Power storage device

Abstract

This power storage device comprises an electrode body in which a first electrode plate and a second electrode plate are wound with a separator interposed therebetween, an electrolyte, a bottomed cylindrical case, and a sealing body. The first electrode plate is provided with a current collector having a first main surface and a second main surface, and a first active material layer provided on the first main surface. The first active material layer includes a first region extending from one width-direction end of the current collector to an intermediate point, and a first main region extending from the intermediate point to the other width-direction end. The mass per unit volume of the first active material layer in the first region is at least 50% and less than 100% of the mass per unit volume of the first active material layer in the first main region, and the proportion of the first region in the first active material layer is 50% or less. The electrode body is accommodated inside the case such that the the first region faces the sealing-body side and the first main region faces the bottom-part side of the case.

Description

Energy storage device Cross-reference of related applications

[0001] This disclosure claims priority rights to Japanese Patent Application No. 2024-218914, filed with the Japan Patent Office on 13 December 2024, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to an energy storage device.

[0003] Patent Document 1 proposes "an electrode plate for an energy storage device, comprising a long sheet-like current collector having a first main surface and a second main surface, and a first active material layer provided on the first main surface, wherein the first main surface has at least one first region that extends along the width direction of the current collector and in which the thickness of the first active material layer is smaller than that of the surrounding area, and the first active material layer is provided on at least a part of the first region."

[0004] Patent Document 2 proposes "an electrode for use in a non-aqueous electrolyte secondary battery, comprising a strip-shaped current collector and a composite layer formed on the current collector, wherein one end edge in the width direction of the current collector is provided with an exposed portion without the composite layer, and the composite layer is such that the average value of the first composite density in the first region, which is the end edge on the exposed portion side, is lower than the average value of the second composite density in the second region, which is the portion of the composite layer other than both ends in the width direction, and the second composite density is substantially constant."

[0005] Patent Document 3 proposes an electrode sheet comprising a holding portion and a blank portion, wherein a positive electrode mixture layer is coated on at least one surface of a current collector, and the holding portion comprises a first positive electrode mixture layer formed in the central part along the length of the electrode sheet and 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 having a lower nickel content than the first positive electrode active material, wherein 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.

[0006] International Publication No. 2024 / 057630, Japanese Patent Publication No. 2015-018765, International Publication No. 2021 / 225303

[0007] To improve the capacity of an energy storage device, it is effective to increase the mass of the active material layer per unit area of ​​the current collector in the electrodes. 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 energy storage device may be reduced.

[0008] One aspect of this disclosure comprises an electrode body formed by winding a first electrode plate and a second electrode plate with a separator in between, an electrolyte, a bottomed cylindrical case for housing the electrode body and the electrolyte, and a sealing body for sealing the opening of the bottomed cylindrical case, wherein the first electrode plate comprises a long sheet-like current collector having a first main surface and a second main surface, and a first active material layer provided on the first main surface, wherein the first active material layer contains an active material and comprises a first region extending from one end in the width direction of the current collector to the midpoint in the width direction, and from the midpoint to the width The present invention relates to an energy storage device having a first main region extending to the other end in a direction, wherein the mass per unit volume of the first active material layer of the first region is smaller than the mass per unit volume of the first active material layer of the first main region and is 50% or more of the mass per unit volume of the first active material layer of the first main region, the proportion of the first region in the first active material layer is 50% or less, and the electrode body is housed in a bottomed cylindrical case with the first region facing the sealing body side and the first main region facing the bottom of the bottomed cylindrical case.

[0009] Another aspect of the present disclosure comprises an electrode body formed by winding a first electrode plate and a second electrode plate with a separator in between, an electrolyte, a bottomed cylindrical case for housing the electrode body and the electrolyte, and a sealing body for sealing the opening of the bottomed cylindrical case, wherein the first electrode plate comprises a long sheet-like current collector having a first main surface and a second main surface, and a first active material layer provided on the first main surface, wherein the first active material layer contains an active material and comprises a first region extending from one end in the width direction of the current collector to the midpoint in the width direction, and the midpoint The present invention relates to an energy storage device having a first main region extending from a point to the other end in the width direction, wherein the mass per unit volume of the first active material layer in the first region is smaller than the mass per unit volume of the first active material layer in the first main region and is 50% or more of the mass per unit volume of the first active material layer in the first main region, the proportion of the first region in the first active material layer is 50% or less, and the average particle diameter of the active material contained in the first region is greater than the average particle diameter of the active material contained in the first main region.

[0010] 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.

[0011] This is a schematic longitudinal cross-sectional view showing an example of an energy storage device according to one embodiment of the present disclosure. It shows a front view (2A) of an example of a positive electrode plate, a cross-sectional view (2B) along the line IIB-IIB in the front view, and a cross-sectional view (2C) along the line IIC-IIC in the front view. This is a front view of another example of a positive electrode plate.

[0012] 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.

[0013] 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.

[0014] The energy storage device according to this disclosure may be a battery or a capacitor. The battery may be a primary battery such as a lithium primary battery, an alkaline storage battery (such as a nickel-metal hydride battery or a nickel-cadmium battery), or a non-aqueous electrolyte secondary battery. The energy storage device according to this disclosure includes an electrode body.

