Electrode for secondary battery and nonaqueous electrolyte secondary battery provided with the electrode

Abstract

The electrode for a secondary battery disclosed herein is any one of a positive electrode and a negative electrode of a secondary battery, and includes a rectangular sheet-shaped electrode current collector and an electrode active material layer formed on the electrode current collector. The electrode current collector has an uncoated portion, in which the electrode active material layer is not formed and the electrode current collector is exposed, at at least one end portion in a long side direction. A length L1 of the electrode active material layer in the long side direction is 300 mm or more. The electrode active material layer has a flat portion in which the thickness is substantially constant, and a sloped portion in which the thickness continuously decreases as it approaches the uncoated portion, in terms of an average film thickness t1. When a position P at which the thickness of the sloped portion becomes 0.8 of the average film thickness t1 of the flat portion is set, a length L2 from a boundary of the electrode active material layer and the uncoated portion to the position P is 0.5 mm to 25 mm.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrode for a secondary battery and a nonaqueous electrolyte secondary battery provided with the electrode. BACKGROUND

[0002] A secondary battery such as a lithium ion secondary battery is preferred to be used as a high-output power source for a vehicle, or a power source for a personal computer and a mobile terminal, because it is lightweight and has a high energy density compared to a conventional battery. In particular, a lithium ion secondary battery is preferred to be used as a high-output power source for driving a vehicle such as an electric vehicle (BEV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like.

[0003] As a typical structure of a positive electrode and a negative electrode (hereinafter, simply referred to as an "electrode" when not particularly distinguishing between the positive electrode and the negative electrode) provided in such a secondary battery, a structure in which an electrode active material layer that takes an electrode active material as a main component is formed on one face or both faces of a rectangular sheet-shaped electrode current collector can be cited. Generally, the electrode current collector has a region (coated portion) in which the electrode active material layer is formed, and a region (uncoated portion) in which the electrode active material layer is not formed at an end portion in the longitudinal direction of the electrode current collector. The uncoated portion provided in the longitudinal direction of the electrode current collector is connected to an electrode terminal for external connection, and is configured to be able to supply electric power to an external device (for example, a vehicle or the like).

[0004] In order to stabilize the performance and quality of a secondary battery at a high level, it is sufficient to reduce the variation in the weight per unit area (unit area weight) of the electrode active material layer. That is, it is preferred that the film thickness of the electrode active material layer be uniformly formed. For example, a manufacturing method of an electrode is disclosed in Patent Literature 1, which brings the unit area weight of the coated start end portion and the coated end portion of the active material layer close to a reference unit area weight.

[0005] Patent Literature 1: Japanese Patent Application Laid-Open (kokai) No. 2015-146232 SUMMARY

[0006] However, in recent years, particularly for a secondary battery for an electric vehicle, there is a demand for further improvement in the cruising distance, and for example, there is a demand for high capacity per unit battery and efficient mounting of a secondary battery with as little gap (dead space) as possible in a limited space. In order to solve the above problem, for example, a study has been made to make the length in the longitudinal direction longer (i.e., elongated) without changing the height (length in the short direction) of the secondary battery.

[0007] When the electrode of the above-described long strip-shaped secondary battery is produced in a manner that the film thickness of the electrode active material layer is uniform as described in Patent Document 1, unevenness of current density can occur near (end portion) and in the central portion of the electrode terminal of the electrode body. Due to this, lithium is deposited at the end portion of the electrode body where the current density is relatively high at the time of charge and discharge, and the durability (particularly, capacity retention rate) of the secondary battery decreases. As a result of intensive studies by the present inventors, it was found that by providing a region where the film thickness of the electrode active material layer is thin at the end portion of the electrode body, the capacity ratio of the positive electrode to the negative electrode can be adjusted to improve the durability. On the other hand, it was also found that in a case where the above-described region where the film thickness is thin is provided excessively large, the volume efficiency of the secondary battery decreases, and the problem that the target energy cannot be satisfied occurs.

[0008] The present application was made in view of the above-described circumstances, and its main object is to provide an electrode that realizes improvement in the durability and volume efficiency of a secondary battery. In addition, another object is to provide a nonaqueous electrolyte secondary battery that has the above-described electrode.

[0009] In order to achieve the above-described object, the electrode for a secondary battery disclosed herein is provided. The electrode for a secondary battery disclosed herein is characterized by being either of the positive and negative electrodes of a secondary battery, and having a rectangular sheet-shaped electrode current collector and an electrode active material layer formed on the electrode current collector. The electrode current collector has an uncoated portion where the electrode active material layer is not formed and the electrode current collector is exposed at at least one end portion in the longitudinal direction. The length LI of the electrode active material layer in the longitudinal direction is 300 mm or more. The electrode active material layer has a flat portion where the thickness is substantially constant in terms of the average film thickness tl, and a sloped portion where the thickness continuously decreases as the uncoated portion is approached. When a position where the thickness of the sloped portion becomes 0.8 of the average film thickness tl of the flat portion is set as P, the length L2 from the boundary between the electrode active material layer and the uncoated portion to the position P is 0.5 mm to 25 mm.

[0010] According to the above-described configuration, by providing a region where the film thickness is thin compared to the flat portion relatively long in the electrode, capacity deterioration due to lithium deposition is suppressed, and the durability (particularly, capacity retention rate) is improved. In addition, by making the above-described region where the film thickness is thin an appropriate length in the electrode, the volume efficiency of the secondary battery can be improved. Therefore, an electrode that realizes improvement in the durability and volume efficiency of a secondary battery can be provided.

