Secondary battery

Abstract

This secondary battery comprises: an electrode body in which a band-shaped first electrode (11) and a band-shaped second electrode having mutually different polarities are wound with a separator interposed therebetween; and an exterior body that accommodates the electrode body. The secondary battery is characterized in that: the first electrode (11) has a core body (30) and a mixture layer (31) disposed on the core body (30); and the resistance of the first electrode (11) at a winding start edge part (31X) of the mixture layer (31) is lower than the resistance of the first electrode (11) at a winding end edge part (31Y) of the mixture layer (31).

Description

secondary battery

[0001] This disclosure relates to secondary batteries.

[0002] Previously, secondary batteries comprising an electrode body in which a positive electrode and a negative electrode are wound with a separator in between, and an outer casing housing the electrode body, have been widely known. Generally, the positive electrode has a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core, and the negative electrode has a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core (see, for example, Patent Document 1).

[0003] Japanese Patent Publication No. 2011-60607

[0004] Incidentally, in secondary batteries, abnormal heat generation can occur inside the battery when a large current flows due to overcharging or an external short circuit. In particular, if heat is generated around the outer periphery of the electrode body, the temperature of the outer casing increases, which may damage the casing. From the perspective of improving battery safety, there is a need for technology that can suppress damage to the casing even when abnormal heat generation occurs inside the battery.

[0005] A secondary battery according to one aspect of the present disclosure comprises an electrode body in which a strip-shaped first electrode and a strip-shaped second electrode having opposite polarities are wound via a separator, and an outer casing for housing the electrode body, wherein the first electrode has a core body and a composite layer disposed on the core body, and the resistance of the first electrode at the beginning end of the winding of the composite layer is smaller than the resistance of the first electrode at the end end of the winding of the composite layer.

[0006] According to one embodiment of the secondary battery of this disclosure, when a large current flows due to overcharging or an external short circuit, heat generation is less likely to occur on the outer periphery of the electrode body. As a result, even if abnormal heat generation occurs inside the battery, damage to the outer casing can be suppressed.

[0007] This is an axial cross-sectional view of a secondary battery, which is an example of an embodiment. This is a diagram showing the positive electrode in an unfolded state, which is also an example of an embodiment.

[0008] In the following, an example of an embodiment of the secondary battery according to this disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc., are examples to facilitate understanding of the present invention and can be appropriately modified according to the specifications of the secondary battery. Furthermore, if the following description includes multiple embodiments and modifications, it is intended from the outset that their characteristic parts may be used in appropriate combinations.

[0009] Furthermore, while the following example illustrates a cylindrical secondary battery in which wound-type electrodes are housed in a bottomed cylindrical casing, the battery casing is not limited to a cylindrical shape. The secondary battery according to this disclosure may, for example, be a prismatic battery equipped with a prismatic casing. The electrodes may also be flattened wound-type electrodes.

[0010] Figure 1 is an axial cross-sectional view of a secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the secondary battery 10 comprises an electrode body 14 in which a first electrode and a second electrode are wound around a separator 13, a non-aqueous electrolyte, and a bottomed cylindrical casing 16 that houses the electrode body 14. The secondary battery 10 also comprises a non-aqueous electrolyte housed in the casing 16 and a sealing body 17 that closes the opening of the casing 16. For convenience of explanation, the side of the sealing body 17 of the secondary battery 10 will be referred to as "upper," and the bottom side of the casing 16 will be referred to as "lower." Furthermore, the following description will focus on the case where the first electrode is the positive electrode 11 and the second electrode is the negative electrode 12.

[0011] The electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound in a spiral shape via a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all elongated strip-shaped bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in the longitudinal and width directions than the positive electrode 11. The separator 13 is formed to be at least slightly larger in dimensions than the positive electrode 11, and for example, two separators are arranged so as to sandwich the positive electrode 11.

[0012] In this embodiment, the electrode body 14 has a positive electrode tab 20 connected to the positive electrode 11 and a negative electrode tab 21 connected to the negative electrode 12. The positive electrode tab 20 is, for example, an aluminum tab, and the negative electrode tab 21 is, for example, a nickel tab. The positive electrode tab 20 is connected to the middle of the positive electrode 11 in the longitudinal direction. The negative electrode tab 21 is provided at one end in the longitudinal direction of the negative electrode 12, which is located on the winding start side of the electrode body 14. Note that the current collection configuration of the positive electrode 11 and the negative electrode 12 is not limited to this. For example, the negative electrode tab 21 may be provided at one end in the longitudinal direction of the negative electrode 12, which is located on the winding end side of the electrode body 14.

