Secondary battery

Abstract

Provided is a secondary battery comprising a wound electrode body. A positive electrode is provided with a positive electrode collector and a positive electrode mixture layer. In the width direction of the positive electrode collector, the positive electrode mixture layer has a low-density region which extends from one end to a middle point and a main region which extends from the middle point to the other end. The mass per unit volume in the low-density region is less than that in the main region. The low-density region includes first and second active materials having first and second particle size distributions. The main region includes third and fourth active materials having third and fourth particle size distributions. In an nth particle size distribution, the particle size D(n)10 for 10% cumulative volume, the particle size D(n)50 for 50% cumulative volume, and the particle size D(n)90 for 90% cumulative volume satisfy D(1)50<D(2)50, D(3)50<D(4)50, and [D(4)90-D(4)10] / D(4)50<[D(2)90-D(2)10] / D(2)50. The electrode body is accommodated inside a case in such a manner that the low-density region faces a sealing body side.

Description

Secondary battery Cross-reference to related applications

[0001] This disclosure claims the benefit of priority of Japanese Patent Application No. 2025-026107, filed on February 20, 2025, with the Japan Patent Office, and incorporates the entire contents of the said patent application herein by reference.

[0002] This disclosure relates to a secondary battery.

[0003] Patent Document 1 proposes an electrode plate for a power storage device that constitutes a wound electrode body including a strip-shaped core body and an active material layer provided on at least one surface of the core body, wherein at one end in the width direction of the core body, a plain portion where the surface of the core body to which a lead is connected is exposed is formed at a position away from one end in the longitudinal direction of the core body disposed on the outer side of the winding of the electrode body, and the active material layer has a thin portion with a layer thickness thinner than that of a second region which is a region other than the first region in at least a part of a first region arranged side by side with the plain portion in the longitudinal direction of the core body.

[0004] Patent Document 2 proposes an electrode used in a non-aqueous electrolyte secondary battery, including a strip-shaped current collector and a mixture layer formed on the current collector, wherein an exposed portion without the mixture layer is provided at one edge in the width direction of the current collector, and the average value of the first mixture density in a first region which is the edge on the exposed portion side is lower than the average value of the second mixture density in a second region which is a portion other than both edges in the width direction of the mixture layer, and the second mixture density is substantially constant, for the electrode for a non-aqueous electrolyte secondary battery.

[0005] Patent Document 3 proposes an electrode sheet including a holding portion and a plain portion on at least one surface of a current collector, on which a positive electrode mixture layer is coated, wherein the holding portion is formed at a central portion along the length direction of the electrode sheet, and includes a first positive electrode mixture layer containing a first positive electrode active material of lithium nickel cobalt manganese oxide, and a second positive electrode mixture layer formed on one or both ends of the first positive electrode mixture layer and containing a second positive electrode active material with a lower nickel content than the first positive electrode active material, and the rolling density (b) of the second positive electrode mixture layer is smaller than the rolling density (a) of the first positive electrode mixture layer, for the electrode sheet.

[0006] International Publication No. 2017 / 77698, Japanese Patent Publication No. 2015-18765, International Publication No. 2021 / 225303

[0007] To improve the capacity of a secondary battery, it is effective to increase the mass of the active material layer per unit area of ​​the current collector in the electrode. However, if such electrodes are wound tightly together to form the electrode body, the permeability of the electrolyte and the heat dissipation of the electrode body will decrease. If the permeability of the electrolyte in the electrode body decreases, it becomes difficult to achieve sufficient battery characteristics. If the heat dissipation of the electrode body decreases, the safety of the secondary battery may be reduced.

[0008] One aspect of the present disclosure comprises an electrode body formed by winding a positive electrode and a negative electrode with a separator in between, an electrolyte, a bottomed case for housing the electrode body and the electrolyte, and a sealing body for sealing the opening of the bottomed case, wherein the positive electrode comprises a long sheet-shaped positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, the positive electrode mixture layer having a low-density region extending from one end in the width direction of the positive electrode current collector to the midpoint in the width direction, and a main region extending from the midpoint to the other end in the width direction, the mass per unit volume of the low-density region being smaller than the mass per unit volume of the main region, the proportion of the low-density region in the positive electrode mixture layer being 50% or less, and the low-density region comprising a first active material having a volume-based first particle size distribution and a second active material having a volume-based second particle size distribution. The main region includes a third active material having a volume-based third particle size distribution and a fourth active material having a volume-based fourth particle size distribution, wherein the particle size D(1)10 at 10% of the cumulative volume, D(1)50 at 50% of the cumulative volume, and D(1)90 at 90% of the cumulative volume in the first particle size distribution; the particle size D(2)10 at 10% of the cumulative volume, D(2)50 at 50% of the cumulative volume, and D(2)90 at 90% of the cumulative volume in the second particle size distribution; the particle size D(3)10 at 10% of the cumulative volume, D(3)50 at 50% of the cumulative volume, and D(3)90 at 90% of the cumulative volume in the third particle size distribution; and the particle size D(4)10 at 10% of the cumulative volume, D(4)50 at 50% of the cumulative volume, and D(4)90 at 90% of the cumulative volume in the fourth particle size distribution. The present invention relates to a secondary battery that satisfies the relationships D(1)50 < D(2)50, D(3)50 < D(4)50, and [D(4)90 - D(4)10] / D(4)50 < [D(2)90 - D(2)10] / D(2)50, wherein the electrode body is housed in a bottomed case with the low-density region facing the sealing body side and the main region facing the bottom of the bottomed case side.

[0009] According to this disclosure, it is possible to improve the permeability of the electrolyte in the electrode and the heat dissipation of the electrode while minimizing the decrease in the energy density of the electrode. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0010] This is a schematic longitudinal cross-sectional view showing an example of a secondary battery according to one embodiment of the present disclosure. It shows a front view of an example of a positive electrode (2A), a cross-sectional view along the line IIB-IIB in the front view (2B), and a cross-sectional view along the line IIC-IIC in the front view (2C). This is a front view of another example of a positive electrode.

[0011] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined, as long as the lower limit is not greater than or equal to the upper limit.

[0012] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0013] The type of secondary battery relating to this disclosure is not particularly limited, but is typically a non-aqueous electrolyte secondary battery capable of achieving high capacity. Non-aqueous electrolyte secondary batteries include lithium-ion secondary batteries, lithium (metal) secondary batteries, and solid-state secondary batteries containing a gel electrolyte or solid electrolyte. In other words, a non-aqueous electrolyte secondary battery may be a liquid-type secondary battery containing an electrolyte, or an all-solid-state secondary battery containing a solid electrolyte.

[0014] A secondary battery comprises an electrode body formed by winding a positive electrode and a negative electrode with a separator in between, an electrolyte, a bottomed case housing the electrode body and the electrolyte, and a sealing body that seals the opening of the bottomed case. In other words, a secondary battery has a wound-type electrode body (electrode group). The cross-sectional shape of the wound-type electrode body perpendicular to the winding axis may be, for example, circular or elliptical, and the outer shape may be, for example, cylindrical, but is not limited to these. The sealing body may be equipped with a safety mechanism such as an exhaust valve that reduces the internal pressure of the case in the event of an abnormality. The cross-sectional shape of the bottomed case perpendicular to the winding axis of the electrode body is a shape that matches the shape of the electrode body, and is usually circular. In that case, the bottomed case is a cylindrical case.