[0015] The type of energy storage device is not particularly limited, but an energy storage device to which this disclosure can be suitably applied is typically a non-aqueous electrolyte secondary battery capable of achieving high capacity. Non-aqueous electrolyte secondary batteries include lithium-ion secondary batteries, solid-state batteries containing gel electrolytes or solid electrolytes, and the like. That is, 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.

[0016] The power storage device includes an electrode body formed by winding a first electrode plate and a second electrode plate with a separator interposed therebetween, an electrolyte, a bottomed cylindrical case that houses the electrode body and the electrolyte, and a sealing body that seals the opening of the bottomed cylindrical case. That is, the power storage device includes a wound electrode body (electrode group). The cross-sectional shape perpendicular to the winding axis of the wound electrode body is, for example, circular or elliptical, and the outer shape may be, for example, cylindrical, but is not limited thereto. The sealing body may be provided with a safety mechanism such as an exhaust valve that reduces the internal pressure of the case in case of an abnormality.

[0017] Hereinafter, the power storage device will be mainly described assuming a lithium-ion secondary battery.

[0018] In a lithium-ion secondary battery, one of the first electrode plate and the second electrode plate corresponds to a strip-shaped positive electrode and the other corresponds to a strip-shaped negative electrode. The separator may be composed of a porous sheet having ion permeability and insulation. Examples of the porous sheet include a microporous thin film, a woven fabric, and a non-woven fabric.

[0019] The first electrode plate includes a long sheet-shaped current collector having a first main surface and a second main surface, and a first active material layer provided on the first main surface. The first active material layer contains an active material and has a first region extending from one end in the width direction Dw of the current collector to an intermediate point in the width direction Dw and a first main region extending from the intermediate point to the other end in the width direction Dw.

[0020] The first electrode plate may have one or more first uncoated portions (exposed portions of the current collector) provided partially along the longitudinal direction of the current collector. In this case, the first region is provided at one or more locations along the longitudinal direction of the current collector, but may be provided over the entire length direction of the current collector except for the first uncoated portion. The first region may be provided over 80% or more (more preferably 98% or more) of the length in the longitudinal direction of the current collector.

[0021] In a specific aspect of the first electrode plate, the first electrode plate can be divided into a strip-shaped region (edge portion) including one end in the width direction Dw of the current collector and a first main region (main portion) other than the first region. In this case, the first region can be provided in the strip-shaped region. The first uncoated portion can be provided at one or more locations along the longitudinal direction of the current collector in the strip-shaped region. The first uncoated portion may be an exposed portion of the current collector having a substantially rectangular shape with a predetermined width in the longitudinal direction of the current collector. The first uncoated portion can be provided at one or more locations or intermittently at a plurality of locations so as to divide the first region provided in the strip-shaped region.

[0022] Here, in the first electrode plate, the mass per unit volume of the first active material layer in the first region is smaller than the mass per unit volume of the first active material layer in the first main region. When a wound electrode body is manufactured using the first electrode plate having such a first region, the first region is disposed on one end face side of the electrode body. Since the first region is a region with a small mass per unit volume, on one end face side of the electrode body where the first region is disposed, the permeability of the electrolyte is relatively high, and the heat dissipation property of the electrode body is also relatively high. The heat generated inside the electrode body is quickly conducted from the inner side of the highly heat-dissipating first region disposed on one end face side of the electrode body to one end side in the width direction Dw of the current collector (i.e., the end face of the electrode body) and released to the outside of the electrode body. Further, the electrolyte extruded to the outside of the electrode body during charging can quickly penetrate from the end face of the electrode body into the first region with high electrolyte permeability and move to the inner side (toward the center of the electrode body) of the first region.

[0023] However, the mass per unit volume of the first active material layer in the first region (hereinafter, also referred to as "volume density M1") needs to be 50% or more of the mass per unit volume of the first active material layer in the first main region (hereinafter, also referred to as "volume density Mm1"), and the ratio of the first region in the first active material layer (i.e., the ratio of the first region in the total of the first region and the first main region) (hereinafter, also referred to as "ratio (1 / (1 + M))") needs to be 50% or less. Thereby, it is possible to improve the electrolyte permeability and the heat dissipation property of the electrode body while suppressing a decrease in the energy density of the electrode body.

[0024] Furthermore, the ratio (1 / (1+M)) can be rephrased as the ratio of the area S1 of the first main surface of the current collector on which the first region is formed to the area S0 of the first main surface of the current collector on which the first active material layer is formed.

[0025] From the viewpoint of ensuring high capacity, the area of ​​all first uncoated portions is sufficiently small compared to the area of ​​the first main surface, for example, 2% or less of the area of ​​the first main surface. Therefore, the ratio (1 / (1+M)) can be rephrased as the ratio of the area S1 of the first main surface of the current collector on which the first region is formed to the total area of ​​the first main surface of the current collector. In other words, "the area S0 of the first main surface of the current collector on which the first active material layer is formed" can be rephrased as "the total area of ​​the first main surface of the current collector".