[0013] Non-aqueous electrolytes are lithium ion conductive. Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0014] A liquid electrolyte (electrolyte solution) comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms of the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0015] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0016] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode tab 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode tab 21 extends through a through-hole in the insulating plate 19 towards the bottom of the outer casing 16. The positive electrode tab 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode tab 21 is connected to the bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative electrode terminal.

[0017] The outer casing 16 is a bottomed cylindrical metal container. The outer casing 16 is made of a material mainly composed of iron, such as carbon steel or stainless steel. The thickness of the outer casing 16 is not particularly limited, and is, for example, 0.2 mm or more and 0.5 mm or less.

[0018] A gasket 28 is provided between the outer casing 16 and the sealing body 17, sealing the inside of the battery. The outer casing 16 has grooves 22 formed by pressing its side surface from the outside. The grooves 22 are preferably formed in an annular shape along the circumferential direction of the outer casing 16, and their upper surface supports the sealing body 17. The upper end of the outer casing 16 is bent inward and crimped to the periphery of the sealing body 17.

[0019] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective radial centers, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27. This interrupts the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0020] Next, with further reference to Figure 2, the positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode body 14 will be described in detail, with particular emphasis on the positive electrode 11. Figure 2 is a plan view showing the positive electrode 11 in an unfolded state.

[0021] [Positive Electrode] As shown in Figures 1 and 2, the positive electrode 11 has a strip-shaped positive electrode core 30 and a positive electrode mixture layer 31 disposed on the positive electrode core 30. The positive electrode 11 also has a portion 32 in the longitudinal middle where the positive electrode mixture layer 31 is not disposed on the positive electrode core 30. A positive electrode tab 20 is connected to the portion 32 where the positive electrode mixture layer is not disposed. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, a binder, etc., and is formed on both sides of the positive electrode core 30, excluding the portion 32 where the mixture layer is not disposed. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc., to the surface of the positive electrode core 30, drying the coating, and then rolling to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0022] The positive electrode core 30 is made of a metal foil obtained by rolling aluminum or an aluminum alloy. The thickness of the positive electrode core 30 is approximately constant along the longitudinal direction of the positive electrode 11, for example, between 5 μm and 20 μm.

[0023] Examples of positive electrode active materials included in the positive electrode mixture layer 31 include particulate lithium metal composite oxides. Lithium metal composite oxides are composite oxides containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal elements constituting the lithium metal composite oxide are, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, and Mn. Examples of suitable composite oxides include lithium metal composite oxides containing Ni, Co, and Mn, or lithium metal composite oxides containing Ni, Co, and Al.

[0024] Examples of conductive agents included in the positive electrode mixture layer 31 include carbon black (CB), acetylene black (AB), Ketjenblack, carbon nanotubes (CNT), graphene, and carbon-based particles such as graphite. These may be used individually or in combination of two or more types. As will be described in detail later, the positive electrode mixture layer 31 of this embodiment contains different types of conductive agents in the longitudinal direction of the positive electrode 11.

[0025] Examples of binders included in the positive electrode mixture layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyimide resins, acrylic resins, polyolefin resins, and polyacrylonitrile (PAN). These may be used individually or in combination of two or more types.

[0026] In this embodiment, the positive electrode 11 has a resistance at the winding start end 31X of the positive electrode mixture layer 31 that is smaller than the resistance at the winding end 31Y of the positive electrode mixture layer 31. As a result, when a large current flows due to overcharging or an external short circuit, the current flows more towards the inner circumference of the positive electrode 11, and the amount of heat generated on the inner circumference of the positive electrode 11 is greater than that generated on the outer circumference. Consequently, the temperature of the side surface of the outer casing 16 is less likely to increase, and damage such as melting of the outer casing 16 can be suppressed. In other words, if the resistance of the positive electrode 11 is constant along the longitudinal direction, when overcharging or an external short circuit occurs, the amount of heat generated on the outer circumference of the positive electrode 11 increases, and the temperature of the side surface of the outer casing 16 may increase. As a result, the side surface of the outer casing 16 may melt, potentially causing damage such as openings or cracks.

