Electricity storage device and method for manufacturing same

Abstract

Provided are: a highly safe power storage device in which deposition of lithium metal is suppressed while maintaining capacity retention rate and high rate characteristics; and a method for manufacturing the power storage device. This electricity storage device is provided with: a positive electrode having a positive electrode current collector foil and a positive electrode mixture layer; a negative electrode having a negative electrode current collector foil and a negative electrode mixture 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, the negative electrode composite material layer is configured from a laminated structure comprising n layers (n is a natural number of 2 or more) from a first layer adjacent to the negative electrode collector foil to an n-th layer adjacent to the separator, the binder content (A) of the n-th layer is 5.0 wt% to 35.0 wt% when the solid content weight of the whole n-th layer is 100 wt%, and the binder content (B) of the n-th layer is 5.0 wt% to 35.0 wt% when the solid content weight of the whole n-th layer is 100 wt%. The binder content (B) of the first layer satisfies the following formula (1): 1.10 < = A / B < = 62.50 when the solid content weight of the entire first layer is set as 100 wt%, and the average resistance value of the negative electrode mixture layer is 15-50 Omega / cm2.

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 suitable as power sources for portable devices such as personal computers and portable terminals and high-output power sources for vehicles such as vehicle drive power sources for battery electric vehicles (BEVs) because they are lightweight and have a high energy density. Among the electrodes included in such power storage devices, there are a current collector foil and a composite material layer formed on the surface of the current collector foil. The composite material layer is mainly composed of an active material. In addition, typically, a conductive additive and a binder are contained in the composite material layer. The binder helps to maintain the electrode structure such as the adhesion between the active material and the current collector foil and the adhesion between the active materials.

[0003] As the use of power storage devices expands and develops, they are required to be able to be charged (or discharged) with a large current in a short time. In response to the above demand, there is a known technique for improving the capacity retention rate of a power storage device by reducing the resistance of the electrode. For example, Japanese Patent Application Laid-Open No. 2018-174096 discloses an electrode having an active material layer in which the content of the conductive additive and the binder on the surface side is higher than the content of the conductive additive and the binder on the current collector substrate 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 closest to the current collector side when the active material layer is divided into six equal parts is lower than the binder content closest to the surface side. This document discloses a technique for reducing the resistance and maintaining the cycle characteristics by further containing carbon nanotubes as a conductive additive in the positive electrode active material layer.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2018-174096

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2022-100812 Summary of the Invention

[0008] However, in a power storage device, a metal (such as metallic lithium) that becomes a charge carrier during charging sometimes precipitates on the surface of the negative electrode composite material layer (i.e., the interface between the negative electrode composite material layer and the separator). In such a case, the charge carriers (such as lithium ions) in the composite material layer decrease, and thus the capacity retention rate after cycling decreases.

[0009] The present disclosure aims to provide a highly safe energy storage device that maintains the capacity retention rate and suppresses the precipitation of a metal that serves as a charge carrier.

[0010] The energy 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 has a laminated structure composed of n layers (n is a natural number of 2 or more) from the first layer adjacent to the current collector foil to the nth layer adjacent to the separator. Here, when the total weight of the solid components of the nth layer is set to 100 wt%, the binder content rate (A) of the nth layer is 8.0 wt% to 25.0 wt%, and the binder content rate (B) of the first layer when the total weight of the solid components of the first layer is set to 100 wt% satisfies the formula (1) 1.10 ≤ A / B ≤ 62.50. In addition, the average resistance value of the first layer adjacent to the current collector foil to the nth layer adjacent to the separator is 15 Ω / cm 2 ~50 Ω / cm 2 .

[0011] In an energy storage device with an excessively low resistance value of the negative electrode (excessively high conductivity of charge carriers), a large amount of charge carriers move rapidly from the positive electrode to the negative electrode during charging. As a result, the precipitation of a metal that serves as a charge carrier is promoted on the surface of the composite material layer. In response, the inventors conducted in-depth research and 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 a metal that serves as a charge carrier can be suppressed. That is, according to the above energy storage device, the resistance value of the surface layer of the negative electrode composite material layer rises appropriately, and the conductivity of charge carriers (such as 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 embedded in the entire negative electrode composite material layer. As a result, the precipitation of a metal that serves as a charge carrier (such as metallic lithium, etc.) can be suppressed, and the capacity retention rate after cycling can be improved. In addition, by making the binder content rates of the surface layer and the bottom layer of the negative electrode composite material layer different, 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 satisfy 1.10 ≤ A / B ≤ 62.50. Thereby, even if the vicinity of the surface layer has a high resistance, the conductivity near the current collector foil is improved, and while the capacity retention rate of the energy storage device can be improved, a decrease in battery performance can be suppressed.

[0012] Regarding the power storage device of one mode 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 any layer on the negative electrode current collector foil side in the thickness direction satisfy the formula (2): 1.10 ≤ C / D ≤ 16.10. Thereby, the difference in the conductivity of charge carriers between two adjacent layers becomes small, and precipitation of metals that become charge carriers between layers can be suppressed.

[0013] In the power storage device of one mode disclosed herein, the negative electrode composite material layer has three or more layers. Thereby, precipitation of metals that become charge carriers can be further reliably suppressed.

[0014] In the power storage device of one mode disclosed herein, the binder content rate increases successively from the first layer to the nth layer in the thickness direction of the negative electrode composite material layer. Thereby, in the negative electrode composite material layer, the resistance value can be gradually changed from near the negative electrode current collector foil to near the separator. In the negative electrode composite material layer, as approaching the negative electrode current collector foil, the resistance value becomes smaller. As a result, charge carriers are easily diffused during charging, and the charge carriers can be uniformly moved to the entire negative electrode composite material layer.

[0015] The manufacturing method of an electrode of one mode disclosed herein is a method for manufacturing an electrode that includes an electrode current collector foil and an electrode composite material layer and has a laminated structure, and the laminated structure is 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. It 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 of the first layer to the nth layer on the electrode current collector foil. In addition, drying after the second layer is performed in such a manner that the binder contained in the composite material composition does not move to other layers due to the convection of the solvent. In addition, the content rate of the binder of the composite material layers of the first layer to the nth layer increases in the order from 1 to n.