[0026] Next, it is preferable that the electrode body is housed in a closed-bottom cylindrical case with the first region facing the sealing body and the first main region facing the bottom of the closed-bottom cylindrical case. With this arrangement, the permeability of the electrolyte in the electrode body and the heat dissipation of the electrode body can be significantly improved. When the electrode body generates heat inside the closed-bottom cylindrical 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 first main region facing the sealing body and the first region facing the bottom of the closed-bottom cylindrical case, the heat generated at the bottom of the closed-bottom cylindrical case will be blocked by the first main region with a high volume density Mm1, 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] A first electrode tab may be connected to the first uncoated area. The first uncoated area is a region in the width direction Dw that does not have a first active material layer from one end to the first main region. If multiple first uncoated areas are formed on the first main surface, a first electrode tab may be connected to each of the multiple first uncoated areas.

[0028] By placing the first electrode tab and the first uncoated section, which are prone to generating heat, on the end face side of the electrode body where the first region with high heat dissipation is located, the heat dissipation of the electrode body can be further enhanced. In addition, by leading the first electrode tab from the end face side of the electrode body where the first region with low volume density M1 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 around the first uncoated area to which the first electrode tab is connected increases. In that case, winding the first electrode plate when constructing the wound body can become difficult. In contrast, when the first uncoated area and the first region with a low volume density M1 are adjacent, winding the first electrode plate becomes relatively easy.

[0030] The length of the first uncoated portion in the width direction Dw and the length Lr1 of the first region in the width direction Dw may be approximately the same. For example, 0.9 ≤ Lex / Lrl ≤ 1.1 may be satisfied. In this case, the length of the first main region Rm1 in the width direction Dw is approximately constant along the longitudinal direction Dleng.

[0031] The width of the longitudinal Dleng of each first uncoated current collector is preferably not excessively large from the viewpoint of maintaining a high energy density of the electrode body, and 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 first electrode tab is attached to the first unpainted portion, it is preferable that the length of the first unpainted portion in the longitudinal direction Dleng of the current collector is greater than the length of the first electrode tab. Furthermore, from the viewpoint of suppressing internal short circuits, it is preferable that a part of the first electrode tab, together with at least a part of the first 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 M1 of the first region is preferably 90% or more of the volume density Mm1 of the first 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 M1 of the first region is preferably 99.5% or less of the mass per unit volume density Mm1 of the first 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 first region R1 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] The first region and the first main region may be formed by preparing two types of slurries (one for the first region and one for the first main region) in which the electrode mixture constituting the first active material layer is dispersed in a liquid component, applying the two types of slurries to the first main surface, drying, and rolling. The slurry for the first region and the slurry for the first main region may be applied using the first and second application apparatuses, respectively.

[0036] The first uncoated portion may be formed by intermittently applying a slurry in which the electrode mixture constituting the first active material layer is dispersed in a liquid component to the first main surface.

[0037] If the current collector is the positive electrode current collector of a lithium-ion secondary battery, the current collector may be made of aluminum foil or aluminum alloy foil. If the current collector is the negative electrode current collector of a lithium-ion secondary battery, the current collector may be made of copper foil or copper alloy foil.

[0038] If the first active material layer provided on the first main surface is the positive electrode active material layer of a lithium-ion secondary battery, the first active material layer may include a positive electrode active material (e.g., lithium-containing transition metal oxide), a conductive agent, a binder, etc. If the first active material layer is the negative electrode active material layer of a lithium-ion secondary battery, the first active material layer may include a negative electrode active material (carbonaceous material, silicon-containing material, etc.), a binder, etc.

[0039] The average particle diameter of the active material contained in the first main region may be greater than the average particle diameter of the active material contained in the first main region. This makes it easier to design the volume density M1 of the first region to be smaller than the volume density Mm1 of the first main region. The average particle diameter refers to the median diameter (D50) in the volume-based particle size distribution of the active material.

[0040] For example, a slurry in which an electrode mixture for the first region is dispersed in a liquid component may be applied to a part of a wide current collector, and a slurry in which an electrode mixture for the first main region is dispersed in a liquid component may be applied to another part of the wide current collector, the two slurry coatings may be dried, and then these coatings may be rolled simultaneously. In this case, the manufacturing process is simple, and a first region and a first main region having a first active material layer of approximately the same thickness can be formed.

[0041] The thickness T1 of the first active material layer in the first region may be, for example, 99.6% or more of the thickness Tm1 of the first active material layer in the first main region. The ratio T1 / Tm1 of the thickness T1 of the first active material layer in the first region to the thickness Tm1 of the first active material layer in the first main region may be, for example, 0.996 or more and 1.004 or less. The fact that T1 and Tm1 satisfy 0.996 ≤ T1 / T2 ≤ 1.004 means that, considering manufacturing variations, T1 = Tm1 (i.e., T1 / Tm1 = 1). The thickness of the first region R1 and the thickness of the first main region Rm1 may be determined as the average value of the thicknesses at any five locations in the first region R1 and the first main region Rm1, respectively.

[0042] In the first main region where the average particle size of the active material is relatively small, a higher density first active material layer can be formed. In such a first main region, the surface area of ​​the active material is relatively large, resulting in a larger reaction rate and a relatively larger amount of heat generated. On the other hand, in the first region where the average particle size of the active material is relatively large, a lower density first active material layer can be formed. In such a first region, the surface area of ​​the active material is relatively small, resulting in a relatively smaller amount of heat generated. Heat moves from high-temperature regions to low-temperature regions. Therefore, heat transfer from the first main region to the first region is significantly promoted.