[0015] In the following, this disclosure will be described primarily with reference to lithium-ion secondary batteries. In a lithium-ion secondary battery equipped with a wound electrode body, a strip-shaped positive electrode and a strip-shaped negative electrode are used. The separator may be composed of a porous sheet having ion permeability and insulating properties.

[0016] (Positive electrode) The positive electrode comprises a long sheet-shaped positive electrode current collector having a first main surface and a second main surface, and a positive electrode mixture layer provided on each main surface. The positive electrode mixture layer contains a positive electrode active material and has a low-density region extending from one end in the width direction Dw of the positive electrode current collector to the midpoint in the width direction Dw, and a main region extending from the midpoint to the other end in the width direction Dw.

[0017] Hereinafter, unless otherwise specified, the "main surface" of the positive electrode current collector refers to one of the first and second main surfaces (i.e., one side). The positive electrode may have positive electrode mixture layers on both the first and second main surfaces. The positive electrode mixture layers on the first and second main surfaces may have similar or different configurations. Typically, the positive electrode mixture layers on the first and second main surfaces have similar configurations, and any cross-section parallel to the thickness direction of the positive electrode has a symmetrical shape with the positive electrode current collector as its central axis, with a probability of, for example, 95% or more (even more than 98%).

[0018] The positive electrode may have one or more uncoated areas (exposed areas of the current collector). In this case, the uncoated areas are partially provided along the longitudinal direction of the positive electrode current collector. As a result, one or more low-density regions are provided along the longitudinal direction of the positive electrode current collector. The low-density regions may be provided over the entire length of the positive electrode current collector, excluding the uncoated areas. The low-density regions may be provided over 80% or more (more precisely, 98% or more) of the longitudinal length of the positive electrode current collector. When a positive electrode mixture layer and one or more uncoated areas are provided on both the first main surface and the second main surface, it is preferable that the uncoated areas on both main surfaces overlap each other in at least a portion.

[0019] From another perspective, the positive electrode can be divided into a strip-shaped region (edge) including one end of the width Dw of the positive electrode current collector, and a main region (essential part) other than the strip-shaped region (edge). In this case, the low-density region corresponds to the strip-shaped region (edge). Unpainted portions may be provided partially in one or more locations along the longitudinal direction of the positive electrode current collector in the strip-shaped region (edge). The unpainted portions may be exposed parts of the positive electrode current collector that are roughly rectangular and have a predetermined width in the longitudinal direction of the positive electrode current collector. Unpainted portions may be provided in one or more locations, or intermittently in multiple locations, so as to divide the first region provided in the strip-shaped region (edge).

[0020] In the positive electrode, the mass per unit volume (i.e., density) of the positive electrode mixture layer in the low-density region is smaller than the mass per unit volume (i.e., density) of the positive electrode mixture layer in the main region. When a wound electrode is fabricated using a positive electrode with such a low-density region, the low-density region is located on one end face side of the electrode.

[0021] Because the low-density region is a region with a small mass per unit volume, the electrolyte permeability is relatively high on one end face of the electrode body where the low-density region is located, and the heat dissipation of the electrode body is also relatively high. Heat generated inside the electrode body is rapidly conducted from the inside of the low-density region with high heat dissipation located on one end face of the electrode body to one end of the positive electrode current collector in the width direction Dw (i.e., the end face of the electrode body) and released to the outside of the electrode body. In addition, the electrolyte pushed to the outside of the electrode body during charging can rapidly penetrate from the end face of the electrode body into the low-density region with high electrolyte permeability and move to the inside of the low-density region (towards the center of the electrode body).

[0022] However, it is preferable that the mass per unit volume of the positive electrode mixture layer in the low-density region (hereinafter also referred to as "volume density M") is 50% or more of the mass per unit volume of the positive electrode mixture layer in the main region (hereinafter also referred to as "volume density Mm").

[0023] Furthermore, the proportion of the low-density region in the positive electrode mixture layer (i.e., the proportion of the low-density region in the sum of the low-density region and the main region) (hereinafter also referred to as "proportion (1 / (1+M))") must be 50% or less. This makes it possible to suppress the decrease in the energy density of the electrode while improving the permeability of the electrolyte in the electrode and the heat dissipation of the electrode.

[0024] Furthermore, the ratio (1 / (1+M)) can be rephrased as the ratio of the area S of the main surface of the positive electrode current collector where the low-density region is formed to the area S0 of the main surface of the positive electrode current collector where the positive electrode mixture layer is formed.

[0025] From the viewpoint of ensuring high capacity, the area of ​​all uncoated portions is sufficiently small compared to the area of ​​the main surface of the positive electrode current collector, for example, less than 2% of the main surface area. Therefore, the ratio (1 / (1+M)) can be rephrased as the ratio of the area S of the main surface of the positive electrode current collector where the low-density region is formed to the total area of ​​the main surface of the positive electrode current collector. In other words, "the area S0 of the main surface of the positive electrode current collector where the positive electrode mixture layer is formed" can be rephrased as "the total area of ​​the main surface of the positive electrode current collector".

[0026] Next, it is preferable that the electrode body is housed in a closed-bottom case with the low-density region facing the sealing body and the main region facing the bottom of the closed-bottom case. This arrangement can significantly improve the permeability of the electrolyte in the electrode body and the heat dissipation of the electrode body. When the electrode body generates heat inside the closed-bottom case, the generated heat, along with the electrolyte, tends to move from the bottom of the case towards the opening side, where the heat capacity is relatively larger. If the electrode body has the main region facing the sealing body and the low-density region facing the bottom of the closed-bottom case, the heat generated at the bottom of the closed-bottom case will be blocked by the main region with a high volume density Mm, making it difficult for it to move towards the opening side of the case. As a result, the normal operation of the safety mechanism provided by the sealing body may be hindered.

[0027] Positive electrode tabs may be connected to the uncoated areas. The uncoated areas are regions in the width direction Dw from one end to the main region that do not have a positive electrode mixture layer. If multiple uncoated areas are formed on the main surface of the positive electrode current collector, positive electrode tabs may be connected to each of the multiple uncoated areas.

[0028] By placing the heat-generating positive electrode tab and uncoated area on the end face side of the electrode body where a low-density region with high heat dissipation is located, the heat dissipation of the electrode body can be further enhanced. In addition, by leading the positive electrode tab from the end face side of the electrode body where the low-density region with low volume density M is located, the shape symmetry of the electrode body is improved. This equalizes the internal pressure of the electrode body and makes it easier to equalize the distribution of electrolyte inside the electrode body.

[0029] Typically, the thickness of the uncoated area around the positive electrode tab is increased. In such cases, winding the positive electrode when constructing the wound body can become difficult. In contrast, when the uncoated area is adjacent to a low-density region with a low volume density M, winding the positive electrode becomes relatively easy.

[0030] The length of the uncoated area in the width direction Dw and the length Lr1 of the low-density region in the width direction Dw may be approximately the same. For example, 0.9 ≤ Lex / Lr1 ≤ 1.1 may be satisfied. In this case, the length of the main region Rm in the width direction Dw is approximately constant along the longitudinal direction Dleng.

[0031] The width of the uncoated portion of the positive electrode current collector in the longitudinal direction Dleng should preferably not be excessively large from the viewpoint of maintaining a high energy density of the electrode body. For example, it is preferably in the range of 10 mm to 90 mm, and more preferably in the range of 20 mm to 60 mm.