[0027] The resistance of the positive electrode 11 can be measured using an electrode resistance meter (device name: RM2610) manufactured by HIOKI E.E. CORPORATION. Specifically, the positive electrode 11 is removed from the secondary battery 10, the measuring probe is brought into contact with the surface of the positive electrode mixture layer 31, and the resistance of the positive electrode 11 is measured using the four-terminal four-probe method. The measurement conditions are as follows. Note that the resistance of the positive electrode 11 at the winding start end 31X of the positive electrode mixture layer 31 refers to the resistance of the positive electrode 11 in the region within 50 mm from the winding start end 31X of the positive electrode mixture layer 31. Similarly, the resistance of the positive electrode 11 at the winding end 31Y of the positive electrode mixture layer 31 refers to the resistance of the positive electrode 11 in the region within 50 mm from the winding end 31Y of the positive electrode mixture layer 31. <Measurement conditions> Measurement current: 100 μA Voltage range: 0.5 V

[0028] The ratio of the resistance of the positive electrode 11 at the winding end 31Y of the positive electrode mixture layer 31 to the resistance of the positive electrode 11 at the winding start end 31X of the positive electrode mixture layer 31 is preferably 1.2 or more, more preferably 1.5 or more, and even more preferably 2.0 or more. By setting this ratio to 1.2 or more, when a large current flows due to overcharging or an external short circuit, the current is more likely to flow towards the inner circumference of the positive electrode 11. As a result, heat generation on the outer circumference of the positive electrode 11 is further suppressed, and damage due to temperature rise on the side surface of the outer casing 16 can be further suppressed. The upper limit of this ratio is not particularly limited and is, for example, 20.

[0029] In this embodiment, the positive electrode mixture layer 31 includes a first positive electrode mixture layer 33 arranged from the winding start end 31X toward the winding end, and a second positive electrode mixture layer 34 arranged from the winding end end 31Y toward the winding start. The resistance of the first positive electrode mixture layer 33 is configured to be smaller than the resistance of the second positive electrode mixture layer 34.

[0030] Furthermore, the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 are arranged in the longitudinal direction of the positive electrode 11 via a non-compounding layer portion 32. In other words, the region closer to the start of winding than the non-compounding layer portion 32 is the first positive electrode mixture layer 33, and the region closer to the end of winding than the non-compounding layer portion 32 is the second positive electrode mixture layer 34. By arranging the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 via a non-compounding layer portion 32 in this way, a positive electrode 11 having the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 can be easily manufactured.

[0031] In the longitudinal direction of the positive electrode 11, the length of the first positive electrode mixture layer 33 is preferably 3% or more of the length of the positive electrode 11, and more preferably 5% or more. In this case, when a large current flows due to overcharging or an external short circuit, the current flows more easily to the inner circumference of the positive electrode 11, and the amount of heat generated on the inner circumference of the positive electrode 11 is greater than the amount of heat generated on the outer circumference. Furthermore, in the longitudinal direction of the positive electrode 11, the length of the first positive electrode mixture layer 33 is preferably 80% or less of the length of the positive electrode 11, and more preferably 70% or less. In this case, when a large current flows due to overcharging or an external short circuit, the heat generation on the outer circumference of the positive electrode 11 is further suppressed, and damage to the outer surface of the casing 16 due to temperature rise can be further suppressed. Therefore, in the longitudinal direction of the positive electrode 11, the length of the first positive electrode mixture layer 33 is preferably 3% or more and 80% or less of the length of the positive electrode 11, and more preferably 5% or more and 70% or less.

[0032] Furthermore, it is preferable that the average thickness of the first positive electrode mixture layer 33 and the average thickness of the second positive electrode mixture layer 34 are approximately the same. In this case, the energy density of the positive electrode 11 can be improved, making it easier to realize a high-capacity secondary battery 10. The thicknesses of the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 are uniform in the longitudinal direction, for example, 50 μm or more and 120 μm or less on one side of the positive electrode core 30. Note that the average thickness of the first positive electrode mixture layer 33 and the average thickness of the second positive electrode mixture layer 34 being approximately the same means that the average thickness T1 of the first positive electrode mixture layer 33 and the average thickness T2 of the second positive electrode mixture layer 34 satisfy the following equation. T1 and T2 are calculated by cross-sectional observation of the positive electrode 11 using a scanning electron microscope (SEM) and are the average values ​​of the thickness at any 50 locations in each layer. -5≦(T1-T2)×100 / {(T1+T2) / 2}≦5

[0033] In this embodiment, the resistance of the first positive electrode mixture layer 33 is made smaller than that of the second positive electrode mixture layer 34 by changing the type of conductive agent in the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34. Specifically, the first positive electrode mixture layer 33 contains a fibrous carbon material as the conductive agent, and the second positive electrode mixture layer 34 contains a particulate carbon material as the conductive agent. Note that the second positive electrode mixture layer 34 does not necessarily have to contain a conductive agent.