[0011] In a preferred one mode of the electrode disclosed herein, the length LI of the electrode active material layer in the longitudinal direction is 600 mm to 1400 mm.

[0012] According to the above-described configuration, an electrode that realizes improvement in the durability and volume efficiency of a secondary battery can be provided even for an electrode of a long strip shape of 600 mm or more.

[0013] To achieve the other objectives described above, a non-aqueous electrolyte secondary battery is provided. The non-aqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein at least one of the positive and negative electrodes is the electrode described above.

[0014] Based on the above configuration, by having electrodes with the aforementioned characteristics, the durability and volumetric efficiency of the secondary battery can be improved. Attached Figure Description

[0015] Figure 1 This is a perspective view schematically illustrating one embodiment of a lithium-ion secondary battery.

[0016] Figure 2 This is an explanatory diagram schematically showing the components that constitute a stacked electrode body in one embodiment.

[0017] Figure 3 This is a perspective view schematically illustrating the configuration of a stacked electrode body in one embodiment.

[0018] Figure 4 This is a cross-sectional schematic diagram illustrating an electrode according to one embodiment.

[0019] Symbol Explanation

[0020] 10 electrodes

[0021] 12-electrode current collector

[0022] 14 Electrode active material layer

[0023] 14A Planar Section

[0024] 14B Inclined section

[0025] 16 Uncoated areas

[0026] 20-layer stacked electrode body

[0027] 30 Battery casing

[0028] 32 Safety valve

[0029] 42 Positive extremes

[0030] 44 Negative extremes

[0031] 50 positive electrode sheets

[0032] 52 Positive current collector

[0033] 52A Uncoated portion of the positive electrode active material layer

[0034] 54 Positive electrode active material layer

[0035] 60 negative electrode sheet

[0036] 62 Negative current collector

[0037] 62A Uncoated portion of the negative electrode active material layer

[0038] 64 Negative Electrode Active Material Layer

[0039] 70 isolation components

[0040] 100 Lithium-ion Secondary Battery Detailed Implementation

[0041] Hereinafter, preferred embodiments of the technology disclosed herein will be described with appropriate reference to the accompanying drawings. It should be noted that matters other than those specifically mentioned in this specification, and matters necessary for implementation (e.g., the general structure and construction process of non-aqueous electrolyte secondary batteries), can be grasped by those skilled in the art based on existing technology in the field. The technical content disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the field.

[0042] It should be noted that the expression "A~B (where A and B are arbitrary values)" in this specification refers to the range above A and below B.

[0043] It should be noted that in this specification, "secondary battery" refers to a general energy storage device capable of repeated charging and discharging. Typical examples include lithium-ion secondary batteries, nickel-metal hydride batteries, lithium-ion capacitors, and double-layer capacitors. Furthermore, in this specification, "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging through the movement of lithium ions between the positive and negative electrodes. Additionally, in this specification, when there is no specific distinction between positive and negative electrodes, they are simply referred to as electrodes.

[0044] Without intending to impose any special limitations, the following detailed explanation uses a lithium-ion secondary battery as an example to illustrate the technology disclosed herein. In the following figures, components and parts that perform the same function are labeled with the same symbols, and repeated descriptions are sometimes omitted or simplified. Furthermore, the symbols X and Y in the figures refer to the short side direction and long side direction of the electrode body, respectively. Additionally, sometimes one direction in the long side direction Y is referred to as the Y1 direction (right-hand direction), and the opposite direction as the Y2 direction (left-hand direction). However, these directions are merely for ease of explanation and do not impose any limitations on the arrangement of the lithium-ion secondary battery.

[0045] Figure 1 The lithium-ion secondary battery 100 shown is a rectangular stacked electrode body 20. Figure 2 and Figure 3The battery is constructed by housing a non-aqueous electrolyte (not shown) within a sealable box-shaped battery casing 30. The battery casing 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 32 designed to release internal pressure when the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, the battery casing 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The battery casing 30 is preferably made of a high-strength, lightweight, and thermally conductive metallic material; examples of such metallic materials include aluminum and stainless steel.

[0046] like Figure 2 As shown, the stacked electrode body 20 is constructed by alternately stacking rectangular sheet-shaped positive electrodes (hereinafter referred to as "positive electrode sheet 50") and rectangular sheet-shaped negative electrodes (hereinafter referred to as "negative electrode sheet 60") with a rectangular sheet-shaped separator 70 sandwiched in between. The positive electrode sheet 50 has a positive electrode active material layer 54 formed on one or both sides of the positive electrode current collector 52. The negative electrode sheet 60 has a negative electrode active material layer 64 formed on one or both sides of the strip-shaped negative electrode current collector 62. At one end of the rectangular positive electrode current collector 52 along the short side direction (X direction) orthogonal to the long side direction, an uncoated portion 52A without the positive electrode active material layer 54 is formed in a strip shape. Similarly, at the other end of the rectangular negative electrode current collector 62 in the direction of the long side, a strip-shaped uncoated portion 62A of the negative electrode active material layer 64 is formed along the direction of the short side.

[0047] The aspect ratio (length of the long side / length of the short side) of the lithium-ion secondary battery 100 is preferably 4 or more, for example. The aspect ratio can be 6 or more, 8 or more, or 10 or more. The upper limit of the aspect ratio can be, for example, 20 or less, or 18 or less. By keeping the aspect ratio within the above range, the secondary battery can be efficiently installed in limited spaces, such as under the floor of a vehicle.