[0043] If the average particle diameter of the active material contained in the first main region is greater than the average particle diameter of the active material contained in the first main region, the average particle diameter of the active material contained in the first main region (median diameter (D50) = dm1 in the volume-based particle size distribution of the active material) may be, for example, 1 μm or more and 17 μm or less, and the average particle diameter of the active material contained in the first region (median diameter (D50) = d1 in the volume-based particle size distribution of the active material) may be, for example, 6 μm or more and 25 μm or less.

[0044] As described above, when the average particle size of the active material contained in the first main region is controlled, the volume density M1 of the first region can be, for example, 90% or more and 99.5% or less of the volume density Mm1 of the first main region.

[0045] Furthermore, the effect of the above configuration on improving the permeability of the electrolyte in the electrode body and the heat dissipation of the electrode body increases as the volume density of the first main region in the first electrode plate increases (in other words, as the energy density of the electrode body increases). Therefore, for example, if the first electrode plate is the positive electrode of a lithium-ion secondary battery, the volume density of the first main region is, for example, 240 g / m³. ２ The above is also acceptable, 260 g / m ２ The above is also acceptable, 280 g / m ２ Anything above that is also acceptable; for example, 240 g / m ２ 350g / m or more ２ It may fall within the following range.

[0046] Furthermore, the energy of the electrode body is, for example, 15 Wh or more in the case of lithium-ion secondary batteries, and may be, for example, 80 Wh 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 cylindrical case having an inner diameter slightly larger than the diameter of the cylinder, thus forming one cell.

[0047] Furthermore, for example, if the first electrode plate is the positive electrode of a lithium-ion secondary battery, the thickness of the first main region (the positive electrode active material layer) 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.

[0048] Furthermore, the effect of the above configuration on improving the permeability of the electrolyte in the electrode body and the heat dissipation of the electrode body increases with increasing number of turns in a wound electrode body. The greater the number of turns in the electrode body, the greater the length of the first electrode plate in the longitudinal direction. The length of the first electrode plate in the longitudinal direction may be, for example, 3000 mm or more.

[0049] The first electrode plate may further comprise a second active material layer provided on the second main surface. The composition of the second active material layer may be the same as or different from that of the first active material layer.

[0050] The configuration of the second main surface can be arbitrarily applied to the configuration of the first main surface. The second main surface may have the same configuration as the first main surface.

[0051] In other words, the second active material layer may contain active material and have a second region extending from one end in the width direction Dw of the current collector to the midpoint in the width direction Dw, and a second main region extending from the midpoint to the other end in the width direction Dw. The mass per unit volume of the second active material layer in the second region (hereinafter also referred to as "volume density M2") is smaller than the mass per unit volume of the second active material layer in the second main region (hereinafter also referred to as "volume density Mm2"), and may be 50% or more of the volume density of the second main region. The proportion of the second region in the second active material layer (i.e., the proportion of the second region in the sum of the second region and the second main region) (hereinafter also referred to as "proportion (2 / (2+M))") may be 50% or less.

[0052] The first and second regions may overlap in at least a portion when viewed from the direction normal to the first principal surface. This ensures a more reliable flow path for the electrolyte in the electrode body and a heat dissipation path from the electrode body, and can significantly improve the permeability of the electrolyte in the electrode body and the heat dissipation of the electrode body.

[0053] The first and second regions may overlap by, for example, 50%, 60%, 70%, 80%, or 90% or more when viewed from the normal direction of the first main surface. In this case, the rigidity of the area where the first and second regions are located can be reduced, making it possible to perform winding smoothly and accurately.

[0054] The following section specifically describes the case where the first electrode plate is the positive electrode plate of a lithium-ion secondary battery. The positive electrode plate (first electrode plate) comprises a long, sheet-like current collector (positive electrode current collector) having a first main surface and a second main surface, a first active material layer (positive electrode active material layer) provided on the first main surface, and a second active material layer (positive electrode active material layer) provided on the second main surface. Since the configuration on the first main surface side and the configuration on the second main surface side are the same, the configuration on the first main surface side will be described below.

[0055] The positive electrode active material layer is composed of a positive electrode mixture. The positive electrode mixture contains the positive electrode active material as an essential component and may contain optional components such as binders, conductive additives, and thickeners. The positive electrode active material layer may also be referred to as the positive electrode mixture layer.

[0056] The positive electrode active material layer can be obtained, for example, by applying a positive electrode slurry in which a positive electrode binder containing particles of a positive electrode active material, a binder, a conductive aid, etc. is dispersed in a dispersion medium onto the surface of a positive electrode current collector, drying it, and rolling the dried coating film. As the dispersion medium, N-methyl-2-pyrrolidone (NMP), cyclohexanone, alcohols, ethers, etc. are used.

[0057] The binder may contain, for example, a fluorine-based polymer. The fluorine-based polymer can exhibit high binding force. The fluorine-based polymer is a general term for polymers having fluorine atoms (F) bonded to carbon atoms constituting the main chain.

