an aluminum / PP laminate film so that each conductive part was exposed, thereby fabricating a laminated bipolar cell. The non-aqueous electrolyte solution used was one in which a supporting salt (LiPF6) was dissolved in an organic mixed solvent (EC:EMC:DMC = 3:3:4) at a concentration of about 1.15 mol / L.

[0072] <Measurement of volume retention rate> The above laminated bipolar cell was subjected to a uniform heat treatment in a thermostat at 25°C for 1 hour. Charge and discharge were repeated twice between 4.2 V and 3.0 V at a constant current of 0.1C. The discharge capacity obtained in the second charge and discharge was taken as the initial discharge capacity. Next, after charging to 4.2 V at a constant current of 3C, discharge was performed to 3.0 V at a constant current of 1C. The above 3C charge and 1C discharge were regarded as one cycle, and this was repeated 100 times. Thereafter, charge and discharge were performed once between 4.2 V and 3.0 V at a constant current of 0.1C, and the obtained discharge capacity was taken as the discharge capacity after cycling. The capacity retention rate (%) was calculated by dividing the discharge capacity after cycling by the initial discharge capacity and multiplying by 100.

[0073] <Examples 2 to 6 and Comparative Examples 1 to 5> As shown in Table 1, it is the same as Example 1 except that the blending ratio of the constituent components (binder) was changed.

[0074]

Table 1

[0075] ​From the above test results, in Comparative Example 1 where the binder content of the first layer to the third layer was the same, the capacity retention rate decreased. Also, in Comparative Example 2 where the binder content of the second layer and the third layer was the same and Comparative Example 4 where the binder content of the second layer was higher than that of the third layer, the capacity retention rate decreased. In contrast, in Examples 1 to 6, it was found that the capacity retention rate was high and the precipitation of metallic lithium could be suppressed. Further, in Comparative Examples 2 to 3 and 5, since the binder content of the second layer or the third layer was high, the average resistance value of the negative electrode became high. Although not shown in detail, in Comparative Examples 2 to 3 and 5, the input / output of the power storage device (i.e., high rate characteristics) significantly decreased, and the battery performance could not be stabilized.

[0076] As described above, specific examples of the technology disclosed herein have been shown in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

[0077] The technology disclosed herein, unless otherwise problematic, each component and each process mentioned herein can be appropriately omitted or appropriately combined. Further, this specification includes the disclosures described in the following sections.

[0078] Item 1: A power storage device comprising a positive electrode having a positive electrode current collector foil and a positive electrode composite layer, a negative electrode having a negative electrode current collector foil and a negative electrode composite layer, and a separator interposed between the positive electrode and the negative electrode, wherein the negative electrode composite layer contains an active material and a binder, and the negative electrode composite layer is composed of a laminated structure of n layers (n is a natural number of 2 or more) from a first layer close to the negative electrode current collector foil to an nth layer close to the separator, where the binder content (A) of the nth layer when the solid content weight of the entire nth layer is 100 wt% is 5.0 wt% or more and 35.0 wt% or less, and the binder content (B) of the first layer when the solid content weight of the entire first layer is 100 wt% satisfies the following formula (1); (1) 1.10 ≦ A / B ≦ 62.50, and the average resistance value of the negative electrode composite layer is 15 Ω / cm2 or more and 50 Ω / cm2 or less.

[0079] Item 2: The negative electrode composite material layer has different binder content ratios from the first layer to the nth layer, and the binder content ratio in each layer gradually increases from the vicinity of the negative electrode current collector foil to the vicinity of the separator in the thickness direction of the negative electrode composite material layer. The power storage device according to Item 1.

[0080] Item 3: The binder content ratio (C) of any layer of the negative electrode composite material layer and the binder content ratio (D) of the layer adjacent to the negative electrode current collector foil side in the thickness direction of the any layer satisfy the following formula (2); (2) 1.10 ≦ C / D ≦ 16.10. The power storage device according to Item 1 or 2.

[0081] Item 4: The number of layers of the nth layer is 3 layers, 4 layers, or 5 layers. The power storage device according to any one of Items 1 to 3.

[0082] Item 5: A method for manufacturing an electrode having a laminated structure composed of n layers (n is a natural number of 2 or more) from the first layer adjacent to the electrode current collector foil to the nth layer on the surface in the thickness direction of the electrode composite material layer including the electrode current collector foil and the electrode composite material layer, the following steps: a step of preparing the composite materials for the first layer to the nth layer; and a step of sequentially laminating and drying the composite material layers from the first layer to the nth layer on the electrode current collector foil, where the drying after the second layer is performed so that the binder contained in the composite material does not move to other layers by the convection of the solvent; The method for manufacturing an electrode, wherein the composite material layers from the first layer to the nth layer have an increasing binder content ratio as 1 to n increases.

Explanation of Reference Numerals

[0083] 1 Lithium-ion battery 10 Battery case 11 Battery case body 12 Sealing plate 13 Safety valve 14 Positive electrode external terminal 15 Negative electrode external terminal 16 Positive electrode internal terminal 17 Negative electrode internal terminal 20 Electrode body 30 Positive electrode 31 Positive current collector foil 31a Non-formed part of the positive composite material layer 32 Positive composite material layer 40 Negative electrode 41 Negative current collector foil 41a Non-formed part of the negative composite material layer 42 Negative composite material layer 42a First layer 42b Second layer 42c Third layer 42d Fourth layer 42n nth layer 43 Intermediate layer 44 Surface layer 50 Separator S10 First process S20 Second process

Claims

1. A power storage device comprising a positive electrode having a positive current collector foil and a positive composite material layer, a negative electrode having a negative current collector foil and a negative composite material layer, and a separator interposed between the positive electrode and the negative electrode, wherein the negative composite material layer contains an active material and a binder, the negative composite material layer is composed of a laminated structure of n layers (n is a natural number of 2 or more) from a first layer adjacent to the negative current collector foil to an nth layer adjacent to the separator, here, when the total weight of the solid content of the nth layer is 100 wt%, the binder content (A) of the nth layer is 5.0 wt% or more and 35.0 wt% or less, the relationship between the binder content (B) of the first layer and the binder content (A) of the nth layer when the total weight of the solid content of the first layer is 100 wt% is represented by the following formula (1); (1) 1.10 ≤ A / B ≤ 62.50 is satisfied, The average resistance value of the negative electrode composite layer is 15 Ω / cm ２ or more and 50 Ω / cm ２ or less, and the power storage device.

2. the binder content (C) of any layer of the negative composite material layer and, the binder content (D) of the layer adjacent to the negative current collector foil side in the thickness direction of the any layer satisfy, the following formula (2); (2) 1.10 ≤ C / D ≤ 16.10 The power storage device according to claim 1, which satisfies the above.

3. The power storage device according to claim 2, wherein the negative composite material layer has three or more layers.

4. The power storage device according to claim 3, wherein the binder content gradually increases from the first layer to the nth layer in the thickness direction of the negative composite material layer.

5. A method for manufacturing an electrode comprising an electrode current collector foil and an electrode composite material layer, the electrode having a laminated structure of n layers (n is a natural number of 2 or more) from a first layer adjacent to the electrode current collector foil to an nth layer on the surface in the thickness direction of the electrode composite material layer, the method comprising the following steps: a step of preparing composite materials for the first layer to the nth layer; and a step of sequentially laminating and drying the composite material layers from the first layer to the nth layer on the electrode current collector foil, wherein the drying of the second layer and subsequent layers is performed so that the binder contained in the composite material does not move to other layers by convection of the solvent; and the composite material layers from the first layer to the nth layer have an increasing binder content as 1 to n increases. ​

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