[0032] When a positive electrode tab is attached to an unpainted portion, it is preferable that the length of the unpainted portion in the longitudinal direction Dleng of the positive electrode current collector is greater than the length of the positive electrode tab. Furthermore, from the viewpoint of suppressing internal short circuits, it is preferable that a portion of the positive electrode tab, together with at least a portion of the unpainted portion, is covered with an insulating material.

[0033] From the viewpoint of maintaining a high energy density of the electrode body, the volume density M of the low-density region is preferably 90% or more of the volume density Mm of the main region, and may also be 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more. Furthermore, from the viewpoint of ensuring the effect of improving the permeability of the electrolyte in the electrode body and the heat dissipation of the electrode body, the volume density M of the low-density region is preferably 99.5% or less of the volume density Mm of the main region.

[0034] From the viewpoint of ensuring higher capacity, the ratio (1 / (1+M)) may be 1% to 10%. This allows for sufficient improvement of electrolyte permeability in the electrode and heat dissipation in the electrode while keeping the decrease in energy density of the electrode body to a minimum. The ratio (1 / (1+M)) may also be 1% to 8%, 1% to 7%, or 1% to 5%. In this case, the length in the width direction Dw of the low-density region is 1% to 10% of the length in the width direction Dw, may also be 1% to 8%, 1% to 7%, or 1% to 5%.

[0035] As the positive electrode current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or perforated sheet) can be used. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but 5 to 20 μm is preferred.

[0036] The positive electrode mixture layer is composed of a positive electrode mixture. The positive electrode mixture contains a positive electrode active material as an essential component and may contain binders, conductive additives, thickeners, etc. as optional components. The positive electrode mixture layer may also be called the positive electrode active material layer.

[0037] The positive electrode mixture layer is obtained, for example, by dispersing a positive electrode mixture containing particles of positive electrode active material, a binder, a conductive additive, etc., in a dispersion medium, coating the surface of a positive electrode current collector with the slurry, drying it, and rolling the dried coating. As the dispersion medium, N-methyl-2-pyrrolidone (NMP), cyclohexanone, alcohols, ethers, etc., can be used.

[0038] The binder may include, for example, a fluorine-based polymer. Fluorine-based polymers can exhibit high binding strength. A fluorine-based polymer is a general term for polymers that have fluorine atoms (F) bonded to carbon atoms that make up the main chain.

[0039] Examples of conductive additives include carbon materials such as graphite, carbon black such as furnace black and acetylene black, carbon fibers (carbon nanotubes (CNTs), carbon fibers other than CNTs), and graphene.

[0040] By controlling the particle size distribution of the positive electrode active material in the low-density region and the particle size distribution of the positive electrode active material in the main region, the volume density M in the low-density region can be designed to be smaller than the volume density Mm in the main region.

[0041] In the following volume-based particle size distribution of each active material constituting the positive electrode active material, a particle size D(n) 10 (n=1, 2, 3, 4) at 10% of the cumulative volume means that 10% of the active material particles in the population have a particle size smaller than this, a particle size D(n) 50 at 50% of the cumulative volume means that 50% of the active material particles in the population have a particle size smaller than this, and a particle size D(n) 90 at 90% of the cumulative volume means that 90% of the active material particles in the population have a particle size smaller than this.

[0042] Furthermore, the value shown by the formula [D(n)90 - D(n)10] / D(n)50 is an indicator of the breadth of the particle size distribution of the active material population. The larger this value, the wider the particle size distribution and the broader the distribution.

[0043] In this disclosure, the low-density region of the positive electrode mixture layer includes a first active material having a volume-based first particle size distribution and a second active material having a volume-based second particle size distribution.

[0044] The main region of the positive electrode mixture layer includes a third active material having a volume-based third particle size distribution and a fourth active material having a volume-based fourth particle size distribution.

[0045] The particle sizes D(1)10 at 10% of the cumulative volume, D(1)50 at 50% of the cumulative volume, and D(1)90 at 90% of the cumulative volume in the first particle size distribution; the particle sizes D(2)10 at 10% of the cumulative volume, D(2)50 at 50% of the cumulative volume, and D(2)90 at 90% of the cumulative volume in the second particle size distribution; the particle sizes D(3)10 at 10% of the cumulative volume, D(3)50 at 50% of the cumulative volume, and D(3)90 at 90% of the cumulative volume in the third particle size distribution; and the particle sizes D(4)10 at 10% of the cumulative volume, D(4)50 at 50% of the cumulative volume, and D(4)90 at 90% of the cumulative volume in the fourth particle size distribution satisfy the following conditions (A) to (C).

[0046] When conditions (A) to (C) are met, the particle size distribution of the positive electrode active material in the low-density region and the particle size distribution of the positive electrode active material in the main region are controlled to a state suitable for achieving the desired volume density M in the low-density region and the desired volume density Mm in the main region.

[0047] Condition (A): D(1)50<D(2)50

[0048] Condition (B): D(3) 50 < D(4) 50

[0049] Condition (C): [D(4)90-D(4)10] / D(4)50<[D(2)90-D(2)10] / D(2)50

[0050] The types and compositions of the first, second, third, and fourth active materials are independent of each other. Any multiple of the first to fourth active materials may be of the same type or composition, or all of them may be of different types or compositions.

[0051] Condition (A) means that in the low-density region, the median diameter of the second active material is larger than that of the first active material. In this case, the volume-based particle size distribution of the positive electrode active material contained in the low-density region has at least two peaks. Of the two peaks with the highest and second highest frequencies, the one with the larger particle size is D(2)50, and the other is D(1)50.

[0052] A preferred median diameter of the first active material is, for example, 1.0 μm ≤ D(1)50 ≤ 6.0 μm, and may also be 0.5 μm ≤ D(1)50 ≤ 7.5 μm.

[0053] A preferred median diameter for the second active material is, for example, 6.0 μm < D(2)50 ≤ 16.0 μm, and may also be 5.0 μm ≤ D(2)50 ≤ 20.0 μm.

[0054] The difference between D(1)50 and D(2)50 may be, for example, 5.0 μm to 8.5 μm or 3.5 μm to 10.0 μm, from the viewpoint of increasing the packing density of the positive electrode active material in the positive electrode mixture layer. From a similar viewpoint, the ratio of D(2)50 / D(1)50 may be, for example, 2.5 to 4.0 or 1.5 to 5.0.

[0055] Condition (B) means that in the main region, the median diameter of the fourth active material is larger than that of the third active material. In this case, the volume-based particle size distribution of the positive electrode active material contained in the main region has at least two peaks. Of the two peaks with the highest and second highest frequencies, the one with the larger particle size is D(4)50, and the other is D(3)50.

[0056] A preferred median diameter for the third active material is, for example, 1.0 μm ≤ D(3)50 ≤ 6.0 μm, and may also be 0.5 μm ≤ D(3)50 ≤ 7.5 μm.

[0057] A preferred median diameter for the fourth active material is, for example, 6.0 μm < D(4)50 ≤ 16.0 μm, and may also be 5.0 μm ≤ D(4)50 ≤ 20.0 μm.

[0058] The difference between D(3)50 and D(4)50 may be, for example, 5.0 μm to 8.5 μm or 3.5 μm to 10.0 μm, from the viewpoint of increasing the packing density of the positive electrode active material in the positive electrode mixture layer. From a similar viewpoint, the ratio of D(4)50 / D(3)50 may be, for example, 2.5 to 4.0 or 1.5 to 5.0.