[0058] Examples of the conductive aid include carbon materials such as graphite, carbon blacks such as furnace black and acetylene black, carbon fibers (carbon nanotubes (CNT), carbon fibers other than CNT), graphene, etc.

[0059] As the positive electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, punching sheet, etc.) is used. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, etc. The thickness of the positive electrode current collector is not particularly limited, but 5 to 20 μm is preferable.

[0060] 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, but may be, for example, a layered rock salt type.

[0061] As the positive electrode active material, for example, a composite oxide containing lithium and transition metals such as Ni, Co, Mn, etc. may be used. Specifically, Li ａ 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 Oｃ Li ａ Ni １－ｂ M ｂ O ｃ Li ａ Mn ２ O 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.

[0062] Among them, Li ａ Ni １－ｂ M ｂ O ２ Lithium nickel composite oxides represented by (M being at least one selected from the group consisting of Mn, Co, and Al, with 0 < a ≤ 1.2 and 0 < b < 0.7) are preferred. From the viewpoint of increasing capacity, it is more preferable that 0 < b < 0.2 is satisfied. From the viewpoint of crystal structure stability, Li containing Co and Al as M is preferred. ａ Ni １－ｂ Co ｄ Al ｅ O ｃ , or Li containing Co and Mn as M ａ Ni １－ｂ Co ｄ Mn ｅ O ｃ (0 < a ≤ 1.2, 0 < b < 0.2, 0 < d < 0.15, 0 < e ≤ 0.1, b = d + e) ​​is even more preferable. Also, Li ａ Ni ｂ Co （１－ｂ－ｃ） Me ｃ M d O e(wherein the formula, 0.9 < a ​​≤ 1.2, 0.3 ≤ b ≤ 0.96, 0.03 ≤ c ≤ 0.70, 1.9 ≤ e ≤ 2.1, and Me / (Ni + Co + M + Me) = d, 0 ≤ d ≤ 0.02.) is also preferred, where Me is at least one of Al and Mn, and M is 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, B.)

[0063] The average particle size d1 of the active material contained in the first region is, for example, 6 μm or more and 25 μm or less, and may be 8 μm or more, 10 μm or more, 11 μm or more, 12 μm or more, or 15 μm or more. Furthermore, from the viewpoint of improving discharge characteristics, d1 may be 25 μm or less, 20 μm or less, or 17 μm or less. d1 may be 6 ≤ d1 (μm) ≤ 25, and preferably 6 ≤ d1 (μm) ≤ 17.

[0064] The average particle size dm1 of the active material contained in the first main region is, for example, 1 μm or more and 17 μm or less, and may be 10 μm or less, 8 μm or less, 6 μm or less, or 5 μm or less. From the viewpoint of improving charge-discharge cycle characteristics, it is preferable that dm1 be 1 μm or more, and may be 3 μm or more. dm1 may be 1 ≤ dm1 (μm) ≤ 10, and preferably 1 ≤ dm1 (μm) ≤ 5.

[0065] The average particle size of the positive electrode active material is preferably such that d1 > dm1, and the d1 / dm1 ratio may be, for example, 2 to 6 or 3 to 5.

[0066] The types (composition formulas) of the positive electrode active materials contained in the first region and the first main region may be the same or different. The positive electrode active materials contained in the first region and the first main region may each independently contain multiple types of positive electrode active materials or contain a single positive electrode active material.

[0067] The median diameter (D50) in the volume-based particle size distribution may be measured by separating the positive electrode active material from the first region and the first main region of the positive electrode active material layer, or it may be determined by image analysis of cross-sectional SEM images of the first region and the first main region of the positive electrode active material layer. Both methods yield approximately the same (without significant difference) median diameter (D50).

[0068] When separating the positive electrode active material from the positive electrode active material layer, the positive electrode active material layer may be peeled off from the first region or the first main region, immersed in a suitable solvent to dissolve or swell components other than the active material particles, such as dispersants, and separated by centrifugation, and this process may be repeated 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 median diameter can be determined.

[0069] When performing image analysis of a cross-sectional SEM image of the positive electrode active material layer, the positive electrode active material layer and the positive electrode current collector are simultaneously cut along the width direction of the positive electrode to obtain cross-sectional samples in the thickness direction of the first region and the first main region. At this time, the cross section may be processed with a cross-section polisher (CP) to obtain the cross-sectional sample. Next, the cross section of the positive electrode active material layer in the cross-sectional sample is observed using a scanning electron microscope (SEM).

[0070] From the contour images of the active material particles in the cross-sectional SEM image, the area enclosed by the contour is determined. The diameter of a circle (equivalent circle) having the same area as the area enclosed by the contour of the active material particles 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 considered as the volume Vi of each particle i. By determining the diameter and volume of the equivalent circles for any 100 or more (preferably 1000 or more) particles, the volume-based particle size distribution and median diameter can be calculated.

[0071] Next, the configuration of the second electrode plate can be arbitrarily applied to the configuration of the first electrode plate as described above. The second electrode plate may have the same configuration as the first electrode plate. However, the configuration of the second electrode plate is arbitrary and can have various configurations different from those of the first electrode plate.

[0072] If the first electrode plate is the positive electrode plate of a lithium-ion secondary battery, then the second electrode plate is the negative electrode plate of the lithium-ion secondary battery. The following section will describe the negative electrode plate of a lithium-ion secondary battery.