[0048] like Figure 2 and Figure 3 As shown, the positive electrode 50 and the negative electrode 60 are stacked in such a manner that their positions are slightly offset in the long side direction, with the uncoated portion 52A of the positive electrode active material layer protruding from one end of the separator 70 in the long side direction and the uncoated portion 62A of the negative electrode active material layer protruding from the other end. As a result, as Figure 3As shown, at one end and the other end along the long side of the stacked electrode body 20, a portion having an uncoated portion 52A of the positive active material layer and a portion having an uncoated portion 62A of the negative active material layer are formed, respectively. The uncoated portions 52A and 62A are electrically connected to the positive terminal 42 and the negative terminal 44, respectively. While not particularly limited, the positive terminal 42 is typically made of aluminum or the like. The negative terminal 44 is made of copper or the like.

[0049] In the stacked electrode body 20, the length of the long side (Y direction) of the negative electrode active material layer 64 is preferably longer than the length of the long side (Y direction) of the positive electrode active material layer 54. In this case, when the positive electrode sheet 50 and the negative electrode sheet 60 are overlapped, the negative electrode active material layer 64 has a opposed portion opposite to the positive electrode active material layer 54 and a non-opposed portion not opposite to the positive electrode active material layer 54. By providing the non-opposed portion in the negative electrode active material layer 64, metal deposition (e.g., lithium deposition) on the negative electrode can be suppressed. On the other hand, if the non-opposed portion is too large, the irreversible capacity increases and the capacity retention rate decreases. From the above viewpoint, the difference in length in the Y direction between the positive electrode active material layer 54 and the negative electrode active material layer 64 (in other words, the phase difference between the positive electrode active material layer 54 and the negative electrode active material layer 64) is preferably about 1 mm to 5 mm (e.g., 1 mm to 3 mm).

[0050] The positive electrode 50 has a positive electrode active material layer 54 on a rectangular positive electrode current collector 52. Examples of materials that can be used as the positive electrode current collector 52 include metals with good conductivity such as aluminum, nickel, titanium, and stainless steel. Aluminum (e.g., aluminum foil) is particularly preferred. The thickness of the positive electrode current collector 52 is not particularly limited, but is, for example, 5 μm to 35 μm, preferably 7 μm to 20 μm.

[0051] The positive electrode active material layer 54 contains at least a positive electrode active material, which is a compound capable of reversibly adsorbing and releasing a chemical species (lithium ions in a lithium-ion secondary battery) that serves as a charge carrier. The positive electrode active material is not particularly limited, and one or more positive electrode active materials commonly used in non-aqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, can be used. For example, lithium composite oxides and lithium transition metal phosphate compounds (e.g., LiFePO4) are preferred as positive electrode active materials. Examples of lithium composite oxides include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, and lithium nickel manganese composite oxides (e.g., LiNi). 0.5 Mn 1.5 O4), lithium nickel manganese cobalt composite oxides (e.g., LiNi) 1 / 3 Co 1 / 3 Mn1 / 3 O2), etc.

[0052] The average particle size of the positive electrode active material is not particularly limited, and can be approximately 0.5 μm to 50 μm, typically 1 μm to 20 μm. It should be noted that in this specification, "average particle size" refers to the particle size (Dsize) that represents the cumulative frequency 50% by volume from the smallest particle side in a particle size distribution based on a general laser diffraction-light scattering method. 50 (also known as median particle size).

[0053] The positive electrode active material layer 54 may also contain substances other than the positive electrode active material, such as conductive materials, binders, etc. As a conductive material, carbon black such as acetylene black (AB) and other carbon materials (such as graphite) are preferred.

[0054] As a binder, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), perfluoroalkoxyalkylene (PFA), ethylene-tetrafluoroethylene polymer (ETFE), ethylene-trifluorochloroethylene copolymer (ECTFE), polyvinyl alcohol (PVA), and polyethylene oxide (PEO) are preferred examples. The solvent for the paste used to form the positive electrode active material layer 54 is a polar non-aqueous solvent (e.g., N-methylpyrrolidone). If the binder has too low an affinity (solubility, dispersibility) for the aforementioned polar non-aqueous solvent, it may be difficult to design an adhesive suitable for coating the paste used to form the positive electrode active material layer. From the above perspective, the binder preferably has excellent affinity for polar non-aqueous solvents. PVdF is an example of such a binder.

[0055] It should be noted that in this specification, "paste" is used as a term that includes forms referred to as "slurry" or "ink".

[0056] The negative electrode sheet 60 has a negative electrode active material layer 64 on the elongated negative electrode current collector 62. The negative electrode current collector 62 is made of a metallic material with good conductivity, such as copper, a copper-based alloy, nickel, titanium, or stainless steel. Copper (e.g., copper foil) is particularly preferred. The thickness of the negative electrode current collector 62 can be approximately 5 μm to 20 μm, and preferably 8 μm to 15 μm.

[0057] The negative electrode active material layer 64 contains at least one negative electrode active material, which is a compound capable of reversibly adsorbing and releasing a chemical species (lithium ions in lithium-ion secondary batteries) that serves as a charge carrier. The aforementioned negative electrode active material is not particularly limited; one or more negative electrode active materials commonly used in non-aqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, can be used. Examples of negative electrode active materials include, for instance, hard carbon, graphite, carbon materials such as boron-added carbon, and lithium titanate.