[0034] The fibrous carbon material contained in the first positive electrode mixture layer 33 may be carbon nanofiber (CNF) or the like, but is preferably carbon nanotube (CNT). Carbon nanotube is a conductive carbon fiber with an outer diameter of several tens of nanometers or less, and has an extremely large aspect ratio (ratio of fiber length to fiber diameter). The average aspect ratio of carbon nanotube is, for example, 20 times or more, and preferably 50 times or more. With carbon nanotube having a high aspect ratio, contact with the positive electrode active material and the positive electrode core 30 becomes linear contact rather than point contact. Therefore, even with a small amount of addition, a good conductive path is formed, and low resistance can be achieved.

[0035] The average fiber diameter of the carbon nanotubes is, for example, 30 nm or less, preferably 20 nm or less. Note that fiber diameter refers to the length in the direction perpendicular to the fiber length direction. An average fiber diameter of 20 nm or less makes it easier to reduce the resistance of the positive electrode mixture layer 31. The lower limit of the average fiber diameter of the carbon nanotubes is not particularly limited, but one example is 1 nm. The average fiber diameter of the carbon nanotubes is determined by image analysis using TEM. The average fiber diameter of the carbon nanotubes is determined by arbitrarily selecting 100 carbon nanotubes, measuring their fiber diameters, and taking the arithmetic mean of the measured values.

[0036] The average fiber length of the carbon nanotubes is, for example, 0.5 μm or more, and may be 1 μm or more. Fiber length refers to the length of the carbon nanotube when stretched in a straight line. An average fiber length of 0.5 μm or more makes it easier to reduce the resistance of the positive electrode mixture layer 31. There is no particular upper limit to the average fiber length of the carbon nanotubes, but one example is 100 μm. The average fiber length of the carbon nanotubes is determined by image analysis using a scanning electron microscope (SEM). The average fiber length of the carbon nanotubes is determined by arbitrarily selecting 100 carbon nanotubes, measuring their lengths, and taking the arithmetic mean of the measured values.

[0037] Carbon nanotubes may be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), and a combination of both may be used. Single-walled carbon nanotubes have a structure in which one layer of graphite sheet is formed into a tube, while multi-walled carbon nanotubes have a structure in which multiple layers of graphite sheet are formed into a tube. An example of a multi-walled carbon nanotube is a double-walled carbon nanotube having a two-layer structure.

[0038] The optimal BET specific surface area for carbon nanotubes varies slightly depending on the type of carbon nanotube, but one example is 200 m². ２ It is 1 / g or more, preferably 250m ２ It is 1 / g or more. The upper limit of the BET specific surface area is not particularly limited, but one example is 2000 m２ / g. The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0039] The content of the fibrous carbon material in the first positive electrode mixture layer 33 is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, based on the total mass of the first positive electrode mixture layer 33. When the content of the fibrous carbon material in the first positive electrode mixture layer 33 is 0.1% by mass or more, it is easy to reduce the resistance of the first positive electrode mixture layer 33. When a large current flows due to overcharge, external short circuit or other reasons, the current is more likely to flow toward the inner peripheral side of the positive electrode 11. As a result, heat generation on the outer peripheral side of the positive electrode 11 is further suppressed, and damage to the exterior body 16 caused by the temperature rise on the side surface thereof can be further suppressed.

[0040] Further, the content of the fibrous carbon material in the first positive electrode mixture layer 33 is preferably 3% by mass or less, more preferably 2.5% by mass or less, based on the total mass of the first positive electrode mixture layer 33. When the content of the fibrous carbon material in the first positive electrode mixture layer 33 is 3% by mass or less, the proportion of the positive electrode active material in the first positive electrode mixture layer 33 increases, making it easy to increase the capacity of the battery. Therefore, the content of the fibrous carbon material in the first positive electrode mixture layer 33 is preferably 0.1% by mass or more and 3% by mass or less based on the total mass of the first positive electrode mixture layer 33, and more preferably 0.3% by mass or more and 2.5% by mass or less.

[0041] The particulate carbon material contained in the second positive electrode mixture layer 34 may be Ketjenblack, furnace black, graphite, etc., but is preferably acetylene black (AB). The average particle diameter of the particulate carbon material is, for example, 50 nm or less, and may be 30 nm or less. The average particle diameter of the particulate carbon material is determined by image analysis using a transmission electron microscope (TEM). The average particle diameter of the particulate carbon material is obtained by arbitrarily selecting 100 particulate carbon material particles, measuring the major diameter of the smallest particle, and arithmetically averaging the measured values. The lower limit of the average particle diameter of the particulate carbon material is not particularly limited, and is 1 nm as an example.