[0059] D(1)50 and D(3)50 may have similar particle sizes. For example, the difference between D(1)50 and D(3)50 may be within ±1.0 μm. Also, D(2)50 and D(4)50 may have similar particle sizes. For example, the difference between D(2)50 and D(4)50 may be within ±1.5 μm. In this case, it becomes easier to achieve the desired volume density M of the low-density region and the desired volume density Mm of the main region without significantly changing the properties of the positive electrode mixture layer in the low-density region and the main region. Therefore, the design of the negative electrode opposite the positive electrode becomes easier, and the battery reaction proceeds more evenly throughout the entire electrode body.

[0060] Condition (C) means that the extent of the second particle size distribution of the second active material contained in the low-density region is greater than the extent of the fourth particle size distribution of the fourth active material contained in the main region. From another perspective, the second particle size distribution is broader than the fourth particle size distribution.

[0061] In the low-density region, the second active material having a relatively large particle size that satisfies condition (A) has a broad distribution, which limits the improvement of the packing density of the positive electrode mixture layer.

[0062] On the other hand, in the main region, the fourth active material having a relatively large particle size that satisfies condition (B) has a sharp distribution, which promotes an improvement in the packing density of the positive electrode mixture layer.

[0063] As a result, the volume density M in the low-density region and the volume density Mm in the main region are controlled so that volume density M < volume density Mm. For example, volume density M is less than 100% of volume density Mm and is controlled to be 90% or more, 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more. Volume density M may also be controlled to be 99.5% or less of volume density Mm.

[0064] Hereinafter, we define [D(2)90 - D(2)10] / D(2)50 = σ(2) and [D(4)90 - D(4)10] / D(4)50 = σ(4). In this case, condition (C) can be rephrased as σ(4) < σ(2). The difference between σ(2) and σ(4) (σ(2) - σ(4)) is preferably 0.1 or more, may be 0.2 or more, may be 0.25 or more, or may be 0.3 or more. The difference (σ(2) - σ(4)) is, for example, 0.5 or less, and may be 0.4 or less. By keeping the difference (σ(2) - σ(4)) within the above range, it becomes easier to achieve the desired volume density M of the low-density region and the desired volume density Mm of the main region.

[0065] For example, σ(4) and σ(2) may satisfy 0.1 ≤ σ(4) ≤ 0.4 and 0.4 < σ(2) ≤ 0.8, 0.2 ≤ σ(4) ≤ 0.35 and 0.45 < σ(2) ≤ 0.7, and 0.25 ≤ σ(4) ≤ 0.3 and 0.5 < σ(2) ≤ 0.65.

[0066] The positive electrode active material included in the low-density region may contain active materials other than the first and second active materials, but from the viewpoint of increasing capacity, it is preferable that the first and second active materials satisfying conditions (A) to (C) account for 90% or more, more preferably 95% or more, and more preferably 99% or more, of the total positive electrode active material.

[0067] The positive electrode active material included in the main region may contain active materials other than the third and fourth active materials, but from the viewpoint of increasing capacity, it is preferable that the third and fourth active materials satisfying conditions (A) to (C) account for 90% or more by mass, more preferably 95% or more by mass, and more preferably 99% or more by mass of the total positive electrode active material.

[0068] The content of the second active material in the total of the first and second active materials is, for example, 50% by mass, preferably 60% by mass, and more preferably 70% by mass or more, in order to easily achieve high volume. That is, from the viewpoint of achieving high volume in the low-density region, it is preferable that the amount of the second active material, which has a relatively large particle size, is greater than that of the first active material. The content of the second active material in the total of the first and second active materials is, for example, 95% by mass or less, may be 90% by mass or less, or 85% by mass or less.

[0069] The content of the fourth active material in the total of the third and fourth active materials is, for example, 50% by mass, preferably 60% by mass, and more preferably 70% by mass or more, in order to easily achieve high volume. That is, from the viewpoint of increasing the volume of the main region, it is preferable that the amount of the fourth active material, which has a relatively large particle size, is greater than that of the third active material. The content of the fourth active material in the total of the third and fourth active materials is, for example, 95% by mass or less, may be 90% by mass or less, or 85% by mass or less.

[0070] The first and second particle size distributions may be determined by separating the positive electrode active material from the low-density region of the positive electrode mixture layer and measuring it, or by image analysis of a cross-sectional SEM image of the low-density region of the positive electrode mixture layer. Both methods yield generally the same (without significant difference) particle size distribution.

[0071] The third and fourth particle size distributions may be measured by separating the positive electrode active material from the main region of the positive electrode mixture layer, or by image analysis of a cross-sectional SEM image of the main region of the positive electrode mixture layer. Both methods yield approximately the same (without significant difference) particle size distribution.

[0072] When separating the positive electrode active material from the positive electrode mixture layer, the mixture layer may be peeled off from the low-density region or the main region, immersed in a suitable solvent to dissolve or swell components other than the positive electrode active material, and separated by centrifugation one or more times. When the separated sample of positive electrode active material is analyzed with a laser diffraction scattering particle size distribution analyzer, the volume-based particle size distribution and D(n)10, D(n)50, and D(n)90 (n=1 to 4) can be determined.

[0073] When analyzing the cross-sectional SEM image of the positive electrode mixture layer, the positive electrode mixture layer and the positive electrode current collector are simultaneously cut along the width direction of the positive electrode to obtain a cross-sectional sample in the thickness direction of the low-density region and the main region. At this time, the cross-section may be processed by a cross-section polisher (CP) to obtain a cross-sectional sample. Next, a scanning electron microscope (SEM) is used to observe the cross-section of the positive electrode mixture layer in the cross-sectional sample.

[0074] From the contour image of the particles of the positive electrode active material in the cross-sectional SEM image, the area surrounded by the contour is determined. The diameter of a circle (equivalent circle) having the same area as the area surrounded by the contour of the particles of the positive electrode active material is determined and taken as the particle size of each particle i. Then, the volume of a sphere having the same diameter as the equivalent circle is regarded as the volume Vi of each particle i. By determining the diameters and volumes of the equivalent circles of any 100 or more (preferably 1000 or more) particles, the particle size distribution based on volume and D(n)10, D(n)50, D(n)90 (n = 1 to 4) can be calculated.

[0075] The positive electrode active material can be a material that reversibly intercalates and deintercalates lithium ions. The positive electrode active material may be, for example, a lithium-containing transition metal oxide. The crystal structure of the lithium-containing transition metal oxide is not particularly limited, and may be, for example, a layered rock salt type. Any lithium-containing transition metal oxide may be used as the first active material, the second active material, the third active material, and the fourth active material.

[0076] As the lithium-containing transition metal oxide, for example, a composite oxide containing lithium and transition metals such as Ni, Co, Mn, etc. may be used. Specifically, Li ｂ , １－ｂ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ [[ID=4​​４ Li ａ Mn ２－ｂ M ｂ O ４、 LiMPO ４、 Li ２ MPO ４ Examples include F (where M is at least one selected from the group consisting of Na, Mg, K, Ca, Rb, Sr, Sc, Y, Ti, Zr, V, Nb, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B). Here, 0 < a ≤ 1.2, 0 < b ≤ 0.9, and 2.0 ≤ c ≤ 2.3. Note that the value of a, which indicates the molar ratio of lithium, increases or decreases with charging and discharging.