[0058] The negative electrode active material is typically in particulate form. The average particle size of the particulate negative electrode active material is not particularly limited, but can typically be 1 μm to 50 μm, for example, 1 μm to 20 μm.

[0059] The negative electrode active material layer 64 may contain substances other than the negative electrode active material, such as conductive materials and binders. Examples of conductive materials include carbon black such as acetylene black and Ketjen black, fumed carbon fiber (VGCF), and carbon nanotubes. Examples of binders include styrene-butadiene rubber (SBR). Furthermore, various additives such as thickeners, dispersants, and conductive materials can be appropriately used; for example, carboxymethyl cellulose (CMC) and methyl cellulose (MC) can be appropriately used as thickeners. Additionally, as an example of a solvent contained in the paste for forming the negative electrode active material layer 64, an aqueous solvent is preferred. An aqueous solvent refers to water or a water-based mixture.

[0060] The capacity ratio of the positive and negative electrodes can be adjusted based on differences in charge carrier acceptance characteristics. Specifically, the capacity C of the positive electrode can be adjusted... c (Ah) and negative electrode capacity C a (Ah) ratio (C) a / C c A value of 1.0 to 2.0 is appropriate, and 1.5 to 1.9 is preferred. It should be noted that the positive electrode capacity C... c (Ah) is defined as the product of the theoretical capacity per unit mass of the positive electrode active material (Ah / g) and the mass (g) of the positive electrode active material. Additionally, the negative electrode capacity C... a (Ah) is also defined as the product of the theoretical capacity per unit mass of the negative electrode active material (Ah / g) and the mass (g) of the negative electrode active material.

[0061] The separator 70 can be used without particular restriction in any type of separator used in this type of secondary battery. Examples include porous sheets (membranes) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. These porous sheets can be single-layer structures or multi-layer structures (e.g., a three-layer structure with PP layers laminated on both sides of a PE layer). A heat-resistant layer (HRL) can also be provided on the separator 70.

[0062] The thickness of the separator 70 is not particularly limited, but is preferably approximately 10 μm or more (typically 15 μm or more, for example 20 μm or more) and less than 100 μm (typically 90 μm or less, for example 80 μm or less). By keeping the average thickness of the separator 70 within the above range, ion permeability becomes better, and micro-short circuits (leakage current) are less likely to occur. Furthermore, the average pore size of the separator 70 is not particularly limited, and can be, for example, from 0.01 μm to 5 μm.

[0063] As a non-aqueous electrolyte, a liquid non-aqueous electrolyte (non-aqueous electrolyte solution) is typically used, in which the supporting salt (e.g., lithium salt, sodium salt, magnesium salt, etc.; lithium salt in lithium-ion secondary batteries) is dissolved or dispersed in a non-aqueous solvent. Alternatively, a non-aqueous electrolyte solution can be formed into a solid state (typically a so-called gel) by adding a polymer to the non-aqueous electrolyte solution.

[0064] As the supporting salt, there are no particular restrictions on the supporting salts used in conventional non-aqueous electrolyte secondary batteries. For example, lithium salts such as LiPF6, LiBF4, LiClO4, LiAsF6, and Li(C2F5SO2)2 can be used. LiPF6 is preferred. The concentration of the supporting salt is preferably 0.1 mol / L or higher, for example, 0.5 mol / L to 1.5 mol / L.

[0065] Examples of non-aqueous solvents include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate, and chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), and methyl butyl carbonate (MBC). Additionally, cyclic esters such as γ-butyrolactone, cyclic sulfones such as sulfolane, cyclic ethers such as dioxolane, chain carboxylic acid esters such as ethyl propionate, and chain ethers such as dimethoxyethane can also be used as non-aqueous solvents. One such non-aqueous solvent can be used alone or in appropriate combinations of two or more. From the perspective of obtaining an electrolyte with low viscosity, high degree of dissociation, and high ionic conductivity, a mixed solvent containing both cyclic and chain carbonates is particularly preferred.

[0066] Figure 4 This is a schematic diagram showing a partial cross-section of the electrodes (positive and / or negative) disclosed herein. (See diagram for example.)Figure 4 As shown, electrode 10 has an electrode active material layer 14 on electrode current collector 12. Electrode current collector 12 has an uncoated portion 16 where the electrode active material layer 14 is not formed and the current collector is exposed. Electrode active material layer 14 has a planar portion 14A with a thickness that is approximately constant based on an average film thickness t1, and an inclined portion 14B whose thickness continuously decreases as it approaches the uncoated portion 16. It should be noted that... Figure 4 As an example, a configuration is shown in which an uncoated portion 16 is provided only at one end (Y1 direction) in the long side direction, but it is not intended to limit the technology disclosed herein to the above configuration.

[0067] The length L1 of the long side of the electrode active material layer 14 is at least 300 mm. For example, the length L1 of the long side of the electrode active material layer 14 can be 400 mm or more, 500 mm or more, or 600 mm or more. By making the long side length L1 longer (elongated), the capacity of each battery cell can be increased. Furthermore, by making the secondary battery elongated, it is preferable to install it in a space with reduced gaps (dead zones) compared to installing multiple smaller batteries as in the past, especially in a vehicle. On the other hand, the upper limit of the length L1 of the long side of the electrode active material layer 14 can be appropriately adjusted according to the design of the product housing the secondary battery; for example, it can be 1400 mm or less, or 1300 mm or less. It should be noted that the length L1 of the long side of the electrode active material layer, as shown in the figure, is the sum of the length L3 of the long side of the planar portion 14A and the length L4 of the long side of the inclined portion 14B.