[0016] In the manufacturing method of the above electrode, in the composite material layer having a laminated structure, 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 accompanied by excessive heating and evaporation of the solvent. Thereby, in the composite material layer, a concentration gradient of the binder from near the current collector foil to near the separator can be produced. As a result, an electrode that suppresses precipitation of metals (such as metallic lithium, etc.) that become charge carriers and improves the capacity retention rate after cycling can be manufactured. In addition, an electrode that can reduce the resistance value on the current collector foil side of the composite material layer and suppress a decrease in conductivity near the current collector foil can be manufactured. Description of the Drawings

[0017] Figure 1It is a schematic cross-sectional view of a lithium-ion secondary battery according to an embodiment disclosed herein.

[0018] Figure 2 It is an exploded perspective view schematically showing a laminated electrode body according to an embodiment disclosed herein.

[0019] Figure 3 It is a schematic cross-sectional view showing a multi-layer laminated structure of a negative composite material layer according to an embodiment disclosed herein.

[0020] Figure 4 It is a schematic cross-sectional view showing a two-layer laminated structure of a negative composite material layer according to an embodiment disclosed herein.

[0021] Figure 5 It is a schematic cross-sectional view showing a three-layer laminated structure of a negative composite material layer according to an embodiment disclosed herein.

[0022] Figure 6 It is a schematic cross-sectional view showing a four-layer laminated structure of a negative composite material layer according to an embodiment disclosed herein.

[0023] Figure 7 It is a flowchart for explaining a manufacturing method of a composite material layer having a multi-layer laminated structure according to an embodiment disclosed herein. Detailed Embodiments

[0024] <Definition of Terms>

[0025] Hereinafter, an embodiment of the power storage device of the present disclosure will be described in detail with reference to the drawings. In this specification, matters other than those specifically mentioned and matters that need to be implemented (for example, the general configuration and manufacturing process of the power storage device that do not characterize the present disclosure) can be grasped as design matters by those skilled in the art based on the prior art in this field. The present disclosure can be implemented based on the content disclosed in this specification and the common general knowledge in this field. It should be noted that in the following drawings, the same reference numerals are given to components and parts that perform the same functions for description. In addition, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships.

[0026] In this specification, the "power storage device" refers to a concept including a device that generates charge and discharge reactions by the movement of charge carriers between a pair of electrodes (a positive electrode and a negative electrode). That is, the power storage device includes batteries such as secondary batteries (for example, 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. In addition, in this specification, the "lithium-ion secondary battery" refers to a power storage device that uses lithium ions as charge carriers and realizes repeated charge and discharge through the movement of charges accompanied by lithium ions between the positive and negative electrodes.

[0027] In this specification, when a numerical range is described as "A to B (where A and B are arbitrary numerical values)", it means "not less than A and not more than B", and includes the meanings of "more than A and less than B", "more than A and not more than B", and "not less than A and less than B".

[0028] <Lithium-ion secondary battery>

[0029] Hereinafter, a lithium-ion secondary battery as one of the embodiments of the present disclosure will be described. Figure 1 It is a schematic longitudinal sectional view showing a lithium-ion secondary battery 1 of one embodiment. It should be noted that in the following description, the symbols L, R, U, and D in the drawings represent the left, right, upper, and lower sides of the lithium-ion secondary battery 1. However, these are only directions for convenience of explanation and do not limit any installation method of the power storage device.

[0030] <Battery case>

[0031] As Figure 1 shown, in the lithium-ion battery 1 of the present embodiment, the shape of the battery case 10 is a cubic shape and a flat square shape. However, the shape of the battery case 10 is not limited thereto, and for example, it may also be a cylindrical shape. The battery case 10 includes a battery case body 11 having an opening and a sealing plate (cover body) for sealing 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 sealed by welding such as laser welding. The material of the battery case 10 may be the same as the material used in conventional such power storage devices, and there is no particular limitation. As an example, the material of the battery case 10 may be a lightweight and highly thermally conductive metal material such as aluminum. However, the configuration of the battery case 10 may also be changed. For example, a flexible laminated 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 opposing 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-walled safety valve 13 and an injection port (not shown) for injecting the electrolytic solution, and the safety valve 13 is set to release the internal pressure when the internal pressure of the battery case rises above a specified level.

[0032] 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 may be used. As the external negative electrode terminal 15, for example, copper or a copper alloy may be used.

[0033] The internal terminals 16 and 17 are made of metal. As the positive internal terminal 16, from the viewpoint of improving the bonding strength with the non-forming portion 31a of the positive composite material layer, for example, aluminum or aluminum alloy can be used. As the negative internal terminal 17, from the viewpoint of improving the bonding strength with the non-forming portion 41a of the negative composite material layer, for example, copper or copper alloy can be used.

[0034] <Electrolyte>

[0035] Regarding the electrolyte, a non-aqueous electrolyte in which a supporting salt is dissolved in an appropriate non-aqueous solvent can be used. A conventionally known non-aqueous electrolyte 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. In addition, as an example of the supporting salt, a lithium salt (such as LiBOB, LiPF 6 etc.) can be used.

[0036] <Electrode body>

[0037] 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 laminated via a rectangular separator. Figure 2 It is an exploded perspective view schematically showing the unit unit constituting the laminated electrode body 20 as one of the present embodiments. In the following description, the symbols LR, T, and UD in the drawings are represented as the width direction, thickness direction, and depth direction of the laminated electrode body 20. However, these are only directions for convenience of explanation and do not limit any setting mode of the lithium ion secondary battery 1. It should be noted that the electrode body 20 is not limited thereto, and it may also be a wound electrode body in which a positive electrode and a negative electrode are wound via a separator. In addition, the number of electrode bodies 20 is not particularly limited. The battery case 10 can accommodate a plurality of electrode bodies (for example, laminating a plurality of the above unit units, etc.).

[0038] The negative electrode 40 of the lithium ion battery 1 of the present embodiment has a composite material layer 42 with a multilayer structure. Figure 3 It is a schematic cross-sectional view showing the laminated structure of the electrode body 20. The positive electrode 30 includes a positive current collector foil 31 and a positive composite material layer 32. In addition, the negative electrode 40 includes a negative current collector foil 41 and a negative composite material layer 42. The separator 50 is interposed between the positive electrode 30 and the negative electrode 40.