[0077] The first active material, second active material, third active material, and fourth active material each independently have the general formula: Li ａ Ni ｂ Co （１－ｂ－ｃ－ｄ） M ｃ Me ｄ O ｅ It may also be a composite oxide (lithium-containing transition metal oxide) represented by . The crystal structure of such a composite oxide may be of the layered rock salt type.

[0078] Here, the general formula may satisfy 0.9 < a ​​≤ 1.2, 0.33 ≤ b ≤ 0.96, 0.03 ≤ c ≤ 0.70, 0 ≤ d ≤ 0.02, and 1.9 ≤ e ≤ 2.1. M is at least one selected from the group consisting of Al and Mn, and Me is a metallic element that is not Li, Ni, Co, or M. Me may be, for example, at least one selected from the group consisting of Na, Mg, K, Ca, Rb, Sr, Sc, Y, Ti, Zr, V, Nb, Fe, Cu, Zn, Cr, Pb, Sb, and B.

[0079] The first active material, the second active material, the third active material, and the fourth active material are each independently composed of Li ａ Ni １－ｂ M ｂ O ２ A lithium nickel composite oxide represented by (where M is at least one selected from the group consisting of Mn, Co, and Al, and 0.9 < a ​​≤ 1.2 and 0 < b < 0.7) may also be used. From the viewpoint of increasing capacity, it is preferable to satisfy 0 < b < 0.2.

[0080] A preferred lithium nickel composite oxide is Li containing Co and Al as M. ａ Ni １－ｂ Co ｃ Al ｄ O ２ (0.9 < a ​​≤ 1.2, 0 < b < 0.2, 0 < c < 0.15, 0 < d ≤ 0.1, b = d + e) ​​are examples.

[0081] Another preferred lithium nickel composite oxide is Li containing Co and Mn as M. ａ Ni １－ｂ Co ｃ Mn ｄ O ２ (0.9 < a ​​≤ 1.2, 0 < b < 0.2, 0 < c < 0.15, 0 < d ≤ 0.1, b = d + e) ​​are examples.

[0082] The low-density region and the main region may be formed by preparing two types of slurries (one for the low-density region and one for the main region) in which the positive electrode mixture constituting the positive electrode mixture layer is dispersed in a liquid component, applying the two types of slurries to the main surface of the positive electrode current collector, drying, and rolling. The slurry for the low-density region and the slurry for the main region may be applied using a first apparatus and a second apparatus, respectively.

[0083] The uncoated portion may be formed by intermittently applying a slurry, in which the positive electrode mixture constituting the positive electrode mixture layer is dispersed in a liquid component, to the main surface of the positive electrode current collector.

[0084] For example, a slurry in which a positive electrode mixture for the low-density region is dispersed in a liquid component may be applied to a portion of a wide positive electrode current collector, and a slurry in which a positive electrode mixture for the main region is dispersed in a liquid component may be applied to another portion of the positive electrode current collector. The two slurry coatings are then dried, and these coatings are rolled simultaneously. In this case, the manufacturing process is simple, and a low-density region and a main region having positive electrode mixture layers of approximately the same thickness can be formed.

[0085] The ratio T / Tm of the thickness T of the positive electrode mixture layer in the low-density region to the thickness Tm of the positive electrode mixture layer in the main region may be, for example, 0.996 or more and 1.004 or less. The fact that T and Tm satisfy 0.996 ≤ T1 / T2 ≤ 1.004 means that, considering manufacturing variations, T = Tm (i.e., T / Tm = 1). Note that the thickness T in the low-density region and the thickness Tm in the main region may be determined as the average value of the thicknesses at any five locations in the low-density region and the main region, respectively.

[0086] The thickness of the positive electrode mixture layer on the first main surface and the second main surface of the positive electrode may be, for example, 50 μm or more, 60 μm or more, 90 μm or more, or within the range of 50 μm to 100 μm.

[0087] The effect of the above configuration on improving electrolyte permeability and heat dissipation in the electrode body increases as the volume density Mm of the main region of the positive electrode mixture layer increases (in other words, as the energy density of the electrode body increases). The volume density Mm of the main region is, for example, 3.5 g / cm³. ３ The above is also acceptable, 3.6 g / cm³. ３ That's fine too.

[0088] The energy density of the electrode body is, for example, 700 Wh / L or more in the case of lithium-ion secondary batteries, and may be, for example, 800 Wh / L or more as capacity increases further. One cell has only one electrode body. For example, a wound electrode body has a cylindrical shape, and only one electrode body is housed in a bottomed cylindrical case having an inner diameter slightly larger than the diameter of the cylinder, thus forming one cell.

[0089] The above configuration improves the permeability of the electrolyte in the electrode body and the heat dissipation of the electrode body, and in the case of a wound electrode body, the effect increases with the number of turns. The larger the number of turns of the electrode body, the larger the length of the positive electrode in the longitudinal direction. The length of the positive electrode in the longitudinal direction may be, for example, 3000 mm or more.

[0090] (Negative electrode) The negative electrode has a long sheet-like negative electrode current collector having a first main surface and a second main surface, and may also have negative electrode mixture layers provided on the first and second main surfaces. However, the negative electrode of a lithium metal secondary battery does not need to have a negative electrode mixture layer.

[0091] The negative electrode mixture layer is composed of a negative electrode mixture. The negative electrode mixture contains a negative electrode active material as an essential component and may contain binders, conductive additives, thickeners, etc. as optional components. The negative electrode active material layer may also be referred to as the negative electrode mixture layer. Alternatively, the negative electrode mixture layer may be composed of at least one selected from the group consisting of lithium metal and lithium alloy.

[0092] The negative electrode mixture layer, composed of a negative electrode mixture, is obtained, for example, by dispersing a negative electrode slurry containing particles of negative electrode active material, a binder, a conductive additive, etc., in a dispersion medium, coating the surface of a negative electrode current collector with the slurry, drying it, and rolling the dried coating. Suitable dispersion media include water, N-methyl-2-pyrrolidone (NMP), cyclohexanone, alcohols, ethers, etc.

[0093] The negative electrode active material used in the negative electrode mixture may be a material that reversibly intercepts and releases lithium ions, or an alloying material. Preferred materials for reversibly intercepting and releasing lithium ions include carbon materials, spinel-type lithium titanium oxide, and spinel-type lithium manganese oxide. Carbon materials may include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon). Alloying materials contain a phase that reversibly forms an alloy with lithium. The phase that reversibly forms an alloy with lithium may be, for example, silicon (silicon phase).

[0094] Examples of binders include acrylic resins and rubber-like materials such as styrene-butadiene copolymer rubber (SBR).

[0095] Examples of conductive additives include carbon black such as furnace black and acetylene black, carbon fibers (carbon nanotubes (CNTs), carbon fibers other than CNTs), and graphene.

[0096] Examples of thickening agents include carboxymethylcellulose (CMC) and its modified forms (including salts such as sodium salts).

[0097] Non-porous conductive substrates and porous conductive substrates are used as the negative electrode current collector. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but 5 to 20 μm is desirable.

[0098] (Electrolyte) The electrolyte may be a liquid electrolyte (electrolyte solution), a gel electrolyte, or a solid electrolyte. A liquid electrolyte is, for example, an electrolyte solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolyte solution may contain known additives.

[0099] For example, liquid non-aqueous electrolytes are prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that undergoes ion dissociation in the electrolyte, and may include, for example, lithium salts. Various additives may be included in the electrolyte. Electrolytes are usually used in liquid form, but they may also be in a state where their fluidity is restricted by gelling agents or other means.