[0042] The content of the particulate carbon material in the second positive electrode mixture layer 34 is not particularly limited as long as the resistance of the second positive electrode mixture layer 34 is in a range larger than the resistance of the first positive electrode mixture layer 33. The content of the particulate carbon material in the second positive electrode mixture layer 34 is, for example, 0% by mass or more and 3% by mass or less, and may be 0.3% by mass or more and 2.5% by mass or less, based on the total mass of the second positive electrode mixture layer 34.

[0043] The positive electrode 11 of the present embodiment can be formed by intermittently applying a first positive electrode mixture slurry constituting the first positive electrode mixture layer 33 and a second positive electrode mixture slurry constituting the second positive electrode mixture layer 34. Note that the method for producing the positive electrode 11 is not limited thereto.

[0044] In the above embodiment, the resistance of the first positive electrode mixture layer 33 is made lower than the resistance of the second positive electrode mixture layer 34 by changing the type of the conductive agent between the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34, but the method for adjusting the resistance is not limited thereto. For example, the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 may contain the same conductive agent, and the resistance may be adjusted by making the content of the conductive agent in the first positive electrode mixture layer 33 larger than the content of the conductive agent in the second positive electrode mixture layer 34. When adjusting the resistance between the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 by the type of conductive agent or the content of the conductive agent, it becomes easy to increase the resistance difference between the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34, and the effect of the present disclosure is exhibited more remarkably.

[0045] Furthermore, in the above embodiment, the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 are arranged in the longitudinal direction of the positive electrode 11 with the mixture layer non-arranged portion 32 interposed therebetween, but the arrangement form of the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 is not limited thereto. For example, the first positive electrode mixture layer 33 or the second positive electrode mixture layer 34 may be arranged so as to straddle the mixture layer non-arranged portion 32. That is, in a region on either side of the mixture layer non-arranged portion 32 in the longitudinal direction of the positive electrode 11, both the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 may be arranged.

[0046] Furthermore, in the above embodiment, the positive electrode mixture layer 31 has only a first positive electrode mixture layer 33 and a second positive electrode mixture layer 34, but the form of the positive electrode mixture layer 31 is not limited thereto. For example, the positive electrode mixture layer 31 may have a layer (third positive electrode mixture layer) between the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34 in the longitudinal direction of the positive electrode 11, with a resistance value different from that of the first positive electrode mixture layer 33 and the second positive electrode mixture layer 34. The resistance of the third positive electrode mixture layer may be lower than that of the first positive electrode mixture layer 33, or higher than that of the first positive electrode mixture layer 33.

[0047] [Negative Electrode] As shown in Figure 1, the negative electrode 12 has a negative electrode core body 40 and a negative electrode mixture layer 41 formed on the negative electrode core body 40. The negative electrode core body 40 can be made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal arranged on its surface. The negative electrode mixture layer 41 contains a negative electrode active material, a binder, and optionally a conductive agent, and is preferably formed on both sides of the negative electrode core body 40. The negative electrode 12 can be manufactured by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode core body 40, drying the coating, and then compressing it to form the negative electrode mixture layer 41 on both sides of the negative electrode core body 40.

[0048] The negative electrode mixture layer 41 preferably contains a carbon material and a silicon-containing material as negative electrode active materials. Including a silicon-containing material as a negative electrode active material makes it easier to achieve high capacity. In addition, as a negative electrode active material, a material containing at least one of an element that alloys with Li, such as Sn, and a material containing said element may be used in combination.

[0049] From the viewpoint of increasing capacity, the silicon-containing material content is preferably 5% by mass or more, and more preferably 10% by mass or more, of the total mass of the negative electrode active material. Furthermore, in high-capacity, high-energy-density batteries, the amount of heat generated by the electrode body 14 tends to increase when a large current flows due to overcharging or external short circuits. Therefore, when the negative electrode active material contains a silicon-containing material, the effects of this disclosure are exhibited more significantly.

[0050] The carbon material that functions as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. In particular, it is preferable to use artificial graphite such as massive artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, or earthy graphite, or a mixture thereof as the carbon material. The average particle size on a volume basis of the carbon material is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less. The particle size distribution of the carbon material (and silicon-containing materials) can be measured using a laser diffraction particle size distribution analyzer (for example, MT3000II manufactured by Microtrac-Bell Co., Ltd.) with water as the dispersion medium.