[0068] The planar portion 14A of the electrode active material layer 14 typically has a relatively constant thickness. The planar portion 14A is formed on the surface of the electrode current collector 12. The average film thickness t1 of the planar portion 14A is not particularly limited, and can be approximately 10 μm to 200 μm, typically 20 μm to 150 μm, for example 40 μm to 100 μm.

[0069] The planar portion 14A here includes the center of the electrode active material layer 14 in the longitudinal direction. The planar portion 14A has a length L3 in the longitudinal direction. The length L3 of the planar portion 14A in the longitudinal direction can be appropriately set such that the length L1 of the electrode active material layer 14 in the longitudinal direction is 300mm to 1400mm. For example, the length L3 of the planar portion 14A in the longitudinal direction can be about 260mm to 1360mm.

[0070] The inclined portion 14B of the electrode active material layer 14 extends from the planar portion 14A. Typically, the thickness of the inclined portion 14B decreases continuously as it approaches the uncoated portion 16 (i.e., the end of the electrode current collector 12 in the Y1 direction). The gradient of the inclined portion 14B is not particularly limited, but is preferably approximately constant. The gradient of the inclined portion 14B can be adjusted according to the viscosity of the paste used to form the electrode active material layer and the conditions of the manufacturing apparatus.

[0071] The average film thickness of the inclined portion 14B is set to be thinner than the average film thickness t1 of the planar portion 14A. The inclined portion 14B has a length L4 in the long side direction. The length L4 of the inclined portion 14B in the long side direction is generally shorter than the length L3 of the planar portion 14A in the long side direction. Although not particularly limited, the length L4 of the inclined portion 14B in the long side direction can be set such that the length L1 of the electrode active material layer 14 in the long side direction is 300 mm to 1400 mm and the length L2 from the uncoated portion 16 to position P (described later) is 0.5 mm to 25 mm. For example, the length L4 of the inclined portion 14B in the long side direction can be about 1 mm to 50 mm.

[0072] In the electrode disclosed herein, the thickness of the inclined portion 14B at position P is 0.8 times the average film thickness t1 of the planar portion 14A. That is, when the film thickness of the inclined portion 14B at position P is set to t2, t2 = 0.8 × t1. From the viewpoint of the durability (capacity retention rate) of the secondary battery, the length L2 from the boundary between the electrode active material layer 14 (more specifically, the inclined portion 14B) and the uncoated portion 16 to the long side direction of position P is preferably 0.5 mm or more, more preferably 1 mm or more, and even more preferably 10 mm or more. From the viewpoint of the volumetric efficiency of the electrode (the volume of the electrode opposing portion relative to the volume of the secondary battery), the length L2 from the uncoated portion 16 to position P is preferably 30 mm or less, more preferably 25 mm or less, and even more preferably 20 mm or less. By adjusting the length L2 from the uncoated portion 16 to position P to the above range, it is possible to achieve a balance between the durability and volumetric efficiency of the secondary battery.

[0073] Not intended to limit the technology disclosed herein, the reasons for the improved durability of secondary batteries are speculated as follows. In a typical secondary battery, charge carriers are absorbed and released between the positive and negative electrodes and the electrolyte, achieving charging and discharging through the resulting electrochemical reactions. During this process, the charge generated by the release of electrolyte ions from the electrode active material moves towards the electrode terminals within the electrode active material layer and electrode current collector and is then removed by an external load. Here, the density of the charge moving within the electrode active material layer and electrode current collector (i.e., current density) deviates within the electrode body. Typically, there is a trend where the current density is relatively higher near the electrode terminals (i.e., at the ends) and relatively lower in areas farther from the electrode terminals (i.e., the center). In particular, when the electrode active material layer is elongated as in the electrode disclosed herein, the deviation in current density between the center and ends of the electrode body becomes significant. Consequently, localized degradation due to lithium deposition occurs in a portion of the electrode, particularly at the ends, reducing the overall durability (capacity retention) of the secondary battery.

[0074] In contrast, according to the technology disclosed herein, a planar portion and an inclined portion are provided in an electrode active material layer of at least 300 mm, and the thickness of the inclined portion is set to be 0.8 times the average film thickness t1 of the planar portion. The length L2 between the position P and the uncoated portion is set to 0.5 mm to 25 mm. By setting the thinner region at the end with high current density to be longer than before, the capacity ratio (negative electrode capacity / positive electrode capacity) can be higher than that in the central portion during charging, suppressing lithium deposition on the negative electrode. Similarly, during discharge, lithium ions released from the negative electrode can be appropriately retained at the positive electrode, suppressing lithium deposition on the positive electrode. Thus, the durability (capacity retention) of the secondary battery is improved.

[0075] Furthermore, the shorter the length L2 from the uncoated portion 16 to position P, the higher the volumetric efficiency of the secondary battery. Based on the inventors' in-depth research, the volumetric efficiency is preferably 80 vol% or more, more preferably 85 vol% or more. Within this range, the volumetric efficiency is higher than that of conventional small secondary batteries, enabling the achievement of the target energy.

[0076] The electrode 10 described above can be fabricated, for example, as follows.