[0100] Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonate esters having unsaturated bonds, such as vinylene carbonate (VC), may also be used. Cyclic carbonate esters having fluorine atoms, such as fluoroethylene carbonate (FEC), may also be used. Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more types.

[0101] Examples of lithium salts include LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０ Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include difluorooxalate borate and dioxalate borate. Examples of imide salts include bisfluorosulfonylimide lithium (LiN(FSO)). ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ Examples include the following. A single lithium salt may be used alone, or two or more may be used in combination. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0102] (Separator) The separator may be composed of a porous sheet having ion permeability and insulating properties. Examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Polyolefins such as polypropylene and polyethylene are preferred as the material for the separator.

[0103] In the following, an example of a secondary battery relating to this disclosure will be specifically described with reference to the drawings. The components of the secondary battery in the example described below can be the components described above. The components of the secondary battery in the example described below can be modified based on the description above. Furthermore, the matters described below may be applied to the above embodiments. Among the components of the secondary battery in the example described below, components that are not essential to the secondary battery relating to this disclosure may be omitted. Note that the figures shown below are schematic and do not accurately reflect the actual shape and number of components.

[0104] The secondary battery 10 in this embodiment is configured as a lithium-ion secondary battery, but is not limited to this. As shown in Figure 1, the secondary battery 10 comprises a wound electrode body 20, a case 40, a sealing body 50, a positive electrode tab 61, and a negative electrode tab 62.

[0105] The electrode body 20 has a positive electrode 21, a negative electrode 29, and a separator 31. The positive electrode 21, the negative electrode 29, and the separator 31 are all in the form of long sheets (or strips). The positive electrode 21 and the negative electrode 29 are wound in a spiral shape with the separator 31 interposed between them, so that the width direction of the long sheets of positive electrode 21 and negative electrode 29 is parallel to the winding axis. A first insulating plate 71 and a second insulating plate 72 are arranged on both sides of the electrode body 20 in the axial direction, respectively. The configuration of the positive electrode 21 will be described in detail later.

[0106] The case 40 is configured as a bottomed cylindrical shape with an opening at one end (the upper end in Figure 1). The case 40 houses the electrode body 20 and the electrolyte (not shown). The case 40 has a stepped portion 41 formed by partially pressing the side wall of the case 40 from the outside. The stepped portion 41 may be formed annularly along the circumferential direction of the case 40 on the side wall of the case 40. In this case, the sealing body 50 can be supported on the opening side of the stepped portion 41.

[0107] The sealing body 50 seals the opening of the case 40. An insulating gasket 81 is placed between the sealing body 50 and the case 40, thereby ensuring the airtightness of the case 40. The sealing body 50 includes a filter 51, a lower valve body 52, an insulating member 53, an upper valve body 54, and a cap 55. In the sealing body 50, these members are stacked in this order. The sealing body 50 is fitted into the opening of the case 40 such that the cap 55 is located on the outside of the case 40. Each of the above-mentioned members constituting the sealing body 50 is, for example, disc-shaped or ring-shaped. Each member, except for the insulating member 53, is electrically connected to one another.

[0108] The positive electrode tab 61 is attached to the first unpainted portion 24 (described later) of the positive electrode 21, and electrically connects the positive electrode 21 and the sealing body 50. Specifically, one end of the positive electrode tab 61 is connected to the first unpainted portion 24, and the other end is connected to the cap 55 of the sealing body 50. The positive electrode tab 61 passes through a through hole formed in the first insulating plate 71. Therefore, the cap 55 of the sealing body 50 functions as an external positive electrode terminal. There may be only one first unpainted portion 24 and one positive electrode tab 61, or there may be multiple of each.

[0109] The negative electrode tab 62 is attached to the negative electrode 29 and electrically connects the negative electrode 29 to the case 40. Specifically, one end of the negative electrode tab 62 is connected to the negative electrode 29, and the other end is connected to the inner bottom surface of the case 40. Thus, the case 40 functions as an external negative electrode terminal. Only one negative electrode tab 62 may be provided, or multiple negative electrode tabs 62 may be provided.

[0110] Next, the configuration of the positive electrode 21 will be explained in detail with reference to Figures 2A to 2C.

[0111] As shown in Figures 2A to 2C, the positive electrode 21 comprises a long sheet-shaped positive electrode current collector 22 having a first main surface 23 and a second main surface 25, a first positive electrode mixture layer 27 provided on the first main surface 23, and a second positive electrode mixture layer 28 provided on the second main surface 25.

[0112] The positive electrode current collector 22 is made of, for example, aluminum foil or aluminum alloy foil. The longitudinal direction Dleng of the positive electrode current collector 22 is the left-right direction in Figure 2A, and the width direction Dw of the positive electrode current collector 22 is the up-down direction in Figure 2A. The longitudinal direction Dleng of the positive electrode current collector 22 coincides with the winding direction of the electrode body 20, and the width direction Dw of the positive electrode current collector 22 coincides with the axial direction of the electrode body 20.

[0113] The first positive electrode mixture layer 27 has a first low-density region R1 extending from one end in the width direction Dw of the positive electrode current collector 22 to the midpoint of the width direction Dw, and a first main region Rm1 extending from the midpoint to the other end in the width direction Dw. The first low-density region R1 is provided at one or more locations along the longitudinal direction Dleng of the positive electrode current collector 22 and is provided over the entire longitudinal direction Dleng of the positive electrode current collector 22, except for the first uncoated portion 24.

[0114] In other words, the first positive electrode mixture layer 27 is divided into an edge portion including one end in the width direction Dw and a main portion other than the edge portion, with the first low-density region R1 constituting the edge portion. The first main region Rm1 is the main portion. The edge portion is provided in a strip shape along the longitudinal direction of the positive electrode current collector 22. The length of the first low-density region R1 in the width direction Dw is, for example, 1% to 10% of the length of the positive electrode current collector 22 in the width direction Dw.

[0115] The first positive electrode mixture layer 27 includes a positive electrode active material (e.g., lithium-containing transition metal oxide), a conductive agent, and a binder. The second low-density region R2 of the second positive electrode mixture layer 28 has the same composition as the first low-density region R1 of the first positive electrode mixture layer 27. The second main region Rm2 of the second positive electrode mixture layer 28 has the same composition as the first main region Rm1 of the first positive electrode mixture layer 27.

[0116] The first unpainted portion 24 is provided at one or more locations along the longitudinal direction of the positive electrode current collector 22. The first unpainted portion 24 is an exposed portion of the positive electrode current collector 22 that is roughly rectangular and has a predetermined width in the longitudinal direction of the positive electrode current collector 22. The length of the first unpainted portion 24 in the width direction Dw is the same as the length of the first low-density region R1 in the width direction Dw. The length L1 of each first unpainted portion 24 in the longitudinal direction Dleng is set, for example, within the range of 10 mm to 90 mm.

[0117] A positive electrode tab 61, shown by a dashed line in Figure 2A, is attached to the first unpainted portion 24, for example, by welding. In the longitudinal direction Dleng, the length L2 of the positive electrode tab 61 is usually smaller than the length L1 of the first unpainted portion 24.

[0118] As shown in Figure 3, the first uncoated portion 24 may be provided at multiple locations on the first main surface 23. In that case, a positive electrode tab 61 is connected to each of the multiple first uncoated portions 24.