[0039] <Negative electrode and negative composite material layer>

[0040] As Figure 2As shown, the negative electrode 40 includes a negative electrode current collector foil 41 having a rectangular shape 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 or both sides (here, both sides) of the negative electrode current collector foil 41. In addition, as Figure 1 and Figure 2 shown, the negative electrode 40 may have a non-formed portion 41a of the negative electrode composite material layer where the negative electrode composite material layer 42 is not formed in a part and the negative electrode current collector foil 41 is exposed. Here, the non-formed portion 41a of the negative electrode composite material layer is provided so as to be exposed from one end of the laminated electrode body 20. A negative electrode internal terminal 17 may be joined to the non-formed portion 41a of the negative electrode composite material layer.

[0041] The negative electrode current collector foil 41 has a rectangular shape. It should be noted that the shape and size 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 material of a known negative electrode current collector foil for 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 viewpoint of balancing the capacity density of the power storage device and the strength of the current collector, the lower limit value 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. In addition, the upper limit value is preferably 40 μm or less, more preferably 25 μm or less, and most preferably 15 μm or less.

[0042] 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 may be used. For example, the negative electrode active material may include carbon materials such as soft carbon (easily graphitizable carbon), amorphous carbon materials, graphite (graphite), hard carbon (difficultly graphitizable carbon), carbon nanotubes, metal oxide materials such as silicon oxide, titanium oxide, vanadium oxide, lithium titanium composite oxide, metal nitride materials such as lithium nitride, lithium cobalt composite nitride, and silicon compounds. Among them, from the viewpoint of improving the energy density, the negative electrode active material 45 is preferably graphite. It should be noted that these negative electrode active materials may be used alone or in combination of two or more. In addition, as long as it is within the range not significantly impairing the technical effects of the present disclosure, a conductive auxiliary agent, an inorganic filler, etc. may also be added to each layer.

[0043] The negative electrode active material may be in a particulate form. In this 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, and thus it is possible to contribute to the high capacity of the power storage device. The upper limit value of the average particle size of the negative electrode active material is preferably 25 μm or less, more preferably 20 μm or less. It should be noted that the average particle size of the active material can be obtained by the 50 volume% particle size (D50 particle size) based on the laser diffraction / scattering method.

[0044] As the binder for the negative electrode composite material layer 42, binders used for the negative electrode of a general lithium ion secondary battery and the negative electrode of a lithium ion capacitor can be used. For example, binders used in non-aqueous pastes include vinylidene halide resins such as polyvinylidene fluoride (PVDF) and polyvinylidene chloride (PVDC), and polyalkylene oxides such as polyethylene oxide (PEO). In addition, when an aqueous paste is used, a water-soluble polymer material or a water-dispersible polymer material is preferably 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, a binder with low cost and less environmental burden is preferred. Therefore, the binder for the negative electrode composite material layer 42 is preferably CMC or SBR. It should be noted that these binders can be used alone or in combination of two or more. In addition, the same type or different types of these binders can be used from the first layer 42a to the negative electrode surface layer.

[0045] Examples of the conductive additive contained 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.

[0046] The negative electrode composite material layer 42 has a laminated structure composed of n layers (n is a natural number of 2 or more) from the first layer adjacent to the negative electrode current collector foil 41 to the nth layer adjacent to the separator 50. In other words, the negative electrode composite material layer 42 of the present embodiment has a multilayer structure formed by laminating at least two or more composite material layers. It should be noted that here, the layer (i.e., the nth layer 42n) of the negative electrode composite material layer 42 adjacent to the separator 50 is defined as the surface layer 44. In addition, 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 adjacent to the negative electrode current collector foil 41 and the surface layer 44 adjacent to the separator 50 is defined as the intermediate layer 43. As Figure 3 shown, 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 n layers of composite material layers (intermediate layer 43 and surface layer 44) provided on the first layer 42a. It should be noted that in this specification, unless otherwise specified, the intermediate layer 43 refers to one layer or multiple layers.

[0047] Within the range that does not significantly impair the technology of the present disclosure, the number of layers of the negative electrode composite material layer 42 is not particularly limited. That is, the negative electrode composite material layer 42 is composed of a laminated structure of two or more layers. For example, as Figure 4As shown, in the case of n = 2, as an embodiment, it becomes a two-layer negative electrode composite material layer 42. It should be noted that in this case, the surface layer 44 refers to the second layer 42b. As Figure 5 shown, in the case of n = 3, as an embodiment, it becomes a three-layer negative electrode composite material layer 42. It should be noted that in this case, the negative electrode intermediate layer 43 refers to the second layer 42b. In addition, the surface layer 44 refers to the third layer 42c. As Figure 6 shown, in the case of n = 4, as an embodiment, it becomes a four-layer negative electrode composite material layer 42. In this case, the intermediate layer 43 refers to the second layer 42b and the third layer 42c. In addition, the surface layer 44 refers to the fourth layer 42d.

[0048] The thickness of each layer from the first layer 42a to the surface layer 44 is not particularly limited. However, from the viewpoint of further reliably suppressing the precipitation of metallic lithium, it is preferable to provide a certain thickness to the surface layer 44. From the viewpoint of maintaining battery performance, 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 still more preferably 60 μm or less. From the viewpoint of suppressing the precipitation of metallic lithium, 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 still more preferably 30 μm or more. In addition, from the viewpoint of maintaining the capacity of the lithium ion battery, it is preferable to provide a certain thickness to each of the first layer 42a or the intermediate layer 43. From the viewpoint of suppressing the precipitation of metallic lithium, 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 still more preferably 160 μm or less. From the viewpoint of improving the capacity retention rate after cycling, 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 still more preferably 50 μm or more.

[0049] It should be noted that the thickness of each layer contained in the negative electrode composite material layer 42 can be measured according to the following steps. First, measurement based on TEM-EELS spectroscopy is carried out under the condition of full discharge, and the binder content of each of the eight measurement points is measured along the thickness direction of the negative electrode composite material layer 42. Secondly, the change amount of the binder content between two adjacent measurement points is calculated. Then, when there are three or more consecutive measurement points where the change amount of the binder content rate exceeds 1.5%, the measurement point at the center of the consecutive measurement points is regarded as the layer switching part. Based on this layer switching part, the thickness of each layer can be measured.