[0077] First, a paste for forming an electrode active material layer is prepared by dispersing materials such as electrode active materials in a suitable solvent (e.g., N-methylpyrrolidone, water, etc.). This paste can be prepared using mixing equipment such as planetary mixers, ball mills, roller mills, dispersers, and kneaders. The solid content concentration of the paste for forming the electrode active material layer can be, for example, 40% to 89% by mass.

[0078] The viscosity V1 of the paste used to form the electrode active material layer can be adjusted to approximately 2000 mPa·s to 34000 mPa·s, typically 3000 mPa·s to 33000 mPa·s, and for example, 5000 mPa·s to 33000 mPa·s. The viscosity V1 can be adjusted, for example, by changing the amount of solid material (binder, etc.) added to the solvent or the mixing time of the paste. By adjusting the viscosity V1 of the paste to an appropriate range, the average film thickness t1 of the planar portion 14A of the electrode active material layer 14 and the length L2 between the position P and the uncoated portion can be appropriately adjusted.

[0079] It should be noted that "viscosity" in this specification refers to shear viscosity (mPa·s), which can be easily measured using a commercially available rotational viscometer (such as the well-known Brookfield Type B viscometer).

[0080] Next, the electrode active material layer forming paste prepared above is applied to the surface of the electrode current collector 12, leaving the end in the Y1 direction uncoated. The paste application can be performed using a coating apparatus such as a die coater, slot coater, comma coater, or gravure coater. In a preferred embodiment, a die coater is prepared, comprising a conveying mechanism for transporting the electrode current collector 12 along its long side and a die for discharging the electrode active material layer forming paste. The die has a discharge section for discharging the paste and a feed valve and a return valve for switching the paste supply. When applying the paste, the feed valve is switched to supply the paste to the discharge section of the die, and the paste is applied to the electrode current collector 12. Conversely, when the paste application is stopped, the return valve is switched to return the paste to a storage tank. By adjusting the opening and closing time difference of the feed valve and the return valve, the length L2 from the uncoated portion 16 of the electrode active material layer 14 to position P can be adjusted. For example, the opening and closing time difference between the feed valve and the return valve can be adjusted within the range of 0 to 750 ms. Alternatively, the length L2 from the uncoated portion 16 of the electrode active material layer 14 to position P can be adjusted according to the conveying speed of the conveying mechanism. For example, the conveying speed can be adjusted to approximately 0.5 m / min to 20.0 m / min.

[0081] Next, an electrode active material layer 14 is formed on the electrode current collector 12 by removing the solvent from the paste coated on the electrode current collector 12 using drying or similar methods. The drying method can be any method conventionally used in such secondary batteries without particular limitations. For example, a heating dryer, a hot air dryer, or an infrared dryer can be used. The drying conditions, such as temperature and time, can be appropriately adjusted considering the type of solvent used and the content of solid components. It should be noted that, in order to adjust the thickness, density, etc., of the electrode active material layer 14 formed on the electrode current collector 12, the active material layer can be pressurized. The pressurization method is not particularly limited; for example, a rolling mill or a flat mill can be used. In this way, a battery can be manufactured... Figure 4 The electrode 10 shown has an electrode active material layer 14 having a planar portion 14A and an inclined portion 14B on the electrode current collector 12.

[0082] The non-aqueous electrolyte secondary battery with electrodes 10 configured as described above achieves a balance between improved durability and ensured volumetric efficiency. Therefore, utilizing these features, it can be used as a power source for driving electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like.

[0083] It should be noted that the above description is provided as an example of a box-shaped lithium-ion secondary battery 100 with stacked electrode bodies. However, the lithium-ion secondary battery 100 can also be configured as a lithium-ion secondary battery with wound electrode bodies. Furthermore, the shape of the lithium-ion secondary battery can also be cylindrical, laminated, etc. The technology disclosed herein can also be applied to non-aqueous electrolyte secondary batteries other than lithium-ion secondary batteries.

[0084] The following describes test examples of the secondary battery disclosed herein, but it is not intended to limit the technology disclosed herein to the technical solutions shown in the above test examples.

[0085] <The Production of Positive Electrode Tablets>

[0086] (Example 1)

[0087] A paste for forming the positive electrode active material layer was prepared by mixing LiMn2O4 (as the positive electrode active material), acetylene black (AB) (as the conductive material), and polyvinylidene fluoride (PVdF) (as the binder) in a mass ratio of 92:4:4 with N-methylpyrrolidone (NMP) (as the solvent) using a planetary mixer. Next, a rectangular aluminum foil was prepared as the positive electrode current collector. The paste for forming the positive electrode active material layer was applied to both sides of the positive electrode current collector (aluminum foil) using a die-coating machine and dried to produce the positive electrode sheet. It should be noted that the paste was applied along the long side of the positive electrode current collector, leaving an uncoated portion without the positive electrode active material layer at the end of the long side. Furthermore, the positive electrode active material layer has a planar portion and an inclined portion, and the average film thickness of the planar portion is 100 μm.

[0088] In Example 1, the length L1 of the positive electrode active material layer in the long direction is 300 mm. Position P is defined as the location where the thickness of the inclined portion is 0.8 times the average film thickness of the planar portion. Then, coating is performed with a length L2 from the uncoated portion of the positive electrode current collector to position P of 0.2 mm. It should be noted that the length L1 of the positive electrode active material layer in the long direction and the length L2 from the uncoated portion to position P are adjusted according to the viscosity of the paste used to form the positive electrode active material layer and the conveying speed. In Example 1, the viscosity of the paste used to form the positive electrode active material layer is 35000 mPa·s, and the conveying speed is 0.5 m / min. It should be noted that the viscosity here refers to the viscosity measured at 25°C using a rheometer with a shear rate of 21.5 s⁻¹. －1 The value obtained through measurement.