[0073] The negative electrode plate (second electrode plate) has a long, sheet-like current collector (negative electrode current collector) having a first main surface and a second main surface, and may include a first active material layer (negative electrode active material layer) provided on the first main surface and a second active material layer (negative electrode active material layer) provided on the second main surface. Note that the negative electrode of a lithium metal secondary battery does not need to have a negative electrode active material layer.

[0074] The negative electrode active material layer is composed of, for example, 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 called the negative electrode mixture layer. Alternatively, the negative electrode active material layer may be composed of at least one selected from the group consisting of lithium metal and lithium alloy.

[0075] The negative electrode active material 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.

[0076] 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).

[0077] Examples of binders include acrylic resins and rubber-like materials such as styrene-butadiene copolymer rubber (SBR).

[0078] 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.

[0079] Examples of thickening agents include carboxymethylcellulose (CMC) and its modified forms (including salts such as sodium salts).

[0080] 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.

[0081] Below, an example of an energy storage device related to this disclosure will be described in detail with reference to the drawings. The components of the energy storage device in the example described below can be the components described above. The components of the energy storage device in the example described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiments. Among the components of the energy storage device in the example described below, components that are not essential to the energy storage device related 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.

[0082] The energy storage device 10 of this embodiment is configured as a lithium-ion secondary battery, but is not limited to this. As shown in Figure 1, the energy storage device 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.

[0083] The electrode body 20 includes a positive electrode plate 21, a negative electrode plate 29, and a separator 31. The positive electrode plate 21, the negative electrode plate 29, and the separator 31 are all in the form of long sheets (or strips). The positive electrode plate 21 and the negative electrode plate 29 are wound in a spiral shape with the separator 31 interposed between them, so that their width directions are 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. The positive electrode plate 21 is an example of a first electrode plate, and the negative electrode plate 29 is an example of a second electrode plate. The configuration of the positive electrode plate 21 will be described in detail later.

[0084] 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.

[0085] 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.

[0086] The positive electrode tab 61 is attached to the first unpainted portion 24 (described later) of the positive electrode plate 21, and electrically connects the positive electrode plate 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. The first unpainted portion 24 and the positive electrode tab 61 may each be provided as one or multiple. The positive electrode tab 61 is an example of a first electrode tab.

[0087] The negative electrode tab 62 is attached to the negative electrode plate 29 and electrically connects the negative electrode plate 29 and the case 40. Specifically, one end of the negative electrode tab 62 is connected to the negative electrode plate 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.

[0088] Next, with reference to Figure 2, the configuration of the positive electrode plate 21 will be explained in detail.

[0089] As shown in Figure 2, the positive electrode plate 21 comprises a long, sheet-like current collector 22 having a first main surface 23 and a second main surface 25, a first active material layer (positive electrode active material layer) 27 provided on the first main surface 23, and a second active material layer (positive electrode active material layer) 28 provided on the second main surface 25.

[0090] The current collector 22 is made of, for example, aluminum foil or aluminum alloy foil. The longitudinal direction Dleng of the current collector 22 is the left-right direction in Figure 2A, and the width direction Dw of the current collector 22 is the up-down direction in Figure 2A. The longitudinal direction Dleng of the current collector 22 coincides with the winding direction of the electrode body 20, and the width direction Dw of the current collector 22 coincides with the axial direction of the electrode body 20.

[0091] The first active material layer 27 has a first region R1 extending from one end in the width direction Dw of the 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 region R1 is provided at one or more locations along the longitudinal direction Dleng of the current collector 22 and is provided over the entire length direction Dleng of the current collector 22, except for the first uncoated portion 24.

[0092] In other words, the first active material 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 region R1 constituting the edge portion. The first region Rm1 is the main portion. The edge portion is provided in a strip shape along the longitudinal direction of the current collector 22. The length of the first region R1 in the width direction Dw is, for example, 1% to 10% of the length of the current collector 22 in the width direction Dw.

[0093] The first active material layer 27 includes a positive electrode active material (e.g., lithium-containing transition metal oxide), a conductive agent, and a binder. The second region R2 and the second main region Rm2 of the second active material layer 28 have the same composition as the first region R1 and the first main region Rm1 of the first active material layer 27, respectively.

[0094] The first unpainted portion 24 is provided at one or more locations along the longitudinal direction of the current collector 22. The first unpainted portion 24 is an exposed portion of the current collector 22 that is roughly rectangular and has a predetermined width in the longitudinal direction of the 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 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.

[0095] 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.

[0096] 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.

[0097] The electrode body 20 is housed in the case with its first region R1 facing the sealing body 50 and its 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.

[0098] The second active material layer 28 has the same configuration as the first active material layer 27. That is, the second active material layer 28 has a second region R2 extending from one end in the width direction Dw of the 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 region R2 is provided at one or more locations along the longitudinal direction Dleng of the current collector 22 and extends over the entire length direction Dleng of the current collector 22, except for the second uncoated portion 26. The length of the second region R2 in the width direction Dw is, for example, 1% to 10% of the length of the width direction Dw of the current collector 22, and is the same as the length Lr1 of the first region R1 in the width direction Dw.