[0050] When the total weight of the solid components of the n-th layer 42n (i.e., the surface layer 44) is set to 100 wt%, the content rate (A) of the binder in the n-th layer 42n (i.e., the surface layer 44) typically satisfies 5.0 wt% to 35.0 wt%. By making the n-th layer 42n (i.e., the surface layer 44) have a high binder content rate (specifically, 5.0 wt% or more), the precipitation of metallic lithium can be suppressed, and the capacity retention rate after cycling can be improved. In addition, by making the binder content rate (A) of the n-th layer 42n (i.e., the surface layer 44) 35.0 wt% or less, the conductivity can be maintained as a whole for the negative electrode and lithium ions can be intercalated. 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. In addition, by reducing the binder content rate, the conduction of lithium ions can be facilitated. Therefore, the upper limit value is preferably 30.0 wt% or less, more preferably 25.0 wt% or less, and further preferably 20.0 wt% or less. It should be noted that if the number of layers increases, the thickness of the composite material layer also increases, and the mechanical strength of the composite material layer becomes higher. On the other hand, a composite material layer with a large thickness has insufficient flexibility and tends to be easily broken in the manufacturing process of the power storage device. In particular, stress concentration occurs near the surface of the composite material layer due to winding or the like. The composite material layer 42 of the present embodiment has a high content rate of the binder as it approaches the surface layer 44. Therefore, the closer to the surface layer 44, the adhesiveness increases, and the occurrence of breakage in the manufacturing process of the power storage device can be reduced.

[0051] The content rate (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 according to the ratio of its content rate (A) of the binder in the surface layer 44 (i.e., the n-th layer 42n). That is, when the total weight of the solid components of the first layer is set to 100 wt%, the formula (1) 1.10 ≤ A / B ≤ 62.50 is satisfied. Thereby, during charging, lithium ions become easy to reach near the current collector foil of the negative electrode composite material layer, and the battery performance can be maintained. In addition, the upper limit value of A / B can be 100 or less. If 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 the battery performance, the upper limit value of A / B can be 45.00 or less, preferably 32.50 or less, more preferably 20.00 or less, and further 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 further preferably 6.50 or more.

[0052] The average resistance value of the negative electrode composite material layer 42 is 15 Ω / cm 2 ~50 Ω / cm 2Thus, the effect of suppressing the precipitation of lithium ions and ensuring conductivity is 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. In addition, 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 further preferably 43 Ω / cm 2 or less.

[0053] The power storage device having the composite material layer 42 with the multi-layered structure in which the binder content rate increases from the current collector foil 41 side to the separator 50 side as described above does not reduce the capacity retention rate and can suppress the precipitation of metallic lithium. Generally, in order to improve the capacity retention rate and high-rate characteristics after cycling, the power storage device suppresses the resistance of the electrode to make lithium ions move easily. However, if the resistance of the electrode is too small, metallic lithium becomes likely to precipitate during charging. The inventors found that since the resistance value of the surface layer of the composite material layer is too low, lithium ions are saturated near the interface between the separator and the composite material layer before being embedded in the entire composite material layer and precipitate as a metal. In contrast, the power storage device of the present embodiment has the composite material layer 42 with a laminated structure. In this laminated composite material layer 42, the binder content rate of the surface layer 44 adjacent to the separator 50 is set in a relatively high range of 5.0 wt% to 35.0 wt%. As a result, 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. In addition, the binder content rate of the layer adjacent 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. As a result, 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 appropriately maintained, and the reduction of the capacity retention rate after cycling can be suppressed. By reducing the binder content rate on the current collector foil 41 side, the average resistance of the entire electrode is reduced (specifically, 50 Ω / cm 2 ), so the battery performance can be maintained. It should be noted that, although described later, the laminated structure of the composite material layer 42 of the present embodiment can also be applied to the positive electrode 30. However, as described herein, the above-mentioned laminated composite material layer 42 shows a higher effect in the negative electrode 40 that embeds lithium ions during charging.

[0054] By adjusting the increase amount of the binder content rate of each layer, it is possible to suppress the reaction of lithium ions and enable the lithium ions to appropriately spread throughout the entire negative electrode composite material layer 42. When the solid component weight of the entire arbitrary layer of the negative electrode composite material layer 42 is set to 100 wt%, the binder content rate (C) of the above-mentioned arbitrary layer and the binder content rate (D) when the solid component weight of the layer adjacent to the above-mentioned arbitrary layer on the negative electrode current collector foil side 41 in the stacking direction (thickness direction T) is set to 100 wt% preferably satisfy the formula (2) 1.10 ≤ C / D ≤ 16.10. Thereby, the gradient of the binder content rate between the layers becomes gentle. The above effect is that the difference in the conductivity of the charge carriers between the adjacent two layers becomes small, and the precipitation of metallic lithium between the layers can be suppressed. 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 can be 1.10 or more, preferably 1.25 or more, more preferably 1.80 or more, and further preferably 2.10 or more. In addition, by setting the upper limit value, the charge carriers can be appropriately inserted when moving toward the current collector foil side, which helps to maintain the high-rate characteristics and the capacity retention rate of the power storage device. The upper limit value of C / D can be 16.10 or less, preferably 15.00 or less, more preferably 10.50 or less, and further preferably 9.00 or less.

[0055] 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 stacking direction (thickness direction T) of the negative electrode composite material layer 42. In the negative electrode composite material layer 42 of the present embodiment, from the first layer 42a adjacent to the negative electrode current collector foil 41 to the nth layer 42n adjacent to the separator 50, each layer has a different binder content rate. In addition, the binder content rate of each layer increases in order from the vicinity of the negative electrode current collector foil 41 toward the vicinity of the opposed separator 50 in the stacking direction (thickness direction T) of the negative electrode composite material layer 42. The thickness and the binder content rate of each layer can be measured according to the steps of the above TEM-EELS chromatographic analysis scan.

[0056] The negative electrode composite material layer 42 of the power storage device disclosed herein is configured as a laminated structure of three or more layers, for example. The more the number of layers of the negative electrode composite material layer 42, the easier the metal serving as the charge carrier diffuses, and it can be uniformly moved to the entire negative electrode composite material layer. Therefore, the precipitation of lithium ions can be further suppressed.