[0051] The silicon-containing material can be any material containing Si, and examples include silicon alloys, silicon compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred. The average particle size by volume of the composite material is generally smaller than the average particle size by volume of graphite. For example, the average particle size by volume of the composite material is between 1 μm and 15 μm. One type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0052] A suitable silicon-containing material (composite material) is a composite particle comprising an ionic conductive phase and a Si phase dispersed within the ionic conductive phase. The ionic conductive phase is, for example, at least one selected from the group consisting of a silicate phase, an amorphous carbon phase, a silicide phase, and a silicon oxide phase. The Si phase is formed by dispersing Si in the form of fine particles. The ionic conductive phase is a continuous phase composed of an aggregate of particles finer than those of the Si phase. The composite material may also have a conductive layer covering the surface of the ionic conductive phase. The conductive layer is composed of a material with higher conductivity than the ionic conductive phase and forms a good conductive path in the negative electrode mixture layer 41.

[0053] An example of a suitable composite material containing Si is one having a sea-island structure in which fine Si particles are dispersed substantially uniformly in an amorphous silicon oxide phase, and the overall general formula is SiO ｘThese are composite particles represented by (0 < x ≤ 2). The main component of silicon oxide may be silicon dioxide. The oxygen content ratio (x) to Si is, for example, 0.5 ≤ x < 2.0, and preferably 0.8 ≤ x ≤ 1.5.

[0054] The binder in the negative electrode mixture layer 41 may be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., as in the positive electrode mixture layer 31, but styrene-butadiene rubber (SBR) is preferred. Furthermore, the negative electrode mixture layer 41 preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. In particular, it is preferable to use SBR in combination with CMC or a salt thereof, PAA or a salt thereof. The negative electrode mixture layer 41 may also contain a conductive agent such as CNT.

[0055] In this embodiment, the negative electrode 12 has a substantially uniform structure along its longitudinal direction and has substantially constant resistance along its longitudinal direction. Similar to the case of the positive electrode 11, the resistance of the negative electrode 12 at the beginning of winding the negative electrode mixture layer 41 may be smaller than the resistance of the negative electrode 12 at the end of winding the negative electrode mixture layer 41. If the resistance of the negative electrode 12 at the beginning of winding the negative electrode mixture layer 41 is smaller than the resistance of the negative electrode 12 at the end of winding the negative electrode mixture layer 41, the positive electrode 11 may have substantially constant resistance along its longitudinal direction.

[0056] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. In addition, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0057] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of the inorganic filler include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by applying a slurry containing the filler to a surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0058] Hereinafter, the present disclosure will be further described with reference to examples, but the present disclosure is not limited to these examples.

[0059] <Example 1> [Production of Positive Electrode] As the positive electrode active material, lithium nickelate containing cobalt and aluminum (LiNi ０．８８ Co ０．０９ Al ０．０３ O ２ ) was used. This positive electrode active material, single-walled carbon nanotubes (CNT) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed at a mass ratio of 100:0.25:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a first positive electrode mixture slurry. Separately, the same positive electrode active material and polyvinylidene fluoride (PVDF) as a binder were mixed at a mass ratio of 100:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a second positive electrode mixture slurry. That is, the second positive electrode mixture slurry of Example 1 does not contain a conductive agent.

[0060] The first positive electrode mixture slurry was applied to both surfaces of a positive electrode core made of aluminum foil with a thickness of 15 µm, and the coating film was dried. Thereafter, the second positive electrode mixture slurry was applied to both surfaces of the positive electrode core while providing an exposed region where the positive electrode core is exposed between the coating film of the first positive electrode mixture slurry, and the coating film was dried. Thereafter, each coating film was rolled using a roller to produce a positive electrode. That is, the produced positive electrode has a first positive electrode mixture layer formed of the first positive electrode mixture slurry on one side in the longitudinal direction, and a second positive electrode mixture layer formed of the second positive electrode mixture slurry on the other side in the longitudinal direction, with the mixture layer non-disposed portion interposed therebetween. The thickness of the first positive electrode mixture layer and the second positive electrode mixture layer was set to be the same.

[0061] In the fabricated positive electrode, the longitudinal length of the first positive electrode mixture layer was 40% of the longitudinal length of the positive electrode. Furthermore, the resistance at the beginning end (R1) and the resistance at the end end (R2) of the positive electrode mixture layer were measured using the method described above and were found to be 18.2 Ω and 253.2 Ω, respectively.