[0089] (Examples 2~7)

[0090] The viscosity and conveying speed of the paste for forming the positive electrode active material layer were adjusted so that the length L1 of the long side of the positive electrode active material layer was 300 mm and the length L2 from the uncoated part to position P was the length shown in Table 1. Otherwise, the positive electrode sheets of Examples 2 to 7 were produced in the same manner as in Example 1.

[0091] Specifically, in Example 2, the viscosity of the paste used to form the positive electrode active material layer was adjusted to 33000 mPa·s, and the conveying speed was adjusted to 0.5 m / min.

[0092] Example 3: The viscosity of the paste used to form the positive electrode active material layer was adjusted to 30000 mPa·s, and the conveying speed was adjusted to 0.7 m / min.

[0093] Example 4: The viscosity of the paste used to form the positive electrode active material layer is adjusted to 20000 mPa·s, and the conveying speed is adjusted to 1.5 m / min.

[0094] Example 5: The viscosity of the paste used to form the positive electrode active material layer is adjusted to 15000 mPa·s, and the conveying speed is adjusted to 2.0 m / min.

[0095] Example 6: The viscosity of the paste used to form the positive electrode active material layer is adjusted to 5000 mPa·s, and the conveying speed is adjusted to 10.0 m / min.

[0096] Example 7: The viscosity of the paste used to form the positive electrode active material layer is adjusted to 2000 mPa·s, and the conveying speed is adjusted to 20.0 m / min.

[0097] (Examples 11 to 17 and Examples 21 to 27)

[0098] The viscosity and conveying speed of the paste for forming the positive electrode active material layer were adjusted so that the length L1 of the long side of the positive electrode active material layer was 625 mm and the length L2 from the uncoated portion to position P was the length shown in Table 1. Otherwise, positive electrode sheets of Examples 11-17 were produced in the same manner as Example 1. The viscosity and conveying speed of the paste for forming the positive electrode active material layer were adjusted so that the length L1 of the long side of the positive electrode active material layer was 1400 mm and the length L2 from the uncoated portion to position P was the length shown in Table 1. Otherwise, positive electrode sheets of Examples 21-27 were produced in the same manner as Example 1. Specifically, in Examples 11 and 21, the viscosity of the paste for forming the positive electrode active material layer was adjusted to 35000 mPa·s, and the conveying speed was adjusted to 0.5 m / min.

[0099] In Examples 12 and 22, the viscosity of the paste used to form the positive electrode active material layer was adjusted to 33000 mPa·s, and the conveying speed was adjusted to 0.5 m / min.

[0100] In Examples 13 and 23, the viscosity of the paste used to form the positive electrode active material layer was adjusted to 30,000 mPa·s, and the conveying speed was adjusted to 0.7 m / min.

[0101] In Examples 14 and 24, the viscosity of the paste used to form the positive electrode active material layer was adjusted to 20,000 mPa·s, and the conveying speed was adjusted to 1.5 m / min.

[0102] In Examples 15 and 25, the viscosity of the paste used to form the positive electrode active material layer was adjusted to 15000 mPa·s, and the conveying speed was adjusted to 2.0 m / min.

[0103] In Examples 16 and 26, the viscosity of the paste used to form the positive electrode active material layer was adjusted to 5000 mPa·s, and the conveying speed was adjusted to 10.0 m / min.

[0104] Examples 17 and 27 show that the viscosity of the paste used to form the positive electrode active material layer is adjusted to 2000 mPa·s and the conveying speed is adjusted to 20.0 m / min.

[0105] (Example for reference)

[0106] As a reference example, an electrode found in conventional secondary batteries is fabricated. Specifically, the positive electrode sheet is fabricated in the same manner as in Example 1, with the length L1 of the long side of the positive electrode active material layer being 250 mm and the length L2 from the uncoated portion to position P being 0.2 mm.

[0107] <Evaluation of the fabrication of lithium-ion secondary batteries>

[0108] Natural lead (C) as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener were mixed with deionized water as the solvent in a mass ratio of 98:1:1 using a planetary mixer to prepare a paste for forming the negative electrode active material layer. Next, a rectangular copper foil was prepared as the negative electrode current collector. The paste for forming the negative electrode active material layer was applied to both sides of the negative electrode current collector (copper foil) using a die-coating machine and dried to produce the negative electrode sheet. It should be noted that the paste was applied along the long side of the negative electrode current collector, leaving an uncoated portion at the end of the current collector without the negative electrode active material layer.

[0109] As a separator, a single-layer porous sheet (17 μm thick) made of polyethylene (PE) is prepared.

[0110] The prepared positive and negative electrode sheets are stacked in a manner that separates them from the prepared separator. At this time, the sheets are cut to a phase difference of 1.5 mm between the positive and negative electrodes and a phase difference of 1.5 mm between the negative electrode and the separator. Next, the positive electrode terminals and negative electrode terminals are connected to the uncoated portions of the positive and negative current collectors, respectively. They are then sandwiched between two laminated films, and the periphery is heat-fused. After injecting a non-aqueous electrolyte, the sheet is sealed, thus producing an evaluation lithium-ion secondary battery. It should be noted that the non-aqueous electrolyte is prepared by dissolving LiPF6, the supporting salt, at a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:30:40.