[0057] In the negative electrode composite material layer 42 of the electrical storage device disclosed herein, the binder content rate of each layer may increase in sequence in the stacking direction (thickness direction T) of the negative electrode composite material layer 42 from the vicinity of the negative electrode current collector foil 41 toward the vicinity of the opposing separator 50. In other words, in the negative electrode composite material layer 42, the binder content rate of each layer may change in such a manner that it increases as it goes from the first layer 42a to the nth layer 42n (i.e., the surface layer 44). Thus, as it approaches the negative electrode current collector foil, the resistance value becomes smaller. That is, the binder content rate of each layer forms a gradient. As a result, during charging, the metal that becomes a charge carrier becomes easily diffusible and can be made to move uniformly throughout the negative electrode composite material layer.

[0058] In addition, 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 becomes smaller as it increases from 1 to n. 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. Thus, as it approaches the separator 50 from the negative electrode current collector foil 41, the resistance value rises more slowly. When lithium ions move between layers, precipitation of metallic lithium due to a sharp change in the resistance value can be reduced.

[0059] Within a range that does not significantly impair the technical effects of the present disclosure, the negative electrode composite material layer 42 may have voids. When the negative electrode composite material layer 42 has voids, the porosity of the negative electrode composite material layer 42 may be 5% to 35%. If the porosity is high, the composite material layer is likely to break and the resistance also rises. Therefore, the porosity of the negative electrode composite material layer 42 is preferably 30% or less. In addition, from the viewpoint of increasing the capacity of the electrical storage device, the porosity of the negative electrode composite material layer 42 is more preferably 20% or less, and most preferably 10% or less. It should be noted that the porosity from the first layer to the negative electrode surface layer may be the same or different.

[0060] Within a range that does not significantly impair the technical effects of the present disclosure, the resistance value of the surface layer 44 is not particularly limited. From the viewpoint of maintaining the conductivity of lithium ions, the upper limit value of the resistance value of the surface layer 44 may be 150 Ω / cm 2 Preferably, it is 140 Ω / cm or less 2 More preferably, it is 130 Ω / cm or less 2 Further preferably, it is 120 Ω / cm or less 2 or less. In addition, from the viewpoint of suppressing precipitation of lithium ions, the lower limit value of the resistance value of the surface layer 44 may be 10 Ω / cm 2 Preferably, it is 15 Ω / cm or more 2 More preferably, it is 20 Ω / cm or more 2 Most preferably, it is 25 Ω / cm or more 2 or more.

[0061] Within the range that does not significantly impair the technical effects of the present disclosure, the resistance value of the first layer 42a is not particularly limited. 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 can be 30 Ω / cm 2 Hereinafter, it is preferably 28 Ω / cm 2 Hereinafter, it is more preferably 27 Ω / cm 2 Hereinafter, it is further preferably 25 Ω / cm 2 Hereinafter. In addition, the lower limit value of the resistance value of the first layer 42a can be 2 Ω / cm 2 or more, and can be 3 Ω / cm 2 or more, and can also be 4 Ω / cm 2 or more, and can further be 5 Ω / cm 2 or more.

[0062] <Positive electrode and positive electrode composite material layer>

[0063] As Figure 2 shown, 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. It should be noted 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. In addition, as Figure 1 and Figure 2 shown, the positive electrode 30 may have a positive electrode composite material layer non-formation part 31a where the positive electrode composite material layer 32 is not formed and the positive electrode current collector foil 31 is exposed. The positive electrode composite material layer non-formation part 31a is provided so as to be exposed from one end of the laminated electrode body 20. The positive electrode internal terminal 16 can be joined to the positive electrode composite material layer non-formation part 31a.

[0064] 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, the positive electrode active material used in a general lithium ion secondary battery can be used. Specifically, the positive electrode active material is a lithium composite metal oxide such as a layered rock salt structure, a spinel structure, or an olivine structure. Examples of the lithium composite metal oxide include LiCoO 2 , LiNiO 2 , LiFeO 2 , LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 (NCM), LiNi 0.5 Mn 1.5 O 4 , LiNi 0.8 Co 0.15 Al 0.05 O 2(NCA), LiCrMO 4 , LiMn 2 O 4 , LiFePO 4 (LFP), etc. It should be noted that these positive electrode active materials can be used alone or in combination of two or more. Among them, from the viewpoint of improving the cycle characteristics of the power storage device, the positive electrode active material is preferably NCM.

[0065] It should be noted that within the range not significantly damaging the technology of the present disclosure, the average resistance value of the positive electrode composite material layer 32 is not particularly limited. 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 Hereinafter, it is more preferably 20 Ω / cm 2 Hereinafter, it is further preferably 15 Ω / cm 2 Hereinafter. In addition, the lower limit value can be 1 Ω / cm 2 or more, can be 2 Ω / cm 2 or more, and can also be 3 Ω / cm 2 or more.

[0066] <Separator>

[0067] The separator 50 disclosed here is a porous sheet with insulating properties, but its shape and size are not particularly limited and can be appropriately determined according to the battery design. Typically, the separator 50 insulates the positive electrode and the negative electrode, so the size of the separator 50 is larger than that of the positive electrode 30 and the negative electrode 40. In addition, the material of the separator 50 can be a known separator for a power storage device without special limitation. For example, the separator 50 is preferably made of polyolefins such as polyethylene or polypropylene, polyester, cellulose, or resins such as polyamide. In addition, within the range not significantly damaging the technical effects of the present disclosure, the surface of the separator 50 can be provided with a heat-resistant layer. From the viewpoint of balancing the capacity density of the power storage device and the strength of the current collector, the lower limit value of the film thickness of the separator 50 is preferably 5 μm or more, more preferably 8 μm or more, and most preferably 10 μm or more. In addition, 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.

[0068] <Resistance ratio of positive and negative electrodes>

[0069] The resistance ratio of the positive and negative electrodes of the power storage device disclosed herein is not particularly limited. If the average resistance value of the positive electrode 30 is α, the resistance value (X) of the surface layer 44 of the negative electrode preferably satisfies the formula (3): 1.0 ≤ X / α ≤ 20.0. In the power storage device having an electrode body that satisfies the above formula (3), the resistance value of the positive electrode composite material layer 31 becomes smaller than the resistance value of the surface layer 44 of the negative electrode composite material layer 42 adjacent to the separator 50. Thus, during charging, lithium ions can move appropriately from the positive electrode to the negative electrode, and therefore 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. In addition, 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.