[0062] [Fabrication of the negative electrode] A material was used as the negative electrode active material, which was a mixture of graphite and Si oxide (SiO) in a mass ratio of 90:10. This negative electrode active material was mixed with styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC) in a mass ratio of 100:1:1, and an appropriate amount of water was added to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was then applied to both sides of the negative electrode core body, which was made of electrolytic copper foil with a thickness of 7.8 μm, except for the vicinity of the winding start end where the negative electrode tabs were connected. After the coating film was dried, it was rolled using a roller and cut to the predetermined electrode size to produce the negative electrode.

[0063] [Preparation of non-aqueous electrolyte] 100 parts by mass of a mixed solvent obtained by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 3:7, to which 5 parts by mass of vinylene carbonate (VC) is added, and LiPF ６ A non-aqueous electrolyte was prepared by dissolving 1.5 mol / liter of [the substance].

[0064] [Preparation of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] Positive electrode tabs and negative electrode tabs were welded to the positive electrode and negative electrode, respectively. The positive electrode, negative electrode, and polyethylene separator were then wound in a spiral shape using a cylindrical winding core to obtain an electrode body. At this time, the winding was carried out so that the first positive electrode mixture layer was positioned on the winding start side. Insulating plates were then placed above and below the electrode body, respectively, and the electrode body was housed in an outer casing made of steel. The negative electrode tab was then welded to the bottom of the bottomed cylindrical outer casing, and the positive electrode tab was welded to the sealing body. After injecting a non-aqueous electrolyte into the outer casing, the opening of the outer casing was sealed with the sealing body via a gasket to prepare the test cell.

[0065] [Evaluation of safety during overcharging] An overcharge test was performed on the fabricated test cells at a current value of 2C in an environment of 25°C until ignition occurred inside the battery. After the test, the sides of the outer casing of the test cells were visually inspected to evaluate whether there was any damage such as openings or cracks. The same test was performed on 10 test cells fabricated under the same conditions, and the percentage of test cells that showed damage was defined as the damage occurrence rate.

[0066] <Example 2> In the preparation of the second positive electrode mixture slurry for the production of the positive electrode, the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 100:0.3:1. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1.

[0067] <Example 3> In the preparation of the second positive electrode mixture slurry for the production of the positive electrode, the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 100:0.75:1. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1.

[0068] <Example 4> In the preparation of the second positive electrode mixture slurry for the production of the positive electrode, the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 100:0.95:1. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1.

[0069] <Example 5> In the preparation of the second positive electrode mixture slurry for the production of the positive electrode, the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 100:1.05:1. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1.

[0070] <Example 6> In the preparation of the first positive electrode mixture slurry for the production of the positive electrode, the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 100:1.05:1. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1.

[0071] <Example 7> In the preparation of the first positive electrode mixture slurry for the production of the positive electrode, the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 100:0.95:1. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1.

[0072] <Example 8> In the preparation of the first positive electrode mixture slurry for the production of the positive electrode, the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 100:0.75:1. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1.

[0073] <Example 9> In the preparation of the first positive electrode mixture slurry for the production of the positive electrode, a test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 100:0.3:1.

[0074] <Comparative Example 1> In the preparation of the positive electrode, the test cell was prepared and evaluated in the same manner as in Example 1, except that the first positive electrode slurry and the second positive electrode slurry were prepared under the same conditions. In Comparative Example 1, the first positive electrode slurry and the second positive electrode slurry were prepared by mixing the positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of 100:1.05:1.

[0075] <Comparative Example 2> In the preparation of the test cell, the test cell was prepared and evaluated in the same manner as in Example 5, except that the first positive electrode mixture layer was wound so that it was positioned towards the winding end. That is, in the positive electrode of the test cell of Comparative Example 2, the resistance of the positive electrode at the winding start end of the positive electrode mixture layer is greater than the resistance of the positive electrode at the winding end end of the positive electrode mixture layer.

[0076] Table 1 shows the damage rate for each test cell. A lower damage rate indicates a safer test cell. Table 1 also shows the resistance at the beginning of the positive electrode mixture layer (R1), the resistance at the end of the positive electrode mixture layer (R2), and the ratio of R2 to R1 (R2 / R1).

[0077]

[0078] As shown in Table 1, the damage rate of the test cells in Examples 1 to 9 is lower than that of the test cells in Comparative Examples 1 and 2. This is presumed to be because the current flows more towards the inner circumference of the positive electrode, and the amount of heat generated on the inner circumference of the positive electrode is greater than that generated on the outer circumference. Furthermore, in the test cells of the examples, the damage rate decreases as the value of R2 / R1 increases.