[0099] The second unpainted portion 26 is provided at one or more locations along the longitudinal direction of the current collector 22. The second unpainted portion 26 is an exposed portion of the current collector 22 that is roughly rectangular and has a predetermined width in the longitudinal direction of the 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 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.

[0100] The second region R2 is located on the back side of the first region R1. When viewed from the direction normal to the first main surface 23, the first region R1 and the second region R2 overlap each other by at least a portion (preferably 90% or more).

[0101] The volume density M1 of the first active material layer in the first region R1 is smaller than the volume density Mm1 of the first active material layer in the first main region Rm1. On the other hand, the thickness of the first region R1 is approximately the same as the thickness of the first main region Rm1.

[0102] Similarly, the volume density M2 of the second active material layer in the second region R2 is smaller than the volume density Mm2 of the second active material layer in the second main region Rm2. On the other hand, the thickness of the second region R2 is approximately the same as the thickness of the second main region Rm2.

[0103] In other words, the density per unit volume of the positive electrode active material layer in the first region R1 and the second region R2 is relatively low, which plays 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 positive electrode active material layer in the first main region R1 and the second main region R2 is relatively high, which plays a role in maintaining a high energy density in the electrode body.

[0104] The average particle diameter d1 of the positive electrode active material contained in the first region R1 and the second region R2 may be larger than the average particle diameter dm1 of the positive electrode active material contained in the first main region Rm1 and the second main region Rm2 (d1 > dm1), and the d1 / dm1 ratio may be, for example, 2 or more and 6 or less.

[0105] [Note] The above description of embodiments discloses the following technologies. (Technical 1) An electrode body formed by winding a first electrode plate and a second electrode plate with a separator in between, an electrolyte, a bottomed cylindrical case for housing the electrode body and the electrolyte, and a sealing body for sealing the opening of the bottomed cylindrical case, wherein the first electrode plate comprises a long sheet-like current collector having a first main surface and a second main surface, and a first active material layer provided on the first main surface, wherein the first active material layer contains an active material and has a first region extending from one end in the width direction of the current collector to the midpoint in the width direction, and a first main region extending from the midpoint to the other end in the width direction, wherein the mass per unit volume of the first active material layer in the first region is smaller than the mass per unit volume of the first active material layer in the first main region, and is 50% or more of the mass per unit volume of the first active material layer in the first main region, and the proportion of the first region in the first active material layer is 50% or less, (Technical 2) The electrode body is housed in a bottomed cylindrical case with the first region facing the sealing body side and the first main region facing the bottom of the bottomed cylindrical case. (Technical 1) The first electrode plate has one or more first uncoated portions partially provided along the longitudinal direction of the current collector, and the first uncoated portion does not have the first active material layer from one end in the width direction to the first main region, the energy storage device according to Technical 1. (Technical 3) The energy storage device according to Technical 1 or 2, wherein the length Lex of the first uncoated portion in the width direction and the length L1 of the first region in the width direction satisfy 0.9 ≤ Lex / Ll ≤ 1.1. (Technical 4) The energy storage device according to any one of Technical 1 to 3, wherein the average particle diameter of the active material contained in the first region is greater than the average particle diameter of the active material contained in the first main region. (Technology 5) The energy storage device according to any one of Technology 1 to 4, wherein the thickness of the first active material layer in the first region is 99.6% or more of the thickness of the first active material layer in the first main region. (Technology 6) The energy storage device according to any one of Technology 1 to 5, wherein the average particle diameter of the active material contained in the first main region is 1 μm or more and 17 μm or less, and the average particle diameter of the active material contained in the first region is 6 μm or more and 25 μm or less.(Technology 7) The energy storage device according to any one of Technology 1 to 6, wherein the mass per unit volume of the first active material layer in the first region is 90% or more and 99.5% or less of the mass per unit volume of the first active material layer in the first main region. (Technology 8) The energy storage device according to any one of Technology 1 to 7, wherein the length of the first electrode plate in the longitudinal direction is 3000 mm or more. (Technology 9) The energy storage device according to any one of Technology 1 to 8, wherein the proportion of the first region in the first active material layer is 1% to 10%. (Technical 10) The first electrode plate further comprises a second active material layer provided on the second main surface, the second active material layer containing the active material and having a second region extending from one end in the width direction of the current collector to the midpoint in the width direction, and a second main region extending from the midpoint to the other end in the width direction, the mass per unit volume of the second active material layer in the second region being smaller than the mass per unit volume of the second active material layer in the second main region and being 50% or more of the mass per unit volume of the second active material layer in the second main region, and the proportion of the second region in the second active material layer being 50% or less, the energy storage device according to any one of Technical 1 to 9. (Technical 11) The energy storage device according to Technical 10, wherein the first region and the second region overlap each other in at least a portion when viewed from the direction normal to the first main surface.(Technical 12) An electrode body formed by winding a first electrode plate and a second electrode plate with a separator in between, an electrolyte, a bottomed cylindrical case for housing the electrode body and the electrolyte, and a sealing body for sealing the opening of the bottomed cylindrical case, wherein the first electrode plate comprises a long sheet-like current collector having a first main surface and a second main surface, and a first active material layer provided on the first main surface, wherein the first active material layer contains an active material and has a first region extending from one end in the width direction of the current collector to the midpoint in the width direction, and a first main region extending from the midpoint to the other end in the width direction, wherein the mass per unit volume of the first active material layer in the first region is smaller than the mass per unit volume of the first active material layer in the first main region, and is 50% or more of the mass per unit volume of the first active material layer in the first main region, and the proportion of the first region in the first active material layer is 50% or less, An energy storage device in which the average particle diameter of the active material contained in the first main region is greater than the average particle diameter of the active material contained in the first main region.