[0070] <Manufacturing Method of Stacked Composite Material Layer>

[0071] One embodiment of the manufacturing method of the electrode disclosed herein includes: a step of preparing composite materials of the first layer to the nth layer and a step of sequentially laminating and drying the composite material layers of 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 of the layers after the second layer is performed in such a manner that the binder contained in the composite material composition does not move to other layers due to the convection of the solvent. In addition, the content rate of the binder in the composite material layers of the first layer to the nth layer increases as it gets larger from 1 to n. 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.

[0072] <First Step S10>

[0073] The manufacturing method of the electrode of the present embodiment first has a preparation for Figure 3Process for the negative electrode composite material of the first layer 42a to the nth layer 42n (i.e., the surface layer 44) shown below (hereinafter also simply referred to as "composite material preparation process"). In the composite material preparation process of the present embodiment, a negative electrode composite material of 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 (ion-exchanged water, organic solvent, etc.) is prepared. In addition, the negative electrode composite material of the first layer 42a to the nth layer 42n at this time is prepared such that the content rate of the binder increases as it increases from 1 to n. Here, the content rate of the binder is determined based on the weight of the binder when the total weight of the solid components of each layer is set to 100 wt%. It should be noted that the negative electrode composite material is a paste-like (slurry-like, ink-like) composition. For example, a conventionally known stirring and mixing device such as a ball mill, a roll mill, a stirrer, a disperser, or a kneader can be appropriately used for kneading the negative electrode composite material. The kneading time can be set to a time when the negative electrode active material and the binder are evenly dispersed. Although it may vary depending on the device configuration and kneading conditions, it is typically 10 minutes to 3 hours, preferably 10 minutes to 30 minutes.

[0074] It should be noted that within a range that does not significantly impair the effects of the present disclosure, the paste-like composition prepared in the composite material preparation process may further contain any components other than those described above. Such arbitrary components are conductive aids and inorganic fillers, etc.

[0075] <Second process S20>

[0076] Here, the negative electrode composite materials from the first layer 42a to the surface layer 44 are 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 process is coated on the negative electrode current collector foil 41 (hereinafter also simply referred to as "first coating process"). For coating the negative electrode composite material, a conventionally known coating device such as a slot coater, a die coater, a comma knife coater, an intaglio printing coater, or a dip coater can be appropriately used. It should be noted that the negative electrode composite material can be coated on both sides of the negative electrode current collector foil 41 or only on one side. In addition, by not coating a part of the negative electrode current collector foil 41 with the negative electrode composite material, a non-formed part 41a of the negative electrode composite material layer can be provided.

[0077] The negative electrode composite material coated on the negative electrode current collector foil 41 in the first coating process is dried to form the first layer 42a (hereinafter also simply referred to as "first drying process"). In the first drying process, the drying means is not particularly limited. For example, a conventionally known drying device such as a hot air drying device, a vacuum drying device, drying air, or infrared rays can be appropriately used.

[0078] Next, as Figures 3 to 5As shown, a negative electrode composite material for the second layer 42b is coated on the surface of the first layer 42a (hereinafter also simply referred to as "the second coating process"). It should be noted that the second coating process is also a process of coating a negative electrode composite material for the nth layer 42n on the surface of the (n - 1)th layer 42n - 1. In the second coating process, the negative electrode composite material can be coated by the same method as in the first coating process. The coating speed of each layer is not particularly limited within the range that does not significantly damage the technology of the present disclosure. The coating speed of the negative electrode composite material after the second layer is preferably 0.5 to 0.8 times the coating speed of the first layer 42a. Since the surfaces of the layers have irregularities, unevenness in the layer thickness can be less likely to occur by reducing the coating speed.

[0079] When drying the negative electrode composite material after the second coating process (i.e., after the second layer), it is carried out in such a way 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 the conventionally known drying methods, with excessive heating and evaporation of the solvent, due to the convection of the solvent, the binder contained in the composite material layer moves unevenly (migrates) to the surface side of the composite material layer. In the method for manufacturing an electrode of the present embodiment, it is carried out by drying for 5 to 15 minutes (for example, in a drying atmosphere, at normal pressure, and at a temperature of 150 °C). Thereby, in the multi-layered electrode composite material layer, the binders of each layer are not homogenized, and each layer can maintain a different binder content rate. It should be noted that as a drying means, dry air is blown in a drying furnace and typically at a temperature below the melting point of the binder (for example, 100 °C to 150 °C), and drying is carried out, for example, within 15 minutes. A drying wind speed of about 1 m / s to 15 m / s is sufficient.

[0080] In the drying after the second layer, by drying it for 5 to 15 minutes, the homogenization of the binder can be suppressed. As a reason therefor, it is considered that the composite material after the second layer before drying has fluidity, and thus the movement of the binder into the voids of the lower layer is suppressed by drying it in a short time (specifically, within 15 minutes).

[0081] In the method for manufacturing an electrode according to an embodiment disclosed herein, the above-described second coating process and second drying process are repeated until any number of layers n (n is a natural number of 2 or more) of the target composite material layer is reached. For example, when n = 3, as Figure 4As shown, a third layer 42c is coated on the surface of the second layer 42b obtained by coating in the second drying process. Then, the coated third layer 42c is dried under the same drying conditions as the second layer 42b (i.e., the second drying process). In this case, in the composite material preparation process, it is preferable to pre-adjust the binder content rate so that the binder content rate of the third layer 42c becomes higher than the binder content rate of the second layer 42b. From the viewpoint of improving manufacturing efficiency by reducing the number of processes, 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.

[0082] After drying, stamping or the like for density adjustment can be performed as needed. The stamping process can be performed, for example, by a roll stamping machine, a flat stamping machine, etc. By performing stamping, an anchoring effect is exerted between the layers of the composite material layer, and thus the strength of the electrode body can be improved. It should be noted that the number of stamping processes is not particularly limited. From the viewpoint of improving manufacturing efficiency by reducing the number of processes, coating and drying can be repeated until the target number of layers n of the composite material layer is reached, and only one stamping is performed at the end. Alternatively, stamping can be performed after coating and drying each layer of the composite material. By performing stamping on each layer one by one, the inflow of the binder into the voids of each layer can be eliminated, and the occurrence of binder migration can be suppressed.