[0079] The present disclosure is further illustrated by the following embodiments. Configuration 1: A secondary battery comprising: an electrode body in which a strip-shaped first electrode and a strip-shaped second electrode having opposite polarities are wound via a separator; and an outer casing for housing the electrode body, wherein the first electrode has a core body and a compound layer disposed on the core body, and the resistance of the first electrode at the winding start end of the compound layer is smaller than the resistance of the first electrode at the winding end of the compound layer. Configuration 2: The secondary battery according to Configuration 1, wherein the ratio of the resistance of the first electrode at the winding end of the compound layer to the resistance of the first electrode at the winding start end of the compound layer is 1.2 or more. Configuration 3: The secondary battery according to Configuration 1 or 2, wherein the compound layer includes: a first compound layer disposed from the winding start end toward the winding end of the compound layer; and a second compound layer disposed from the winding end toward the winding start end of the compound layer, and the resistance of the first compound layer is smaller than the resistance of the second compound layer. Configuration 4: The secondary battery according to Configuration 3, wherein the first composite layer contains a fibrous carbon material as a conductive agent. Configuration 5: The secondary battery according to Configuration 4, wherein the fibrous carbon material is a carbon nanotube. Configuration 6: The secondary battery according to Configuration 4 or 5, wherein the content of the fibrous carbon material in the first composite layer is 0.1% by mass or more and 3% by mass or less of the total mass of the first composite layer. Configuration 7: The secondary battery according to any one of Configurations 3 to 6, wherein in the longitudinal direction of the first electrode, the length of the first composite layer is 5% or more and 70% or less of the length of the first electrode. Configuration 8: The secondary battery according to any one of Configurations 3 to 7, wherein the average thickness of the first composite layer and the average thickness of the second composite layer are substantially the same. Configuration 9: The secondary battery according to any one of Configurations 3 to 8, wherein the first electrode has a portion on the core body where the compounding layer is not disposed, and the first compounding layer and the second compounding layer are disposed in the longitudinal direction of the first electrode via the portion where the compounding layer is not disposed. Configuration 10: The secondary battery according to any one of Configurations 1 to 9, wherein the first electrode is the positive electrode and the second electrode is the negative electrode.

[0080] 10 Secondary battery, 11 Positive electrode (first electrode), 12 Negative electrode (second electrode), 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode tab, 21 Negative electrode tab, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core body, 31 Positive electrode mixture layer, 31X Winding start end, 31Y Winding end, 32 Area without mixture layer, 33 First positive electrode mixture layer, 34 Second positive electrode mixture layer, 40 Negative electrode core body, 41 Negative electrode mixture layer

Claims

1. A secondary battery comprising: an electrode body in which a strip-shaped first electrode and a strip-shaped second electrode having opposite polarities are wound with a separator in between; and an outer casing for housing the electrode body, wherein the first electrode has a core body and a composite layer disposed on the core body, and the resistance of the first electrode at the beginning end of the winding of the composite layer is smaller than the resistance of the first electrode at the end end of the winding of the composite layer.

2. The secondary battery according to claim 1, wherein the ratio of the resistance of the first electrode at the end of the winding of the mixture layer to the resistance of the first electrode at the beginning of the winding of the mixture layer is 1.2 or more.

3. The secondary battery according to claim 1, wherein the composite layer comprises a first composite layer arranged from the winding start end toward the winding end, and a second composite layer arranged from the winding end toward the winding start, wherein the resistance of the first composite layer is less than the resistance of the second composite layer.

4. The secondary battery according to claim 3, wherein the first composite layer contains a fibrous carbon material as a conductive agent.

5. The secondary battery according to claim 4, wherein the fibrous carbon material is a carbon nanotube.

6. The secondary battery according to claim 4, wherein the content of fibrous carbon material in the first mixture layer is 0.1% by mass or more and 3% by mass or less of the total mass of the first mixture layer.

7. The secondary battery according to claim 3, wherein the length of the first composite layer in the longitudinal direction of the first electrode is 5% or more and 70% or less of the length of the first electrode.

8. The secondary battery according to claim 3, wherein the average thickness of the first mixture layer and the average thickness of the second mixture layer are substantially the same.

9. The secondary battery according to claim 3, wherein the first electrode has a portion on the core body where the compounding layer is not disposed, and the first compounding layer and the second compounding layer are disposed in the longitudinal direction of the first electrode via the portion where the compounding layer is not disposed.

10. The secondary battery according to claim 1, wherein the first electrode is a positive electrode and the second electrode is a negative electrode.

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