[0119] The electrode body 20 is housed in the case with the first low-density region R1 facing the sealing body 50 and the first main region Rm1 facing the bottom of the case 40. As a result, the end of the positive electrode tab 61 is led out from the end face of the electrode body 20 on the sealing body 50 side. The led-out end of the positive electrode tab 61 is connected to the sealing body 50.

[0120] The second positive electrode mixture layer 28 has the same configuration as the first positive electrode mixture layer 27. That is, the second positive electrode mixture layer 28 has a second low-density region R2 extending from one end in the width direction Dw of the positive electrode current collector 22 to the midpoint in the width direction Dw, and a second main region Rm2 extending from the midpoint to the other end in the width direction Dw. The second low-density region R2 is provided at one or more locations along the longitudinal direction Dleng of the positive electrode current collector 22 and extends over the entire length direction Dleng of the positive electrode current collector 22, except for the second uncoated portion 26. The length of the second low-density region R2 in the width direction Dw is, for example, 1% to 10% of the length of the width direction Dw of the positive electrode current collector 22, and is the same as the length Lr1 of the first low-density region R1 in the width direction Dw.

[0121] The second unpainted portion 26 is provided at one or more locations along the longitudinal direction of the positive electrode current collector 22. The second unpainted portion 26 is an exposed portion of the positive electrode current collector 22 that is roughly rectangular and has a predetermined width in the longitudinal direction of the positive electrode current collector 22. The length of the second unpainted portion 24 in the width direction Dw is the same as the length of the second low-density region R2 in the width direction Dw. The length of the second unpainted portion 26 in the longitudinal direction Dleng is the same as the length L1 of the first unpainted portion 24 in the longitudinal direction Dleng.

[0122] The second low-density region R2 is located on the back side of the first low-density region R1. The first low-density region R1 and the second low-density region R2 overlap each other by at least a portion (preferably 90% or more) when viewed from the direction normal to the first main surface 23.

[0123] The volume density M1 of the first low-density region R1 is smaller than the volume density Mm1 of the first main region Rm1. On the other hand, the thickness of the first low-density region R1 is approximately the same as the thickness of the first main region Rm1.

[0124] Similarly, the volume density M2 of the second low-density region R2 is smaller than the volume density Mm2 of the second main region Rm2. On the other hand, the thickness of the second low-density region R2 is approximately the same as the thickness of the second main region Rm2.

[0125] In other words, the density per unit volume of the first low-density region R1 and the second low-density region R2 is relatively low, playing a role in improving the permeability of the electrolyte in the electrode body and the heat dissipation of the electrode body. On the other hand, the density per unit volume of the first main region Rm1 and the second main region Rm2 is relatively high, playing a role in maintaining a high energy density in the electrode body. Experimental example

[0126] The following four types of LiNi ｂ Co （１－ｂ－ｃ） Mn ｃ O ２ A composite oxide having a layered rock salt-type crystalline structure represented by [formula] was prepared. Composite oxides (A) and (B) are examples of the fourth active material. Composite oxide (C) is an example of the second active material. Composite oxide (D) is an example of the first or third active material.

[0127] (Composite oxide (A)) In the volume-based particle size distribution, the particle size D10 at 10% cumulative volume = 8.2 μm, the particle size D50 at 50% cumulative volume = 9.3 μm, and the particle size D90 at 90% cumulative volume = 10.7 μm. [D90 - D10] / D50 = 0.27

[0128] (Composite oxide (B)) In the volume-based particle size distribution, the particle size D10 at 10% cumulative volume = 8.0 μm, the particle size D50 at 50% cumulative volume = 9.2 μm, and the particle size D90 at 90% cumulative volume = 10.6 μm. [D90 - D10] / D50 = 0.28

[0129] (Composite oxide (C)) In the volume-based particle size distribution, the particle size D10 at 10% cumulative volume = 6.7 μm, the particle size D50 at 50% cumulative volume = 8.9 μm, and the particle size D90 at 90% cumulative volume = 11.9 μm. [D90 - D10] / D50 = 0.58

[0130] (Composite oxide (D)) In the volume-based particle size distribution, the particle size D10 at 10% cumulative volume is 1.9 μm, the particle size D50 at 50% cumulative volume is 4.0 μm, and the particle size D90 at 90% cumulative volume is 7.7 μm.

[0131] A positive electrode with the following configuration was prepared: (Positive electrode X) A positive electrode slurry was prepared by mixing 80 parts by mass of composite oxide (A), 20 parts by mass of composite oxide (D), 1 part by mass of polyvinylidene fluoride, 1 part by mass of acetylene black, and N-methyl-2-pyrrolidone (NMP). Next, the positive electrode slurry was applied to the surface of an aluminum foil, which was to be used as the positive electrode current collector. After the coating was dried, the foil was rolled to form positive electrode mixture layers on both sides of the aluminum foil.

[0132] (Positive electrode Y) A positive electrode slurry was prepared by mixing 80 parts by mass of composite oxide (B), 20 parts by mass of composite oxide (D), 1 part by mass of polyvinylidene fluoride, 1 part by mass of acetylene black, and N-methyl-2-pyrrolidone (NMP). Next, the positive electrode slurry was applied to the surface of an aluminum foil, which was to be used as the positive electrode current collector. After the coating was dried, the foil was rolled to form positive electrode mixture layers on both sides of the aluminum foil.

[0133] (Positive electrode Z) A positive electrode slurry was prepared by mixing 80 parts by mass of composite oxide (C), 20 parts by mass of composite oxide (D), 1 part by mass of polyvinylidene fluoride, 1 part by mass of acetylene black, and N-methyl-2-pyrrolidone (NMP). Next, the positive electrode slurry was applied to the surface of an aluminum foil, which was to be used as the positive electrode current collector. After the coating was dried, the foil was rolled to form positive electrode mixture layers on both sides of the aluminum foil.

[0134] Table 1 shows the mass per unit volume of the positive electrode mixture layer of positive electrode Y and positive electrode Z, when the mass per unit volume of the positive electrode mixture layer of positive electrode X is set to 100.

[0135]

[0136] From Table 1, it can be seen that the positive electrode mixture layers of positive electrodes X and Y are suitable as the main region, and the positive electrode mixture layer of positive electrode Z is suitable as the low-density region. Furthermore, it can be seen that when providing both a low-density region and a main region in a single positive electrode, it is important that the low-density region and the main region satisfy the previously described conditions (A) to (C).