[0083] 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, the uneven distribution of the binder contained in the composite material due to the convection of the solvent can be suppressed. Thereby, a concentration gradient of the binder from near the current collector foil to near the separator can be produced. It should be noted that regarding the method for manufacturing an electrode of the present embodiment, the positive electrode 30 can also be manufactured based on this as a reference.

[0084] As described above, the preferred embodiments of the present disclosure have been described with reference to the drawings, but such descriptions are not limiting matters, and various changes can of course be made. For example, the configuration of the negative electrode composite material layer 42 of the negative electrode 40 described above can be applied to the positive electrode composite material layer 32. However, in this case, various materials are generally changed to materials that can 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, the resistance value at the interface between the separator and the positive electrode composite material layer becomes high, so the rapid movement of lithium ions from the positive electrode to the negative electrode during charging can be suppressed.

[0085] Examples

[0086] Hereinafter, examples and comparative examples will be shown to specifically describe the power storage device of the present disclosure. Here, the case where n = 3, that is, the negative electrode composite material layer having a three-layer laminated structure, will be described as an example. It should be noted that the present disclosure is not intended to limit the content shown in the above examples.

[0087] <Example 1: Fabrication of the Positive Electrode Sheet>

[0088] 97.5 wt% of NCM powder as the positive electrode active material, 1.5 wt% of PVDF as the binder, and 1.0 wt% of CNT as the conductive additive were mixed to prepare a paste for forming the positive electrode composite layer. It should be noted that N-methyl-2-pyrrolidone (NMP) was used as the solvent. Next, an aluminum foil with a thickness of 13 μm was prepared. The above-mentioned paste for forming the positive electrode composite layer was coated on both sides of the aluminum foil. At this time, the aluminum foil and the positive electrode composite layers on both of its sides were adjusted so that the total thickness (i.e., the average film thickness of the positive electrode) was 140 μm, and coating was carried out. After coating the positive electrode composite layer and natural drying, heat drying was carried out at 120 °C. Then, leaving the conductive part, it was cut to be 45 cm in length and 35 cm in width to fabricate the positive electrode sheet.

[0089] <Example 1: Preparation of the Composite Material for the Negative Electrode Sheet>

[0090] 96.6 wt% of graphite particles as the negative electrode active material and 0.4 wt% of SBR as the binder were mixed to prepare a paste for forming the first layer. 95.6 wt% of graphite particles as the negative electrode active material and 4.4 wt% of SBR as the binder were mixed to prepare a paste for forming the second layer. 92.0 wt% of graphite particles as the negative electrode active material and 8.0 wt% of SBR as the binder were mixed to prepare a paste for forming the third layer. It should be noted that ion-exchanged water was used as the solvent.

[0091] <Example 1: Coating and Drying of the First Layer>

[0092] Next, a copper foil with a thickness of 8 μm was prepared. The above-mentioned paste for forming the first layer was coated on both sides of the copper foil. In addition, at this time, the thickness of the first layer on one side was adjusted to be 60 μm for coating. After coating the first layer and natural drying, heat drying was carried out at 120 °C.

[0093] <Example 1: Coating and Drying of the Second to Third Layers>

[0094] After drying the first layer, the paste for forming the second layer is coated on the surface of the first layer. At this time, the coating is performed while adjusting the thickness of the single-sided second layer to 60 μm. After coating the second layer, drying is performed under the conditions of a drying atmosphere, normal pressure, a temperature of 150 °C, and 5 to 15 minutes. Further, after drying the second layer, the paste for forming the third layer is coated on the surface of the second layer. At this time, the coating is performed while adjusting the thickness of the single-sided third layer to 60 μm. It should be noted that a comma coater (registered trademark) is used for coating each layer. After coating the third layer, drying is performed under the conditions of a drying atmosphere, normal pressure, a temperature of 150 °C, and 5 to 15 minutes, and then stamping is performed. Then, the conductive portion is left, and it is cut to a length of 50 cm and a width of 40 cm to produce a negative electrode sheet.

[0095] <Fabrication of laminated three-electrode cell>

[0096] In the measurement of the resistance value and the capacity 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 is used. The reference electrode is fabricated using a platinum wire and has a conductive portion and a coating portion coated with LFP. Two porous polyolefin sheets with a length of 53 cm, a width of 43 cm, a thickness of 16 μm, and a material of polyethylene are prepared as separators. The above positive electrode sheet, separator, reference electrode, separator, and negative electrode sheet are overlapped in this order, and sealed with an aluminum / PP laminated film so that each conductive portion is exposed to fabricate a laminated three-electrode cell. It should be noted that the non-aqueous electrolyte used is a solution in which a supporting salt (LiPF 6 ) is dissolved in an organic mixed solvent (EC:EMC:DMC = 3:3:4) at a concentration of approximately 1.15 mol / L. Terminals are connected to the reference electrode and the negative electrode of the laminated three-electrode cell, and pre-charging is performed at 0.6C until the state of charge (SOC) reaches 50%.

[0097] <Measurement of resistance value>

[0098] AC impedance measurement method (frequency range: 10000 Hz to 10 mHz, applied voltage: 30 mV) is performed at 25 °C to measure the resistance value. The resistance values from 1000 Hz to 50 mHz are used as the positive electrode resistance value / negative electrode resistance value, and each resistance value (Ω / cm 2 ) is calculated by dividing by the area (cm 2 ) of the above positive and negative electrode sheets.

[0099] <Fabrication of laminated two-electrode cell>

[0100] In the measurement of the capacity retention rate shown below, a laminated bipolar unit composed of a positive electrode plate, a negative electrode plate, a separator, and a non-aqueous electrolyte is used. One porous polyolefin sheet with a length of 53 cm, a width of 43 cm, a thickness of 16 μm, and a material of polyethylene is prepared as the separator. The above-mentioned positive electrode plate, separator, and negative electrode plate are overlapped in this order, and are sealed with an aluminum / PP laminated film so that each conductive part is exposed to produce a laminated bipolar unit. It should be noted that as the non-aqueous electrolyte, a solution in which a supporting salt (LiPF 6 ) is dissolved in an organic mixed solvent (EC:EMC:DMC = 3:3:4) at a concentration of approximately 1.15 mol / L is used.