[0106] This disclosure can be used in energy storage devices such as lithium-ion secondary batteries.

[0107] 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.

[0108] 10: Energy storage device 20: Electrode body 21: Positive electrode plate (first electrode plate) 22: Current collector 23: First main surface 24: First uncoated area 25: Second main surface 26: Second uncoated area 27: First active material layer 28: Second active material layer 29: Negative electrode plate (second electrode plate) 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 (first electrode tab) 62: Negative electrode tab 71: First insulating plate 72: Second insulating plate 81: Gasket L1: Length of the first uncoated area L2: Length of the positive electrode tab R1: First region R2: Second region Tmin1: Minimum thickness of the first active material layer Tmin2: Maximum thickness of the first active material layer Tmax1: Minimum thickness of the second active material layer Tmax2: Maximum thickness of the second active material layer

Claims

1. An electrode body formed by winding a first electrode plate and a second electrode plate with a separator in between, an electrolyte, a bottomed cylindrical case for housing the electrode body and the electrolyte, and a sealing body for sealing the opening of the bottomed cylindrical case, wherein the first electrode plate comprises a long sheet-like current collector having a first main surface and a second main surface, and a first active material layer provided on the first main surface, wherein the first active material layer contains active material and has a first region extending from one end in the width direction of the current collector to the midpoint in the width direction, and a first main region extending from the midpoint to the other end in the width direction, wherein the mass per unit volume of the first active material layer in the first region is smaller than the mass per unit volume of the first active material layer in the first main region, and is 50% or more of the mass per unit volume of the first active material layer in the first main region, and the proportion of the first region in the first active material layer is 50% or less, An energy storage device in which the electrode body is housed in a bottomed cylindrical case with the first region facing the sealing body side and the first main region facing the bottom of the bottomed cylindrical case side.

2. The energy storage device according to claim 1, wherein the first electrode plate has one or more first uncoated portions partially provided along the longitudinal direction of the current collector, and the first uncoated portions do not have the first active material layer from one end in the width direction to the first main region.

3. The energy storage device according to claim 1, wherein the length Lex in the width direction of the first uncoated portion and the length L1 in the width direction of the first region satisfy 0.9 ≤ Lex / Ll ≤ 1.

1.

4. The energy storage device according to claim 1, wherein the average particle diameter of the active material contained in the first main region is greater than the average particle diameter of the active material contained in the first main region.

5. The energy storage device according to claim 1, wherein the thickness of the first active material layer in the first region is 99.6% or more of the thickness of the first active material layer in the first main region.

6. The energy storage device according to claim 1, wherein the average particle diameter of the active material contained in the first main region is 1 μm or more and 17 μm or less, and the average particle diameter of the active material contained in the first region is 6 μm or more and 25 μm or less.

7. The energy storage device according to claim 1, wherein the mass per unit volume of the first active material layer in the first region is 90% or more and 99.5% or less of the mass per unit volume of the first active material layer in the first main region.

8. The energy storage device according to claim 1, wherein the length of the first electrode plate in the longitudinal direction is 3,000 mm or more.

9. The energy storage device according to claim 1, wherein the proportion of the first region in the first active material layer is 1% to 10%.

10. The energy storage device according to claim 1, wherein the first electrode plate further comprises a second active material layer provided on the second main surface, the second active material layer containing the active material and having a second region extending from one end in the width direction of the current collector to the midpoint in the width direction, and a second main region extending from the midpoint to the other end in the width direction, the mass per unit volume of the second active material layer in the second region being smaller than the mass per unit volume of the second active material layer in the second main region and being 50% or more of the mass per unit volume of the second active material layer in the second main region, and the proportion of the second region in the second active material layer being 50% or less.

11. The energy storage device according to claim 10, wherein at least a portion of the first region and the second region overlap each other when viewed from the direction normal to the first main surface.

12. The electrode body comprises an electrode body formed by winding a first electrode plate and a second electrode plate with a separator in between, an electrolyte, a bottomed cylindrical case for housing the electrode body and the electrolyte, and a sealing body for sealing the opening of the bottomed cylindrical case, wherein the first electrode plate comprises a long sheet-like current collector having a first main surface and a second main surface, and a first active material layer provided on the first main surface, wherein the first active material layer contains active material and has a first region extending from one end in the width direction of the current collector to the midpoint in the width direction, and a first main region extending from the midpoint to the other end in the width direction, wherein the mass per unit volume of the first active material layer in the first region is smaller than the mass per unit volume of the first active material layer in the first main region, and is 50% or more of the mass per unit volume of the first active material layer in the first main region, and the proportion of the first region in the first active material layer is 50% or less, An energy storage device in which the average particle diameter of the active material contained in the first main region is greater than the average particle diameter of the active material contained in the first main region.

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