[0111] <Calculation of volumetric efficiency>

[0112] The volumetric efficiency of each of the above-described examples was calculated using lithium-ion secondary batteries. The volumetric efficiency (vol%) was calculated using the following formula: Volumetric efficiency (vol%) = (Volume of the positive electrode / Total volume of the secondary battery) × 100. The results are shown in Table 1.

[0113] It should be noted that a volumetric efficiency of 85 vol% or higher is rated as "◎", 80 vol% or higher as "○", and less than 80 vol% as "×". The results are shown in Table 1.

[0114] <Measurement of Temperature Difference in Secondary Batteries>

[0115] The examples fabricated above were evaluated using lithium-ion secondary batteries subjected to initial charge-discharge treatment. Subsequently, the batteries were charged at a constant current of 1.5C, and the temperatures of the central and terminal portions (electrode terminals) of the secondary batteries were measured using a non-contact thermometer after 30 minutes. At this point, with the temperature difference between the central and terminal portions of the secondary battery within 20°C, it can be evaluated that the current density unevenness between the terminal and central portions was suppressed. The results are shown in Table 1.

[0116] It should be noted that "1C" refers to the current value that can fully charge the battery capacity (Ah) predicted by the theoretical capacity of the positive electrode active material in 1 hour.

[0117] <Post-cycle capacity retention>

[0118] At 25°C, for each example, lithium-ion secondary batteries were evaluated by constant current charging (CC charging) at 1C until SOC reached 95%, followed by constant current discharging (CC discharging) at 1.0C until SOC reached 5%. The discharge capacity during CC discharging was taken as the initial capacity. Next, at 25°C, the batteries were CC charged at 1C until SOC reached 95%, followed by CC discharging at 1.0C until SOC reached 5%. This charge-discharge cycle was considered one cycle, and 100 cycles were performed. The discharge capacity of the 100th cycle was taken as the post-cycle capacity and calculated using the same method as the initial capacity. The capacity retention rate (%) was calculated as an indicator of durability using the following formula: Capacity retention rate (%) = (Post-cycle capacity / Initial capacity) × 100. The results are shown in Table 1.

[0119] [Table 1]

[0120] Table 1

[0121]

[0122] As shown in Table 1, it can be seen that for Examples 1-7 with a long side length L1 of 300 mm and Examples 11-17 with a length L1 of 625 mm, from the viewpoint of capacity retention, a length L2 from the uncoated portion to position P of 0.5 mm or more is sufficient. In particular, when the length L2 from the uncoated portion to position P is 10 mm or more, the capacity retention exceeds 90%, ensuring good capacity retention. On the other hand, from the viewpoint of volumetric efficiency, as long as the length L2 from the uncoated portion to position P is 25 mm or less, a volumetric efficiency of 85 vol% or more can be achieved. The shorter the length L2 from the uncoated portion to position P, the higher the volumetric efficiency.

[0123] Therefore, if both capacity retention and volumetric efficiency are taken into account, the length L2 from the uncoated portion to position P is preferably 0.5 mm to 25 mm, more preferably 1 mm to 25 mm.

[0124] Furthermore, as shown in Table 1, in Examples 21-27 of secondary batteries where the length L1 of the electrode active material layer is 1400 mm and is particularly elongated, from the viewpoint of capacity retention, the length L2 from the uncoated portion to position P only needs to be 0.5 mm or more. On the other hand, from the viewpoint of volumetric efficiency, as long as the length L2 from the uncoated portion to position P is 25 mm or less, a volumetric efficiency of 85 vol% or more can be achieved, and the shorter the length L2 from the uncoated portion to position P, the higher the volumetric efficiency.

[0125] Therefore, if both capacity retention and volumetric efficiency are taken into account, the length L2 from the uncoated portion to position P is preferably 0.5 mm to 25 mm.

[0126] Based on the above results, a secondary battery with an electrode having a length L1 of 300 mm or more in the long side direction of the electrode active material layer, having a planar portion and an inclined portion, and having a length L2 of 0.5 mm to 25 mm from the boundary between the electrode active material layer and the uncoated portion to position P is a secondary battery that achieves improved durability and volumetric efficiency.

[0127] The specific examples of the present invention have been described in detail above, but these are merely illustrative and do not limit the scope of protection claimed. The technology described in the scope of protection includes technologies obtained by various modifications and alterations to the specific examples described above.

Claims

1. An electrode for a secondary battery, characterized in that, It is either the positive or negative electrode of a secondary battery, having a rectangular sheet-shaped electrode current collector and an electrode active material layer formed on the electrode current collector. The electrode current collector has an uncoated portion exposed at at least one end in the long side direction, where the electrode active material layer is not formed. The length L1 of the long side of the electrode active material layer is 600 mm to 1400 mm. The electrode active material layer has a planar portion with a generally constant thickness based on the average film thickness t1, and an inclined portion with a thickness that continuously decreases as it approaches the uncoated portion. When the position where the thickness of the inclined portion is 0.8 of the average film thickness t1 of the planar portion is set as P, The length L2 from the boundary between the electrode active material layer and the uncoated portion to the position P is 10 mm to 25 mm.

2. A non-aqueous electrolyte secondary battery, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. At least one of the positive and negative electrodes is the electrode described in claim 1.

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