[0101] <Measurement of capacity retention rate>

[0102] The above-mentioned laminated bipolar unit is subjected to heat treatment for 1 hour in a constant temperature bath at 25°C. Charge and discharge are repeated twice between 4.2 V and 3.0 V at a constant current of 0.1C. The discharge capacity obtained from the second charge and discharge is used as the initial discharge capacity. Next, after charging to 4.2 V at a constant current of 3C, discharge is performed at a constant current of 1C to 3.0 V. The above-mentioned 3C charge and 1C discharge are regarded as one cycle, and this is repeated 100 times. Then, charge and discharge are performed once between 4.2 V and 3.0 V at a constant current of 0.1C, and the obtained discharge capacity is used as the discharge capacity after cycling. The capacity retention rate (%) is calculated as a percentage by dividing the discharge capacity after cycling by the initial discharge capacity.

[0103] <Examples 2 to 6 and Comparative Examples 1 to 5>

[0104] As shown in Table 1, except for changing the mixing ratio of the constituent components (binder), it is the same as in Example 1.

[0105] Table 1

[0106]

[0107] Based on the above test results, in Comparative Example 1 where the binder content rates of the first to third layers are the same, the capacity retention rate decreases. In addition, the capacity retention rates of Comparative Example 2 where the binder content rates of the second and third layers are the same and Comparative Example 4 where the binder content rate of the second layer is higher than that of the third layer decrease. In contrast, in Examples 1 to 6, a high capacity retention rate is found and the precipitation of metallic lithium can be suppressed. In addition, in Comparative Examples 2 to 3 and 5, since the binder content rate of the second layer or the third layer is high, the average resistance value of the negative electrode becomes high. In Comparative Examples 2 to 3 and 5, although not shown in detail, the input and output of the power storage device (i.e., the high-rate characteristics) are significantly reduced, and the battery performance cannot be stabilized.

[0108] Specific examples of the technology disclosed herein are shown in detail above, but these are merely illustrative and do not limit the scope of the claimed invention. The technology recited in the scope of the claimed invention includes technologies obtained by making various modifications and changes to the specific examples illustrated above.

[0109] In the technology disclosed herein, within the range where no particular problem occurs, each component and each process mentioned herein can be appropriately omitted or appropriately combined. In addition, this specification includes the disclosures recited below.

[0110] Item 1: An electrical 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 has a laminated structure, the laminated structure is composed of n layers (n is a natural number of 2 or more) from the first layer adjacent to the current collector foil to the nth layer adjacent to the separator, here, when the total weight of the solid components of the nth layer is set to 100 wt%, the binder content rate (A) of the nth layer is 5.0 wt% to 35.0 wt%, and the binder content rate (B) of the first layer when the total weight of the solid components of the first layer is set to 100 wt% satisfies the formula (1) 1.10 ≤ A / B ≤ 62.50, and the average resistance value of the negative composite material layer is 15 Ω / cm 2 ~50 Ω / cm 2 。

[0111] Item 2: The electrical storage device according to Item 1, wherein the negative composite material layer has different binder content rates in the first layer to the nth layer, and in the thickness direction of the negative composite material layer, the binder content rate of each layer increases in order from the vicinity of the negative current collector foil to the vicinity of the separator.

[0112] Item 3: The electrical storage device according to Item 1 or 2, wherein the binder content rate (C) of any layer of the negative composite material layer and the binder content rate (D) of the layer adjacent to the any layer on the negative current collector foil side in the thickness direction satisfy the following formula (2): 1.10 ≤ C / D ≤ 16.10 (2).

[0113] Item 4: The electrical storage device according to any one of Items 1 to 3, wherein the number of layers of the nth layer is 3 layers, 4 layers or 5 layers.

[0114] Item 5: A method for manufacturing an electrode, the electrode having a current collector foil for the electrode and an electrode composite layer and having a laminated structure, the laminated structure being composed of n layers (n being a natural number of 2 or more) from the first layer adjacent to the current collector foil for the electrode to the nth layer on the surface in the thickness direction of the electrode composite layer, the manufacturing method including 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 for the first layer to the nth layer on the current collector foil for the electrode, wherein drying after the second layer is performed in such a manner that the binder contained in the composite material does not move to other layers due to the convection of the solvent, and the content rate of the binder in the composite material layers for the first layer to the nth layer increases in the order from 1 to n.

Claims

1. An electric storage device comprising: a positive electrode having a positive electrode collector foil and a positive electrode composite material layer, a negative electrode having a negative electrode 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 active material and a binder. The negative electrode composite material layer is composed of a stacked structure, and the stacked structure is composed of n layers from a first layer adjacent to the negative electrode current collector foil to an nth layer adjacent to the separator, where n is a natural number greater than or equal to 2. in, When the total weight of the solid content of the n-th layer is 100 wt %, the binder content A of the n-th layer is 5.0 wt % to 35.0 wt %, When the total weight of the solid content of the first layer is 100 wt %, the relationship between the binder content B of the first layer and the binder content A of the nth layer satisfies the following formula (1): 1.10≤A / B≤62.50 (1) The average resistance of the negative electrode composite material layer is 15Ω / cm 2 ~50Ω / cm 2 .

2. The power storage device according to claim 1, wherein The binder content C of any layer of the negative electrode composite material layer and the binder content D of the layer adjacent to the arbitrary layer on the negative electrode collector foil side in the thickness direction satisfy the following formula (2): 1.10≤C / D≤16.10 (2).

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

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

5. A method for manufacturing an electrode, wherein the electrode comprises an electrode collector foil and an electrode composite material layer and has a laminated structure, wherein the laminated structure is composed of n layers from a first layer adjacent to the electrode collector foil to an nth layer on the surface of the electrode composite material layer in a thickness direction, wherein n is a natural number greater than or equal to 2, The manufacturing method comprises the following steps: The process of preparing the first to nth layers of composite material, and The step of sequentially stacking the first to n-th composite material layers on the electrode collector foil and drying them, wherein: The drying of the second layer and beyond is carried out in such a way that the binder contained in the composite material does not migrate to other layers due to the convection of the solvent. The binder content of the first to nth composite material layers increases from 1 to n.

Images