[0137] 《Note》 The above description of embodiments discloses the following technology. (Technology 1) An electrode body comprising a positive electrode and a negative electrode wound around a separator, an electrolyte, a bottomed case housing the electrode body and the electrolyte, and a sealing body sealing the opening of the bottomed case, wherein the positive electrode comprises a long sheet-shaped positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, wherein the positive electrode mixture layer has a low-density region extending from one end in the width direction of the positive electrode current collector to the midpoint in the width direction, and a main region extending from the midpoint to the other end in the width direction, wherein the mass per unit volume of the low-density region is smaller than the mass per unit volume of the main region, the proportion of the low-density region in the positive electrode mixture layer is 50% or less, and the low-density region includes a first active material having a volume-based first particle size distribution and a second active material having a volume-based second particle size distribution. The main region includes a third active material having a volume-based third particle size distribution and a fourth active material having a volume-based fourth particle size distribution, wherein the particle size D(1)10 at 10% of the cumulative volume, D(1)50 at 50% of the cumulative volume, and D(1)90 at 90% of the cumulative volume in the first particle size distribution; the particle size D(2)10 at 10% of the cumulative volume, D(2)50 at 50% of the cumulative volume, and D(2)90 at 90% of the cumulative volume in the second particle size distribution; the particle size D(3)10 at 10% of the cumulative volume, D(3)50 at 50% of the cumulative volume, and D(3)90 at 90% of the cumulative volume in the third particle size distribution; and the particle size D(4)10 at 10% of the cumulative volume, D(4)50 at 50% of the cumulative volume, and D(4)90 at 90% of the cumulative volume in the fourth particle size distribution. A secondary battery satisfying the relationships D(1)50 < D(2)50, D(3)50 < D(4)50, and [D(4)90 - D(4)10] / D(4)50 < [D(2)90 - D(2)10] / D(2)50, wherein the electrode body is housed in the bottomed case with the low-density region facing the sealing body side and the main region facing the bottom of the bottomed case side. (Technical 2) The secondary battery according to Technical 1, satisfying 0.1 ≤ [D(4)90 - D(4)10)] / D(4)50 ≤ 0.4 and 0.4 < [D(2)90 - D(2)10] / D(2)50 ≤ 0.8.(Technology 3) A secondary battery according to Technology 1 or 2, satisfying 1.0 μm ≤ D(1) 50 ≤ 6.0 μm, 6.0 μm < D(2) 50 ≤ 16.0 μm, 1.0 μm ≤ D(3) 50 ≤ 6.0 μm, and 6.0 μm < D(4) 50 ≤ 16.0 μm. (Technology 4) A secondary battery according to any one of Technology 1 to 3, wherein the positive electrode has one or more uncoated portions, and the uncoated portions do not have the positive electrode mixture layer from one end in the width direction to the main region. (Technology 5) A secondary battery according to any one of Technology 1 to 4, wherein the length Lex of the uncoated portion in the width direction and the length L1 of the low-density region in the width direction satisfy 0.9 ≤ Lex / Ll ≤ 1.1. (Technical 6) A secondary battery according to any one of Technical 1 to 5, wherein the length of the positive electrode in the longitudinal direction is 3000 mm or more. (Technical 7) A secondary battery according to any one of Technical 1 to 6, wherein the proportion of the low-density region in the positive electrode mixture layer is 1% to 10%. (Technical 8) The first active material, the second active material, the third active material and the fourth active material are each independently of the general formula: Li. ａ Ni ｂ Co （１－ｂ－ｃ－ｄ） M ｃ Me ｄ O ｅ A composite oxide represented by the general formula, wherein the general formula satisfies 0.9 < a ​​≤ 1.2, 0.33 ≤ b ≤ 0.96, 0.03 ≤ c ≤ 0.70, 1.9 ≤ e ≤ 2.1, and 0 ≤ d ≤ 0.02, where M is at least one selected from the group consisting of Al and Mn, and Me is a metallic element that is not Li, Ni, Co, or M, as described in any one of the techniques 1 to 7.

[0138] This disclosure can be used, for example, in non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries.

[0139] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0140] 10: Secondary battery 20: Electrode body 21: Positive electrode 22: Positive electrode current collector 23: First main surface 24: First uncoated area 25: Second main surface 26: Second uncoated area 27: First positive electrode mixture layer 28: Second positive electrode mixture layer 29: Negative electrode 31: Separator 40: Case 41: Stepped section 50: Sealing body 51: Filter 52: Lower valve body 53: Insulating member 54: Upper valve body 55: Cap 61: Positive electrode tab 62: Negative electrode tab 71: First insulating plate 72: Second insulating plate 81: Gasket L1: Length of first uncoated area L2: Length of positive electrode tab R1: First low-density region R2: Second low-density region

Claims

1. The electrode comprises an electrode body formed by winding a positive electrode and a negative electrode with a separator in between, an electrolyte, a bottomed case for housing the electrode body and the electrolyte, and a sealing body for sealing the opening of the bottomed case, wherein the positive electrode comprises a long sheet-shaped positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector, the positive electrode mixture layer having a low-density region extending from one end in the width direction of the positive electrode current collector to the midpoint in the width direction, and a main region extending from the midpoint to the other end in the width direction, the mass per unit volume of the low-density region being smaller than the mass per unit volume of the main region, the proportion of the low-density region in the positive electrode mixture layer being 50% or less, and the low-density region comprising a first active material having a volume-based first particle size distribution and a second active material having a volume-based second particle size distribution. The main region includes a third active material having a volume-based third particle size distribution and a fourth active material having a volume-based fourth particle size distribution, wherein the particle size D(1)10 at 10% of the cumulative volume, D(1)50 at 50% of the cumulative volume, and D(1)90 at 90% of the cumulative volume in the first particle size distribution; the particle size D(2)10 at 10% of the cumulative volume, D(2)50 at 50% of the cumulative volume, and D(2)90 at 90% of the cumulative volume in the second particle size distribution; the particle size D(3)10 at 10% of the cumulative volume, D(3)50 at 50% of the cumulative volume, and D(3)90 at 90% of the cumulative volume in the third particle size distribution; and the particle size D(4)10 at 10% of the cumulative volume, D(4)50 at 50% of the cumulative volume, and D(4)90 at 90% of the cumulative volume in the fourth particle size distribution. A secondary battery in which the electrode body satisfies the relationships D(1)50 < D(2)50, D(3)50 < D(4)50, and [D(4)90 - D(4)10] / D(4)50 < [D(2)90 - D(2)10] / D(2)50, and the electrode body is housed in a bottomed case with the low-density region facing the sealing body side and the main region facing the bottom of the bottomed case side.

2. The secondary battery according to claim 1, satisfying 0.1 ≤ [D(4)90 - D(4)10)] / D(4)50 ≤ 0.4 and 0.4 < [D(2)90 - D(2)10)] / D(2)50 ≤ 0.

8.

3. The secondary battery according to claim 1, satisfying 1.0 μm ≤ D(1)50 ≤ 6.0 μm, 6.0 μm < D(2)50 ≤ 16.0 μm, 1.0 μm ≤ D(3)50 ≤ 6.0 μm, and 6.0 μm < D(4)50 ≤ 16.0 μm.

4. The secondary battery according to claim 1, wherein the positive electrode has one or more uncoated portions, and the uncoated portions do not have the positive electrode mixture layer from one end in the width direction to the main region.

5. The secondary battery according to claim 1, wherein the length Lex of the uncoated portion in the width direction and the length L1 of the low-density region in the width direction satisfy 0.9 ≤ Lex / Ll ≤ 1.

1.

6. The secondary battery according to claim 1, wherein the longitudinal length of the positive electrode is 3,000 mm or more.

7. The secondary battery according to claim 1, wherein the proportion of the low-density region in the positive electrode mixture layer is 1% to 10%.

8. The first active material, the second active material, the third active material and the fourth active material each independently have the general formula: Li ａ Ni ｂ Co （１－ｂ－ｃ－ｄ） M ｃ Me ｄ O ｅ The secondary battery according to claim 1, wherein the composite oxide is represented by the general formula, the general formula satisfies 0.9 < a ​​≤ 1.2, 0.33 ≤ b ≤ 0.96, 0.03 ≤ c ≤ 0.70, 1.9 ≤ e ≤ 2.1, and 0 ≤ d ≤ 0.02, M is at least one selected from the group consisting of Al and Mn, and Me is a metallic element that is not Li, Ni, Co, or M.

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